The Monopod Bucket Foundation - GL Group

The Monopod 

Bucket Foundation 

Hamburg Offshore Wind 2009 

12. May 2009 

Recent Experience and Challenges 

Ahead 

Christian LeBlanc Bakmar 

Offshore Technology, DONG Energy Power 

www.dongenergy.com

GENERATION - CHRLE 

Agenda 

�� 1. Introduction 

Background and history of the monopod bucket 

foundation concept. 

�� 2. Motivation 

Why is there an interest in commercialising the monopod 

bucket foundation? 

�� 3. Recent experience 

Successful installation of The Mobile Met Mast at Horns 

Rev 2 Offshore Wind Farm. 

�� 4. Challenges ahead 

What are the main challenges for commercialisation of the 

monopod suction caisson. 

�� 5. Concluding remarks 

2


Introduction 

The Monopod Bucket Foundation: 

�� A hybrid of a monopile and a 

gravity based foundation. 

�� Cost efficiency is improved if 

the ratio diameter/skirt length is 

approximately 1. 

3


The installation technology 

Downward pressure on 

lid, due to suction 

Reduction of tip resistance, due to 

flow induced in soil 

4 

Skirt tip injection for 

vertical alignment and reduced 

tip resistance


Previous research & development, since 2001 

Development has been undertaken MBD Offshore Power A/S. 

Test field in Frederikshavn: 

�� Medium-scale testing in sand 

(2x2 m and 4x4 m) 

Aalborg University: 

�� Laboratory testing in sand 

�� Numerical modelling 

Tests performed: 

�� Installation tests 

�� Monotonic moment loading tests 

�� Cyclic moment loading tests 

Methods and techniques derived for: 

�� Installation using suction 

�� Vertical installation using skirt tip injection 

�� Static and cyclic moment resistance 

5


Previous prototypes 

Frederikshavn, Denmark 

2003 

�� Designed for 3 MW Vestas turbine 

�� Successfull installation 

�� Post-installation monitoring program was conducted 

Skirt dimensions: D = 12 m, L = 6 m 

Wilhelmshaven, Germany 

2005 

�� Designed for a 5-MW Enercon turbine 

�� The installation failed. The installation barge 

floated sideways and colided with the bucket 

during the installation process. The impact with 

the bucket skirt initiated buckling after only 7 m 

penetration. 

Skirt dimensions: D = 16 m, L = 15 m 

6


MBD Offshore Power A/S 

October 2006 

MBD Offshore Power A/S became a subsidy of DONG Energy. 

The group holds patents encompassing the installation 

technology. 

DONG Energy is promoting the further development of the 

monopod bucket foundation through MBD Offshore Power A/S. 

7


Agenda 














8


Motivation 

Commercialization of the monopod bucket foundation: 

��We estimate that the bucket foundation can be more costeffective 

than the monopile. 

��The monopod bucket foundation is a feasible foundation 

concept for several sites in DONG Energy's current offshore 

portfolio. 

��The concept is environmentally friendly 

(silent and reversible installation). 

��Patented technology - possible commercialization of IPR and 

use in other projects. 

9


Some pros and cons 

Pros 

�� Less steel is required compared to the monopile 

�� Installable without the use of heavy cranes 

�� No transition piece is required 

�� No hammer is required 

�� Stiffer structure than the monopile, resulting in less dynamic 

amplification of loads. Important for deeper waters 

�� The need for scour protection can be eliminated 

�� The installation process can be reversed 

�� Silent installation process - no pile driving is required 

�� Floatable foundation - limited storage area needed during fabrication 

Cons 

�� More complicated structure to fabricate 

�� Require grouting beneath the bucket lid 

�� Can only be used in certain types of soil 

�� Float-out to site requires sufficient water depth in the harbor 

�� More “high-tech” than the monopile – more can go wrong 

�� More vulnerable concept 

10


DONG Energy's Offshore Portfolio 

In operation 

Under construction 

Use of monopod bucket foundations: 

