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POAC 81
PROCEEDINGS
COMPTES RENDUS
VOL. II

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Prof. Bernard Michel
Dep. de genie civil
Universite Laval
Cite universitaire
Quebec, Canada
G1K 7P4
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genie maritime dans I' Arctique ", Quebec, Canada, 1981.

POAC 81
The Sixth International Conference on Port and
Ocean Engineering under Arctic Conditions
Sixieme conference internationale sur Ie
genie maritime dans I' Arctique
Quebec, Canada
July 27-31, 1981
Du 27 au 31 juillet 1981
Proceedings
Comptes rendus
Volume II
Universite Laval, Quebec, Canada
Ministere de I'Environnement, Gouvernement du Quebec

SPONSORS - PARRAINEE PAR
Ministere de I'Environnement, Gouvernement du Quebec
Universite Laval, Quebec, Canada
CO-SPONSORS - CO-PARRAINEE PAR
Department of Transport, Ottawa
Ministere du Transport, Ottawa
Canadian Coast Guard, Ottawa
Garde cOtiere canadienne, Ottawa
Department of Northern and Indian Affairs, Ottawa
Ministere des Affaires indiennes et du Nord, Ottawa
National Research Council of Canada, Ottawa
Conseil national de recherches du Canada, Ottawa
Canadian Committee on Oceanography
Comite canadien sur I'oceanographie
Arctic Petroleum Operators Association
Association des operateurs petroliers de I'Arctique
Eastern Petroleum Operators Association
Association des operateurs petroliers de l'Est
Canadian Society for Civil Engineering
Societe canadienne de genie civil
Order of Engineers of Quebec
Ordre des ingenieurs du Quebec

INTERNATIONAL COMMITTEE - COMITE INTERNATIONAL
Dr. P. Bruun, The Norwegian Institute of Technology, Trondheim, Norway
(Chairman)
Dr. T. Carstens, River and Harbor Lab., Trondheim, Norway
Dr. R. Dempster, Memorial University of Newfoundland, St.John's, Nfld.
Mr. E. Ernstons, Swedish Board of Maritime Works, Stockholm, Sweden
Dr. K. Horikawa, University of Tokyo, Tokyo, Japan
Mr. A. Juliusson, University of Iceland, Reykjavik, Iceland
Dr. M. Maattanen, University of Oulu, Finland
Dr. B. Michel, Universite Laval, Quebec, Canada
Dr. W.M. Sackinger, University of Alaska, Fairbanks, USA
Dr. P. Tryde, Denmark's Technical University, Lyngby, Denmark
ORGANIZING COMMITTEE - COMITE D'ORGANISATION
Dr. B. Michel, professeur de Mecanique des glaces, Universite Laval (president)
Dr. Y. Ouellet. professeur de Genie maritime, Universite Laval (co-president)
M. B. Harvey, sous-ministre adjoint. Environnement Quebec (tresorier)
Dr. D. Carter, ingenieur-consultant, Quebec
Mr. K. Charbonneau,Chief, Conference Services, National Research Council of Canada,
M. J. Dery,
Dr. M. Frenette,
M. J.P. Godin,
Mr. G.D. Hobson,
Dr. O.H. Loken.
Dr. R. Peters,
Dr. J-L. Verrette,
Ottawa
Jacques Dery & Associes Inc., Montreal
president. Societe canadienne de genie civil
directeur regional, Garde cOtiere canadienne. Quebec
Department of Energy. Mines and Resources, Ottawa
Director, Environment Division, Department of Northern and Indian
Affairs, Ottawa
Associate Dean, Memorial University of Newfoundland, St.John's,
Nfld.
professeur d'Hydrodynamique, Universite Laval
LADIES' COMMITTEE - COMITE FEMININ
Mme Mariette Michel (presidente)
Mme Ghislaine Carter
Mme Monique Frenette
Mme Suzanne Godin
Mme Suzanne Harvey
Mme Madeleine Ouellet
Mme Claire Verreault
Mme Marielle Verrette
SECRETARIES - SECRETAIRES
Mme Jeanne Roy
Mme Diane Dussault

TABLE OF CONTENTS - TABLE DES MATIERES
PAGE
SPONSORS - PARRAINS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV
COMMITTEES - COMITES ...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V
PRESENTED PAPERS - ARTICLES PRESENTES
OPENING SESSION - SESSION D'OUVERTURE
A. Soucy, Societe d'Energie de la Environmental Impact of Hydro Plants in
Baie James, Canada Northern Quebec- Vol. III
B. Johansson, Canadian Marine
Drilling Ltd, Canada
Dome Petroleum Operations in the
Beaufort Sea- Vol. III
Session A1 MARINE STRUCTURES - STRUCTURES MARINES
P. Bruun and G. Moe, Norwegian
Institute of Technology, Norway
V. Di Tella, G. Sebastiani,
Tecnomare S.p.A., Italy
C. Maclean, Panarctic Oils Ltd,
W. Semotiuk, Engineered Urethanes
Ltd., A. Strandberg and
D. M. Masterson, Fenco Consultants
Ltd., Canada
B.R. Wasilewski, Gulf Canada
Resources Inc., J. C. Bruce, Albery,
Pullerits, Dickson & Associates,
Canada
C. A. Wortley, University of
Wisconsin, U.S.A.
J. V. Danys, Professional Engineer,
Canada
L. D. Brooks, Chevron Oil Field
Research Co., U.S.A.
- Preliminary titles
Titres provisoires
Design Criteria for Nearshore and
Offshore Structures under Arctic
Conditions
Production System in Arctic Waters
by using a Fully Integrated TSG Platform
Ice Platforms with Urethane Foam Cells
in the Neutral Axis Zone and their
Application in Arctic Offshore Drilling
Conceptual Design for a Mobile Arctic
Gravity Platform
Marine Piling and Boat Harbor Structure
Design for Ice Conditions
Offshore Structures on Weak
Foundations Exposed to Large Ice Forces
Ice Resistance Equation for Fixed
Conical Structures
39
49
60
70
80
90

Session 81 NAVIGATION IN COLD REGIONS­
NAVIGATION DANS LES REGIONS FROIDES
J. Lewis, Arctec Incorporated, U.S.A.
P. McCallister and Carl Argiroff,
U.S. Army Engineer Detroit District,
U.S.A.
S. Skarborn, Albery, Pullerits,
Dickson & Associates, Canada
K. Takekuma, Mitsubishi Heavy
Industries Ltd., Japan, P. Noble and
A. Nawwar, Arctec Canada Ltd.,
Canada
G. Lijestrom and K. Lindberg,
Gotaverken Arendal, Sweden
D. Maxutov, U.S.S.R. Institute of the
Arctic and Antarctic, and O. Kossov,
Institute of Systems Studies, Moscow
On the State of Commercial Arctic Marine
Transportation
Extension of the Navigation Season
on the Great Lakes and St. Lawrence
Seaway System
Marine Transportation in Arctic Waters
Transit Analysis for Delivery of Large
Barges to Arctic Destinations
Performance of Icebreaker Ymer on
the Swedish Arctic Expedition "Ymer 80"
Experience of Development and
Introduction of Ice Free Ports for Cargo
Ships under Arctic Conditions'
Session C1 REMOTE SURVEILLANCE AND INSTRUMENTATION­
TELEMETRIE ET TECHNOLOGIE DES MESURES
J. Rossiter, Huntec (70) Ltd, Canada
L. A. LeShack, LeShack Associates
Ltd., U.S.A.
J.D. Wheeler, Exxon Production
Research Co., U.S.A.
C. J. Bowley and J. C. Barnes,
Environmental Research &
Technology Inc., U.S.A.
E.J. Langham, J.E. Glynn and
D.A. Sherstone, National Hydrology
Research Institute, Canada
W.B. Jonasson, Petro-Canada and
C. Durand, Centre de developpement
des transports, Canada
Remote Surveillance and Instrumentation
in Sea Ice'
Correlation of Under-Ice Roughness
with Satellite and Airborne Thermal
Infrared Data
Ridge Statistics from Aerial
Stereophotography'
Comparison of Sea Ice Features in the
Beaufort and Bering Seas Using Slar
and Landsat Data
Comparison of Pseudo-Parallax Effect
and Cross-Correlation for the
Computation of Ice Surface Velocities
in Northern Waters
An Ice Hazard Detection System -
Preliminary Investigations'
100
107
117
136
145
Vol. III
Vol. III
156
Vol. III
166
178
Vol. III

Session A2 ICE MECHANICS - MECANIQUE DES GLACES
B. Michel, Universite Laval, Canada Advances in Ice Mechanics 189
T. D. Ralston, Exxon Production Plastic Limit Analysis of Ice Splitting
Research Company, U.S.A. Failure 205
N.K. Sinha, National Research Constant Stress Rate Deformation
Council of Canada, Canada Modulus of Ice 216
R.M.W. Frederking and G.W. Timco, Mid-Winter Mechanical Properties of Ice
National Research Council of Canada, in the Southern Beaufort Sea
Canada 225
V.W. Neth and D.M. Masterson, Relationship Between In-Situ Confined
Fenco Consultants ltd., Canada Compressive and Unconfined Laboratory
Strength of Sea Water Ice> Vol. III
Session 82 NAVIGATION IN COLD REGIONS-
NAVIGATION DANS LES REGIONS FROIDES
C. D. McKindra and T.C. Lutton,
Coast Guard Headquarters, U.S.A.
J. Sandkvist, University of Lulea,
Sweden
Statistical Analysis of Broken Ice
Dimensions Generated During 140'
WTGB Icebreaking Trails
Conditions in Brash Ice Covered
Channels with Repeated Passages
H. Okamoto, K. Nozawa, H. Kawakami Dynamic Ice Loads and Stress Analysis
and F. Yamamoto, Kawasaki Heavy on the Propeller of the Arctic Ship;
Industries ltd., Japan Model Test in Ice 253
T. Sasajima, Mitsubishi Heavy
Industries ltd., Japan, V. Bulat and
I. Glen, Arctec Canada ltd., Canada
An Experimental Investigation of
Two Candidate Propeller Designs for
Ice Capable Vessels
R. F. Carlson, J. P. Zarling and Engineering for Vessel Ice Accretion
C. I. Hok, University of Alaska, U.S.A. with Particular Reference to the Alaskan
Fishing Fleet 276
Session C2 MARINE STRUCTURES - STRUCTURES MARINES
J. l. Dery, Groupe Lavalin, Canada
K.D. Vaudrey, Vaudrey & Associates
Inc., and R.E. Potter, Sohio Petroleum
Company, U.S.A.
T. Yamaguchi, H. Yoshida,
N. Yashima and M. Ando, Mitsui
Engineering & Shipbuilding Co. ltd.,
Japan
Design of Wharves for Winter Navigation
in the St. Lawrence River
Ice Defence for Natural Barrier Islands
During Freezeup
Field Test Study of "Pack Ice Barrier"
C. A. Wortley, University of Wisconsin, Dock Floats Subjected to Ice
U.S.A.
235
244
263
286
302
313
323

A. Shak, Tetra Tech Inc., U.S.A., Design Factors for Rubble Mound
M.T. Czerniak and J.1. Colling Structures Under Ice and Wave Attack" Vol. III
Session A3 ICE MECHANICS - MECANIQUE DES GLACES
T. P. Taylor, Mobil Research and
Development Corporation, U.S.A.
Y. S. Wang, Exxon Production
Research Company, U.S.A.
N. Urabe and A. Yoshitake, Technical
Research Center Nippon Kokan K.K.,
Japan
J. J. Kolle, Flow Research Company
Kent, U.S.A.
A. C. T. Chen, Exxon Production
Research Company, U.S.A.
P. C. Xirouchakis, Massachusetts
Institute of Technology, and
W. St. Lawrence, Cold Regions
Research and Engineering Laboratory,
U.S.A.
An Experimental Investigation of the
Crushing Strength of Ice
Uniaxial Compression Testing of Arctic
Sea Ice
Fracture Toughness of Sea Ice­
In-Situ Measurement and its
Application -
Fracture Thoughness of Ice;
Crystallographic Anisotropy
Transverse Pressure Effects on an
Embedded Ice Pressure Sensor
On the Acoustic Emission and
Deformation Response of Finite
Ice Plates
T. S. Vinson, Oregon State University, Mechanical Properties of Low Density
U.S.A., and T. Chaichanavong, Ice under Cyclic Axial Loading
Kasetsart University, Thailand 395
Session 83 METEOROLOGY AND OCEANOGRAPHY -
METEOROLOGIE ET OCEANOGRAPHIE
E.F. Roots, Environment Canada,
Canada
T. L. Kozo, Tetra Tech Inc., U.S.A.
T.S. Murty, Institute of Ocean
Sciences, M.1. EI-Sabh and
J.M. Briand, Universite du Quebec
a Rimouski, Canada
D. O. Hodgins, Seaconsult Marine
Research Ltd., and H. G. Westergard,
Aquitaine Company of Canada Ltd.,
Canada
S. K. Liu and J. J. Leendertse,
The Rand Corporation, U.S.A.
Oceanography and Meteorology in
the Arctic"
Surface Wind Direction Anomalies Along
the Alaskan Beaufort Sea Coast
Influence of an Ice Layer on Storm
Surge Amplitudes
Internal Waves in Davis Strait and their
Measurement with a Real-Time System
A Three-Dimensional Model of Norton
Sound Under Ice Cover
332
346
356
366
375
385
Vol. III
415
423
433

V. R. Neralla, ARMF, Canada and
M. L. Khandekar
Water Current Calculations for
Modelling of Sea Ice Movement·
Session C3 MARINE STRUCTURES - STRUCTURES MARINES
I. Holand, The Norwegian Institute Risk Assessment of Offshore Structures
of Technology, Norway Experience and Principles
N. Urabe and A. Yoshitake, Technical Steel Selection System and Reliability
Research Center, Nippon Kokan K.K., Analysis of Structures in Cold Regions
Japan
E. Eranti, State University of New York Dynamic Ice-Structure Interaction
at Buffalo, U.S.A., F. D. Haynes, U.S. Analysis for Narrow Vertical Structures
Army Cold Regions Research, U.S.A.,
M. Maattanen, University of Oulu,
Finland, and T.T. Soong, State University
of New York at Buffalo, U.S.A.
D.V. Reddy, Memorial University of Response of Offshore Towers to
Newfoundland, P.S. Cheema, College Nonstationary Ice Forces
of Trades & Technology, and
M. Arockiasamy, Memorial University
of Newfoundland, Canada
M. Maattanen, University of Oulu, Experiences with Vibration Isolated
Finland Lighthouses
X. Jizu, Tianjin University, China, Dynamic Response of a Jacket Platform
and B. J. Leira, Norwegian Institute Subjected to Ice Floe Loads
of Technology, Norway
H. Nakajima, N. Koma and M. Inoue, The Ice Force Acting on a Cylindrical Pile
Technical Research Center, Japan
Session A4 ICE MECHANICS - MECANIQUE DES GLACES
T. Tabata, Hokkaido Universiy, and
K. Tusima, Toyama University, Japan
H. Saeki and A. Ozaki, Hokkaido
University, and Y. Kubo,
CR Engineering Laboratory, Japan
J.-P. Nadreau and B. Michel,
Universite Laval, Canada
K. Cederwall, University of Lulea,
Sweden
P. R. Johnson, P.E., Consulting
Engineer, U.S.A.
Friction Measurements of Sea Ice on
some Plastics and Coatings
Experimental Study on Flexural
Strength and Elastic Modulus of Sea Ice
Creep of S2 Ice Beams and Plates
Behaviour of a Reinforced Ice-Cover
with Regard to Creep
The Reaction of a Floating Ice Sheet
to Simple Loads and Certain Classes
of Vehicles and Machines
Vol. III
444
462
472
480
491
502
517
526
536
548
562
571

IN VOLUME II
DANS LE VOLUME II
Session 84 SEA ICE CONDITIONS - CONDITIONS DE LA GLACE DE MER
E. Leavitt, Intera Environmental
Consultants Ltd., Canada, J. Sykes,
University of Waterloo, and T.T. Wong,
Intera Environmental Consultants Ltd.,
Canada
A Sea Ice Model Developed For Use
in a Real Time Forecast System
Pages
R.T. Lowry, J.T. Sutton, G. J. Wessels A Sea Ice Model Developed for Use
and W.C. Jefferies, Intera Environ- Real Time Forecast System, Part II:
mental Consultants Ltd., Canada Extraction of Imaging Radar Data 589
D. J. Agerton, Shell Oil Company,
U.S.A.
R. S. Pritchard and M. D. Coon,
Flow Research Company, U.S.A.
R. Colony and A. S. Thorndike,
University of Washington, U.S.A.
Large Winter Ice Movements in the
Nearshore Alaskan Beaufort Sea
Canadian Beaufort Sea Ice
Characterization
Sea Ice Strains During 1979
Session C4 MARINE STRUCTURES - STRUCTURES MARINES
M. Metge, B. Danielewicz and
R. Hoare, Dome Petroleum Ltd.,
Canada
J. D. Wheeler, Exxon Production
Research Company, U.S.A.
On Measuring Large Scale Ice Forces;
Hans Island 1980
Probability Distributions for Structure
Loading by Multiyear Ice Floes
A. B. Cammaert, Acres-Santa Fe Inc., Impact of Large Ice Floes and Icebergs
and G. P. Tsinker, Acres Consulting on Marine Structures
Services Ltd., Canada 653
M. Rojansky and B. C. Gerwick, Failure Modes and Forces of Pressure
University of California, U.S.A. Ridges Acting on Cylindrical Towers 663
A. Prodanovic, Exxon Production Upper Bounds of Ridge Crushing
Research Co., U.S.A.
pressure on Structures' Vol. III
Session AS MARINE FOUNDATIONS AND SCOUR -
FONDATIONS MARINES ET AFFOUILLEMENTS
H. Kivisild, Fenco, Canada
Marine Foundations'
Vol. III
G. R. Pilkington, Dome Petroleum Methods of Determining Pipeline
Limited, and R. W. Marcellus, Canada Trench Depths in the Canadian
Marine Engineering Ltd., Canada Beaufort Sea
674
581
599
609
619
629
643

R. Abdelnour and D. Lapp, Arctec
Canada Ltd., S. Haider and
S.B. Shinde, Esso Resources Canada
Ltd., and B. Wright, Gulf Canada,
Canada
R. Lien, Continental Shelf Institute,
Norway
Model Tests of Sea Bottom Scouring
Sea-Bed Features in the Blaaenga
Area, Weddell Sea, Antarctica
T.R. Chari, Memorial University of
Newfoundland, and S.M. Abdel-Gawad,
Static Penetration Resistance of Soils
University of Windsor, Canada 717
H. Youssef, University of Montreal,
and R. Kuhlemeyer, University of
Calgary, Canada
Dynamic and Static Creep Testing
of Ice and Frozen Soils
Session 85 SEA ICE CONDITIONS - CONDITIONS DE LA GLACE DE MER
W. M. Sackinger, University of Alaska, A Review of Technology for Alaskan
U.S.A. Offshore Petroleum Recovery 735
R.G. Sisodiya, Gulf Research and
Development Co., and K.D. Vaudrey,
Vaudrey & Associates, U.S.A.
Beaufort Sea First-Year Ice
Features Survey - 1979
D.F. Dickins, DF Dickins Engineering, Multi-Year Pressure Ridge Study
and V.F. Wetzel, Suncor Inc., Canada Queen Elizabeth Islands
L. Wolfson and W. M. Evans, ARCO
Oil and Gas Company, U.S.A.
J. R. Kreider and M. E. Thro,
Shell Development Company, U.S.A.
G. F.N. Cox, US Army CRREL, and
W.S. Dihn, Sea Ice Consultants,
U.S.A.
Session C5 WAVE AND ICE MECHANICS
HOULE ET MECANIQUE DES GLACES
Ice Studies Aid in the Successful
Completion of the Norton Sound
C.O.S.T. Well
Statistical Techniques for the Analysis of
Sea Ice Pressure Ridge Distributions
Summer Ice Conditions in the Prudhoe
Bay Area, 1953-75
J. Ploeg, National Research Council On the Importance of Defining Wave
of Canada, Canada Climates 809
P. F. Andersen, Consulting Engineer, Surface Agitation in Ice Prone Waters
Canada 820
A. Lachapelle, Atmospheric Winds and Waves Lancaster Sound
Environment Service, Canada 830
688
706
726
755
765
776
789
799

D. Carter, Consultant, Y. Ouellet,
Universite Laval, and P. Pay,
Transport Canada, Canada
B.D. Pratte and G.W. Timco, National
Research Council of Canada, Canada
Fracture of a Solid Ice Cover by
Wind-Induced or Ship-Generated
Waves
A New Model Basin for the Testing of
Ice-Structure Interactions
Session A6 SEA ICE DRIFT - DERIVE DES GLACES DE MER
W.D. Hibler III, U.S. Army Cold
Regions Research, U.S.A., I. Udin
and A. Ullerstig
W.B. Tucker III and W.D. Hibler III,
U.S. Army Cold Regions Research,
U.S.A.
R. Zorn, Danish Hydraulic Institute,
and H. H. Valeur, Danish
Meteorological Institute, Denmark
R. R. Rumer, A. Wake and
S-H. Chieh, State University of New
York at Buffalo, and R. D. Crissman,
GAl Consultants Inc., U.S.A.
W.W. Denner, Memorial University of
Newfoundland, Canada
Modeling Mesoscale Ice Dynamics
Using a Viscous Plastic
Constitutive Law·
Preliminary Results of Ice Modeling
in the East Greenland Area
Pack Ice Drift and Weather Impact
Development of an Ice Transport
Model for Great Lakes Application
Numerical Modeling of Labrador Pack
Ice Dynamics·
Session 86 OIL SPILLS - POLLUTION PAR LE PETROLE
E. Palosuo, University of Helsinki,
Finland
A. Kovacs, U.S.A. CRREL,
R. M. Morey, Morey Research Co. Inc.,
D. F. Cundy, U.S. Coast Guard Res.,
and G. Dicoff, U.S.A. CRREL, U.S.A.
J. D. Malcolm, Memorial University of
Newfoundland, and A. B. Cammaert,
Acres-Santa Fe Incorporated, Canada
G. A. Robilliard and M. Busdosh,
Woodward-Clyde Consultants, U.S.A.
K. Horikawa and N. Mimura,
University of Tokyo, Japan
The Biologically Important Areas
in the Arctic Ocean
Pooling of Oil under Sea Ice
Movement of Oil and Gaz Spills under
Sea Ice
Need for Real World Assessment of the
Environmental Effects of Oil Spills in
Ice-Infested Marine Environments
Environmental Aspects of Heated Water
Discharged from Coastal Power Stations
843
857
Vol. III
867
879
892
Vol. III
902
912
923
937
945

Session C6 INTERACTION BETWEEN ICE AND SHORE­
INTERACTION ENTRE LA GLACE ET LES COTES
J-C. Dionne, Universite Laval, Canada
J. R. Harper and EH Owens,
Woodward-Clyde Consultants,
Canada
L'action des glaces sur les littoraux
Analysis of Ice-Override Potential Along
the Beaufort Seacoast of Alaska
A. Kovacs and D. S. Sodhi, Sea Ice Piling at Fairway Rock,
U.S. CRREL, U.S.A. Bering Strait, Alaska: Observations
and Theoretical Analysis 985
A. Kovacs and G.F.N. Cox, Norton Sound Grounded Rubble Fields
U.S. CRREL, U.S.A. and Shore Ice Pile-Ups* Vol. III
B.W. Graham, Esso Resources Ice Rubble Field Stability*
Canada Ltd., Canada, and S.B. Shinde Vol. III
Session A7 ICEBERGS -
P. Ball, H. A. Gaskill, Memorial
University of Newfoundland, and
R.J. Lopez, Canada
D. V. Reddy and P. S. Cheema,
Memorial University of Newfoundland,
Canada
S. D. Smith, Bedford Institute of
Oceanography, and E. G. Banke,
Martec Ltd, Canada
R. T. Lowry, Intera Environmental
Consultants Ltd, Canada, and
J. S. Miller
Environmental Data Requirements for
a Real Time Iceberg Motion Model*
Simulation of Shapes of Icebergs
and their Impact Probabilities *
A Numerical Model of Iceberg Drift
Iceberg Mapping in Lancaster Sound
with Synthetic Aperture Radar*
T. R. Chari and H. P. Green, Memorial Iceberg Scour Studies in Medium
University of Newfoundland, Canada Dense Sands 1012
Session B7 ICE CONDITIONS - CONDITIONS DE LA GLACE
J. D. Miller, Petro-Canada, Canada
M. Lepparanta and E. Palosuo,
University of Helsinki, Finland
R. W. Reimer, J. C. Schedvin and
R. S. Pritchard, Flow Research
Company, U.S.A.
A Sensitivity Analysis of a Simple
Model of Seasonal Sea Ice Growth
Studies of Sea Ice Ridging with
a Ship-Borne Laser Profilometer
Chukchi Sea Ice Motion
955
974
Vol. III
Vol. III
1001
Vol. III
1020
1031
1038

P. McComber, Universite du Quebec
a Chicoutimi, Canada
J.-L. Laforte, P. C. Luan and J. Druez,
Universite du Quebec a Chicoutimi,
Canada
W. R. McLeod, Marathon Oil
Company, U.S.A.
Numerical Simulation of Ice Accretion
Using the Element Method
The Effects of an Electric Field on the
Microstructure and Mechanical
Properties of Glaze and Rime
Atmospheric Superstructure Ice
Accumulation Measurements
Session C7 ICE CONTROL MEASURES -
METHODES DE CONTROLE DES GLACES
J.W. Smith, University of Toronto,
a.M. Kaustinen and R. O'Caliaghan,
Polar Gas Project, and F. Brennan,
Canada
I. K. Hill and A. B. Cammaert, Acres
Consulting Services, and D. R. Miller,
Arctic Pilot Project, Canada
K. Haggkvist, University of Lulea,
Sweden
Arctic Marine Heat Transfer Experiment
for the Polar Gas Project>
A Laboratory Study of Heat Transfer
to an Ice Cover from a Warm Water
Discharge
Combination of a Sinking Warm Water
Discharge and Air Bubble Curtains for
Ice Reducing Purposes
1047
1057
1067
Vol. III
1094
1104
G.D. Fonstad, Alberta Environment, The Explosive Demolition of Floating
R. Gerard and B. Stimpson, University Ice Sheets
of Alberta, Canada 1114
D. B. Coveney, National Research
Council of Canada, Canada
Cutting Ice with "High" Pressure
Water Jets 1124

E. Leavitt
J. Sykes*
T.T. Wong
A Sea Ice Model Developed For Use In A
Real Time Forecast System
INTERA Environmental
Consultants Ltd.
Calgary, Alberta
Canada
ABSTRACT: An ice mechanics model developed by INTERA Environmental Consultants
for unconsol idated pack ice is described. This model is intended for use in a
real time forecast system in support of winter dril I ing operations in the Beaufort
Sea.
The model solves the momentum balance for sea ice using the Galerkin
finite element method. The momentum equation follows an Eulerian formulation and
Includes internal ice stress, air and water stresses, Coriol is force, inertial
force and ocean ti It terms. The rodel uses a plastic constitutive law with a
normal flow rule and viscous closure at small strain rates. Boundary conditions
can be specified using velocity or strain rate components.
The ice thickness distribution is described using a four category rodel
wh i ch a I lows red i str i but i on of ice between categor i es due to both thermodynam i ca I
and mechanical effects. Ice strength is calculated as a function of the ice
th I ckness d i str i but i on and the red i str i but I on funct I on. The th i ckness
distribution equation is solved using a Lagrangian formulation which enables the
tracking of ice features and avoids the requirement of specifying a numerical
dispersion term.
A sample rode I run demonstrates how the finite element discretization
can be matched to irregular boundary shapes. The use of the Lagrangian tracking
scheme is illustrated by comparing the calculated positions of a data buoy with
its observed trajectory.
*Dr. Sykes is an assistant professor in the Department of Civil Engineering,
University of Waterloo, Waterloo, Ontario
581

1. I NTRODUCTI ON
Petroleum industry dri II ing operations outside the fast-ice zone in the
Beaufort Sea have been restricted to the ice-free season. However, Dome Petroleum
is evaluating dri Iiship designs which would be capable of operating in winter ice
conditions. I n a cooperat ive program, Dome and the Government of Canada have
funded the development of an ice mechanics model for use in a forecast system to
support year round dril ling operations.
The objective of the joint program was to develop a model that would
provide accurate 24 hour forecasts of ice velocities, ice convergence, and ice
deformation at specific sites with high resolution. The 24 hour period is
specified in order to provide sufficient warning for the drillship to be
disconnected from the hole in an orderly fashion. An additional requirement was
to track the location of hazardous features such as multiyear floes or areas of
heavy ridging.
The model ing effort was spl it into two phases: INTERA was given
responsibi I ity for developing a fine scale site specific model and the Atmospheric
Environmental Service (AES) was to develop a regional scale ice model that would
be used to predict boundary conditions for the site specific model. The exact
domain of each model was to be determined during model development. An important
constraint on both models was that they be capable of producing forecasts using
readily available input data.
Data for model val idation were =llected during December 1979 in the
Beaufort Sea. The measurement program included deployment of 5 buoys by Dome
which monitored position and atmospheric pressure, and ice observations using the
AES and Canadian Center for Remote Sensing airborne radars. Analysis of the radar
data is described in a companion paper [11. Ocean currents were also measured at
one buoy and the Frozen Sea Research Group =nducted a CTD survey of the region in
I ate November.
The mode ling proj ect was comp I eted in March 1981. The formu I at i on of
the site specific model and the solution technique adopted are described in this
paper. A sample ice model run using the data is included to illustrate the
capabil ities of the model.
2. MODEL FORMULATION
The modeling of the mechanical and thermodynamic behaviour of sea ice as
a =ntinuum requires equations relating the forces acting upon the ice as well as
equations describing the resultant redistribution of the ice (such as ridging and
rafting). Fol lowing [21 the momentum equation used to describe the ice motion is:
582

Boundary velocities were set equal to zero along the coast and zero
norma I stra in rate boundar i es were assumed on the ice- ice boundary (F i gure 1).
Twenty time steps that increased in length from 900 seconds to 3 hours were used
in this simulation.
The geostrophic winds were interpolated for the grid positions using a
surface pressure distribution prepared by AES using avai lable buoy and shore
stat ion pressure data comb i ned wi th the Canad ian Meteorolog ica I Center surface
ana I ys is. Ocean currents at the bottom of the mixed I ayer were assumed to be
zero.
Values of drag coefficient and turning angles were 0.0008, 0.005, 25°
and 20° respectively for the air and water stress calculations. The Initial ice
th i ckness proport ions were assumed constant over the mode I doma i n and were set
equal to 0.05, 0.05, 0.60 and 0.30 for the open, thin, flat and rubble categories,
in that order. The th i ckness I eve I s for the four categor i es were 0.0 to O. 1 m,
0.1 m to 0.5 m, 0.5 to 0.9 m and 0.9 to 20 m. These thicknesses are based on a
I imited set of observations and are only approximate.
As indicated in Figure 1, one of the thickness nodes was positioned at
the location of a Dome buoy. The predicted trajectory and velocity components are
compared with the observed va lues in Figures 2 to 4. I n genera I, af ter the
initial six hours of simulation, excellect agreement is achieved in the y­
component of the velocity. However, the model tends to over-predict the x­
component.
There are severa I poss i b I e reasons for th i s discrepancy. First I y, the
reduction of the assumed drag coefficients and/or an increase in the turning angle
for air stress would reduce the xvelocity component and provide a better match
with the observations. Secondly, examination of the wind data showed that the x
component of the air stress was large across the entire model domain. The large
fetch would require a very high ice strength to balance this air stress if the ice
is to remain relatively motionless. If the ice was less compact than assumed in
this simulation in the western half of the modeled area the effective fetch would
be reduced. Consequently, sma I ler ice velocities would be calculated.
5. SUMMARY
This paper demonstrated that the INTERA fine scale ice model is capable
of predicting ice velocities, areas of convergence and ice deformation. The model
can also forecast trajectories of specific ice features. Initial tests indicate
that model predictions reflect the observed behaviour. Further testing is being
continued to cal ibrate the model parameterizations with the data from December
1979.
586

Besides providing forecast support for dril I ing operations the forecasts
could also provide useful input to transportation systems. For transportation the
emphasis would be to forecast areas of convergence and ice deformation for input
into route selection.
6. ACKNOWLEDGEMENT
Th is project was supported under a contract from the Department of
Supp lies and Serv ices, Government of Canada. Add i tiona I support was prov i ded by
Department of the Environment, Dome Petroleum Limited, Ministry of Transport and
Energy Mines and Resources.
The authors acknowledge the efforts of the many INTERA co-workers who
have worked together on this project to ensure its success.
REFERENCES
1. Lowry, R.T., J.T. Sutton and G.J. Wessels. 1981. A Sea Ice Model Developed
for Use in a Real Time Forecast System, Part I I: Extraction of Imaging
Radar Data. To be presented at The Sixth Conference on Port and Ocean
Engineering under Arctic Conditions, Quebec City.
2. Rothbrock, D.A. 1975. The Steady Drift of an Incompressible Ice Cover in the
Arctic Ocean. CI imate of the Arctic.
3. Coon, M.D. 1980. A Review of AIDJEX Model ing. Sea Ice Processes and Models,
Ed. R.S. Pritchard. University of Washington Press. 12-27.
4. Hibler III, W.D. 1979. A Dynamic Thermodynamic Sea Ice Model. Journal of
Physical Oceanography. Vol. 9: 815-846.
5. Reimer, R., Pritchard, R., and M. Coon. 1980. Consistent Reduction of Ice
Thickness Distribution to a Few Categories. Prepared for INTERA
Environmental Consultants Ltd. by FLOW Research Company. 29 pp.
6. Pritchard, R.S. and M.D. Coon. 1981. Four Component Ice Characterization for
the Southern Canad i an Beau fort Sea. To be presented at the Sixth
Conference on Port and Ocean Engineering under Arctic Conditions, Quebec
City.
7.
588
Irons, B.M., and R.C. Tuck.
Computer Iteration.
Engineering. Vol. 1:
1969. AVers i on of the Aitken Acce I erator for
Internat iona I Journal for Numer ical Methods in
275-278.

1. INI'ROJXJCTIOO
An ice mechanics !1Ddel has teen developed by Intera Environmental
Consultants Ltd. as part of a co-operative program between the Atr.'Dshperic
Environment Service (AES) and rolE Canada (Leavitt et al., 1981). 'lliis I!Ddel is
intended to te used in a real time forecast system, in support of drilling
operations in winter pack ice in the Beaufort Sea. 'llie data needed to
initialize, operate and update the !1Ddel can te divided into two categories.
First, there are atmospheric and oceanographic data, such as winds,
ter.peratures, currents, etc. Second are the ice data, sud! as type,
concentration, motion, ridging, etc. 'lliis paper deals with the use of imaging
radars to supply these ice data, necessary for the !1Ddel.
Imaging radars were d!osen for this program tecause they provide ice
data at an appropriate resolution, independent of light and atmospheric
conditions. Further, X-band radars yield !!Ore information on ice type than do
optical sensors or longer wavelength radars, especially if the resolution (data
rate) is COIlParable (LcMry et al., 1979). 'llie techniques were developed for the
analysis of radar imagery of ice to provide data for a !1Ddel. 'lliis paper is an
attenpt to descrite the techniques evolved, and outlines the developments needed
before radar data can be used in operational !!Odelling work.
2. Dl\.TA SETS
2.1 SLlIR Dl\.TA
'llie SLlIR data used in this analysis were collected using the AES
AN/APS 94E SLlIR. 'Ihe SLlIR data, with mud! lCMer resolution but mud! larger
swath width (100 krn per side, both sides operating), were intended to te used
with a "Regional Scale" !1Ddel. Four flights were COIlPleted, to cover the same
two periods for whid! SAR data were oollected, naMely, 2, 6, 14, and 16 Decel!ber
1979. Figure 1 shOlolS the approximate extent of the area imaged by the SLlIR.
Also shown is the polar stereographic grid that was used in this I!Ddelling
effort (See Leavitt et al., 1981).
2 • 2 SAA DA.TA
SAR data were oollected with the SAR-S80 system (Inkster et al., 1979)
operated in the X-band (0.032 m wavelength), wide swath !!Ode. In this !!Ode, 22
km of slant range data are collected on each pass of the aircraft. 'llie
resolutioo cell of this radar is approximately square, and has an area of just
over 3 m 2 • Data were oollected so that a ground range of just CNer 24 kIn was
covered.
'lWo !!Odelling periods were established for the SAR data: 2 to 6
Decel!ber 1979, and 14 to 17 December 1979. An area of approximately
10 000 km 2 was imaged 00 each day of the two periods, using four flight lines
of just over 100 kIn in length. A nominal 20% overlap was planned, to allew for
drift in the navigatioo. The areas covered are shown in Figure 2. Most passes
had one end extended well onto land to aid in geometric calibratioo of the
image. Because of poor image quality, data fran DecentJer 2nd were discarded.
590

A second program was developed to find the latitude and longitude of
al¥ point on the inage. '!his nade use of measurements, fran the inagery, of the
distance to the biO closest timing narks. By o.:xrparing the position of points
on tw:> different days, ice IlDtion vectors could t:e calculated. Further, by
o.:xrparing the calculated position of the same point on biO different passes, INS
errors could t:e observed. '!his was ir.portant t:ecause self-consistent radar
r.nsaics could not t:e constructed for all data sets. By cbserving the "notion"
of stationary ice features, including land, confidence limits were estimated for
IlDtion vectors.
3.2 SAR GIDlETRIC CALIBRATIrn
Before SAR inagery can t:e calibrated geanetrically, using the CXlI!p.lter
program, the following data nust t:e known for eam pass:
1. Starting latitude and longtitude;
2. Final latitude and longtitude;
3. Flying height;
4. Time delay and record interval of the radar; and
5. Length and width of the SAR inagery.
Ideally, the inagery will contain nany well-scattered control points
on land whim can t:e located on an NI'S reference map. Fran the control nap, the
latitude and longtitude of a control point are calculated and converted to real
reference distances in the along-track and across-track directions. Similarly,
measurements are taken fran the inagery in the along-track and across-track
directions and converted to real inage distances.
Using all the control points, a least-squares method is used to
OOIlP1te unique scales and offsets in both the along-track and across-track
directions. '!he result is two linear equations for the inagery, with reference
values dependent upon the measured image values. In this manner, the location
of any unknown point can t:e determined, provided one of the follOWing parameters
is known:
1. Target's location on an NTS nap;
2. Target's latitude and longitude;
3. Target's along-track and across-track real distances
from the beginning of the flight; or
4. Target's along-track and across-track inage distances.
4. ICE MOl'Irn
4.1 SLAR ANALYSIS
Ice mtion during the first period (Decer.tJer 2nd to 6th) and the
second period (December 14th to 16th) was determined by measuring the change in
position of identifiable ice features. A set of approxinately 60 ice features
was located on both pairs of inagery. Latitudes and longitudes of eam feature
were calculated by neasuring the distance to the biO adjacent time r.arks, and
interpolating exact positions using the INS data. '!his process assumed constant
along-track and across-track scale and no INS drift.
594

LARGE WINTER ICE MOVEMENTS
IN THE NEARSHORE ALASKAN BEAUFORT SEA
DAVID J. AGERTON SHELL OIL COMPANY HOUSTON, TEXAS, USA
ABSTRACT
Observations of a mid-winter storm in the Alaska Beaufort Sea and associated
ice movements and deformations are discussed and compared with simple mathematical
force models. Landsat imagery and weather records from previous years are reviewed
for indications of similar large ice movements.
INTRODUCTION
Ice movements are of interest because their rate, in part, determines
structure loading. Also, movements create ice ridges, pile-ups, and leads which can
load structures and disrupt over-the-ice logistics operations. The magnitude of ice
movements determines, in part, the probability of ice features, such as multiyear
floes, colliding with offshore structures in winter.
In this paper, "large" movements are those exceeding 50 meters (m) per day.
Some exceed 300 m per day. "Winter" and "nearshore" refer to movements which occur
from December through May in waters up to 20 m deep.
Past Investigations of Large Winter Ice Movements
In protected, nearshore areas of the Beaufort Sea offshore of the North
Slope of Alaska, winter ice movements are generally small and slow. The largest 30day
excursion measured shoreward of the coastal barrier islands by industry in a
three-year winter program was less than 4 m, and the fastest rate was about 1.5 m/hr
(.001 ft/sec) [1]. In exposed areas, generally outside the barrier islands in water
depths between 6 and 20 m, winter ice movements are larger, but are still constrained.
In 20 m water depths, the largest measured excursion was about 70 m and the maximum
rate was about 50 m/hr (0.05 ft/sec) [2]. These maximums are two to three orders of
magnitude greater than their respective medians. In shallower water depths, the
599

magnitudes and rates of ice movement decrease. In deeper water depths, they increase.
At a location 24 km offshore, CRREL investigators measured about 1 km displacement
during a moderate storm [3]. A correlation between large ice movements and the
direction and sequence of storms in the nearshore Beaufort Sea has been observed [2].
Although threshold windspeeds appeared to be required for large ice movements to
occur, above windspeeds of 13 m/sec (20 knots), ice displacements do not correlate
with local windspeeds.
Shapiro [4] measured very rapid ice movements in the Chukchi Sea off
Barrow in waters less than 20 m deep during an intense storm in late December 1973.
The ice was 0.6 m thick. Onshore radar recorded ice feature position. Calculated
ice movement rates were as high as 2.3 m/sec (7.6 ft/sec) in winds 26 m/sec (50
knots), reportedly gusting to 52 m/sec. 2.3 m/sec is a much higher rate than observed
in the Beaufort Sea. And, it is higher than might be expected. The arctic oceanographer's
rule-of-thumb is that free-floating ice drifts at three percent of the wind
speed. By this rule, the ice would have moved at 0.76 m/sec. The high-speed wind
gusts and local ocean currents might have increased floe speed beyond that expected
for the free-floating case. Considering it was December and the ice was probably
compact instead of free-floating, the high floe speeds are even more surprising.
March 1979 Field Observations
On March 17, 1979, a large extratropical cyclone developed in the Arctic
Ocean northeast of the Beaufort Sea. The next day, it drifted southwestward and
intensified. At Deadhorse airport, about 16 km inland, winds were 17 m/sec (33 knots)
from 240 to 260°. The coastline is oriented at about 300°, so that a component of
the wind blew offshore. It is common knowledge that Eskimos stay off the North Slope
ice during strong southwesterly winds because leads form and large ice movements
typically occur.
The storm coincided with several oil industry and government projects. So,
an unusual amount of data was available to describe the storm and its effects. In
addition to onshore coastal meteorological stations, an array of meteorological buoys
was deployed in the Beaufort Sea and Arctic Ocean. The day preceding the storm, and
again several days after the storm, NASA Lewis Research Center flew aerial photo and
side-looking airborne radar (SLAR) missions in the area. Two days after the storm, a
field team arrived to survey first-year ice features [5]. Refrozen leads, offset
seismic roads, pressure and shear ridges, and a large ice pile-up appeared newly
formed. The Landsat satellite passed over the area on March 12 and again on March
20. Fortunately, both days were cloud free. In this same area, on March 29, Gulf
R&D Co. had aerial photos made covering 500 km of flightline.
600

shear ridge surveyed in March and seen on Landsat. It might have formed on November
26, when a moderately intense easterly storm occurred with winds from gO-100°.
The shear ridge struck an angle of 300°, so a component of the wind was normal to it,
but the major component of the wind was parallel to it--just what would be expected
for creation of a shear ridge.
Unfortunately, where the ice was rough, the SLAR image return saturated the
photographic paper and detail was obliterated. It was impossible to differentiate
between areas of low, rough rubble and areas of high ridging. Ridges and narrow
leads could also give indistinguishable returns. Consequently, although the SLAR
imagery fixed the date of initial formation of a major offshore shear ridge, it was
not helpful in determining the magnitude and direction of subsequent displacements,
specifically those which occurred in March.
Photography
Aerial photos showed the magnitude, direction, and general location of
nearshore ice movements. They showed patterns of ice failure. And, they established
the location of ridges for a summer ice gouge survey by the USGS.
principal flight line locations.
Reference 11 shows
Ice movements near Cross Island. NASA photos from March 16 showed straiqht
seismic roads east of the island. A Gulf R&D Co. photo from March 29, showed these
roads offset repeatedly by ice movements and numerous leads, changes attributed to
the March 17 storm. Several large shear displacements were evident: 250 m at 130°
and 110 m at 150°. The displacements occurred 2 to 3 km offshore in a sector between
NNE and E of the island. Water depth here is roughly 14 m. Figure 3 is a line
FIGURE 3. DRAWING OF SEISMIC ROAD DISPLACE­
MENTS, LEADS, AND FAILURE PATTERN
IN ICE EAST OF CROSS ISLAND,
MARCH 1979.
drawing tracing the offset seismic
roads, shear failure lines, and
ridged areas. It shows the wedgelike
local ice failure pattern.
Ice movements north
of Narwhal Island. Figure 4 is a
line drawing from Gulf photos
offshore Narwhal Island [5] showing
larger movements. Based on seismic
road dispacement, ice moved eastward
1200 m along a line oriented at
300°. In the process, a 21 m high
grounded ice pile-up formed, which
the field team dubbed "Ice Mountain". Water depth here was 18 m. Further offshore,
we know from Landsat that the ice moved 2500 m eastward. Closer to land, the ice was
604

DISCUSSION OF MATHEMATICAL MODELS
Grounded Ice Pile-ups
The forces acting to create grounded ridges and pile-ups are generally
thought to be low [12]. Most equations describing forces during ridging and pile-up
result in predictions of forces less than 0.3 mega newtons/m (MN/m) (20 kips per
linear foot). Vaudrey [5] estimated average forces during pile-up of Ice Mountain as
being about 0.09 MN/m (6 kips/ft or 8 psi). He assumed that most of the work done
during ridging was stored as gravitational potential energy, and the energy expended
in friction and in creating new surfaces by fracture was small. Fracture energy loss
in ridging appears negligable. But, friction forces may be of the same order of
magnitude as gravity forces [12]. Kovacs and Sodhi [13] estimated the frictional
force for an ice sheet being pushed up an inclined surface as a function of friction
coefficient and geometry. Applied to Ice Mountain, their equation results in a
calculated frictional force of 0.03 to 0.07 MN/m (2-5 kips/ft), depending on parameter
assumptions. However, the frictional force along the shear boundary surface of
the moving ice sheet was not estimated because the normal forces along the two
surfaces shearing past one another were not known.
Ice Dynamics
Generally, the principal force moving ice is wind stress. The floe which
created Ice Mountain appeared on Landsat to be about 1.6 km wide by 24 km long,
assuming the western-most lead was a boundary. The forces which might have acted on
this floe at a moment in time in the March storm were considered by estimating the
inertial, drag, Coriolis, and deformation forces. Driving forces transmitted from
adjacent floes were not considered.
It was assumed that ice movement occurred during the 8-hour storm peak when
winds exceeded 15 m/sec, that acceleration was sinusoidal, and that velocity peaked
when half the 1200 m movement had occurred. This resulted in a total movement time
of 4.4 hours and a peak velocity of 0.15 m/sec (0.5 ft/sec). This velocity was an
order of magnitude greater than values measured in this area.
Estimated coriolis, inertia, and water drag were 3 to 5 percent of wind
drag on the ice. Thus, wind drag, deformation forces and driviriq forces from
adjacent floes should about balance. Estimated wind stress on the floe was 37MN
(8300 kips). The estimated pile-up and friction force for Ice Mountain was 13 MN
(3000 kips) or about 35% of the wind drag. Photos showed many other areas of ice
deformation,along the floe's southern boundary so that 13 MN should probably represent
much less than 35% of the total driving forces. The floe was probably driven by
the ice pack northwest of it as well as by local wind stress.
606

Landsat Imagery 1973-1978:
OTHER LARGE ICE MOVEMENTS
Landsat imagery from 6 winters was examined for nearshore
leads to see how often they occurred in the lease sale area. Landsat could not
provide a complete data sample because of low pass frequency and inability to image
ice through cloud cover. The inferred ice movement and storm which appears most
likely to have caused the leads by virtue of its date and wind direction are
summarized for three examples below. A fourth example was discussed in reference 2.
Identification of the associated storm may have been limited by poor wind data.
March 14, 1973. The image is depicted by a line drawing in Figure 5. A
refrozen lead 1 km wide occurred in 20 m of water 6 km N.E. of Cross Island. N.E. of
Pole Island 7km, a 2 km wide lead extended into 17 m of water. A large area of pack
ice had been displaced eastwards.
and February 2.
Moderate westerly storms occurred on January 29
- .......... _--... _ 2O-fotIETERS _ ... ---:
MARCH 14, 1973
FIGURE 5. LINE DRAWING OF 3/14/73 LANDSAT IMAGE
SHOWING PATTERN OF LEADS.
KM
March 10, 1974. An
older, refrozen lead system
was visible bordering on the
20 m water depth contour.
Estimated eastward displacement
is 5 km at locations 8 km
North of Pole Island and 25 km
North of Pingok Island.
Harrison Bay was free of leads
in this and in other images.
The winter of 1973-74 had the
most intense storms of those
examined: five westerly
storms with winds exceeding 50
knots. Storms preceeding
March 10 occurred on January
13 and January 27.
April 19, 1975. A lead 1 km wide at the 20 m depth contour offshore
Alaska Island was visible. Again, offshore areas west of Cross Island are lead free.
A weak westerly storm occurred on April 14. It was preceeded by an intense easterly
storm on March 28.
SUMMARY
Despite limitations in frequency and resolution, Landsat imagery has
provided data on past ice movements in the nearshore Alaskan Beaufort Sea. Ice
displacements of 1 to 5 km have been observed on Landsat images and aerial photos in
607

R. S. Pritchard, Sr. Research Scientist
M. D. Coon, Sr. Research Scientist
CANADIAN BEAUFORT SEA ICE CHARACTERIZATION
Flow Research Company
Kent, Washington 98031 U.S.A.
Abstract
A model characterizing the Canadian Beaufort Sea ice cover in terms of fractions of
coverage of open water (including new ice) and thin, flat and rubbled ice is presented.
The minimum and maximum thicknesses that define each category vary in time
to account for thermal growth, except for the fixed thickness separating open water
(new ice) from thin ice. This characterization is intended to be part of a complete
ice dynamics model. Ice strength for a plasticity model is determined from the ice
conditions. A specific function that redistributes only the thinnest ice available
is introduced. This constitutive law can describe the essential physics of rafting
and ridging processes and is so simple that it can be integrated analytically.
Examples of model behavior with no deformation, isotropic opening, uniaxial closing
and pure shearing allow the effects of several parameters on the response to be
isolated. This result simplifies the determination of material constants when
observed sea ice behavior is simulated. Future validation of model performance is
suggested as actual data become available.
Acknowledgement
This work was supported by AES and DSS of the Canadian government and Dome Petroleum
through a subcontract from Intera Environmental Consultants, Calgary, Alberta. We
gratefully acknowledge the help of E. Leavitt, technical monitor, for a critical
review of the work.
609

compression and pure shear. The response is acceptable for the material constants
chosen. Ice strength depends strongly on the amounts of open water and thin ice
present. As the fractions of open water and thin ice are depleted, the strength
jumps. Values are 43 N/m for open water, 1.8 x 10 4 N/m for thin ice, 3.3 x 10 4 N/m
for flat ice and infinity for rubbled ice. These values depend on other material
parameters but are representative of values expected to allow accurate simulation of
observed ice motions and deformations.
While the behavior of this model is reasonable, no attempt has yet been made to
compare it with observed data. This critical step should be taken as soon as data
are available. The model is rather simple to modify because the effects of various
material constants can be isolated. This fact will allow constants to be tuned to
match observations and the model's performance to be evaluated qualitatively.
References
1. Coon, M. 0., Maykut, G. A., Pritchard, R. 5., Rothrock, D. A., and Thorndike,
A. S. (1974) "Modeling the Pack Ice as an Elastic-Plastic Material," in AIDJEX
Bulletin 24, University of Washington, Seattle, pp. 1-105. ---
2. Thorndike, A. 5., Rothrock, D. A., Maykut, G. A., and Colony, R. (1978) "The
Thickness Distribution of Sea Ice," J. Geophysical Research, Vol. 80, No. 33,
pp. 4101-4513.
3. Rothrock, D. A. (1979) "Modeling Sea-Ice Features and Processes," l:.
Glaciology, Vol. 24, No. 90, pp. 359-375.
4. Coon, M. D. (1980) "A Review of AIDJEX Modeling," in Sea Ice Processes and
Models, R. S. Pritchard (ed), University of Washington Press, Seattle, pp. 12-33.
5. Pritchard, R. s. (1981) "Mechanical Behavior of Pack Ice," in Mechanical
Behaviour of Structured Media, A.P.S. Selvadurai (ed), Elsevier, Amsterdam, to
appear.
6. Reimer, R., Pritchard, R., and Coon, M. (1980) "Consistent Reduction of Ice
Thickness Distribution to a Few Categories," Flow Research Report No. 167, Flow
Research Company, Kent, Washington.
7. Nye, J. F. (1976) itA Coordinate System for Two-Dimensional Stress and
Strain-Rate and its Application to the Deformation of Sea Ice," in AIDJEX
Bulletin 33, University of Washington, Seattle, pp. 131-143. ----
8. Rothrock, D. A. (1975) "The Energetics of the Plastic Deformation of Pack Ice
by Ridging," J. Geophysical Research, Vol. 80, No. 33, pp. 4514-4519.
9. Rothrock, D. A., and Hall, R. T. (1975) "Testing the Redistribution of Sea Ice
Thickness from ERTS Photographs," AIDJEX Bulletin 29, University of Washington,
Seattle, Washington, pp. 1-19.
618

R. Colony
A. S. Thorndike
ABSTRACT
SEA ICE STRAINS DURING 1979
Polar Science Center
University of Washington
Seattle, Washington 98105
A number of drifting data buoys were operational in the Arctic Basin from
U.S.A.
February through December 1979. Using the procedure of optimal interpolation, esti­
mates of ice motion and gradients of ice motion were made at selected locations in
the basin. Seasonal and regional patterns of strain are shown for both daily and
longer periods of deformation. Strain and rotation statistics are compared to
measurements made during the Arctic Ice Dynamics Joint Experiment.
INTRODUCTION
Sea ice moves largely in response to stresses exerted by winds and ocean cur­
rents. Even when these stresses vary smoothly in space the response of the ice pack
can be uneven because of its granular character. The grains, single ice floes or
groups of floes acting together and ranging in size from 10 to 10 5 meters, move
rigidly with the only deformation occurring at the grain boundaries. A snapshot of
the velocity field at any particular time would reveal a field which remained con­
stant across a single grain (or varied linearly to account for rigid rotation of the
grain) and which was discontinuous at the grain boundaries. At the grain boundaries
energy is dissipated in building pressure ridges and in friction. When grains move
apart new area of open water is exposed leading to vigorous exchange of heat between
the ocean and atmosphere and rapid growth of new ice. Because of the range of grain
sizes and the irregular geometry of the grain boundaries it is not feasible to
resolve the deformation completely. Instead techniques are needed to characterize
the deformational activity from limited measurements of ice motion. Toward this end
we have taken recent measurements of ice motion which are widely separated in space
and calculated a number of deformation indices from them.
Evidence from earlier work suggests that such indices contain useful information
about the behavior of the ice pack. In our analysis of the Arctic Ice Dynamics Joint
619

DATA SET
An array of automatic data buoys was established and maintained in the Arctic
Basin as a part of the First GARP Global Experiment. The first buoys were deployed
on 19 January 1979. The complete array of about 15 buoys operated from the first of
March 1979. Our analysis is restricted to the period 1 March - 31 December 1979.
The measurement program is expected to continue for several more years. Objectives
of the program are to provide measurements of surface atmospheric pressure and to
define the large scale field of motion of sea ice. A data report [41, available from
the authors, describes the measurement program, data processing procedure, and avail­
able data sets.
The deformation quantities are determined using techniques of optimal interpola­
tion given in Gandin [51. The N measurements zi of displacement u i at points Xi with
measurement errors E.
1-
The estimator is
where the C1. i satisfy
Here
Zi - u i are used to estimate the displacement U at the point x.
N
E
i=l
1'. • U.U.
1-J 1- J
£.u.
1- J
N
U = E C1. Z
i i
i=l
o for i # j, and
The correlation 1'ij between displacements at xi and Xj is taken to be a function of
the separation 1'ij = R(lxi-Xjl). Similarly Sj is the correlation between displacement
at x. and at X; B. = R(lx-x./). An early sketch of the correlation function R
J J J
was given in [11. For these calculations R(a) = q2 exp [-(a/Z)21 was used with q = 5 km
and Z = 600 km. The above description for scalar quantities is readily extended to
vectors, producing in the end estimates for the displacement components U and V. The
measured displacements zi were taken over one day intervals; zi = Xi(t + 1/2) -
X.(t - 1/2) and x. = X.(t) where X. was the trajectory of the i th buoy.
1- 1- 1- 1-
(1)
(2)
621

Note that the matrix M = rr .. + a 2 o. ]-1 depends on the measurement positions
L'LJ 'L.tJ
xi but not on the position x where the interpolated value is desired. Thus the gra-
dients of the estimated displacement can be found by combining equations (1) and (2)
and differentiating,
N N ds.
:E :E Mij zi a:i=l
j=l
We use the symbol U x for this quantity and u y ' V x ' Vy for the other derivatives of
the estimated displacement field.
The position measurements, obtained by satellite Doppler positioning techniques
had essentially zero mean random errors with a standard deviation of about 300 meters ..
This contributes to an interpolation error for the daily displacement which typically
had standard deviation less than 500 meters at points within the region defined by
the outer buoys. Extrapolations to regions beyond the outer buoys had much larger
estimation errors. The nine grid points used in this study, Figure 2, were within
the buoy array for the ten month period.
STATISTICS OF DAILY STRAIN
Figure 2. The grid points used
in the strain study. This set of
grid points was always inside the
outer perimeter of FGGE buoys.
The drift of the AIDJEX main camp
is shown for May 1975 through
April 1976.
Spectral analysis of the velocity and velocity gradient time series shows little
energy associated with periods of less than one day. This observation allows us to
regard the daily displacement, u, and the instantaneous velocity, i, as nearly inter­
changeable. Therefore in the description of strain which follows one can either think
of the quantities, u x ' u y ' Vx and Vy as representing gradients of the estimated daily
displacement (strain) which are dimensionless, or as gradients of the estimated
622

- ':::>' -
Figure 4. The deformation
ellipses and total rotation angle
for the selected grid points.
The angle is measured clockwise
from lines parallel with the date
line, 180·. March 1979 through
December 1979.
We can interpret the deformation as points initially defining a circle of unit area
being rotated through an arc and then mapped onto an ellipse. The area of the
resulting ellipse indicates net convergence or divergence; the eccentricity of the
ellipse is a measure of net shearing; and the orientation of the ellipse indicates
the principal directions of the total deformation.
The magnitude of the major and minor axes of the ellipses of Figure 4 can be com­
pared to Figure 1, the AIDJEX array which accumulated strain for 360 days. Again we
see similarities in the amount of total shear. A pattern of orientation of the ellip­
ses is not clearly discernible. Perhaps there is a suggestion the major axis of the
ellipse tends to be alligned with the coast. This may be due to either stress propa­
gating out from the coast or the dynamic topography of the underlying ocean.
The time evolution of the cumulative strain is seen in the time series of the
major axis, minor axis, and the orientation of the major axis. The time series of the
major and minor axes for grid point G, Figure 5, are qualitatively different from
AIDJEX and some of the other grid points, in that the curves are not monotonic.
Figure 5a clearly shows a prolonged extension (May through mid August) followed by a
period of contraction (mid August through mid October) in that same direction, Figure
5b.
Figure 4 shows the ellipses G, H, and I to have similar configurations. However
Figures 6a,b and 7a,b reveal marked differences in how they evolved to that final con­
figuration. On time scales of tens of days the deformations at points roughly 500 km
apart appear to be poorly correlated.
As seen in the mean daily rotations, the region shows a net clockwise rotation.
Time series of rotation show in general the steady evolution of the total rotation.
The net divergence is small in most cases, and no pattern is suggested. In fact it
appears that one cannot even determine the sign of the divergence for the entire region.
625

DISCUSSION
Our conclusion for the AIDJEX study that long term deformations were strongly
organized over scales of 100-500 kilometers cannot, on the basis of these observations,
be extended to much larger scales. Perhaps observations from subsequent years will
confirm the pattern noted for the principal directions of the long term deformation.
The results to date suggest the following tentative statements about the annual
deformation:
i) this part of the ice pack has a net clockwise rotation of about 40°-60° with
the larger rotations appearing at the lower latitudes,
ii) the long term deformation in this region is approximately non-divergent,
iii) there is a net stretching of order 80% in one principal direction and con­
traction of order 40% in the other.
The general impression is that the deformations of regions 500 kilometers or more
apart evolve more or less independently of each other, organized only weakly by some
very large scale influence (such as the long term circulation of the atmosphere or
ocean or the dynamic constraints imposed by the boundaries). This lack of organiza­
tion is puzzling at first considering the well documented large scale organization
of the velocity field itself consisting of a general clockwise circulation in this
part of the basin. Daily velocities at different points in fact have strong positive
correlations out to large distances (e.g., R(500 km) z 0.5). The paradox is explained
by noting that the process of differentiation in going from displacement to deforma­
tion inevitably accentuates the smaller length scale variations in the field of
motion. This reduces the distance over which correlation is felt.
Acknowledgment This work was supported by the National Oceanic and Atmospheric
Administration Grant NA80-AA-D-00015, which was funded in part by the Global Atmospheric
Research Program and the Office of Climate Dynamics, Division of Atmospheric
Sciences and the Meteorology Program, Division of Polar Programs, of the National
Science Foundation, and the Office of Naval Research, Arctic Programs.
References
[1] Thorndike, A. S. and R. Colony, 1980. Large-scale ice motion in the Beaufort
Sea during AIDJEX, April 1975-April 1976, in Sea Ice Processes and ModeLs,
R. S. Pritchard, Ed. University of Washington Press, Seattle, Washington (249-260).
[2] Rothrock, D. A. and R. T. Hall, 1975. Testing the redistribution of sea ice
thickness from ERTS photographs, AIDJEX BuLLetin. 29, July (1-19).
[3] Rothrock, D. A., R. Colony and A. S. Thorndike, 1980. Testing pack ice constitutive
laws with stress divergence measurements, in Sea Ice Processes and ModeLs,
R. S. Pritchard, Ed. University of Washington Press, Seattle, Washington (102-112).
[4] Thorndike, A. S. and R. Colony, 1980. Arctic Ocean Buoy Program, Data Report,
19 January 1979 - 31 December 1979. Polar Science Center, University of Washington
(131 pages).
627

