1. THE MONOPOD BUCKET FOUNDATION Recent experience and challenges ahead Christian LeBlanc1, Kim Ahle1, Søren A. Nielsen 2, Lars B. Ibsen 3
1) DONG Energy, 2) MBD Offshore Power, 3) Aalborg University
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Abstracts
2009 Prototype
The Mobile Met Mast
In March 2009, NearshoreLAB A/S, a subsidiary of DONG Energy, successfully installed a prototype of a monopod bucket foundation at Horns Rev 2 Offshore Wind Farm, Denmark, to support a met-mast. The prototype is called the “Mobile Met-Mast” as the met-mast can be transferred to another location by reversing the installation process.
Specs
Total height: 38 m
Weight: 165 tons
Skirt length: 6 m
Skirt diameter: 12 m
Fabricated by Bladt Industries A/S, Aalborg, Denmark, in August 2008
Following the successful installation of a prototype of a monopod bucket foundation at Horns Rev 2 Offshore Wind Farm, Denmark, in 2009, DONG Energy is developing a strategy for commercialization. The monopod bucket foundation is a promising foundation concept for offshore wind turbines that can be installed using suction assisted penetration, combined with other installation methods. This is a self-installing foundation concept, avoiding the use of expensive jack-ups and scour protection, and thus significantly reduces installation costs. Further research, development and prototype testing are required before the monopod bucket foundation can be commercialized.
Up-ending at Horns Rev 2 Offshore Wind Farm using MS SeaPower by A2Sea
Successful offshore installation
Background
The foundation was floated to site using 2 tug boats
and up-ended using MS Seapower by A2SEA vessel. The water depth was on the lower limit feasible for installation. Inflation/deflation of air in the bucket skirt was necessary during up-ending.
The monopod bucket foundation, also called “monopod suction caisson”, is a promising foundation concept for offshore wind turbines which has the potential to be cost-effective in certain soil conditions. Depending on the skirt length and diameter, the bucket foundation can have a bearing capacity similar to that of a monopile, a gravity foundation or in between.
Pros
Less steel required compared to the monopile
Installable without heavy offshore cranes
Rapid offshore installation is possible
No transition piece or hammer required
Scour protection can be eliminated
Silent and reversible installation
Cons
Complicated structure to fabricate
Grouting required beneath bucket lid
Only suitable for certain types of soil
More “high-tech” than the monopile
Penetration velocity: 2 m/hour
Installed with a 0.1 degree inclination out of vertical
Time from jacking-up to complete installation: 32 hours
Room for much optimization - estimated that the operation can be performed in less than 10 hours
Local scour holes, approximately 1m deep, had developed on two sides of the bucket. Monitoring of the scour development continues.
Launching of the Mobile Met Mast at Bladt Industries fabrication yard.
Design
The geometry of bucket foundations falls into the category of thin shell structures as the ratio between the bucket diameter and wall thickness is very large. Buckling is therefore a major design consideration.
The Mobile Met Mast was designed to withstand structural buckling on the basis of advanced numerical analyses, including
three-dimensional measurements of the as-built skirt imperfections.
Challenges ahead
Following the successful installation of the prototype, DONG Energy and MBD Offshore Power A/S, a subsidiary of DONG Energy, are developing a R&D strategy for commercialization. Further research, development and prototype testing are required before the monopod bucket foundation can be successfully commercialized. Techniques to minimize the out-of-roundness of the caisson skirt, excavate soil beneath the bucket lid, minimize the risk of unforeseen rocks or large stones, as well as techniques for floating installation, including up-ending, lowering to seabed and positioning are currently being studied. Furthermore, physical tests to obtain more accurate predictions of buckling loads and scour development are planned. Dong Energy are planning to install a second prototype in 2010/2011.
The Installation Technology
The monopod bucket foundation is installed using suction assisted penetration, whereby suction is applied within the bucket subsequent to an initial self-weight penetration, combined with other installation methods. Special techniques are used to ensure verticality during installation. The suction installation gives the possibility of having a self-installing foundation concept, avoiding the use of expensive jack-ups, installation vessels and scour protection.
Downward pressure on lid, due to suction
Reduction of tip resistance, due to flow induced in soil
Skirt tip injection for vertical alignment and reduced tip resistance
European Offshore Wind 2009 Conference & Exhibition, 14 – 16 September, Stockholm, Sweden