Non-applicable 

Partly or maybe applicable 

Applicable 

Walney II 

Walney I 

West of Duddon Sands 

Scarweather Sands 

Wigtown Bay 

Burbo 

Barrow 

Shell Flat 

Gunfleet Sands 

Westermost Rough 

London Array II 

London Array I 

11 

Frederikshavn Offshore 

Horns Rev II 

Horns Rev I 

Borkum Riffgrund II 

Borkum Riffgrund I 

Tunø Knob 

Vindeby 

Nysted 

Middelgrunden


Agenda 














12


The Mobile Met Mast 

"The Mobile Met Mast" is a prototype of 

a monopod bucket foundation designed 

as a support structure for a met-mast. 

Purpose: 

�� To gain confidence that a monopod 

bucket foundation can be successfully 

installed offshore. 

�� To obtain a movable met-mast, 

which can be used in several offshore 

wind farms. 

Specs 

��Total height: 38 m 

��Weight: 165 tons 

��Skirt length: 6 m 

��Skirt diameter: 12 m 

Fabricated in Aalborg in August 2008. 

Installed at Horns Rev 2 Offshore wind 

farm in March 2009. 

13


Fabrication 

�� Fabricated by Bladt Industries A/S, 

Denmark 

�� Steel structure 

�� Plate girder lid 

�� Cold rolled and welded skirt 

14


Fabrication 

�� The geometric skirt imperfertions was measured by a 3D 

point cloud laser scanner 

�� The maximum out-of-roundness was ± 50mm 

�� The largest imperfections were along the vertical weldings 

15


Pumping Equipment 

Pumping equipment was installed for: 

�� Adding suction in the bucket 

�� Skirt tip injection 

�� Adding or emptying water from parts of the 

foundation body 

�� Adding air into the bucket skirt 

16


Launching 

17


Site for installation 

Horns Rev 2 

Wind turbines: 

��91 Siemens 2.3MW 

��200 MW 

Scheduled installation: 

��- 2008: Foundations 

��- 2009: Turbines 

The Mobile Met Mast 

��3 installation tests were 

planned at different 

locations 

(depending on weather) 

��Was only installed at the 

final location 

��No data from CPT or 

borings are available (yet) 

Horns Rev 1 

18 

Esbjerg 

Denmark


Float out to site 

�� Floated to site using 2 tug boats 

�� 40 m 3 water was pumped into the 

head of the Mobile Met-Mast in order 

to ensure a horizontal orientation 

when floating 

19


Up-ending procedure 

�� Up-ended with M/S SeaPower by A2SEA 

�� The lift was performed with a heave-compensator 

�� Water depth (15 m) was close to the lower limit – only 1 meter of 

water below skirt during up-ending. 

�� Inflation/deflation of air in the bucket skirt was necessary during 

up-ending 

20

GENERATION - HCASA 

21



22



23


Adding connections 

24


Installation 

�� A penetration velocity of 2 m/ 

hour was obtained 

�� After 2.5 m penetration, the 

foundation started tilting. At 4 m 

penetration, the process was 

reversed until 3 m penetration. 

Then the penetration process 

was repeated without tilting 

problems. We believe the tilt 

problem was caused by a stone 

along the skirt circumference. 

�� The flow induced in the soil was 

blocked, indicating that a clay 

layer was present. Installation 

continued. 

�� The foundation was successfully 

installed with a 0.1 degree 

inclination out of vertical. 

25


Scour development 

�� Scour is critical during and 

after installation. 

�� During up-ending and 

installation there were 

currents of ± 1 knot. 

�� Local scour holes, 

approximately 1m deep, 

had developed on two sides 

of the bucket. 

�� No scour protection has 

been installed. 

�� Monitoring of the scour 

development continues. 

26


Installation time 

�� Time from jacking-up to complete installation was 32 hours. 

�� In comparison, the first monopile was installed at Horns Rev II in 31 

hours and at Horns Rev I in over 2.5 days. 

�� Mounting and unmounting of connections was very time consuming. 