References, continued
(5) Gandin, L. S., 1965. The Ob.iective Analysis of Meteorological FieUs.
Leningrad, 1963, English translation, Israel Program for Scientific Translations.
Jerusalem (242 pages).
(6) Colony, R., 1978. Daily rate of strain of the AIDJEX manned triangle, AIDJEX
Bulletin, 39, May (85-110).
628

is equal to the product of the effective failure pressure of the ice by the diameter
of the platform and by the thickness of the ice. The design of the structure is
therefore governed by the value of ice failure pressure averaged over the full contact
area. Our knowledge of this effective ice failure pressure over large areas will
significantly affect the safety and cost of future production platforms.
SCALE EFFECT ON ICE STRENGTH
It has been known for a long time that the measured strength of ice is some function
of the size of the sample. (Weeks & Assur 1969, Metge 1977, Gold 1978). In fact,
Weeks and Assur (1969) state that, "an understanding of the scale effect in ice testing
is essential before a thorough scientific basis can be developed for the utiliza­
tion of small scale testing in engineering design problems". Their statement applies
just as well today as it did 12 years ago.
In the case of production platforms, the ice failure area is typically 3000 m 2 and
the failure pressure must be deduced from indentation tests involving a few square
2
centimetres or at the very most 3 m • Extrapolating from 3 to 3000 m2 is dubious
indeed, especially since the scale effect phenomenon is not yet well understood.
As an example of how widely opinions may vary regarding scale effect, consider the
following relationships between the failure pressure P, the ice thickness h and the
width of the failure area D: Weeks and Assur (1969) based on data by Butiagin
thought that Po(D-O. S h-O. S • Hirayama et al (1974) show that PeCD-O. S h+O· 1 • More
extensive tests by Saeki et al (1977) indicate hCD-o· S hO.O. While Gold (1978)
found that PI(D-0. 4 hO.O seems to fit data from a variety of sources. More recently,
Kry (1979) has shown that the effect of D (in terms of number of zones). on P decreased
when D becomes large and eventually that PO( DO. 0.
Knowing how important P is to the design of production platforms, and knowing the
amount of uncertainty in methods of extrapolating small scale tests to large scale
ice failure, it becomes obviously very attractive to try and measure large scale
failure pressures directly in the field.
STATE OF THE ART IN ICE FORCE MEASUREMENTS
Many measurements of ice forces on narrow structures such as bridge piers (Neill
1976), lighthouses and drilling platform legs (Blenkarn 1970) have been made. The
measured ice failure pressures for narrow structures range from 1 to 3 MPa.
To date, however there has been no direct measurement of ice forces on wide struc­
tures. In order to evaluate ice forces on their artificial islands, oil companies in
Canada and Alaska have used ice stress sensors embedded in the ice sheet surrounding
630

the artificial island, (Metge et al 1975). Using the ice stress at a few points and
an elastic model of stresses in a plate around a fixed object, one can derive an
estimate of the total ice forces. Very few recorded stress events (if any) actually
involved ice failure because most island locations have been in landfast ice. A
measurement of ice stress during ice failure is reported by Sackinger 1979. The
disadvantages of such measurement methods are:
-the questionable accuracy of the stress sensors
-the possible additional error introduced in deriving total ice forces from
local ice pressures.
THE HANS ISLAND PROJECT
During 1979, Dome Petroleum initiated a project with the aim of measuring full scale
multi-year ice forces on stationary obstructions (Metge 1979). Several different
possibilities for such measurements were reviewed, such as:
-instrumented test structure
-giant "nutcracker" type of device using hydraulic jacks to fail the ice
over large areas
-instrumentation of natural rocks or islands
-instrumentation of the ice failing against natural rocks or islands
As a result of this study, it was found that it might be possible to measure large
scale ice forces by measuring the deceleration of large fast moving floes as they
impact a natural obstruction. At the same time, a survey was made of all the rocks
and small islands in the Canadian Arctic which might be used to measure full scale
multi-year ice forces (Roche 1979).
After air photo surveys of the most likely locations, it was realized that Hans
Island, located in the middle of Kennedy Channel between Greenland and Ellesmere
Island at 81° latitude North, provided an ideal platform from which to make measurements
of ice floe decelerations during impact and therefore determine large scale ice
forces. During August 1980, a field party of five stayed on Hans Island for three
weeks and measured the decelerations of floes up to 6 km in diameter as they impacted
the island at speeds up to 0.6 m/s.
2. HANS ISLAND FOR ICE RESEARCH
Hans Island (Figure 1) is a location unique in the world and extremely well suited to
determining ice design criteria for Beaufort Sea platforms. It is a steep island, 1
km in diameter and 150 m high which stands in the middle of Kennedy Channel. The ice
conditions in Kennedy Channel in July and August are similar to the worst ice condi-
631

tions expected in the Beaufort Sea during a summer polar pack invasion, i.e., large
multi-year floes up to 20 km in diameter, with average ice thicknesses up to 7 m
moving down the channel and impacting Hans Island at speeds over 1 mls (due to high
winds and currents in the channel).
Even ice islands have been known to impact Hans Island as when ice island WH5, about
8 x 20 km, became stuck between Hans Island and Ellesmere Island for several months
(Nutt 1966).
The highest ever recorded ice pile-up (25 m) was recorded at Hans Island in 1974 by
M. Dunbar, when a 7 m thick 16 km by 10 km multi-year floe, again became stuck
against Hans Island. The floe was held up by the island and kept moving back and
forth with the tide, grinding against the island for over a month (Kovacs 1979).
The disadvantages of Hans Island are: the difficult logistics, the very short season
(the ice breaks-up about July 27 on the average and the weather closes in a month
later), and the high frequency of fog.
3. SOME QUALITATIVE OBSERVATIONS DURING ICE IMPACT
During the summer of 1980, eight impacts of multi-year floes larger than 10 km 2 on
Hans Island were recorded over the 21 day field work.
In most cases the mode of ice failure at the interaction zone could be called a
"crushing" mode, with very little evidence of any flexural failures. The failed ice
showed evidence of a combination of "flaking" and "crushing".
In some cases the large floe was preceeded by a "cushion" of smaller floes which were
squeezed against the island and absorbed the impact by ridging and rafting.
In other cases the large floes were split by the island upon impact. This occurred
in particular when the floe was made-up of several smaller multi-year pieces in a
matrix of first year ice, a not uncommon occurrence.
4. IMPACT FORCE MEASUREMENTS: METHODOLOGY
GENERAL PRINCIPLES:
When a large independent ice floe collides with a rigid obstruction, it decelerates
rapidly due to the force required for ice failure. The interaction dissipates the
kinetic energy of the floe through work done to fail the ice. Wind drag, current
drag and Coriolis force may also contribute to the interaction. It is usually assumed
that the energy of deformation inside the ice floe is negligible compared to
632

points, A and B. After the fact, the location of G can be determined from the shape
of the floe and its thickness distribution and the distances AG and BG can be meas­
ured. The acceleration of G can then be calculated from the two equations:
... ... -t ...
a G = a A + w AG
... ... -t ...
a G - a G + w • BG
Note here that while the two accelerations a A and a B are required to calculate a G ,
they, as an added benefit, give the value of w.
In summary, the measurements required to apply equation (l) are: area of floe,
average ice thickness, representative ice bottom roughness, shape and thickness
distribution of the floe to determine tbe location of G, and either acceleration of
center of mass G, or acceleration of A (magnitude and direction), acceleration of B
(magnitude and direction), direction of AB.
It is clear that there would be a great advantage in determining the location of G
before the impact and measuring the acceleration of G only.
In the special case where w stays constant throughout the impact, the acceleration is
the same for any point over the floe. This happens in particular when a floe with no
initial angular velocity hits the island "head on", and comes to a complete stop. In
this case, one acceleration measurement is sufficient to calculate the force.
b) The application of equation (2) requires knowledge of: the magnitude of the
velocity of the centre of mass of the floe at two given instants (it's direction is
not necessary), the angular velocity of the floe at those two instants, the mass and
moment of inertia of the floe, the displacement of the center of mass between the two
instants.
Note that this method only gives the average magnitude of the component of the resul­
tant force which is parallel to the displacement ds, it does not give either the
magnitude of the force or its direction.
Equation (2) is therefore generally of limited use, however, it can be very useful,
in the special case where a floe with no initial angular velocity comes to a complete
stop against the island. In this case, equation (2) reduces to I: F • d; a 1/2 MV 2 •
... ... 0
Furthermore, in general in this case, F is parallel to ds, and, if the total displacement
of G between the time of impact and the time of stopping is S , the total
penetration, equation 2 then simplifies to:
634
F • S a 1/2 MV 2
o
(4)

sumably is obtained by multiplying the "ice failure pressure" corresponding to the
worst possible "failure mode" and the worst probable ice, by an adequate safety factor,
or "load factor".
Almost always, the definition of "ice pressure" has implicitly corresponded to the
case of a head-on collision without rotation; i.e. to the case where the force, F,
the velocity of the centre of mass V G , the velocity of ice at the point of contact
VA' and the normal to the contact area n, are all on the same line. The 'effective ice
pressure is then simply defined as the ice force divided by the width of the contact
area and by the ice thickness. However, in a more general case, illustrated in
Figure 2, the directions of V G , VA' F and n are all different, and they all change
during the impact. Defining the ice pressure in this case is more difficult: we
know the force F and the ice thickness, but what width of contact area W should we
use? There are four conceivable definitions of the effective ice pressure.
1. Using the width in the direction of the ice force, F, PI - _----!F'--_
h W cos a
2. Using the width in the direction of motion of the ice which is failing, VA
P =
2
3. Using the maximum width P 3
F cos (0 - a)
h W cos 0
po cos a
hW
(wi th a friction factor f = tan S)
As an example, using co = 15°, a - 30°, 0 = 60°, would give: PI
1.73 F/wh, P 3 = 0.87 F/wh. Which one is correct?
1.15 F/Wh, P 2
The word "pressure", i.e. "the force per unit area exerted by a fluid", implies that
the ice pressure should be the force per unit area in the direction normal to the
contact surface.
In a more practical way, to calculate the overall required lateral resistance of a
structure, the engineer needs the design effective pressure P, normal to the face of
the structure; he then calculates a maximum normal force Fn = P x Wand if nece­
ssary the maximum shear force as F s = P x W x f where f is a sui table friction
factor. Therefore definition 3 is most appropriate, i.e., if during the measurement
of an impact, the ice force F and its angle a with the contact zone are known then
the effective friction factor is f = tan a and the effective ice pressure is:
P = F cos S!Wh
Note that the effective friction factor during crushing may not be the ice/structure
friction factor. If the sliding is occurring within the crushed ice, the friction
factor f would be closer to the "internal friction" of crushed ice when it is consid-
636

C. Theodolites
If the impact lasts long enough, it is possible to use two theodolites mounted on top
of the island. Two easily identifiable points on the approaching floe can be select­
ed and the azimuth and declination of the points recorded as a function of time
during the impact. If enough measurements are made, the motion of the centre of mass
of the floe can be calculated as well as its deceleration.
The deceleration obtained in this case is the average deceleration over the time
required for three readings of azimuth and declination.
D. Photogrammetry
Photographs taken at regular intervals during the impact can also provide a measure
of the deceleration. The photographs can be taken from the island itself or from a
helicopter hovering over the contact zone. In this way the photographs provide a
record not only of the translation and rotation of the floe but also of the ice
failure mode and the width and shape of the failure zone.
After the impact, the following measurements can be made in order to characterize the
ice floe.
a) Floe size - Using photographic or surveying techniques.
b) Ice thickness - Three methods were used during the 1980 Hans Island project:
direct measurements at augered holes, surveys of freeboard with rod and level, and
using an impulse radar system mounted on the helicopter.
c) Ice strength - The FENCO borehole jack system was used to provide an index of the
ice strength, in order to be able to apply the data to other locations and different
ice. This method provides a "confined crushing strength" and a modulus of elasticity.
d) Crystallography, salinity, temperature, wind velocity and direction as well as
current velocity and direction should also be recorded.
6. EFFECTS OF WIDTH AND THICKNESS ON ICE PRESSURE
The effect of size on the effective ice pressure was discussed briefly in the introduction.
It was shown that the "size effect" is not yet well understood and that
there are many varied opinions on the effects of the width D of the failure area and
the ice thickness h.
The following discussion is an attempt at better understanding this scale effect and
638

REFERENCES:
Blenkarn, K.A. (1970), "Measurements And Analysis Of Ice Forces On Cook Inlet Struc­
tures", Offshore Technology Conference, paper 1261, Houston, U.S.A., Vol. II, pp 365-
380.
Gold, L. (1978), "Ice Pressures And Bearing Capacity", Geotechnical Engineering For
Cold Regions, Edited by Andersland and Anderson, McGraw-Hill, ISBN 0-07-001615-1.
Hirayama, et al (1974), "An Investigation Of Ice Forces On Vertical Structures",
University Of Iowa, Institute Of Hydraulic Research, Report 158, Iowa City.
Kovacs, A., and Sodhi, D.S. (1979), "Shore Ice Pile-Up And Ride-Up", Workshop On
Problems Of The Seasonal Ice Zone. Naval postgraduate school, Monterey California,
February 26-March I, 1979.
Kry, R. (1977), "Implications Of Structure Width For Design Ice Forces", Int. Union
Of Theoretical And Applied Mechanics, Symposium on the Physics of Ice Mechanics Of
Ice, Copenhagen, August 6-10.
Metge, M. et aI, (1975), "On Recording Stresses In Ice", Proceedings of 3rd Interna­
tional Symposim of IAHR On Ice Problems, Hanover, U.S.A., pp 459-468.
Metge, M. (1977), "Recent Field Testing Program", Workshop On The Mechanical Proper­
ties Of Ice. NRC Technical Memorandum Number 121.
Metge, M. (1979), "Full Scale Ice Force Measurements", Report For Dome Petroleum
Ltd., August 12, 1979 (proprietary).
Michel, B. (1978), "Ice Mechanics", Les Presses de I 'Universite Laval Quebec, ISBN
0-7746-6896-8.
Neill, C.R. (1976), "Dynamic Ice Forces On Piers And Piles". An assessment of design
guidelines in the light of recent research. Canadian Journal Of Civil Engineering,
Vol 3, pp 305-341.
Nutt, D.C., (1966), "The Drift Of Ice Island WHS", ARCTIC 9 (3), pp 244-262.
Roche, C. (1979), "A Selection Of Islands For Use As A Test Platform", Report by C­
Core for Dome Petroleum Ltd.
Sackinger, W., et al (1979), "Ice Stress Near Grounded Structures", Proceedings Of
5th International Conference on POAC 79, Vol I, pp 57-73.
Saeki, H., Hamanaka, K., Ozaki, A. (1977), "Experimental Study Of The Ice Forces On
A Pile", proceedings of POAC Conference 1977, pp 695-706.
Weeks, W. and Assur, A., (1969), "Fracture Of Lake And Sea Ice", CRREL Research
Report 269.
640

642
a)
NUMBER OF INDEPENDENT ZONES
b)
c)
n • D/4h
Pi • f(h,D), WITH D· 4h ... Pj. flh ONLY)
FIGURE 3. THEORY OF SCALE EFFECT ON ICE PRESSURE ON WIDE
STRUCTURES

ABSTRACT
PROBABILITY DISTRIBUTIONS FOR STRUCTURE LOADING
J. D. Wheeler
Research Advisor
BY MULTIYEAR ICE FLOES
Exxon Production Research Company
Houston, Texas
Possible encounter with multiyear ice floes must be allowed for in the design of
bottom-founded offshore structures in the arctic. Given a knowledge of sea-ice
mechanics and of the failure mode imposed by structure geometry on a multiyear
floe, there will be uncertainty as to the diameter, thickness and number of such
floes that may impact a structure in different seasons of the year. This paper
describes procedures for calculating probability distributions for multiyear ice
loads on a structure during open-pack conditions (e.g., breakup, summer invasions,
seasonal pack) and consolidated-pack conditions (winter). Information on floe
diameters and thickness, areal ice coverage, ice movement and ice strength are
combined in a Monte Carlo calculation to develop the probability of exceedance
(risk) versus loading in each season. Variation of ice thickness through the year
and variation in point of contact between structure and floe are allowed for.
Sensitivity of results to several types of environmental data is examined using
reasonable but fictitious values for environmental parameters.
Introduction
In the design of offshore structures for arctic service, consideration must be
given to possible encounter with multiyear ice floes. In this paper, Monte Carlo
calculation procedures are outlined for estimation of load-risk curves for rapidice-movement
conditions (summer) and for limited-ice-movement conditions (wintertime
fast ice). The calculation procedures provide a framework for converting
field observations on multiyear ice into probability distributions for ice loading
and for evaluating the sensitivity of design-level loads to various input items.
Distribution of Multiyear-Floe Forces in Summer
"Summer" is defined here as the period from June 1 through November 1, which includes
breakup, gross open-water season and freezeup. Summer ice coverage and multiyear
fraction can be gotten, for example, from ice charts for a location of interest.
Such coverage data is used in the calculation procedure to be described by assuming
rough equality between the areal fraction of sea surface that is covered by multiyear
ice and the fraction of total distance moved by a partial ice cover that brings
multiyear ice against a fixed structure [6]. This assumption together with an
estimate of ice-movement rate, permits translation of partial coverage by multiyear
ice into an equivalent distance of multiyear ice that will impact a fixed structure.
To implement the assumption, it is necessary to specify time series through the 643

MOVE FLOE
PAST STRUC. 1----=:::::1:------1
FIGURE 2. SUMMER CALCULATIONS
DATA
__ MYR. COVER
DRIFT RATE
-- FLOE THICKNESS
IN WINTER PACK
-- MAX. THICKNESS I
-- FLOE DIAMETERS I
__ MELT AND
FREEZE RATES
crushing strength against the structure. Following this period, the stress developed
is the lesser of those from edge-failure or splitting [3]. Thickness,
stress and structure diameter (D) give structure load. Another floe failure will
not occur until the present floe is moved past the structure by ice motion. If
this movement consumes the movement estimate for the current week, time is shifted
forward one week; if not, another floe is sampled.
The procedure of Figure 2 is continued until completion of a summer. The maximum
force exerted on the structure during the entire summer is stored. The entire
procedure is then repeated for 1000 summers. This population is histogrammed to
determine the distribution of maximum multiyear loading for a randomly chosen
summer. The development of a population of maximum forces for an entire summer
amounts to an integration over the three distributed quantities (floe diameter,
thickness and impact point on the structure), under the constraints that the sum of
floe diameters in any week match the ice movement specified for the week and that
floe thickness in any week be consistent with both the thickness distribution for
the pack (Figure 3), a melting rate derived from air-temperature measurements and
location water depth. It is these time-varying constraints that distinguish the
procedure in Figure 2 from that used for multiyear ridges in [6]. In the ridgeloading
work, no allowance was made for variation through the time period of
interest (one summer) or statistics associated with individual ridge failures.
645

Distribution of Multiyear-Floe Forces in Winter
Winter is defined for calculations here as the period November through May. As
with summer calculations, it is necessary to specify an areal coverage of multiyear
ice and an ice-movement rate for each week. To make use of values input for multiyear
coverage and movement rate, the assumption is again invoked that there will be
rough equality between the areal fraction of multiyear ice and the fraction of
multiyear ice along any linear path across the ice [6]. Intuitively, this assumption
seems most likely to be satisfied when the multiyear ice is uniformly
distributed over the area to which the specified fractional coverage is taken to
apply. A detailed instance of uniform areal distribution of multiyear floes would
be for each multiyear floe to have connected to it an area that is free of multiyear
floes, in such a way that the areal percent of multiyear coverage is maintained
in the vicinity of every floe. In summer, the floe-free area around each floe
would contain water and vestigial annual ice. In winter, the floe-free area would
consist of annual ice. The size of the floe-free area would increase with floe
diameter. This picture does not preclude multiyear floes being in contact. There
can be clusters of 2-4 multiyear floes and arbitrarily long lines of multiyear
floes. The picture is somewhat like a thin section of cellular tissue, with
multiyear floes being the cell nuclei and the floe-free area the cytoplasm, the
whole cell being contained within a flexible membrane. In such a picture, the
distance between floes can vary widely for any specified floe-size distribution,
but the variation is constrained by the requirement that areal coverage be preserved
"in the small"; i.e., in the vicinity of each floe.
The above picture of detailed uniformity of multiyear-floe areal distribution
brings consideration of floe spacing into the calculation procedure to be described
below. Such consideration permits allowance for the fact that, in wintertime fast
ice, the total movement and the multiyear coverage are not great enough for a
structure to be contacted by a multiyear floe in every week, or even in every
winter. In the summer calculation, it is tacitly assumed that ice movement in any
week is always sufficient for multiyear floes to contact a structure, if such floes
are present. The relatively limited movement in wintertime fast ice also invites
consideration of percent-coverage values that are appropriate over areas that may
contain several multiyear floes but are comparable in size to a total winter's
movement. Historical information on multiyear ice coverage seems primarily to
consist of visual estimates by trained ice observers from airplanes or ships. The
area to which the visual estimates apply has dimensions on the order of miles, a
region much larger than that swept out by a winter's movement in the fast ice. Any
localized concentrations of multiyear ice are not recorded. Aerial photography of
the fast ice indicates that it is not infrequent for multiyear ice to occur in
patches, as indicated schematically in Figure 4. The floes in these patches could
in some cases be fragments of larger floes that, weakened by summer melting, broke
apart during the fall storms that moved them into the fast-ice zone. In any case,
it is possible to define as a "multiyear area" the photographed area between the
start and finish of such a multiyear patch. The multiyear coverage within such a
multiyear area of flightline can be determined with reasonable accuracy to give a
value of multiyear coverage that applies over a relatively small area in the waterdepth
range spanned by the photography. Outside the multiyear areas, the ice is
entirely first-year. A localised estimate of the chance that there will be a
multiyear hazard in winter can be gotten from the ratio of total multiyear area to
photographed area. Coverage values in multiyear areas of the kind defined in
Figure 4 seem more appropriate for quantification of the mUltiyear-ice hazard in
wintertime fast ice than visual averages over broad areas.
The wintertime calculation flow for a specified location is given in Figure 5.
Winter too is marched through, week by week. At the beginning of each week.a multiyear
floe is sampled. Its thickness in the current week is gotten by sampllng
647

a FLOGAP of ice past a structure to determine whether or not there is structurefloe
contact in the current week. The average daily rate does not determine the
rate of loading when structure-floe contact is made and so does not determine the
crushing strength that the floe will exert against the structure. For a given
hourly movement rate, ice strength is determined from laboratory and field data on
strength variation with strain rate, as outlined in [4]. The strength determination
is made for both annual (A) and multiyear (M) ice. The fraction of structure
diameter that is loaded by each of these two ice types is determined by random
sampling of the point along the structure diameter that is impacted by the center
of the multiyear floe. For the force calculaton, strain rate and load are determined
from the following two expressions.
Strain rate = Ice velocity/(2Df D )
Force = fcICx(DfD)t
where D is structure diameter, fD is the factor by which grounded rubble around
the island increases its effective diameter, fc is a contact factor [4], I is an
indentation factor, C x is the unconfined compressive strength of the ice and t is
the ice thickness. Edge or splitting failure [3] is not considered in the winter
calculations because ice floes are assumed to be confined in winter by the annual
ice sheet. This should be conservative treatment of early winter, when the annual
ice is thin. Following the force calculation, the ice is moved the remaining
distance assigned to the current week, with time advance and/or selection of
another FLOGAP as discussed above. As with the "summer" calculations, the maximum
loading experienced by the specified structure in each winter season is stored and
the calculation repeated for several thousand winters to obtain a population of
maximum annual winter loads.
Example Calculations
Calculations indicating sensitivity or results to input quantities were run for
schematic summer and winter cases. For summer, a structure in 60 feet of water was
considered. The distance of multiyear ice moving past the structure in every year
was gotten for each summer week using the ice-coverage and movement values from
Figure 1 plus the assumption that 50% of the ice is multiyear during an invasion
and 12.5% is multiyear otherwise. Floe diameters and thicknesses were sampled from
Figure 3. Maximum ice thickness in the winter pack was specified to be 16 feet.
The procedure of [3] was used to determine the mode of failure and stress imposed
for each floe impacting the structure. The contact factor[f c in Eq. (2)] was set
at 0.6 and indentation factor(I) at 3.0 [4]. Grounded rubble was assumed to be
absent in the summer period. This summertime base case gave the solid line in
Figure 7 for probability of exceedance versus normalized maximum summer load.
Structure loads have been normalized by the product CxD from Eq.(2) to give units
in Figure 7 of square inches per foot of structure diameter. Sensitivity to the
product of ice coverage and ice movement was considered by arbitrarily dividing
this product by 100 for each summer week. This gives the dashed curve(triangles)
below the base-case curve in Figure 7. The reduction in ice movement makes little
difference in the annual-risk range likely for design (0.1-0.01). Increasing the
assumed maximum thickness of multiyear ice from 16 to 24 feet gives a corresponding
50% increase in load for the 0.1-0.01 range of annual risk. This correspondence
plus the steepness of the curves in Figure 7 indicate that the calculation procedure
of Figure 2 gives loads associated with the maximum thickness of multiyear ice that
can reach the structure. This maximum will be the lesser of water depth and the
maximum multiyear thickness assumed to exist in the pack ice.
650
(1)
(2)

Change in Calculation Input Change in Load at Specified Annual Risk
Risk = 0.1 Risk = 0.01
KIPL1000 % KIPL1000 %
No change = Base Case 55 0 78 0
Increase hourly rate x 10 107 95 151 95
MYr cover = 0.375 66 20 94 20
MYr cover = 0.125 in
10% of years 51 - 7 73 - 6
Median Floe diam. = 250 ft. 53 - 3 70 -10
=1000 ft. 57 3 82 5
MYr strength = 1.2 annual 57 3 84 8
Increase max. myr thickness
to 24 ft. 61 11 99 27
The 95% increase in load with a ten-fold increase in ice velocity follows from
Eqs.(l) and (3);10 raised to the 0.291 power is 1.95. The large increase indicates
the need in these calculations for accurate determination of the relation between
stress and strain rate and of ice movement rates in winter. The 20% load increase
with a three-fold increase in multiyear ice coverage also stands out in the tabulation.
However. available information indicates that 37.5% multiyear coverage in
wintertime fast ice is rare. which reduces the significance of the load increase
shown. The load-risk curve for 37.5% multiyear cover is plotted in Figure 8. The
fourth table entry was obtained by assuming 1/8 multiyear coverage near the
structure in only 10% of the winters. The coverage for other winters was uniformly
distributed between zero and 1/8. The load-risk curve for this case is also
plotted in Figure 8. The median floe diameter in Figure 3 is about 500 feet. Use
of distributions parallel to the one in Figure 3 through median values of 250 and
1000 feet changes loads in the 1-10% risk range by no more than 10%. Increasing
multiyear strength by 9%. to 1.2 times the annual strength. gives essentially the
same percentage increase in load at 0.01 annual risk but a lower increase in load
at 0.1 annual risk. This is consistent with the low-risk events occurring when
the structure diameter is almost completely spanned by a multiyear floe. Similarly
a 50% increase in maximum allowable floe thickness to 24 feet has greater effect
on the low-risk loading events. which are associated with a larger cross-section
of multiyear ice impinging on the structure.
REFERENCES
1. Herbert. W .• Geographical J .• 136(4).511533. December. 1970
2. Kniskern. F .. E .• Potcsky. G.J. 7'Frost Degree Day. Related Ice Thickness ..• ". Tech.
Rpt. TR-60. Oceanogr. Prediction Div .• US Naval Oceanogr. Office. July. 1965
3. Ralston. T.D .• "Plastic Limit Analysis of Ice-Splitting Failure".POAC 81 Preprints.
4. Wang. V.S •• "Tech. Seminar on Alaskan Beaufort Sea Gravel Island Design".Exxon
Company. U.S.A •• October 18. 1979
5. Weeks. W.F .• et al."Characterization of Surface Roughness and Floe Geometry ••. ".
AIDJEX Symp. on Sea Ice Processes .•.• Preprints. Vol.II.September 6-9. 1977
6. Wheeler. J.D .• "Probabilistic Force Calculations for Structures in Ice-Covered
Seas". Proceedings. POAC 79. p.1111-26. Vol.II. Trondheim. August. 1979
652

A. B. CAmmaert,
Manager, Engineering Studies
G. P. Tsinker,
Lead Engineer
ABSTRACT
IMPACT OF LARGE ICE FLOES AND
ICEBERGS ON MARINE STRUCTURES
Acres-Santa Fe Incorporated
Calgary
Acres Consulting Services Limited
Niagara Falls
In recent engineering studies it has been necessary to calculate the
impact of large ice features on various types of marine structures.
A simple approach was developed which relates kinetic energy dissi­
pation to progressive crushing of the ice.
When a large ice floe or iceberg collides with a massive structure
the contact zone will fail by crushing, and will increase in size as
the resisting force is steadily increased. The kinetic energy is
then decreased until an equilibrium point is reached.
An analysis of an ice floe impact on a sluice gate will first be
presented to illustrate the methods.
The particular case of a "blocky" iceberg colliding with either a
cylindrical or a conical gravity platform will then be analyzed.
For typical iceberg characteristics and structure dimensions, it
will be demonstrated that a gravity structure could be designed to
withstand iceberg loadings. The effect of structure movements will
also be considered.
Canada
Canada
653

INTRODUCTION
In recent engineering studies conducted by Acres it has been necessary to calculate
the impact of large ice features on several different types of marine structures and
a simple design approach was adopted. The approach is based on kinetic energy
diSSipation, which is similar to that used by other researchers in the calculation
of the impact of an ice floe hitting a pier (Michel, 1978), and the determination of
iceberg scour (Chari, 1981).
In these studies the following design assumptions have been used.
- When an ice feature collides with a massive structure, the leading edge of the ice
feature will be progressively crushed, and the force acting on the structure
during impact will be that of a steady increase from zero to a certain maximum
value, as a result of kinetic energy dissipation during the ice crushing period.
- The shape of the ice feature has been arbitrarily simplified for preliminary
calculations.
- The deformation of the structure has been neglected, since it is normally an
insignificant portion of overall displacement.
- Ice feature dimensions and strengths are such that buckling and bending failures
will not occur, and a compression or crushing failure is assumed.
- For large ice features such as iceberqs the volume of water in motion with the ice
feature will have influence on impact energy. The virtual displacement of the ice
feature is obtained by adding this volume to the ice feature displacement.
The following analysis represents two different case studies I that of an individual
ice floe in collision with a sluice gate, and an iceberg impact with an offshore
gravity platform.
ICE FLOE IMPACT AGAINST
A SLUICE GATE
When an ice sluice gate (Figure 1) is partially open the water level overtops it.
At this moment an ice floe which moves with velocity V can strike the top of the
gate and create an impact load F in addition to the hydrostatic load. The collision
654

where k is assumed to have a value of 1.5. This coefficient is commonly used for
calculation of the hydrodynamic mass of ships in berthing energy calculations
(Canadian Coast Guard, 1977), and for nblocky· shapes, it is likely a reasonable
estimate.
The maximum value of penetration "m can be determined by equating the kinetic
energy of the ice floe to the energy absorbed by the crushing of the icel
wv 2 = 1. 5Wi v2
2g "2"q
Bfcr
cosa
"m
JXdx o
where Ek = kinetic energy of the ice floe
Hence,
Ec = energy dissipated in crushing.
and the maximum impact force Fm is expressed as
Representative values of all variables for a typical case study are given in
Table 1.
Table
Impact on Sluice Gate
Ice floe dimensions
Ice floe displacement
Ice floe velocity
Ice crushing strength
Sluice gate slope
Maximum penetration
Maximum impact force
656
B = 5 m, L = 15 m, D 1 m
Wi = 67.5 tonnes
V = 0.20 m/s
fer - 1.0 MPa
c:a 70·
"m = 1.7 em
Fm 24.4 tonnes
(4)
(5)
(6)
(7)

For a cylindrical platform, the maximum impact force Fm is determined as follows:
It is assumed that the arc length (acb) is approximately equal to the chord length
(ab), if the total penetration is small.
contact area is (Figure 2)
where D = iceberg depth
R = platform radius
L
I BLOCKY I BERG
ELEVATION
PLAN
Figure 2 - Iceberg Impact on Cylindrical Platform
658
For a penetration of distance x, the
CYLINDRICAL
PLATFORM
(9)

The impact load is then defined as
and the energy dissipated during crushing is
Xm
Ec = 2Df crJi( 2Rx - x 2 )1/2 dx (11 )
o
Equations (8) and (11) can be solved numerically, or an approximate expression for
Xm can be found by dropping the x 2 term in (11) (if >

Again, for small penetrations, it is as'sumed that the area (abck) in Figure 3 is
equal to the projected parabolic area (akc), so that
AX = 2. (ac)(kf) =.! tanSx
3 3
and Rl is the platform radius at the point of impact, which is
R _ (d - di)
tanS
where R radius of platform base
base angle
water depth
depth of iceberg impact
The parameters Fx ' E c ' xm and Fm are defined as
.! tanSfcrx ( 2Rlx - x2 )1/2
3 '
(2R - x 2 ) 1/2dx
I
! tanSfcrXm ( 2Rlxm - Xro2 )1/2
3
Of particular application to possible production platforms for the Hibernia
,
discovery, the numerical values given in Table 2 are substituted. The platform
dimensions are those for an approximate water depth of 80 m and are similar to those
discussed bY Jarlan (1981). The' maximum credible' iceberg anticipated for this
depth can have a displacement of approximately 1.2 x 10 7 tonnes. The above
analysis indicates that for the cylindrical platform, the expected impact force is
7.6 x 10 5 tonnes, and for a conical platform is 1.4 x 10 5 tonnes.
660
( 17)
( 18)
( 19)
(20)
(15)
( 16)

Table 2
Design Example, Iceberg Impact
on Gravity Platform
Iceberg displacement, Wi
Iceberg depth, D
Iceberg draft, di
Water depth, d
Iceberg velocity, V
Ice strength, fcr
Platform radius at base, R
Platform base angle,
Maximum penetration, xm
Maximum impact force, Fm
Platform displacement (est)
Coefficient of friction
Factor of safety versus sliding
Cylindrical Platform
1.2 x 107 tonnes
75 m
60 m
80 m
1.0 mls
5.0 MPa
60 m
90·
0.88 m
7.6 x 105
1.1 x 10 6
0.70
1.01
tonnes
tonnes
Conical Platform
1.2 x 107 tonnes
75 m
60 m
80 m
1.0 mls
5.0 MPa
100 m
40·
7.81 m
1.4 x 10 5 tonnes
1.1 x 10 6 tonnes
0.70
5.35
Again simplifying the design process, the factor of safety against sliding is
calculated for each case, as indicated in Table 2. This shows that the =nical
platform, in particular, could be designed to resist iceberg impact forces. If the
structure can be designed to accommodate limited horizontal movement, a higher
factor of safety can be assured.
Naturally these calculations are preliminary only, since the nature of iceberg-
structure interaction is still poorly understood. A more detailed study would
involve a much more careful analysis of iceberg displacements, velocities and
strengths. The nature of the contact face during impact is especially sensitive, as
the above analysis demonstrates.
CONCLUSIONS
Using a simplified model for ice-structure interaction, it is possible to develop
impact forces on offshore and marine structures. The analysis is based on individ­
ual ice features colliding with structures, where the crushing of ice dissipates the
kinetic energy of the ice feature. The analysis indicates as an example, that large
gravity platforms for the Hibernia field could possibly be designed to absorb ice-
berg impact. However, a more careful evaluation of the design variables must be
performed to achieve more confidence in the results.
661

ACIQIOWLEDGMEN'l'S
The authors wish to thank Mr. David B. Sampson, General Manager of Acres-Santa Fe,
for his support of this publication.
REFERENCES
Canadian Coast Guard (1977)
"Termpol Code-Code of Recommended Standards for Prevention of Pollution in Marine
Terminal Systems"
Transport Canada, 1977
Chari, T. R. (1979)
"Geotechnical Aspects of Iceberg Scours on Ocean Floors"
Canadian Geotechnical Journal, Vol 16, No.2, pp 379-390.
Jarlan, G.E. and Lehalleur, J.D.
"A Prestressed Concrete Fixed Drilling and Production Platform for the Hibernia Oil
Field Development"
Symposium on Production and Transportation Systems for the Hibernia Discovery,
St. John's, February, 1981
Lewis, J. (1981)
·Icebergs on the Grand Banks: Oil and Gas Considerations"
World Oil, January 1981
Michel, B. (1978)
"Ice Mechanics·
Les presses de l'Universite Laval, Quebec, 1978
662

M. Rojansky
B. C. Gerwick
Abstract
Failure Modes and Forces of Pressure Ridges
acting on Cylindrical Towers
University of California, Berkeley USA
This study is directed at the potential failure modes and sequences of failure
of a typical sea ice pressure ridge impinging against the vertical cylindrical
shaft of an offshore gravity platform. From these analyses, the maxima forces
acting on the structure can be bounded.
It was found that in the case of short ridges, the sheet behind the ridge will
fail in crushing and create a rubble field. For longer ridges, the failure will
be due to flexure. The critical length is a function of the geometry and kinematics
of each given situation.
Since actual pressure ridges will often have both consolidated and unconsolidated
zones, a transformed section has been adopted.
The indentation problem is addressed analytically, and the indentation factor
is shown to vary non-linearly with the aspect ratio, being comPl'-rable to previously
published empirical data for both large and small aspec1l ratios but lying
below them for intermediate ratios.
A ridge which is incorporated within an isolated ice floe may exert a significant
impact force when it impinges on a structure. The analysis for impact
considers a series of increments for each of which the variations in apparent
crushing strength due to changing strain rates and aspect ratios are considered.
A numerical example is presented in order to demonstrate the principle
that the "minimum required energy to failure concept" should be adopted in
order to determine the mode or modes of failure and the maxima forces
imposed on the structure.
Introduction
Continued studies of the potential interaction between sea ice and structures
in Arctic and sub-Arctic regions have demonstrated that pressure ridges are a
dominant feature in determining the maxima forces to be resisted. These ridges
consist of both consolidated and unconsolidated ice and move with different
velocities depending on the amount of open water.
663

Although many previously published papers have recommended that a vertical
cylindrical shaft was unsuitable for an environment which includes multi-year
pressure ridges, due to the alledgedly high forces developed by the ice in the
crushing mode; however, the vertical tower has a number of advantages for
specific sites, including the minimization of lateral and vertical wave forces,
low wave run-up, relative freedom from problems occasioned by adfreeze and
ice over-ride. Therefore it seemed worthwhile to reexamine the modes of
failure and determine an envelope or upper bound of the forces actually developed.
Failure Seguence
Previously published reports of structures in Cook Inlet and the Baltic Sea,
as well as model tests show that when a pressure ridge impinges against a vertical
indentor, (Fig. I) the principal failure of the ridge does not occur in crushing of
the contact zone between the ice and structure, but most often occurs in flexural
cracking opposite or even at some distance laterally from the structure. Such
cracking may occur repetitively and may result in a pile up of broken ice, ie. a
rubble field behind the ridge. A sequence of failure modes is observed, of which
complete failure in crushing is seldom if ever dominant. This contrasts with the
failure of a uniform ice sheet against such a vertical shaft, which is characterized
by crushing and shear.
This phenomenon of multi-modal failure is an example of the "minimum
energy to failure concept" and is the underlying assumption behind the modal
analyses presented in this paper. The ridge is assumed to fail first at a mode
and at a location which requires the least amount of energy to failure. Subsequent
failures will then similarly be determined, based on the structural and
strength characteristics of the new ice formation.
One way to identify a possible failure sequence is to assume various possible
failure mechanisms and search for the critical one. The resulting failure mode
and force will then be a function of the mechanical properties of the materials
involved as well as the geometry and kinematics of the event. Particular attention
is directed in this study to failures in crushing and in flexure.
Mechanical Properties of Ice
The mechanical properties of ice are obviously critical to any study of
failure mechanisms and imposed forces. Although ice varies widely, there
has fortunately been a great deal of research and testing of mechanical properties
in recent years, much of which has been published. For the purpose of this
paper, relevant information from references 3, 9 and 10 will be used.
The most important aspects of mechanical properties to this study are:
a. The crushing strength varies with the rate of loading (strain rate)
such that three behavior zones, ductile, transition, and brittle are
identified. (Fig. 2)
b. Sea ice is approximately 5 times as strong in compression as in
flexure or tention. (Fig. 3)
To simplify the analysis, the following assumptions have been made:
664
3.. Although sea ice is anistropic and non-homogeneous, the analytical
model can be based on homogeneous and isotropic behavior because
the failure against a vertical cylinder is essentially planar, and the
structure of a multi-year ridge consists essentially of crystals
oriented as vertical colwnns.

Under the assumptions described earlier the maximum positive moment will
occur at the point of interaction and will be equal to:
... p COShAL -COS)'L
M : 4>.)( SlnhAL +SII'l).L eq.3
The location of the maximum negative moment will vary, however
for short ridges its magnitude may approach the magnitude of the maximum
positive moment. For these cases the maximum negative moment will occur
at the end of the ridge and will be given by:
- P sinh L. SI " L
M =-X)( 51'1 AL+SII'1). eq.4
The failure force P can be found by equating the maximum externally
applied moment to the maximum internal moment capacity.
Ml1llx= -¥-
where: 17£ The flexural strength of ice
C The distance to the extreme "fiber"
Once the initial crack has formed, the ridge separates into two shorter
ridges which are assumed to be simply supported at the center (Fig. 4 b).
The new maximum moment is most likely to occur at the fixed end and it is
equal to:
The resulting interaction force is found as described earlier, using
eq.5. A comparison between the two interaction forces (P versus P')
shows that for all practical cases, P' is smaller than P. Hence, for the
stipulated flexural failure the maximum interaction force is associated
with the formation of the initial crack.
Analysis for Failure in Crushing
Crushing failure can be defined as a failure in shear due to excessive
compression. To examine this failure mechanism we will evaluate the
case of ice impinging against a flat indentor. This is a two dimensional
idealization of a three dimensional problem. However, the same logic
could be applied in the latter case. Several studies have been conducted
in order to find relevant expressions which make it possible to predict
the magnitude of the ice-structure interaction force when crushing failure
takes place (Refs. 1,4,6,8). The various proposed relationships are
similar to each other: thus here we use an expression from Ref. 8. This
expression for the interaction force is as follows:
F = fcX1"v>

SUMMARY
The previous discussion offers a rational approach towards the analysis of
the ice-structure interaction problem. In analyzing the interaction forces, the
minimum required energy to failure concept can be beneficially adopted rather
than considering the maximum force due to a single failure mode. Particular
attention should be given to the development of a rational failure sequence.
Acknowledgement
The study which formed a basis for this paper was performed under a grant by
Mobil Research and Development Corp., whose support is gratefully acknowledged.
REFERENCES
1. Acres/Santa Fe-Pomeroy - Feasibility study offshore drilling in the
Beaufort Sea, APOA 1971
2. Bercha, F. G., Stenning, D. M. - Arctic offshore deepwater ice-structure
interactions, OTC 3632
3. Blenkarn, K. A. - Measurements and analysis of ice forces on Cook Inlet
structures, OTC 1261
4. Croasdale, K. R. - Ice forces on fixed rigid structures - Calgary,
Canada, 1978
5. Hetenyi, M. - Beams on elastic foundation, University of Michigan
Press, 1971.
6. Korzhavin, K. N. - Action of ice on engineering structures, Siberian
Dept. of the Head of Science, Novosibirsk, USSR 1962.
7. Michel, B. - Ice pressure on engineering structures, CREEL, U. S.
Army, Hanover, NH 1970.
8. Ralston, T. D. - Sea ice loads - Technical Seminar on Alaskan Beaufort
Sea Gravel Island Design, EXXON, Houston, TX 1979
9. Vaudrey, K. D. - Study of related properties of floating sea ice sheets
and summary of elastic and viscoelastic analysis - U. S. Navy,
Civil Engineering Laboratory, Port Hueneme, CA 1977
10. Wang, Y. S. - Sea ice properties - Technical Seminar on Alaskan
Beaufort Sea Gravel Island Design - EXXON, Houston, TX 1979.
670

of determining the total depth of disruption of the soil below the score exists and
this will be the topic of future research. Multi-year ice, ice islands or ice
island fragments generally cause a single, or a few, smooth, wide furrows, while
first year keels cause multiple rake-like furrows in the sea floor.
In the Canadian Beaufort Sea, a relatively stable land fast ice zone forms early in
the winter and remains with little movement for most of the winter; little scoring
would be expected in this zone during the winter season. Bordering this zone close
to the 20 m water depth is the shear zone (Croasdale and Marcellus 1977) which
separates the highly mobile pack ice further offshore and the landfast ice; significant
scoring occurs each year in the shear zone. In deep water few scores occur, as
only extreme keels are able to reach to the seabed and beyond 47 m it is believed
that no current scoring is occurring, as a 47 m keel is the maximum keel that has
been observed.
Once a score has been formed, dependant on its cross-section it may, immediately, be
partially filled-in by the scored sediments as illustrated in Figure 1. Other in­
filling processes (shown in Figure 1) such as preferential infilling (Barnes and
Reimnitz 1979) and superimposition (Lewis 1977a) could cause the disappearance of
scores which is much greater than expected by infilling at the average sedimentation
rate. Sedimentation and score infilling are qui te distinct processes. Uniform
sedimentation by river sediments or sediments reworked by the scoring process results
in minimal infilling of the scores (Figure lB). Immediate infilling, superimposition
and preferential infilling cause the most significant infilling of scores (see
Figure 1).
Immediate infilling is a function of the characteristics of the soils scored. The
probability of superimposition infilling is a function of the frequency of scoring
in a given area. Preferential infilling however may be the result of a number of
physical environmental occurrences, such as normal and tidal currents, wave induced
currents, ice keel induced currents or turbidity currents (which could be caused by
earthquake activity). Thus in shallow waters, off the Mackenzie Delta score infill­
ing will occur almost continuously, whereas in deep water, infilling will be episo­
dic, occurring only during extreme environmental conditions; in very deep water
probably no wave induced infilling occurs. Also scores behave like sediments traps
on the seabed, whereby the seabed roughness dictates the quantity of sediment trap­
ped in a specific area during a certain interval of time.
Thus one would expect to observe very few scores on the seabed near shore due to
both a low scoring rate and a high fill-in rate, a zone of maximum scoring in 15 to
35 m water depths and numerous scores on the seabed in water depths beyond 40 m dne
to a low fill-in rate.
676

AVOIDANCE OF SCORING
It is possible to avoid having to trench the pipeline by directing its route through
areas that are not scored due to the local topography, i.e. route pipelines down
natural depressions in the seafloor.
There are several extinct river valleys or channels in the Beaufort Sea. It was
thus felt that it may be possible to run a pipeline along these low points in the
seafloor. A study (Marcellus 1980) indicated that it is not economical unless the
valley is close to the required route. Alternative techniques of protecting pipe­
lines in ice infested waters are discussed by Marcellus and Palmer (1979).
METHODS OF DETERMINING PIPELINE BURIAL DEPTHS
Burial Below The Saturated Scored Zone (SSZ)
Based on the discussion in the previous chapter a pipeline set below the SSZ (in
areas where seabed erosion is not occurring) would be below the deepest scour that
has occurred since the transgression period for that particular water depth.
In water depths where there is 10,000 years of sediment and scoring has occurred
during the entire time period, the bottom of the SSZ would have been formed 10,000
years ago and current scores may no longer reach to this depth. In shallow water,
assuming no erosion, the sediments have been deposited for about 3,000 years, so the
depth of the SSZ indicates the deepest scores that have occurred during this period
of time. Thus in shallow and medium water depths the bottom of the SSZ is probably
a reasonable TOP depth but in deep water it is probably unreasonably conservative.
Burial Below The Deepest Scores
Over the past few years, large quantities of data have been obtained from echo
sounding and side scan sonar traces of scores of the seafloor. Echo sounding data
gives the depths of scores and the deepest score along a pipeline route can be
obtained from these data. Does the depth of the deepest score indicate a safe TOP
depth? Figure 3 shows the deepest scores from APOA 32; more recent data are present­
ed in APOA 122, but these are confidential.
In extremely shallow water scores are rapidly obliterated due to the high level of
hydrodynamic activity in this water depth. In deep water the observed scores are
the result of scoring which occurred when the water level was some tens of metres
below its current level and the resulting burial depth would then be extremely
conservative. In the mid-range water depths where the scoring rate is high (around
the shear zone) the deepest score which has occurred in the past 50 years may have
678

een significantly infilled. Thus in these water depths burial below the deepest
score is probably inadequate.
Therefore it is felt that the maximum score depth cannot be used to select a safe
TOP depth at this time. This method is useful for comparative purposes.
Score Dating
If scores could be dated directly, then it would be possible to calculate the
return periods for scores of various depths. In this method, it is necessary to
identify the original score surface and date the sediment immediately above this
surface. In medium water depths where scoring is very active and scores are filled
by the disturbed sediments from the SSZ, it is not expected that dating will work.
In deep water where scoring is very infrequent and scores are probably filled by
river sediment, dating may be possible. A score dating project by Dome in 1975
(Dome, 1975) in intermediate water depth was unsuccessful, presumably for the reasons
stated above.
Repetitive Mapping
In this method a given area on the seafloor is surveyed by Side Scan Sonar every
year or every few years, and the number of new scores which have occurred during the
interval are counted. This provides the number of scores per year per kilometer for
the specific area. LeWis (1977b) reports on data obtained between 1971 and 1974
near Pullen Island in the Canadian Beaufort Sea. His findings for the 15-20 m water
depth showed 10 new scores during three years at one site and 11 new scores during
two years at the other site. Considering the areas surveyed, an average number of
0.35 ::!: .12 scores per year per nautical mile, or 0.19 ::!: 0.07 scores/year/km was
calculated. Knowing the score depth distribution and the number of scores/year/km,
the return period for scoring can be estimated by the follOWing equation, (Weeks, et
al,1980);
d - {In (l/NTL»/{-k)
where: d is the score depth predicted (m), L is the length of pipeline route (km),
T is the return period desired (years), N is the average number of scores/year/km, k
is a parameter equal to the reciprocal of the sample mean score depth for a given
water depth (m- 1 ), In is the natural logorithm.
Using an N of 0.19 scores/year/km, k - 2.5 (from Lewis 1977b), T = 100 years and L =
78 km, we find that for the water depth range 15-20 m the TOP depth for a pipeline
should be 2.9 m. This is plotted in Figure 2.
679

The main problem with this method is the short time base of available data and the
resulting relatively poor score statistics. For a given area the yearly variations
in the local ice features scoring the seabed affect the number of new scores seen
during that period. In this respect an averaging of data over a long period should
give a better representation of the scoring rate for a given area. Secondly this
method uses the observed score depth distribution on the seabed with no correction
for infilling of the new s'cores. Therefore based on the previous discussion this
method may tend to under or over predict the required TOP depth for a pipeline for a
given return period. We view these data as extremely useful since they are the only
data which use direct measurements of current scoring to obtain TOP depths for
specific return periods, but better statistics are required.
Scoring Equilibrium Analysis
The basis of this analysis (Lewis 1977b) is that the number of scores in an area is
in steady state and thus the number of new impacts in the area equals the number of
scores infilled plus the number of impacts superimposed on existing scores. For the
infilling rate Lewis used the average "sedimentation" rate based on the total mud
thickness at a given site divided by the time available for deposition of mud in
that area. However, considering the previous discussions this is clearly not cor­
rect.
Lewis' (1977b) basic equation was used to obtain the following equation:
where:
d (In(V/(un o + (V pno)/k»/-k
d is the predicted score depth for a return period of T years
V = l/LT
L is the total length of pipeline (km)
u is the sedimentation rate (mm/year)
no is the number of scores/km in the area of interest about to be
filled in
This equation was used to obtain the line shown in Figure 2. The data used in the
analysis was Lewis' (1977b) data for a pipeline route from the Kopanoar Wellsite to
shore which incidently goes through the Pullen sites surveyed by repetitive mapping
(discussed above). It is seen that for the 15-20 m water depth range Lewis' analysis
indicates a TOP depth which is less than the point for the repetitive mapping method,
which is based on an observed scoring rate in the indicated water depth range. This
would then indicate that the infilling rate used by Lewis is lower than the actual
infilling rate occurring at the Pullen site. By fitting Lewis' method to the ob­
served point from repetitive mapping a new infilling rate can be determined which is
680

2.8 times the sedimentation rate used by Lewis. If the discussion in the chapter on
origin and disappearance of seabed ice scores is correct, we would expect that the
scoring equilibrium analysis as documented by Lewis would overpredict the amount of
scoring in deep water (where fill-in rate is less than the sedimentation rate) and
underpredict the amount of scoring in shallow water (where fill-in rate is far
greater than sedimentation rate), which is indeed noted in comparison with the other
method presented.
If, in line with our previous discussion, a high fill-in rate is assumed in shallow
water (Barnes and Reimnitz (1979) reported infilling of .6 m in one score in 13 m
of water in one year), and very little fill-in in deep water, Lewis' method can be
made to better fit the observations (Marcellus 1981). However, much more information
is needed on score infilling rates before these results can be used to select
safe TOP depths.
Ice Keel/Score Statistics Method
This method uses the measured keel depth distribution passing over a point on the
sea floor (obtained by upward looking profiler, submarine profiler, laser profiler)
and the measured scour depth distribution, observed on the seafloor.
The number of keels greater than depth D metres passing a point on the seafloor/year
can be represented by:
-D/Do
N()D} = Noe (c.f. Wadhams 1975)
Where No is the total number of keels/year and Do is a constant.
The number of keels pasing over a 1 km long pipeline is f times the above, where:
f 1000/ (w/2)
where w is the width or length of the keel (m). W can vary from the length of a
keel (approximately 2000 m) to width of keel (D/tan 35°), where D is the ice keel
depth; here an average value for f is used.
The distribution of scores of depth d on the sea floor is given by:
e- kd
(Lewis 1977b)
Thus the probability of a score d metres deep occurring in a water depth of D is
given by:
681

-0/00 -kd
p(d) a Noe fe scores/year/km of depth d
This method uses current ice feature data, features which must be causing the curr­
ently observed scoring. It allows for extreme events provided they are from the
observed keel distribution (one can use submarine data for the area North of Dome's
well sites in the permanent pack ice and also up along the Banks Island coast which
show extreme features); here the keel distribution is from Wadhams (1975) laser
data to demonstrate the method as it is public. The model assumes that the distribu­
tion of score depths is constant with time. This is true if all scores fill-in
uniformly with time. It is conservative if the shallow scores fill in faster than
the deep scores and non-conservative if the deep scores fill in faster; in fact, the
assumption is probably conservative. The factor f is not known, but can be bracket­
ed. The method also allows for the different soil types and local bathymetry, as it
uses measured score depth distributions.
Thus the TOP depth for a pipeline of length L and return period T is given by:
d = In (LT N f e-D/Do)/k
o
Assuming a pipeline of length 78 km, a value of No of 2300 keels/year derived from
Wadhams (1975), Do = 2.5 and Lewis' (1977b), k for different water depths, the TOP
depths as indicated in Figure 2 were obtained for 100 year return period.
Here we have utilized pack ice ridge statistics to cover the entire route from shore
to Kopanoar. This is acceptable at and beyond the shear zone; however, within the
landfast ice, which is relatively stationary throughout the winter, these statis­
tics over estimate the occurrence of scoring and hence TOP depths.
Ice Keel-Soil Interaction
Theoretical studies have been conducted (Kivisild et aI, 1981) to calculate the
forces involved in scoring, the available environmental forces and the integrity of
ice keels. The study shows that a narrow, deep multi-year keel being pushed by pack
ice under extreme storm conditions could cause very deep scores in soft seabed
soils.
The difficulty of using the method is correctly quantifying the driving force on the
ice keel which to date is the topic of extensive research, the probability of this
force occurring, and the probability of the ice keel running aground. Estimating
score return periods using this method would be difficult at this time.
683

TOP Depth Optimization
Optimum pipeline TOP depths should be based on minimizing the costs over the life of
the pipeline. The total cost of the pipeline is given by the sum of installation
and trenching costs plus the cost of a disruption times the probability of a disruption.
Thus:
Cost C = I + A(d + 0 )2 L + B p(d) LT
where: I is the cost of installing the pipeline of length L, A(d + 0 )2 is the cost
of trenching the pipeline, 0 m in diameter, d metres (TOP) into the seabed, B is the
cost of a disruption and p(d) is the probability of a disruption.
To minimize the total cost of the pipeline we set:
dC/dd = 0 2A (d + 0) L + B P (d) LT/dd
Using the expression for p(d) from the ice keel/score statistics method above we
get:
2A (d + 0) B k f N o
Where T is the lifetime of the pipeline in years.
-0/00 -kd
Te e
The results of evaluating this expression for A = $20,000 and B = $1.5 billion for
a pipeline route from Kopanoar to shore are presented in Figure 3.
DISCUSSION
This paper presents methods of calculating TOP depths for protecting subsea pipe­
lines in ice infested waters.
The SSZ and deepest score depth data are obtained by taking observations of the sea
floor. However, these data must be considered with reservations. It is felt that
setting the TOP just below the SSZ in shallow to moderate water depth (depths where
there is current scoring) is reasonable; but in deep water where no (or very little)
active scoring is taking place, the thickness of the SSZ is a relic of former times
and setting the TOP below this depth is far too conservative. A similar argument
can be made for the deepest score. In shallow and medium water depth scores are
filled in rapidly and the "deepest" score may be significantly reduced by infilling.
In deep water the deepest score may be relic and setting the TOP below this depth
would be too conservative. These two methods are felt to be useful for comparative
purposes only in deep water, but the SSZ-is a reasonable TOP depth in shallow water.
684

The score equilibrium analysis as proposed by Lewis is invalid if the sedimentation
rate is used for score fill-in. It predicts a deep TOP in deep water and probably
inadequate TOP depths in shallow water. However, if the score fill-in rate is
reduced to zero in deep water (i.e., uniform sedimentation only) then this method
can be made to give more reasonable TOP depths, but these values cannot be con­
sidered quantitative or useful for engineering design at present.
The repetitive mosaic work is extremely useful as this gives the only actual available
information on current scoring. However, the method must be conducted for
many years to generate adequate statistics.
Score dating cannot be used in medium water depths where scoring is frequent, but
may be useful for dating the deep scores in water depths of more than 40 m. This
could be invaluable to rationalize no burial in deep water as indicated by the ice
keel/score statistics method discussed here.
At present it is felt that the keel/score statistics method is the most reliable.
Recent results worked out by Dome utilize about 7 years of data (confidential) for
the keel statistics. Site specific score depth distributions were used thus incor­
porating the effects of soil type and local bathymetry.
It is interesting to note that the results in Figures 2 and 3 show some agreement
between the methods disscused. This, coupled with an understanding of the physics of
the particular methods, provides credibility for the TOP depths indicated. The big
discrepancy is the indicated TOP depth in deep water between the SSZ, deepest score
and score equilibrium methods and the ice keel/score statistics methods. However,
it has been rationalized here that the first three methods mentioned will greatly
overestimate the required TOP depth in deep water. Likewise, the ice keel/score
statistics method will overestimate the TOP depth in shallow water due to the data
used and the ice morphology in the Beaufort Sea. It is felt that the "SSZ" depth
should be used in shallow water and the ice keel/score statistics method in water
depths beyond 20 m.
One possible problem with ice score statistics methods of predicting the TOP depth
is that the score depth may not indicate the total depth of disturbance in the
seabed; however, this is not a problem with the SSZ method. This problem will be
the topic of future research.
In summary we feel that significant advances have been made in score research over
the past few years, and statistical methods are being developed which will allow for
sound engineering and safe design.
685

