www.dongenergy.com
The Monopod
Bucket Foundation
Hamburg Offshore Wind 2009
12. May 2009
Recent Experience and Challenges
Ahead
Christian LeBlanc Bakmar
Offshore Technology, DONG Energy Power
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.
GENERATION - CHRLE
5. Concluding remarks
2
Introduction
The Monopod Bucket Foundation:
A hybrid of a monopile and a
gravity based foundation.
GENERATION - CHRLE
Cost efficiency is improved if
the ratio diameter/skirt length is
approximately 1.
3
The installation technology
GENERATION - CHRLE
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
GENERATION - CHRLE
Methods and techniques derived for:
Installation using suction
Vertical installation using skirt tip injection
Static and cyclic moment resistance
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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.
GENERATION - CHRLE
Skirt dimensions: D = 16 m, L = 15 m
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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.
GENERATION - CHRLE
DONG Energy is promoting the further development of the
monopod bucket foundation through MBD Offshore Power A/S.
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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.
GENERATION - CHRLE
5. Concluding remarks
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).
GENERATION - CHRLE
Patented technology - possible commercialization of IPR and
use in other projects.
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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
GENERATION - CHRLE
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
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DONG Energy's Offshore Portfolio
In operation
Under construction
Use of monopod bucket foundations:
Non-applicable
Frederikshavn Offshore
Partly or maybe applicable
Applicable
Tunø Knob
Horns Rev II
Wigtown Bay
Middelgrunden
Horns Rev I
Walney II
Walney I
West of Duddon Sands
Burbo
Barrow
Shell Flat
Westermost Rough
Gunfleet Sands
GENERATION - CHRLE
Scarweather Sands
London Array II
London Array I
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Vindeby
Nysted
Borkum Riffgrund II
Borkum Riffgrund I
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.
GENERATION - CHRLE
5. Concluding remarks
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.
GENERATION - CHRLE
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.
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Fabrication
Fabricated by Bladt Industries A/S,
Denmark
Steel structure
Plate girder lid
GENERATION - CHRLE
Cold rolled and welded skirt
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Fabrication
The geometric skirt imperfertions was measured by a 3D
point cloud laser scanner
The maximum out-of-roundness was ± 50mm
GENERATION - CHRLE
The largest imperfections were along the vertical weldings
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Pumping Equipment
Pumping equipment was installed for:
GENERATION - CHRLE
Adding suction in the bucket
Skirt tip injection
Adding or emptying water from parts of the
foundation body
Adding air into the bucket skirt
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GENERATION - CHRLE
Launching
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Site for installation
Denmark
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
Horns Rev 1
GENERATION - CHRLE
No data from CPT or
borings are available (yet)
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Esbjerg
Float out to site
Floated to site using 2 tug boats
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40 m3 water was pumped into the
head of the Mobile Met-Mast in order
to ensure a horizontal orientation
when floating
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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.
GENERATION - CHRLE
Inflation/deflation of air in the bucket skirt was necessary during
up-ending
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GENERATION - HCASA
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GENERATION - CHRLE
Up-ending procedure
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GENERATION - CHRLE
Up-ending procedure
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Adding connections
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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.
GENERATION - CHRLE
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.
GENERATION - CHRLE
Monitoring of the scour
development continues.
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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.
Time distribution
Other
5.5 h
GENERATION - CHRLE
Jack-up and
preload
1.5 h
Mounting
connections
11.5 h
Preparation of
lifting equip.
2h
Up-ending
foundation; 1.5
h
Installation
10 h
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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.
GENERATION - CHRLE
5. Concluding remarks
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
GENERATION - CHRLE
b) increase the cost-efficiency of the foundation concept
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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.
GENERATION - CHRLE
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.
GENERATION - CHRLE
Acustic Corer by PanGeo Subsea
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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?
GENERATION - CHRLE
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?
Monopile and transition pieces on a barge,
Horns Rev 2 Offshore Wind Farm
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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.
GENERATION - CHRLE
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.
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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.
GENERATION - CHRLE
Expected installation date:
Late 2010 / Early 2011
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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.
GENERATION - CHRLE
5. Concluding remarks
35
GENERATION - CHRLE
Technology jump?
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GENERATION - CHRLE
Thank you for listening
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