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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

165

Abstracts

Successful offshore installation

Challenges ahead

Background

European Offshore Wind 2009 Conference & Exhibition, 14 – 16 September, Stockholm, Sweden

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

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

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

The Installation Technology

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

Design

2009 Prototype

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.

Launching of the Mobile Met Mast at Bladt Industries fabrication yard.

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.

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.

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.

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.

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