LIST OF REFERENCES
Arctic Petroleum Operator's Association (APOA) 1119, "Analysis of Records Showing Sea
Bottom Scouring", prepared for Gulf Oil Canada Ltd.
APOA 1132, "Analyze 1971 Scour Records And Run 1972 Program", prepared for Gulf
Canada Ltd.
APOA 11122, "Investigation Of Sea-Bed Scouring In The Beaufort Sea Phase IU", pre­
pared for Gulf Oil Canada Ltd., by Maclaren Atlantic Ltd., December 1977.
APOA 11151, "Ice Scour Mosaic Study", prepared for Gulf Oil Canada Ltd., by J. Shearer,
May 1979.
APOA 11158, "Ice Scour Mosaic Study", prepared for Gulf Oil Canada Ltd., by J. Shearer,
May 1980.
Barnes, P. and Reimnitz, E., "Ice Gouge Obliteration And Sediment Redistribution
Event' 1977-1978 Beaufort Sea, Alaska", Report 79-848, U.S. Dept. Of The Interior
Geological Survey, Menlo Park, California 1979.
Blasco, S., 1980, Private Communication.
Croasdale, K.R. and Marcellus, R.W., "Ice And Wave Action On Artificial Islands In
The Beaufort Sea", The Canadian Journal Of Civil Engineering, Vol. 5, pp 98-113,
March 1978.
Dome (1975), "The Dating Of Ice Scours In The Beaufort Sea", Kenting.
Kivisild, H.R., Pilkington, G.R. and Iyer, S.H., "An Analytical Study Of Ice Scour
On The Sea Bottom", ASHE Conference, Houston, January 1981.
Kovacs, A. and Mellor, M., "Sea Ice Morphology And Ice As A Geologic Agent In the
Southern Beaufort Sea". CRREL, 1974, AINA Symposium On The Beaufort Sea Coastal And
Shelf Research, San Francisco.
Lewis, C.F .M., "The Frequency And Magnitude Of Drift-Ice Groundings From Ice-Scour
Tracks In The Canadian Beaufort Sea", POAC 1977a.
Lewis, C.F.M., "Bottom Scour By Sea Ice In The Southern Beaufort Sea", Dept. Of
Fisheries And The Environment, Beaufort Sea Technical Report 23 (draft), Beaufort
Sea Project, Victoria, B.C., 1977b.
686

Marcellus, R.W., and Palmer, A.C., "Shore Crossing Techniques For Offshore Pipelines
In Arctic Regions", POAC '79, Trondheim Norway, August 13-17, 1979.
Marcellus, R.W., "Preliminary Order-Df-Magnitude Cost Comparison Of Alternative
Pipeline Routes For Similar Pipelay And Trenching Methods From The Kopanoar Well­
site To Shore", Canada Marine Engineering Report, prepared for Dome Petroleum Limited,
June 1980, (proprietary).
Marcellus, R.W., "Ice Score Return Periods And Pipeline Trench Depths In The Canadian
Beaufort Sea", Canada Marine Engineering Report, prepared for Dome Petroleum Limited,
April 1981, (proprietary).
O'Conner, M.J. and Associates Ltd., "Development Of A Proposed Model To Account For
The Surficial Geology Of The Southern Beaufort Sea", March 30, 1980.
Wadhams, P., "Sea Ice Morphology In The Beaufort Sea", Beaufort Sea Project, Techni­
cal Report Number 36, December 1975.
Weeks, W.F., Barnes, P. W" Rearic, D., and Reimnitz, E., "Statistical Aspects Of
Ice Gouging On The Alaskan Shelf Of The Beaufort Sea", (draft), CRREL, New Hampshire,
1980.
ACKNOWLEDGEMENTS
The authors are very grateful to Bruce Dunwoody for his contribution to the develop­
ment of the optimization method discussed above. The authors would also like to
acknowledge Dome Petroleum for supporting the work upon which this paper is based
and Beth Weber for the patience she has shown in typing this paper.
687

IoKlDEL TESTS OF
SEA BOTTOM SCOURING
R. Abde1nour, Vice President Arctec Canada Limited Canada
D. Lapp, Project Engineer Arctec Canada Limited
S. Haider, Senior Marine Geotechnical Engineer, Petro Canada
S.B. Shinde, Senior Research Engineer Esso Resources Canada Limited
B. Wright, Coordinator, Frontier Development, Gulf Canada
Canada
Canada
Canada
Canada
ABSTRACT
A series of model scale tests of ice sea bottom scouring were conducted as part
of the Arctic Petroleum Operator' s Assoc ia t ion project to acquire experimenta 1 data on:
°scouring resistance forces
°pressure distribution in the soil
°pressure distribution on the m0de1 front face
°shape and characteristics of the scour profile.
The test variables included two model scales of 1 :25 and 1 :50, two model shapes
an inverted pyramid and a prism; three soil materials, sand, sandy silt and silty
clay. Each of these parameters were tested at three cut depths in a levelled soil
bottom with three towing velocities.
1 • INTRODUCTION
The objectives of the test program were:
°To determine the resistance force required for an ice mass to scour under
various conditions of shape of the ice mass, soil type, cut depth and
towing velocity.
°To determine the pressure distribution measured on the ice model front face
as well as within the surrounding soil in both horizontal and vertical dimensions
related to the model.
°To observe the behaviour of the soil during the scouring procedure and determine
the scour profile characteristics behind the ice mass relative to its
shape and cut depth.
°To correlate the experimental results obtained with available published work
including model and full scale tests.
688

The test program provided significant information that covered most of the
above objectives.
The ice scouring tests were conducted at the ARCTEC CANADA LIMITED hydraulic
basin in Kanata, which is 18.3 m long, 6.1 m wide and has a depth of 1 meter. The
tests consisted of towing two model scale ice feature shapes "keels" constructed
from steel plates and instrumented with force and pressure sensors, through three
sea bottom conditions.
The program was divided into three phases, each phase relating to a different
sea bottom soil type. Each phase included four to seven test days in which the
following variables were investigated:
°mode1 shape an inverted pyramid and rectangular prismatic shape
°mode1 scale model scales of A = 50 and A = 25
°cutting depth three cutting depths
°towing velocity: three velocities.
Table 1 is a summary of the complete test program which describes the number of
runs performed in each soil and the model shape used.
TABLE 1 - SUMMARY OF TEST PROGRAM
Vt1/bl SOil
nST
•
NO. OF Rutl$
/lK)OEl LJAn Of TESTING
1979 PER TEST' PER SOil
I Sand 1 Pyramid 25, 26 JUlie 9
Safld Pyr'amld 28 June 9
Sand 3 large Prismatic 3, 4, 5 July 6
Sand 4 Small Pnsmatlc 6 July 9
II Silt 5 Pyram1d 30 July 9
Slit 6 Small Prismatic 21 August 9
13. 18 Sept.
SIlt 7 large Prismatic 16. 21 Sept. 1
Silt 8 Pyramid 24 Sept. 4
--
III Clay 9 Pyramid 29 October 9
Cldy 10 Pyramid 31 October 0'
Clay 11 Snlll11 PrlSmatic 5 Noyen'ber 9
Chy 12 Small Prismatic 7 Noventler 9
Clay 13 large PriSAlatlc 9 Noventler 9
Clay 14 large PrlSmatic 13 Novenber 9
Clay 15 Pyramid·-Front
face only 15 NO'i'eniler 9
'I:ddl tests consisted of three mdentor cut depths and three velocities.
Numilt!fS shown In lhis colwm are the ones Iiitlere data was obtained with
tH.,02lJtahlt! accuri\Cv.
'Nll1t: runs were conducted but results dre suspected due to improper preparation
uf the c1dY.
The combination of these parameters would have resulted in 144 runs. However,
some of the problems encountered reduced the test runs to 110. It was clearly not
possible to investigate all possible "keel" shapes for economic reasons, and so the
two shapes were chosen to represent extremes within which all other shape configurations
were assumed to lie.
33
23
54
689

The indentor models were rigidly fixed to the carriage allowing only one degree
of freedom. This implies that the ice mass being modelled is large relative to the
scour force and that there is therefore no movement generated by the scour in other
directions.
Three discrete soil depths were chosen rather than a continuous slope for ease
of soil preparation and interpretation of results. Three artificially prepared
soils were used; the first was sand, the second was sandy silt (termed silt in
the paper) and third was silty clay (termed clay in the paper). All soils
were obtained from the Ottawa area.
In order to fulfill the experiment objectives, the program was designed and
executed to concentrate on two specific activities:
1) The study of the magnitude and extent of horizontal and vertical stresses
induced by a moving ice feature including:
(A) Indentor Model - horizontal and vertical forces on model faces and total
forces of the whole model; - compressive pressure on front face at various
locations.
(B) Soil - lateral extent of total stresses beyond the scour track; - lateral
extent of porewater pressure changes in the soil beyond the scour track;
- the zone of pressure influence in the soil; - extent and magnitude of
axial total soil pressure within the scouring dimensions.
2) Visual documentation of the interaction between the indentor and the soil
including:
o failure and shearing patterns
o surcharge behaviour and dispersal
o eddy effects on soil
o apparent and original scour depths
o deposition of material along scour tracks.
2. EXPERIMENT PREPARATIONS
The preparation for the execution of the experiments consisted of the following:
o design and construction of the ice scouring models (indentors)
o design of the pressure transducers and piezometers
o overall instrumentation
o soils type selection and preparation procedures.
2.1 Design Construction and Instrumentation of Ice Scouring Models
Two model shapes were selected for the experiments, one an inverted pyramid and
the other a rectangular prism. For the two scales, two models were required for the
690

2.1.2 RectanguU7r FTismatic Models
Two models were built to simulate a prismatic ice feature at 1:25 and 1 :50
scales. The models differed in width and length by a factor of 2. The shape contrasts
with the pyramid in that the sides are vertical. The basic construction of
the prismatic model was similar to the pyramid except that there were three detached
faces versus two. These three detached faces were connected to the rigid portion
of the model through force blocks. The bottom face had a force block measuring in
the vertical direction (2 axis). The force blocks for the side and front faces
measured horizontal forces perpendicular to and parallel to the direction of travel­
Y and X axis - respectively. A diagram of this arrangement is shown in Figure 3.
Force blocks also measured the forces on the whole model in the X and Z directions.
FIGURE 3 - SECTIONAL VIEW OF SMALL PRISMATIC MODEL
JI- l PLANE
L, CARRIAGE
fORCE BLOCK ALLOCATION
}-
LOCATIONS OF PRESSURE TRANSDUCERS ON FRONT FACE
OF SMALL PRISMATIC MODEL
fRONT fACE
693

Pressures on the front face of each model were measured in a similar manner to
the pyramid model. However, the transducers were located in different positions as
Figure 3 shows. The larger area of the front face permitted a transducer to be fitted
at the corner of the model so that variation in pressure on the face could be
measured in two different planes, if desired.
2.2 Soil Description and Basin Preparation
Three soils were used over the course of the test program. Each was selected
on the basis of grain size analysis to simulate an ideal sand, silt and clay. Figure
4 shows the grain size distribution of the three soils finally selected. The
three materials were classified as fine sand, sandy silt and silty clay.
Each soil was monitored and its properties logged. The following soil properties
were determined over the course of the test program depending on soil type:
°grain size - sieve and hydrometer (clay only)
°bul k dens ity
°water content
°dry density
°angle of internal friction (for sand and silt only)
°Atteberg limits lfor sand and silt only)
°shear strength by direct shear method, triaxial test, Swedish fall cone and
cone penetrometer.
FIGURE 4 - GRAIN SIZE DISTRIBUTIONS
OF SOIL MATERIALS USED DURING PRESENT PROGRAM
694
,10 .. IV w.oeo.... 100 'III"IN"U 1 •• 1
II I I I I II I I

Before testing commenced, the test basin was prepared so that some control
over the properties of the soils could be achieved.
This primarily consisted of constructing a filter for the soil to control its
drainage characteristics. First, a bed of 25 mm stone was laid on the floor of the
basin to a depth of approximately 0.17 meters. At each end of the basin a slot was
left to act as a collecting area for the draining water. Water draining at either
end of the basin was subsequently recirculated into the basin using a pump. Second,
a mat of filter cloth was laid on top of the stone to screen out the soil but permit
water to pass through into the stone.
The soil depths in the basin were chosen to be approximately 30 cm deeper than
the cut depth. The soil level depth was believed to be enough to dissipate any
effect the floor of the basin might have had on the forces experienced by the models.
The soil was laid in three stepped depths, each constituting a run, matching the
planned scouring depths for each model.
3. TEST EXECUTION
3.1 Testing Procedures
The testing program consisted of towing each of the three models in each of the
three phases represented by the three soil types. In each phase a model was towed
through three thicknesses of soil over three tracks, each of which represented a
different towing velocity . Figure 5 shows the prismatic model being tested in the
basin and Figure 6 shows the testing details in the basin.
FIGURE 5 - THE LARGE PRISMATIC MODEL DURING TESTING
695

FIGURE 7 - FLOW CHART OF TEST PROCEDURES
No
697

In addition to these lateral horizontal pressures perpendicular to the track,
a single pressure cell was installed at the end of Run 3 within the cutting depth
located on the centerline of the model along the axis of movement. The cell was
placed so that it was just ahead of the point where the final test run was completed.
After the installation of the model and the soil the instrumentation was
completed, a check was made to see that all force blocks, pressure cells and
piezometers were operational. Following this, the test was executed at the specified
velocity.
After the run was completed, the model was removed and reinstalled into the
next test position. Soil instrumentation was removed and placed into the appropriate
positions for the next run. This procedure was repeated until all three
speeds had been run.
The scours left by the three runs were then profiled and photographed.
The soil was then reworked after all instrumentation in the soil was removed.
The model just tested was disassembled and the force blocks and pressure cells were
installed in the next model. The procedure just described was repeated for the new
model with the reworked soil.
After all the models had been tested, the soil was removed from the basin and
a new soil was placed.
3.2 Parameters Measured
During and after test execution the following parameters were measured:
°scour trench depths
°resistance forces
°pressure measurements
°scour profiles.
For scour trench depth, the original scour depth was recorded and compared to
the apparent past test depth. Resistance forces were measured in both horizontal
and vertical directions including forces on the front face, bottom (for prismatic
models) and one of the two sides. In addition, total along track horizontal and
vertical forces on the model were recorded. The output of the resistance traces
from the force blocks were recorded in analogue form on a 28 channel recorder and
displayed on the oscillograph recorder at the same time. The peak average force
for each run was reported. Figure 8 shows an example of the force versus time and
penetrations.
Pressure measurements were made using transducers to record pressures on the
front face of each model at varying locations depending upon the model size and
shape.
698

The friction factor between the model and the soil was not varied during these
experiments, therefore its effect on the measured force was not investigated.
The relationship between the previous seven parameters is given by:
where:
f (q"
F Inertial Force
yL 3 Gravitational Force
q,
CiyL =
Frictional Force
Gravity Force
Cohesion Force
Gravity Force
V2 External Force
gL Gravity Force
C
yL
Dimensionless Force
Dimensionless Friction
Dimensionless Cohesion
Dimensionless Velocity
This equation was used to obtain dimensional and nondimensional relationships
for each group of results obtained for geometrically similar models tested in the
same soil type. Therefore, the analysis of the data was done separately for each
of the soil types and each of the two model geometries tested, pyramid and prismatic
models.
4.3 Conclusions
Based on the results and their analysis the following have been obtained:
°Empirical relationships describing the resistance force versus the soil
properties and the speed of the indentor for all three soils tested. By
comparison with other published work in this field, the results showed some
agreement for the part of the work where similar studies have been undertaken.
°The pressure data of the gauges installed at the indentor force, the pressure
distribution and the average pressure at the front face were calculated and
cOl'related to the total resistance measured at the same face.
°The data of the soil Dressure gauges and the piezometers within the soil.
Despite the small number of gauges used, the zero pressure distribution
line, relative to the model location was abstracted.
701

7. REFERENCES AND BIBLIOGRAPHY
[1] Brooks, L.D., "Another Hypothesis about Iceberg Draft", POAC 79, Proceeding
Volume 1, Page 24=152, Trondheim, Norway, August 1973.
[2] Schuring, D.J. and Emori, R.I., "Soil Deforming Processes and Dimensional
Analysis", Society of Automotive Engineers Report 897C, September 1964.
[3] O'Callaghan, J.R., McCullun, P.J., "Soil Mechanics in Relation to Earth
Moving Machinery", Proc. Inst. Mech. Engineers 1964-65.
[4] Kovacs, A. and Mellors, M., "Sea Ice Morphology and Ice as a Geologic Agent
in the Southern Beaufort Sea", in the Coast and Shelf of the Beaufort Sea,
Proceedings of a Symposium on Beaufort Sea Coast and Shelf Research, December
1974.
[5] Chari, T. and Allen, J.H., "An Analytical Model and Laboratory Tests on Iceberg
Sediment Interaction", IEEE International Conference on Engineering in the
Ocean Environment, Volume I, August 1974.
[6] Abdelnour, R. and Lapp, D., "Model Tests of Sea Bottom Ice Scouring", APOA
Report 151, ARCTEC CANADA Final Report 356C-3, November 1980.
705

Reidar Lien
Geological Enqineer
SEA BED FEATURES IN THE BLAAENGA AREA,
WEDDELL SEA, ANTARCTICA
Continental Shelf
Institute
Norway
ABSTRACT
Data for this contribution were gathered during two expeditions in the summer
seasons 1976/77 and 197B/79, and consist of records with echo sounder and side-scan
sonar. From these data we have constructed a tentative bathymetric map of the area,
and the sea floor has been classified into four groups of sea bed features.
Two of these are well known and widely described in the literature: plough marks
from grounded icebergs, and conventional undisturbed sea bed. The other two, in
spite of a comprehensive study, are not found in the literature. These features
consist of a washboard pattern, and a hummocky, mosaiclike, sea bed pattern.
The different features are described and shown on record sections. Further some
record sections with special phenomena such as tracks of wobbling icebergs,
arresting icebergs, multi-keeled icebergs etc. are shown. Finally the different
patterns and phenomena will be discussed with reference to their process of
formation.
INTRODUCTION
In 1976/77 and 1978/79 the Continental Shelf Institute participated in expeditions
administrated by the Norwegian Polar Institute to the Weddell sea. Data for the
following contribution were gathered during these expeditions. These data consist
of registrations with echo sounder and side-scan sonar from the Blaaenga area on
the Dronning Maud Land coast. The area with side-scan sonar profiles and tentative
bathymetry is shown in Fig. 1. In 1977 this area was covered by the ice shelf, but
in 1978 the shelf had calved and the area was free of ice.
706

LEGEND:"-:-- Side-scan sonar profile lines with dots each 10 min.
'-_ _ _ _ FIg. - Location of profile section shown in text
uf
300
15°3(1
Fig. 1 Approximate bathymetric map with location of side-scan sonograms and
figure sections.
SEA BED FEATURES
In the area four groups of sea bed features have been registered :
a) Washboard pattern with lateral ridges
b) Unsystematic furrows
c) Hummocky sea bed
d) Undisturbed sea bed.
Washboard pattern with lateral ridges
An example of this feature is shown in Fig_ 2. The straight parallel stripes have
different spacing and represent ridges on the sea floor. Most of the stripes seem
quite persistent. The washboard pattern represent roughness on the sea floor, in
the form of undulations. The spacing between the grooves, and the grooves themselves
may be of different sizes. The grooves form different patterns because of
their geometrical form, but normally they are parallel for long ranges.
707

Fig. 4 Hummocky sea bed.
DISTRIBUTION OF THE SEA BED FEATURES
The different sea bed features are mapped along the profiles in Fig. 5. On the
shallowest bank area the records are dominated by unsystematic furrows. The majority
of these furrows disappear below 300-320 m, and the deepest registered was found at
a depth of about 380 m below sea level.
The hummocky sea bed feature is found mainly in the lee side slope of the bank in
relation to the direction of movement of the shelf ice. It was also found in a
trough near the southern border of the investigated area. The features are found in
depths between about 290 and 350 m.
The washboard pattern with lateral ridges has been recorded from the south- eastern
slope of the bank. This pattern was recorded to a waterdepth of 400 m, but may
extend deeper since there are no registrations from greater water depths. The upper
limit of the feature is diffuse because there is a gradual transition to a sea
floor dominated by the unsystematic furrows as the water becomes shallower. The
furrows seem to be superimposed on the washbord pattern. This latter pattern has
been registered up to about 280 m before the unsystematic furrows completely seem
to extinguish them.
Undisturbed even sea bed is registered only along one profile in the lower part of
the western slope of the central bank of the area. The water depth for this pattern
varies between 330 and 380 m along the profile.
709

Another possibility is that no ridges were originally formed. The bathymetric map
of the area is not very reliable, so if the mentioned ridge is sharp, it is possible
that the icebergs have ploughed through it and transported the material in front of
the iceberg all through the ridge.
Fig. 6 Plough marks lacking the rims.
However, this theory is more doubtful since we can follow large plough marks without
lateral ridges for several hundred meters, and that would probably have given an
excessive accumulation of material in the front of the icebergs.
Some places the plough marks are flat and shallow compared with its width (Fig. 7).
The reason for this may be that possible earlier peaks and roughness have shedded
during the grounding until the iceberg has got a shape that will be stable to
further influences. The explanation may also be that the bottom of the iceberg
represent the original bottom of the ice shelf.
Forming of the washboard pattern with lateral ridges
The washboard pattern is underlying the plough marks and must therefore be older. In
the literature there is not any description of this pattern. However, similar
patterns formed by single icebergs north of Alaska (1) have been described. Corresponding
impressions are also recorded in this area (Fig. 7). Reimnitz et al. (1)
explain the formation of this pattern by wobbling of the iceberg after its grounding.
The impressions in Fig. 7 may also be explained in that way, and the spacing
between the crossing ridges in the plough marks is due to the icebergs' stability
and their drifting velocity. Further the scour may be directed by the topography
like conventional plough marks.
711

Fig. 7 Plough marks from wobbling icebergs.
The washboard pattern with lateral ridges, however, seems to be quite unaffected by
the topography. In Fig. 8 a sketch of the records from the three close spacing
profiles in the south-eastern corner of the investigated area is shown (Fig. 1).
This sketch is a line drawing from the profiles, and shows the main features from
the records. The records nearly overlap. In this sketch we can, without too many
assumptions, follow the parallel stripes for several hundred of meters without
notable curvature. The elevation along the profiles is about 20 m.
The direction of the parallel stripes seems to be the same on all profiles where
they are registered, and the direction is at right angles to the barrier front. The
pattern of crossing ridges and grooves between the parallel stripes (the washboard
pattern) also seems to be persistent, and each groove is similar to the foregoing.
The change in this pattern is gradual and often takes place during several hundred
meters, or there is no change at all within one profile.
The formation of this pattern is supposed to be similar to that described by
Reimnitz et al. (1), but here the forces that cause the drift, and the drifting ice
itself must have been of another magnitude.
712

Fig. 8 Sketch of approximate mosaic.
The ploughing ice might have been icebergs bounded by mighty sea ice or pressed
together to a firmer body, but not closer than that the icebergs could have minor
individual movements . The drifting forces may be wind and current acting on the sea
ice and the icebergs, they may also be represented by the shelf ice acting on the
ice mass .
acting
force
1
- II '
A
i \
: \ ICEBERG
! \
.I I
i J
i I
:--_J ________ _
\
equilibrium
water surface
-
- ' -
Fig. 9 Suggested formation of the washboard pattern with lateral ridges; A) the
mechanism after grounding, B) resulting pattern.
The pattern is thought to be formed by the pushing of the ice mass when the bottom
of it is grounding on the sea floor (Fig. 9a). As the forces are tremendous, the
ice mass is always pushed forward with its upper part. A point in the lower corner
of the grounded iceberg will be the rotation center. As this motion goes on, the
iceberg will be more and more unstable and the stress on the sea floor contact will
8
713

increase. When the action between the ice and seafloor is equal to the strength of
the ice or sea bottom, or the friction between these, a break will occur at
the contact, and the iceberg will move toward equilibrium. This jump may be
initiated by tidal movements, but this is not supposed to be necessary. At each such
jump there will be a ridge that delineate the front of the iceberg. In that way the
grooves and ridges may be of different shapes, dependent on the shape of the iceberg's
front. In Fig. 10 is shown a front of an arrested iceberg that would have
formed an edged pattern, while the plough marks in Fig. 7 show smoother forms.
Fig. 10 Plough mark from an iceberg with irregular lower front.
The pattern may also have been formed by the back sides of the icebergs if the
icebergs are floating with an inclination larger than the gradient of the sea floor.
The gradient of the sea floor i s here about 1:150. Thi s supposed inclination may be
due to ram s of the iceberg or other irregularities. The mechanism itself is thought
to be similar to the foregoing description.
When a front of such icebergs is moved along there will be icebergs of different
sizes and stabilities in the ice mass, and the time when the break between sea
floor and iceberg occurs, will be individual . In that way the space between
the grooves may be different for each iceberg (Fig. 9b).
The stripes or parallel ridges in the direction of motion of the icemass are thought
to represent soil pressed up between the icebergs. If the position among the
714

icebergs themselves and their form is steady, these ridges will be persistent. The
small, less persistent, stripes may represent unevenesses of the icebergs' fronts
or bottom like those of the ploughmark shown in Fig. 10. These stripes may be more
local since the front or bottom of the iceberg that touch the sea floor may change
by shedding, thawing, freezing or tilting of the iceberg as it is moved up the
slope. These factors will also affect the geometry of the grooves.
Origin of the hummocky sea bed features
The hummocky sea bed features are generally found in lee side slopes. The features
some places resemble the plough marks with crossing ridges which are shown in Fig.
7, but closer investigations show no continuous ridges across the lines where the
pattern to a certain degree shows orientation. The crossing lines are broken and
reminds us of an unstructured mosaic feature. Reimnitz et al. (1). describe
comparable imprints from the shelf outside Alaska. These are supposed formed in
overconsolidated sediments at or near the surface by icebergs placing blocks of
sediments in a random manner along its track.
Since the feature concerned is generally recorded in slopes, there may also be an
element of slide in the explanation. The sea floor might have been disturbed by
icebergs, which have initiated slides or small sediment movements.
The hummocky sea floor may also be explained by more or less sliding in areas with
rich accumulation. These areas in the lee side slope of the bank are likely to be
such areas. It is supposed that the shelf ice has been grounded on the bank before
1979 and the areas lie in the distal side of a moraine accumulation that forms the
top of the bank (Haisey, G.H, pers.com. 1980). The pattern then might have been
formed during a rich supply of sediments by random distribution of the material and
more or less local slides in the slopes.
Undisturbed sea floor
Along one profile the foregoing type of sea floor gradually changes to rather
undisturbed soils. This profile is crossing the trough west of the central height
of the bank, and the undisturbed sea floor is situated in water depths from about
330 to 380 m. Why this area is apparently undisturbed by icebergs, may be because
the water depths or locations have not been favourable. But this seems strange
since iceberg plough marks are neighbouring this area to the west on corresponding
depths. The transition between these features seems to be in the bottom of the
trough of about 380 m water depth. Since plough marks have been formed in the western
slope of the trough, plough marks have in all likelihood also been formed in the
715

eastern slope of the trough. These marks might, however, have been blurred by fines
from the depositing to the east, further up the slope. This sedimentation of fines
is most likely concentrated from its place of release, downslope to the bottom of
the trough, and gradually decrease upward the opposite slope.
Age of the features
The age of the plough marks is difficult to deduce, but from the registrations they
seem recent, especially in the shallowest areas. However, the sedimentation of soils
in the area generally seems to be sparse (Maisey, G.H. pers. com. 1980), so they may
be quite old though they look fresh.
Further, the ice shelf's natural depth in the area is measured by radio echo sounder
and side-scan sonar (2). These measurements revealed a depth of the ice shelf of
about 200 m below sea level. This may imply that few of the iceberg plough marks are
made by local icebergs in recent time, since the shallowest height in the area is
about 240 m. In the south-western corner of the area, where the water depths are
less than 160 m, the ice shelf was grounded on the sea floor in 1979. Accordingly
the plough marks, if they are recent, are presumably made by icebergs drifted from
regions with thicker shelf ice, or remnants of icebergs which have got a greater
draught than the ice shelf. If the plough marks are ancient, however, they may also
be originated in times with lower sea levels.
As to the washboard pattern with lateral ridges, this is clearly formed before the
plough marks, since these gradually blot out the pattern in water depths from about
300 m and shallower. Further this pattern is probably formed during lower sea levels,
because the pattern is registered at water depths of at least 400 m referred to the
sea level of today. And masses of icebergs with draughts of more than 400 mare
rather improbable.
LITERATURE
(1) REIMNITZ, E., BARNES, P.W. and ALPHA, T.R., 1973: Bottom features and processes
related to drifting ice on the Arctic shelf, Alaska. Department of the interior
United States Geological Survey.
(2) FOSSUM, B.A. and KLEPSVIK, J.O.: Studies of icebergs and iceshelf using sidescanning
sonar. In prep.
716

is required for tests inside bore holes. In site-specific foundation studies, static
penetrstion tests appear to be extremely relevant and useful.
With a view to develop a quick and economical way of testing surficial seafloor
soils, a free fall penetrometer with 4S cm 2 nominal cross section, a 60· cone and a
62S cm 2 friction sleeve was developed at Memorial University. A description of the
penetrometer and results from its sea trials are reported by Chari et al (1978, 1979)
As a part of the ongoing research on penetrometers, quasi-static and free fall tests
were conducted in the laboratory to facilitate an interpretation of the data from the
marine penetrometer. This paper relates to the quasi-static tests with the standard
10 cm 2 "Fugro" type penetrometer and the 4S cm 2 Memorial University Penetrometer.
A study of the two penetrometers in the free fall mode has been reported by Chaudhuri
(1979).
EXPERIMENTAL ASSEMBLY
The layout of the experimental facility is shown in Fig. 1. A detailed descrip­
tion of the assembly is given by Abdel-Gawad (1979). The penetrometer was activated
718
FIG 1: THE EXPERIMENTAL FACILITY

through a hydraulic system and was set to move at 20 _/sec in accordance with ASTK
recommendations. The output from the penetrometer was continuously recorded on an
analog recorder. The soil target was prepared in a steel bin which was a 1 m cube
open at the top. Silica sand and modelling clay were used as the representative sam­
ples of cohesionless and cohesive soils. Table 1 gives the physical properties of
the soils used.
Comparison was made of the experimental results and with the different theoret­
ical analyses available in the literature. Fig. 2 shows a summary of the different
soil failure mechanisms suggested in the literature. The theories of Meyerhof
(1961), Nawatzki and Karafiath (1978) and Durgunoglu and Mitchell (1975) were used in
this analysis.
De Beer (1945)
Meyerhof (1951,1961)
TEST RESULTS ON SAND
Terzoghi (1943)
Nowotzki and Korofiath
(1972, 1978)
Q
FIG. 2: DIFFERENT MODES OF SOIL FAILURE
Q
Biarez et 01 (1961)
Hu (1965)
Durgunoglu and Mitchell
(1973. 1975)
Typical experimental results for dry silica-70 sand are presented in Fig. 3.
Similar results were obtained for the different types of soil. Theoretical values
using the peak friction angle from triaxial tests are also presented on the same
figure. The analysis of these tests leads to the following conclusion:
719

Theoretical values of Nowatzki and Karafiath are found to be consistently lower
than the experimental values. The difference increases with increasing depths of
penetration. This theory is the most conservative of all.
Values obtained from the theory of Durgunoglu and Mitchell are the nearest to the
the experimental values. This is true for the various soil types and penetrometer
variables. These values are intermediate between the values obtained by the other two
theoretical methods. This can be explained with reference to the failure mechanism
shown in Fig. 2. It can be seen from the figure that the failure mode suggested by
Durgunoglu and Mitchell is an intermediate case between Meyerhof' s failure shape and
that of Nowatzki and Karafiath.
TESTS ON CLAY
To choose an appropriate theoretical analysis for cohesive soils, results of the
penetration tests performed on the clay target were compared with the theoretical
values of Meyerhof (1961) and Durgunoglu and Mitchell (1975). The numerical technique
suggested by Nowatzki and Karafiath (1978) is not applicable for cohesive soils.
Typical results of penetration tests in clay are presented in Fig. 4.
Theoretical values obtained by Meyerhof' s theory and that of Durgunoglu and Mitchell
are also shown on this figure.
It may be seen from the data presented that Meyerhof' s theoretical values are
greater than the experimental values for relative depths greater than 4 and less than
the experimental values for D/B less than 4. A similar phenomenon was observed and
explained previously in the case of cohesionless soils. The values of Durgunoglu and
Mitchell were found to be in better agreement with the experimental values for stiff
clay and medium stiff clay. But for soft clay the agreement is not good. The theo­
retical values was found to be higher than the experimental values in soft clay
indicating the effect of soil compressibility on the static penetration resistance.
The ratios of predicted to measured penetration resistance are presented in
table 2. It may be seen that these ratios are always greater than unity for
Meyerhof's theory. Comparison with the theoretical values of Durgunoglu and Mitchell
gives ratios which are close to unity for a dense deposit, indicating the validity of
this method for general shear conditions. However, for low densities, these ratios
are larger than one, indicating the significant influence of soil compressibility on
penetration resistance. The use of bearing capacity factors formulated for general
shear conditions will cause overestimation of the penetration resistance of
compressible deposits.
721

In compressible soils, the shesr surface is restricted to s smaller zone around
the penetrometer tip as suggested by Vesic (1963) for punching or local shear
failures.
CONCLUSIONS
Among the various theoretical bearing capacity solutions available for the anal­
ysis of the penetrometer problem, only those of Meyerhof (1961), Nowatzik and
Karafiath (1978) and Durgunoglu and Mitchell (1975) take into account the penetrometer
roughness, base apex angle and relative depth all of which are factors influencing the
penetration resistance.
As was seen in this analysis, Meyerhof' s method overestimates the penetration
resistance in the deep foundation zone and is conservative in the shallow foundation
region, while the method of Nawatzki and Karafiath is always conservative. The agree­
ment between measured and predicted values using the theory of Durgunoglu and Mitchell
for cohesionless and cohesion soils was reasonably good. This method can be used for
predicting the static penetration resistance of relatively imcompressible soils.
ACKNOWLEDGEMENTS
The authors wish to thank Professor C.D. diCenzo, Dean of Engineering, for his
constant encouragement and support of the research project. Thanks are also due to
Dr. G.R. Peters, Associate Dean and Group Leader of Ocean Engineering for his con­
structive comments at various stages of the project. The assistance of Professor W.G.
Smith in the development of the hardware is acknowledged with gratitude. Funding for
the project is provided by a grant A-3710 from the Natural Sciences and Engineering
Research Council Canada.
NOMENCLATURE
B: Width of foundation; diameter of penetrometer
c: Soil cohesion
D: Depth of foundation; depth of penetration
I: Density Index (Relative density of sand)
qf: Unit penetration resistance kPa
w: Water content (moistrue content) %
723

a: Semi Apex angle of cone
Y dry : Dry density of soil
6: Soil/foundation or soi1/pentrometer friction
+: Angle of soil shear resistance
REFERENCES
Abde1-Gawad, S.M. (1979) "Static Penetration Resistance of Soils", M.Eng. Thesis,
Memorial University of Newfoundland, St. John's, Nf1d., 225 p.
Biarez, J., Burel, M. and Wack, B. (1961), "Contribution a l'etude de 1a force
portante des foundations", Proceedings of the 5th International Conference on Soil
Mechanics and Foundation Engineering, Paris, pp. 603-609.
Chari, T.R., Smith, W.G. and Zielinski, A. (1978), "Use of Free Fall Penetrometer in
Sea Floor Engineering", Conference Record, Ocean 78, IEEE-MTS Conference, pp. 686-
691.
Chari, T.R., Muthukrishnaiah, K. and Zielinski, A. (1979), "Performance Evaluation of
a Free Fall Penetrometer", First Canadian Conference on Marine Geotechnical
Engineering, Calgary, Alberta, April 1979, pp. 203-210.
Chaudhuri, S.N. (1979), "Free Fall Impact Penetration Tests on Soils", M.Eng. Thesis,
Memorial University of Newfoundland, St. John's, Nf1d., 134 p.
De Beer, E.E. (1945), "Etudes des Fonditions sur Pilotis et des Fonditions Directes",
Anna1es Des Travaux Publics de Belgique 46, pp. 1-78.
De Ruiter, J. (1975), "The Use of In-Situ Testing for North Sea Soil Studies",
Preprints, Offshore Europe 75, Aberdeen, pp. 219.1-219.10.
Durgunog1u, H.T. and Mitchell, J.K. (1973), "Static Penetration Resistance of Soils",
Research Report Prepared for NASA Headquarters, Washington, D.C., April 1973,
University of California, Berkeley.
Durgunog1u, H.T. and Mitchell, J.K. (1975), "Static Penetration Resistance of SOils,
I-Analysis, II-Evaluation", Proceedings of the Conference on In-Situ Measurement of
Soil Properties, ASCE, Vol. I, pp. 172-189.
ESOPT, (1974), European Symposium on Penetration Testing, Stockholm, June 1974, Vol.
I, State-Of-The-Art Report.
Ferguson, G.H., McClelland, B. and Bell, W.D. (1977), "Seafloor Cone Penetrometer for
the Deep Penetration Measurements of Ocean Sediment Strength", The 9th Offshore
Technology Conference, OTC 2787, Vol. I, pp. 471-478.
Hu, G.C. (1965), "Bearing Capacity of Foundation with Overburden Shear", Sols- SOils,
Vol. I, No. 13, June 1965, pp. 11-18.
Meyerhof, G.G. (1951), "The Ultimate Bearing Capacity of Foundations", Geotechnique,
Vol. 2, pp. 301-322.
724

Meyerhof, G.G. (1961), "The Ultimate Bearing Capacity of Wedge-Shaped Foundations",
Proceedings, 5th International Conference on Soil Mechanics and Foundation
Engineering, Vol. 2, pp. 105-109.
Nowatzki, E.A. and Karafiath, L.L. (1972), "The Effect of Cone Angle on Penetration
Resistance", Highway Research Board, No. 405, pp. 51-59.
Nowatzki, E.A. and Karafiath, L.L. (1978), "Soil Mechanics for Offroad Vehicle
ngineering", Series on Rock and Soil Mechanics, Vol. 2 (1974/77), No.5, Trans. Tech.
Publications.
Sanglerat, G. (1972), "The Penetrometer and Soil Exploration", Developments in
Geotechnical Engineering, Elsevier Publishing Company.
Terzaghi, K. (1943), "Theoretical Soil Mechanics", (Ninth Printing), John Wiley & Sons
Inc., New York.
Vesic, A.S. (1963), "Bearing Capacity of Deep Foundations in Sand", Highway Research
Board Record, No. 39, pp. 112-135.
Zuidberg, H.M. (1975), "The Sea Calf, a Submersible Cone Penetrometer Rig", Marine
Geotechnology, Vol. 1, No.1, pp. 15-32.
725

Hamdy Youssef
Roger Kuhlemeyer
ABSTRACl'
DYNAMIC AND STATIC CREEP TESTING
OF ICE AND FROZEN SOILS
Genie Civil. Ecole Polytechnique de Montreal
The University of Montreal. Quebec
Department of Civil Engineering
The University of Calgary. Alberta
Canada
Canada
The behavior of frozen ground under dynamic loading has received very little
attention to date. so that design information related to machine foundations in
cold regions still very scarce. The objective of this paper is to demonstrate
the importance of predicting the creep response of frozen ground (ice and frozen
soils) to dynamic loadings; to present the state of knowledge in this area and
to suggest a testing technique for investigation of the effect of the torsional
vibration on the static creep rate of ice and frozen soils. as well as determi­
ning their dynamic properties. this testing technique has been developed by the
authors at the University of Calgary. Alberta. Canada.
INTRODUCTION
Frozen grounds can be defined as that half space in which stresses and strains
arising due to the influence of an external load. are not constant; but vary with
time. giving rise to a relaxation of stresses and creep (an increase in the strain
with the passage of time). The main cause of the rheological processes in frozen
soils is particulary due to their internal bonds. in which ice. which is an ideal­
ly flowing solid plays a major role. (Tsytovich 1975).
According to Ladanyi (1974). a basic foundation design philosophy in permafrost
is based on predicting the delayed and long-term capacity of the foundation and its
time dependent settlement due to combined creep and consolidation. Predicting of
time-dependent bearing capacity of frozen soils supporting static loading founda­
tions is relatively easy. provided the creep and creep failure properties of the
soil are known. On the other hand as the prediction of the delayed settlement is
concerned. the frozen soil is considered as a quasi-single phase medium with ma-
726

thematically well defined creep properties, neglecting the fact that one portion of
the observed creep is actuallY due to vOlumetric strain (Ladanyi 1974, 1981).
In projects involving construction of machine foundations in cold regions, the
sub-and-super structures will be constructed first and then the machines (e.g. ge­
nerators and compressors) will be situated in place. These components will deter­
mine the magnitude of the static load which the frozen ground has to support over
a long term, usuallY the life time of the structure.
PREDICTION OF CREEP DEFORMATION DUE TO STATIC LOAD (STATIC CREEP)
Under this static load, instantaneously, elastic deformation (i.e. strain) will
occur, and with time continuous deformation (creep) will progress; firstlY at a de­
creasing rate (stage I - Fig. 1) then followed by a steady state creep (stage II -
Fig. 1). The long-term strength is represented by the end of this stage (point B -
Fig. 1), the load has to be designed so that creep failure (accelerating creep -
stage III) will not occur.
Ladanyi (1972) developed a unified engineering theory of time, temperature and
normal pressure dependent deformation and strength of frozen soils, which enabled
the creep and creep failure information to be expressed in a relativelY simple ana­
lYtical form, using a minimum number of experimental parameters. This theory has
been highly credited and widelY adopted in recent years by researchers and practi­
ce engineers for prediction the static creep of frozen ground. More recently, Li
and Andersland (1980) investigated experimently the effect of cyclic loadings on
the creep parameters in Ladanyi's theory and they found that the cyclic load acce­
lerate the static creep rate, this effects appear to be included in the creep para­
meter n and the proof stress 0c'
Ladanyi (1972) assumed that for long-term deformations, the frozen soil behavior
is generallY dominated by a secondary creep, and the creep curve could be correctlY
approximated by a straight line having the intercept E(i) at time t - 0, as shown in
Fig. 1. Therefore, for a constant stress and temperature the total strain E can be
predicted as:
where:
E = Young's modulus, ° = uniaxial normal stress, Ek = arbitrary small strain, Ok =
proof stress for E k , Ec = arbitrary creep rate, 0c = proof stress for Ec' and k,n =
creep parameters.
727

3. Response Prediction
The analysis of the response of frozen ground to dynamic loadings requires: (1)
determining appropriate material properties (which are available) and (2) selecting
a suitable analytical techniques to predict ground response to machine foundation
(Vinson 1978). As previously mentioned these analytical techniques do not exist as
yet, and the designer has to use the dynamic soil properties of the frozen ground as
an input to suitable analytical techniques developed for unfrozen soils with great
careto insure that the assumptions on which the techniques are based are not violated.
Due to the fast development of Northern Canada and Alaska; the joint U.S.-Canadian
committee in the Northern Civil Engineering Research workshop (Carlson et al. 1978)
stated that there is a need for research in: (1) basic studies on the interaction
between high-frequency dynamic foundation loads and frozen ground, and (2) reduction
in shear and adfreeze strength and possible acceleration of creep caused by vibratory
loadings.
The above discussion aimed to place emphasis on the necessity for analytical tech­
niques which should be developed specifically for predicting the response of frozen
ground to dynamic loadings and especially for the dynamic creep. These analytical
techniques should be based on and verified by experimental and field data, which are
lacking. Therefore, the first step to acheive this goal is the development of expe­
rimental apparatus to provide this needed information. The ideal apparatus should be
capable of permitting investigation the influence of the following parameters: (1)
temperature, (2) strain or deformation with time, (3) axial and confining stresses,
(4) frequency and amplitude for longitudinal and torsional vibration, (5) different
material type and composition, and (6) dynamic properties during different stages of
the test. The authors have developed an experimental apparatus at the University of
Calgary, Alberta, Canada to investigate most of the above mentioned parameters, a
breif description of Calgary apparatus is given below.
4. Calgary Apparatus
In connection with the design of the proposed Mackenzie Valley pipe line, this ap­
paratus has been developed to study in the laboratory the effect of vibration on the
static creep rate of ice and frozen soils as well as their dynamic properties.
The new (1976) Drnevich free-free torsional resonant column apparatus has been
used as a forced vibration device. This device has been modified at the University
of Calgary to permit applying vertical loads without influencing the free torsional
vibration characteristics of the sample apparatus system. The testing procedures
which are to be followed and which the apparatus is designed for is according to the
line of thought as described before and expressed in Fig. 1. Two vertical identical
730

Fig. 2. CALGARY APPARATUS
SUGGESTED TESTING TECHNIQUE FOR EXPERIMENTAL
INVESTIGATION OF STATIC AND DYNAMIC CREEP, AS WELL AS
THE DYNAMIC PROPERTIES OF ICE AND FROZEN SOILS .
cylindrical samples of ice or frozen soils are subjected to the same constant tempe­
rature and vertical static loading conditions. After time tsd (Fig. 1) the two spe­
cimens will strain to Esd. At this time a steady- state sinusoidal torsional vibra­
tion is superimposed to one specimen (in the left triaxial cell in Fig. 2) while the
other specimen is subjected to static loading only. The torsional vibration is ap­
plied to the bottom end of the dynamically tested specimen while the top end is free
(to r otate) except for a light , relatively rigid cap ; the input motion and output
sample r esonance are both observed and measured with piezoelectric t r ansducers atta­
ched to the cap and base plates of the specimen. The axial deformati ons of the two
specimens are recorded with time , which allow obtaining the static and dynamic creep
curves as shown in Fig . 1 ; for di ffer ent types of ice and frozen soil s . The dynamic
properties can be measured and evaluated during different stages of the test from
the resonant column theory (Drnevich 1976a) and by using Drnevich (1976b , 1978) reso-
731

ACKNOWLEDGEMENTS
The authors wish to acknowledge the work contribution by Mr. R. French, of Geo­
Phsi-Co. Ltd - Calgary, Alberta, during the initial development of Calgary Appara­
tus. Financial support from the University of Calgary and the National Research
Council of Canada is gratefully acknowledged. The authors are grateful to Prof.
B. Ladanyi of Ecole Polytechnique de Montreal for the help provided during prepara­
tion of this paper.
REFERENCES
(1) ANDERSLAND, 0., SAYLES, F. and LADANYI, B. (1978), "Mechanical Properties of
Frozen Ground", Chp. 5 in "Geotechnical Engineering :for Cold Regions", Mc
Graw-Hill Book Co., N.Y., U.S.A.
(2) BARKAN, D. (1962), "Dynamics of Bases and Foundations", McGraw-Hill Co., N.Y.
(3) CARLSON, R. and MORGENSTERN, N. (1978), "Northern Civil Engineering Research
Workshop Report", Editor, The Univ. of Alberta, Edmonton, March 20, 1918.
(4) DRNEVICH, V.P. (1976a), "Free-Free Resonant Column Apparatus Operating Manual",
Soil Dyn. Inst. Inc., Lexington, Kentucky, U.S.A.
(5) DRNEVICH, V.P. (1916b), "A user's Manual for the FORTRAN Computer Program En­
titled RESCOL4", Dept. of civil Engg., Univ. of Kentucky, U.S.A.
(6) DRNEVICH, V.P. (1978), "Resonant Column Test", Report No. S-78-6, Geotech. Lab.
U.S. Army WES, Vicksburg, Miss., U.S.A.
(7) IVANOV, P. L. (1980), "Vibrocreep and Loose soil strength under cyclic loading
Action", Int. Symp. on Soils Under Cyclic and Trans. Ldg., Swansea, U.K.
(8) LADANYI, B. (1972), "An Engineering Theory of Creep of Frozen SoilS", Canadian
Geot. J., 9, 63.
(9) LADANYI, B. (1914), "Bearing Capacity o:f Frozen Soils", Proc. 21th Canadian
Geot. Conf., Edmonton, Canada.
(10) LADANYI, B. (1981), "Mechanical Behavior of Frozen Soils", Proc. Int. Symp. on
the Mech. Behav. of Structured Media, Carleton Univ., Ottawa, Canada.
(11) LI, J. and ANDERSLAND, o. (1980), "Creep Behavior of Frozen Sands Under Cyclic
Loading Conditions", Proc. 2nd Int. Symp. on Ground Freezing, Trondheim,
Norway.
(12) PANDE, G. and SHARMA, K. (1980), "A Micro-Structural Model for Soils Under
Cyclic Loading", Proc. Int. Symp. on Soils Under Cyclic and Tran. Ldg.,
Swansea, England.
(13) SAYLES, F. (1974), "Triaxial Constant Strain Rate Tests and Triaxial Creep
Tests on Frozen ottawa Sand", U.S. Army CRREL, Hanover, N.H., U.S.A.
(14) SHETTY, D., MURA, and MESH II (1915), "Analysis of Creep Deformation Under Cyclic
loading Conditions", Material Science and Engg., 20-261-266, Elsevier Seq.,
Netherlands.
733

(15) STEVENS, H. (1975), "The Response of Frozen Soils to Vibratory Loads", U.S.
Army CRREL, Tech. Rep. 265, Hanover, N.H., U.S.A.
(16) TRIMBLE, J.R. (1977), "A Comparison of the Creep Deformation on Naturally
Frozen Soils Under Static and Repeated Loadings", M.Sc. Thesis, Queen's
Uni v., Canada.
(17) TSYTOVICH, N. (1975), "The Mechanics of Frozen Ground", McGraw-Hill Co., N.Y.
(18) TURCOTT-RIDS (1980), "Resonant Column Testing of Frozen Soils", M.Sc. Thesis,
McGill Univ., Montreal, Canada.
(19) VINSON, T. (1978), "Response of Frozen Ground to Dynamic Loading", Chap. 8,
in "Geotechnical Engg. for Cold Regions", McGraw-Hill Co., N.Y., U.S.A.
(20) YOUSSEF, Hamdy (1979), "Development of a Testing Apparatus for Static and
734
Dynamic Creep Testing of Ice and Frozen Soils", M.Sc. Thesis, The Univ.
of Calgary, Alberta, Canada.

William M. Sackinger
Associate Professor
Abstract
A Review of Technology for
Alaskan Offshore Petroleum Recovery
Geophysical Institute
University of Alaska
Fairbanks, Alaska 99701
USA
A comprehensive summary is presented of all of the environmental
hazards which relate to the design and deployment of petroleum produc­
tion systems for the Alaskan Beaufort, Chukchi, and Bering Seas.
Environmental factors which control the design choices are identified for
each area, and the most recently available values for the magnitude of
these hazards are discussed. Both structure designs and icebreaking
vessel operations are shown to be dependent upon these selected para­
meters. Design example calculations are shown to be dependent upon
these selected parameters. Design example calculations are made for
structures in the Bering, Chukchi, and Beaufort offshore lease areas.
Finally, research which would lead to optimization of such structures
is suggested.
Introduction
The offshore regions of Alaska, most of which are subject to
moving sea ice for at least part of the year, have become prominent as
major unexplored areas from which future domestic petroleum may be
produced. In Figure 1, the geologic basins of the Bering, Chukchi,
and Beaufort Seas are broadly illustrated. In the Beaufort Sea, many
areas are within reasonable distance of the trans-Alaskan pipeline's
northern terminal; other more remote regions in the Chukchi and
Bering Seas do not have ready access to an existing transportation link,
735

736
implying that marine transportation (including pipelines and/or
tanker loading facilities) will be required if oil is discovered in
these ice-infested seas.
Production structures, oil gathering and storage facilities,
tanker loading terminals, ice-breaking tankers, and ice-breaking
support vessels will be required. Designs will often be domina­
ted by ice considerations, but in some regions (such as the
southern Bering Sea) other environmental hazards such as waves,
earthquakes, and seafloor sediment conditions must also be taken
into account, and in fact may dictate the final designs. In this
paper, the important environmental hazards for each area are iden­
tified and quantified, to the extent permitted by the present
incomplete state of knowledge of these remote regions, and some
sample calculations of ice forces on structures are made for several
ice-dominated areas. In assembling this information, reference is
made to the results of many researchers, and much of their work
has appeared in previous POAC Conferences. Finally, suggestions
are made for future research which would make offshore produc­
tion from these areas of severe ice problems technologically prac­
tical.
Environmental Hazards
In the course of structure design, the meteorological vari­
ables of wind, temperature and pressure are, of themselves, of
little direct consequence, but they are responsible for waves,
storm tides, ocean currents, structure ice accretion, and sea ice
movement, all of which must be included in structure design calcu­
lations. In Figure 2, the inter-relationships of these n secondary"
environmental forces are shown. Bathymetry is an important factor
in assessing the feasibility of several types of offshore structures
for Alaskan waters. In Table I, the bathymetry range for the
several basins is given. For shallow water, up to 20 meters in
depth, the artificial island has proven to be useful in the Canadian
and Alaskan Beaufort Sea. The caisson-retained artificial fill island
concept might be extended to greater depths, such as 30-40 meters,
but the cost of such an approach would have to be weighed against

other options such as conical gravity structures. For deeper water
in the Chukchi Sea (40 meters) these latter appear promising, and
for the 100 to 150 m depths of the Bering Sea, steel or concrete
slender structures are being considered.
Wave-loading and current-loading calculations have become rather
refined and well-understood for offshore platforms in non-ice-covered
areas, and will not be repeated here. The impact of an isolated ice
floe driven by high waves against the leg of an offshore platform has
not yet been carefully analyzed, however.
A widely-quoted source of information for maximum wave heights
in the Alaskan offshore areas is the Climatic Atlas of the Outer Contin­
ental Shelf Waters and Coastal Regions of Alaska [II. For the several
regions of the Bering Sea, Figure 3 shows the wave heights which are
predicted, based on calculations using the approach of Thom [2,3 I.
Such calculations, however, do not include the possibility that wind
fetch and also wave height are reduced by the possible presence of an
ice cover. Furthermore, shallow-water effects (which would be expec­
ted in Norton Sound [20 meters nominal depth]) and non-linear wave
effects are not included. Extreme waves of 40 meters for a 100-year
return period are difficult to imagine in the Bering Sea, but clearly
the establishment of a realistic extreme wave height is of importance
for the Navarin, St. George, Zemchug, Bristol and St. Matthew Basin
regions, which are frequently exposed directly to weather systems
from the southwest.
Ice accretion on offshore structure and service vessels can also
be a serious hazard, greatly increasing their weight and elevating the
center of gravity. Very large loads can accumulate; for example,
drilling operations in Lower Cook Inlet were interrupted when a semi­
submersible rig accumulated an estimated 450 metric tons of vertical
load due to ice during winter operations. Freezing spray commonly
occurs [1] when the air temperature is between -2°C and -18°C, the
sea surface temperature is below +5 0 C, and the wind speed is greater
than about 11 m/sec. Using weather data from St. Paul [II, in the
Pribilov Islands adjacent to the St. George Basin of the Bering Sea,
one finds for example that in January the air temperature is less than
737

738
-8°C and the average wind speed is greater than 11 meters/sec. for 9%
of the time. The sea surface temperature in that region is less than
+loC for 10% of the time. For the combination of these conditions, spray
icing will accumulate at rates from 3.5 cm to over 14 cm thickness in 24
hours [1). Although exploratory drilling in summer in that area would
not encounter this hazard, winter drilling or year-round production
must take superstructure icing into account. Designs could allow for
this additional gravity and wind loading, or, alternatively, special
surface coatings with low adfreeze bonding (such as PTFE) could be
used (4). Waste heat and special y designed enclosures rna y also be
utilized. An additional type of ice accumulation can be expected from
floating frazil ice particles (5) which adhere to the legs of a structure
near the waterline. The adhesion rate is a function of the turbulence
around the structure legs, and in protected Arctic harbors, has been
related to the tidal cycle. Further research is needed to determined the
degree of frazil ice adhesion which would be encountered under open
ocean conditions in the marginal ice zone.
There are, broadly speaking, four categories of moving ice hazards,
in order of increasing severity: the undeformed sheet ice; annual ice
ridges; multi-year ridges; and ice islands. Each of these ice forms
produces lateral and vertical loads on structures, which are determined
by the ice thickness, the ice velocity, and the mechanical properties
of the ice. The details of the several modes of ice/structure interac­
tions will be mentioned in a later section of this paper.
A convenient geographical division may be made at the Bering
Strait, on the basis of ice severity. In the Bering Sea south of the
Bering Strait, multi-year ice is rarely encountered, although for some
winter weather conditions the sea ice (presumably including both annual
and multi-year broken ice floes) does flow southward from the Chukchi
Sea through the Strait (6). Additional field observations have been
planned to assess this condition. In the Chukchi and Beaufort Seas
north of the Bering Strait, multi-year ice is commonly encountered, but
ice islands are found infrequently.
Regardless of the choice of type of offshore structure, problems are
associated with its installation. It is far easier to install most structures

742
A comparison of this result for a multi-year ridge with that of
annual ice in the Bering Sea shows that the lateral force is nearly 8 times
greater in the Chukchi and Beaufort Sea pack ice region than in the most
hazardous part of the Bering Sea. The comparison with sea ice in Cook
Cook Inlet can also be made; assuming an ice thickness of 1.3 meters in
Cook Inlet, and crushing failure, lateral forces of about 22,460 kN are
predicted by using Korzhavin's formula and the same assumptions for
other parameters as in the Bering Sea. The ratio of horizontal forces
in the Chukchi Sea to those in Cook Inlet are, in this simplified example,
61/1. Obviously, structures for the Chukchi Sea must be much more
robust and have a very wide foundation to counteract the overturning
moment. For discussion of these details, the reader is referred to the
thesis by Karp [15 I.
One final observation must be made. A search for a structure
coating with low friction coefficient would appear to have a potentially
dramatic benefit, reducing the calculated horizontal force to perhaps as
low as 913 ,600 kN in the Chukchi Sea, or only 40 times greater than
the Cook Inlet situation. Changes to lower cone angles would also be
of some benefit. Further research on low-friction coatings for structures
subject to moving ice loads is strongly recommended.
One could criticize the assumptions made in Karp's analysis, pointing
out (as he did) (15) that multi-year keel depths of 52 meters probably
exist, since scouring of the seafloor by ice has been observed to this
depth. Moreover, the flexural strength of multi-year ice is known to be
widely variable. Finally, ice islands have not yet been considered.
These and related supporting research studies remain for the future [17 I.
Geological Hazards
A comprehensive book by G. D. Sharma [181 has recently been
published which provides much useful information on sediment size
distribution in the Alaskan offshore areas. For the St. George, Norton,
and Beaufort Basins, research has revealed few seafloor problems that
would be considered severe for offshore foundation design [171, with
the notable exception of the very fine gas-charged sediments in Norton
Sound, as described by Thor and Nelson [191. Formation and growth

of gas craters under gravity structures could occur there, if no provi­
sion were made for normal gas evolution to the surface. Geological
reconnaissance and geotechnical properties studies remain to be in the
other areas.
Earthquakes have been measured for many decades, and most earth­
quakes occur along the Aleutian arc. Activity in St. George Basin and
the Seward Peninsula seems to be limited to events of less than magnitude
7.0 Richter. The transfer of energy from the epicenter to a particular
site, and the seafloor accelerations at the site, remain to be determined,
however. Seed (20) has studied liquefaction of marine soils due to earth""
quakes and has concluded that for most cases an acceleration of the
order of 0.16 g, and a magnitude 7.5 Richter, is required for soil lique­
faction. This result has not been verified for gas-charged marine sedi­
ments, however. Although foundation design and earthquake excitation
of modes of vibration of the structure should be considered, these prob­
lems are not peculiar to the Arctic, nor does the Arctic present any
obvious problems, with the possible exceptions of the gas-charged sedi­
ments of Norton Basin, and the presence of ice adjacent to vibrating
structures during earthquakes.
The offshore permafrost and in-situ gas hydrates of the Beaufort
and Chukchi Seas will call for special drilling techniques and cementing
procedures, which are, however, beyond the scope of this paper (17).
Research Frontiers in the Alaskan Offshore Areas
On 30 June - 2 July 1980, this writer convened a small research
planning workshop, on behalf of the U.S. Department of Energy, at
Sandia National Laboratories, Albuquerque, New Mexico. The report
of the workshop (21) identified 60 engineering problem areas for further
research; these are listed in the Appendix. The workshop was preceded
by a more comprehensive review report (17), covering some of the
material discussed in this paper, and also identifying research needs.
Conclusions
1. Although lateral forces caused by wave loading are likely to
dominate over ice forces in the southern Bering Sea, an accur­
ate assessment of extreme wave heights remains to be completed
for the Bering Sea.
743

744
2. Spray ice buildup on structures, ships, and aircraft will occur for
an appreciable percentage of the time during the winter months in
the Bering Sea. This must be prevented or accommodated with
special designs.
3. Rafted annual ice floes up to 10 m thick in the Bering Sea could
produce lateral forces some eight times greater than those in Cook
Inlet. The probability of encounter between these floes and a
production structure is quite low but remains to be established.
4. Multi-year ice ridges breaking against a conical structure in flexure
represent a typical, severe ice loading condition in the pack ice of
the Chukchi and Beaufort Seas. A sample calculation gives lateral
forces ranging from 40 to 61 times larger than those due to ice
crushing in Cook Inlet, depending upon the friction between ice
and structure.
5. Many technology research directions remain to be addressed in order
to design and emplace safe and economical offshore production struc­
tures in the pack ice of the Beaufort, Chukchi and Bering Seas.
Acknowledgements
Much of this material has been reviewed with the support of the
U.S. Department of Energy, Division of Oil, Gas and Shale Technology.
This study was supported by the Bureau of Land Management through
interagency agreement with the National Oceanic and Atmospheric Adminis­
tration, under which a multi-year program responding to needs of petro­
leum development of the Alaskan Continental Shelf is managed by the
Outer Continental Shelf Environmental Assessment Program Office. The
review of this manuscript by G. Weller was most helpful.

