By the end of October 2011, Danish contractor MT Højgaard had installed approximately half of the 75 monopile foundations for the Lincs Offshore Wind Farm, which is located off the coast of Lincolnshire in England. The remaining foundations are to be installed between now and the first half of 2012.
The foundations consist of monopiles, which are hollow steel pipes approximately 5m diameter and between 37 and 46 metres long, with variable wall thickness typically between 52mm and 84mm and weighing between 291 and 391 tonnes. The monopiles are driven between 17 and 26 metres into the seabed (which is mainly weathered chalk), using a hydraulic hammer on board the jackup vessel Resolution, operating from Great Yarmouth outer harbour.
Specially designed hydraulic pile gripper and pile follower
The monopiles have been optimised in length to be as short as possible, and this, combined with the height of the jackup vessel above water level, requires the use of an 80 tonne pile follower to make the overall length sufficient for temporary stability during the initial stages of piling, while the top of the monopile / follower is held by a specially designed hydraulic pile gripper mounted at the stern of the jackup vessel.
On top of each monopile, MT Højgaard installs a transition piece, which is 24 metres high, weighs around 280 tonnes and functions as a support structure for the wind turbine itself.
The total weight of the steelwork in the foundations is about 46 thousand tonnes.
Conical grouted connections
Recent developments in design of offshore foundations have required a conical grouted connection between the transition piece and monopile, which increases the complexity of design, fabrication, and installation from the earlier cylindrical connections used in the industry.
MT Højgaard is among the world's most experienced companies in installing foundations for offshore wind turbines. There is a substantial amount of development and design and logistics work involved in the project, since the sea depth in the area varies from 8 to 16 metres. Each monopile is unique and designed and fabricated for the precise location where it will be installed. The steel foundations are fabricated in Holland and Belgium and sailed to Great Yarmouth.
MTH and consultant Rambøll have been working on the turnkey design and fabrication of these foundations for two years, and the design is a synthesis of current best practices for both permanent and temporary works.
The Lincs OWF client is a consortium of Centrica, DONG Energy and Siemens Project Ventures. The 270 MW wind farm will supply electricity to 200,000 British households.
Ed Garvey, ICE representative for Denmark, is manager of MT Højgaard's offshore design and engineering department.