Vertically Loaded Plate Anchors for Deepwater Applications

Vertically Loaded Plate Anchors
for Deepwater Applications
Don Murff
OTRC, Texas A&M University, USA
Mark Randolph, Sarah Elkhatib
COFS, University of Western Australia
Harry Kolk
Fugro Engineers, The Netherlands
Rod Ruinen
Vryhof Anchors, The Netherlands
Pål Strøm
det Norske Veritas, Norway
Colin Thorne
University of Sydney, Australia
Project sponsored by
American Petroleum
Institute
&
Deepstar Project
Undertaken jointly by
OTRC, NGI and COFS
+ volunteers!

Vertically Loaded Anchors
Bruce DENNLA Vryhof Stevmanta

Motivation for the Study
Seabottom
Mudline Force

Phase I - Data Collection
• Identify and collect relevant references
• Develop a collection of prediction methods
• Create a database of VLA test data and
applications

Phase II - Evaluate Current Practice
• Industry prediction exercise of hypothetical
(idealized) case studies.
• Industry prediction exercise of actual case
studies.
• Numerical studies of anchor performance under
multi-axial loading and comparison with industry
prediction methods.

• References
Phase I Results
– More than 80 references collected
• Prediction Methods
– Several existing methods were documented for analysis of anchor
line behavior, installation, and holding capacity.
– Methodologies ranged from strictly empirical to advanced numerical
methods.
• A database of VLA applications and experiments was
compiled, including:
– Offshore field experiments
– Onshore field or laboratory experiments
– Full scale applications

Year
1995
1996
1998
1999
1999
2000
2002
2003
2004
Field & Type
Nkossa FSO
Liuhua 11-1
Voador P27 Semi-FPU
Marlim South EPS
FPSO-II
Roncador P36 Semi-FPU
Marlim P40 Semi-FPU
Roncador FPSO
Fluminese FPSO
Marlim FPSO
Field Applications
Location
Gulf of Guinea
South China Sea
Offshore Brazil
Offshore Brazil
Offshore Brazil
Offshore Brazil
Offshore Brazil
Offshore Brazil
Offshore Brazil
Water
depth
(m)
1125
310
530
1215
1350
1080
1150 to
1475
700
1210
Anchor type
SBM ‘Mag’
Bruce FFTS Mk4
Stevmanta
Bruce DENNLA
Stevmanta
Stevmanta
Stevmanta
Stevmanta
Stevmanta
Fluke
area
(m 2 )
16.4
11
10
13
13
14
11
13
Operator
Elf
Amoco
Petrobras
Petrobras
Petrobras
Petrobras
Petrobras
Shell
Petrobras

Industry Prediction Exercise
Hypothetical Cases
B
H
D 1
C
D
θ f
D 2
Idealized Drag Anchor Geometry
Case 1 Base case
Case 2 Vary anchor weight (weight x 2)
Case 3 Vary anchor line diameter (diameter x 2)
Case 4 Vary fluke shank angle (decrease from 50 to 35 degrees)
Case 5 Vary shank cross-section (increase cross-section area x 2.25)
Case 6 Vary fluke aspect ratio (change fluke area from a 2:1 rectangle to a square)
Case 7 Vary soil profile (change from linear increase with depth to uniform strength)
G
CG
E
θ 1
A
F

Comparisons Among Predictors
Base Case
Shackle Load, KN
Shackle Depth, m
0
10
20
30
40
50
60
70
3000
2500
2000
1500
1000
500
0
Drag Distance, m
0 100 200 300 400 500 600
Predictor 1
Predictor 2
Predictor 3
Predictor 4
Predictor 5
0 100 200 300 400 500 600
Drag Distance, m
Predictor 1
Predictor 2
Predictor 3
Predictor 4
Predictor 5

Comparisons Among Cases
Predictor 3
Shackle Load, KN
Shackle Depth, m
0
10
20
30
40
50
60
70
80
4000
3500
3000
2500
2000
1500
1000
500
0
Drag Distance, m
0 100 200 300 400 500
Case 1
Case 2
Case 3
Case 4
Case 5
Case 6
Case 7
0 100 200 300 400 500
Drag Distance, m
Case 1
Case 2
Case 3
Case 4
Case 5
Case 6
Case 7