746
13. Schwarz, J., and W. F. Weeks (1977). "Engineering properties
of sea ice", Journal of Glaciology, 19, pp. 499 531.
14. Wang, Y. S. "Sea ice properties", Technical Seminar on Alaskan
Beaufort Sea Gravel Island Design, Exxon USA, Anchorage,
October IS, 1979.
15. Karp, L. B. "Concept Development of a Concrete Structure
Founded in the Ice-Stressed Chukchi Sea: A Case of Ice/Structure
Interaction in an Offshore Arctic Region", D. Eng. thesis,
University of California, Berkeley, August 1980.
16. Frederking, R. "Dynamic ice forces on an inclined structure", in
Physics and Mechanics of Ice, P. Tryde (ed.), Springer-Verlag,
Berlin, 1980, pp. 104-116.
17. Sackinger, W. M. "A review of technolo gy for arctic offshore oil
and gas recovery", U. S. Department of Energy, Division of
Fossil Fuel Extraction, Contract No. DE-A COI-80ET14317,
Vol. I, June 6, 1980, 97pp.
18. Sharma, G. D. The Alaskan Shelf, Springer-Verlag, New York,
1979.
19. Thor, D. R. and I-l. Nelson. "A summary of interacting, surficial
geologic processes and potential geologic hazards in the Norton
Basin, northern Bering Sea", Proc. of the Offshore Technology
Conference, Houston, TX (1979), pp. 377-385, (OTC 3400).
20. Seed, H. B. "Soil liquefaction and cyclic mobility evaluation for
level ground during earthquakes", J. Geotechnical Eng. Div.,
Proc. ASCE, Vol. 205, No. GT2, February 1979, pp. 201 255.
21. Sackinger, W. M. "Report of the workshop on arctic oil and gas
recovery", U.S. Department of Energy, Office of Oil, Final
Report for Contract No. DE-ACOI-80ET14317, September 1980,
38pp.

Appendix I
Recommended research topics in support of Arctic offshore petroleum
recovery: The results of a workshop at Sandia National Laboratories,
Albuquerque, New Mexico, 30 June - 2 July 1980.
1. Verify hindcast models for Arctic areas that include the influence
of ice cover on wave and surge generation, and shallow water
effects.
2. Investigate methods to date ice gouges of the seafloor.
3. Investigate mechanics of ice gouging as a function of sediment
type.
4. Conduct confined and unconfined tests to provide data on strength
and stress/strain behavior of sea ice and freshwater ice as a
function of environmental parameters and loading rates.
5. Conduct in-situ beam tests to obtain flexural strength data.
6. Conduct large scale and small scale tests on the same sea ice to
investigate size effects.
7. Investigate analytical procedures to predict size effects.
8. Obtain data on dielectric and electromagnetic properties of sea ice.
9. Obtain engineering data on frictional forces between ice and other
materials.
10. Obtain data on adhesion bond strength.
11. Collect in-situ measurements of first-year and multi-year ridge
geometry and properties.
12. Measure ice drift velocity.
13. Develop and verify regional ice dynamics models.
14. Conduct systematic stereo photography, laser overflights, and
sonar measurement programs to obtain ridge profile distributions.
15. Verify use of passive microwave imagery to identify multi-year ice.
16. Process available satellite imagery to provide data on multi-year
ice movement and floe size distribution.
17. Develop physical model of pack ice edge movement.
747

748
18. Compile pack ice edge data to verify the ice edge movement model.
19. Obtain field measurements of ice state, e.g., crystal structure,
composition, temperature, and snow cover.
20. Utilize ships of opportunity and specific icebreaker voyages to
collect data on ice conditions.
21. Conduct oceanographic/meteorological measurement program.
22. Conduct systematic side-scan sonar and bathymetric surveys in
the Beaufort Sea to water depths of 80 m and in the Chukchi Sea.
23. Sponsor a logistics base at the Naval Arctic Research Laboratory,
Barrow, and an equivalent operating base at Nome to conduct
field studies in the Chukchi and Bering Seas.
24. Improve understanding of failure modes of a variety of ice features
against various types of structures.
25. Investigate the influence of random flaws and nonsimultaneous
structure contact on total ice forces.
26. Develop a model of driving force and average ridge-building
forces across a wider structure front.
27. Describe local ice pressures as a function of ice mechanical
properties, ice velocity, and contact geometry.
28. Investigate the characteristics and effects of rubble fields around
structures.
29. Consider test structure verification of ice/structure interaction
models.
30. Conduct model tests and develop predictive theories for ice
rideup on islands and structures.
31. Develop improved techniques to predict wave runup and overtopping.
32. Investigate non-linear surface effects for wave diffraction theory.
33. Evaluate impact loads upon structures from storm-driven ice floes.
34. Conduct studies to investigate ice actions on slope protection
systems.
35. Improve prediction of ice ridge penetration resistance for icebreakers.
36. Develop models of ship/multiyear ice ridge breaching.

37. Develop model to predict ice pressure-induced delays in ship
transit.
38. Deveop a verified model for high energy penetration, resistance,
and pressure on a ship's hull pursuant to collision with an ice
feature.
39. Accelerate development of models to predict ice forces on propellers
and conduct full scale verification trials.
40. Conduct full scale and model tests to improve propulsion efficiency
in ice.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
Extend existing models of ship/terminal interactions in ice to more
severe ice conditions of the Arctic offshore. Verify with model
tests.
Develop mathematical models and physical modeling techniques to
improve ship maneuvering performance in ice.
Develop models to predict variations in ship performance in level
ice due to changes in ice strength, hull shape, and snow cover.
The characterization of the seafloor should be completed in the
regions of the Arctic offshore which will be subject to petroleum
develop men t •
The mechanical properties of sediments on the sea bed should be
determined.
Geomorphological processes acting upon the seafloor, such as ice
scour, must be described more precisely •
The ground motion offshore • which results from earthquakes should
be measured.
A model of thaw and subsidence for a warm subsea pipeline buried
in offshore permafrost should be developed.
Frost heave in marine soils and gravels containing seawater
deserves further investigation.
The procedures for predicting behavior of pilings and island
foundations installed above subsea permafrost should be refined.
Effects of artificial islands and causeways upon natural coastal
processes should continue to be studied.
The location and extent of ice-bonded subsea permafrost should
be determined for the Arctic offshore basins which are expected
to be developed.
749

750
53. A remote sensing technique for detection of gas hydrates, in-situ,
should be perfected.
54. The location of active subsurface faults should be determined.
55. Both theoretical and remote-sensing techniques should be applied
to ascertain the extent of thaw of gas hydrates during production,
and the possible resulting subsidence.
56. The technology of cementing casing in regions where gas hydrates
have been penetrated should be perfected.
57. Instrumentation for both laboratory and in-situ measurements of
ice properties should be reviewed and perfected.
58. Methods should be developed for rapid and easy assessment of
ice ridge profiles and degree of consolidation.
59. Diagnostic instrumentation should be developed to measure the
interaction of ice with prototype or operational artificial islands,
structures, and subsea pipelines.
60. An advanced ice surveillance system to be used in an operational
mode for ship routing in ice-infested waters should be designed.

FIG. 1. Selected Arctic Alaskan Offshore Basins.

IG. 2. Major Technical Factors Related to Arctic Offshore Oil and Gas Recovery
LAND
PIPELINES
IN
PERMAFROST

R. G. Sisodiya
K. D. Vaudrey*
Abstract
BEAUFORT SEA FIRST-YEAR
ICE FEA TURES SURVEY - 1979
Gulf Research and
Development Co.
Vaudrey & Associates
Houston, TX, USA
Missouri City, TX, USA
A joint industry study was conducted on first-year ice features in the Alaskan Beaufort
Sea during March-April 1979 to determine ice feature geometry and internal characteristics as
well as assess winter ice conditions in the lease sale area offshore Prudhoe Bay.
Data from the field investigation consisted of sail and keel profiling of ridges by standard land
surveying and sonar techniques, respectively, while internal composition was defined by ice
augering.
Over 300 miles of stereo aerial photography was flown both perpendicular and parallel to the
coast outside of the barrier island chain •.
I. Introduction
Since first-year ice ridges and grounded features form each winter in the Alaskan Beaufort Sea,
they may pose an operational problem for transportation and pipeline installations. In some
instances, a consolidated first-year ridge moving against a structure may provide the design
load.
Several joint industry projects have utilized aerial photography and laser profilometry to
determine first-year ridge populations; but data are limited on geometry statistics and
consolidation properties. Only a few individual first-year ridges have been investigated (I), (2),
(3), (4). Ridging frequency data have been determined for large offshore areas in the Alaskan
Beaufort Sea (4), (5), (6), (7), but may not be applicable for the nearshore area of interest.
*Formerly with Gulf Research and Development Co.
755

Gulf Research and Development Company, along with seven participating companies, performed
a joint industry study to investigate first-year ice features in the Prudhoe Bay region of the
Alaskan Beaufort Sea during late winter 1979. The objectives of this program were: I) to
provide statistical information on ice features that may be correlated to predict ridge-structure
interaction; 2) to determine ridge and rubble pile geometries to help establish pileup and
loading design criteria; and 3) to determine internal ridge characteristics to provide insight on
possible models of first-year ridge behavior.
2. Field Investigation
An on-the-ice survey of thirteen first-year ridges and two rubble piles was performed,
determining sail profiles by standard surveying, keel profiles by sonar techniques (fathameter
with 3 0 transducer), and internal composition by ice augering. Ice features studied are shown
on the map in Figure I. Twenty-two complete cross-sections were obtained with seven of the
ridges having two or more profiles eoch.
The average keel depth to sail height (K/S) ratio for 17 floating ridge sections was calculated as
K/S = 5.5 ! 1.2. All ridge sections with sail heights greater than 15 feet were grounded. The
average K/S ratio lies between the results of Kan (8), who found an average K/S = 7.6 for five
ridges, and Kovacs (I), who calculated K/S = 4.9 for five profiles from three separate ridges.
For floating ridges 113-12 the sail height is plotted versus keel depth in Figure 2, showing a well­
correlated linear relationship given by a least squares straight line fit of the data points.
Assuming that keel depths are distributed normally about the mean defined by the straight line,
95% of the keel depth observations should lie within the confidence intervals shown in Figure 2
as dashed lines.
A method using the program data was established for determining a critical sail height (Scr) for
a given water depth, representing a boundary between floating and grounded ridges. The
critical sail height can be plotted as a function of water depth as shown in Figure 3 as a straight
line least squares fit minimizing sail height error.
At least two augerholes were drilled into the keel of each ridge section to determine internal
composition, and in most cases, solid, competent ice existed up to 6-8 feet below sea level,
indicating that the keels were not very well-bonded below a single, late-winter ice sheet
thickness. Where rafting occurs extensively, free water was generally absent at the ice sheet
layer boundaries. Therefore, large refrozen ice floes approximately 18-24 feet thick may exist
between and on the slopes of ridges. For example, Ridge 9 shown in Figure 4 had a rafted floe
along its southern boundary greater than 22 feet thick, indicating at least four five-foot ice
756

sheet layers rafted on top of each other. In addition to qualitative keel information, nine ice
cores were extracted from several ridge slopes to record temperature and salinity profiles.
The single most interesting feature studied was Ice Mountain, a large rubble pile (350' x 1100'),
2.5 miles north of Narwhal Island in 59 feet of water (Figure 5). Probably formed by a single
storm on St. Patrick's Day, a series of 3-4 pileups occurred with progressively less sail height.
The maximum height measured was 72 feet. A formation of Ice Mountain is associated
with a movement of more than 4000 feet which is estimated by studying seismic road
separations immediately to the west, presented in more detail by Agerton (9).
Another interesting, rubble pile occurred in 12' water depth inside the barrier islands within
Prudhoe Bay, five miles south of Reindeer Island. The most dramatic ice feature inside the
island during 1979, Rubble Pile (300' x I 000'), formed during early winter when the young ice
sheet was only 4-8 inches thick. The moximum height of the pileup was 24 feet (Figure 6).
Shear ridge was the third special first-year ice feature, extending from east of Narwhal to just
northeast of Cross Island, approximately two miles offshore of the islands in 50-60 feet of
water. Formed primarily by shearing action, the ice was ground up and refrozen, maintaining
the shape of an almost vertical, straight wall 10 feet high.
3. Aerial Photography
Over 300 miles of stereo aerial photography was flown midway through the field investigation
with the flight lines shown in Figure I. Flightline mosaics were developed and visually
interpreted for aerial coverage of smooth ice, rubble, or ridges for each one-mile segment.
Every third stereo pair of photographs was selected for centerline digitization and appeared to
be sufficiently random for meaningful statistical analysis. Digitizing every third stereo pair
permitted 50% coverage of each flightline due to 60% overlap between consecutive photo­
graphs. Eighteen interesting special features located off the centerline were selected for
additional digitization across or along them.
To provide meaningful statistics-suitable criteria must be selected to scan the data and identify
ridges and two-dimensional ice features. Previous criteria, including the "two-foot drop" used
by Hibler (5) and others and the 50% Rayleigh criterion, have all overestimated the number of
ridges. To determine ice volume estimates in pileup prediction on or force transmitted to,on
offshore structure, it is necessary to differentiate between linear and two-dimensional ice
features. Lowry and Wadhams (10) have developed such a criterion, but it assumes that all
ridges have similar slopes which may not be the case.
760

FIGURE 5 - ICE MOUNTAIN, LOOKING SOUTH
FIGURE 6 - RUBBLE PILE, SHOWING THE WESTERN EXTREMITY
LOOKING NORTH
761

Part of the analysis in this study utilized a modified Rayleigh criterion (Criterion A), while
another (Criterion B) was developed to distinguish between ridges and areal ice features.
Criterion A. An ice feature starts if a data point is found to exceed a height of 3 feet.
Subsequent data points are scanned to find the maximum height until a point is found to end the
feature, indicating an elevation lower than one foot above smooth ice, or if it is in a trough
which dropped by 80% of the maximum height. Criterion A is summarized in Figure 7.
Criterion B. An ice feature ends only when a trough wider than 10 feet and lower than one foot
high is encountered. Ridges were separated from other ice features by comparing the ratio of
the height above the one-foot elevation and width of feature between one-foot elevations. An
ice feature is assumed to be: I) rubble field if the maximum height-to-width ratio of the
feature was less than 0.075; 2) rafted block if the ratio was greater than 0.5 and if the
maximum height was less than 7 feet; 3) ridge, otherwise. These limiting conditions on
maximum height-to-width ratios and maximum heights were selected from a parameter study
verified by visually interpreting features from flight line V2. Criterion B is summarized in
Figure 8.
Histograms of ice features or ridges by height categories and population using both criteria
were generated, and Table I shows the average population comparison between east-west and
north-south f1ightlines. Additional analysis to compare the ability of these criteria and others
to predict extreme feature recurrences has been performed by Kreider and Thro (II).
TABLE I -AVERAGE POPULATION COMPARISON BETWEEN LINES
PERPENDICULAR AND PARALLEL TO THE COAST
Perpendicular to Coost
(Combining V I, V2, V3, and VS)
Parallel to Coast
(Combining H3, H4, HSB, HSA
H6B, H6A, H7B, and H7A)
4. Conclusions
Population Per One-Fifth Mile
Criterion A Criterion B Criterion B
Ice Feature Ice Feature Ridges
2.96 1.82 0.53
1.95 1.52 0.43
I) A new ice features identification criterion was developed to differentiate between linear
ridges an two-dimensional ice features (e.g., rubble piles).
2) A relationship has been established between sail height and keel depth for floating
762
ridges, while another relationship has been found to estimate the critical sail height
representing the boundary between flooting and grounded ridges for a given water depth.

3) Rafted ice appeared to have greater areal coverage and thickness than previously
considered.
4) Most first-year ridges did not appear to be well-consolidated below six to eight feet
below sea level wthin the keel.
5) Large winter ice movements may occur in the outer perimeter of the landfast ice zone.
5. Acknowledgments
The authors thank the participating companies (Arco, Chevron, Conoco, Exxon, Gulf, Mobil,
Phillips, Shell) and their representatives for making the field program possible. A special
acknowledgment goes to Gulf Research and Development Co. for their permission to present
this paper.
6 References
I) Kovacs, A. (1971) "On Pressured Sea Ice," Sea Ice-Proceedings of an International
Conference, Reykjavik, Iceland, p. 276-295.
2) Rigby F. and A. Hanson (1976) "Evolution of a Large Arctic Pressure Ridge," AIDJEX
Bulletin No. 34, Seattle, WA., p. 43-71.
3) Weeks, W. F. and A. Kovacs (1970) "The Morphology and Physical Properties of Pressure
Ridges: Barrow, Alaska - April 1969, "IAHR Ice Symposium Proceedings, Reykjavik,
Iceland.
4) Weeks, W. F., et al. (1971) "Pressure Ridge Characteristics in the Arctic Coastal
Environment," PO AC-7 I Proceedings, Trondheim, Norway.
5) Hibler, W. D. III, Mock. S. J., Tucker, W. B. III (1974) "Classification and Variation fo Sea
Ice Ridging in the Western Arctic Basin", Journal of Geophysical Research, Vol. 79,
pg. 2735-43.
6) Wadhams, P. (1976) "Sea Ice Topography in the Beaufort Sea and its Effects on Oil
Containment", AIDJEX Bulletin No. 33, pg. I-52.
7) Tucker, W. B. III and Westhall, V. M. (1973) "Arctic Sea Ice Ridge Frequency Distribution
Derived from Laser Profiles", AIDJEX Bulletin No. 21, pg. 171-ISO.
S) Kan, T. K., et al. (1973) "Sonar Mapping of the Underside of Pack Ice", AIDJEX Bulletin
No. 21, Seattle, WA., p. 155-169.
9) Agerton, D. J. (19SI) "Major Nearshore Winter Ice Movements in the Alaskan Beaufort
Sea", POAC-SI, Quebec City, Canada.
10) Lowry, R. T., and Wadhams, P. (1979) "On the Statistical Distribution of Pressure Ridges
in Sea Ice", Journal of Geophysical Research, Vol. S4, pg. 24S7-94.
II) Kreider, J. R., and M. E. Thro (l9SI) "Statistical Techniques for the Analysis of Sea Ice
Pressure Ridge Distributions", POAC-SI Proceedings, Quebec City, Canada.
764

D.F. Dickins
V.F. Wetzel
MULTI-YEAR PRESSURE RIDGE STUDY
QUEEN ELIZABETH ISLANDS
DF Dickins Engineering
Suncor Inc.
Canada
Canada
ABSTRACT
The study of multi-year pressure ridges within the Arctic Islands was conducted
as Arctic Petroleum Operators Association Project 102. Sun Oil Company limited (now
known as Suncor Inc.) was operator of the project and Norcor Engineering and Research
limited was consultant. This project had as its primary objective the obtaining of
fundamental data on multi-year pressure ridges in the Queen Elizabeth Islands of the
Canadian Arctic. From a base camp on the ice in the Maclean Strait, 8 km west of
Ellef Ringnes Island, the geometry and sail/keel depths of 12 free floating multiyear
ridges was investigated. A rotating echo sounder transducer was used to
determine the below water profile.
A total of 20 ridge cross sections were obtained between May 11 and June 13, 1976.
The mean keel/sail ratio of 5.6 ± 2.2 is larger and more variable than indicated
from previous ridge studies conducted in more southern latitudes. A maximum keel
of 37 meters was observed. Underwater keel profiles were characterized by a distinct
asymmetry, with little correspondence between the location of maximum sail height
and maximum keel depth along a particular ridge. Ice specific gravity, calculated
from buoyancy considerations and a mean keel/sail ratio of 9.32, was 0.92. This
corresponds almost exactly with quoted values for mUlti-year ice. The results of
this study confirmed the abscence of significant void spaces in multi-year pressure
ridges. This was in keeping with surface observations of fractured sails which
clearly showed total consolidation of the original blocks. A trend toward higher
sail/keel ratios with decreasing sail slope was evident, and this is postulated to be
a manifestation of the ageing process over successive melt seasons.
Further work will be required to better define the variability of keel/sail ratios
and to determine the maximum keel depths that could be encountered in that region.
This information is essential if we are to develop economic and environmentally
safe designs for sub-sea production systems and pipelines in the Sverdrup Basin.
765

INTRODUCTION
Starting with the first exploratory hole drilled in the Arctic Islands in 1969,
(Panarctic Drake Point K-67-A) to the Whitefish discovery of 1979, considerable
interest has been shown in the development of specialized offshore drilling
structures, pipelines, and suitable routes for ice strengthened tankers. Eventually,
in the production phase, bottom founded well heads, terminal facilities and connector
pipelines will have to be constructed. With safety as one of the primary environmental
and operational concerns the probability of bottom scouring and impact from multiyear
pressure ridge keels will be a major factor considered in the design of any
facility or structure in this region.
The study of multi-year pressure ridges was initiated by Sun Oil Company limited (now
known as Suncor Inc.) through the Arctic Petroleum Operators Association as Project
102. Canadian Superior, Gulf Canada Resources Inc. and Panarctic Oils were early
participants with Phillips Petroleum and PetroCanada joining the study about a year
later. Sun Oil Company limited was operator of the project with Norcor Engineering
and Research limited as its consultant.
Prior to this study, no investigation had been conducted concerning pressure ridge
geometry in the High Arctic. The depth of ridge keels in areas of interest had to
be estimated by applying a height to depth ratio of 1 to 3.2 (l, 2, 3). This ratio
was determined from multi-year pressure ridge studies in the Beaufort Sea, and does
not necessarily apply at more northern latitudes. The principal objective of this
study was to determine the sail to keel ratio for various types of ice ridges commonly
encountered in the Sverdrup Basin.
766

The large degree of asymmetry typical of most of the ridges, meant that the maximum
keel depth was often offset from the sail centre-line by 20 to 30 m. Figures 2 to 5
show four different cross sectional profiles obtained along the length of one ridge.
Profile 1 keel profile shows the excellent match between sonar data from two
different sides. The asymmetry of the keel is important, as it indicates the uncertainty
involved in estimating the entire ridge geometry from a half profile. The
second profile, conducted 28 m further west, incorporated the ridge high point of
6.4 m. Here, the deepest keel of the programme was observed at 40 m. There was
still a faint sonar return at this depth, but lack of more sonar rods prevented a
transducer mounting lower than 30 m. This extremely deep section is probably a
localized feature, because on the matching north profile, the return was lost at 35 m.
The third profile was undertaken to prove whether the keel depth was at all proportional
to the sail at a particular point. A position was chosen off the main point
of the ridge where the sail was very wide (60 m) and low (3 m). No significant keel
was expected here, but the sonar record showed a keel more massive than Profile I,
extending 32 m into the water. With the transducer perpendicular to the ridge, the
keel bottom was below the limit of sonar again, i.e., > 40 m. The nearly vertical
face indicates that part of the original keel was missing.
Profile 4 was conducted still further to the East where the ridge was extremely broad
and only had a 2 m sail projecting above the general elevation of the multi-year floe.
As on Profiles 1 and 2, this keel was asymmetric, with most of the mass to the north
of centre. The keel is extremely irregular and could not have experienced significant
ablation.
769

etween 30 and 50 m depth (8). Other oceanographic measurements, taken in Parry
Channel, show a similar temperature stratification at the same depth range.
Consequently, the majority of ridges in the study area have their keels within
water below its freezing point during much of the winter period. The open water
season in the summer, north of 78°N is very short and often non-existent. Compared
to ridges in more southern areas, there seems to be much less opportunity for keel
depletion in the Queen Elizabeth Islands area.
For impact purposes, the total mass of a typical ridge is of interest. The three
areas of the Ridge 13 profile were averaged. When applied to a 200 m long ridge,
the mass would be 340 (10)6 kg • A corresponding smooth piece of mUlti-year ice 6 m
thick x 200 m x 100 m (approximate ridge keel width) would weigh 140 (10)6 kg • The
presence of the ridge has effectively increased the ice mass by a factor of 2.4.
CONCLUSIONS
The data presented in this paper is an extraction from a report containing the entire
body of current knowledge about multi-year pressure ridge geometry in the Queen
Elizabeth Islands of the Canadian Arctic.
The mean sail/keel ratio found in this study was 1 to 5.6 ± 2.2 (population of 17).
This value is considerably larger and more variable than indicated by previous ridge
studies at more southern latitudes. The mean total ice thickness of the ridges
studies here was 25.3 m with a maximum thickness of 46 m. It is theorized that the
different aging process of ridges in the islands area, due to cold air and water
temperatures over most of the year, contributes to the large sail to keel ratios
found here.
The ridges studies were generally in hydrostatic equilibrium and underwater profiles
were characterized by a distinct asymmetry. A trend toward higher sail/keel ratios
with decreasing sail side slope angle was evident, and this is postulated to be a
result of the aging process over successive melt seasons.
Due to the high sail/keel ratios found in this study, considerable re-thinking may
be necessary to accurately present the operational and design constraints imposed
by ridging in the Arctic Islands area. With similar ratios even small ridges of 3 m
sail height, not previously considered a severe obstacle, may contain over 20 m of ice.
774

REFERENCES
1. KOVACS, A., 1972: On Pressured Sea Ice, Sea Ice Conference Proceedings,
Reykjavik, Iceland.
2. KOVACS, A., 1973: Structure of a Multi-Year Pressure Ridge, ARCTIC, Volume 26,
No.1.
3. KOVACS, A., DICKINS, D.F., WRIGHT, B., 1975: An Investigation of Multi-Year
Pressure Ridges and Shore Pile-Ups. A.P.O.A. Project 89 by NORCOR Engineering
for GULF CANADA LTD.
4. WETZEL, V.F., 1971-75: Statistical Study of Late Winter Ice Thickness
Distribution in the Arctic Islands, A.P.O.A. Project 96 - proprietary data.
5. ANDERSON, D.L., 1960: The Physical Constants of Sea Ice, Research, Volume 13,
pp. 310-18.
6. DOAKE, C.S.M., 1976: Thermodynamics of the Interaction Between Ice Shelves and
the Sea, Polar Record, Volume 18, No. 112.
7. SVERDRUP, H.U., no date: Arctic Sea Ice, Encyclopedia Arctica, Vol. 7,
unpublished manuscript.
8. FUJINO, K., LEWIS, E.L., PERKIN, R.G., 1974: The Freezing Point of Seawater at
Pressures up to 100 Bars, Journal of Geophysical Research, Volume 79, No.
12.
9. DICKINS, D.F., 1976: Multi-Year Pressure Ridge Study, Queen Elizabeth Islands,
A.P.O.A. Project 102 report by Norcor Engineering and Research Ltd. to
Sun Oil Company Ltd.
775

L. Wolfson,
Senior Drilling Engineer
-Ice Studies Aid in the Successful COmpletion
of the Norton Sound C.O.S.T. Well-
ARCO Oil and Gas Company
W. M. Evans,
Senior Staff Drilling Engineer ARCO Oil and Gas Company
Abstract
U.S.A.
U.S.A.
ARCO Oil and Gas Company was the operator of a C.O.S.T. Well drilled in Norton Sound
during the open water season of 1980. Satellite imagery were used to document
historical ice breakup and ice freezeup periods. Meteorological data were used to
determine causal effects during these periods and forecast models were developed to
predict ice breakup and ice freezeup. Historical and real time ice and meteorological
data were used to forecast and advise of favorable operating conditions for 1980. Other
studies and activities which led to the successful completion of the well included:
(1) determination of extreme and normal meteorological and oceanographic conditions in
the open water season; (2) establishment of supply facilities available and scheme for
supply support for the well; (3) sea floor condition; (4) establishing a well prognosis
which included well design, drilling time and cost estimate; and (5) the rig
mobilization and demobilization plan.
Introduction
In August 1978, ARCO Oil and Gas Company, as operator for a group of petroleum industry
participants, initiated plans to drill a Continental Offshore Stratigraphic Test
(C.O.S. T.) Well in the Norton Sound Basin, Offshore Alaska, Figure 1. To facil itate the
planning and operations of the C.O.S.T. well, several studies and various activities
were initiated.
776

A jack-up drilling vessel, the Dan Prince, was chosen to drill the C.O.S.T. well. The
Dan Prince was capable of operating only in an ice-free environment. To determine if
sufficient time was available to complete the well in one season, a study to document
the historical ice-free season was conducted. To take maximum advantage of the open
water season, forecast models were developed to predict the onset of ice breakup and ice
freezeup.
Additional studies and activities were initiated to aid in logistical and operational
planning and the permitting process. A site specific seismic survey was conducted to
determine if potential shallow drilling hazards were present and also prepare the
drilling prognosis. Sea floor soil conditions were evaluated to determine if the soils
would provide an adequate foundation for the rig. A tow efficiency study was conducted
to assist in planning mobilization and demobilization of the rig. Extreme and normal
meteorological and oceanographic conditions were established to evaluate the suitability
of the rig for the environment in this area and to evaluate supply schemes.
Logistical requirements were determined to assure timely supply of material and
movement of personnel.
Ice Breakup and Ice Freezeup Documentation
Since the drilling vessel was able to operate only in an ice-free environment, the
vessel's owners, insurance underwriters and regulatory agencies needed assurance that
operations would be conducted only during the ice-free season. This dictated the need
to establish adequate forecasting of the ice breakup and ice freezeup periods.
Historical ice breakup and ice freezeup periods and the associated meteorological data
were documented and used for developing ice forecasting models for this area.
Ice conditions in the Northern Bering Sea and Norton Sound are quite varied with respect
to ice features, concentration, and movement on a spatial and temporal basis. The area
of the C.O.S.T. well site can be ice-free at certain times, but ice from other areas can
be driven into the area by changing meteorological conditions. To establish the ice
breakup and ice freezeup periods in areas in the vicinity and at the C.O.S.T. well site,
four areas which are illustrated in Figure 2 were studied: Area 1, Northern Bering Sea
(St. Lawrence Island to the Bering Strait); Area 2, Norton Sound (East of St. Lawrence
Island); Area 3, St. Lawrence (South of a line from St. Lawrence Island to the Yukon
Delta); and Area 4, Bays and Inlets (Go1vin Bay and Norton Bay).[1]
777

if it can be used for long range forecasts to a certain degree of accuracy. In
mid-March, the first ice breakup forecast was issued using the pressure data for the
first two weeks of March. The first freeze up forecast was issued on the first of
September and pressure data for the last two weeks of August were used in the forecast
model. Subsequent forecasts were made at two week intervals and the pressure data used
were the data used for the previous forecasts plus the pressure data recorded during the
most recent two week period. For the 1972-1978 data base, the models developed
accurately predicted the ice breakup and ice freeze up dates established in the study.
It was recognized that such a small data base (seven years) for relatively average
weather conditions during the seven years might not be representative for years that
deviate considerably from average.
To test the accuracy of the forecast models, a study of the performance of the models
for predicting ice breakup and ice freezeup for the 1979 season was initiated.[2,3] As
noted previously, ice conditions during 1979 were much less severe. The ice breakup
model performed poorly; the ice freezeup model was more accurate than the ice breakup
model but did not approach the degree of accuracy established in the 1972-1978 study.
This 1979 study illustrated the need of the forecasters to use the predictive models
only as a guide. The forecasts from the models would have to be modified by the forecast
personnel relying on their personal experience using all available meteorological and
satellite data on a real time basis. These modified forecasts are referred to as
subjective forecasts.
Both model and subjective semi-monthly forecasts were issued beginning on March 19,
1980 for support of the 1980 drilling program.[4] The subjective forecasts for ice free
condtions in Area 2 (which includes the C.O.S.T. well site) were usually within one week
of the actual ice free date. The model forecasts for ice free conditions varied from
one and one-half weeks to three weeks.
Semi-monthly forecasts for ice freezeup were initiated in late August. As with the ice
breakup forecasts, the model forecasts varied with the most accurate forecast being the
first forecast on September 1 (within 4 days) which had the least data base. The error
in subsequent forecasts increased. Subjective forecasts were very accurate and the
first forecast issued in late August was within 2 days of the inception of freeze up in
Norton Sound. Subsequent subjective forecasts were not in error.
779

Utilization of Ice Data for Actual Operations
What is important is not only what the forecasts were and the accuracy of the forecasts,
but how the operator used these data to mobilize the jack-up rig to Norton Sound. The
first subjective forecast indicated that open water conditions would be expected in
Area 2 by June 10 and the area should be ice free by June 28. 8y mid-March, 1980, ARCO
anticipated bein9 able to mobilize the jack-up rig in time to start operations by
mid-June. A meeting was held with the contractor and marine surveyor in mid-March to
review all prior work that had taken place in regard to establishing the ice free season
variations and how the forecasts of ice breakup for 1980 would be utilized.
Subsequently, real time satellite data and ice charts prepared from the satellite data
were reviewed on a systematic basis by operator and contractor personnel. Cloud cover
prevented monitoring the C.O.S.T. well site from satellite photos shortly before the
rig arrived at the site. During this time, overflights were made of the area to assure
that the site was ice free and to monitor the movement of ice in Area 1 (Northern Bering
Sea). The drilling rig was secured on location in Norton Sound on June 13, 1980 without
encountering sea ice.
Ice freezeup forecasts which were initiated in late August indicated that ice would
start to form in Norton Sound in late October. Ice was first observed on October 27,
1980. Operations were completed on September 29, 1980, thus the well activities were
finished prior to ice forming in the area.
Other Studies and Activities
A. Site Specific Seismic Surveys
Sea floor, shallow seismic and deep seismic surveys were made in the summer of 1979.
Sea floor surveys were used to determine the possibility of encountering sea floor
hazards such as boulders and gas seeps. Shallow seismic surveys were used to determine
(1) the thickness and continuity of shallow formations, (2) if faults were present and,
(3) if shallow high pressure gas zones were present. Data from the deep seismic surveys
were used to establish (1) if the formations to be penetrated would be expected to be
normally or abnormally pressured, (2) the casing program, and (3) the expected
formation drillability. The formation drillability data were used in conjunction with
the proposed drilling, well logging and coring program, to establish the expected total
well time. The expected well time was determined to be 126 days and the actual total
well time was 107 days.
780

B. Geotechnical Data[S]
In 1979, cores were taken at the sea floor to correlate the shallow soil conditions with
the shallow seismic survey. These data were used to establish the suitability of the
soil conditions for supporting the legs of the jack-up rig. In establishing the
suitability for jack-up rigs, soils need to be of sufficient strength to support the
weight of the legs but allow for some leg penetration to prevent the drilling rig from
sliding if subjected to high horizontal loading from environmental conditions. The
corehole data were also used to plan the setting of the shallow casing string. This
casing string acts as a foundation for the other casing strings.
C. Tow Efficiency Studies[6,7]
Previous experience offshore Alaska indicated that very high localized wind conditions
could develop quickly offshore, especially in close proximity to mountain ranges. The
jack-up rig was located in the Lower Cook Inlet and was to be towed south along the
Alaska Peninsula through Unimak Pass enroute to Norton Sound (Figure 4). A tow
efficiency study was initiated to determine the expected time to mobilize the vessel to
Norton Sound. Six different starting times were used beginning with the first week in
May and the first day of five successive weeks. Weather data used were for the years
1974 through 1979. The study showed very little variation in expected tow times because
the weather conditions were not expected to be severe.
One does not know for sure what weather conditions might be encountered during tow
operations. The tow efficiency study also documented "safe havens" along the route
where the rig could be diverted in the event of a storm of such magnitude which would
dictate going to a sheltered area. The results of the tow efficiency study and the
identified "safe havens" were reviewed with operator and contractor personnel involved
with the operations prior to the rig leaving the Lower Cook Inlet. Marine forecasts
were disseminated to the tug and barge personnel on a routine basis during the tow. No
adverse weather conditions were experienced during the tow to Norton Sound. The
expected tow time was 13 days but the actual tow time was 10.4 days. A tow speed of 4.7
knots in calm seas was used in the simulation. The approximate average tow speed was
S.2 knots.
On completion of drilling the C.O.S.T. well, there was the possibility of moving the rig
back to the Lower Cook Inlet. A similar tow efficiency study was made for moving the
rig to the Lower Cook Inlet using eight different starting dates from September 14
781

through November 12. The study indicated that adverse weather conditions could be
expected and possible delays enroute could result. The rig was subsequently
demobilized to Dutch Harbor in the Aleutian Islands and then released by ARCO. Tow time
from Norton Sound to Dutch Harbor was approximately 5.3 days.
While the rig did not have to be diverted to "safe havens", the above studies were
instrumental in informing the contractor and marine surveyor of the difficulty that
might be experienced in such tow operations offshore Alaska. For subsequent equipment
mobilization in these areas, ARCO will initiate similar studies.
D. Extreme and Normal Meteorological and Oceanographic Conditions[8,9]
Extreme meteorological and oceanographic conditions were determined for the open water
months of June through October to evaluate the suitability of the jack-up rig for use
at the particular water depth, in the given soil conditions. Marine surveyors make
calculations to assure (1) the rig will not fail by overturning, (2) environmental
forces will not exceed the yield strength of the legs, (3) there is sufficient soil
strength to prevent the lower support (cans or mat) from penetratin9 the soil further
during environmental loading and, (4) the rig will not slide along the sea floor under
certain environmental conditions. The calculations are made for certain calculated le9
and can (or mat) penetrations of the sea floor and for a given deck or barge clearance.
In areas where oil activities have been carried out for a number of years, the 50 year
extreme event storm is sometimes used. In frontier areas, some marine surveyors use 100
year extreme event storm data.
An extreme event analysis was made by hindcasting severe storms which affected the
Norton Sound area in the time frame of 1955 through 1978 (24 years). Extreme events for
various return intervals were determined. The 100 year return interval extreme event
was used to determine the suitability of the jack-up rig Dan Prince for operations in
Norton Sound.[10]
A hindcast of normal wind and wave conditions was made to determine the expected
environmental operating conditions in the open water months of June through October.
The hindcast involved determining wind and wave conditions for consecutive 6-hour
periods during a particular year. A total of three years were hindcast. These data
were used to determine (1) the expected environmental conditions when the rig arrived
on location, (2) if supply of the rig would be difficult due to expected environmental
conditions, and (3) the expected environmental conditions when operations would have to
be terminated.
782

Even though the environmental conditions for Norton Sound are milder than most areas
offshore Alaska, the hindcast study did provide data to evaluate potential problems
during raising the barge at the start of operations and lowering the barge at the end
of operations. Raising and lowering the barge of a jack-up rig requires mild sea states
in order to prevent high impact loads of the legs with the sea floor. The hindcast data
were also used to determine the feasibility of using a supply barge moored close to the
jack-up rig.
E. Logistics
Logistical operations involved the use of two helicopters, a helicopter base and
expeditor office at the Nome airfield, a 1ightering tug and barge for transporting
fresh water from Nome to supply boats offshore, 2 supply boats and a large supply barge.
Water depth at Nome (approximately 6 feet) prevented the use of regular draft supply
boats (16-19 feet draft) at the Nome harbor. The 1i9htering barge was approximately
120 feet long, 40 feet wide, 8 feet high and had a draft of approximately 3 feet.
A supply barge (400 ft. long, 76 ft. wide, 20 ft. high) was the major supply base for
drilling materials and was moored approximately three miles from the rig. The barge was
equipped in Seattle and towed to the well site. The barge was equipped with living
quarters, a sewage treatment plant, fresh water and fuel oil tanks, refrigeration
equipment, generator, crane, forklift, bulk storage tanks for mud and cement, casing
and miscellaneous drilling equipment.
Mooring requirements of a barge in an open seaway are greater than mooring requirements
of a barge in protected waters. A mooring study was made and a single point mooring
system was designed for the barge.[II] The mooring system was designed to withstand a
25-year return interval storm. In the event the mooring system failed, an emergency
anchor was attached to a wire1ine and winch system which would allow for rapid
deployment of the emergency anchor. The emergency anchor was incorporated into the
mooring system design to prevent the barge from drifting free in the event the primary
system failed. Also, a complete backup mooring system was available in the event the
primary mooring system was lost.
These supply arrangements allowed for activities to be conducted on the rig without
delays due to inadequate supplies.
783

Conclusions
The C.O.S.T. well was successfully completed on September 29, 1980. The utilization of
the ice studies, site survey and geotechnical data, meteorological and oceanographic
data and the supply scheme allowed for drilling personnel to utilize the drilling rig
and drilling scheme with a minimum of problems.
Acknowledgements
The authors wish to acknowledge the contributions of Kwang U. Park, ARCO Oil and Gas
Company and Gary M. Wohl, Oceanographic Services, Inc.
REFERENCES
1. Oceanographic Services, Inc: "Freezeup and Breakup Forecasting, Norton Sound",
July, 1979
2. Oceanographic Services, Inc.: "Norton Sound Ice Forecasting, 1979 Breakup and
Freezeup Predictions and Documentation", March, 1980
3. Oceanographic Services, Inc.: "Norton Sound Ice Forecasting, 1979 Breakup and
Freezeup Statistical Verification and Supplement", March, 1980
4. Oceanographic Services, Inc.: "Norton Sound Ice Forecasting, 1980 Breakup and
Freezeup Predictions and Documentation (Preliminary Report)", December, 1980
5. Woodward-Clyde Consultants: "Geotechnical Investigation Program and Siting
Study for Jack-up Rig Footing Behavior in Norton Sound, Offshore Alaska",
October, 1979
6. Oceanroutes, Inc.: "Tug/Tow Simulation; Kachemak Bay, Alaska To Norton Sound
Alaska," April,1980
7. Oceanroutes, Inc.: "Tug/Tow Simulation; Norton Sound, Alaska to Kachemak Bay,
Alaska," May, 1980
8. Oceanographic Services, Inc.: "Environmental Study Norton Sound," December,
1978
9. Oceanographic Services, Inc.; "Extreme Event Analysis, Norton Sound C.O.S. T.
Well Site," March, 1980
10. Noble Denton & Associates Inc.: "Self-Elevating Offshore Drilling Platform "Dan
Prince", Recommendations for Operations Offshore Alaska in Areas of the Cook
In 1 et and Norton Sound," February, 1980
11. J. Ray McDermott & Co. Inc.: "Mooring System Design for Supply/Storage Barge,
Norton Sound, Alaska," April 21, 1980
784

.....
'"
BERING STRAIT
NORTHERN
BERING SEA
AREA
3
ST. LAWRENCE AREA
2
NORTON SOUND
AREA
FIGURE 2. - ICE FORECAST AREAS
NORTON SOUND

LEGEND
Ice Concentration in Octas (8ths)
IF: Ice Free
OW: Open Water
F: First Year Ice
V: Young Ice
N: Nilas
O.W.
BERING STRAIT
6-7
l-:h',-SF
7-8
IV, 6-7
6-7
IV, S--6F
fiGURE 3 .. TYPICAL ICE CHART

788
FIGURE 4 . TOW SIMULATION ROUTING

J. R. Kreider
M. E. Thro
STATISTICAL TECHNIQUES FOR THE ANALYSIS OF
SEA ICE PRESSURE RIDGE DISTRIBUTIONS
Shell Development Company
Shell Development Company
U.S.A.
U.S.A.
A B S T R ACT
Techniques to obtain pressure ridge statistics are evaluated using stereoaerial
photography data from the nearshore Alaskan Beaufort Sea. Four ridge definitions
are compared. Similar probability distributions for ridge height result, but
significant differences for the number of ridges per mile occur. A first-order negative
exponential distribution is found to fit the sail height data. A Type I Extreme
Value Distribution for maximum ridge heights is proposed and found to fit existing
data.
I N T ROD U C T ION
In April 1979, Gulf Research and Development Company, as administrator for a
joint industry project supported by eight oil companies, acquired stereo-aerial photography
in the nearshore Alaskan Beaufort Sea [1]. Centerline profiles of these photographs
were digitized, from which statistics on first-year pressure ridges and rubble
were computed. These statistics can be used to establish extreme ridge thicknesses
for structural design and to plan surface logistics for winter operations.
Determining pressure ridge statistics requires a ridge definition to identify
independent ridges. Several different ridge definitions and probability distributions
for ridge height have been used previously. The objective of this paper is to
assess the effect of analysis technique on the calculated ridge statistics.
A N A L Y SIS T E C H N I QUE S
D a t a S e 1 e c t ion
Figure 1 shows the flightlines in the nearshore Alaskan Beaufort Sea near
Prudhoe Bay. Centerline profiles are digitized from one-mile stereo models of the
photographs [1]. Fifty percent of the total line length is digitized by alternately
digitizing and skipping one-mile segments.
789

z
0
I-
U
z
::>
l.L
>-
I-
(f)
Z
W
0
>-
I- 10-2
-.J
!D
([
!D
0
a:
D..
10-3
3
o SOX RRYLE I GH
D 2-FT DROP
+ RRYLE I GH \I ITH
7" SLOPE
)( \-FT TROUGH
ELEVATION
)(
W x
6- x
S = 0.625 FT -\---
5 7 9 11
RIDGE HEIGHT
h (FT)
Figure 2a
13 0 100 200
R lOGE HE I GHT
SQUARED h 2 (FT2)
Figure 2b
given in Table 1, indicate that both distributions should be accepted for all defini­
tions at the 95 percent significance level.
For the nearshore zone, the data fit equation (3) up to a ridge height of
approximately ten feet. At larger ridge heights, a few single ridges distort the
distribution. This distortion may be due to a relatively small data sample or to the
fact that the larger sail heights are associated with grounded ridges, which will have
proportionately larger sails for the same volume of deformed ice.
Previous investigators have found both definitions to apply to sail and keel
data. Hibler et a1. [10] finds that equation (1) fits both keel depth and sail height
distributions in the Central Arctic Basin. Wadhams [5] finds that equation (1) fits
keel data and that equation (3) fits sail data for the same area in the Arctic Ocean
northeast of Greenland. Wadhams [4] and Tucker et a1. [6] find that equation (3) best
fits sail data in the coastal Beaufort Sea. Our results indicate that equation (3)
provides a better fit, although both distributions pass a Chi-squared test.
ENG I NEE R I N G CON SID ERA T ION S
Rid g e S ail S tat i s tic s
An important consideration for engineering applications is the probability
of exceedance, or nonexceedance, for a given ridge height. If the probability dis­
tributions for ridge occurrence and ridge height are known, the probability of non­
exceedance can be calculated as follows:
793

5. Wadhams, P. 1977. "A Comparison of Sonar and Laser Profiles along Corresponding
Tracks in the Arctic Ocean". AIDJEX/ICSI Symposium. Sea Ice Processes and
Models, University of Washington Press, 283-299.
6. Tucker, W. B. III, W. F. Weeks, and M. D. Frank. 1979. Sea Ice Ridging over the
Alaskan Continental Shelf. CRREL Report 79-8.
7. Tucker, W. B. III and V. H. Westhall. 1973. Arctic Sea Ice Ridge Frequency
Distribution Derived from Laser Profiles. AIDJEX Bulletin, 21, 171-180.
8. Hibler, W. D. III, S. J. Mock, and W. B. Tucker III. 1974. Classification and
Variation of Sea Ice Ridging in the Western Arctic Basin, Journal of Geophysical
Research, 79, 18, 2735-2743.
9. Lowry, R. T. and P. Wadhams. 1979. On the Statistical Distribution of Pressure
Ridges in Sea Ice. Journal of Geophysical Research, 84 (C5), 2487-2494.
10. Hibler, W. D. III, W. F. Weeks, and S. J. Mock. 1972. Statistical Aspects of
Sea-Ice Ridge Distributions, Journal of Geophysical Research, 77 (30), 5954-
5970.
11. Hibler, W. D. III. 1975. Statistical Variations in Arctic Sea Ice Ridging and
Deformation Rates. Proceedings of the Ice Technology Symposium, Montreal, 9-11,
April, pp. Jl-J19, Society of Naval Architects and Marine Engineers, New York.
12. Keinonen, A. 1976. The Shape and Size of Ridges in the Baltic According to
Measurements and Calculations, Winter Navigation Research Board Report No. 17,
Helsinki, Finland.
13. Ackley, S. F., W. D. Hibler III, F. K. Kugzruk, A. Kovaks, and W. F. Weeks.
1974. Thickness and Roughness Variations of Arctic Multiyear Sea Ice. AIDJEX
Bulletin, 25, 75-96.
798

Gordon F.N. Cox·
W.S. Delm
Abstract
SlM1ER ICE CONDITIOOS IN 1HE
PRUDHOE BAY AREA, 1953-75
US Army Cold Regions Research
and Engineering Laboratory
Sea Ice Consultants
A detailed knowledge of the summer ice conditions is required for planning offshore
petroleum operations in the Prudhoe Bay area. Statistics on breakup and freezeup
dates and the number of open water days are needed to plan and assess the feasibility
of fill island and causeway construction, pipeline laying, platform installation,
and other summer activities such as seismic vessel operations. Breakup and freezeup
data are also needed to schedule winter operations.
Long-term, site-specific statistics on the summer ice conditions in the Harrison
Bay - Camden Bay area are presented in probalistic terms. The statistics are based
on twenty-three years of ice observations acquired by commercial ships and icebreakers,
ice reconnaissance flights, and various satellites. Data is given on
breakup and freezeup dates, the first occurrence of open water, and the number of
continuous and total open water days. The impact of the summer ice conditions on
petroleum activities in the study area are also briefly discussed.
*Most of this work was prepared while employed by Amoco Production Company,
Research Center, Tulsa, OK. 799
USA
USA

1. Introduction
A historical perspective of the summer ice conditions along the north coast of Alaska
is needed to evaluate the feasibility and cost of operating in that area during the
summer. Long-term statistics on river break-up and overflow, sea ice break-up,
floe size, ice concentrations, and pack ice invasions are needed for planning: barge
and ship navigation; seismic vessel operations; platform towing and installation;
gravel island and causeway construction; and pipe laying operations. Break-up and
freeze-up data are also needed to schedule winter operations.
In 1976 the Research Department of Amoco Production Company and Sea Ice Consultants
performed an in-depth analysis of the summer ice conditions along the central,
third portion of the north Alaskan coast. The study area extended from Harrison
Bay to Camden Bay, out to the continental slope (Figure 1). Twenty-three years
(1953 to 1975) of summer ice conditions data were compiled for the study area from
all available data sources and site-specific ice statistics were generated for
twenty sites inside the 20-meter isobath. The results were later made available
to interested parties as Alaskan Oil and Gas Association, Project 35.
This paper discusses the data and methods used in the Amoco study. Statistics on
sea ice break-up and freeze-up dates, the first occurrence of open-water, and the
number of continuous and total open-water days are presented for each of the twenty
sites. The impact of the summer ice conditions on petroleum activities in the
study area is also discussed.
2. Previous Work
Regional information on the summer ice conditions off the Alaskan north coast has
been summarized by Potocsky (1). Potocsky used the Naval Oceanographic Office
Annual Ice Reports to calculate the mean, median, ranges of the l5-day mean, and
extreme southern and northern positions of the pack ice edge. This was done for
semi-monthly periods at 50 intervals of longitude along the coast. Landsat
satellite imagery have also been used by several investigators to determine the
extent and variation of the open-water season off the central Alaskan coast (2,3).
While these studies provide an overview of the ice conditions, the results do not
lend themselves to detailed site-specific analyses. The ice charts in the Navy
annual ice reports which were used by Potocsky only provide a regional description
of the ice conditions. Even though the resolution of Landsat emagery is more than
adequate, the Landsat data base is limited by few years of operation and the low
frequency of passes over an area of interest. Often the ice is obscured on Landsat
imagery by cloud cover.
3. Data Sources
In order to obtain an adequate data base for detailed, site-specific analyses of
the summer ice conditions, it was necessary to consider all available ice data
sources. The data sources used in this investigation are listed below:
800
1. Original U.S. and Canadian ice observer flight logs and messages
2. U.S. and Canadian annual ice reconnaissance reports
3. Commerical and government ship reports
4. Satellite imagery
a. NOAA and ESSA SR (Scanning Radiometer)

00
o
....
71°30'
71°
70°30'
70 0
N
Beaufor Sea
Figure 1: Study Area and Twenty Sites Chosen for Statistical Analyses.
144°W
71° 30'

00
o
'"
71"30'
70"30'
70"N
o 7
T5T
Beaufor Sea
4MY
3FT
PD3
Figure 2: Example of an Ice Chart Used in This Study, WM) Nomenclature
is used to describe the ice conditions (7),
Undercast
144"W
144"W
71"30'

Because of a lack of observational data for some periods, approximately 5% of the
ice charts were prepared by hindcasting the ice conditions. Hindcasts were performed
using surface pressure charts, mean wind data, and temperature records. Previous and
subsequent ice conditions were taken into consideration.
After the ice charts were prepared, data on average weekly ice concentration, ice
type, and floe size were obtained for 20 sites in the study area (Figure 1) and
presented in time series form for subsequent statistical analyses. One week was
defined as a six-day period so that the results could be compared to the U.S.
Navy annual ice reports. The first period or week represents the first six days in
June, and so on (Table 1).
Period Time
1 June 1 to 6
2 7 12
3 13 18
4 19 24
5 25 30
6 July 1 to 6
7 7 12
8 13 18
9 19 24
10 25 30
11 31 5
12 August 6 to 11
13 12 17
14 18 23
15 24 29
16 30 4
17 September 5 to 10
18 11 16
19 17 22
20 23 28
21 29 4
22 October 5 to 10
23 11 16
24 17 22
25 23 28
Table 1: Time during summer season corresponding to a given period.
5. Statistical Ana1lses
Various types of statistical analyses were performed as the required ice
statistic depends on the nature of the operation of interest. Due to space
limitations on the length of this paper, only a few general statistics will be
presented here. These include statistics on break-up, the first occurrence of
open-water, freeze-up, and the number of continuous and total open-water days
804

during the summer. Other useful statistics for a 10-meter water depth site inside
the barrier islands near Prudhoe Bay have been presented by Wheeler (S).
The past occurrence of break-up by a given period was determined for each of the
20 sites in the study. Break-up was defined as the first time the ice
concentration dropped below S oktas (S/S ice cover) following the winter season.
The results are presented in probalistic terms in Table 2. For example, there is
a 10% probability that break-up will occur by at least the 4th period (June 19 to
24) at Site 1. The occurrence of the earliest and latest observed break-up are
also given.
Similar statistics on the first occurrence of open-water and freeze-up are presented
in Table 2. The first occurrence of open-water was defined as the first time the
ice concentration dropped below 1 okta (l/S ice cover) following the winter season.
Freeze-up was defined as the first-time the ice concentration equalled S oktas
and remained S oktas until the twenty-fifth period or until data for that year were
no longer available.
The probability of having a given number of continuous and total open-water six-day
periods was also computed for each of the 20 sites. The number of continuous
open-water periods was defined as the maximum number of consecutive open-water
periods without any intervening pack ice invasions. The total number of open-water
periods included all the open-water periods during the summer regardless of number
of pack ice invasions. These results are summarized in Table 3. For example, at
Site 1, there is a 50% probability of having 3 or more continuous open-water periods
and 5 or more total open-water periods during the summer.
6. Discussion
Examining the 50% probability data in Table 2, it appears that on the average
break-up in the study area takes place during the 6-th and 7-th periods, that is,
between July 1 and 12. The ice first begins to break-up offshore and about a week
later begins to break-up along the coast and inside the barrier islands. In general,
break-up in the area has been observed to have occurred as early as the middle of
June and as late as the beginning of August.
The first appearance of open-water in the study area is more variable. The 50%
probability data in Table 2 indicate that, on the average, open-water first appears
close to the coast between July 7 and IS. Farther offshore the first occurrence
of open-water does not usually take place until August 12 to 23, about a month
later. Even though the ice first begins to break-up offshore, ice deterioration
is accelerated near the coast due to river over-flooding of the sea ice. Both
water and sediment on the ice surface reduces the ice albedo and enhances melting.
There also appears to be a tendency to have open-water offshore (close to the
20-meter isobath) first in the eastern part of the study area. The earliest
open-water has been observed near the coastal sites is between June 25 and 30.
It should be emphasized that even near the coast there are about one in ten years
with no open-water and, farther offshore near the 20-meter isobath, as many as
one in four years may have no open-water.
The ice cover usually freezes over and remains in tact by the 22-nd and 23-rd
periods. Freeze-up first takes place between October 5 and 10 near the coast and
about a week later offshore. In general, the earliest freeze-ups have occurred
between September 17 and ::lS, and about one in four years, freeze-up is not
permanent until after October 2S.
805

00 PROBABILITY OF OCCURRENCE
0
a-
BREAK-UP FIRST OPEN-WATER FREEZE-UP
SITE 10% 50% 90% E L 10% 50% 90% E L 10% 50% N-F E
1 4 6 10 4 10 9 13 18 9 9%* 20 22 25% 20
2 4 7 9 5 10 7 8 11 5 14 20 22 12% 20
3 4 6 10 4 11 11 16 9 20%* 20 23 27% 20
4 5 7 10 3 10 9 13 19 7 9%* 20 23 27% 20
5 5 6 8 5 10 5 7 9 5 11 20 22 11% 20
6 4 6 10 4 11 10 14 9 27% 20 23 25% 20
7 5 7 9 5 10 6 8 10 5 13 20 22 17% 20
8 5 6 9 3 10 5 7 9 5 11 20 22 12% 20
9 5 7 9 4 10 9 12 18 8 9%* 20 23 22% 20
10 5 7 9 4 10 8 11 8 12%* 21 23 22% 20
11 1 6 9 1 10 11 15 5 15%* 20 22 27% 19
12 5 7 9 5 10 7 9 11 6 13 20 22 12% 20
13 6 7 9 5 10 8 10 15 5 9%* 20 22 20% 20
14 4 7 9 4 10 8 11 15 5 5%* 20 22 20% 20
15 2 6 9 1 10 9 15 5 19%* 20 22 27% 19
16 5 7 9 5 10 5 8 11 5 15 21 22 12% 19
17 4 7 9 3 10 9 12 5 15%* 20 23 22% 19
18 3 6 9 2 9 9 13 20 5 9%* 19 23 22% 19
19 2 6 9 2 10 8 14 20 5 9%* 19 23 22% 18
20 5 7 9 4 9 7 10 14 7 4%* 20 22 12% 20
10% E Earliest
50% Percent Probability L Latest
90% * Percent of years with no open-water
N-F Percent of years with freeze-up after the 25th period
Table 2: Period of break-up, first open-water, and freeze-up at different probability levels.