�� Room for much optimization - it is estimated that the operation could 

be performed in less than 10 hours. 

Other 

5.5 h 

Jack-up and 

preload 

1.5 h 

Time distribution 

Mounting 

connections 

11.5 h 

Installation 

10 h 

Preparation of 

lifting equip. 

2 h 

Up-ending 

foundation; 1.5 

h 

27


Agenda 














28


Further research & development 

Motivation for further research & development: 

Foundations are a vital part of an offshore wind farm – 

foundation failure is crucial! 

Statistically, 1 of 3 prototypes have failed so far. This clearly 

underlines the vulnerability of the concept. 

The monopod bucket foundation will not be commercialized 

before we are completely confident that we are able to handle 

all risks successfully! 

We are not there yet! 

Further research, development and prototype testing are 

necessary in order to: 

a)� eliminate all major risks associated with the foundation 

concept 

b)� increase the cost-efficiency of the foundation concept 

29


Managing risks 

Installation failure; complete penetration cannot be obtained 

Accurate prediction of the installation process is important in order to 

ensure a successful installation. Our experience is mainly in installation of 

monopod bucket foundations in sand. 

Further work is undertaken to gain knowledge with: 

�� Installation in layered soils 

�� Installation in clays and silts 

�� The effects of skirt imperfections on penetration resistance 

Structural failure; collapse of the bucket during installation 

Monopod bucket foundations fall into the category of thin shell structures 

and are therefore particularly exposed to structural buckling. 

Further work is undertaken in order to: 

�� obtain accurate prediction of buckling limits 

�� investigate ways in increasing the robustness of future buckets in a costeffective 

manner, e.g. by addition of stiffeners 

30


Managing risks 

Unforeseen ground conditions; risk of installation failure due to e.g. 

boulders in the soil. 

The geotechnical site investigations may be extended in order to 

minimize the risk of unforeseen ground conditions. 

For example, acoustic intensity imaging may be used to determine 

sub-seabed stratigraphy and identify buried geohazards, such as 

boulders, e.g. the Acustic Corer by PanGeo Subsea, which is a 

seabed deployed unit with subsurface scanning sonar heads 

attached to a boom rotating to cover 360 degrees. 

31 

Acustic Corer by PanGeo Subsea


Concept development 

Design and fabrication 

The bucket lid: 

�� Plate girder lid 

�� Conical transition 

�� Concrete lid + dry dock construction 

The bucket skirt: 

�� methods to reduce geometrical imperfections 

�� use of stiffeners? 

�� other? 

Installation methodology 

There are several possibilities for installing a 

monopod bucket foundation: 

�� Horizontal floating transport + up-ending using a 

crane or an external pile gripper on a jack-up/vessel 

or by buoyancy alone. 

�� Vertical transport on barge or jack-up + lift with 

crane on jack-up 

�� Vertical floating transport in arrays + lift by vessel 

�� Other? 

32 

Monopile and transition pieces on a barge, 

Horns Rev 2 Offshore Wind Farm


Technical development 

Coupling device 

Adding connections, hoses etc., is very time-consuming. 

A module for rapid coupling needs to be developed. The module 

will attach to the bucket lid and contain pumping equipment, 

sensors, hoses etc. 

Lid excavation 

The installed capacity of the bucket foundation may be 

significantly increased, if it is possible to excavate soil beneath the 

lid and penetrate the bucket further into the ground. This may 

also eliminate the need for scour protection. 

33


Future prototypes 

Onshore test bucket, diameter 8m 

The primary purpose is to develop and test a lid 

excavation system and coupling device. 

Installation site: Onshore test facility in Frederikshavn 

Commencement date: fall 2009 

Full-scale prototype 

Offshore installation of a full-scale prototype, probably 

for a 3.6 MW wind turbine. 

Installation site: To be decided 

A location at Borkum Riffgrund is a possibility. 

Expected installation date: 

Late 2010 / Early 2011 

34


Agenda 














35


Technology jump? 

36


Thank you for listening 

37

The Monopod Bucket Foundation - GL Group

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?