Shackle Load, KN
Predicted Depth, m
0
10
20
30
40
50
60
70
80
90
4000
3500
3000
2500
2000
1500
1000
500
0
Case
1
Case
1
Parameter Effects Among Predictors
Case
2
Case
2
Case
3
Case
3
Case
4
Case
4
Case
5
Case
5
Case
6
Case
6
Case
7
Case
7
Case
1. Base
2. Vary
weight
3. Vary line
diameter
4. Vary flukeshank
angle
5. Vary shank
cross-section
6. Vary fluke
aspect ratio
7. Vary soil
profile
Parameter
Mean
COV
Mean
COV
Mean
COV
Mean
COV
Mean
COV
Mean
COV
Mean
COV
Depth
ratio
z/z case1
1
0
1.00
0.006
0.54
0.036
0.38
0.26
1.06
0.035
1.13
0.27
0.49
0.049
Shank
force
ratio
T/T case1
1
0
1.01
0.007
0.57
0.040
0.29
0.31
1.20
0.065
1.15
0.27
0.26
0.204
Mudline
force
ratio
T/T case1
1
0
1.01
0.008
0.58
0.033
0.28
0.32
1.19
0.062
1.17
0.28
0.27
0.193

Simulation Results vs Design Chart Predictions
Design Chart Equation for Soft Clay
0.
92
T hc = 48W
1.
53
W = 2. 31 A
T = 13
As
nhc
u
T h c =
103. 7
A
where T hc = anchor holding capacity (kN); W = anchor weight (kN); and A= fluke area (m 2 )
Base Case Capacity by Chart: 864 kN
Capacity by Simulation:
Mean = 2985 kN with range of 1963 kN to 3811 kN
Plate Uplift Capacity for Normal Loading
At 20 m depth T nhc = 1755 kN Performance Ratio = 1755/864 = 2.0
At 50 m depth T nhc = 4390 kN Performance Ratio = 4390/2985 = 1.5
1.
41

Location
Operator
Water depth
Anchor type
Weight
Fluke area
Industry Prediction Exercise
Case Histories
Case 1
Gulf of Mexico
Omega
Marine
Unknown
Vryhof Stevpris
68.6 kN
9 m 2
Case 2
Gulf of Mexico
Aker Maritime JIP
91m
Bruce DENNLA
12.7 kN
5 m 2
Case 3
Gulf of Mexico
Aker Maritime JIP
91 m
Vryhof Stevmanta
31.6 kN
5 m 2
Case 4
Centrifuge UWA
(80g)
COFS
NA
Based on Stevpris
373 kN
(prototype)
21 m 2

Results of the Case Studies Prediction Exercise
Mudline Load, KN
Shackle Depth, m
0
5
10
15
20
25
30
35
40
1600
1400
1200
1000
800
600
400
200
0
Drag Distance, m
0 50 100 150 200 250
Predictor 2
Predictor 3
Predictor 4
Predictor 5
Measured
0 50 100 150 200 250
Drag Distance, m
Predictor 2
Predictor 3
Predictor 4
Predictor 5
Measured
Mudline Load, KN
1600
1400
1200
1000
800
600
400
200
0
0 10 20 30 40
Depth, m
Predictor 2
Predictor 3
Predictor 4
Predictor 5
Measured

Drag =
Empirical Method Predictions
vs Field Measurements
Case
1
2
3
Quantity
Measured
Chart
Measured
Chart
Measured
Chart
49. 4
A
0.
48
Drag
distance (m)
75
141
77
102
77
110
M u d l i n e C a p a c i t y =
Penetration
depth (m)
21.3
27.7
21
20.1
24.0
21.6
Depth =
103. 7
A
Mudline load
(kN)
1.
41
1983
2260
1445
886
1053
1089
9. 7
A
0.
48

Advanced Numerical Analysis

Advanced Numerical Analysis of Anchor
Capacity Under Multi- Axial Loading
Global finite element mesh
Finite element mesh
detail near plate anchor
Model properties
Soil: linear elastic, Tresca plastic
E/s u = 500
υ = 0.49
Plate: rigid body
Interface: τ max = αs u
α varied from 0 to 1
Plate geometry & loading
t
M
F n
L
F s

Analysis Results - Capacity Factors for Pure
Normal, Shear & Moment Loading
Aspect
ratio
L/t = 7
L/t = 20
Value
of α
0
0.2
0.4
0.6
0.8
1
Bonded
Bonded
N n
11.15
11.24
11.32
11.39
11.45
11.49
11.58
11.33
F
⎛ + ⎞
= max
t 1 α
N
n
n = 3π
+ 2 ⎜α
+ ⎟
Lsu
L ⎝ 2 ⎠
FE Results UB Approximations (α = 1)
N s
2.10
2.58
3.04
3.50
3.95
4.36
4.49
3.21
N m
1.48
1.56
1.61
1.63
1.65
1.67
1.74
1.71
Aspect
ratio, L/t
F
t
N s max ⎛ ⎞
s = = 2⎜α
+
N t i p ⎟ ≈ 2α
+ 15
Ls
u ⎝
L ⎠
6
7
8
9
10
15
20
t
L
12.39
12.23
12.11
12.03
11.96
11.91
11.75
11.67
Upper Bound Analytical Approximations (after O’Neill et al, 2003)
5
N n
N s
5.00
4.50
4.14
3.88
3.67
3.50
3.00
2.75
N m
1.63
1.61
1.60
1.60
1.59
1.59
1.58
1.57
⎡ 2
M
⎛ ⎞
⎤
= max π
t
N
m = ⎢1
+ ⎜ ⎟ ⎥
2
L
s 2 ⎢⎣
⎝
L
u
⎠ ⎥⎦