PROBAILITY OF OCCURRENCE
CONTINUOUS OPEN-WATER TOTAL OPEN-WATER
SITE 10% 50% 90% 10% 50% 90%
1 10 3 1 11 5 1
2 16 10 4 16 11 7
3 10 4 0 10 4 0
4 11 3 1 12 4 1
5 15 12 5 15 12 8
6 10 3 0 10 5 0
7 15 7 3 15 11 7
8 15 12 5 15 12 9
9 12 5 1 12 6 1
10 12 4 0 14 6 0
11 11 3 0 11 4 0
12 14 7 2 15 9 5
13 13 4 1 13 7 1
14 12 4 1 14 6 3
15 9 3 0 10 4 0
16 14 7 4 14 10 5
17 10 3 0 11 5 0
18 11 4 1 11 5 1
19 11 4 0 12 5 0
20 12 5 1 13 7 1
Table 3: Number of 6-day periods with open-water
at different probability levels.
Data on the number of open-water periods in Table 3 show that, on the average,
there are about SO to 70 days of open-water close to the coast and inside the
barrier islands. As one moves offshore to more exposed areas inside the 20-meter
isobth, the number of open-water days rapidly decreases to about 20 to 30 days.
There are even fewer consecutive days of open-water without pack ice invasions.
At the 90% probability level, we only have 30 to SO days of open-water close to
the coast and up to 6 days offshore.
The variability and limited number of open-water days offshore have a serious
impact of offshore arctic petroleum operations in this area. Due to the variability
in break-up and freeze-up dates, scheduling both winter and summer operations is
difficult. It is necessary to begin an operation early while recognizing that
the operation may not even get off the ground. Early mobilization of equipment
and personnel and delays increase operating costs.
The limited number of open-water days and time lost due to equipment failures and
storms may require that projects, such as artificial gravel island construction, be
conducted over several seasons. A fill island that can be constructed in 30 to SO
days in MacKenzie Bay (9) may take two or more years to build in the Prudhoe Bay
area. Winter standby costs will further increase the cost of construction.
807

1.0 Introduction
The successful design and operation of marine structures is highly
dependent on a complete knowledge of the wave climate. The term "marine
structures" is used here to include coastal works and offshore platforms,
as well as vessels, ranging from sailing boats to VLCC's. Since the
design and operation of these structures relies heavily on the results
of physical model experiments, hydraulics laboratories and ship towing
tanks can be expected to be equally interested in having more complete
definitions of the wave climate available, as an input to their model
studies.
Unfortunately similar statements have been made by many other
authors, at many times during the last few decades, but the number of
structures which have failed, or the number of vessels which have capsized,
due to the occurrence of "unexpected wave conditions", is still
much larger than the total number of pleas which have been made to
increase and to improve the measurement systems of wind generated water
waves.
Even today there is no area of the world's oceans, where adequate
observations of the sea state are being made, or have been made over
sufficiently long periods of time, to provide a satisfactory estimate
of the wave conditions. Some wave recording stations have indeed been
operational for several decades and their records can be used to predict
with a good degree of accuracy the wave heights and periods, which are
expected to occur in the areas of those stations with a given frequency
of occurrence. The safe design and operation of marine structures,
however, requires a much more complete knowledge of the wave climate
than the "wave height" and the associated "period". The present trend
of designing and building structures in depths of water well beyond the
breaker zone, has led to the requirement of defining the design wave
conditions in much greater detail than was considered necessary only
ten or twenty years ago.
Some of the additional parameters which are known to be important
in the design of marine structures include the wave steepness, the
asymmetry of wave profile, the joint distribution of wave heights and
periods, the wave direction, the crest lengths of waves, wave grouping
(amplitude and period) and the spectral shape. While some of these
additional parameters can still be obtained from re-analysinghistorical,
instrumented wave records, others first require the development of more
sophisticated instrumentation packages.
810

There exists therefore today an urgent need not only to enlarge
significantly the extent of wave measuring programmes, but also to
develop new instrumentation packages and more comprehensive analysis
programmes, so that in the future design wave conditions can be predicted
more completely and more reliably.
The following presentation will trace the development of some of
the above listed factors and consider suggested definitions or techniques
aimed at improving the analysis of wave records.
2.0 Wave Modelling Techniques
Much of the need for a more complete description of the wave climate
is a direct result of laboratory experiments, particularly of model
tests of large, deep water structures. The use of irregular waves has
identified many factors, other than the wave height and period, which
affect the design and operation of marine structures.
Variable speed, electric motors, traditionally used to drive wave
boards in laboratory wave tanks or basins, have been replaced by much
more versatile and powerful hydraulic actuators. The use of fast,
digital computers to control these hydraulic systems has made it possible
to reproduce even the most complex wave patterns. Indeed, the problem
of producing realistic model sea states has been moved from the laboratory
to nature. It is now largely the lack of a more complete definition
of the ocean wave conditions which limits the development of techniques
to simulate accurately ocean sea states.
A survey in 1979 of some 250 institutes [11 indicated that about
50% of the 150 respondents to a questionnaire on wave generation and
analysis techniques, reported irregular wave generating facilities.
Interestingly, the hydraulics laboratories reported a considerably higher
utilization of their irregular wave equipment than the ship towing tanks.
This difference is probably partly due to the different nature of ship
model testing, mostly a fairly linear process, as compared to the testing
of coastal and offshore structures, or for that matter the behaviour
of large, moored vessels, where the non-linear effects become very important.
It may, however, also be because of a basic difference in the
philosophy of wave model testing, the deterministic versus the random
approach. Johnson and Takezawa [21 review the various methods presently
used, in a state-of-the-art paper at this years' International Towing
Tank Conference.
Whatever methods are used to generate irregular waves in a laboratory
tank, the design of marine structures has benefitted greatly from
811

To obtain a direct measurement of the wave steepness requires a more
sophisticated sensor than is presently available. The ongoing development
work to produce a sensor capable of determining wave direction will
probably also be able to record the wave steepness directly.
Kjeldsen further makes a plea for using the zero-down crossing
analysis, rather than the zero-up crossing method, arguing that the
zero-down crossing analysis produces a wave height which is physically
more relevant, in particular with regard to the capsizing of vessels
and shock pressures on structures.
Another example of the impact of using irregular waves can be found
in the effect of wave grouping on the design of structures. The phenomenon
of wave grouping was known to affect moored vessels, harbours or
other coastal structures with a natural response period close to the
wave grouping period. More recently, laboratory wave tests have confirmed
that the reliability of rubble mound breakwaters is also greatly
affected by the occurrence of wave grouping [6,7]. Again, by means of
laboratory experiments, new parameters have been defined, capable of
describing the sea state more completely.
The Danish Hydraulics Institute has reported the results of experiments
using short-crested waves [8], and their effects on the mooring
forces of vessels. They concluded that in order to evaluate the feasibility
of using exposed offshore mooring platforms, it is essential to
use a three-dimensional wave generation system, which simulates the
short-crested waves. No wave measuring system is presently available
to provide prototype information of the crest lengths of wind generated
water waves.
Much improvement is already possible, however, by more extensive
analysis procedures of instrumented wave records. The routine reporting
of additional parameters will contribute significantly to a better
understanding of the required input wave conditions for model tests. As
soon as possible, international agreement should be reached on the definition
of such additional parameters and which of these to include in
the normal wave climate reporting procedures. Only the most important
of these have been included in the following chapters.
3.0 Wave Heights and Wave Periods
The selection of input wave conditions for the design of marine
structures requires a knowledge of the short term statistics, as well
as the long term statistics or extreme values of the various parameters
defining the wave climate. Many papers and books have been written
813

on both subjects, but today there still exists a great deal of contro­
versy on which method to use.
The long term statistic methods fall into two categories. The
simplest procedure, mostly used by engineers, is to plot all available
measured data on some carefully selected graph paper, so that the data
will most closely follow a straight line. Commonly used probability
laws on which the scales of graph paper are based, are log-normal,
Weibull, Gumbel or Rayleigh distributions. Lately, the Weibull distri­
bution appears to find the greatest following and several studies of
long periods of wave records have indicated good fits.
The second method of determining extreme values involves the use
of a model, which has been calibrated for a data base of recorded wave
parameters. The most frequently used approach is the hindcasting technique;
some of the recent models in this method involve reasonablyaccurate
descriptions of the physical wave generation process. Both methods
have their problems and limitations. Common to both, is the problem
that most data bases are too short and that often the available data
are from a different population than for which the extreme values are
required.
The short term features of wave conditions involve not only the
distribution of individual wave heights within a storm, but also of
course the distribution of wave periods, jointly with the wave heights,
the wave steepness as already mentioned earlier, wave grouping phenomena
and spectral shapes.
Longuet-Higgins paper in 1952 [9] on the distribution of the
heights of sea waves has become a classic. He derived that the probability
density of the wave height is given by the Rayleigh distribution.
He also discussed in various subsequent papers the distribution of wave
periods and in 1975 published a paper on the joint distribution of wave
periods and wave heights [10]. Fig. 2 shows graphically this joint distribution.
Longuet-Higgins and several others have compared the theo­
retical curves with recorded data sets for various parts of the ocean
and in general good agreements have been found.
4.0 Spectral Shapes
The use of irregular waves for model testing is largely based on
a spectral input, although this method has some serious limitations.
The two most widely used spectral formulations are the Pierson-Moskowitz
[11] and the Jonswap spectra [12].
8"

or in words: GF is the standard deviation of the SIWEH about its mean
and normalized with respect to this mean. Fig. 3 illustrates some
examples of wave trains with a common spectral density function, but
different grouping factors 1 also shown are the SIWEH spectra for these
wave trains, which indicate substantial differences.
These proposed formulations for describing wave grouping are find­
ing a certain amount of support from other institutes [16].
The definition of the grouping factor, using the SIWEH-spectral
density function does not only provide a valuable new parameter to des­
cribe the wave conditions, but it can also be used to generate sea
states in laboratory wave tanks which have the same degree of wave group­
ing as measured in a particular area. This is most important to improve
the design and operation of marine structures.
6.0 Conclusions
The increasing requirement for designing and building structures
in deep water has led to the need for a more complete definition of the
wave climate. Much work has been done over the past few years to
develop new analysis techniques and to formulate new models or parame­
ters. The most urgent requirement at the present time appears to be
reaching international agreement on a new set of parameters, which
should be included routinely in all wave data analysis programmes. The
sooner such an agreement is reached, the earlier new data bases can be
developed, either from existing wave records or from new recordings,
which can then be used to develop short and long term statistics of
these new parameters.
References
1. Ploeg, J. and Funke, E.R., "A Survey of 'Random' Wave Generation
Techniques", Proc. Seventeenth Coastal Engineering Conference,
Sydney, Australia, 1980.
2. Johnson, B. and Takezawa,S., "State of the Art Review of Irregular
Wave Generation and Analysis", 16th International Towing Tank
Conference, 1981.
3. Funke, E.R. and Mansard, E.P.D., "SPLSH - A Program for the Synthe­
sis of Episodic Waves", National Research Council, Hydraulics
Laboratory Technical Report LTR-HY-65, 1979.
4. Kjeldsen, S.P. and Myrhaug, D., "Interaction and Breaking of
818
Gravity Water Waves in Deep Water", River and Harbour Laboratory,
Trondheim, Norway, 1978.

5. Kjeldsen, S.P. and Myrhaug,D., "Kinematics and Dynamics of Breaking
Waves", River and Harbour Laboratory, Trondheim, Norway, 1975.
6. Johnson, R.R. et aI, "Effects of Wave Grouping on Breakwater
Stability", 16th Coastal Engineering Conference, Hamburg, Germany,
1975.
7. Goda, Y., "Numerical Experiments on Wave Statistics with Spectral
Simulation", Port and Harbour Research Institute, 1970.
S. Kirkegaard, J. et aI, "Effects of Directional Sea in Model Testing",
ASCE Specialty Conference, Ports 'SO, 19S0.
9. Longuet-Higgins, M.S., "On the Statistical Distribution of the
Heights of Sea Waves", Journal of Marine Research, 1952.
10. Longuet-Higgins, M.S., "On the Joint Distribution of the Periods
and Amplitudes of Sea Waves", Journal of Geophysical Research,
1975.
11. Pierson, W.J. and Moskowitz, L., "A Proposed Spectral Form for
Fully Developed Wind Seas Based on the Similarity Theory of
Kitaigorodskii", Journal of Geophysical Research, 1964.
12. Hasselman, K. et aI, "Measurements of Wind-Wave Growth and Swell
Decay during JONSWAP", Deutsche Hydrographische Zeitschift, 1973.
13. Thompson, E.F., "Energy Spectra in Shallow U.S. Coastal Waters",
Coastal Engineering Research Centre, Fort Belvoir, 19S0.
14. Funke, E.R. and Mansard, E.P.D., "Synthesis of Realistic Sea States
in a Laboratory Flume", National Research Council, Hydraulics
Laboratory Technical Report LTR-HY-66, 1979.
15. Funke, E.R. and Mansard, E.P.D., "'Random' Wave Generation by
Means of a Generalized Computer Software System", Proc. XIX IAHR
Congress, New Delhi, India, 19S1.
16. Houmb, O.G., "On the Wave Climate of the North Sea and the Problems
of Determining Design and Operational Conditions for this Area",
Wave Information Workshop, Bedford Institute of Oceanography,
Halifax, 19S0.
S19

Mass transportation of water in the direction of wave propagation is
an inherent feature of wave action and Wiegel and Johnson provide the
following expression for its velocity:
V max
in which z is the water depth (rated negative from the water level
downwards). This expression has been substantiated by observations [2J.
,-
Since "H/L equals the maximum gradient of the water surface it
follows that the velocity of mass transportation will decrease rapidly
with decreasing wave steepness.
The theoretical upper limit of the H/L ratio equals 0,14 or 1/7 as
determined by Mitchel and Havelock and quoted by Wiegel and Johnson [2J.
When testing with plunger type wave machines it was not possible to
produce waves of such steepness but waves with H/L = 1/10 could be
produced.
In the wave period range of 1.5 to 2.5 seconds waves of steepness 1/10
produce surface mass transport of more than 1,000 meters per hour and
waves of steepness 1/20 produce 200 to 300 meters per hour. In
comparison swell waves of this period when occurring in nature would
have waves of steepness about 1/30 and provide surface mass transpor­
tation of only 50 meters per hour which would be difficult to
destinguish from random currents often present.
Decay of wave energy and wave height due to fluid friction is very
small and in the wave range being considered here would be insignificant.
Wiegel and Johnson [2J.
The viscosity of water increasingly charged with ice slush would
increase but the resulting damping is still small for waves of any
appreciable length.
The wave pattern generated by a machine or several machines in line
form a wave train. Wave height reduction is caused by diffraction of
waves or spreading of the wave train during progression as it occurs
when waves move through a gap into a sheltered region. Relative
diffraction losses will be reduced when the breadth of wave generation
is increased. We have found that a breadth of wave generation equal to
approximately 6.5 L will suffice to provide a far-reaching wave train
as also confirmed by calculation procedures.
822
(2)

with wave machine breadth equal to 6.5 L, the wave height reduction
along centre line will reach 0.6 H at a distance from the generator of
approximately 75 L but further reduction of wave height down to 0.5 H
will take place only very slowly over the next 1000 L so that one half
wave height will be mainlained over a very long distance.
3. Suspended Ice in the Wave Train
In agitated water the initial ice formations, discoids, are very small
but from these will grow the needle - like fragments or spicules
recognized as frazil ice. Masses of spicules form slush ice.
Due to the relatively high deformation drag associated with the motion
of small particles in a viscous fluid, the rising velocity of the
smallest ice pieces are negligible in comparison with the mixing
currents and it is only the larger ice pieces that tend to accumulate
at surface due to buoyancy forces. For this reason ice may be distri­
buted throughout a surface layer of considerable thickness.
Studies of ice formation in water that has been pre-cooled to the
freezing point temperature and then subjected to surface cooling and
wave action was carried out in conrection with earlier wave tests as
discussed in "Ice Free Harbours" [1] in which primitive tests of rough
accuracy are discussed. The amount of ice formed under wave action was
found to be of the order of 250 grammes/hour per square meter of
surface for each 1 0 Celcius that the air temperature is below the
freezing point.
4. Ice Transportation Capacity of the Wave Train
The capacity of the wave train for transporting surface formed ice
would depend not only on the velocity of its transportation but also
on the capacity of the surface waters for holding formed ice in
suspension. As the amount of ice in the wave train grows, a greater
proportion of it would congregate near the surface and a layer
extending from surface to the level where the energy level has been
halved coinciding with the halving of the mass transportation velocity
would contain a great majority of the suspended ice.
In order to obtain a practical basis for comparing wave trains of
different make-up we have chosen to define this layer thickness as the
"active layer" of the wave train and to define as the "Reach" of
823

the wave train the distance covered by the mass transport by the time
the ice content of the active layer has reached ten percent (10% ice
spicules and go% water).
The active layer thickness would by this definition be derived from
and becomes
e4fiz/L = 0.5
Z = -0.055L
a
The surface velocity of the mass transportation according to (2) will
be decreasing along the wave train towards
_ 1 2
V s = ( II 20) x C = 0.025 C
and average velocity of the mass transportation within the limits of
the active layer can be estimated as
Va = 0.02 C
Let RK denote the "Reach" in meters of the wave train at a given
design temperature K in 0 Celcius and let t denote the time variant
in hours, then
(3)
RK V
a
x t x 3600 72 x t
and -K 250 t 0.1 10 6
x x = x Z
a
x
or
RK
1580
L C
=K
(meters) (4)
Since basic concepts regarding ice contents and layer thickness were
arbitrarily chosen the expression (4) should only be used for the
purpose of comparing wave trains. Absolute expressions of capabilities
of waves can only be developed with increased knowledge of ice
behavior in ,waves.
5. Practicable Wave Machines
Wave machine installations for ice prevention in harbours, channels,
navigation lock approaches, stagnant waters etc. must be designed to
suit basic requirements including
Power efficiency and automation
Infrequent and easy maintenance
Survival against environmental forces
Model tests with floating wave machines were carried out several years
ago as discussed in "Ice Free Harbours" llJ. Based on one of the models
824

6. Ice Removal by Wave Train, Tabulation
The table of wave trains is based on calculated results and generally
accepted formula and data. The column REACH is prepared to compare the
ice clearing capacities of different wave trains under Arctic condi­
tions but should not be considered as an absolute measure of these
capacities.
x xx xxx
Wave Characteristics Wave Train Wave Machine Installation
Period Velo Length Height Mean REACH Com- Dis- Horse
-city Trans ato bined place -power
-port -30 C Breadth -ment
x
Sec mlsec m m mlhour m m Tonnes HP
1.5 2.34 3,51 0.35 168 433 3x 8m 10 T
1. 75 2.73 4.78 0.48 196 689 3x12m 28 T
2.0 3.12 6.24 0.62 225 1,030 3x12m 48 T
2.25 3.28 7.37 0.74 236 1,280 4x12m 95 T
2.5 3.90 9.75 0.98 281 2,010 3x20m 193 T
3.0---- 4.68 14.0 1.40 337 3,470 3xl30m 600 T
9 HP
25 HP
45 HP
90 HP
260 HP
980 HP
3.5 5.46 19.1 1.91 393 5,510 3x40m 1480 T 2700 HP
4.0 6.24 25.0 2.50 449 9,870 3x50m 3160 T 6400 HP
The table is based on all waves starting out with the same original
steepness of H/L = 1/10
xx Mass transportation figures represent the calculated mass transport
velocity in the wave train at a distance from the wave generators of
at least 100 L and a depth below surface of 0.055 L.
The REACH is based on _2 0 C cold sea water and a continuing air tempera­
tur of -30 0 C and represents the distance from the wave machine covered
by the mass transport before or by the time the ice content of the sur­
face waters has reached ten percent (10% ice spicules and 90% water).
xxx A wave machine installation as planned may consist of several,
usually three, individual machines each with its own engine, but,
alligned and linked together and attached to the sea bottom. The Dis­
placement Tonnages represent the weight of equipment, pontoons and
machinery of all the wave machine installation. The horsepower figures
represent the combined power rating of all of the engines of the
installa ticn.
827

eventually have to cope with either
Fast ice
Open pack ice
Closed pack ice
The first two conditions hold no problems because the wavetrain can
maintain an open channel through stationary ice and the disposal rates
for the slush ice amongst the ice floes at sea are relatively small.
However, icebreaker service will be needed to maintain access if
closed pack ice moves across the harbour antrance.
It is estimated that inflow of ice into the harbour due to moderate
tidal inflow can be held at bay by the disposal wave train because
Doppler effect will steepen the waves against counter flow.
8. References
1 Andersen, Per F. (1972) "Ice Free Harbours" The Engineering
Journal, July-August 1972. The Journal of the Engineering
Institute of Canada.
2 Wiegel, R.L. and Johnson, J.W. (1951) "Elements of Wave Theory"
Proceedings First Conference Coastal Engineering, Long Beach,
California.
829

WIllDS AIID WAVES III LAllCASTD SOUIID
A11IOSPIIBRIC DVIIlOIIIIBlIIT SnVICE DOiflfSVIBW, OIlTARIO,
CAIIADA
This paper presents an overview of a derived wind and wave climatology of
Lancaster Sound in the Canadian Arctic.
Hourly wind speeds were synthesized at three locations in Lancaster Sound.
The geostrophic equations were used to obtain wind data from 33 years of gridded
6-hourly sea-level pressure data compiled by the Fleet Numerical Oceanography
Center in Monterey, California. Hourly wind values were obtained by linear inter­
polation of the 6-hourly 'u' and 'v ' components of the pressure gradient. The 21-
year wave hindcasts were based on the Bretschneider equations using geographical
features and average minimum monthly ice conditions to limit the fetch.
Statistics of the wind and wave climatologies are presented in terms of fre­
quency distributions and extremes.
There are obvious drawbacks in using gridded pressure as a data base. Many of
these drawbacks are identified and some are examined in detail. As part of this
examination, comparisons of derived data versus real data at Resolute and Ocean
Weather Station Bravo are made and case studies of particular significant storms
in the Lancaster Sound - Baffin Bay area are discussed.
830

1. BACGIlOIIIID
The increasing interest in offshore oil and gas exploration during the 1970's
and 1980' s has led to a nlDDber of studies which seek to determine the possible
hazards to and from the environment associated with exploration activities. One of
the areas currently of high interest is Lancaster Sound.
Lancaster Sound is located between Devon Island and northern Baffin Island
and connects with Baffin Bay at its eastern end (see Figure 1). It is one of a
series of sounds and straits known collectively as Parry Channel, this latter be­
ing a proposed shipping route for Liquid Natural Gas (LNG) [I].
A study performed in 1977 [2] looked in a preliminary fashion, at different
meteorological and oceanographic facets of Lancaster Sound. Similar techniques
were used in a parallel study of northwestern Baffin Bay [3]. The present study
concentrates on winds and waves in Lancaster Sound.
2. DATA SOUItCBS
The wind and wave data used in this study were derived from a set of gridded
6-hourly hemispherical pressure maps compiled by the Fleet Numerical Oceanography
Center in Monterey, California. The grid interval is 381 km (see Figure 1). The
gridded pressure data began in January 1946 and ended in December 1978.
Three locations in Lancaster Sound were chosen for calculating winds and
waves. They are 77°W, 83°W and 89°W all three along the latitude 74.25°N, and will
henceforth be referred to as positions A, Band C respectively (see Figure 1). As
well, winds were computed at 56.5°N, 51.0oW and at 74.7°N, 95.0 o W which correspond
to Ocean Weather Station "Bravo" and to Resolute respectively. In the tables,
positions SB and R will refer to computed data while OWSB and YRB will refer to
observed data for "Bravo" and Resolute respectively.
The Summary of Synoptic Meteorological Observations (SSMO) tables compiled to
1972 for the Lancaster Sound - Northwestern Baffin Bay and OWSB were used for ver­
ification of the computed winds and waves for the marine areas. The Atmospheric
Environment Service data archives were used to compare winds computed for
Resolute.
831

Individual cases were assessed using information compiled by Thomson and
Vickers ([4), and personal communication) on the 81 gale force storms in North­
western Baffin Bay over the period 1969-79.
Winds at a specific location were computed using Bessel's function in cubic
form. A Sixteen-point grid array was used to obtain 'u' and 'v' gradients at a
specific location.
Wind speed and direction were obtained from the u and v components. Hourly
values were obtained by linear interpolation of the u and v components. No inter­
polation was done if four or more 6-hourly consecutive maps were missing. This
last condition only occurred three times in 33 years, none of which were in the
June to October period.
4. WAVE COIIPOTATIOIIS
Hourly wave height and period data were produced under contract by Hydrotech­
no logy Limited. Due to initial difficulties with the gridded pressure data and to
time constraints, the wave data were computed for the years 1956-71 and 1974-78,
for the five-month periods June to October. The wave hindcasts were based on the
Bretschneider equations. Wind fetches were limited by geography and minimum ob­
served ice conditions for each month (see Figure 2). No limitation was imposed due
to isobaric curvature. Fetches were adjusted when coastal or channel topography
warranted changes.
5. VERIFICAnOli or PIlOCBDUUS
a) Stoma
Two specific storms are considered. One provides good verification of the
gridded pressure data, the other provides poor verification. These storms are de­
picted in Figures 3 and 4 where the lines of pressure are taken from a hand analy­
sis of data and the grid values are indicated below the appropriate square. All
values are in kiloPascals.
832

• Pressure grid positions
e Lancaster Sound sites
Figure 1. Geographical reference map
June - - -- August - ........... .
September - all open water October
Figure 2. Boundary of minimum ice edge by month
for June to September

The November 24, 1976 storm (Figure 3) shows the same direction of the gradi­
ent from both methods, but the hand analysis indicates a much stronger gradient in
the north than does the numerical data. This particular double-low pressure case
is unusual in that the northern low pressure was developed in Baffin Bay rather
than advected from upstream.
The September 26, 1974 storm (Figure 4) was also a notable one as described
by Thomson and Vickers [4). "One of the most intense storms in the period 1969-79
to affect the study area, this is a type of storm that advects warm air into Baf­
fin Bay and produces strong southeasterly winds. The storm is all the more signi­
ficant since at the time Baffin Bay was ice-free which allowed a long fetch length
(840 km) for waves to build." In this case the numerical data agrees very closely
with the hand analysis.
b) Ceostrophic ViDds st Location A.
A comparison of hand abstracted geostrophic winds (Thomson and Vickers, per­
sonal communication) and computed values was carried out for 381 cases.
Of the 381 pairs of wind direction at location A, 80% were within 20· of each
other and 91% were within 30·. A difference of 30· between the two methods is
viewed as being reasonably acceptable. Only 3% of the pairs had a deviation great­
er than 50·.
The wind speed did not fare as well. The hand abstracted winds varied from
one-half to five times the values of the numerically computed winds with an aver­
age close to one and one-half times the numerical value. This result will be dis­
cussed later.
c) .. solute ViDds.
Tables 1 and 2 give a comparison of winds recorded at Resolute (YRB) and
winds computed from the grid pressure data at position R (Resolute). Table 1 lists
the percentage frequency of directions by month. The calm class does not appear in
the computed winds since the occurrence of a zero gradient is very rare in gridded
data (4 calms in 37,956 cases). Table 2 lists the percentage frequency by 5 mls
speed classes by month (o

Figure 3. Surface pressure analysis and numerical grid
values for 1800 GMT,November 24, 1976.
Figure 4. Surface pressure analysis and numerical grid
values for 0000 GMT, September 26, 1974.

836
TABLE 1
PERCENTAGE FREQUENCY OF WINO DIRECTIONS
AT RESOWn
1953-1978
JURE JULY AUG. SEPT. OCT. NOV.
N YRa 22.6 17.2 18.0 31.5 25.6 21.8
It 23.0 19.7 20.1 26.1 21.5 23.4
NE YRB 8.2 7.0 5.2 10.3 10.0 9.6
It 9.0 11.9 9.4 9.1 9.1 10.0
E YRB 13.8 13.5 19.2 12.6 15.5 18.5
It 8.6 8.8 12.6 7.7 7.8 9.0
SE YRB 10.6 14.6 14.7 7.3 10.1 9.9
It 4.6 6.2 11.3 6.1 9.2 7.5
S YRB 8.0 9.1 9.0 7.0 7.9 6.3
It 10.1 11.4 12.2 8.4 10.9 9.2
SW YRB 2.2 1.5 1.8 3.0 3.1 1.7
It 7.8 10.1 7.9 7.1 6.4 6.5
W YRB 15.9 19.5 14.6 8.8 8.4 6.4
It 13.9 15.0 1l.0 1l.8 lJ.O 11.1
NW YRB 14.2 12.0 11.6 14.8 13.9 15.2
It 23.0 16.9 15.7 23.9 22.1 23.3
CAu{ YRB 4.6 5.7 6.0 4.7 5.6 10.6
'It 0 0 0 0 0 0
TABLE 2
PERCENTAGE FREQUENCY OF WIRD SPEEDS
AT RESOwn
1953-1978
_/8 JURE JULY AUG. SEPT. OCT. lIOI'.
0-5 Yl!.8 45.5 50.1 47.3 37.8 41.3 53.2
It 36.7 44.5 45.7 33.9 28.7 29.2
5-10 YRB 39.0 36.6 37.0 43.3 37.5 29.8
It 47.9 46.3 42.5 45.6 44.5 46.0
10-15 YRB lJ.6 11.0 11.5 16.6 16.5 12.4
It lJ.7 8.8 10.6 17.1 22.2 20.0
15-20 YRB 1.8 2.1 3.5 2.1 4.4 3.9
It 1.4 0.4 1.2 3.0 4.3 4.2
20-25 YRB 0.1 0.2 0.4 0.2 0.3 0.5
R 0.1 0 0.1 0.3 0.4 0.6
25-30 YRB 0 0 0 0.03 0.06 O.lJ
R 0.10 0 0 0.06 0.03 0.6
30-35 Yl!.8 0 0 0 0 0 0.3
It 0 0 0 0 0 0
35-40 Yl!.8 0 0 0 0 0 0
R 0 0 0 0 0 0

For both real and computed data, 100% represents 3100 to 3200 values per
month. Thus the 0.03% for wind speeds above 30 mls in Table 2 represents one oc­
currence of such winds for each category, both of which occurred on November 20,
1965. On this date the maximum observed wind was easterly at 35.8 mls and the
maximum computed wind was easterly at 24.7 m/s.
Easterly to southeasterly winds at Resolute are known to be poor indicators
of the strength of the pressure gradient due to local topography. In these condi­
tions, observed winds tend to be greater than might normally be expected if ter­
rain were not considered.
d) Ocean weather Statio. Bravo.
OWS Bravo was felt to hold the better opportunity for comparing the proce­
dures used. Comparisons were primarily accomplished by using "Percentage Exceed­
ance" graphs.
Figures 5 and 6 show the percentage exceedance of winds both observed at
Bravo and computed at Bravo from the pressure grid for August and October. Inset
in each figure are tables of frequency of direction. The calm class for the SSKO
statistics has not been considered. The wind speed plots are quite close and the
wind direction percentages are more accurate than using persistence and a random
distribution of winds which would allot 12.5% to each compass direction.
6. VALIDITY OF PROCBDUUS
The different comparisons of the previous section demonstrate that the method
used in this study does not yield absolute answers.
Individual storms can be mis-analyzed by numerical techniques for computing
pressure grid maps. However one of the important aspects for wave generation in
this topographically rugged area is wind direction. This parameter has been cap­
tured fairly well by the numerical method since 91% of the wind cases compared
were within 30° of each other. Of the 3% of the cases where direction differed by
more than 50°, half involved difficult pressure patterns in which a displacement
of 50 to 100 km on the map would result in a totally different wind which would
agree with the numerical wind within 30°; such a level of uncertainty can be con-
837

30°; such a level of uncertainty can be considered acceptable in view of the fact
that the grid spacing of the numerical data is 381 km.
The large differences observed in the wind speeds for the case comparisons is
understandable to a point in that numerically computed winds were expected to be
lower than a true geostrophic wind as a result of numerical smoothing techniques
normally incorporated. Only 45 of the 381 cases yielded a numerical wind greater
than the hand abstracted wind with the largest difference not exceeding 6 m/s.
The winds obtained by numerical means for Resolute provided a good indication
of conditions on a frequency basis for this location. The lack of terrain influen­
ces in the numerical winds is evident in the E, SE, and SW direction which are the
directions of the major topographical obstructions at Resolute.
Where topography was not a factor, at ship Bravo, the results are encourag­
ing. Computed winds were a good approximation of observed winds.
7. LAllCASTBIl somm
There are very few actual observations taken from ships in Lancaster Sound.
The SSMO Tables summarize the available information to the end of 1972 on a grid
square basis. Two such areas are pertinent to this study, areas 11, and 12 (see
Figure 1). Area 11 has a total of 263 observations for the month of August and 230
for September while area 12 has 158 for August, 143 for September and 66 for
October.
Figure 7 compares the winds computed for August at A with area 12 winds.
Figure 8 compares the waves computed for August at A with area 12 waves. Frequency
tables of the waves computed at A and B are given in tables 3 and 4 respectively,
as well as theoretical wave heights for specific return periods based on Gumbel
statistics and their extremes (1.8 times the significant wave heights).
8. DlSCUSSIOil
The data presented for the three locations provide a first guess estimate of
sea state conditions in Lancaster Sound. Due to the many variables involved in the
procedure, too many qualifiers could be applied to these results at this time.
839

One of the most significant variables pertinent to this area is ice cover.
Minimum ice conditions were specified in this study in order to obtain results
which would approximate the higher extremes of the wave climatology of the area.
The results obtained appear to provide a reasonable estimate, slightly on the high
side, of actual conditions encountered in the area.
Validation of techniques by using ship information is a questionable method
given the sparseness of information. Only long-term, continuous information from a
fixed location can remedy this situation.
REFEBElfCES
1. Smiley, B.D. and A.R. Milne, 1979: LNG Transport in Parry Channel: Possible
environmental hazards. Institute of Ocean Sciences, Patricia Bay,
Sydney, B.C. 47 pp.
2. Duck, P.J., M.O. Berry and D.W. Phillips, 1977: A Preliminary Analysis of
Weather and Weather-Related Factors in Lancaster Sound. Unpublished
Manuscript. Project Report No. 32, Meteorological Applications Branch,
Atmospheric Environment Service, Downsview, Ont. 29 pp.
3. Maxwell, J.B., P.J. Duck, R.B. Thomson and G.G. Vickers, 1980: The Climate of
Northwestern Baffin Bay. Unpublished Manuscript. Canadian Climate Centre
Report No. 80-2. Atmospheric Environment Service, Downsview, Ont. 10Spp.
4. Thomson, R.B. and G.G. Vickers, 1980: Extreme Storm Study of Northwestern
842
Baffin Bay. To be published by Atmospheric Environment Service, Downs­
view, Ont. 28 pp.

D. Carter, D.Sc.
Y. Ouellet, D.Sc.
P. Pay, P. Eng.
Abstract
FRACTURE OF A SOLID ICE COVER BY
WIND-INDUCED OR SHIP-GENERATED WAVES.
Consultant, 1281 Bishop, Quebec.
Professor, Universite Laval.
Transport Canada, Waterways Development
Canada
Canada
Ottawa
The paper presents the basic equations governing the propagation of waves in
ice-covered waters of any arbitrary finite depth. Practical relationships are also
proposed to easily evaluate the modifications of the wave characteristics while
entering the ice-covered waters, and the minimum wave amplitude necessary for the
fracture of the ice bordering the open waters.
Resume
La presente communication rappelle les equations de base qui gouvernent la
propagation des vagues dans des eaux, de profondeur arbitraire, recouvertes de
glace. Des relations pratiques sont aussi proposees pour permettre une evaluation
facile des modifications subies par les vagues lors de leur entree dans les eaux
recouvertes de glace ainsi que l'amplitude suffisante pour briser la glace qui
borde les eaux libres.
Introduction
In the St. Lawrence ship channel, large pieces of border ice are often observed,
during winter, breaking off through the action of wind-induced or ship-generated
waves. The broken ice floes, not only, cause hazardous conditions for shipping but
frequently telescop and compress to form heavy ice jams and packs, with subsequent
sharp increase in upstream water levels.
The present study will aim, first, at determining the modifications undergone by
the waves that arrive at the ice edge and propagate into the ice-covered waters.
Then, we will try to estimate the minimum wave amplitude necessary to cause fracture
of bordering ice cover.
843

where the sinusoidal ice displacement is given by:
n
After lengthy algebraical manipulations, we finally obtain:
In open water, the celerity, Co' of a surface wave, including surface tension
effects, may be expressed by the following equation (Lamb 1932):
k s
C =_0_
o [ P
+ -L tanh (k d)]
k 0
o
!
Figure 1 summarizes how C i varies with wave period for different ice thicknesses
and water depths. It can be seen that for long period wave the phase velocities in
ice-covered and open waters are practically the same. All graphs in this paper were
plotted with E = 6 x 10 9 N I m 2 , a = 15 x 10 5 N I m 2 and \I - 0,3, representative
values for natural ice covers, (Carter, 1970).
1.2 - Wave Length
By considering that the wave impinging on the ice edge acts as a forcing term in
the equation of motion of the ice, then we may argue that the edge of the ice responds
essentially at a frequency corresponding to the period of the incident wave. Thus,
by this simple argument, it must be concluded that the wave period remain unchanged
while the wave pass from the open water to the ice-covered water. In order to satisfy
the basic equation:
A
i = [10]
the wave length must be modified as required by the phase velocity relation for ice­
covered water, according to Eq. [8].
Figure 2 shows how Ai varies with wave period for various ice thicknesses and
water depths.
[]]
[8]
[9]
845

Experimental data have shown that the average ice thickness on Lake St. Peter
can be related to a semi-empirical formula (Carter 1977):
h = 0,024 1 1 / 2 [28J
From the very few available data, Carter and Ouellet (1980) have estimated that,
for a bulk carrier type ship sailing in the middle of the navigation channel, the
average wave height reaching the bordering ice cover can be evaluated by:
H = 0,037 V
for ship speed less than 12 knots.
Figure 6 shows the allowable speed predicted for "lakers" which generate the
highest waves for a given velocity.
Conclusions
The application of the theoretical results to Lake St. Peter using average values
seems to yield promising insights into a better understanding of the complex problem
of the erosion of border ice under the action of waves from passing ships.
However, even though the results achieved by the application of the proposed
theory are very optimistic, it is well recognized that a comparison with experimental
data is necessary to properly assess its validity.
Acknowledgements
The authors wish to thank Transport Canada for permission to present this
publication. Advice and assistance from Mr. N.E. Eryuzlu, Transport Canada. Waterways
Development Division, are also greatly acknowledged, Part of the materials of this
paper is obtained from our study financed by D.S,S, and Transport Canada.
Notations
The subscripts 0 and i refer respectively to open and ice-covered waters.
C Phase velocity (m/s)
E Elastic modulus of ice (N/m 2 )
EI Average kinetic energy of water per unit area (N-m/m2)
E2 Average potential energy of water per unit area (N-m/m2)
E3 Average kinetic energy of ice per unit area (N-m/m2)
E4 Average potential energy of ice per unit area (N-m/m2)
H Wave height (m)
1 Freezing index (OC-days)
M Flexural . . d . t f 1 t h 3 E
r1g1 1 y 0 a p a e: 12 (1-v2) (N-m)
T Wave period (s)
[29J
849

U
V
a
d
g
h
k
P
s
t
x, y, z
n
A
\)
P
Pi
a
w
References
Group velocity (m/s)
Ship speed (knots)
Wave amplitude (m)
Water depth (m)
Gravitational acceleration (9,8 m/s2)
Ice thcikness (m)
Wave number 2 n/A (rad/m)
Pressure at the ice-water interface (N/m2)
Surface tension of water (0,074 N/m)
Time (s)
Cartesian coordinates
Sinusoidal displacement of ice plate (m)
Wave length (m)
Poisson's ratio: 0,3
Density of water (kg/m 3 )
Density of ice (kg/m3)
Stress in ice sheet (N/m 2 )
Angular frequency, 2n/T, (rad/s)
Velocity potential (m 2 /s).
Carter, D. and Ouellet, Y. 1980 "Fracture of a Solid Ice Cover by Wind-Induced or
Ship-Generated Waves". Report prepared for Ministry of Transport under contract
lSV79-000ll, 90 pages.
Carter, D. 1977 "Ice Thickness in the St. Lawrence Waterway" Report prepared
for Ministry of Transport, File No.: 80l0-ll5lCGAA, 22 pages.
Carter, D. 1971 "Lois et mecanismes de l'apparente fracture fragile de la glace
de riviere et de lac" These de doctorat, Dep. de Genie civil, Section Mecanique des
Glaces, Univ. Laval, Quebec.
Ewing, M. and Crary, A.P. 1934 "Propagation of Elastic Waves in Ice" Physics,S, 2,
pp. 181-184.
Lamb, H. 1932 "Hydrodynamics" Cambridge University Press, 6th Edition 1975.
Peters, A.S. 1950 "The Effect of a Floating Mat on Water Waves" Commun. Pure
Appl. Math., Vol. 3, pp. 319-354.
Robin, G. de Q. 1963 "Wave Propagation through Fields of Pack Ice" Phil. Trans.
Roy. Soc. London, Ser. A, 255 (1057), pp. 313-339.
850

Wadhams, A. 1973 "Attenuation of Swell by Sea Ice" Jour. Geophys. Res., Vol. 78,
No. 78, pp. 3552-3561.
Weitz, M. and Keller, J.B. 1950 "Reflection of Water Waves from Floating Ice in
Water of Finite Depth" Commun. Pure Appl. Math. Vol. 3, pp. 305-318.
851

B.D. Pratte and G.W. Timco
Research Officers
ABSTRACT
A NEW MODEL BASIN FOR THE TESTING
OF ICE-STRUCTURE INTERACTIONS
Hydraul ies Laboratory
National Research Council
Ottawa
Canada
The Hydraulics Laboratory of the National Research Council of Canada has con­
structed a facility for model testing the dynamic interaction between a structure and
an ice cover. The facility consists of a refrigerated chamber (29 m long, 10 m wide
and 1.2 m deep in which carbamide (urea) doped model ice is grown. The design of the
test facility has many novel features including duet work along one side of the
chamber which blows cold air at various speeds across the ice surface to hasten the
ice formation, a moveable insulated curtain which separates the main tank from the
rigging area, and full instrumentation of the air, ice and solution for monitoring
the changes in temperature and thermal state during the seeding process, growth and
warm-up of the ice sheet. In this paper the features and operational characteristics
of this test facility are described.
857

1.0 Introduction
With the rapidly escalating activity In Canada's Ice covered waters, It is
necessary that design data on the interactive forces and displacements between struc­
tures and ice covers be obtained. Not only must ships and drilling rigs move through
the Ice, but also fixed structures such as drilling platforms, artificial islands,
docks and breakwaters, etc. must be able to withstand the forces of ice covers moving
past them. To better understand the forces which a structure experiences by relative
motion between it and an ice cover, a few laboratories have constructed modelling
facilities to simulate to scale the ice-structure Interactions. Host of these facil­
ities, however, are mainly concerned with the testing of ship models in both uniform
and broken ice covers. This type of model testing has greatly advanced the science
of designing efficient ice breaking hull forms. To date, however, only a handful of
tests have been performed on measuring the forces which a moving Ice sheet can exert
on a stationary structure. This, in spite of the increasing importance of such
stationary platforms in Arctic regions. For ice covers moving past fixed or moored
structures, the relative speed is considerably lower than for ships in ice, but the
forces may be much greater due to the large size and blunt shape of many structures.
Recently the Hydraulics laboratory of the National Research Council of Canada in
Ottawa constructed a refrigerated cold room which will be used to study such
ice-structure interactions on a model scale. In addition, this facility will be
used to quantitatively document mechanical properties and growth characteristics of
model ice so that more meaningful results can be obtained from tests on small model
structures. Upon completion of the carriage in late 1981, basic research on forces
and motions of structures moving relative to the ice cover will begin. Such research
will complement the state of knowledge and be used to assist commercial ice tank
laboratories in evaluating their own testing on proposed Arctic structures. This
paper describes the features and operational characteristics of this test facility.
2.0 General layout
Fig. 1 shows the general layout of the facil ity. The cold room, which is built
inside the existing laboratory, is 29 m long, 10 m wide and 4.9 m high. Its walls
consist of 10 cm thick polyurethane foam insulation (R20) between enamelled steel
exterior and white fibreglass reinforced plastic (FRP) interior wall panelling.
The basin itself is 21 m long, 7 m wide and 1.2 m deep to the top of the walls.
The tank is filled with a carbamide (urea) solution [1,2] to a depth of 1 m. Both
the basin walls (0.4 m thick) and the basin floor (0.2 m thick) are constructed of
heavily reinforced poured concrete (5000 psi (34.5 HPa) minimum compressive strength}.
The entire tank is sealed with a black trowel-on rubberized ashphalt coating TP90V,
and then painted Polarcote white. Because several of the existing ice tanks have
B58

Photo I View of Ice Basin from Northwest Corner showing
Air Supply Outlets, Ceil ing Mounted Thermocouples,
Service Carriage, Walkway, Insulated Curtain
Stowed Along Right Wall, and Board Frozen into Ice
where Curtain is Normally Drawn during the Freezing
Process
861

862
Photo 2 Testing Ice Sheet Properties. View from Southwes t
End Showing Ductwork, Service Carriage, Evaporators
and Stowed Curtain

Photo 5 Clearing the Ice Sheet Using Scraper Screen
on Service Carriage - Movement to Right
Photo 6 Pushing the Old Ice Sheet over Ramp into the
Melt Pit (Pump and Spray Bar at Right)
865

After ensuring that an ice skin covers the whole surface of the solution, the fans
are restarted and the ice sheet Is grown at an ambient air temperature of -21°C.
Following the freeze, the refrigeration system is shut off and the room temperature is
allowed to rise to O°C. This procedure warms the ice sheet and aids in reducing its
strength for the model studies.
After the testing with the ice sheet is complete, the ice is removed from the
tank in the following way. The air bubbler pipe near each side wall is started in
order to free up the ice from the walls. The water from the melt pit is then pumped
into the main tank until the water in the tank is overflowing back into the melt pit.
Using the screen attached to the service carriage (Photo 5), the ice is manually
pushed into the melt pit (Photo 6), where it is melted using the heater and spray bar
system. Once the ice is melted, the pump and heater are shut off and the plumbing
lines are drained.
4.0 Future Work
The main structures carriage will be operational this year. It will be propelled
by two pinions driving onto racks mounted on each rail, and powered by a 40 HP d.c.
motor. The carriage speeds will be 0 to 50 cm/s for structures testing, with higher
speeds available if required. The carriage will be exceedingly stiff with a natural
frequency above 25 Hz and the ability to exert up to 5 tons horizontal force. Load
cells will be used to mount the structures to be tested to the carriage. The car­
riage speed and all force data will be recorded either on-board or in the control room
for analysis by computer.
Basic research on forces exerted by moving Ice sheets on various elementary
structures will be carried out. Such structures include circular piles, sloping
cones, vertical and sloping faces such as the shores of islands, breakwaters, etc.
More complicated designs will also be tested as they evolve for Arctic applications.
5.0 References
1. Timco, G.W., "The Mechanical and Morphological Properties of Doped Ice: A Search
for a Better Structurally Simulated Ice for Model Test Basins", Proc. POAC '79,
Vol. I, pp. 719-739, Trondheim, Norway, 1979.
2. Ti meo, G. W., "A Compar i son of Severa 1 Chem i ca 11 y-Doped Types of Mode 1 Ice", Proc.
IAHR Symp. on Ice (in press), Quebec City, Canada 1981.
3. Sandell, D.A., "Urea Ice Growth in a Large Test Basin", Proc. IAHR Symp. on Ice
866
(in press), Quebec City, Canada, 1981.

W.B. Tucker III, Geologist
and
Research
W.D. Hibler III, Physicist
PRELIMINARY RESULTS OF ICE MODELING IN THE
ABSTRACT
EAST GREENLAND AREA
U.S. Army Cold Regions
Research and Engineering
Laboratory, Hanover, NH
A sea ice model which employs a viscous-plastic constitutive law has been applied
to the East Greenland area. The model is run on a 40 km spatial scale at 1/4 day
time steps for a 60-day period using forcing data beginning 1 October 1979. Preliminary
results verify that the model predicts reasonable thicknesses and velocities
well within the ice margin. Separate simulations show that thermodynamics only and
free drift with thermodynamics produce inadequate results. In particular, the free
drift simulation produces unrealistic ice trajectories with excessive drift toward
the coast and unreasonable nearshore thicknesses. The net results of these simulations
tend to verify that internal ice stress, thermodynamics, and ice import must
be considered to properly model this region.
INTRODUCTION
The East Greenland area is unique to this hemisphere because it is an area of
confluence of polar and temperate systems for both atmosphere and ocean. The complex
nature of air-sea interaction here is further complicated by the presence of sea ice.
This ice cover is highly variable, with large changes in the extent occuring both on
a seasonal and interannual basis.
Wadhams [11 has extensively reviewed the literature which deals with the sea ice
in this area. It is well known that the major components which govern the ice balance
are the thermodynamic balance at the sea surface, the wind and water stresses upon
the ice, the Coriolis force and the internal ice stress. Additionally, there is a
flux of ice into the East Greenland area from the Arctic basin which contributes to
the mass balance [21. The relative importance of these components to the region has
not been made clear. More recent studies [1,31 have identified smaller scale proces­
ses such as wave induced pulverization and warm eddies, which may contribute to the
ice edge location on a synoptic scale.
To begin to sort out the role of some of these processes, a first order sea ice
modeling study seemed relevant. Initially, we decided that it would be most appropriate
to apply a model that has been used successfully in studies of the sea ice in the
USA
867

A 40 km, 31 x 45 grid was established for the simulations. The grid location is
shown in Figure 1. As the model allows for free boundaries (which allow inflow and
outflow), these are designated for the Fram Strait (north), the Denmark Strait (south)
and the eastern boundary. In order to satisfy stability criteria, a 1/4 day (21600 s)
time step was required. All other parameters were identical to those used by Hibler
[4] in the Arctic basin study with exception of the Coriolis parameter and P*, a
constant used in determining ice strength. Here, the Coriolis term was calculated
for each grid point location. The constant p* was set to four times that used in
the Arctic basin simulations (5.0·10 3 Nm- 1 ). This change was implemented when initial
tests showed the ice velocities to be excessive, presumably due to the large magni­
tudes and variability of the daily winds. The previous Arctic simulations [4] used
8-day averaged winds which inherently provided spatially and temporally smoothed
fields.
Due to computer time limitations at our facility, we were limited to a 60-day
simulation period. We chose the period October through November, 1979 because it is
a season of relatively rapid ice expansion and because position data for drifting
buoys located on the ice were available for this time period [7]. For the initial ice
field, compactness was digitized from the published ice chart [8] for 2 October 1979.
Thickness was estimated by allowing it to vary linearly with latitude, 1.0 m at 67°N
to 3.2 m at 83°N. These estimates seemed reasonable based on data reported from
submarine transects of the area [9,10].
Four basic simulations were performed in this study. Three were designed to test
the response of the model 1) to thermodynamics only (no ice dynamics), 2) to ice
import through the Fram Strait, and 3) to ice interaction (the importance of the ice
stress term). The final test was designed to evaluate the general performance of
the completed model in this area. For the thermodynamics only run, all ice velocities
were set to zero and the ice was allowed to grow and ablate according to the modified
ice growth rates for the 60 day period. The response of the model to ice inflow from
the Arctic basin was evaluated by not assigning constant thickness to the northern
free boundary cells as was the normal procedure. Without this specification (called
the no inflow case), the model calculates open cell thicknesses as an average of the
thicknesses in the adjacent cells inside the boundary and will be forced to run out
of ice if large southward advection occurs. (Thus, this is not a true no inflow
test; we would have to specify zero thicknesses for the open cells. This is more a
test of a true northern open boundary). In the ice interaction test (referred to as
free drift case), the ice strength was set to zero, effectively damping out the in­
ternal ice stress. The final simulation (the standard case) included specification
of inflow cell thicknesses, ice strength, and thermodynamics.
870

Current and Wind Fields.
RESULTS AND DISCUSSION
The 60-day averaged wind velocity and ocean current fields are shown in Figures
2 and 3. The most significant feature of the wind field is that the narrow band of
northerly winds follows the continent almost precisely, indicating the large influence
of topography on the sea level pressure field. Also noteworthy is the fact that the
winds immediately to the east of this high velocity stream are southerly over most of
its length. On the other hand, the current field is quite smooth as would be expected
from a temporally constant dynawic height field.
Ice Thickness.
The 2 October 1979 ice thickness field used to initiate the model runs is shown
in Figure 4. All thickness fields represent the average ice thickness for each grid
cell, that is the product of the actual ice thickness and the compactness (0.0 tol.O).
We chose the 0.25 m contour to represent the ice edge because thinner ice is subject
Figure 2. 60-day averaged wind velocity
field.
Figure 3. Current velocity field.
871

)
Figure 4. Initial average thickness
field.
Figure 5. Average thickness field
for the thermodynamics only simulation
after 60 days. Dashed line
is actual ice edge position for
2 December 1979. Contour values
are in meters.
to large variations due to growth and ablation. Later studies will examine the
variability and diffusivity of the ice edge in detail.
Figure 5 shows the result of the 60 day thermodynamic simulation. Also shown' is
the observed ice edge position for 2 December 1979 [8]. This simulation shows that
thin ice «1.0 m) has extended to the south and east, but the thicker ice categories
have changed very little. Previous simulations that excluded the oceanic heat flux
showed that thin ice covered the entire grid. The net effect then is that the param­
eterized oceanic heat flux is completely dominating the ice edge location in this
simulation. It is also apparent that too much ice production is taking place in the
Denmark Strait area (lower part of grid), possibly indicating a lack of proper ocean
heat flux parameterization in that region unless advection is the mechanism responsi­
ble for moving this ice closer to the coast. The fact that the growth rates for
thicker ice are much smaller than for thin ice and open water is evidenced by the
small change in the location of the thicker ice contours (>1.0 m).
The effect of the ice dynamics on the thickness field can be appreciated from
Figure 6 which shows the 60-day thickness field for the standard run. Several
872

Figure 8. Average thickness field for
free drift simulation.
" .....
Figure 10. 60-day average velocity
field for standard simulation.
874
Figure 9. 60-day average velocity
field for free drift simulation.
which signifies that ice import from the
Arctic basin is important to the region. A
longer simulation will verify whether the ice
to the south also becomes thinner as the ice
there continues to advect out of the southern
boundary.
The final thickness plot, shown in Figure
8, is the result of the free drift simulation.
with zero strength, thus no ice interaction
or resistance to deformation, the ice builds
to physically unrealistic thicknesses along
the coast. The necessity of allowing for ice
interaction in this region for any modeling
effort is clearly demonstrated by this figure.
Free drift may be applicable on a localized
scale for very short term forecasts but it is
certainly not sufficient for the region as a
whole over time periods on the order of months.

PACK ICE DRIFT AND WEATHER IMPACT
A Pilot Study off East Greenland
Ren€ Zorn Danish Hydraulic Institute Denmark
Hans H. Valeur Danish Meteorological Institute Denmark
ABSTRACT
Several mathematical models on ice drift have been developed since N.N. Zubov published
his formulae. Common for most of them have been the lack of sufficient verification
data, since most ice observations are too insufficient to allow evaluation of short
term fluctuations compared with weather data. The aim of this paper was to validate
the above mentioned drift ice theory. Unfortunately it turned out that after obtaining
ice drift observations it was the weather data that were insufficient. Instead it is
shown that ice drift information might provide wind information to verify weather
charts.
An environmental research programme in August and September 1980 for the benefit of
possible future oil prospecting in and off East Greenland created a possibility to
investigate the ice conditions off East Greenland between 6S o N and 78 0 N as related to
weather conditions.
Two drift bouys were deployed in the area, and 22 reconnaissance flights were executed.
The data from those platforms supplied with data from satellite images were compared
with the actual weather maps and climatic current data.
1. INTRODUCTION
In connection with a Marine Geophysical Survey Prngramme and Environmen­
tal Studies Offshore East Greenland, an operational ice reconnaissance
and drift ice study was executed in 1980 for Geological Survey of Green­
land (GGU) and Grenland Technical Organization (GTO) both under the
Ministry for Greenland.
Results from the above programmes have been used for preparation of the
present paper.
879

2. GENERAL
During August and September 1980 ice reconnaissance flights were
carried out 2 or 3 times a week along the East Coast of Greenland /1/,
departing from Reykjavik, Iceland or Narssarssuaq, Greenland. During
and after the flights ice charts were prepared and transmitted to the
Danish Meteorological Institute (DMI) in Copenhagen, Denmark. DMI also
made interpretations of satellite images (NOAA 6) and weather analysis
and forecast charts. All results were transmitted daily from Denmark
to the survey vessels offshore East Greenland via radio facsimile,
Fig. 1.
Fig. 1. Communication Diagram
880

The Environmental Study Offshore East Greenland 1979 and 1980, /2/,
which included a drift ice study was based on board a survey vessel
which was mainly operating in the area offshore Scoresbysund and Me­
stersvig.
The object of these experiments was to determine drift ice patterns and
velocities by means of an automatic NIMBUS-6 station placed on an ice
flow (ice island).
The drift study covered the period August 3 to August 10, 1979; and
August 20, 1980, to February 16, 1981 and results are given in Section
3. Selected ice observations have been analysed together with weather
analyses for the same period to determine sea ice drift and the re­
sults are given in Section 4.
3. NIMBUS-6 DRIFT ICE STUDY
On August 3, 1979, an automatic NIMBUS-6 station was placed on an ice
floe at position 73 0 36'N/19 0 52'W. The dimensions of the ice floe (a
floe berg?) were approximately 60 x 40 m with a total thickness of
about 30 m.
The station has been tracked until August 10, 1979, when the ice floe
was grounded for half a year until the station was lost, Fig. 2. The
-1
mean drift speed has been calculated to about 0.13 m'sec in the pe-
riod of drift.
On August 20, 1980, a second automatic NIMBUS-6 station was placed on
an ice floe (ice island) at position 71 0 55'N/21 o 33'W. The approximate
dimensions of this floe were about 450 x 150 m, with a height of 6 m
above the sea surface and a draft of about 20 m.
The station has been tracked from August 20, 1980, to February 16, 1981,
by satellite, and from August 20 to September 21, 1980, also by air­
craft. The track is shown in Fig. 2.
881

• SATELLITE OBSERVATIONS
o AIRCRAFT OBSERVATIONS
DAY I MONTH -YEAR
'50 100IO;M
f---+I------il
Fig. 2 Drift of automatic NIMBUS-6 stations placed on ice floes off
882
East Greenland August 1979 and August 1980 to February 1981.

The drift of the ice island reflects the currents at 20 m depth which
may well be different from the surface currents. The time lag between
wind and current grows with depth, so ice island drift will not provide
information of immediate wind conditions, but will show mean current
conditions through some period of time.
4. ANALYSIS OF ICE OBSERVATIONS
The ice charts of August 18 and September 11, 1980 (Figs. 4, 5 and 6)
show the typical distribution of sea ice during the period of investiga­
tion. As usual for that time of the year open water is prevalent in the
fjords and along the coast, while a 100-200 km wide belt of pack ice of
various concentration is present outside. Figs. 5 and 6 depict the ice
concentration on September 11, 1980 as derived from aerial reconnais­
sance and satellite images, respectively. Generally the two sources
agree reasonably well, yet discrepancies, especially regarding concen­
tration indications, do occur now and then.
Fig. 4 Satellite image, August 18, 1980.
884
(Numbers indicate ice con­
centrations in tenths,
dotted areas indicate less
than 1/10 ice concentration,
heavily shaded areas indica­
te 10/10 ice concentration).

It is a well known fact that the ice drift is determined by wind (di­
rectly and through wind generated currents) '- with a time lag of about
one day - and by the gradient and tidal currents. Therefore, under
calm weather conditions (Fig. 7) the drift pattern should be simple
with a weak negative vorticity, determined by the East Greenland Current
as was the case in the interval August 24-25, 1980 (Fig. 8) where the
drift was towards the S and the SSE, the speed varying from 0.06 m.sec- l
near the coast to more than 0.32 m.sec- l close to the ice edge.
Fig. 7 Weather Chart, August
25, 1980 1200 GMT
Fig. 8 Ice drift, August 24-25,
1980.
(Open arrows indicate movement of
ice edge, while solid arrows indicate
drift of individual floes)
On the other hand, wind opposing the gradient current will reduce the
drift (e.g. August 25-27, 1980 (Fig. 9) when the winds through the pre­
ceding day and night, Fig. 3, were from the Sand SW thus causing a
drift towards the East), while downcurrent winds will increase the drift
velocity (e.g. the southernmost current vector on Fig. 11, August 28-29,
1980) .
886

Fig. 9 Ice drift, August 25-27,
1980.
\ - . -
iJ :
Fig. 10 Ice drift, August 27-28,
1980.
In a region like the present where weather data is utterly sparse, the
weather analysis is as a rule extremely uncertain, whence ice drift
vectors could be of considerable value in adjusting wind calculations.
However, to avoid hidden subjectivity in stating the impact of wind on
the ice drift, the weather charts in the present study have deliberately
been analysed by a very experienced analyst (Class II meteorologist
mr. Leif Rasmussen) not knowing the ice drift.
The drift August 27-28 (Fig. 10) may serve as an example of how the ana­
lysis may be adjusted: North of 76 0 30'N the drift was generally towards
the E and the NE, which might indicate that the calculated northerly
wind of 10 kts on August 27 at position 77 0 20'N/13 0 W (Fig. 3) actually
should be 20 kts from the South.
-
......
887

On the other hand the calculated 20 kts wind from the south at position
7S o N/13 0 W may actually have been zero or weak northerly, causing a drift
speed of nearly 0.43 m·sec- l around this position. A displacement of the
indicated high pressure of about 150 nm towards the NNE on the weather
map would fit these winds. The drift pattern August 28-29 (Fig. 11) is
similar to that of August 27-28, 1980.
Fig. 11 Ice drift, August 28-29, 1980.
The other period selected for drift investigation September 8-12, also
shows great local variations and seems to confirm the feasibility of
using ice floes as a tool to adjust the weather analysis.
On September 8-9, 1980 (Fig. 12) the drift was weak and diverging around
7SoN. North of this latitude the drift seems to have been slightly to­
wards the North in spite of indicated strong wind on September 7, 1980
from northerly directions (Fig. 3). This may go to show that the pres­
sure difference indicated by the isobars in the Greenland Sea area on
September 7 and 9 (Fig. 13) was actually concentrated in the eastern
8M

part. Hence, the isobars should be moved eastwards indicating a weak
gradient at the positions indicated in Fig. 3 with calm weather and
later southerly winds.
Fig. 12 Ice drift, September 8-9,
1980.
Fig. 13 Weather chart, September
7, 1980.
September 9-11 and 11-12, 1980 (Figs. 14 and 15) showed a marked nega­
tive rotation with considerable local variations. Only south of 75N the
drift seems to have had a southerly direction (up to 0.32 m.sec-lat the
edge), while the tendency north of this latitude was more a drift to-·
wards the East.
889

Fig. 14 Ice drift, September
5. CONCLUSION
9-11, 1980.
Fig. 15 Ice drift, September
11-12, 1980.
The velocity values as derived from the present drift ice study agree
fairly well with those found by Malmberg et al. /3/. In the inner shelf
region a general decreasing current component with depth is a common
feature, with maximum currents of about 0.35 m'sec- l at the surface.
The weather data were too sparse to allow any test of drift models to be
made in this pilot study. However, the study did show the complexity of
the drift pattern, - similar to that found further to the south, by
Malmberg et al., but much more irregular than found by Vinje /4/.
Further it appears that the drift of ice floes may serve to verify historical
weather charts thus proving a tool to improve the weather analysis.
890

6. REFERENCES
/1/ GGU, Ice Reconnaissance along the East Coast of Greenland, 1980.
December 1980.
/2/ GTO, Environmental Studies Offshore East Greenland, 1980. April
1980.
/3/ S.-A. Malmberg, H.G. Gade and H.E. Sweers, Current Velocities and
Volume Transports in the East Greenland Current of Cap Nordenskjold
in August-September 1965, Reykjavik 1972.
/4/ Torgny E. Vinje, Sea Ice Studies in the Spitsbergen-Greenland Area.
Oslo 1977.
891

t. K
P = p*....!. N
to
in which p* and K are empirical constants and to is a reference ice thickness. p* is
presumably related to the yield strength of the macroscopic ice medium. The pressure
modifier, N K , serves to reduce the ice pressure substantially when the ice area concentration
is less than one.
Model Calibration
The model was applied to Lake Erie for the period January 15, 1979 to January 17,
1979 ( a period of 49hours) during which significant freezing and movement of the ice
field had been observed to occur. Side-looking airborne radar images (SLAR) and the
accompanying ice chart products produced by the U.S. Coast Guard Ice Navigation Center
in Cleveland, Ohio were the principal observational data available for model calibration
(see Figures 1 and 2).
During this freezing period the magnitude of Em was computed differently for
ice covered areas than for open water areas. For ice covered areas, Em was represented
as follows,
where Ta = local air temperature and a = an empirical coefficient.
areas, Em was given by,
(5)
(6)
For open water
where a = an empirical coefficient. The wind speed and the air temperature at any
position over the lake were obtained by weighting the observed values at n meteorological
stations (in this case, Toledo, Cleveland, and Buffalo) using a weighting procedure
[11) and correcting overload wind measurements to overlake values [13),
The following values of the constants were utilized in this calibration effort:
p* = 5000 N/m2; to = 0.5 m; K = 20; e = 2.0; k 2 (max) = 10 10 Ns/m 2 ; k 2 (min) = 104 Ns/m 2 .
e denotes the ratio of the lengths of the principal axes of the assumed two-dimensional
stress ellipse for the ice medium [9). Only the constants a and a from equations
(6) and (7) were varied in this calibration effort: case (1) [a = 4x10- 6 , a = 2x10- 5 );
case (2) [a = 3x10- 6 , a = 6X10- 5 ); case (3) [a = 3x10- 6 , a = 3x10- 4 ). The water
currents were assumed to be zero throughout the basin and the no-slip condition was
applied to the ice velocity component tangential to the lake boundaries. However,
the component of the ice velocity normal to an impenetrable boundary was expressed
896
(7)

differently depending on the direction of ice motion at the boundary. On a boundary
from which ice mass moves lakeward (upwind boundary), the boundary condition requires
that the gradient of the normal velocity component be zero, while at a downwind
boundary, the normal ice velocity component must be zero. The initial conditions for
N(x,y,o) and t.(x,y,o) were obtained from the SLAR image and accompanying ice chart
1 +
product for January 15, 1979 (see Figure 1). The meteorological data, Va and Ta'
were corrected every 3 hours according to the input of observed values at Toledo,
Cleveland, and Buffalo. The initial ice velocities, Vi(x,y,o), were set equal to
zero since the transient period for ice mass acceleration had been previously found
to be on the order of 15 minutes [16].
The model output after 49 hours is shown in Figures 3, 4, and 5. These figures
portray the ice condition for January 17, 1979 in terms of ice drift velocities, ice
area concentration, ice thickness, and ice pressure. The ice pressure field is
presented as the product of the local computed pressure and the local ice thickness
(pt i ) which is representative of the hull pressure to be encountered by vessels transiting
the ice field. Since the ice pressure has been parameterized in the present
model and exact values are unknown, the plots of pt i are only qualitative. However,
in this manner, it is possible to identify regions of high value of pt i which may be
useful in charting vessel tracks through the ice field. The computed ice condition
for January 17, 1979 is in reasonable agreement with the observed conditions as portrayed
by the SLAR image and accompanying ice chart product for that same day (see
Figure 2). Only the computed values of Nand ti can be compared with the data from
Figure 2.
Summary
A fairly elaborate and versatile numerical model has been developed for use in
forecasting ice conditions in the Great Lakes including a graphical output-retrieval
scheme. The application of the model has been demonstrated by calibration against an
observed ice transport episode in Lake Erie during January, 1979. A second calibration
is being conducted for an observed ice tranport episode in Lake Erie (March, 1979)
during which significant ice melting occurred. The heat exchange occurring in the
prototype could be dealt with more explicitly [19], but the computational effort and
data requirements would be substantially greater.
Further calibration efforts to selected portions of one of the Great Lakes is
recommended. Better observational data are needed for comparison with model output
in order to make further adjustments in model coefficients and to reveal more fully
the use of the model as a forecasting tool.
897

[I)
[2)
[3)
[4)
[5)
[6)
[7)
[8)
[9)
[10)
[11)
[12)
[13)
[14)
[15)
[16)
[17)
[181
[19)
898
REFERENCES
Campbell, W.J., "The Wind-Driven Circulation of Ice and Water in a Polar Ocean",
J. Geophys. Res., Vol. 70, 1965, pp. 3279-3301.
Campbell, W.J., and Rasmussen, L.A., "A Numerical Model for Sea Ice Dynamics
Incorporating Three Alternative Ice Constitutive Laws", Sea Ice, Proc. of an
Internat. Conf. Reykjavik, Iceland, May, 1971, pp. 176-189.
Coon, M.D., "Mechanical Behavicr of Compacted Arctic Ice Floes", J. Petro. Tech.
Vol. 26, 1974, pp. 466-470.
Coon, M.D., Maykut, G.A., Pritchard, D.S., and Thorndike, A.S., "Modeling the
Pack Ice as an Elastic Plastic Material", AIDJEX Bulletin, No. 24, 1974, pp.
1-103.
Doronin, Y.P., "On a Method of Calculating the Compactness and Drift of Ice
Floes", AIDJEX Bulletin, No.3, 1970, pp. 22-39.
Glen, J.W., "Thoughts on a Viscous Model for Sea Ice", AIDJEX Bulletin, No.2,
1970, pp. 18-27.
Hibler, W.O., "Differential Sea Ice Drift II: Comparison of Mesoscale Strain
Measurements to Linear Drift Theory Predictions", J. Glaciology, Vol. 13,
No. 69, 1974, pp. 457-471.
Hibler, W.O., "A Viscous Sea Ice Law as a Stocastic Average of Plasticity",
J. Geophys. Res., Vol. 82, 1977, pp. 3932-3938.
Hibler, W.D .. "Modeling Pack Ice as a Viscous-Plastic Continuum: Some Preliminary
Results", Proc. of a Symp. on Sea Ice Processes and Models, Unv. of
Washington, Seattle, Wash., 1977, pp. 46-55.
Neralla, V.R., and Liu, W.S., "A Simple Model to Calculate the Compactness of
Ice Floes", J. Glaciology, Vol. 24, No. 90, 1979.
Platzman, G.W., "The Dynamic Prediction of Wind Tides on Lake Erie", Meteorological
Monographs, Vol. 4, No. 26:1-44, 1963.
Pritchard, R.S., "An Elastic-Plastic Constitutive Law for Sea Ice", J. Applied
Mech., Vol. 42, No.2, 1975, pp. 379-384.
Resio, D. T., and Vincent, C.L., "Estimation of Winds over the Great Lakes",
Miscellaneous paper, H-76-12, U.S. Army Corps of Engineers, Vicksburg, Mississippi,
1976.
Rothrock, D.A., "The Mechanical Behavior of Pack Ice", Annual Review of Earth
and Planetary Sciences, Vol. 80, No.3, 1975, pp. 317-342.
Rumer, R.R., Crissman, R.D., and Wake, A., "Ice Transport in Great Lakes",
Contract No. 03-78-B01-104, Great Lakes Environmental Research Lab., NOAA, Ann
Arbor, Mich., Sept. 1980.
Rumer, R.R., Wake, A., Chieh, S-H, Fukumori, E., and Tang, G., "Internal Resistance
of Lake Ice", Contract No. NA79RAC00124, Great Lakes Environmental Research
Lab. NOAA, Ann Arbor, Mich., Sept. 1981.
Udin, I., and Ullerstig, A., "A Numerical Model for Forecasting the Ice Motion
in the Bay and Sea of Bothnia", Research Report No. 18, Winter Navigation
Board, Norrkoping, Sweden, 1976.
U.S. Coast Guard, "Lake Erie Ice Charts", U.S. Coast Guard Ice Navigation
Center, Cleveland, Ohio, 1979-1980.
Wake A., and Rumer, R.R., "Modeling the Ice Regime of Lake Erie", Proc. ASCE.
Vol. 105, No. HY7, 1979, pp. 827-844.
ACKNOWLEDGMENT
This work is supported by the Great Lakes Environmental
Research Laboratory, National Oceanic and Atmospheric Administration,
U.S. Dept. of Commerce, Ann Arbor, Michigan.

900
Fiqure 2. Lake Erie Ice Condition on January 17, 1979 (t=49.Z hrs)
Methodology for Computing Nand ti from Ice Charts
Code from Ice Chart: alaZa3a 4
nln Zn 3
a
1
a
Z
a
3
a
4
tenths of area with ice thickness, t
1
=0.5m
tenths of area with ice thickness, t
Z
=0.ZZ5m
tenths of area with ice thickness, t
3
=0 . 075m
tenths of area with ice thickness, t
4
=0.OZm
1
N(x,y) = To E a n
ti(x,y) = (E antn)/( E an)

THE BIOLOGICALLY IMPORTANT AREAS IN THE
ARCTIC OCEAN
Erkki Palosuo, prof.emer. University of Helsinki Finland
ABSTRACT
The shelf between Spitzbergen and Franz Josef Land is considered as
a biologically important area in the open sea, and the Mackenzie
Estuary and the Stefansson Sound are considered important in inshore
areas. A general view of the biological productivity and its signifi-
cance to the Arctic Basin ecosystem is given using these examples.
1. INTRODUCTION
Several scientists who have been working in the Arctic have noted how
certain animals gather in particular places, known, for example, as
hunting areas. Those who have had the opportunity to study the pri-
mary biolog¥ and the rather simple food chains in the sea have
realized that the Arctic Ocean is a fascinating environment, which
has only just begun to be understood. Recent expeditions have shown
that some particular areas are very important to the ecosystem of the
Arctic.
Of these "cradles of life", I shall mention the following:
- the shelf between Spitzberegen and Franz Josef Land, a center of
902
the local polar bear population (Fig.l),

ut scare in the open sea.
It is well known that the majority of nutrients reach the Arctic
Ocean from Siberia, but the author has no information concerning the
amounts involved. In the Stefansson Sound and its nearshore zone,
the energy budgets for production indicate that about half of the
carbon input is from terrestial sources, which include shoreline
erosion and transport of humus detritus by rivers (13). It is impor­
tant to note that the humus represents a large fraction of the ener­
gy requirements of nearshore invertebrates. Its importance can be
seen in the low 14C content of organisms during winter, when the
flow of humus into sea is reduced, (report of prof. Donald M. Schell).
In the summer, convection brings the nutrients from shallow bottom
up to the upper water layer without any upwelling or other phenomena.
The turbidity following this convection can considerably reduce the
primary production.
There is very little information available on the effect of a river
on the sea. In the Mackenzie Estuary, the influence of the river
seems to reach some 10 km out from the outer banks, where the gyra­
tory circulation in the Beaufort Sea is dominant (8). The same situ­
ation occurs in the Baltic Sea, too.
We can conclude that estuaries and the nearshore zone from many
distinct areas, in which the primary production and animal life
depend on local condi tions·.
3. THE CENTRAL PART OF THE ARCTIC BASIN.
3.1. Nutrients
The quantity of nutrients in the central part of the Arctic Basin
varies according to the way they have entered the basin. As mention­
ed above, most nutrients originate inland and are carried by rivers
to the sea. Water from adjacent seas also enters the Basin. The
Atlantic Gulf stream flows to the west of Spitzbergen and the sub­
merges under the less saline Arctic water along the continental
shelf to the north of the Siberian, Alaskan and Canadian Arctic (16b).
Uppwelling finally raises this relatively warm watermass to the sur­
face. Water from the Bering Sea eneters the Chukchie and Beaufort Seas.
905

tion under the ice usually reaches a depth of 10 to 20 m. Such a
shallow depth is favourable to the development of a bloom and thus
to the whole primary productivity.
In the shelf-break area of the Bering Sea, Dr. Alexander made the
very significant observation that the necessary stratification and
chlorophyll concentration occurred off the edge of the ice up to
25 km seawards. The effect of melting ice on the stratification is
much higher than the warming of water in summer or upwelling at the
ice edge caused by wind-driven off-ice Ekman transport.
There have been large annual variations in the seasonal extent of
the ice edge in the Bering Sea (1). The same has also been observed
in the other parts of the Arctic Ocean. The relationship between
the spatial and temporal position of the seasonal ice edge might be
very important in determing wheter sufficient nutrients are available
to support the summer bloom, and in determing the rate of increase
in the phytoplankton community.
3.3. The role of ice algae
An extraordinary phenomen occurs in the Arctic and in the Antarctic
seas: the blooming of ice algae in and under the ice in the early
summer. As shown by Meguro et.al. (11) the diatoms reproduce in brine
solutions trapped in the microfissures between fine crystals of sea
ice. they form a brownish layer near the bottom of ice.
The main poin! of interest is firstly that they develop during the
summer. The light intensity in the ice is more favourable for photo­
synthesis than in the water under the ice. Meguro found no diatom
activity at a distance greater than 0.3 m from the bottom of the ice.
He postulated that diatoms are frozen into sea ice as it is formed
and that they then grow in the early summer as a result of increases
in nutrient and light.
Ice algae bloom in the first year ice only. The surface warming of
the ice causes brine to descend from the upper layers and none solar
radiation reaches the second year ice below the older ice. Thus no
ice flora will develope on the under side of the second year ice.
Selective absobtion increase the temperature and therefore the
porosity of the layer until the ice disintegrates. Second year ice
907

later forms at the base of the ice sheet and there is no evidence
908
of the summer plankton layer being transferred into the second year
ice. The chlorophyll content in the brown layers is from 40
to more than 100 times greater than that in the sea water below the
ice (10,16). An explanation to this is probably a considerable ex­
change of salt and nutrients between the lower layers of ice in
which the algae occur and the seawater beneath it (9).
Later, when the ice has disappeared, a general phytoplankton primary
production can take place. According to the observations of English
(6) on drift station "Alpha", in 1956-58 in the high Arctic, phyto­
plankton appear in the water for only a short time in summer. The
chlorophyll concentrations suddenly increased at the end of June,
but had already decreased in September. The average daily production
for the summer was 5 to 6 mg c/m 2 d (14). Compared with values from
other oceans, especially with Atlantic, this is rather low.
The average annual production in the nearshore zones is 20 g C/m 2 a
(7), and less in the open sea. However, the high production of ice
algae means that the total primary production of the Arctic Ocean
is not necessarily the smallest of all the oceans.
4. REQUIREMENTS FOR A "CRADLE OF LIFE"
The geographical areas in which the primary productivity is high are
often marked by an abundance of mammals or other animals at the top
of the food chain. The simplest food chain is, e.g. when the phyto­
plankton is consumed by zooplankton which again is eaten by a bow­
head whale. Slightly more complicated food chains arise when small
fish and Arctic cod consume the zooplankton and then seals and beluga
whales devour the fish.
The king of the Arctic, the polar bear, is at the top of the food
chain. He catches seals from ice floes. A rich primary production in
the upper water layer, a shallow sea area for rich bottom fauna, an
abundance of seals and ice floes are needed to maintain the polar
bear population. One of the areas is the shelf between Spitzbergen
and Franz Josef Land (Fig.l). Its western part is only 200 to 300 m
deep. A continous flow of drift is through the sounds between the
islands renews the ice field. The drifting floes are mostly first
year ice with many ridges (Figs 4 to 5). The original thickness of

910
the sheet ice was 1.5 m, but in July it had melted to a thickness
of 1 m. It is natural that Kongsoya and the two other islands belong­
ing to the Kong Karls Land in the western part of the area form the
mating place of the polar bear population. Polar bear cubs are born
and raised up on these big islands (16).
5. ENVIRONMENTAL CONCERN OVER POTENTIAL OIL SPILLS
Protection of the Arctic Ocean and the Arctic in general must start
from the fact that the Arctic ecosystem is relatively simple, with
a small number of species and few feeding links. As Dunbar (4) has
mentioned, the simplicity of the food chain has important effects
upon the stability and vulnerability of populations. It has been
found that Arctic populations fluctuate widely, and has been argued
that simple ecosystems are more vulnerable to change than complex
ecosystems in temperate regions. The slow growth rate in Arctic
ecosystem may well be dependent upon both food and temperature. This
means that to animal and plant populations takes a long time to
repair.
Special attention must be paid to the areas which are conductive to
biological production. It is obvious that the influence of these
areas on the Arctic life is not limited to the areas themselves.
They also provide energy for other regions.
With regard to oil spillage, we must consider the microbes as decom­
posers of oil. One surprising fact was found during "Ymer-80" expedi­
tion: despite the low temperature of the water, the number of bacte­
ria was e.S high as in the seas on middle latitude, even at great
depths (16e), and were active at all depths. On other hand, decompo­
sition by microbes can be slow, and the oil can remain for a long
time in an ice covered area. If we allow oil transport during the
short period of fundamental energy incorporation by the Arctic eco­
system, an oil spill might be a catastrophe for the whole Arctic
Ocean. In first place, it would be disastrous to the basic level of
the ecosystem; its external effects on birds and mammals wouls take
second place

REFERENCES
1. Alexander, Vera and N.J. Niebauer. 1981. Limnology and Oceano­
graphy. (In press.)
2. Andersson, L. and D. Dyrssen. 1980. Report on the chemistry of
seawater, XXIV. Department of analytical and marinen chemistry
Chalmers University of Technology and university of GOteborg.
3. Contributions to the Swedish Arctic Expedition "Ymer-80".
(Report of Thor Larsen)
4. Dunbar, M.J •• 1971. MCGill-Queen's University Press.
5. Dunton,K. and Susan Schonberg. 1979. Environmental Assesment of
the Alaskan Continental Shelf. Annual Reoprt (A.C.Broad). Nat.
Oceanic Atmos. Admin., Boulder, Co.
6. English,T.S •• 1961. AINA, Sci.Rep. 15.
7. Fogg,G.E .• 1977. Phil. Trans. R. Soc. London, B 279.
8. Fraker.M.A., C.D.Gordon, J. McDonald, J.K.B.Ford and G.Cambers.
1979. Fisheries and Marine Service, Techn. Rep. 863.
9. Martin, S •• 1970. Geophysical Fluid Dynamics, 1: 143-160.
10. Maykut,G.A. and N.Untersteiner. 1971. Journ. Geophys., Vol.76:
1550-75.
11. Meguro H., I.Kuniyuki and H.Fukushima. 1967. Arctic, Vol.20:114
12. Niemi,A. 1973. Acta Botanica Fennica, 100.
13.Schell,D.M.,1980. Beaufort Sea Winter watch, Spec. Bull. 29:25-30.
14. Strickland,J.D.H •• 1960. Fish.Res.Bd. Canada, Bull. 122.
15. Sverdrup,H.U •. 1953. Joun.Cons.lnt.Explor.Mer. 18:287-295.
16. Ymer 1981, Arsbok. (Expedition "Ymer-80"in Swedish).
a.Palosuo,E. (Ice around "Ymer"): 46-50,
b. Aagard,K., A.Foldvik and B.Rudels. (Physical oceanography):
110-121,
c. Dyrssen,D. (Chemie of the Arctic Ocean): 122-130,
d. Hernroth,L. and L.Edler. (Planktonologically studies): 155-158,
e. Kjellberg,S. (Microbiologically studies): 159-162.
911

Austin Kovacs
Rexford M. Morey
Donald F. Cundy
Gary Decoff
POOLING OF OIL UNDER SEA ICE
U.S.A. Cold Regions Res. and Eng. Lab
Morey Research Co., Inc.
U.S. Coast Guard Res. & Development Center
U.S.A. Cold Regions Res. and Eng. Lab
Abstract
Ice thickness profiles were constructed for six fast ice locations in the V1C1nity
of Prudhoe Bay, Alaska, using a radar echo sounding system. The sounding data revealed
in detail the undulating relief of the bottom of the sea ice in which oil could
pool up if released under the ice. In general, ice bottom morphology was found to reflect
variation of the surface snow cover thickness and ice deformation. However, at
several sites the ice bottom relief could not be correlated with these factors. Slush
ice accumulations of up to 0.5 m were apparently the cause of this bottom roughness.
Estimates of the volume of oil that could pool up in the ice bottom relief range from
20,000 to 60,000 m3 /km 2• For undeformed fast ice with no bottom slush ice growth the
potential pooling capacity varied from about 10,000 to 35,000 m3 /km 2• The effect of
slush ice relief and structure on potential under-ice oil pooling is for the most part
unknown.
Introduction
Offshore oil and gas exploration has been underway for some 10 years in the
southern Beaufort Sea. Seismic studies and drilling results indicate significant oil
and gas fields, and one can anticipate that within 5 years offshore resource production
will be underway, with the attendant risk of oil being released under the sea
ice. The first production will probably take place in areas that are seasonally
covered by relatively undeformed fast ice.
Information on under-ice relief is extremely limited and not well documented.
Most ice thickness information consists of spot measurements, which are of no value in
assessing under-ice oil pooling potential. Even where profile information exists it
generally comprises ice thickness measurements that were taken along a single traverse
of limited length and is therefore of uncertain value. Holtsmark [6), Roots [14) and
Barnes et al. [1) showed the effect of snow cover thickness variation on the growth of
sea ice. Hanson [4) described the accumulation and variation in snow cover thickness
on sea ice during the course of a year. Hibler [5) reviewed the thermal processes responsible
for sea ice growth and decay and discussed the complex effects of varying
snow cover thickness on the ice growth and decay process. His review and the aforementioned
field studies clearly show that because winter snow cover acts as an
insulator, reducing heat exchange from the sea water through the sea ice to the atmosphere,
sea ice growth and therefore thickness vary inversely with snow cover depth
variation. As a result the underside of undeformed annual sea ice has a rolling, hummocky
topography that tends to mirror the long-term snow accumulation patterns on the
ice surface.
912
USA
USA
USA
USA

Ice thickness profile data were collected using a single antenna impulse radar
sounding system. This system was similar to the one used by Kovacs [7] in 1976 to
profile the thickness of both first-year and multi-year sea ice. It was from this
study that the first assessment of the volume of oil which could pool up under sea
ice was made. The radar profile depth data were calibrated against drill hole ice
thickness measurements. An example of the radar sounding data as displayed on a
graphic record is given in Figure 2. A more detailed description of the sounding
system has been given in Kovacs [7].
At Tigvariak Island a 20- by 150-m section of a recently plowed snow-free sea
ice runway was profiled. Snow depths near the runway varied from 24 to 46 cm over a
distance of 120 m. The mean depth was 34 cm with a standard deviation of 7 cm. Ice
thickness was measured along 18 lines 1.1 m apart running parallel to the long axis
of the runway. From the digitized radar ice thickness data cross sections were constructed
(Fig. 3). The under-ice depressions seen in Figure 3 occurred at about 40-m
intervals and in general could be correlated with the major snowdrift relief observed
alongside the runway. The ice thickness data were also used to make a contour map of
the under-ice relief (Fig. 4), in which ice thickness from 1.5 to 1.65 m is shown at
contour intervals of 5 cm. The mean ice thickness was 1.55 m, with a standard deviation
of 0.03 m. The shaded areas represent ice which is less than 1.55 m thick, i.e.
where oil could be expected to accumulate. The black areas represent pockets where
the ice is also less than 1.55 m thick but where surrounding thicker ice (white areas)
might prevent the influx and accumulation of oil. Chances are, however, that some of
these pockets would also fill with oil, as the surrounding ice appeared to be of no
more than average thickness.
For the 20- by 150-m runway area profiled, the quantity of oil which could be expected
to pool up under ice of less than the mean thickness was found to be 0.0320
m 3 /m 2 or 32,000 m 3 /km 2 •
Analysis of the 18 ice profiles in 30-, 60-, 90-, 120- and 150-m-long segments
resulted in the findings shown in Table I. This table shows that mean ice thickness
data from an area 20 m wide and 30 m long are not representative of the total runway
area, but that data from an area 60 m or more long do give a representative mean. An
important finding is that a single traverse only 30, 60 or 90 m long is not reliable
for determining the under-ice oil pooling capacity. For example, the storage volume
above the mean thickness for each of the 18 30-m-long traverses varied from 18,300
m3/km 2 to 56,300 m3/km 2• For the 6D-m-long segments the volume varied from 20,700 to
46,800 m 3/km 2 and for the 90-m-long segments it varied from 25,000 to 39,800 m 3 /km 2 •
Table I shows, as would be expected, that with increasing traverse length the standard
deviation of the potential storage volume which can be determined from a single
traverse decreases. The data in Table I are shown graphically in Figure 5, from which
one can infer that a single ice thickness profile 150 m or more long would have provided
data from which the potential under-ice oil pooling capacity of the local sea
ice area could be determined with a high degree of confidence.
914
Table. I. Mean sea ice thickness and potential under-ice oil pooling
volume for 18 ice profile segments obtained at Tigvariak Island.
Profile length Mean thickness Std dev Mean vol Std dev
(m) (m) (m) (m 3 /km 2 ) (m 3 /km 2 )
30 1.497 0.026 33,200 7,660
60 1.541 0.028 29,700 6,750
90 1.539 0.036 31,100 3,540
120 1.544 0.031 29,500 2,240
150 1.546 0.031 32,000 1,590
1.537* 31,100*
* Mean of means.

Figure 7. Aerial view of undisturbed terrain at the Prudhoe Bay west dock before
the area was cleared of snow and ice blocks. The rectangle encloses the
area profiled for ice thickness. A refrozen crack about 2 rn wide runs between
A and B.
At the west dock site (Fig. 7) we graded off the snow and the uplifted ice
blocks from the sea ice surface. The deformed ice typically was less than 1/2 m high
and consisted of minor ice blocks 15 Cm thick randomly scattered in a sinuous line,
i . e. the blocks did not form an ice pile per se. The profile area was 127 rn wide by
160 m long . Seventy-eight 160-m-long profile lines 1.65 m apart were run. The radar
data showed the mean ice thickness to be 1.83 m with a standard deviation of 0.15 m.
The pocket volume above the mean thickness was 1237 m 3 • This translates into a potential
oil pooling capacity of 60,500 m 3 /km 2 or nearly twice that of the ice area profiled
at Tigvariak Island. This higher storage capacity resulted in part from a
marked variation in snow cover depth, particularly around the uplifted ice, and in
part from deformation features, including ridge keels and refrozen cracks, some about
2 m wide, in which the ice was only about 1 m thick. Contour maps were constructed to
provide insight into the under-ice relief . Figure 8 is a map showing where the ice
thickness was less than (black and shaded areas) or more than (white areas) the mean .
In the shaded areas oil would probably flow under the ice . The black areas are isolated
by deeper surrounding ice (white areas) and might not be reached by the oil;
they represent less than 5% of the total volume above the mean depth .
When the relief map (Fig. 8) was placed over the near vertical aerial view of
the undisturbed study area (Fig . 7) we found that the ice ridges in the northern portion
of the study area corresponded to the white areas of the map, i . e . where thicker
ice existed. In addition , where there were deep snowdrifts along the ridges the ice
was thinner, as it was under some but not all of the snow sastrugi . The location of a
refrozen crack about 2 m wide that passed thr ough the profile area from A to B was
clearly evident on the contour map and indeed overlies this feature ' s surface manifestation,
which is traceable in Figur e 7. One large snowdrift located at position C in
Figure 8 was found to have thinner ice underneath.
917

References
1. Barnes, P.W., E. Reimnitz, L.J. Toimil and H.R. Hill (1979) Fast ice thickness
and snow depth relationships related to oil entrapment potential, Prudhoe
Bay, Alaska, 5th Int. Conf. on Port and Ocean Engineering under Arctic Conditions,
Norwegian Institute of Technology.
2. Cox, J.C., L.A. Schultz, R.P. Johnson and R.A. Shelsby (1980) The transport
and behavior of oil spilled in and under sea ice, Arctec Inc., Report 460C.
3. Greene, G.D., P.J. Leinonen and D. Mackay (1977) An exploratory study of the
behaviour of crude oil spills under ice, the Canadian JOl1rnal of Chemical
Engineering, Vol. 55.
4. Hanson, A.M. (1980) The snow cover of sea ice during the Arctic Ice Dynamics
Joint Experiment, 1975 to 1976, Arctic and Alpine Research, Vol. 12, No.2.
5. Hibler, W.D. III (1980) Sea ice growth, drift, and decay, In: Dynamics of Snow
and Ice Masses, ed. S. Colbeck, Academic Press Inc.
6. Holtsmark, B.E. (1955) Insulating effect of a snow cover on the growth of young
sea ice, Arctic, Vol. 81, No.1.
7. Kovacs, A. (1977) Sea ice thickness profiling and under ice oil entrapment,
9th Annual Offshore Technology Conference, Houston, Texas, Paper OTC2949.
8. Kovacs, A. and R.M. Morey (1978) Radar anisotropy of sea ice due to preferred
azimuthal orientation of the horizontal c-axes of ice crystals, Journal of
Geophysical Research, Vol. 83, No. C12.
9. Larson, L. (1980) Sediment-laden sea ice: Concepts, problems and approaches,
Outer Continental Shelf Environmental Assessment Program, Special Bulletin
No. 29, ed. D.M. Schell, Arctic Project Office, University of Alaska.
10. Malcolm, J.D. (1979) Studies of oil spill behaviour under ice, Proc. Workshop
on Oil, Ice and Gas, Toronto, Ontario.
11. Matthews, J.B. (1980) - Under-ice current regimes in the nearshore Beaufort
Sea, In: Beaufort Sea Winter Watch Ecological Processes in the Nearshore
Environment, Outer Continental Shelf Environmental Assessment Program,
Special Bulletin No. 29, ed. D.M. Schell, Arctic Project Office, University
of Alaska.
12. Reimnitz, E. and K. Dunton (1979) Diving observations on the soft ice layer
under the fast ice at DS-ll in Stefansson Sound boulder patch, U.S. Geological
Survey, Annual Report, 1979, Attachment D.
13. Reimnitz, E. and R. Ross (1979) Lag deposits of boulders in Stefansson Sound,
Beaufort Sea, Alaska, U.S. Geol. Survey Open File Report 79-1205.
14. Roots, F. (1971) Shore fast sea ice, Canadian Polar Continental Shelf Project
Report 71-2.
15. Weeks, W.F. and A.J. Gow (1979) Crystal alignments in the fast ice of arctic
Alaska, U.S. Army Cold Regions Research and Engineering Laboratory, CRREL
Report 79-22.
922

J. D. Malcolm
Associate Professor
A. B. Cammaert
Manager, Engineering Studies
ABSTRACT
MOVEMENT OF OIL AND GAS SPILLS
UNDER SEA ICE
Faculty of Engineering
Memorial University
St. John's, Newfoundland
Acres-Santa Fe Incorporated
Calgary
Canada
Canada
Laboratory studies related to the complex behavior of oil-well blowout products
under Arctic ice are described. The studies include the disposition of crude oil
and gas bubbles under smooth ice and the effects of currents on crude oil behavior
under undulating ice covers when the undulations contain gas.
The potential for ice fracture by a well blowout is reviewed, along with a brief
survey of literature on sea ice roughness. Hypothesis concerning the behavior of
well blowout products under Arctic conditions are discussed.
INTRODUCTION
The potential for Arctic oil spills is increasing dramatically with the level of
petroleum exploration and development acti vi ty. Response to an oil spill in the
Arctic is expected to be slowed by the remoteness, accessibility, severe climate,
and a lack of fundamental knowledge of oil spill behavior under Arctic conditions.
Oil spilled in the Arctic is estimated by Boyd [1 J to remain there for a period of
the order of 50 years because of the slow rate of biological degradation at near-
zero temperatures. During its lifetime, the spill will be diffused widely by the
highly dynamic pack ice. Speculation on the area effected by an Arctic oil spill
has received continuous attention for almost 10 years, largely because of the poten­
tially catastrophic consequences. A lowering of the natural albedo of the Arctic
ice by a widely dispersed spill may lead to permanent removal of the Arctic Ocean
923

ice pack [2] with dramatic reordering of climate and temperatures over the entire
globe. Many other more localized consequences are also possible.
The transport of oil under the ice, particularly from an uncontrolled subsea well
blowout, would represent one of the most efficient mechanisms for diffusing oil over
a large area. Almost all known spill cleanup techniques developed within the past
decade will prove useless until the oil migrates to the top surface of the ice.
Monitoring of the spills' progress is possible, if the spill forms a well-behaved
coherent pool beneath the ice, but detection is unlikely if the spilled oil breaks
up into many widely dispersed globules. It is the purpose of this paper to report
on several laboratory studies related to the motion of well blowout products under
Arctic ice. These studies have been conducted over the past 3 years. We have
resisted the temptation to speculate on the implications of our laboratory results
for the areal extent of an Arctic well blowout.
THE POTENTIAL FOR ICE
FRACTURE BY A WELL BLOWOUT
An Arctic oil well blowout in ice-covered waters would release a m1xture of gas and
oil which would rise to the bottom of the ice cover and spread out beneath it. The
volume of gas released is expected to exceed the volume of oil by a factor of about
150. As the gas moves out under the ice, driven by either existing currents in the
area or the horizontal currents generated as the blowout plume impacts the ice
cover, the gas will displace the water from beneath the ice cover. At some distance
from the plume impact zone, the plume currents may be reduced to the point where the
gas comes to rest, relative to the ice cover.
The forces exerted by a trapped gas bubble under thin ice, which tend to lift and
fracture the ice sheet, have been analyzed by Topham [3]. Assumptions in the model
include isotropic elastic properties for the ice sheet, which in reality is noniso­
tropic, exhibiting both plastic and elastic properties dependent on the past history
and climatic conditions. The analysis considered the ice to be thin, and deflec­
tions small, and fracture was shown to occur either at the bubble center or just
beyond the bubble edge--depending on bubble depth, ice thickness, and material
properties assumed for the ice.
In general, the flattened spherical shape of a gas bubble is truncated at some angle
of contact with the solid which supports the bubble, and the contact angle could be
in the range from 0 degrees to 180 degrees, depending on the surface. In the case
924

The shear zone or seasonal ice zone features the greatest degree of roughness since
it is characterized by hummocks and ridges. Wadhams [7] reports on the topography
of the Beaufort Sea ice cover giving mean ridge height and spacing from airborne
laser profiles. Ridges can act to contain the area of spill, and occur with a mean
spacing of 1.5/km to 2.1/km in the Beaufort Sea with heaving ridging up to 8/km.
Keel depths can average 21.2 m for light ridging and slow drift, 26.8 m for heavier
ridging and fast drift with a 10-year maximum depth of 32.8 m. Wadhams estimates a
vertical roughness scale of approximately 5 em for ice floes between ridges.
The ice cover features in the vicinity of a well blowout cannot be predicted.
However, laboratory studies can provide insight into the physical processes involved
and in particular, can test hypotheses about the relative importance of various
factors. In what follows we describe small experimental spills under both smooth
horizontal ice covers and undulating ice covers.
THE DISPOSITION OF CRUDE OIL
AND GAS BUBBLES UNDER ICE
Laboratory studies consisting largely of photographic observations have been
conducted on the disposition of finite quantities of oil and air under smooth hori-
zontal ice, or rather, its physical analog--plate glass. Air bubbles, like oil
drops, lack sufficient pressure to squeeze out the thin water film separating the
bubble from the glass. The gas bubbles are thus guaranteed to possess a contact
angle of 180 degrees and the bubble thickness under horizontal glass or ice is given
by equation (1) or (2).
An air bubble 40 to 50 mL was first formed under a glass plate submerged 10 to 15 rom
below the free surface in a tank of water. Crude oil drops with volume 0.5 mL were
released at 10- to 15-s intervals 25 em beneath the gas bubble.
The arrival of the first drop of oil at the gas/water interface creates a miniature
spill of gas and oil with volume ratio about 150:1. The oil drop was found to
remain intact for several seconds, presumably waiting for the thin film of water
surrounding it to drain away. The oil drop then coalesces with the gas/water inter­
face and spreads over it to form either a very thin film extending to the edge of
the bubble, or a thick lens of oil confined to the bottom of the gas bubble. In the
case of spreading crudes, the film covering the bubble can be extremely thin,
exhibiting some colors of the spectrum, but a thicker lens of crude near the origi­
nal oil drop location remains.
926

The arrival of a successive train of oil drops at the gas bubble interface produce a
growing oil lens in the gas bubble after each drop coalesces with the lens. Drops
that land near the edge of the gas bubble slide upward along the bubble contour
since the bubble surface is not horizontal, but curved. As the drop slides along
the bubble contour, the film of water preventing immediate coalescence is gradually
draining away. Some drops slide off the gas bubble and onto the ice (or glass in
our experiments) before coalescence takes place. In general, coalescence time is
short for the first oil drop then increases when a thick lens is present in the
gas/water interface. Coalescence time can range from 1 to 2 s to 15 to 20 s and
longer. As the oil lens thickens to about 2 mm, the gas bubble thickness is corres­
pondingly decreased. When a thick oil lens covers the bottom of the gas bubble, oil
drops tend to slide off the bubble more readily, and in the process drag oil from
the lens up the side of the gas bubble. Oil drops shed from the gas bubble accumu­
late on the glass surface and can coalesce to form a puddle, which is connected to
the lens on the bottom of the gas bubble. This occurs when the addition of oil,
drop by drop has reduced the ratio of gas to oil close to 1:1.
Figure 1 illustrates the appearance of a gas bubble under glass with a quantity of a
nonspreading crude oil forming a lens in the bubble interface. The crude oil is a
mixture of Trinidad (24.8 percent), BCF (56.2 percent) and Leona (19.0 percent)
supplied by Ultramar Canada Ltd. Figure 2 shows another gas bubble with a spreading
crude oil lens. The oil is Tijuana Light and was supplied by Imperial Oil Ltd.
Figure 3 shows the lens formed by Guanipa crude, and a thin oil film surrounds the
lens. In Figure 4, the lens size has been increased by additional Guanipa, and a
number of oil drops have slid from the gas bubble contour to take-up positions
beside it.
The experiments just described confirm the two-dimensional analytical study of
Topham [8J, which suggest several stable configurations of oil and gas beneath a
planar ice sheet. OUr experiments provide additional insight into the physical
mechanisms controlling the various stable configurations observed.
The mobility of the gas bubbles beneath smooth glass or ice is such that great care
in leveling the plate had to be exercised to ensure a stable bubble position. The
bubble buoyancy exceeds that of crude oil by a order of magnitude, and earlier
studies on the mobility of crude oil under ice [9J, demonstrate sustained oil motion
along a surface tilted as little as 2 degrees to the horizontal plane.
927

Under rough sea ice the gas bubbles are expected to find their way into undulations,
displacing the water. Whether the undulations will be completely filled with gas
depends on the gas volume available. The effect of currents on crude oil motion
under undulating ice covers when the undulations contain gas is described below.
EFFECT OF CURRENTS ON OIL AND
GAS SPILLS UNDER SEA ICE
Testing was carried out in the Acres' ice flume facility. The flume is a se1f-
contained recirculating system measuring 12 m long, 1.2 m high and 1.2 m wide. The
flume and recirculating line are fully insulated with 10 em of styrofoam. One wall
incorporates a 5.5 m length of acrylic windows behind hinged insulation panels,
allowing visual inspection of test events. Bottom windows were installed to permit
viewing and photographic lighting. A vinyl liner was installed to prevent leakage
and fouling of flume walls with oil.
A variable-speed pump provides continuous flow from 0 to 0.3 m 3 /s. Velocities
under the ice cover were measured with a miniature flow meter. The ice cover was
formed by circulation of air at a minimum temperature of -20·C at a velocity of
7.4 m/s over the water or ice surface.
refrigeration system.
Air was cooled by a conventional 21 kW
The flume was filled with water to a depth of 0.6 m and a sufficient quantity of
salt was added to the freshwater to bring the salinity to 32 pro mille. The water
was cooled to the freezing point (approximately -1. S·C) over a period of 2 days.
The miniature flowmeter was frozen in place in preparation for the test runs.
Thickness measurement scales were frozen in the cover at half-meter intervals along
the length of the flume to measure ice thickness at undulation crests and troughs.
At the same time insulated 20-cm diameter gas/oil injection ports were frozen in the
cover at the trough locations.
Undulations were formed by placing 2.5-cm thick sheets of rigid polyurethane insula­
tion on the crest areas of the undulations, halting the growth of the ice cover at
the desired thickness. The uninsu1ated trough areas of the cover were allowed to
continue growing until the desired undulation shape had been attained. Undulation
wave lengths were 1 m long for the 15-cm deep undulations and 1.5 m long for the
7.5-cm and 2-cm deep undulations.
928

An air piston and curved delivery tube was used to release air through the injection
ports directly below the center of the depressions in the ice cover. After the
desired volume of gas (air was substituted for natural gas for safety purposes) was
placed in the depression under the =ver, the required volume of oil was added
(proportional to the volume of gas) by gravity feed through the curved delivery
tube. The spreading and interaction of the oil and gas were observed under static
conditions, and the oil was allowed to cool.
The flow was gradually increased in steps. The flow was allowed to stabilize at
each step and the behavior of the oil and gas was observed. Tests were performed
for gas/oil ratios of 150:1 and 15:1. Undulation amplitudes tested were 15 em,
7.5 em, and 2 em.
Gas and oil were injected into the 15-cm undulation at a gas/oil ratio of 150: 1.
Following the injection of 9 L of air, 60 mL of oil was injected. The undulation
was approximately one-third full. When the oil was being injected it rose from the
tip of the injection system at the bottom of the flume in a series of pinched off
jets (pinched off in the form of pendent drops). The pendent drops were character­
istically 1 em or 2 cm in diameter at the most, and they rose to the undersurface of
the ice as a series of drops under the action of buoyant forces. The drops would
impact in the ice surface and bounce and deform slightly, gradually roll up the
underside of the curved ice surface to meet the air/water interface at the bottom of
the air pocket. The amount of oil injected was not large (60 mL) and the oil pool
collected at the upstream end of the air/water interface in the undulation. After
the oil was injected the pump was turned on and the velocity increased gradually.
As the flow increased to 14 em/s, the oil began to migrate downstream across the
undulation. At 18 em/s, most of the oil had been herded against the downstream edge
of the gas pocket under the ice. At 30 em/s, it was observed that the oil slick
gradually circulated within the air/water interface, but the oil motion was (J"t
particularly rapid; no oil drops were broken off from the oil slick and swept down­
stream in the high-current flows and, in fact, no motion of the air within the
pocket or waves on the air/water interface were observed whatsoever.
Figure 5 illustrates the sequence of observations on the next test. Gas and oil
were injected at a gas/oil ratio of 150: 1 into a second 15 cm undulation located
upstream from the undulation used in the previous test. A larger volume of gas was
injected in the depression (12 L of air with 80 mL of oil). Initially the air/water
929

interface was at approximately two-thirds of the undulation depth. At a flow velo­
city of 14 em/s, the oil slick started to migrate to the downstream edge of the gas
pocket in the undulation. As the flow was increased in steps beyond 24 em/s, the
oil was definitely herded against the downstream edge of the undulation. At
27 cm/s, fine air bubbles were visible in the flow indicating entrainment of air
from the flume headbox. At a flow of 30 em/s, the slick thickened against the
downstream edge of the gas pocket against the ice. Air entrained in the flow began
filling the undulation. With approximately 2 em of ice below the gas/water inter­
face, the slick was forced out of the undulation. rippling of the oil/water inter­
face and shedding of oil droplets from the slick were observed. The slick elongated
and large droplets of oil were broken off. The oil moved slowly and erratically
under the rough ice cover. The oil seemed to alternately stick to the rough ice
surface and break free.
Additional oil (540 mL) was added to the 15-cm undulation used previously to bring
the gas/oil ratio to 15: 1 with the undulation half full. The oil slick completely
covered the bottom of the gas pocket, but the slick thickness was less than 0.5 em.
As the flow was increased to 30 em/s, no reaction to the flow was observed beyond
migration of oil downstream and slight thickening of the oil slick. Undoubtedly,
the oil slick was protected from the high flows by the depth of the undulation below
the gas/water interface.
A 7.5-cm undulation was filled two-thirds full with 30 L of air and 200 mL of addi­
tional oil. The flow was increased in steps to 18 em/so at which point, significant
herding of smaller slicks to form one large slick at the downstream edge of the
depression occurred. The slick thickened to form a 7.5-mm thick lens. At 25 em/s,
a steady supply of air from upstream entered the undulation and was vented to main­
tain an air pocket thickness of 6 em, leaving a 1.5 em depth of ice (to the tip of
the keel) below the air/water line. The slick rotated to align itself against the
downstream edge of the air/water/ice contact line. At 30 em/s no significant change
had occurred. At 44 em/s, l-mm diameter oil droplets were entrained in the flow.
A 2-cm undulation was filled with 15 L of air and 100 mL of oil to make it half
full. The slick configuration was as observed before; randomly distributed thin
patches of oil.
At a flow of 12 cm/s some herding of oil was observed at the downstream end of the
shallow depression. Herding was more evident at 18 em/s, but the oil was still
930

contained in the depression. At 21 cm/s, a droplet of oil was sheared away from the
downstream edge of the main slick of oil which was starting to flow slowly under the
ice out of the air filled part of the undulation. At 23 em/s, the shedding of drop­
lets from the main slick was more pronounced. At 25 em/s the major portion of the
patch had moved down out of the undulation and under the ice at a velocity of
0.5 cm/s and was elongating and breaking up under the shearing action of the flow.
At 30 cm/s, interfacial waves on the remaining oil slick were observed and shedding
of 2-cm diameter droplets (larger than the previously observed droplets) was
observed. The slick was 0.5 to 1 em thick. More air had collected in the undula­
tion leaving a 0.5 em depth of ice below the air in the undulation.
From the short series of tests performed it can be concluded that the spreading of
oil under an ice cover, when released in the presence of gas, depends on the effec­
tive configuration of the underside of the ice cover. The volume of gas and undula­
tion geometry combine to determine the behavior of the oil slick on the gas/water
interface.
For the meter-long undulations tested, if the gas volume is small or the undulation
depth is large enough that the gas/water interface is more than about 5 em above the
bottom of the undulation of lower ice surface, the oil slick seemed to be fully
protected from the flow. The only effect on the oil slick at velocities as high as
44 em/s was herding or migration of the oil slick to the downstream limit of the
gas/water interface below the gas pocket.
However, when the ice extended only 1.5 em below the gas/water interface, the oil
slick was forced out of the undulation at a flow velocity of 44 em/s and migrated
under the ice cover. When the ice cover extended 0.5 em to 1.0 em below the
gas/water interface, a flow velocity of 25 em/s was able to force the oil out of the
undulation. (It should be noted that the size of the ice lip was difficult to
observe under a fluctuating water level.)
A relationship between flow velocity and depth of ice below the gas/water interface
can not be determined from the limited amount of data available and it is suspected
that the slope at the water line and wave length of the undulation is equally
important. Further investigation should be directed toward the configuration of the
contact line between the gas pocket, water level and ice cover to determine the
effect of the depth and slope of the ice surface that the oil must descend to be
forced from the undulation at a given flow velocity.
931

CONCLUSIONS
In the event of a well blowout under Arctic ice, there is evidence that the blowout
products will be spread out under the ice rather than fracture it. Laboratory
studies reported in this paper indicate that gas will fill the under-ice undulations
near the blowout zone, probably creating a continuous gas/water interface for the
oil to spread on in the form of thin lenses. At a greater distance from the blow­
out, the gas bubble will be fragmented and oil and gas will become trapped in the
ice cover undulations. In this region the oil lenses will be partially protected
from currents by the ice keels below the gas/water line in each undulation.
Future studies directed toward understanding the blowout generated currents, and the
action of a continuously discharged oil and gas mixture under rough ice are
recommended. Only when detailed topographic mapping of the local under-ice surface,
together with detailed knowledge of local currents, becomes available for a particu­
lar well site, then forecasting of the areal extent and ultimate disposition of well
blowout products may become viable.
ACKNOWLEDGMENTS
The experiments on oil and air under glass were conducted by students C. R. Dutton,
C. D. Elliott, J. Hillman at Memorial University with funding provided by the
Natural Sciences and Engineering Research Council of Canada, and Imperial Oil
Limited. The authors are also grateful to Canadian Marine Drilling Ltd., and the
Environmental Protection Service for funding the flume study at Acres laboratories,
and permitting timely release of the results.
932

934
FIGURE I. AIR BUBBLE UNDER GLASS WITH
NON -SPREADING CRUDE OIL LENS AND
DROPLETS, VIEWED AT OBLIQUE ANGLE .
FIGURE 2. AIR BUBBLE UNDER GLASS WITH
SPREADING CRUDE OIL LENS

FIGURE 3 . AIR BUBBLE UNDER GLASS WITH GUANIPA
CRUDE OIL LENS AND DROPLETS, VIEWED
FROM BELOW AT OBLIQUE ANGLE
FIGURE 4. SAME AIR BUBBLE AS IN FIGURE 3 . OIL
LENS IS LARGER. OIL DROPS BESIDE
BUBBLE SLID BENEATH LENS
935

936
SALINE
WATER
INSULATED COVER
INSU LATED BOTTOM
0.45m
0) FLUME CROSS-SECTION AND GAS POCKET
b) OIL INJECTION
WATER FLOW
( 30 em I s )
o
d) OIL DROPLET SHEDDING
•
FIGURE 5 . MECHAN ISMS OF OIL SLICK
MOTION IN FLUME TESTS
0 . 6m
UlllIllIllIDUfLLW1lJJJJJ I IIIIill
WATER FLOW
( UP TO 24 em / s)
c) OIL HERDING
WATER FLOW
( > 30 em Is )
e) OIL MIGRATION

NEED FOR REAL WORLD ASSESSMENT OF THE
ENVIRONMENTAL EFFECTS OF OIL SPILLS
IN ICE-INFESTED MARINE ENVIRONMENTS
Gordon A. Robilliard, Senior Project Scientist Woodward-Clyde Consultants U.S.A.
Michael Busdosh, Senior Staff Scientist Woodward-Clyde Consultants U.S.A.
ABSTRACT
The increase in oil-related activities in the arctic will result in one or more
significant oil spills in ice-infested marine waters. These spills will pro-
bably be persistent and difficult to contain or clean up. Most of the effort to
date has dealt with the fate of oil and developing counter-measures. Relatively
little has been done to assess the real-world environmental, especially ecological,
impacts of the few spills that have occurred to provide a basis for predicting
realistic impacts of future spills. The paucity of impact assessment data appears
to be related to the logistic and safety aspects of sampling spills in ice-infested
waters. The sparse information that is available suggests that the ecological
consequences are not different (or more or less) significant than those resulting
from temperate or tropical spills; in fact, the data from the latter spills are
directly useful in predicting the types of impacts that may occur in polar areas.
The rate and time scale of oil degradation and impacts seem to be longer in
the arctic, primarily due to the cold. However, for at least some macro­
invertebrates, biological processes such as growth, sexual maturation and
others seem to take longer so the overall consequences are probably similar
in all marine environments. With the increase in the oil exploration activities
in arctic regions, accompanied by the concerns of regulators and citizens over
oil spills, increased effort must be placed on documenting the "real world" impact
of oil spills in arctic waters. These data will provide a basis for predicting
with confidence the realistic environmental effects of oil spills in ice-infested
marine environments.
937

INTRODUCTION
The increase of oil exploration production and transportation in arctic
and sub-arctic marine environments will result in one of more significant oil
spills. The oil spilled in these cold, ice-infested or ice-covered waters is
likely to be persistent, become widespread and be especially hard to contain or
clean up [1). Most research and technology development efforts have been
directed to identifying the chemical and physical fate of spilled oil and to a
means of locating, containing and cleaning up spilled oil, especially when ice
is present (2). While this effort is a necessary step in developing effective,
efficient counter-measures, it is also important to decide if these counter­
measures are required for environmental reasons and, if so, where, when and how?
To answer these latter questions we need a better understanding of the actual
environmental effects and consequences of oil spills in ice-infested marine waters.
At present, it is nearly impossible on the basis of existing data from actual
spills to make a realistic prediction about the ecological consequences of an
oil spill in arctic marine environments. Assessments of such spills have
generally been based on "worst case" scenarios while the "most likely case"
scenario receives little attention. For example, the "standard" worst case
scenario is a large spill of crude oil spreading over a large area at freeze-up
in a biologically productive and sensitive habitat. Another worst case scenario
involves a large spill occurring in the Lancaster Sound-northern Baffin Bay
area during the summer when the murres are all flightless and would be unable
to escape oiling. Furthermore, the environmental consequences of these worst
cases in the marine environment have generally been based on impacts
associated with spills in temperate, ice-free environments or, at best, with
spills in northern environments where ice is present on the water for a few
weeks at the most [3,4). Whether or not this is justified is simply not known
with confidence. Very few actual spills in ice-affected environments have been
publicly documented and thus there are few data to use as a basis for predicting
impacts of hypothetical spills.
In most hypothetical impact assessments of polar oil spills, the implicit
assumption is that since oil is not rapidly weathered or biodegraded in arctic
environments, it will have significantly greater ecological consequences than
in warmer waters where oil weathers and biodegrades faster. That is, the
ecological functioning of arctic environments is different somehow than it is
in warmer waters. However, we submit that though oil may degrade slowly (i.e. over
several years or growing seasons) in the arctic, many biological processes such
as growth, sexual maturation, etc. involving individual marine organisms as
well as populations also seem to operate slowly (i.e. on the order of decades).
938

We hypothesize that the environmental consequences of oil spills in arctic
marine environments may be no more (or less) significant ecologically than in
similar habitats in warmer waters once differences in the rate and time scale
for ecological processes are recognized.
REVIEW OF SOME SPILLS IN ICE-INFESTED WATERS
There have been a number of planned as well as accidental oil spills in
ice-infested waters, polar and otherwise. However the information on size and
cause of the spill or type and source of oil is often sketchy and for many
spills, is available only through word of mouth making the accuracy of the
information questionable. It seems clear though that most of the spills have
been (l)accidental, (2) of small volume, and (3) from ships or machinery
exploring for oil or engaged in marine transportation.
In most cases, the major environmental assessment effort of accidental
(and planned) spills has emphasized the chemical and physical fate of the oil
and the efficiency of any countermeasures [1,2,3,5,6,7,8]. Carstens and
Stendstad [8] found that the shore fauna in the immediate vicinity of a
diesel spill was wiped out but that the birds and benthic fauna were apparently
unharmed. However, the benthic fauna had moderately high levels of aromatic­
hydrocarbons in the tissue 67 days after the spill was discovered. A gasoline
and diesel fuel spill on the sea ice at Deception Bay resulted in a loss of
about 50% of the nearshore bivalves and 5% of the pOlychaetes [5]. Fucus and mussels
were more affected by the cleanup measure (burning) than by the oil. The overall
short-term biological damage appeared slight and localized; no estimate of long
term damage or recovery was made. Petersen [9] found that Bunker C spilled in
Melville Bay off Greenland underwent little microbial degradation after 4 weeks
and that weathering was slow due to low temperatures and low light levels. He
reported no adverse biological impacts though he expected the oil might remain
in the bottom sediments for some time.
A few investigators have tested the effects of oil on arctic marine
organisms in field experiements rather than in laboratory situations. Busdosh, Atlas
and their colleagues [12,13,14,15] found that in general, amphipods, pOlychaetes
and other benthic organisms not unexpectedly choose oil-free sediments over oiled
ones and survival was better in the clean sediments. However, after several
weeks to months the differences began to disappear and the organisms' selectivity
was less definite. Busdosh et al also found that acute toxicity disappeared
939

elatively rapidly (i . e. within a period of days to weeks) and that toxicity
appeared to be associated with the lighter hydrocarbon fractions. In these
cold waters bacteria tended to degrade the oil more slowly than in warmer
waters, but they were nevertheless fairly efficient over longer periods .
These results from field experiments are generally supported by numerous
studies conducted in the laboratory under more controlled, though less
environmentally realistic, conditions. We have not attempted to review that
literature here.
OIL SPILLS IN ANTARCTICA
The antarctic benthic community is generally pristine relative to hydro­
carbon pollution due to man's activities. However, in at least two areas where
man has been active, hydrocarbon pollution has been found. Clark and Law [10]
compared aromatic and aliphatic hydrocarbon loads in benthic organisms
(predators) from Signy Island, a pristine area, and King Edward Cove, South
Georgia, a whaling camp abandoned in 1965 after 61 years of use. They found
significantly higher levels of petroleum hydrocarbons in the benthic organisms
at King Edward Cove, probably due to spills from the whaling ships in years gone
by. Platt and Mackie [11] measured the high levels of aromatic hydrocarbons in
sediments of King Edward Cove, but because they did not analyze other pristine
antarctic sediments, they attributed the high levels to worldwide distribution of
pollutants rather than to local pollution.
At Winter Quarters Bay, McMurdo Sound, Antarctica in August-September 1974,
one of us (GAR) observed and photographed (Figure 1) a large bed (about 75m by 25m)
940
Figure 1. Bivalve ( Laternula ) shells in Winter Quarters
Bay, McMurdo Sound, Antarctica

of clam shells, piled several deep on the surface of the bottom. The area is
a shallow depression about ISO-200m long by about 100m wide, at about the 2Sm
depth, and surrounded by a rim about 3-l0m high. The shells, probably Laternula,
were lO-20cm long, and both valves were often side by side. It appeared as though
the clams had been dug up, died, and the shells left in place. However, few
shells were broken indicating that they were not physically removed such as can
occur when icebergs gouge the bottom . We also noted that there was a
considerable amount of trash on the bottom, primarily from the McMurdo Station
garbage dump on the sea ice above and that the fauna was extremely depauparate
compared to physically similar areas nearby (Figure 2).
Figure 2. Sponges, Anemones and other Benthic Organisms at
Hut Point, Ross Island, Antarctica. Photo at same depth about
1 km north from Figure 1.
The surface sediment was reddish-brown to light brown silt, but the
subsurface sediments were black and smelled strongly of petroleum hydrocarbons.
Subsequently, (December 3-6, 1974) samples were obtained from this area as well
as two other sites, one in 3m of water nearshore in Winter Quarters Bay and the other
across McMurdo Sound at New Harbour (30m depth). There was no obvious smell of oil
in the sediments at either place. These three samples were analyzed for
hydrocarbons to determine the amount and type of hydrocarbon fractions present.
The sediments from the clam bed in Winter Quarters Bay contained approximately
0.23% petroleum hydrocarbon by dry weight of sediment. Although a small amount
of the hydrocarbons apparently was biogenic in origin, most of it appeared to be
lubricating oil and possibly heavy residual or Bunker C fuel. No diesel fuel was
present. The hydrocarbon content of the other two samples was approximately
941

0.02 and 0.03%, respectively, for New Harbour and Winter Quarters Bay stations
and most of it was biogenic in origin.
There seems to be little doubt that the presence of a large number of dead
clams and the high level of petroluem hydrocarbons in the sediments are related.
We hypothesize that the petroleum hydrocarbons were introduced to the environment
rather suddenly although we do not know the exact mechanism. It is possible
that they were in containers that were dumped overboard as part of the trash
that is piled in the McMurdo garbage heap, much of which gets dumped into Winter
Quarters Bay when the ice breaks up. It may have also been discharged overboard
during the offloading of fuel from the tanker vessels to the shore-based storage
tanks each summer; these tankers anchor directly over the clam bed area. After
the oil was introduced into the substratum, it probably reached levels high
enough to affect the clams and they attempted to escape the area by crawling
out of and on top of the sediment. They were unable to escape from the area and
finally died. This would explain why most of the shells were unbroken. It is
difficult to tell how long ago the clams died, but several years (1978) later divers
reported that there was no apparent change in the community and the sediments
still smelled strongly of oil. We expect that it will be several years at
least before clams and other deep burrowing forms will once again inhabit this
area, though polychaetes and amphipods are abundant in the thin uncontaminated
surface sediment layer (Oliver, personal communication).
CONCLUSIONS
The general conclusions from these and several ongoing studies including
the BIOS program at Pond Inlet, Baffin Island, under the auspices of the Arctic
Marine Oilspill Programme, seem to be three. First, quantitatively sampling and
assessing the biological consequences of oil spills in ice-infested marine en­
vironments is extremely difficult, largely due to the logistic and safety aspects
of getting to and then working in the ice-infested environment. Second, the
impacts do not appear different, or more or less significant, than those in more
temperate areas; that is, the information based on many, well-documented spills in
sub-arctic, temperate and tropical waters seems to be applicable in principle
though not always in detail. Third, the physical, chemical and ecological
degradation of oil in these cold environments is slower than in temperate areas
so that the time periods over which toxic effects may occur and community recover
is delayed are longer. However, this may not be ecologically significant in the
sense that normal ecological processes often occur more slowly and the final
community response may be similar albeit slower than that described in ice-free
942

environments. Only better documentation of actual spills will provide the final
test for this hypothesis and a basis for predicting with some confidence the
realistic impacts of and recovery from oil spills in ice-infested marine waters.
ACKNOWLEDGMENTS
We are grateful to several colleagues, especially Jon Percy and Eric
Schrier, who provided information about the few arctic oil spills and to
John Oliver who first brought our attention to the presence of oil in the
Winter Quarters Bay sediments and provided the sediment samples for analysis. We
are also grateful to Ed Owens and Jim Sartor for reviewing the paper. We
acknowledge Woodward-Clyde Consultants for providing the typing, graphic and
financial support to prepare this paper.
REFERENCES
[1]. Anon. 1980. An Oilspill in Pack Ice. C-Core Contract Report
(Contract No. 055 79-00007) to Environment Canada, Environmental
Protection Service, Ottawa.
[2]. D. Mackay & S. Paterson (eds.). 1979. Oil, Ice and Gas. Publication
No. EE-14, Institute of Environmental Studies, University of Toronto,
Toronto.
[3]. Arctec, Inc. 1977. Bouchard #65 Oil Spill in Ice-Covered Waters of
Buzzards Bay. Prepared for Alaskan OCSEAP, National Oceanic and
Atmospheric Administration, Boulder, Colorado.
[4]. J. Vandermeulen (ed.) 1978. Proceedings of SympOSium on Recovery
Potential of Oiled Marine Northern Environments. Journal Fish, Research
Board of Canada. 35 (5).
[5]. R. O. Ramseier, G. S. Gantcheff, and L. COlby. 1973. Oil Spill at
Deception Bay, Hudson Strait. Scientific Series No. 29, Inland Waters
Directorate, Environmen't Canada.
[6]. F. G. Barber. 1970. Oil Spills in Ice: Some Cleanup Options. Arctic
23:285-286.
[7]. F. G. Barber. 1971. An Oiled Arctic Shore. Arctic 24:229.
[8]. T. Carstands and E. Sendstad. 1979. Oil Spill on the Shore of an
Ice-Covered Fjord in Spitsbergen. POAC 79, Proceeding of Port and
Ocean Engineering Under Arctic Conditions Conference.
943