Load Interactions & the Generalized Model
Parallel, Parellel, Ns, N or Moment, Nm, factor
s , or Moment, Nm , factor
5
4
3
2
1
0
Normal-parallel
Normal-moment
L/t = = 7, 7, fully rough
0 2 4 6 8 10 12
Normal capacity factor, Nn Nn Bransby and O’Neill (1999) equation with exponents from least squares fit of
FEM results.
1
3.
68
1.
37
3.
74
⎛
⎡
⎤1.
22
F ⎞ ⎛ ⎞ ⎛ ⎞
=
⎢
+
⎥
⎜
n
M
F
f ⎟ +
−1
= 0
max ⎢
⎜
⎟
⎜
s
⎟
⎝
F
n ⎠
max
max ⎥
⎣
⎝
M ⎠ ⎝
F
s ⎠
⎦
F n
M
F s
F n

Parallel Capacity Factor, N sN
s
5.00
4.50
4.00
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
FEM Results vs Generalized Model
0 5 10 15
Normal Capacity Factor, Nn Nn Parallel Capacity Capacity Factor, N sNs
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
FEM-COFS,L/t=7
Eqs. Failure 2-5, fn, L/t=7 L/t = 7
FEM-OTRC, L/t=10
Eqs. Failure 2-5, fn, L/t=10 L/t = 10
Failure Eqs. 2-5,L/t=7 fn, L/t = 7
FEM-COFS, L/t=7
Failure Eqs. 2-5, fn, L/t=6 L/t = 6
FEM-OTRC, L/t=6
Moment Capacity Factor, N mNm
1.8
1.6
1.4
1.2
0.8
0.6
0.4
0.2
0 0.5 1 1.5 2
Moment Capacity Factor, NmNm
2
1
0
0 5 10 15
Normal Capacity Capacity Factor, Factor, NnNn
FEM-COFS, L/t=7
Failure Eqs. 2-5, fn, L/t L/t=7 = 7
FEM- OTRC, L/t=6
Failure Eqs. 2-5, fn, L/t L/t=6 = 6

Predicted Load Interactions
Normalised Resultant resultant Force/LWSu force, F/Asu 14
12
10
8
6
4
2
0
N n = 11.49
t
Centroid
0 20 40 60 80
L
Load Inclination from Vertical, Degrees
b
b/L=0
b/L=0.1
b/L=0.2
b/L=0.3
b/L=0.4
b/L=0.5
Load inclination from vertical, θ (deg)
θ
F
N s = 4.36

Interpretation of Hypothetical Study Results
c
m d
Fluke
θ f
d
f bn
f bs
f dn
f ds
e
b
Shackle
Shank
a
Direction of travel
F
η
λ

Hypothetical Predictions Compared
to The Failure Function
Failure Function Function
4
3.5
3
2.5
2
1.5
1
0.5
0
-10 -5 0 5 10
Force Angle with Shank, Degrees
Inferred from Hypothetical
Data
Inferred from Hypothetical
Data, No moment
Failure function

Summary and Conclusions
Hypothetical Studies
• Predictions among five models were qualitatively similar.
• Parameters with greatest effects are:
– Anchor line diameter
– Fluke-shank angle
– Fluke aspect ratio
– Soil strength profile
• Model predictions for depth and load
>> empirical chart predictions.

Summary and Conclusions
Field Studies
• Model predictions matched measurements well (with
model adjustments).
• Plate anchors showed a linear depth vs drag distance.
• Linear load vs depth behavior predicted well by models.
• Chart predictions compared well with measurements
but….
– field anchors may not have been dragged sufficiently far to
maximize capacity.

Summary and Conclusions
FEM Studies
• FEM studies were carried out to assess load coupling
– Strongest coupling is between normal load and moment.
– FEM results agree well with upper bound analytical results.
• The FEM load interaction results gave an excellent fit to
the Bransby-O’Neill yield equation.
• FEM results were compared with inferred load capacities
from the hypothetical studies concluding that:
– A Priori prediction of anchor trajectories is subject to significant
uncertainty; and
– With calibration to specific situations the simplified methods can
work well.

Acknowledgements
• API Advisory Committee and Chairman Phillipe Jeanjean
• Deepstar Joint Industry Project
• U.S. Minerals Management Service
• Australian Research Council’s Research Centre’s
Program