[9]. H. K. Petersen. 1978. Fate and Effect of Bunker C Oil Spilled by the
[10] .
[ll] .
[12] .
[13] .
[14] .
[15] .
944
USNS Potomac in Melville Bay-Greenland 1977. Proc. Conference on
Assessment of Ecological Impacts of Oil Spills. AIBS. Keystone, Colorado.
A. Clarke and R. Law. 1981. Aliphatic and Aromatic Hydrocarbons in
Benthic Invertebrates from Two Sites in Antarctica. Marine Pollution
Bulletin. 12(1):10-14.
H. M. Platt and P. R. Mackie. 1979. Analysis of Aliphatic and Aromatic
Hydrocarbons in Antarctic Marine Sediments Layers. Nature (London)
280:576-578.
R. M. Atlas and M. Busdosh. 1976. Microbial Degradation of Petroleum
in the Arctic. Proceedings of the Third International Biodegradation
Symposium. Applied Science Publishers Ltd. London pp. 79-85.
R. M. Atlas, A. Horowitz and M. Busdosh. 1978. Prudhoe Bay Crude Oil
in Arctic Marine Ice, Water and Sediment Ecosystems: Degradation and
Interaction with Microbial and Benthic Communities. Journal Fish,
Research Board Canada. 35(5):585-590.
M. Busdosh, K. W. Dobra, A. Horowitz. S. E. Neff and R. M. Atlas.
1978. Potential Long-term Effects of Prudhoe Bay Crude Oil in Arctic
Sediments on Indigenous Benthic Invertebrate Communities. Proc.
Conference on Assessment of Ecological Impacts of Oil Spills. AIBS Keystone,
Colorado.
M. Busdosh. 1978. The Effects of Prudhoe Crude Oil Fractions on the
Arctic Amphipods Boeckosimus affinis and Gammarus zaddachi.
Dissertation submitted to Department of Biology, University of
Louisville, Louisville, Kentucky III pp.

istics of surface water temperature distribution These are: 1) large scale warm water
patches are formed in the coastal region with the width extending beyond 1.5 km from
the shoreline, and 2) the temperature distribution pattern is classified into three
categories; that is, the first and second ones are closely correlated to the northward
and southward currents and the third one is the transitional stage from the northward
to the southward or vi ce versa. In Fi gure 7, Case A 1 demonstrates a typi ca 1 temperature
pattern induced by the northward current, Cases A2 and A3 represent transitional stages
and Case A4 demonstrates a typical one induced by the southward current. That is to
say, the observation period in September was fortunately set to catch the three stages
of temperature distribution patterns. On the other hand, the pattern observed in
December was only southward.
Through the present investigations, it is realized that the heated water discharged
through the outlets is convected by the large scale alternating current appeared in the
coastal region and is diffused during the convective movement.
The time required to finish the measurement of surface temperature distribution
by using a boat was about 2 hours for one run. Hence a question was arised on the
reliability of measuring technique. In order to check this problem, the airborne infrared
scanning image was taken during the period of Case A4 in Figure 7. The both patterns
derived are basically the same in spite of using different techniques.
(3) Vertical distribution of water temperature: Figure 9 shows the vertical distribution
of water temperature along the three lines set from shore to offshore at the south
outlet, the Ottozawa River, and the Kumakawa River. The present measurement was made
in the morning of September 16, when the southward current was predominant. Therefore
the temperature condition was the same as in Figure 7 (d). From these diagrams it is
recognized that the discharged warm water diffuses within a surface layer of 2 to 3 m
thickness with a layer of temperature discontinuity. The surface temperature decreases
with the distance from the outlet. Figure 10 indicates temperature records of a thermister
chain set at the site of B-N in December 1979. Along the chain eleven thermisters
were installed and the three records of thermisters at the depths of O.Sm, 2m
and 8m from the surface were analyzed. Abrupt temperature rises at O.Sm and 2m
depths are clearly shown in this diagram and are certainly caused by the warm water
patch passing through the measuring site.
(4) Mixing between the warm water and the circumference water: Another interesting
result obtained in due course of measurements is the horizontal temperature discontinuity.
Figure 11 shows the records of two thermisters set at O.Sm and 2.0m depths
below the water surface on the boat and pulled along the measuring line parallel to the
coastline at the distance of 1.5 km on December 14, 1979. These records indicate that
949

the mixing of warm water with surrounding water is not active as we expected and a warm
water front is used to be formed. The temperature difference reaches to 2 to 3 degrees
C and the above front appears to be slick along which dust and foam are gathering.
CONCLUSIons
In the first part of this paper. the authors reviewed the general trend of energy
consumption in Japan and pointed out that the tremendous amount of heated water has
been discharged to the nearshore area and has given some environmental impacts to the
coastal region. However the direct impact of heated water to the marine growth has not
yet be clarified.
On the other hand. great efforts have been devoted to carry out i ntensi ve fi eld
observations. through which a number of information on the diffusion and dispersion of
heated water discharged from the outlet of power stations. The authors also have done
a series of field investigations in September and December 1979. at the Fukushima
Nuclear Power Station, and have realized that the warm water is convected by the long
oscillatory current with the predominant period of 2 to 3 days and diffused during the
movement of warm water patch. The stated oscillatory current seems to be generated by
the migrating front in spring and autumn. and by other unknown reasons.
In order to understand the details of diffusion and disperson processes of warm
water more precisely and to evaluate the environmental impact of warm water on marine
life. synthetic investigations are recommended to accomplish in the near future. However
some approaches have recently been tested to find out its possible influence. For
example. research engineers at the Fukushima Fishery Experiment Station are trying to
catch the behaviour of salmons who are coming back to and going up their mother river
(the Kuma River). the mouth of which is presently affected by the warm water discharged
from the power station. Such joint efforts between engineering scientists and biologist
are cordially appreciated to expand our knowledges on the present complicated phenomena
and to solve the practical problems.
ACKtIOWLEDGEt1EIITS
The present investigation was carried out under the Scientific Research Grant of
the Ministry of Education. Science and Culture. Japan. The authors acknowledge to the
personnel at the Tokyo Electric Company. who kindly permits them to use their valuable
data.
REFERENCES
Nakamura, Y. (1979): Long period oscillation of coastal current. Chapter II. Section
950

L'ACTION DES GLACES SUR LES LITTORAUX
Jean-Claude DIONNE
Departement de Geographie, Universite Laval, Quebec
ABSTRACT
The termglaaiel refers to all processes, forms, sediments or features related
to drift ice action, including icebergs, in the various sedimentary environments.
Glaciel processes occur on about 200 000 km of the world shoreline in both hemi­
spheres. In Canada, approximately 90% of the marine and lacustrine shorelines are
subjected to drift ice action. The subarctic regions and the mid-latitude temperate
regions with cold winters, especially the macrotidal environments, are the most
exposed to ice processes.
Drift ice activity is relatively complex and varies in intensity with latitude
and the changing environment. In general, drift ice is considered an important agent
of erosion, transportation, sedimentation and protection. Drift ice erodes shores
in unconsolidated deposits, and picks up large quantities of sediments which are
dispersed and scattered over various distances according to the melting rate of
floes. Every year, millions of tons of sediments are displaced and distributed by
ice along cold region shorelines giving them a particular aspect. In addition, ice­
push action commonly destroys beaches and alters unconsolidated shorelines. From
a practical point of view, drift ice often acts as a negative process destroying
shore defences, walls and dykes made of unconsolidated material. Drift ice also
constitutes a serious obstacle to coastal navigation and causes problems for access
to harbours. In addition, drift ice commonly fills navigation channels and port
basins with sediments. Drift ice is a universal process that should be considered
seriously in planning the development of cold region shorelines and the design of
new harbours and facilities.
955

La sedimentation glacielle est certainement aussi importante que l'erosion,
puisqu'elle en est la contrepartie. Son caractere positif n'est pas toujours sou­
haitable; c'est Ie cas des apports de sediments dans les chenaux de navigation,
les voies d'approche des installations portuaires et les bassins de mouillage pres
des quais.
La sedimentation glacielle presente plusieurs facettes suivant les milieux.
II existe des differences fondamentales par exemple, entre Ie bas et Ie haut estran
(plage), entre les rivages a pente faible et a pente raide, entre les rivages ro­
cheux et ceux en materiel meuble. Ces differences ont ete mises en evidence dans
diverses publications [22]. Rappelons simplement que si les glaces deposent leur
charge detritique au hasard de la fonte, elle en abandonnent la plus grande partie
dans les zones littorales et prelittorales.
Au Quebec par exemple, dans les zones intertidales du Saint-Laurent et de la
baie de James, les glaces abandonnent chaque annee plusieurs millions de tonnes
de debris [19,27]. L'apport de blocs erratiques constitue probablement la manifes­
tation la plus evidente. Sur la rive sud du Saint-Laurent, les estrans sont fre­
quemment couverts de blocs cristallins provenant du Bouclier canadien situe sur la
rive nord, entre 10 et 35 km de distance. Or, ces blocs reposent directement sur
des sediments marins fins post-glaciaires de plusieurs metres d'epaisseur. La meme
situation prevaut sur la cote orientale de la baie de James ou des centaines de
milliers de cailloux capitonnent les estrans argileux, vaseux ou sableux. Le con­
traste frappant entre ces elements grossiers et les substrats meubles qui les por­
tent met en evidence Ie role majeur des glaces dans la sedimentation littorale des
regions froides.
Les glaces transportent aussi de grandes quantites de sediments dont la taille
est inferieure a celIe des blocs (25 cm). On trouve frequemment, a la surface des
estrans et des marais littoraux, des semis ou des ilots de galets, gravier, sable,
vase ou argile qui contrastent avec les substrats qui les portent. Dans plusieurs
baies ou rentrants des rives du Saint-Laurent et de la baie de James par exemple,
les apports glaciels de toutes sortes sont si abondants qu'ils jettent parfois dans
l'ombre l'action des vagues et des courants. La meme situation prevaut dans plu­
sieurs autres regions froides dans Ie monde.
Le role sedimentologique des glaces flottantes n'est pas restreint aux seuls
apports de sediments. Les glaces influencent parfois profondement la sedimentation
963

deterres et exposes au froid; il s'en suit une augmentation considerable de la
mortalite. Les vegetaux n'echappent pas a 1a destruction par 1es glaces. Les
plantes halophi1es et 1es algues sont fauchees ou arrachees par 1es glaces; cer­
tains peup1ements peuvent meme etre dangereusement decimes. De plus, arbres et
arbustes a la limite des hautes mers souffrent aussi de l'erosion glacielle.
Ces quelques exemples montrent 1a necessite d'etudier serieusement le glacie1
en milieu littoral, en particulier sur 1es cotes ou i1 existe des amenagements
pour 1a navigation ou pour l'exp1oitation des richesses nature11es. C'est pour­
quoi 1es deve10ppements en ce sens dans la mer de Beaufort et 1a plate-forme du
Labrador tiennent compte de plus en plus des effets nefastes des glaces flottantes
et des icebergs [12, 53].
CONCLUSION
En conclusion, rappe10ns que si l'action des glaces est relativement bien
connue dans ses gran des 1ignes, i1 reste encore beaucoup a faire loca1ement pour
preciser ou quantifier cette activite et trouver des solutions efficaces pour
contrer 1es effets negatifs des glaces. Heuseusement, de plus en plus de specia-
1istes prennent conscience de ces prob1emes et oeuvrent ales resoudre. Rappe10ns
aussi que 1es regions po1aires ne sont pas forcement 1es plus serieusement menacees.
Les littoraux des regions subarctiques et temperees a hiver froid, plus peuplees
et plus 1argement amenagees, souffrent davantage des effets negatifs des glaces.
967

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971

51. PRESTWICK, J., 1886: Geology; Vol. I, Chemical and Physical; Oxford,
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of the Beaufort Sea, J.C. Reed et J.E. Sater, eds.,
Arctic Inst. North America, p. 301-351.
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972

64. WASHINGTON, 1958a: Arctic Sea ice; Proc. on Arctic sea ice, (Easton,
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Proc. 4th Conf. Coastal Eng. (Berkeley, Calif.),
p. 201-206.
973

John R. Harper
and
E.H. Owens
ANALYSIS OF ICE-QVERRIDE POTENTIAL
ALONG THE BEAUFORT SEA COAST OF ALASKA
WOODWARD-CLYDE CONSULTANTS
16 Bastion Square
Victoria, B.C.
vaw 1H9
ABSTRACT
Ice override, the process of sea ice thrusting landward across beaches and bar­
rier islands, has been identified as a potential hazard to drilling and exploration
activities along the Beaufort and Chukchi Sea coasts of Alaska. In order to better
define the risk associated with ice override as an envircnmenta1 hazard, the objec­
tives of this study were to (1) estimate the frequency of override occurrence along
the Beaufort and Chukchi Sea coasts, and (2) delineate areas or regions of high ice­
override potential. The results are based on an objective analysis of vertical aerial
photographs and indicate that ice override occurs infrequently along both the Beaufort
and Chukchi Sea coasts of Alaska. Evidence of 11 override events was observed on the
611 km of Beaufort Sea coasts that were surveyed. Evidence of 9 override events was
noted on the 970 km of Chukchi Sea coasts that were surveyed. Return periods for ice­
override events, obtained by combining the frequency of occurrence estimates in a
probability model, are estimated at 93 years for the Beaufort Sea coast and 215 years
for the Chukchi Sea coast. Return periods for the more severe override events are es­
timated at 341 years and 323 years for the Beaufort and Chukchi Sea coasts respect­
ively. Within the framework of the overall probability of occurrence of ice override,
it is recognized that some locations may have higher levels of override potential.
The study delineates three shoreline segments, two on the Chukchi coast and one on the
Beaufort coast immediately east of Barrow, as having a greater potential of override
occurrence, and suggests that the coastline east of the Colville River has a slightly
lower level of override potential.
974

INTRODUCTION
Ice override is the process by which sea ice moves landward across the shore
zone as an unbroken sheet and penetrates substantial distances inland from the high­
water line. As such, ice override represents a potential hazard to structures placed
on or near the shore. It is important to distinguish iae override from iae push,
which also involves the landward movement of ice but which is limited to the zone of
normal wave activity (usually

along Tapkaluk Island (20 km east of Barrow), where l-m thick ice overrode approxi­
mately 3 km of coast and penetrated inland as much as 150 m [2]. This override event
occurred during mid-winter (January, 1978) and is thought to have resulted from a
large shore lead closing against the shore. Other more severe events have been docu­
mented in the Canadian arctic, where one massive override of l-m thick ice penetrated
as much as 185 m inland from +he high-water line [8]. Even thin ice «20 cm) can
penetrate substantial distances inland, up to 100 m; however, analysis of the re­
ported events suggests that a 30-60 m override-penetration distance is more typical
[3]. In plan form, the ice often may appear as long tongues or fingers, as the pene­
tration distance often exceeds the alongshore width of the override.
There is no particular site condition which is consistently associated with lo­
cations of ice-override, although Owens and McCann [6] note that in the Canadian
arctic, ice override tends to be concentrated near major promontories along the coast.
Override events have occurred in areas of low nearshore gradients of moderate offshore
ice regimes and where offshore submarine bars are present.
The existing reports on ice-override events provide little information on either
the frequency of ice override as a process, or the type of site conditions that fa­
vovr ice override. It is difficult to establish the frequency from the existing in­
formation, as there exists an element of uncertainty whether events have simply been
unnoticed in the past, or if ice override is actually a rare event (both in time and
space).
AIR PHOTO ANALYSIS
Air photo interpretation was used to develop an ice-override data base that
could be used to define the frequency of events. Low-level air photos (Tables 1
and 2) were examined for direct evidence (i.e., ice on the beaches and backshore) and
indirect evidence of ice-override events (i.e., ice-thrust scars on the beach, sedi­
ment push-piles or gravel mounds, striations in the beach sediments). Where evidence
of an ice override was noted, relevant information on the override characteristics
(penetration distance, alongshore width, etc.) and site characteristics (foreshore
slope, distance to 20 m contour, offshore ice-regime severity) were noted (see
Tables 3 and 4).
The survey technique and the resulting data base incorporate limitations with
regard to the interpretations that can be made. The frequency values may be an
underestimate, as (1) some of the override events may not be visible on the photos,
either because the remnant scars were too small to be resolved, or because the beaches
976

The air-photo survey data were used to quantify the risk-evaluation analysis.
The data were combined into a probability model to estimate the likelihood of an over­
ride event affecting a given section of shoreline in a given length of time. The re­
sults show that the probability of override occurrence along a 0.5-km segment of shore
in a 20-year period ranges between 0.056 and 0.192 for the Beaufort Sea coast, and
0.060 and 0.088 for the Chukchi Sea coast. The lower value of each probability range
represents a probability associated with the more severe events. The corresponding
return periods for the occurrence of ice override range from 93 to 341 years for the
Beaufort Sea coast, and 215 to 323 years for the Chukchi Sea coast. The longer re­
turn period of each range is associated with the more severe events.
REGIONAL VARIATIONS IN OVERRIDE
It should be noted that the frequency estimates (and associated probabilities)
are average estimates for a random segment of coastline. The actual estimates for
specific segments of coastline may be higher or lower than the average estimates,
depending upon temporal and spatial factors applicable to the given segments.
Beaufort Sea Coast
The distribution of override events shows a relatively high concentration in the
coastal segment immediately east of Point Barrow (Fig. 1). Six (54 percent) of the
observations were concentrated on 27 percent of the surveyed shoreline. Of special
interest is that 5 of the 11 events were identified on Tapkaluk Island (Fig. 1, events
6 and 7), Martin Island (events 1 and 8), and Igalik Island (event 9), which are the
islands where three large 1978 overrides occurred [2].
Chukchi Sea Coast
The distribution of events shows a distinct concentration of events near Point
Barrow, and also in the vicinity of Point Lay (Fig. 2); 78 percent of the events were
concentrated within these two segments, which comprised 25 percent of surveyed shore­
line (11 percent in the Point Lay area and 14 percent in the Point Barrow area). The
concentration of events near Point Lay is likely associated with a locally more in­
tense offshore-ice regime [7] and a slight flexure in the coastline.
Discussion
It is clear that considerable spatial variation exists within the overall fre­
quency estimates of ice override on both the Beaufort and Chukchi Sea coasts. Both
980

00
.... '"
..
AUF 0 T 5 E
Figure 1. The combined distribution of published • and surveyed CD ice override events
along the Beaufort Sea coast; event numbers are keyed to Table 3.

Figure 2. The combined distribution of published .. and surveyed CD ice override events
along the Chukchi Sea coast; event numbers are keyed to Table 4.

the published and surveyed override data identify three zones of high override poten­
tial - one on the Beaufort Sea coast immediately east of Barrow, a second near Barrow
on the Chukchi Sea coast, and a third near Point Lay on the Chukchi Sea coast. The
trends are especially important in terms of an interpretation of previous override re­
ports. The implication is that the Barrow region, both to the east and southwest of
Barrow, is not representative of the Alaskan coast as a whole, and that observations
on both the frequency and severity of override events should not be extended to other
sEctions of the coast without qualifications.
The ice-override data base is not sufficiently large to delineate site condi­
tions which are conducive to ice override, although there appears to be a general as­
sociation of high override frequency with zones of intensive offshore ice movements
and with large-scale promontories not protected by offshore shoals. Intuitively, one
would expect that the following factors would increase the probability of ice override
in a particular area: severe offshore ice regimes, steep offshore and nearshore grad­
ients, the absence of offshore bars and shoals, and low backshore elevations.
SUMMARY AND CONCLUSIONS
Ice override is known to occur throughout the arctic, and has been documented
along the Alaskan arctic coast. Ice override is capable of penetrating substantial
distances inland from the coast, up to 150 m in some cases, and as such must be re­
garded as a potential environmental hazard to structures placed near the shore zone.
The level of risk associated with this process along the Alaskan arctic coast is rela­
tively low, because ice-override is rare both in time and space. It should be empha­
sized, however, that these results are derived for the Alaskan arctic coast, and ice
override is a significant modifying process in other areas of the arctic (e.g., the
south coast of Viscount Melville Sound is completely dominated by ice-override scars).
The following specific conclusions may be drawn from the study:
(1) An objective inventory of ice-override events, derived from
vertical air-photo interpretation, provides an estimated frequency
of ice override as 0.018 events/km of shoreline for the
Beaufort Sea coast, and as 0.009 events/km of shoreline for
the Chukchi Sea coast. The combined frequency for the entire
Alaskan arctic coast is 0.013 events/km.
(2) The probability of override in any given O.s-km section of
coast is low. For the Beaufort Sea coast, the probability
of override occurrence in a 20-year time period ranges from
0.19 to 0.056, with corresponding return periods of 341 to 93
years. For the Chukchi Sea coast, the probability of override
during a similar 20-year interval ranges from 0.06 to
0.088, with corresponding return periods of 323 to 215 years.
(3) Analysis of the vertical aerial photographs has tentatively
defined three zones of high override potential. The zones are
%3

located (a) near Point Lay on the Chukchi Sea coast, (b) near and including
Point Barrow on the Chukchi Sea coast, and (c) on the segment
of shoreline immediately east of Point Barrow (to Cape Simpson) on the
Beaufort Sea coast. The important implications are that the Barrow
area is not typical of the Alaskan arctic coast as a whole, in terms of
override potential, and that observations from the Barrow area on both
the frequency and the severity of events should not be extended to
other areas without qualification.
ACKNOWLEDGEMENTS
Funding for this study was provided through a contract to Woodward-Clyde Consul­
tants from Chevron, U.S.A., who also gave permission to publish the results.
Ram Kulkarni of Woodward-Clyde Consultants performed the probability analysis.
REFERENCES
[1] A1esta1o, J., and Haikio, J., 1976. Ice features and ice-thrust shore forms at
Luodonse1ka, Gulf of Bothnia in winter 1972-1973. Fennia 144, Geographical
Society of Fin1arld, Helsinki, 24 p.
[2] Hanson, A., Metzner, R., and Shapiro, L., 1978. Ice shove in the Point Barrow
area. Arctic Project Bulletin #22, (OCSEAP), p. 4-8.
[3] Harper, J.R., 1980. Analysis of ice override potential along the Beaufort Sea
coast of Alaska. Woodward-Clyde Consultants, Anchorage, Alaska, unpublished
proprietary report to Chevron, U.S.A., San Francisco, 114 p.
[4] Kovacs, A., and Sodhi, D.S., 1979. Ice pile-up and ride-up on arctic and subarctic
beaches. Proc. of 5th International Conference on Port and Ocean Engineering
under Arctic Conditions, Norwegian Institute of Technology,
Trondheim, p. 127-146.
[5] ,1979. Shore ice pile-up and ride-up, field observations, models,
theoretical analyses. O.N.R. Workshop on Problems of the Seasonal Sea Ice
Zone, Naval Postgraduate School, Monterey, California, 84 p.
[6] Owens, E.H., and McCann, S.B., 1970. The role of ice in the Arctic beach environment
with special reference to Cape Ricketts, southwest Devon Island,
N.W.T., Canada. American Journal of Science, 268(5), p. 397-414.
[7] Stringer, W.N., 1978. Morphology of Beaufort, Chukchi and Bering Seas nearshore
ice conditions by means of satellite and aerial remote sensing. NOAA­
OSCEAP, Boulder, Colo., Annual Report of Principal Investigators,
Vol. I (218 p) and Vol. II (576 p).
[8] Taylor, R.B., 1977. The occurrence of grounded ice ridges and shore ice piling
along the northern coast of Somerset Island, N.W.T. Arctic, 31(2),
p. 133-149.
984

Austin Kovacs, Research Civil Engineer
Devinder S. Sodhi, Research Hydraulic Engineer
SEA ICE PILING AT FAIRWAY ROCK,
BERING STRAIT, ALASKA:
OBSERVATIONS AND THEORETICAL ANALYSES
U.S. Army Cold Regions
Research and Engineering
Laboratory
Abstract
Information on sea ice conditions in the Bering Strait and the icefoot formation
around Fairway Rock, located in the strait, is presented. Cross-sectional profiles
of Fairway Rock and the relief of the icefoot are given along with theoretical
analyses of the possible forces active during icefoot formation. It is shown that
the ice cover most likely fails in flexure as opposed to crushing or buckling, as the
former requires less force. Field observations reveal that the Fairway Rock icefoot
is massive, with ridges up to 15 m high, a seaward face only 20' from vertical, and
interior ridge slopes averaging 33'. The icefoot is believed to be grounded and its
width ranges from less than 10 to over 100 meters.
Introduction
In the design of offshore structures to be placed in arctic waters, major consideration
is being given to determining the loads developed during ice failure
against a structure. This can result in the creation of an ice rubble field through
which forces can be transmitted to the structure during subsequent sea ice movement.
The phenomena of ice pile-up and override are also being considered.
A variety of analytical and model studies have been made to investigate ice
forces on offshore structures, but they are inconclusive as their results have not
been verified by field measurements. This paper presents the results of a study of
the general configuration of ice rubble around Fairway Rock, Alaska. The purpose of
the investigation was to acquire data on the morphology of the sea ice rubble surrounding
the rock that would be applicable to offshore structures in general and
particularly to those placed in deep water. Estimates of the relative force levels
required to form the observed rubble are also given. The surface relief across a
number of sea ice rubble fields formed in the shear zone along the west side of
Prince of Wales Shoal is also described for the purpose of documenting the size of
ice features which may impact offshore structures placed in the northern Bering Sea.
Bering Strait
The Bering Strait is 85 km wide and has an irregular bottom, with a depth of 52
m near the western side and about 60 m near the eastern side. On the western side of
the strait is the bold topography of Cape Dezhneva and on the eastern side is the
formidable landscape of Cape Prince of Wales. Winds in the strait tend to be
funneled and accelerated in northerly or southerly directions by these headlands.
Within the Bering Strait are three islands (Figure 1): Little and Big Diomede
Islands and Fairway Rock. Fairway Rock, situated about 24 km west-southwest of Cape
Prince of Wales, Alaska, is a 350-m-diameter igneous rock that rises almost vertically
out of 50-m-deep water (Bloom, personal communication) to a height of about 165
USA
985

Figure 2. Ice conditions in Bering Strait on 7 March 1973. Note the splitting of an
ice floe moving past Fairway Rock, and the turbulent wind wakes downstream
of various landforms.
Coachman and Aagaard [3] show that transport through the Bering Strait is well
correlated with regional east-west atmospheric pressure differences: "When a strong
E-W pressure gradient lies over the strait and extends in a north-south direction
from the Chukchi Sea to the central Bering Sea, entirely crossing the northern Bering
Shelf, extensive northerly winds move water southward off the shelf . This produces a
sea-level slope down to the south which, together with the northerly winds, drives
southward transport. The atmospheric pressure pattern causing this condition is always
a str ong low located a considerable distance southeast of the strait (e.g. over
Kodiak) together with the Siberian high being centered some distance west of the
strait."
Movement of ice southward through the Bering Strait is therefore driven by
northerly winds and southerly current flow. The resulting coupling of the wind and
current produces drag forces on the ice which exceed its arching strength. Thus the
987

Seo
Figure 3. Area map of the Bering Strait .
Dashed lines indicate zone of
major ice stream which occurs
during large southern ice drift .
Stippled area is zone of maximum
drift.
/
current produces drag forces on the ice
which exceed its arching strength.
Thus the arch or jammed-up ice bridging
the strait fails and moves rapidly
southward (Figure 2).
Under strong driving forces acting
over a prolonged period of time, sea
ice in a belt 100 or more km wide, extending
from the strait northward along
the east side of the Chukchi Sea to Pt
Barrow, gradually moves southward (Figure
3). This movement may eventually
bring multi-year ice floes into the
northern Bering Sea.
Once the Bering Strait ice arch
collapses , in excess of 60,000 km2 of
sea ice from the Chukchi Sea can move
southward at high velocities in a relatively
short period of time [1]. Fairway
Rock is situated in the center of
the major ice floe stream through the
strait. As a result the rock is frequently
impacted by either southerlyor
northerly-moving ice floes throughout
the winter season.
Field Reconnaissance
On 26 April 1980 a reconnaissance
of Fairway Rock was made. Low cloud
cover, high winds and associated turbulence
around the rock caused the small
aircraft to be thrown around and prevented
a close-up inspection of the
icefoot . Nevertheless, the photos taken show an impressive accumulation of pressured
ice forming the icefoot, particularly on the north and south sides. At the time of
the reconnaissance, open water surrounded the island. Off to the south significant
open water and a diffused pack were noted, whereas to the north the pack ice could be
988
Figure 4. South side of Fairway Rock .

Figure 5. North side of Fairway Rock.
Figure 6. East side of Fairway Rock. Dark objects on
top of island are large propane gas tanks
and a generator installation.
seen in an apparent holding line between the Diomede Islands and Cape Prince of
Wales . The northerly winds, while high, were not strong enough to drive the pack ice
south against the prevailing southerly current .
A view of the south side of the rock is shown in Figure 4. Note the steep face
of the icefoot and the slope of the top of the island. The northern half of the island
is seen caked with snow, and along the base the rock surface is covered with a
layer of glaze ice (Figure 5). In this photo the large rock talus area is visible,
as is the shear face of the massive icefoot. An east view of the rock (Figure 6)
shows portions of the vertical rock face at sea level and thick icing accumulations
covering the rock to an apparent height in excess of 40 m above sea level.
989

Figure 7. Aerial view of Fairway Rock. Elevation profiles were made, from
stereographic photo analysis, along the lines shown. North is to the
right .
Aerial photography of Fairway Rock was obtained on 14 March 1980. An aerial
view of Fairway Rock is shown in Figure 7. The ridge systems composing the southern
icefoot are quite apparent due to the favorable sun angle . The general bluntness of
the icefoot is striking, as is the narrow width of the icefoot on the west and southeast
sides of the rock. In these areas the rock wall seems to drop vertically into
the sea. Unlike the view of the rock shown in Figure 4 the south face of the rock at
this time was caked with snow and/or glaze ice . The top of the rock is clearly windswept;
visible are shadows from a cluster of propane tanks and a small generator used
to power salinity, temperature, conductivity and current instruments on the sea
bottom (Bloom, personal correspondence) .
990

1
/1
/1
/1
/ I
/ I
I I
Figure 14. Geometry of wedge-shaped ice sheet with idealized
load distribution parameters. Note the distribution
of the stress 0xx along a line at x distance from
the apex.
Table I. Buckling pressure for different values of wedge-angle
a. R/Lp and ice thickness h.
R/L P
h L 0.1 1.0 10
P
Buckling pressure
m (in. ) m ( ft) MPa ( psi) MPa ( psi) MPa
a = 30·
0.3 ( 11.8) 3.98 (13) 4.84 (702) 1.01 (146) 0.58
0.5 (19.7) 5.85 (19) 6.45 (906) 1.30 (189) 0.74
1.0 (39.4) 9.81 (32) 8.84 (1281) 1.85 (268) 1.05
1.5 (59) 13.30 (44) 10.82 (1570) 2.26 (328) 1.29
a = 90 0
0.3 (11.8) 3.98 (13) 5.78 (838) 1.09 (158) 0.53
0.5 (19.7) 5.83 (19) 7.46 (1082) 1.41 (204) 0.74
1.0 (39.4) 9.81 (32) 10.55 (1529) 1.99 (289) 1.05
1.5 (59) 13.30 (44) 12.92 ( 1873) 2.44 (354) 1.28
a = 180 0
0.3 (11.8) 3.98 (13) 9.23 (1339) 1.36 (198) 0.51
0.5 (19.7) 5.83 (19) 11.92 (1729) 1.76 (255) 0.66
1.0 (39.4) 9.81 (32) 16.86 (2445) 2.49 (361) 0.93
1.5 (59.0) 13.30 (44) 20.64 (2995) 3.05 (442) 1.14
996
( psi)
(84)
(108)
(153)
( 187)
(77)
(107)
(152)
(186)
(74)
(96)
(135)
(166)

to its effective diameter. In order for this to be so the submarine slope needs to
be relatively steep. At Fairway Rock it is reasonable to assume that the shallowest
submarine slope was at or near the angle of repose of the rock talus.
Fairway Rock appeared encrusted with a thick layer of glaze ice that extended up
to 40 m above sea level. For offshore structures placed in these waters this phenomenon
needs to be considered from a loading as well as an operational hindrance
standpoint.
The force levels given here are only estimates based upon assumed ice data and
geometry considerations. Larger forces are possible at local contact pOints where
stress is concentrated. The calculated effective pressures due to crushing and
flexural failure of ice 1.5 m thick against Fairway Rock were estimated to be 3000
kPa (435 psi) and 414 kPa (60 psi), respectively. Since the ice cover in the
vicinity of Fairway Rock is believed to comprise floes not sufficiently confined to
move against the rock and crush against it over its entire width, these effective
pressures are considered to be high. Out of the two possible modes of failure, the
ice cover would most likely fail in flexure as opposed to crushing, since it requires
less force.
We have presented expressions to show that buckling is also entirely possible at
areas of local sheet ice contact, even when the far-field ice pressure is relatively
low. Indeed, we envision that all three failure modes will occur randomly during ice
failure against a large structure or during ice pack deformation.
Acknowledgments
Support for this study was provided by Gulf Oil Canada Resources Incorporated
and in part by the Bureau of Land Management/National Oceanic and Atmospheric
Administration's Alaska Outer Continental Shelf Environmental Assessment Program.
References
1. Ahlnas, K. and G. Wendler (1979) Sea-ice observations by satellite in the
Bering, Chukchi and Beaufort Seas, Proceedings of the Fifth International
Conference on Port and Ocean Engineering Under Arctic Conditions, Norwegian
Institute of Technology, Trondheim, Norway.
2. Bloom, G.L. and D.E. McDougal (1967) Bering Strait unattended oceanographic
telemetry system utilizing a commercial LCC-25 strontium 90 generator, The
New Thrust Seaward, Transactions of the Third Annual Marine Technological
Society Conference and Exhibit, 5-7 June, San Diego, Cal., Marine
Technological Society, Washington, D.C.
3. Coachman, L.K. and K. Aagaard (1979) Re-evaluation of water transports in the
vicinity of Bering Strait, preprint of paper to appear in Bering Sea
Symposium, Hood, Ed.
4. Croasdale, K.R. (1980) Ice forces on fixed, rigid structures, Working group on
ice forces on structures: A state-of-the-art report, T. Carstens, Ed.,
CRREL Special Report 80-26.
5. Kovacs, A. and M. Mellor (1974) Sea ice morphology and ice as a geologic
agent in the southern Beaufort Sea, The Coast and Shelf of the Beaufort
Sea, Reed and Sater, Eds., Arctic Institute of North America, Arlington,
Va.
6. Kovacs, A. and D.S. Sodhi (1980) Shore ice pile-up and ride-up, field
observations, models, theoretical analyses, Cold Regions Science and
Technology, Vol. 2.
7. Kry, P.R. (1977) Ice rubble fields in the vicinity of artificial islands,
Proceedings of the Fourth International Conference on Port and Ocean
999

Engineering Under Arctic Conditions, Memorial University of Newfoundland,
St. John's, Newfoundland, Canada.
8. Michel, B. (1970) Ice pressure on engineering structures, CRREL Monograph
III-BIb.
9. Michel, B. and N. Toussaint (1976) Mechanism and theory of indentation of ice
plates, Symposium on Applied Glaciology, Cambridge, England, Journal of
Glaciology, Vol. 19, No. 81.
10. Parmerter, R.R. and T.D. Coon (1973) Model of pressure ridge formation in
sea ice, Journal of Geophysical Research, Vol. 77, No. 33.
11. Shumway, G., D.G. More and G.B. Dowling (1964) Fairway Rock in Bering
Strait, Papers in Marine Geology, Shepard Commemorative Volume, R.L.
Miller, Ed., Macmillan.
12. Sodhi, D.S. (1979) Buckling analysis of a wedge-shaped floating ice sheet,
Fifth International Conference on Port and Ocean Engineering Under Arctic
Conditions, Norwegian Institute of Technology, Trondheim, Norway.
13. Sodhi, D.S. and H.E. Hamza (1977) Buckling analysis of a semi-infinite ice
sheet, Fourth International Conference on Port and Ocean Engineering under
Arctic Conditions, Memorial University of Newfoundland, St. John's,
Newfoundland, Sept 26-30, 1977.
14. Wang, Y.S. (1978) Buckling analysis of a semi-infinite ice sheet moving against
cylindrical structures, International Association of Hydraulic Research
Symposium on Ice Problems, Lulea, Sweden.
15. Wang, Y.S. (1978) Buckling of a half ice sheet against a cylinder, Proceedings,
ASCE Journal of Engineering Mechanics Division, EMS.
1000

Stuart D. Smith
and
Erik G. Banke*
ABSTRACT
A NUMERICAL MODEL OF ICEBERG DRIFT
Atlantic Oceanographic Laboratory
Bedford Institute of Oceanography
Dartmouth, Nova Scotia B2Y 4A2
The movement of icebergs under the influence of winds and currents
has been hindcast using a simple numerical model. Air and water drag
coefficients have been adjusted to give a best fit to the observed drift
in five of seven cases investigated, while in two other cases the observed
winds and currents cannot explain the observed track.
INTRODUCTION
Exploration activity in iceberg infested waters off the east coast
of Canada has brought new urgency to problems of tracking and predicting
iceberg movement. It will eventually be necessary to predict these
motions in the vicinity of a rig or structure over distances of a few
tens of kilometers and time scales of about one day.
In the present study we attempt to hindcast the motion of icebergs
using data which have been taken on a routine basis. The purposes of
this study are to determine whether, or in what circumstances, these
data are adequate to describe the dynamics of iceberg motion, and to
infer what additional data may be necessary to better describe the
dynamics. In addition we are able to examine the relative influence of
winds and currents, and will in a future paper use this model to predict
what influence towing forces would have had on the modelled tracks.
*Nowat: Martec Ltd., 1526 Dresden Row, Halifax, N.S.
Canada
1001

Coriolis Force
This fictitious force allows for the rotation of the earth, which
makes a floating object appear to drift in a circular path in the
absence of drag or other applied forces. Corio lis force also acts on
the water in which the iceberg floats. The water surface is assumed to
slope in geostrophic balance with the Coriolis force, so that an iceberg
moving with the water would experience no net deflection. Only the
velocity of the iceberg relative to the water
V
l'
+ + +
V - W = -w l'
is used to compute the Coriolis force
F = Mf X V
C l'
where the Coriolis vector
If I = 2Qsin'"
is directed vertically upward, Q = 7.27 X 10- 5 rn s-l is the earth's
rate of rotation, and", is the latitude. No adjustments to the esti­
mated mass M are made in optimizing the fit of the model to the observed
track, but a proportional increase (decrease) of both air and water drag
coefficients together is equivalent to a proportional decrease (increase)
in the mass.
MODELLING PROCEDURE
For each track to be modelled the initial wind, current, position,
and velocity are supplied, and a description of the iceberg in terms of
sail and keel area, mass, and assumed air and water drag coefficients.
At six second intervals a computed force balance is used to update the
velocity and position. New data are read in at regular hourly intervals
in the examples to be shown, but the model allows these data to be
specified at irregular intervals in 10 minute steps.
The observed track is plotted starting at time zero as a line with
a square symbol at each observed point and a heavier symbol at every
twelfth point. The computed track is plotted in 10 minute line segments
with a symbol every hour and a heavier symbol every 12 hours. Any
number of models of the same track can be plotted on a page, each being
identified by a different symbol, and a number of tracks with different
(6)
(7)
(8)
1003

choices of C a and C w can be compared subjectively. At each observed
point (i.e. hourly), the magnitude of the vector difference between
observed and computed drift since the previous observation is used to
find an rms error over the entire track. Selecting C a and C w for
minimum rms errors in the individual (hourly) drift segments is an
objective method which must be checked to ensure that it corresponds to
a reasonable cumulative drift track.
MODELLING OF OBSERVED ICEBERG TRACKS
A number of iceberg drift observations have been made available to
us (see Acknowledgements). About half of the tracks supplied were not
used because the data were incomplete or irregular in time. In six
cases we had estimates of the height, width, and mass of the iceberg
(Table 1), a sketch of its above-water shape, and an hourly log of
iceberg range and bearing, and wind and current speed and direction,
measured at the drillship Pelican. Currents at 50 metre depth were used
when available, since we feel that currents at the only other available
depth (IS m) may be subject to influences from the ship's hull. In
cases where the draft was not known the keel area was estimated to be
four times the sail area.
The variability of current between the drill ship and an iceberg at
2 to 20 km distance is obviously a major source of uncertainty, and
success of our model depends on the currents being uniform over the
separation distance. An array of current meters would be required to
properly define the currents at the iceberg location. From the level of
success achievable in a number of modelling attempts in a particular
area, we hope to be able to estimate how dense such an array would have
to be.
Iceberg K007
Although this was the last of the group chronologically, its track
is especially interesting and will be discussed in more detail than the
others. This was the largest of the group, and drifted close to the
ship so that the measured currents should be representative of those
acting on the iceberg.
1005

1008
We concentrated on modelling a 54 hour segment of the track which
is nearest the ship. The iceberg was grounded for 19 hours of this
period, which we modelled kinematically (not dynamically) by simply
setting the velocity to zero during the appropriate period. An optimum
track with rms error 0.5 km was obtained with C = 0.55 and C - 0.57
a w
(Figure 1).
The effects of variations in the drag coefficients are illustrated
in Figure 2. In the upper group, the air and water drag coefficients
are both doubled and then both halved with the track becoming longer and
straighter, or shorter and curlier. In the lower group, the coeffi­
cients are each (in turn) incremented up and down by 0.1. Increasing C w
moves the end point after 54 hours in a southeasterly direction, while
increasing C a moves it in a west-southwesterly direction. Figure 3
shows that the wind drift and the current drift (with winds and currents
in turn set to zero) are of similar importance in determining the total
drift.
Iceberg
F018
F025
K016A
K016B
K024
S012
K007
TABLE 2. PRELIMINARY RESULTS OF MODELLING ICEBERG TRACKS
Best rms error Length of Track
C
a
km in 1 hr hr.
km
0.05 0.55 0.51 22
9
0.1 0.6 0.37 17
7
10
11
t t
36
40
t t
18
25
1.0 0.3 1.05 23
33
0.55 0.57 0.51 54*
35
* Aground for 19 hours of this period
t Unable to model this track
Distance (km)
due to
current wind
8
5
9
13
10
16
20
1
2
2
t
t
17
15
Range
km
8-17
15-20
18-25
18
15-19
22-28
2-15
Table 2 lists the results to date. In general the current appears
to have a slightly larger effect than the wind on the drift. The track
of iceberg K016 was split into a 10-hour period which was modelled
(K016A) and a 36-hour period during which the iceberg continued to drift

1010
in a southerly direction, while the currents were slow and circled in a
clockwise direction, and the winds were light. The track of iceberg
K024 similarly could not be modelled because the observed drift was to
the southeast while the modelled track would give a slow, clockwise
motion driven mainly by the currents.
DISCUSSION
The results given here are preliminary and development of the model
is continuing. Even with sparse data we are able to achieve some success
by using an adaptive modelling approach in which the observed track is
used to overcome deficiencies in knowledge of the size and drag characteris­
tics of an iceberg. In a minority of cases the measured currents and
winds appear not to be related to the drift tracks.
Many hundreds of iceberg tracks have been observed from drill ships
and rigs off the coast of Labrador and elsewhere, and we plan to continue
testing and improving our model as more of these data become available.
In particular, if currents at a number of locations and depths are measured,
an interpolation to the iceberg location will be attempted. Separate
water drag calculations for several depth ranges (e.g. Mountain, 1980)
would then be possible.
We are presently experimenting with the addition of towing forces
to ,the successfully modelled tracks. This will allow us to estimate how
much deflection could have been achieved if a particular towing force
had been exerted over a particular time interval. Another area of
ongoing development is in automating the selection of the best drag
coefficients, and in further developing objective criteria for a "best"
fit.
Forecasting
In order to use this model for forecasting, it would be necessary
to forecast the currents for several hours. While tidal components of
current can be identified and forecast accurately, and forecasts of
wind-driven (Ekman) currents can be attempted, variations associated
with large-scale ocean circulation cannot be forecast by a local model.
Wind forecasts are routinely available, and can be improved if a number
of high-quality offshore surface and radiosonde weather observations are
added to the regular meteorological network. Site-specific forecast

services are based on the large scale numerical forecasts, and the
offshore weather data could be more fully used to feed and tune-up these
numerical forecasts.
Development of the present model into a forecast mode can proceed
in two steps: (1) Using only part of the track to optimize the drag
coefficients, and then using observed winds and currents to hindcast the
rest of the track with these coefficients, and (2) Replacing the winds
and currents with forecasts based on data available at the time when
the forecasting mode is started. This approach would allow separation
of errors into those associated with iceberg dynamics and those due to
wind and current forecasting errors.
While it may be some time before iceberg drift models become
operationally useful, the present practice is to tow icebergs if they
appear to endanger an operation, based on extrapolation of the observed
drift rate. It should be possible to develop an adaptive model to help
in decision making by giving an estimate the effects of anticipated
changes in winds and currents, and the possible effectiveness of towing.
ACKNOWLEDGEMENTS
The data were collected by MacLaren Marex (now Canplan) Ltd., under
contract to Total Eastcan Ltd., and were made available to us by P.E.R.
Vandall of Resource Management Branch, Dept. of Energy, Mines and Re­
sources, Ottawa.
REFERENCE
Mountain, D.G., 1980: On predicting iceberg drift. Cold Regions
Science and Technology, !, 273-282.
1011

ICEBERG SCOUR STUDIES IN MEDIUM DENSE SANDS
T. R. Chari Faculty of Engineering & Applied Science
Memorial University of Newfoundland
H. P. Green St. John's, Newfoundland Canada
ABSTRACT
INTRODUCTION
The problem of iceberg scours on Canada's east coast is a major
hazard in the extraction of the offshore hydrocarbon resources. Various
production systems which take into account the severe environmental fac­
tors such as the heavy seas, ice and icebergs are under consideration
for the Hibernia field on the Grand Banks. In any system, all seafloor
structures are to be located below the zone of iceberg scours. However,
the estimation of the maximum scour depths is still an aspect of the
problem not fully understood. A model for iceberg scouring in clays has
been suggested earlier and is now modified to include cohesionless
soils. Laboratory tests were conducted with a 50 cm wide model, using
medium dense sand as the representative seabed material.
these experiments are discussed.
Results of
The problem of seabed scouring by icebergs and the consequent threat to the pipe­
lines and buried installations on the Canadian eastern seaboard is well recognized.
Scours as deep as 6.5 m have been measured (Harris and Jollymore, 1974) in some
locations. Geological surveys of the different oceans have revealed scour-like fea­
tures in various locations where icebergs and ice are no longer present day phenomena.
The Northeast Atlantic west of the British Isles is a typical example, where furrows
up to about 25 m wide have been observed (Belderson et aI, 1973) with depths of 5 m
and lengths of 2 \an in waters of 140 m to 500 m. Such observations have lead to some
1012

of safety of 1.5 to 3 usually adopted in s t ructural and Geotechnical engineering
designs, this difference is not considered to be very significant.
LABORATORY MODEL TESTS
The significance of the laboratory tests, their interpretation and relevance to
the mathematical model have been discussed elsewhere in detail. (Chari 1975, 1980).
Results of tests in a tilting flume using a cohesive sediment as the representative
seafloor soil were also reported . Laboratory experiments on iceberg scouring are
designed to verify only the nature and type of soil resistance forces. These tests do
not duplicate the entire mathematical model as given by Eqns . [IJ and [2]. The mathe­
matical model consists of different components, the kinetic energy of the moving ice­
berg, the hydrodynamic drag, the soil resistance forces and the principle of energy
balance. The problems of scale modelling all these properties, particularly the soil
strength, precludes the possibility of duplicating the entire scour phenomenon in the
laboratory . Further, all the concepts except the nature and type of soil resistance
are well established principles in Applied Mathematics and in engineering requiring no
further experimental verification . Thus the laboratory experiments have been designed
to measure the forces and pressures on a physical model and also inside the soil
medium when the idealized model is towed at constant speeds into a sloping bed of the
sediment. Laboratory measurements attempt to justify the right hand side of Eqns .
[1 J and [2 J which then validates the entire mathematical model. Recent tests in the
FIG. 2: LABORATORY TEST FACILITY
1015

parison of the theoretical and experimental values for the tests in sand. It is ob­
served that the measured soil resistance is higher than the theoretical values for the
cohesionless soils as well. However, the maximum difference is in the order of 10% as
against 30% for the clays reported earlier (Chari, 1980). A similar phenomenon has
been reported by Krause (1974) for experiments in loose sands. Consistent with the
above observations, it was postulated (Chari, 1980) that there would be a movement of
the soil ahead and below the actual limits of the scour. This was demonstrated by
placing pressure cells inside the soil mass which recorded pressures when the iceberg
model passed above. To confirm these observations further, in the recent tests with
the cohesionless soil, a model pipeline was instrumented and placed in the soil at
different locations relative to the scour boundary (Fig. 4). A set of typical results
is shown in Fig. 5. These confirm that there is a movement of the soil below and
ahead of a scouring iceberg. Maximum pressures of 12.5 kPa were measured in these
experiments. Further work is in progress to delineate the zone of soil movement and
also to evaluate the scale effects.
CONCLUSIONS
The model for iceberg scouring in soft clayey sediments was extended to fric­
tional soils. There are some problems in the precise evaluation of the type of fric­
tional forces between the Boil and iceberg. However, preliminary computations show
that for an iceberg of 10 x 10 9 kg the scour depth would be overestimated in the
FIG. 4: PIPELINE MODEL EMBEDDED IN SOIL
1017

John D. Miller
Senior Environmental
Analyst - Ice
ABSTRACT:
A SENSITIVITY ANALYSIS OF A SIMPLE MODEL OF
SEASONAL SEA ICE GROWTH
Petro-Canada
Box 2844
Calgary, Alberta
T2P 3E3
Canada
Use is made of a simple model of seasonal sea ice growth to evaluate the sensitivity
of ice growth to variations in the climatological forcing functions. The
model is an energy conservation, equilibrium surface temperature simulation that
has been found in previous studies to realistically reproduce first year sea ice
growth in Arctic locations. By employing a standard test case in conjunction
with varying climatological conditions the growth response of the ice to the new
conditions is evaluated. A series of simulations to evaluate the effects of
changes in air temperature, wind speed, incoming solar radiation, snow density
and snow depth are performed and the results reported upon.
INTRODUCTION
With the increasing development of the Arctic regions the need has become apparent
for a simple yet rigorous method to estimate the growth response of a sea
ice cover and the associated energy fluxes. As well, a knowledge of the thickness,
properties and coverage of young ice is important in studies of climatic
change, regional energy balance, ice dynamics, ice forecasting, ice engineering,
remote sensing response, petroleum extraction and marine transportation. To this
end a simple model of heat transport through young sea ice coupled with the energy
exchange mechanisms in the lower atmosphere has been developed to estimate ice
growth and decay in association with the ice surface micrometeorology. With this
model the sensitivity of ice growth to changing environmental conditions may be
evaluated.
MODEL DEVELOPMENT
The use of climatological models to predict sea ice growth offers a variety of
advantages over empirical or analytical techniques. Empirical techniques are
limited in that a host of energy exchanges are subsumed and expressed in terms of
a surrogate variable or variables and a relatively simple functional relationship.
This simplicity defies an understanding of process interaction resulting
in a 'black box' approach which makes it impossible to investigate, or masks the
effect of, changes in other related parameters. Analytical approaches include
more effects but solutions are often limited to cases where relatively simple
boundary conditions exist.
1020

Maykut and Untersteiner (1969) presented a comprehensive one dimensional thermodynamic
model of sea ice in which the turbulent energy fluxes, radiation fluxes,
ocean heat fluxes, snow accumulation and density, surface albedo and ice salinity
are treated as time dependent model inputs. With these the energy balance is calculated
as a function of an effective surface temperature and the ice mass changes
(accretion/ablation) calculated at the ice boundaries. A pioneering stage in the
modelling of ocean-ice-atmosphere interaction, it remains a most comprehensive
treatment of ice thermal behaviour.
Since this work others have developed models which attempt to interrelate climatological
processes to the ice and its growth. Notable works include those of
Goddard (1974), Semtner (1976), Washington et al (1976) and Maykut (1978).
For this work a simulation of climatological fluxes and ice growth employing energy
conservation methodology was adopted. The complete development of the model is
presented in Miller (1979, 1980) and is summarized here. The model attempts to
combine the synoptic scale climatic variables with site characteristics to evaluate
the surface energy balance. This balance is given by the energy conservation
equation (equation 1) with the magnitude and direction of its components resulting
from the thermal, radiative and aerodynamic properties of the site.
Q* + FH + FE + FI + FA = 0 (1)
where Q* net radiation flux
FH sensible heat flux
FE evaporative heat flux
FI conductive heat flux
FA energy flux due to ablation
The components of the net radiation flux can be expressed as the sum of four radiation
terms: incoming and outgoing (reflected) shortwave radiation and the incoming
and outgoing longwave radiation. In the model the incoming shortwave radiation
is required as a specified input. The outgoing radiation is expressed, in
the presence of a snow cover, as:
Kt = (Hi (2)
where a = surface albedo
Under snowfree conditions penetration of the shortwave radiation occurs into the
ice. To allow for this phenomenon the approach of Maykut and Untersteiner (1969) is
employed in which a predetermined percentage of the net shortwave radiation is
assigned to penetration of the ice and the remaining portion used to determine
the surface energy balance. This percentage is fixed during the snow free period.
The net shortwave (K*) is then calculated as:
K* = (1-a) (1-i) Ki (3)
where i = penetrating fraction
Incoming longwave radiation is estimated using an empirical relation developed
by Idso and Jackson (1969).
Lj= fUT 4(1-0.261 exp (-7.77 x 10 -4(T -273)2)
a a
(4)
1021

The ice thermal conductivity exhibits a temperature and salinity dependency as
defined by Untersteiner's (1961) functional relation. Snow thermal conductivity
is determined from the snow density ( P s )' using Van Dusen's (1927) equation.
The average ice salinity (parts per thousand) is estimated on the basis of ice
thickness using the empirical relationships obtained by Cox and Weeks (1974).
In accordance with the observations of Thorpe et al (1973) and Banke et al (1976)
who found that CD»CH»C E , in the model C H is assigned the value 1.1 x 10- 3 with
C E set to 0.6 x 10- 3 in accordance with their observations.
Air density is fixed at 1.32 kg/m 3 , the specific heat capacity of air at 1010 J
kg- 1 ·C-l and the latent heat of vaporization at 2.533 x 10 6 J/kg. Ice density
is assumed to be 920 kg/m 3 , the latent heat of formation fixed at 2.72 x 105 J /kg
while the latent heat for melting of snow and ice is taken as 3.344 x 105 J/kg.
The ocean temperature beneath the ice is taken as the salinity determined freezing
point, a feature supported by observational data of Lake and Lewis (1971);
Doherty and Kester's (1974) relationship between freezing point and salinity is
used.
MODEL RESULTS
The model has been previously tested against ice growth data at three Arctic sites
which provide a significant range of climatic environments. Nine years of simulations
at Eureka N.W.T. (80 0 00'N, 85 0 46'W), Frobisher Bay, N.W.T. (63 0 45', 68°33'W)
and Resolute N.W.T. (74 0 43'N, 94 0 59'W) are presented and evaluated in Miller (1979).
It was found that the model accurately reproduced the pattern and magnitude of ice
growth under a variety of climatological conditions.
The test site chosen for the sensitivity analysis was Resolute N.W.T. which features
a relatively long period of ice record (since freeze-up 1955), an average
ice season of greater than 280 days and operates as a first order climatological
station. Climatological normals were obtained from the Atmospheric Environment
Service (1972, 1976) records and are summarized in Table One, while summary ice
growth data were found in Richardson and Burns (1975).
The sensitivity analysis was performed by first defining a normal or standard case
and defining the ice growth season on a daily basis (see Figure One). Specified
parameters were then changed, singly or in unison, and the ice season simulated.
Changes between the standard case and the simulated case were then identified and
evaluated to determine the effect of the variation of the parameter under investigation.
The standard case was run using the data presented in Table One. Daily values
were obtained by employing a cubic spline interpolation on the monthly mean values
of temperature, incoming solar radiation and wind speed; the snowfall was allowed
to accumulate in two snowfalls per month with a density of 310 kg/m 3 • An initial
ice thickness of 29 cm and starting date of October 01 (Julian day 274) were used.
With these data a maximum ice thickness of 178.4 cm occurs on Julian day 173. The
snow is ablated over six days while 117.8 cm of ice is lost during the next sixty
three days yielding an end of season thickness of 60.6 cm and a sixty nine day
melt period. Comparison with the Richardson and Burn (1975) data show a very reasonable
similarity between the simulated standard condition and the statistically
derived normals.
1025

7 Lake, R.A. and E.L. Lewis 1971. The microclimate beneath growing
sea ice. Proc. I.A.H.R. Conf. (T. Karlsson, Ed.) Reykjavick, May 10-13,
1971, N.R.C., 241-244.
8 Maykut, G.A. 1978. Energy exchange over young sea ice in the
central arctic. J. Geophys. Res., 83(C7): 3646-3658.
9 Maykut, G.A. and N. Untersteiner 1969. Numerical prediction of the
thermodynamic response of arctic sea ice to environmental changes, Rand
Memorandum RM-6093-PR, Nov. 1969, 173 p.
10 Miller, J.D. 1979. An equilibrium surface temperature climate model
applied to first year sea ice growth. Unpublished Masters Thesis, Carleton
University, 182 p.
11 Miller, J.D. 1980. A simple model of seasonal sea ice growth. ASME
80-WA/HT-20, 8 p.
12 Richardson, F.A. and B.M. Burns 1975. Ice thickness climatology for
Canadian stations. A.E.S. Publication Ice 1-75, 60 p.
13 Semtner Jr., A.J. 1976. A model for the thermodynamic growth of
sea ice in numerical investigations of climate. J. Phys. Oceanography, 6:
379-389.
14 Thorpe, M.R., Banke, E.G., and S.D. Smith 1973. Eddy correlation
measurements of evaporation and sensible heat flux over arctic sea ice.
J. Geophys. Res., 78(18): 3573-3584.
15 Untersteiner, N. 1961. On the mass and heat budget of the arctic
sea ice. Arch. Meteorol. Geophys. Bioklimatol., A, 12, 151-182.
16 Van Dusen, M.S. 1929. IntI. Crit. Tables, 5: 216.216.
17 Washington, W.M., A.J. Semtner Jr., C. Parkinson and L. Morrison,
1976. On the development of a seasonal change sea-ice model. J. Phys.
Oceanogr., 6: 679-685.
18
(14):
1030
Weller, G. 1972.
28-30.
Radiation flux investigation. AIDJEX Bulletin

Mr. Matti Lepparanta
Prof. Erkki Palosuo
STUDIES OF SEA ICE RIDGING WITH A
SHIP-BORNE LASER PROFILOMETER
Institute of Marine Research
University of Helsinki
Abstract
Finland
Finland
Distributions of the height and spacing of ridge sails have been
observed with a laser profilometer mounted on the deck of ice­
breakers. The laser beam is directed down to one side of the ship
and it hits the surface at a distance of 15-20 m from the ship. The
laser is in use in the Baltio Sea in a long-term program for mapping
ridge statistics. The results show that the average sail height is
typically 45-55 cm and linear ridge density 5-10 km- 1 , when the cut­
off height is defined as 30 cm; both sail heights and spacings
fit well the negative exponential distribution. The mass of ridged
ice is estimated to be typically 0.2-0.6 times the level ice mass.
The laser was used in July 1980 in the area from Svalbard to Franz
Josef Land between the latitudes of 78 0 N and 82 0 N during the Ymer-80
expedition. The method worked well in fipst-year ice but in heavier
ice the ship's run was too uneven.
1. Introduction
Spatial features of sea ice ridges are presently measured mainly
through linear profiling. Upper portions have been recorded with
airborne lasers (e.g., [3]). In the Baltic Sea a laser profilometer
has been used in the last three years from icebreakers and the
1031

5. Concluding remarks
Spatial distribution of sea ice ridges can be observed by using a
laser profilometer aboard an icebreaker. The laser beam is directed
down to one side of the ship and it hits the surface at a distance of
15-20 m from the ship. The method has been tested in the Gulf of
Bothnia, Baltic Sea, and taken in use in a long-term program for
mapping ridge statistics. The measurements can be done while the ice­
breaker is performing her routine assistance work.
Aboard 16 MW icebreakers the method works well in first-year ice.
This is especially the case in the marginal ice zone and subarctic
seas where the level ice sheet and totally frozen layer of ridges are
not thick.
6. References
1. Gudkovic, Z.M. & M.A. Romanov 1976: Method for calculation the
distribution of ice thickness in the Arctic seas during the
winter period. - In: Krutskih, B.A., Z.M. Gudkovic & A.L.
Sokolov (eds.): Ice Forecasting Techniques for the Arctic
Seas, pp. 1-48. New Delhi (translated from Russian) •
2. Hibler, W.D.,III, W.F. Weeks & S.J. Mock 1972: Statistical aspects
of sea ice ridge distributions. - J. Geophys. Res. 77:5954-70.
3. Ketchum, R.D., Jr. 1971: Airborne laser profiling of the Arctic
pack ice. - Remote Sensing Environ. 2:41-52.
4. Kirillov, A.A. 1957: Calculation of hummockness in determining ice
volume. - Probl. Arctic 2:53-58.
5. Lepparanta, M. 1981a: On the structure and mechanics of pack ice
in the Bothnian Bay. - Finnish Mar. Res. 248.
6. -"- 1981b: Statistical features of sea ice ridging in the Gulf of
Bothnia. - Styrelsen f8r Vintersj8fartsforskning, Forskningsrapport
32. Helsinki.
7. Makinen, E., A. Keinonen & A. Laine 1976: Ice resistance measurements
with IB APU in the Baltic Sea. - Ocean Engng. 3:267-91.
8. Palosuo, E. 1975: Formation and structure of ice ridges in the
Baltic. - Styrelsen f8r Vintersj8fartsforskning, Forskningsrapport
12. Helsinki.
9. Wadhams, P. 1980: A comparison of sonar and laser profiles along
corresponding tracks in the Arctic Ocean. - In: Pritchard, R.
(ed.): Proc. ICSI/AIDJEX Symp. on Sea Ice Processes and
Models, University of Washington.
1037

CHUKCHI SEA ICE MOTION
R. W. Reimer
and
J. C. Schedvin, Research Scientist
R. S. Pritchard, Sr. Research Scientist
Flow Research Company
Kent, Washington 98031 U.S.A.
Abstract
Chukchi Sea ice motion in response to ocean currents is simulated to determine if
oiled ice could be transported from the Alaskan North Slope oil fields over 1000 km
southward into the Bering Sea. This is the first such detailed simulation of the
ice behavior in this region. Reversals of the typically northward ocean currents
through the Bering Strait and Chukchi Sea provide the dominant driving force that
induces breakouts and large-scale southward ice motions. Therefore, we use an ocean
current model coupled with the AIOJEX elastic-plastic model to describe ice
behavior. The ocean current field is extrapolated over the Chukchi Sea from data
collected at seven current meters during the winter of 1976-77. Since large-scale
cumulative motions depend significantly on ice strength, strength is varied from
zero to 10 6 N/m. For zero strength (free drift), ice is transported from Point
Barrow to within 100 km of the Bering Strait; for a strength of 10 4 N/m, ice is
transported from Point Barrow nearly to Cape Lisburne; for a strength of 105 N/m,
only 100 km of motion is simulated; and finally, for a strength of 10 6 N/m, no
motion occurs. This simulation shows that it is unlikely that oiled ice could be
carried southward from Point Barrow through the Bering Strait. Even at the lowest
strength limit (free drift), the oiled ice does not reach the Bering Strait during
the winter of 1976-77.
Acknowledgement
This study was funded by the Bureau of Land Management through interagency agreement
with the National Oceanic and Atmospheric Administration as part of the Outer
Continental Shelf Environmental Assessment Program.
1038

Introduction
As part of the concern over oil spill hazards in the offshore Prudhoe Bay lease
areas, consideration must be given to the possibility of spilled oil being
transported through the Bering Strait and into the Bering Sea. Even though Prudhoe
Bay and the Bering Strait are separated by over 1000 km, it is possible to envision
a sequence of events which could lead to oiled pack ice passing through the Bering
Strait. This paper addresses the likelihood of such large-scale motions in the
Chukchi Sea by determining for the first time the velocity field of the Chukchi Sea
ice cover during a strong southward motion. Specifically, we ask: Given that oiled
ice is near Point Barrow, what is the likelihood of its being transported southward
across the Chukchi Sea and through the Bering Strait? This question cannot be
answered with certainty because of limitations in our knowledge of ice behavior and
ocean currents in the Chukchi Sea. However, in our simulations, we determine ice
trajectories for conditions that tend to maximize the southward flow of the pack
ice. As a result, the model simulations provide a worst case for the transport of
oiled ice through the Bering Strait.
Previous work has shown that reversals of generally northward-flowing currents are
the cause of large, southward ice motions [1]. Drag due to ocean currents has been
identified as the major driving force for the breakout of the Chukchi Sea ice pack
through the Bering Strait, with wind stress and traction at the northern boundary of
the Chukchi Sea playing only secondary roles [2]. On the basis of these results, we
calculate ice velocity fields in the Chukchi Sea during a time of large-scale
current reversal using a realistic model of the Chukchi Sea currents to provide the
current drag forces. From the ice velocity fields we are able to calculate the
southward trajectory of sea ice through the course of one winter.
Ice-Ocean Model
In this work a mathematical model which is capable of being driven by winds and
ocean currents is used to simulate ice behavior. Forces on the ice occur as a
result of divergence of internal ice stress, drag on its bottom surface by ocean
currents and across its top surface by winds, sea-surface tilt, and Coriolis
acceleration. This allows a range of inputs to be introduced to find a range of
motions. Following Reimer et al. [2], we neglect wind stress and the stress applied
by the Beaufort Sea ice pack, as these were found to have only a minor influence on
the southward transport of ice in the Chukchi Sea. We consider variations in ice
conditions and currents to determine how each affects the ice behavior. Pack ice is
modeled as an elastic-plastic material, a continuum on the scale of tens of
kilometers. The ice model used is essentially the same as that developed by
1039

AIDJEX [3], but strength is taken as a constant for each simulation and does not
vary as the ice deforms [4].
In order to develop a realistic model of the ocean currents, a study was made of
available oceanographic current data. Much of this information is collected and
summarized by Coachman et al. [5]. More recent data from current meter moorings are
discussed by Tripp et al. [6] and Coachman and Aagaard [7]. The later data include
observations from a 5-month period during the winter of 1976-77 when the Chukchi Sea
was ice covered. Dur ocean current model is an extrapolation of these current meter
measurements in accord with the data prior to 1975.
The ocean current model is generated by dividing the Chukchi Sea into a number of
generally north-south trending regions. These regions extend from the Bering Strait
to an arc passing from Wrangel Island to Point Barrow. The boundaries of the
regions follow the expected current paths; therefore, the northward or southward
transport in each region is constant (northward and southward are taken to mean away
from and toward the Bering Strait, respectively). Fig. 1 shows the boundaries of
the current path regions and the locations of current meters NC-l through NC-7 in
the Cape Lisburne section and NC-IO in the Bering Strait. The north-south water
transport in each region is determined by multiplying the cross-sectional area of
the region at the current meter by the perpendicular velocity component measured by
the meter. Local current magnitudes, at locations other than current meter
stations, are determined from the local transport, conserved in each region, divided
by the local cross-sectional area of the region. Current meter NC-IO is used to
determine the transport through the eastern half of the Bering Strait. The
transport through the western half of the Bering Strait is the difference between
the Cape Lisburne section transport and that through the eastern half of the
Strait. This allows a nonuniform flow through the Bering Strait that corresponds to
reported observations.
The current field constructed from the model for 4 February 1977 is shown in
Fig. 2. This is the largest observed southward current event during the 1976-77
experiment when the Chukchi Sea was ice covered; southward current speeds at Cape
Lisburne reached values of 0.5 m/s. Using this current distribution, the ice
velocity field is calculated for the entire Chukchi Sea. Separate calculations are
made for four values of ice strength, p*. With zero ice strength, free drift
occurs. In free drift, the ice velocity field is identical to the local current
distribution shown in Fig. 2. Fig. 3 shows the ice velocity fields for p* = 10 4
and 105 N/m. When p* is set to 10 6 N/m, no ice motion occurs.
1040

Figure 1. Current Meter Locations.
Transport Sections. and Current Path
Regions
1 m/s
8. Strength p* = 104N/m
1 m/s
Figure 2. Ocean Current Field on
4 February 1977. Also Free Drift
Ice Velocity Field.
1 m/s
b. Strength p* = 10 5 N/m
Figure 3. Ice Velocity Field Forced by Ocean Current Field of Figure 2.
1041

Ice Transport
Using the current meter data for the winter of 19.76-77 [7] a daily ice velocity
field is calculated for each ice strength. The daily ice motions are then
accumulated to determine ice trajectories over a period of time. To simplify
computations, the dependence of ice velocity at Cape Lisburne on ice strength and
local current velocity is scaled by dimensional analysis. This requires that, in
the ice transport calculations which follow, the ocean current field is assumed
always to vary spatially as shown in Fig. 1. In this current velocity field, the
current speed off Cape Lisburne, v g ' corresponds to the speed at current meter
NC-7. Fig. 4 shows the ice speed, v, calculated from the various daily ice velocity
simulations (e.g., see Fig. 3) at the location of NC-7 as a function of v for ice
strengths p* = 0, 10 4 , and 10 5 N/m. As would be expected, ice velocity g
decreases toward zero with increasing ice strength. It is also observed that for
p* = 10 5 N/m, there is a threshold ocean current of about 0.10 m/s below which the
ice does not move. The threshold is negligible for p* = 104 N/m, and is greater
than 0.5 m/s for p* = 10 6 N/m.
The calculated dependence of ice velocity on current velocity allows us to predict
ice movement through the course of the winter of 1976-77 when the history of ocean
currents is known. From the current meter measurements, we obtain a time history of
v g ' and from velocity field solutions (Fig. 4) for a given value of p*, we
calculate a corresponding time history of ice speeds, v. In determining the ice
velocity, the values of Vg are set to zero if they are northward. Thus, we
prohibit northward ice motion and our results provide an upper bound to southward
0.50
0.40
Iii
E 0.30
:;
"tl
OJ '" Q.
(/)
.l,l '"
1042
0.20
0.10
Ocean Currents Vg Im/s)
Figure 4. Ice Speed as a Function of
Ocean Current at Constant p* .

is 10 5 N/m or greater, but almost all the way to the Bering Strait if the ice
strength is zero. When the strength is 10 4 N/m, ice is transported from Point
Barrow nearly to Cape Lisburne. This analysis prohibits northward ice motions in an
attempt to estimate conservatively whether or not oiled ice can be transported
through the Bering Strait.
When comparing our simulated ice motions, we conclude that large southward
transports, such as the one recorded during January 1976 [2], are possible only when
ocean currents are much larger than observed during the winter of 1976-77 and when
the ice along the Alaskan coast is very weak. Therefore, we believe that breakouts
and large-scale southward pack ice motions are a result of a combination of
processes. Deformations must weaken the ice along the coast by the formation of
open water, the ocean currents must reverse their northward flow, and a persistent,
fast (0.4 to 0.5 m/s) southward flow must develop. The process of breakout is more
complex than that envisioned at the inception of this study. It requires that a
hardening/softening plastic model of sea ice be used to account for open water
created during deformations. Because a sequence of events is important for large
motions of the pack ice, simulations should be performed using a time history of
oceanic and atmospheric data to provide the driving forces for pack ice deformation
and transport.
This study represents the first substantial research effort aimed at simulating
Chukchi Sea ice motions on length scales on the order of tens of kilometers with
daily time resolution. Our results complement past studies and provide guidance for
directing future research efforts.
References
1. Pritchard, R. 5., and Reimer, R. W. (1979) "Ice Flow Through Straits," POAC '79,
Vol. 3, Trondheim, Norway, pp. 61-74.
2. Reimer, R. w., Schedvin, J. C., and Pritchard, R. S. (1980) Ice Motion in the
Chukchi Sea, Flow Research Report No. 168, Flow Research Company, Kent,
Washington.
3. Coon, M. D., Maykut, G. A., Pritchard, R. 5., Rothrock, D. A., and Thorndike,
A. S. (1974) "Modeling the Pack Ice as an Elastic-Plastic Material," AIDJEX
Bulletin 24, University of Washington, Seattle, Washington, pp. 1-105-.-----
4. Pritchard, R. S. (1980) "A Simulation of Winter Ice Dynamics in the Beaufort
Sea," in Sea Ice Processes and Models, ed. R. s. Pritchard, University of
Washington Press, Seattle, Washington, pp. 49-61.
5. Coachman, L. K., Aagaard, K., and Tripp, R. B. (1975) Bering Strait, The
Regional Physical Oceanography, University of Washington Press, Seattle,
washington.
1045

6. Tripp, R. B., Coachman, L. K., Aagaard, K., and Schumacher, J. D. (1978) "Low
Frequency Components of Flow in the Bering Strait System," EDS Transactions
Vol. 59, No. 12, American Geophysical Union, pp. 1091.
7. Coachman, L. K., and Aagaard, K. (1981) "Re-Evaluation of Water Transports in
the Vicinity of Bering Strait," in The Eastern Bering Sea Shelf: OceanographY
and Resources, Vol. 1, ed. D. W. Hood and J. A. Calder, u.S. Dept. of Commerce,
National Oceanic and Atmospheric Administration, Office of Marine Pollution
Assessment, Juneau, Alaska, pp. 95-110.
8. Pritchard, R. S. (1981) "Mechanical Behavior of Pack Ice," in Mechanical
Behaviour of Structured Media, ed. A. P. S. Selvadurai, Elsevier, Amsterdam, in
press.
1046

Pierre McComber
NUMERICAL SIMULATION OF ICE ACCRETION
USING THE FINITE ELEMENT METHOD
Universite du Quebec
a Chicoutimi
ABSTRACT
Canada
The rate at which ice grows on a cylinder exposed to supercooled droplets is a func­
tion of the number and sizes of droplets impinging on different points of the surface.
The shape of the ice accretion, however, modifies the air velocities upstream of the
cylinder and therefore the droplet trajectories as well. Because of the resulting
irregular boundary, the finite element technique is appropriate the solve this prob­
lem numerically. To account for the change in shape of the obstacle as the ice grows,
a two-dimensional finite element grid, modified at each step of the time integration,
is used to solve both the air velocity field and the droplet velocity field. From
the droplet speeds, the amount of supercooled water impinging on different points of
the cylinder and the resulting ice thickness are calculated.
Some results of the numerical simulation are compared to samples of ice accretion ob­
tained in a wind tunnel. The comparison indicates that the numerical simulation is
adequate to predict the shape and thicknesses of accretions formed in dry growth
conditions.
1. INTRODUCTION
Atmospheric ice accretion on different structures is presently being investigated by
different research groups to solve various environmental problems [lJ. One of these
problems is related to the increasing importance of electrical energy transport lines
in northern regions where they are exposed to adverse weather conditions. In parti­
cular, it has become important for certain utility companies to gather meteorological
data on this phenomenon. Since the adequate instrumentation is usually not available
where and when it occurs, one has to try to determine the meteorological conditions
of formation from an ice accretion sample. For this purpose, an accurate numerical
1047

+ 0.102 R 0·955
e
0.2 < Re < 2
Equations 3, 4, 5, 6 and 7 are non-linear, and therefore require an iterative method
of solution. A Newton-Raphson sheme was used to obtain convergence of the non-linear
equations. The velocity of air V was taken as the initial velocity for the droplet
a
-+field
V • This corresponds to the case of negligible inertia (K = 0). The calcula-
e
tion then proceeds from lower values of K to larger ones using the results of the
-+-
last iteration as the intitial value of V for the next iteration. With such a mee
thod convergence was obtained in less than four or five iterations for each successi-
ve value of K. This procedure offers the advantage that the results obtained for
increasing values of K can be filed as they are calculated. Since K is the only pa­
rameter involving the droplet diameter in the solution, the results can be readily ex­
tended to a droplet diameter distribution.
Droplets impingement. The droplet velocity field was used to determine local impinge­
ment efficiency S on the cYlinder by the following relation applicable for a «L:
-+- -+-
V .dl
e
for S > 0 (7)
The local impingement efficiency, when multiplied by the liquid water content wand
the free stream velocity Yo' gives the mass flux of water impinging on the surface of
the iced-covered cylinder. The maximum angle of collection 8 m is obtained when S
reaches zero.
Droplet diameter spectrum. Attempts were made at
first to use a statistical distribution to fit the
droplet diameter spectrum. Because of the importan
ce of the few large droplets in the total volume,
for which the statistical distributions tried were
not very accurate, it was better to use directly
the spectrum obtained by experiments as shown in
Table 1. The total water collected by the ice co­
vered cylinder was calculated by a summation of the
collection for different diameters.
Ice accretion volume and shape. Whenever super­
cooled droplets hit an object, they can freeze lo­
cally without runbacks or they can spread on the
surface before freezing. The first condition is
called dry growth and results in the formation of
Mean
diameter
5.1
15.3
25.5
35.7
45.9
56.1
66.3
76.5
86.7
96.9
107.1
117.3
127.5
137.7
TABLE 1
Number of
(Ilm) droplets
945
1093
481
211
74
38
30
13
19
11
10
3
1
1
1051

(a) 0 kV/cm (c) -1 0 kV/cm
(d) -IS kV/cm (e) -20 kV/cm (f) 20 kVrms/cm
Figure 4. - Elongation of air bubbles under applied electric field.
(cutting plane in the cross section of the accretion)
The appearance of large bubbles in ice obtained with a dc negative applied field
(Fig. 3) may be attributed to two factors. The first factor may be the increase of
impact speed of small droplets due to the electrostatic attraction exerted by the
electric field upon the polarization charges of the droplets . This effect should be
accompanied by an increase in the collection efficiency of small droplets. As a re­
sult, the deposit temperature should increase. On the other hand, an increase in the
deposit temperature will produce a decrease in the bubble concentration or an increa­
se in the transmittance [6J; this is inconsistent with the results shown in Fig . 2,
1061

Wilfred R. McLeod,
Tedmical Coosultant
A'lMOOPHERIC SlJPERS'1'ROCTU ICE ACaMUIATICN
MFJ\SURI:MENl'S
Marathon Oil Conpany U.S.A.
Ice acamulatioo neasurements are reported which were rrade 00 Middletoo Island
in the Gulf of Alaska at a 1G-meter elevatioo wring the winter of 1975-1976,
and 00 St. Paul Island in the Bering Sea wring the fall of 1976, the winter of
1976-1977, and the fall and winter of 1978-1979 at 10-, 20-, and 30-meter
elevatioos. These events are reported along with neteorological cooditioos
prevalent before, during and after icing. The data are used to establish
preliminary nethods for design rea:JlllE!ndatioos for offshore a1:!!ospheric icing on
derricks, flare booms, antennas, quarters and superstructures 00 an offshore
platform located in subarctic or Arctic regioos.
1067

This paper attempts to relate the incidence of atmospheric icing to the better
studied sea spray iCing phernneoon. According to Minsk (1977) in his review of
both atmospheric and sea spray icing research, relatively little is known
about atmospheric icing in polar regions, save some continental data which are
inapplicable to the arctic marine environment. Generally, the atmospheric icing
phenomenon is considered less severe than the sea spray icing problem and
hence is ignored. '1t1is paper, through a series of new physical measurements on
two small subarctic islands, seeks to prOlTide a better scientific basis for
determining the physical circumstances that lead to significant atmospheric icing
events. In addition to recognizing those meteorological conditions during which
icing events are likely to occur, we wish also to quantify the rnagnitooe and
frequency of ice aCCl.DllUlation which may be attributed purely to atm:>spheric
icing.
In earlier studies, it often appears that the sea spray and atnDspheric icing
phenanenon are confounded in shipboard measurements. No original work has been
done here in the field of sea spray icing; such data are presented in this
discussion merely to provide a useful reference to the relative rnagnitooe and
conditions for the two distinct types of icing.
CLIMA'IOIffiICAL cnISIDERATICNS
KOnishi and Saito noted that a two-year cycle seemed evident in the wind, ice and
tenperature patterns in the Bering Sea for the period 1960-71. The range of
tenperature variations (based on May values) was on the order of 3.6 0
around 35.6 0
F centered
F, with odd years being warmer and even years cooler. Exceptions
were found in 1965 and 1971. During these periods, the northerly winds also
prevailed as they did in the "low" tenperature even years. Evidently, what is
reflected are differences in the position of the Aleutian low and the North
Pacific high. Generally, storms enter the Bering Sea fran the southwest or
south, sometimes fran Kanchatka on the west, or fran the Gulf of Alaska on the
southeast. Many storms die as they encounter ice fields in winter, others
continue across the Alaska peninsula into the Gulf of Alaska, and sane lIDVe
northeastward up the Kuskokwim and Yukon Rivers (Figure 1).
1068

Sea spray Icing Considerations
we should briefly consider sea spray icing to distin;Juish it clearly fran the
atnospheric icing phenanenon discussed later. At least a portion of the sea
spray icing reported in the literature may, in fact, be due to atnospheric
icing.
Studies of ship icing show that maximum icin;J generally occurs in the rear of low
pressure areas, durin;J north, northwest and west winds; however, a sub-maximum of
icing events may also occur in the forward part of a low with northwest or east
winds. According to Borisenkov and Pchelko (1972), 57% of the 442 reported cases
of ship icing in the Beril'l3 Sea occurred in the rear of a low pressure area,
while 23% of the events were reported in the forward part of the low; all other
cases accounted for 11%. The icil'l3 period defined by these reports is fran
December through March, with corresponding frequency of ship icing of 20%. This
is not so strikil'l3 a low pressure systan dependence as is found elsewhere. In
the sea of Japan, for exanple, 93% of the icing events occur in the rear of a low
pressure systan.
It is even more interesting to note that these icing events reported by Borisen­
kov and Pchelko are almost all clustered about the Pribilofs, with the balance
occurril'l3 to the southeast as far as western Bristol Bay. Only about 11 events
occurred outside this area. Thus, these figures are truly representative of the
study area, the southeast Bering Sea. Certainly these data are biased towards
fishing grounds since the reporting vessels tend to concentrate in these areas.
However, in view of the storm tracks noted and the probable increase in icing
events trailing and leading these storms, it seans likely that the Pribilofs and
the area to the southeast do, in fact, represent more severe icing regions. We
conjecture that a relatively more severe atmosphere icing hazard might be found
here, too.
It sb:luld also be noted that, as can be inferred fran Mertins (1968) data, thick
accumulations of sea spray icil'l3 would most probably result fran strOn;J winds
blowil'l3 for an extended period, 3 - 6 hours, Figure 2. Upper air tE!1peratures of
O· F or less at the 850 mb level are also indicative of atmospheric icing. Thus,
presence of a cold trough at these levels may be taken as a short-term prediction
factor.
1069

location such as the Mscni region (see Table 1, which gives the types of icing
that occurred in three height zones [Glukhov 1972) on a tower at OOOinsk, near
Mscni). On the other ham, the general relationship of rime ice and mixture
aCCl.lDUlation will likely hold true reganUess of location. If one Cbubles the
values measured on the OOOinsk tower at the 25-meter height, a thickness of about
4 an dianeter and a mass of 320 glm result. It is questionable whether the
extreme glaze accumulation of 4 - 6 indies (10 - 15 em) reported in Great Britain
in 1940 would occur offshore since the vertical theD1lal gradient over water is
less extreme than Cl\l'er lam and, therefore, the conditions for superoooled rain
falling en a cold accumulating surface are less likely. Nonetheless, a thickness
of 5 an might reasonably be expected.
Table 1. Occurrence (i) of Type of Ice by Height
SOft rime ice predaninated to a height of 100 m (328 ft).
Type of Ice
Height SOft Glaze Han] Mixture
Range (m) Rime Rime
0-100 23 5 5 2
100-200 29 31 25 23
200-300 48 64 70 75
No. of cases 227 237 633 396
OUr own evaluations of icing rate variation with height are given in Figure
4. The data used the monthly values for the two d:>servation years, 1976-77
and 1978-79. We include in Figure 4 both the linear regression line for total
seasonal accumulation and for mean monthly accumulation. Both of these relation­
ships support the hypothesis of an increase in ice accretion with elevation,
though individual monthly accretions may not reflect this trend. Of course, only
two years of data are insufficient to give anything IIDre than preliminary design
criteria.
CaIplter Analysis of NOM Data
Of several stations considered in our OCIIputer analysis, COld Bay and St. Paul
were chosen as the IIDst typical of the Bering Sea marine environnent. Figure
1071

Coosistently, the dew point depressioo for all events but three shown in Figure
10 lies within 8" F. Are these then legitimate events? In this case, why
does the observer not note icing events at tE!lTperatures aboITe 32" F? What is
evidently needed to verify these data are contact tenperature measurements to
coofirm that either the icing sensor probe is significantly below 32" F, or
continuous air tenperature data to show that the tE!lTperature does not fall
intermittently below freezing in these time intervals notwithstanding the IDurly
data.
Total Precipitatioo Ananalies
Of primary ooncem here is the aRl'lrent discrepancy between the equivalent
precipitatioo recx>rded by the Roseroount detector and the rain gauge 00 St. Paul
(e.g., Figure 8, the events of 29 December 1976, for which ooly a trace is
recx>rded in the rain gauge). 'nle problem of underestimating precipitatioo in the
Arctic is a general ooe. The official precipitation statistics in the canadian
Arctic have already been questioned by Walker and Lake (1975). 'nley have given
values of 9% for the official underestimatioo of rainfall, and 40% for the
corresponding snowfall underestimatioo. Limited Arctic runoff studies also
support such underestimation of precipitatioo by official recx>rds. 'nle cbserver
00 St. Paul states that the dnm-type rain collector is, in all probability,
unusually susceptible to underestimatioo of total precipitation due to the
frequent high winds which blow precipitatioo

Table II
Air Dewpoint
Date Wind '1'EnF. Depression Precipitation
1;7 (3 hrs) E to SE 31· F 2· F SI'rIW
1/11 (6 hrs) E to NE 32· F o· to 1· F snow
1/16 (1 hr) calm 32· F o· snow
1/17 (2 hrs) calm to N 32· F 1· snow
Hence, these criteria, while primitive at this stage, suggest that we could
greatly reduce the number of precipitation events which rust be considered as
possible icing occurrences and concentrate on the ice equivalence problem.
Satellite Ptotographs might also be used to judge an event's ocrurrence with
respect to a given low pressure system's position, as discussed earlier.
'lbe reliability of hindcasting methods for Arctic waters is sanewhat questionable
at this time. Further field measurements in the areas under evaluation where
icing tends to be highly cumulative are desirable in order to verify usable
techniques. Nonetheless, it is possible to attenpt a preliminary approadl to
oatpUter modeling of ice forecasts, Figure 12. Data available include ship
forecasts together with the NOM weather tapes fran the Pribilof Islands, the
Aleutians and the surrounding oontinental statiCl'ls.
Hindcasting of atJrospheric glaze fOITRatiCl'l is sensitive to the accurate pre­
diction of precipitation amount and capture efficiencies of individual structural
members. This requires estimates of precipitatiCl'l particle sizes and knowledge
of CDnditions aloft.
1078
Hindcasting of icing fran fogs similarly requires infoITRation Cl'l the vertical
distribution of liquid water, estimation of the height of low level atmospheric
inversions, and infoITRation giving fog depth versus wind speed.
Hindcasting of icing fran snow accretion will probably need to be based Cl'l
primitive enpirical aSSUllptions about the ability of a structure to build up snow
aCCUlllllation. In general, this may not prove to be a problem as much for diffuse

members (e.g., masts or derricks), as for drifting problems between ooildings or
pieces of dec:k-m:xmted equipnent.
'nle author acknowledges Dr. Arnold Court, Professor of Climatology at california
State University, Northridge, california; and David T. Hodder, Chief Scientist,
Geoscientific Systems and COnsulting, Playa Del Ray, california, for their
assistance in developing the views expressed in this paper. The author also
acknowledges James Pruter and Anna Frisby of the National Weather Service for
their assistance at the Bering Sea Test Site during the data rollection phase.
BIBLIver, NH.
2. Borisenkov, E. P., ed.; I. G. pchelko, ed. (1972) Indicators for forecasting
ship icing (Metodichski ukazaniia po preduprezhdeniiu ugrozy obleden­
eniia sudov), Leningrad, Arkticneskii i antarkticheskii nauchnoissle­
dovatel'skii institut, 81 p. (in Russian).
3. Chaine', P. M. (1972) Estimating the ice accretion hazard, Atnospheric
Environment Service, Toronto.
4. Chaine', P. M. & P. Skeates (1974) Wind and ice loading criteria selection,
Industrial Meteorological - Study III, Toronto
5. Chaine', P. M., A. R. Wayman, and D. A. Bondy, (1975). In Cloud Icing James
Bay and Churchill Falls Power Projects. Industrial Meteorolog ical,
Study VII. Atnospheric Envirorment Service, Toronto.
6. Court, A. (1960). Reliability of hourly precipitation data, Journal of
Geq>hYsical Research, 65 (12), p. 4017.
1079

7. DeAngelis, R. M. (1974) Superstructure icing, Mariners Weather Log, 18 (1),
p. 1-7.
8. Dunbar, M. (1964) Geographical distribution of superstructure icing in the
Northern Hemisphere, Report No. Misc. G-15, Directorate of Physical
Researdl, Defense Research Board, Canada.
9. Glukhov, V. G. (1971) Evaluation of ice loads on high structures from
aerological observations (K otsenka gololednykh nagruzok na vysotnye
sooruzheniia po dannym aerologidleskikh nabliudenni). Trudy, Vol 283
GQJ, p. 3-11, Leningrad, Gidraneteoizdat (in Russian), (Translaticn:
Soviet Hydrology, selected Papers, p. 223-8, Issue No.3, 1971).
10. Guttman, N. B. (1971) Study of worldwide occurrence of fog, thunderstotlDS,
supercooled low clouds and freezing temperatures. NAVAIR 5O-1C-60,
Naval Weather service Coomand.
11. Konishi, R. & M. Saito, (1974) The relationship between ice and weather
conditions in the eastern Bering sea. In: Oceanography of the Bering
Sea, Institute of Marine Science, University of Alaska, Fairbanks,
Alaska.
12. Kuroiwa, Oaisuke (1965) Icing and snow accretion on electric wires, Researdl
Report 123, U.s. ADny Cold Regicns Research and Engineering LaboratoJ:Y,
Hanover, NH.
13. Lenhard, R. W. (1955) An indirect method for estimating the weight of glaze
on wires, Bulletin of the AMS, 36 (1), p. 1-5.
14. McKay, G. A. & H. A. Thoopson (1969) Estimating the hazard of ice accretion
in Canada from climatological data, Jnl. of Applied Met. 8(6), p.
927-935.
15. McLeod, W. R. (1977) Atnospheric Superstructure Ice Accumulation Measure­
1080
ments, Paper No. 2950, Offshore Tedlnology Conference, Houstcn, Texas.

16. Mertins, H. o. (1968) Icing on fishing vessels due to spray, Marine Observer
32(221), p. 128-30.
17. Minsk, L. D. (1977) Ice accumulation on ocean structures, CRREL Report
77-17 , U • S • Army Cold RegiO'1s Researcn and Engineering Laboratory ,
Hanover, NH.
18. Walker, E. R. & R. A. Lake (1975) Rmoff in the canadian Arctic Ardlipelago.
In: Climate of the Arctic, University of Alaska, Fairbanks, Alaska.
1081

9
INPUT WEATHER DATA FROM STATIONS SURROUNDING THE STUDY
6-HOURLY WINDS
AIR AND DEWPOINT TEMPERATURES (36°ro 20" F)
VISIBILITIES AND CEILINGS
PRECIPITATION AMONTS AND TYPES
MEAN MONTHLY SURFACE WATER TEMPERATURES (NOAA)
8 IF SPRAY, IS TEMPERATURE
IN GLAZE
OR RIME FORMATION RANGE?
COMPILE ICE
ACCRETION RATES
APPLY CORRECTION FACTOR FOR ELEVATION,
ICING THICKNESS, WIND SPEED,AND
STRUCTURAL MEMBER TYPE
PROJECTED ICING HINDCASTING PROCEDURE
Figure 12
1093

I. K. Hill, Head of Hydraulic
Department
A. B. Cammaert, Project Engineer
D. R. Miller, Naval Architect
ABSTRACT
A LABORATORY STUDY OF HEAT TRANSFER
TO AN ICE COVER FROM A WARM WATER DISCHARGE
Acres Consulting Services
Acres Consulting Services
Arctic Pilot Project
Canada
Canada
Canada
A warm water discharge has been proposed to limit ice buildup around an Arctic
liquid natural gas terminal. One approach consists of high velocity jet diffusers
inside a floating curtain surrounding the termlnal area, resulting in the suppression
of ice growth.
Water temperatures and velocities were predicted from a thermal plume model.
Calculation of the effect of the flow on ice thickness requires knowledge of the
rate of heat transfer between the water and ice. Experimental flume observations
were made to confirm the relatlonship used for the heat transfer analyses.
This paper describes the experimental program conducted in an ice flume at the Acres
laboratories. The heat transfer rate was obtalned by measuring the temperature
profile through the ice cover and the change in ice thickness with time.
Tests were conducted with both freshwater and saline ice. The parameters of ice
thickness, water depth, water velocity and water temperature were varied. The test
data are presented and compared with the analytical results.
INTRODUCTION
The Canadian High Arctic is anticipated to contain significant reserves of gas and
oil. In late 1976 the Arctic Pilot Project (APP) was formed, bringing together
Petro-Canada, Nova - an Alberta Corporation, Melville Shipping, and Dome Petroleum.
1094

The concept of the Project was a gas transportation system on the smallest scale
possible which would be commercially viable. It is intended to evolve and
demonstrate appropriate technology and to provide definitive cost and performance
information for facilities in the high Arctic.
The system throughput will be 7 x 10 6 m 3 gas per day, obtained from the Drake Point
field on the Sabine Peninsula of Melville Island.
The Project requires two liquid natural gas vessels designed and constructed to the
requirements of Arctic Ice Class 7 vessels. At intervals of 9 to 16 days, depending
on ice conditions, these vessels will transport the liquefied natural gas by way of
Parry Channel and Baffin Bay to a receiving terminal on the east coast.
At the time of writing, the Arctic pilot Project has made an application to the
Canad1an Government for permission to export the natural gas. An environmental
hearing was held in Resolute, Northwest Territories, in April 1980 to consider the
environmental impact north of 60 0 N. This report has been received and has concluded
that the impacts are acceptable provided certain conditions are complied with by the
Project proponents. The projected start-up date of the Project is early 1986.
GENERAL LOCATION AND ENVIRONMENT
Melville Island is located in the Parry Island group of the Queen Elizabeth Islands.
On the southeastern portion of the island at 75 0 N, 108 0 50'W, is Bridport Inlet, the
site of the terminal facilities. The average mean temperature in February is about
_35 0 C, and in July about +4 0 C, with winter temperatures going as low as _52 0 C
for up to 3 days at a time. A typical year at Bridport Inlet would have about
6,440 freezing degree C days.
Ice problems will be very severe for year-round operation of a marine term1nal at
such a northern location. with the exposure of open water after each ship passage,
ice production is accelerated resulting in a greater volume of ice than under un­
disturbed conditions. Uneven distribution of ice can cause manoeuvering difficulty
if the vessel attempts to berth unassisted.
A previous publication (Carnrnaert et aI, 1979) has ident1fied and quantified this
augmented growth. An active ice management system was required to control or in­
hibit the ice growth to some reduced thickness, allowing the vessels to dock
successfully. It was concluded that the most promising method for this
1095

The layer of moving water was expected to be a few metres thick and held under the
ice by a very weak buoyancy force induced by additional heat added by the jets.
Extensive literature is available on heat transfer to a flat plate (L.C. Thomas,
1980) under various boundary conditions. The behavior of an ice-water interface is
modified ,however ,by the melting or freezing process which,in addition to mass
transfer,results in local changes to the density of the fluid at the interface.
Literature available on heat transfer to ice largely relates to fully developed
turbulent flow such as a river (Cowley and Lavender, 1974).
A common assumption in heat transfer analyses is that the coefficient of thermal
expansion is independent of temperature in the boundary layer. This is not valid,
for a water-ice interface at salinities of around 25 0/00. At this salinity the
coefficient of thermal expansion at the freezing point is zero and is negative at
still lower salinities.
In the prototype a temperature differential between the bulk flow and the interface
of the order of 2 0 C is expected and the depth of the moving layer is about 3 m
for typical jet designs. In the first 100 m of the ice management zone the under­
ice boundary layer will thicken to between 1 and 2 metres.
The model tests were designed to determine whether the standard heat transfer
formulae developed for other situations and used for the feasibility analyses of
this Project (Acres, December 1978) were sufficiently reliable to ensure basic
feas1bility of the Project. It was recognized that to obtain data for design
purposes, additional test1ng beyond the work described might well be required.
For forced convection with buoyancy effects the system can be described by the
geometric shape, Reynolds number Re, Prandtl number,Pr,and the Grashof number,Gr
which relate viscous to buoyancy forces. The heat transfer coefficient is
described in terms of the Nusselt number,Nu. In the test program water was used
so that the Prandtl number,which is dependent on fluid properties only,was similar
in the model and prototype. In the prototype the longitudinal Reynolds number
ranges of order 10 7 , implying a fully turbulent boundary layer over most of the
area,with an area close to the dock of potential laminar flow, particularly if the
final design used low velocities. Dynamic and thermal similarity require a laboratory
Reynolds number in the same range, that is, either turbulent or laminar. Because
of the initial turbulence in the water the transition Reynolds number in the flume
will be close to the lower limit of 3 x 105. Tests were conducted either side of
this transition value.
1099

The relative importance of buoyancy vis-a-vis forced convection is similar for
similar values of the dimensionless ratio Gr/Re 2 which for the same fluid reduces
to similarity of 6T.L/V2 where L and V are characteristic lengths and velocities
and 6T is the temperature differential. This is equivalent to similarity of the
densiometric Froude number or the Richardson number. Because of the nonlinear
temperature density relationship in water it was felt desirable to maintain similar
temperatures in the model and prototype. Similarity of the ratio Gr/Re 2 then
required that the velocity scale be reduced as the square root of the length scale.
This implied, if large values of Re were to be maintained, that as large a test
facility as possible would be required. Because of the high cost of meeting this
requirement in full, tests were carried out over the desired ranges of Re and
Gr/Re 2 separately without covering the extremes of high Re and high Gr/Re 2
simultaneously. Because of the nonlinearity of the density temperature relationship
6T was varied independently. Tests were also conducted using freshwater and salt­
water, which results in a large variation in Grashof number as indicated by a change
from destabilization of the flow by cooling for seawater to a stabilizing of the
flow in freshwater. The intent of the range of tests selected was to determine
whether,for the conditions expected in the prototype,the preliminary analyses used
were adequate for feasibility. The actual range of parameters examined is given in
Table 2.
Parameters
Velocity (m/s)
Ice Thickness (cm)
Water Temperature
(oC)
Ice Growth Rate
(crn/h)
Temperature Gragient
Through Ice ( C/m)
Nusselt number
Reynold's number
Heat Transfer
Coefficient,
W/m2 oc
TABLE 2 - RANGE OF TEST PARAMETERS EXAMINED
Smooth Freshwater
0.214 to 0.016
17.8 to 3.3
5.20 to 0.36
0.410 to -1.811
211 to 68
6,688 to 411
633,700 to
24,400
988 to 47
Rough Freshwater
0.269 to 0.116
18.0 to 6.9
1.96 to 0.20
-0.119 to -1.500
74 to 36
10,590 to 3,096
764,500 to
325,700
1,200 to 350
Sallne
0.225 to 0.020
20.5 to 6.1
4.79 to 1. 25
-0.643 to -4.500
149 to 5
6,483 to 1,009
675,800 to
64,900
706 to 112
The test runs were carried out by initially forming a solid cover to the desired
thickness with three strings of temperature probes frozen into the cover as
indicated in Figure 1.
1100

Haggkvist, Kenneth
Research Engineer
ABSTRACT
COMBINATION OF A SINKING WARM WATER DISCHARGE AND
AIR BUBBLE CURTAINS FOR ICE REDUCING PURPOSES
Water Resources Engineering
University of Lulea
WREL
Sweden
The work described in this report indicates that a considerable ice reduction effect
in a limited area can be obtained by combining a sinking warm water discharge and
a bottom located air bubble discharge. A laboratory experiment and a theoretical
analysis are presented. In an example based on the conditions in Lulea Harbour
in northern Sweden it is shown that over an area of 50 by 500 metres, the ice
thickness can be reduced to less than 0.2 metres, which is less than 25 % of the
maximum natural ice thickness.
INTRODUCTION
Harbours in northern Sweden (northern Bothnian Gulf) are since 1970 open for yearround
navigation. However, due to repeated breaking and freezing of the ice masses
in fairways and at quays, navigation problems occur during the winter period. The
problems are accentuated in quay areas, where berthing manoeuvres are made difficult
and time-consuming.
Two methods, frequently used for ice reducing purposes in limited areas, are respectively
surface discharge of warm water and transport of "warmer" bottom water to the
surface by means of a bottom located air discharge.
The harbours of the northern Bothnian Gulf are mostly situated in shallow river estuaries,
with very low salinity and homogenous water temperature very close to the
freezing point from surface to bottom. The methods mentioned above are separately not
effective for ice suppression. The surface discharge of warm water is rapidly mixed
with the surrounding water, thereby being heavier than the ambient water, and sinks
to the bottom. Further, the "warm" bottom water, necessary for an air discharge
arrangement to be effective, is not available. If, on the other hand, the two mentioned
methods are combined, a portion of the released sinking warm water can be brought to
the surface by means of a (bottom) air discharge.
In order to make a study of the combination of a sinking surface water discharge and
air-bubble curtains a laboratory experiment has been performed at WREL (Division of
Water Resources Engineering, University of Lulea).
1104

a) Run 1; Two parallell air curtains
B=O.5mi [):O.5mi 8/0=1
A-A
b) Run 2; One air bubble curtain parallell to the flume wall.
Air pipes
... Surface discharge of water heavier than ambient (flume) water.
=i Ambient (flume) flow.
x Downstream distance from discharge
B Width between air-pipes
D Flume water depth
The water volume between the air-pipes and the air-pipe/flume wall respectively is
below termed the box.
Before (and after) each run the ambient water temperature was measured with operating
air curtains. The discharge of colder water was then started and its temperature was
measured. When steady state conditions were reached the water temperature was measured
over the flume width at three different depths and for various downstream distances.
Listed below are the numerical values of the parameters characterizing the experiment.
Flume water depth
Flume water velocity
Sinking plume discharge rate
Initial temperature difference between discharge
and ambient water
Initial density difference
1106
0.5 m
0.01 - 0.02 m/s
0.3.10- 3 m 3 /s

G.D. Fonstad
R. Gerard
B. Stimpson
THE EXPLOSIVE DEMOLITION OF
FLOATING ICE SHEETS
Hydraulic Engineer, River Engineering Branch
Alberta Environment
Professor, Dept. of Civil Engineering
Associate Professor, Dept. of Mineral Engineering
University of Alberta
Edmonton
Alberta
Canada
Abstract
Demolition of floating ice sheets is a common technique used to clear shipping
lanes, construct temporary port facilities in Arctic and Antarctic environments and
to mitigate ice jam effects on inland waterways both before and after ice jam formation.
Mellor carried out a review and analysis, on the data existing to 1972, of the
effects of point charges on floating ice sheets. On the basis of this analysis Mellor
made preliminary recommendations of the optimum charge size and placement depth as a
function of ice thickness.
In this paper a series of tests conducted to confirm Mellor's analysis and to
determine the optimum spacing of charges in a row are described. The appropriate dimensionless
terms are derived, and equations giving the optimum ice sheet demolition
parameters are given.
Introduction
Ice has an adverse impact on the operation of ports and navigable waterways.
Various methods of keeping ports open have been developed over the years. These include
icebreakers, explosive destruction of the ice and, more recently, the use of
air cushion vehicles. In Arctic and Antarctic environments explosives have been used
in the clearing of channels and the construction of temporary port facilities.
On inland waterways in cold regions ice jams are a constant concern. In many
instances efforts are made to weaken the ice cover at critical locations by blasting
in advance of breakup. If an ice jam does form, blasting is often helpful in removing
the jam.
In carrying out such explosive demolitions it is desirable to have some knowl-
1114

and can be incorporated into the constants implicit in a function such as Equation 2.
It has been shown [3] that the energy released upon detonation is proportional to the
weight W of the explosive charge, and can thus be substituted for the energy E. Also,
experience has shown that, except perhaps for very shallow depths, the depth of water
has little influence on the results.
Hence Equation 2 can be reduced to:
R _ (ti' d , 1 ,do, Ro)
K2 - f K2 K2 gK 2 K2 K2
••• (3)
where K2 = w 1 / J , which is the well known 'cube root' scaling criteria for blast effects,
applied to the case of floating ice.
Information Available in the Literature
Although explosives have been used for clearing ice for over 200 years [4],
little information could be found which dated prior to the 1960's. Cole [1] laid the
groundwork for ice demolition investigations through his study of underwater explosions,
but it was not until the 1960's that reported experimentation on ice demolition
was conducted in the western world. During the 1960's and early 1970's a number
of such studies were reported. Those prior to 1968 have been reviewed by Bolsenga [4].
Other information was included in the data compilation by Mellor [5], and some additional
results are given by Nikolayev [6].
Four cratering mechanisms can be identified from this past work. First, the
impact of the shock wave may cause compressive failure of the ice sheet [6], though
that this occurs is not generally agreed upon. For instance, Barash [7] notes that
the shock wave causes the ice to "rise in the shape of a dome". Kurtz et.al. [2]
noted a similar ice dome but from analysis of the high speed photography of their experiments,
considered that the ice may not be shattered by the passage of the shock
wave, as there was no indication of the upper ice surface spa1ling. Second, the gas
bubble vents through the ice sheet to the atmosphere. At this time, ice is thrown out
either "mostly upward or mostly radially depending on the phase of the gas bubble"[7].
The third cratering mechanism is a water surge through the crater following the venting
gases. As the gas bubble vents, water rushes in to fill the void, and fluid momentum
causes it to surge through the crater. A fourth and final mechanism has been noted
though only for 'deep' charge placements in shallow water: this is a second water
surge through the ice which is "always very muddy" [2].
It has been noted in the literature that even with these several means of ejecting
ice from the crater, upwards of 90% of the ice still remains in it.
These early studies determined by trial the charge weight and the optimum depth
1116

on Drummond Lake in the Chilcotin region of central British Columbia, on ice which
was approximately 0.37 m thick. In all some 86 separate single shots were fired.
These included four explosive types: a military plastic explosive with a PETN base,
for the majority of the tests; and three commercial explosives manufactured by
Canadian Industries Limited.: 40% Forcite, Amex II and Hydromex, which are respectively
a dynamite, a blasting agent and a bulk slurry explosive.
Analysis
In order to analyse how well Mellor's regression equation predicted the independent
Chilcotin data it was considered insufficient to scale from the curves given
by Mellor, especially with his recommendation for caution. The data used by Mellor
was collected and re-analysed by regression using the same model as Mellor. During
this repeat analysis it was confirmed that the terms with coefficients b 1 and b 4
could indeed be omitted as their contribution to the multiple correlation coefficient
was minimal. The analysis predicted a mean scaled crater radius of 2.1? m/kg 1 / 3 , with
a standard error of estimate of 0.55 m/kg 1 / 3 , and a multiple correlation coefficient
of 0.69, which is compatab1e to that found by Mellor, as it should be.
The regression equation obtained was:
2 2 3 2
Y = 1.55 + 7.73{X 2 ) - 0.419{X 1 ) - 14.8{X 2 ) + 0.141{X 1 ) - 0.551{X 1 X 2 )
2 3
+ 1.32{X 1 X 2 ) + 6.59{X 2 ) (6)
This equation was used to calculate scaled crater radius for Mellor's data, for
which the comparison between predicted and observed crater radius is shown in Figure
2. The equation underpredicts larger values of scaled crater radius and overpredicts
the smaller ones. The skew of the data about the expected best fit slope of. 1.0 indicates
that there may be a factor unaccounted for in the analysis. An attempt was
made to take this skew out by including the gravity term of equation 3 in a separate
regression analysis. There was, however, only a slight improvement in the correlation
coefficient, and the skew remained. The Chi1cotin data were analysed in similar
fashion, with the results shown in Figure 3. The three lowest Hydromex shots were
omitted from the analysis for the line of best fit, as three other shots out of eight
total Hydromex shots had misfired, and it is considered that the three lowest shown
in Figure 3 had only partially detonated. From Figure 3 it can be seen that Equation
6 generally - overpredicts - the scaled radii observed and there is a suggestion that
the same skew noted in Figure 2 exists. At the time of writing, attempts to explain
the general overprediction and to improve the relationship are continuing.
1119

for both the Hydromex and the Amex misfired, even though careful attention was paid
to ensuring the water integrity of the charges. It is thought that either water seepage
into the charges, or insufficient loading density caused these charges to misfire.
Depending on the operation undertaken, though blasting agents and slurries are less
expensive than other explosives, the care required to ensure the charges are watertight
may detract from the speed at which the operation can be conducted.
For any application of the optimum ice demolition conditions given herein, it
is recommended that a few trial shots be made before undertaking the main program.
Acknowledgements
The writers would like to express their appreciation to the Canadian Defence
Research Establishment - Suffied, who financed and assisted in the investigation. A
special thanks is due to Mr. G.K. Briosi, of that establishment, who assisted in the
data collection throughout the field experiments. The field tests were carried out by
1 Combat Engineer Regiment, Canadian Armed Forces, under the technical supervision of
the writers. Mrs. S. Notton, of Alberta Environment drafted the figures.
References Cited
1. Cole, R.H.,(1948),'Underwater Explosions', Princeton University Press.
2. Kurtz, M.K., R.H. Benfer, W.G. Christopher, G.E. Frankenstein, G. Van Wyhe and
E.A. Roguski ,(1966), 'Consolidated Report, Operation Breakup,
FY-66, Ice Cratering Experiments, Blaire lake Alaska', NCG/TM
66-7, U.S. Army Nuclear Cratering Group, lawrence Radiation
laboratory, livermore, California.
3. Baker, W.E., P.S. Westine and F.T. Dodge,(1973),'Similarity Methods in Engineering
Dynamics', Spartan Books, Hayden Book Co. Inc., Rochelle Park,
New Jersey.
4. Bolsenga, S.J.,(1968),'River Ice Jams', Research Report 5-5, U.S. Army Corps of
Engineers, lake Survey District, Detroit.
5. Mellor, M.,(1972),'Data for Ice Blasting', CRREl Technical Note, U.S. Army Corps
of Engineers, Cold Regions Research and Engineering laboratory,
(CRREl), Hanover, New Hampshire.
6. Nikolayev, S.Ye.,(1970),'Blasting Fast Ice in the Antarctic', Soviet Antarctic
Expedition, Information Bulletin. (Translation by the U.S. Army
CRREl, February, 1973).
1122

7. Barash, R.M., (1966),'Ice Breaking By Explosives', NOLTR 66-229, U.S. Naval
Ordnance Laboratory, White Oak, Maryland. (Extracts from a
Confidential report, 1962,'Underwater Explosions Beneath Ice',
NOLTR 62-96, U.S. Naval Ordnance Laboratory, White Oak, Maryland.
8 Personal Communication,(1980), Dr. M. Mellor.
1123

D. B. Coveney
Research Officer
ABSTRACT
CUTTING ICE WITH
"HIGH" PRESSURE
WATER JETS
National Research Council
of Canada
Canada
A high pressure jet of water can be used to cut a slot into or through
a sheet of ice, thereby substantially weakening the ice sheet. Such weakening
could be particularly useful to enhance the ice breaking capabilities and/or to
reduce the overall power and fuel requirements of an ice breaking vessel. Although
water jet cutting is less efficient in material removal than mechanical modes of
cutting, its ability to cut with a substantial mechanical stand-off from the ice
sheet and with a concentrated, high level of power input into the ice would provide
significant practical advantages for the water jet cutting method.
This paper describes the ice cutting performance of small to moderate
scale water jets in fresh water ice and of small scale water jets in a simulated
sea ice. The majority of cuts produced a narrow, clean kerf, indicative of erosion
in a ductile material, while other cuts produced a wide spalled trench, indicative
of spalling in a brittle material. Still others produced a combination of the two
modes of cutting, with a wide, shallow trench and a narrow, deep kerf below the
trench. The causes and the effects of these characteristics on ice cutting per­
formance are discussed, along with the effects of jet traverse speed, nozzle
diameter, nozzle pressure, nozzle stand-off, ice characteristics and the overall
scale of the system. An empirical relationship, derived by regression analysis, is
presented correlating the jet penetration to the power in the jet, the jet traverse
speed, the nozzle stand-off and the estimated ice temperature.
1124

1.0 INTRODUCTION
Cutting a slot into a sheet of ice can reduce its flexural strength con­
siderably. Such a slot or multiple slots should be useful in easing the passage of
an ice-breaking vessel through ice fields. The substantial weakening of an ice
sheet by cutting one or more grooves in the ice by means of a high pressure water
jet has been proposed as a possible means of extending current ice breaking capabil­
ities and reducing fuel consumption. A relatively simple device, the high pressure
water jet, used as a cutting tool, has the potential for development into a rugged,
practical system for notching ice ahead of an ice-breaking vessel. Although mech­
anical modes of cutting can remove material more efficiently, a water jet has the
advantage of non-mechanical contact and can cut with a substantial stand-off from
the material being cut. This characteristic along with the ability to introduce a
concentrated, high level of power into the material would provide significant prac­
tical advantages for the water jet cutting method when used to assist ice breaking.
Previous work by the Gas Dynamics Laboratory of the Division of Mechanical
Engineering of the National Research Council of Canada in cutting a variety of mater­
ials with high pressure water jets and a few water jet cuts in ice at the University
of Missouri at Rolla during frozen soil cutting trials for the U. S. Army Cold
Regions Research and Engineering Laboratory [1] led to exploratory small scale ice
cutting trials in the Gas Dynamics Laboratory [1]. While these initial trials
showed that ice indeed could be cut with high pressure water jets, extrapolation of
the results to a full scale system was impractical. After a subsequent series of
field tests by CRREL at very high pressures [1], the Gas Dynamics Laboratory in col­
laboration with CRREL made various series of cuts in ice ranging from floating
ice [2] to manufactured ice to lock wall ice collars with a pumping system about one
full order of magnitude larger than the laboratory system. These cuts covered a
fairly wide range of conditions, from relatively high speed shallow penetration cuts
to low speed relatively deep penetration cuts. Extrapolating about two orders of
magnitude from these results, while not at all reliable, did indicate that a realis­
tic full scale system might be possible.
To further investigate the potential cutting ability of water jets in ice,
larger scale field tests were initiated [3] [4][7] by the Low Temperature Laboratory
of the Division of Mechanical Engineering of the National Research Council of Canada
and conducted in collaboration with the Gas Dynamics Laboratory. Through the course
of this investigation, the field tests were supplemented by further fairly small
scale laboratory tests [5][6] including one series of cuts in a simulated sea
ice [6].
1125

2.0 ICE CUTTING SYSTEM
Cutting ice with a water jet is achieved by impacting a high velocity jet
of water onto the ice. The resulting velocity and directional changes apply forces
to the ice sufficient to fracture some of the weaker bonds between and within crys­
tals. By traversing the jet across the surface of the ice a slot can be cut into
or even through the ice. Figure 1 shows a water jet cutting such a slot in a
floating ice sheet, and Figure 2 shows the kerf cut by the water jet.
1126
Figure 1: Water Jet Cutting of
Floating Ice Sheet
Figure 2: Kerf Cut by
Water Jet
Typically, the jet is produced by accelerating high pressure water
through a convergent steel nozzle. The high pressure water is supplied by a pump­
ing system usually drawing the water for the jet from under the ice sheet . For
most of our field testing the swing of a hydraulic crane with a telescoping boom
was used to traverse the cutting nozzle.
Suitable and accessible test sites were selected, on a spring-fed pond
for the first series of field t ests (March 1977) [3] and on the Ottawa River for
the second and third series (February 1978 [4] and February 1979 [7] respectively).
For the field tests the ice thickness ranged up to about 0.7 metre.

frequently thrown considerable distances; the small particles usually were ejected
in the spray of spent water from the jet. Except for the shallowest cut all cuts of
the third series resulted in a narrow fairly clean kerf about 13 mm wide, similar to
the cuts of the first series.
Of the measurable cuts in the ice blocks, most produced a spalled groove,
a few produced a recognizable kerf and the remainder simply melted a shallow groove
in the ice. It was noticed that kerf cuts only occurred at the higher nozzle pres­
sures (above 48 MFa) and at the higher ice temperatures (above -SoC), while spalling
occurred between 7 and S2 MPa nozzle pressure and a melted groove was often found at
nozzle pressures from 4 to 21 MFa.
Cuts in the laboratory fresh water ice sheets varied from deep, narrow,
clean kerf cuts at the higher nozzle pressures to shallow, widely spalled cuts at
the lower nozzle pressures. No sharply defined change in mode occurred; rather,
there was a gradual increase in spalling as the pressure was lowered.
For the simulated sea ice most of the cuts were essentially clean kerf
cuts with a small degree of surface spalling. However, for the first seven tests,
as the pressure was reduced below about 40 MFa, the spalling became wider and
deeper until at about 13 MFa only a spalled trench was produced.
4.0 ICE CHARACTERISTICS
The fresh water ices cut in the field tests were generally a type of nat­
ural ice commonly found on lakes and rivers, having many layers of snow ice with an
underlying layer of clear ice. For the first series the ice temperature was OoC
and the top layers had candled, while for the second series the ice near the top
surface varied from about -lloC to _2 o C and for the third series it ranged from
about -18 o C to _lloC. The colder ice was harder and stronger and obviously more
difficult to cut, while the candled top layers of ice cut eaSily.
All the fresh water ice made in the laboratory was clear with the charac­
teristics of ice grown unidirectionally from the free surface. The blocks of ice,
having been stored in a chest freezer, were at a uniform temperature throughout
varying from _18 o C to OOC. However, for the July/August 1978 tests, the three sep­
arate ice sheets, near their top surface, ranged from about _17 o C to OOC.
There was a variety of ices cut in the tests from the early studies of
NRC and USA CRREL [1][2], ranging from ice blocks to floating ice sheets to lock
wall ice collars. However, all were apparently at or very near to OOC and there­
fore relatively easy to cut.
The saline ice produced for these tests was a first approximation facsim­
ile of first year sea ice. Its salinity at 5 ppt was in the same range as the
"typical average figure" of 4 ppt cited by Pounder [8]. With its many inclusions
1128

and underlying fragile structure, this ice should have been more susceptible to
water jet cutting than fresh water ice.
5.0 ANALYSIS OF TEST DATA
For the cuts in fresh water ice and for those in simulated sea ice, separ­
ate relationships between the jet penetration and the jet parameters have been
derived by multiple linear regression analyses.
From the general expression
Y = f (u, d, p, s)
where: Y average penetration (cm)
u = nozzle traverse speed (km/h)
d nozzle diameter (mm)
p nozzle pressure (MFa)
s = average nozzle stand-off distance (cm)
and assuming that a zero value for this function would result in a zero depth of
cut, a first approximation was obtained by applying multiple linear regression anal­
ysis to a logarithmic transformation of this expression to yield ultimately an
equation of the form:
(1)
E
s (2)
For the cutting of fresh water ice, analyses of this type were conducted
both on individual series of tests and on combinations of data from groups of test
series, including published and unpublished data from the early studies of NRC and
USA CRREL [1] [2]. These analyses yielded exponents for nozzle traverse speed con­
sistently near -0.5. Whenever the data covered a sufficient range of nozzle diam­
eter and pressure, the exponent for nozzle diameter tended to range between 1.5 and
2, while that for nozzle pressure tended to vary about 1.5. There was a general
lack of useful correlation to nozzle stand-off.
It was recognized that the exponents for nozzle diameter and pressure were
close to those that appear in the relationship describing the physical jet property
of hydraulic power,
HP = C d 2 p3/2
where: HP hydraulic power (kW)
C dimensional constant
d nozzle diameter
p nozzle pressure
With the simple relationship,
ru
Y = f (HP)
(mm)
(MFa)
consistently good, highly significallt correlations were obtained both for individual
(3)
(4)
1129

clear demarcation between kerfing and spalling, the results of this test program
suggest that about 40 MPa was needed to cut a kerf without excessive spalling in
either fresh water ice or in the simulated sea ice. Still higher pressures gener­
ally produced cleaner cuts. For equivalent conditions a spalled cut tended to be
shallower than a kerf cut. A few small scale cuts simply melted a groove in the
ice.
For the first few simulated sea ice tests, when the ice was still cold,
cutting through the hard surface layer, in all cases, resulted in some degree of
surface spalling, becoming more pronounced as the nozzle pressure was reduced until,
at 13 MPa, only spalling occurred. Cutting into or through the underlying softer
ice produced clean kerf cuts.
For the fresh water ice cutting regression analysis, equation (5) provides
a statistically excellent fit to the entire body of data. A comparison of the slope
of equation (6) to that of equation (5) indicates that penetration in the simulated
sea ice was more than double that in fresh water ice when the jet parameters were
similar. Apparently, the brine and air inclusions within the saline ice structure
did permit easier jet penetration into this ice, as expected.
7.0 CONCLUSIONS
A large number of cuts have now been made in fresh water ice with small to
moderate scale water jets; a few have also been made in a simulated sea ice with
small scale water jets. In the majority of cases a narrow, clean kerf was cut in
both types of ice. However, below about 40 MPa nozzle pressure the cut consisted
mostly of a wide spalled trench. The kerf was apparently produced by erosion in a
ductile material while the spalled trench was apparently produced by brittle frac-
ture.
As the fresh water ice temperature dropped substantially below freezing,
a considerable reduction in penetration capability occurred. This was apparently
due to an increase in ice strength. A first approximation of this effect was
obtained by applying an empirical correction factor to the penetration - jet para­
meters relationship based on the estimated temperature of the ice near the surface.
This factor enabled the data from the entire range of temperatures to be explained
by a single highly significant regression equation.
With all the fresh water ice cutting data taken together, equation (5)
represents a statistically excellent fit to the data. It confirms that the jet
parameters, hydraulic power and the square root of traverse speed, are the important
factors and that ice strength can be taken into account by a simple empirical ice
T /20
temperature factor (e i ). Use of this entire body of data has also revealed
that nozzle stand-off does have a statistically significant effect, albeit a small
one.
1132

7. Coveney, D. B.
Brierley, W. H.
8. Pounder, E. R.
1134
"Cutting Cold River Ice with Water Jets during the
Winter 1978-79". LTR-LT-108, National Research Coun­
cil of Canada, January 1980.
"The Physics of Ice". Pergamon Press Ltd., London,
England, 1965.