The installed cost of an offshore wind turbine is about 50% higher than an onshore machine and foundations make a significant contribution to the extra cost. Typically, they account for around 20% of the total cost, compared with around 6% for an onshore machine. Corus believes it may be able to achieve savings of £100,000, or more, which represents a worthwhile reduction of around 2-3% on the installed cost of an offshore wind turbine.
The key lies in combining foundation, tower and turbine installation offshore as one operation. In addition to reducing component costs, there are also savings in the time taken in offshore assembly. There are also significant safety benefits associated with the concept. At the heart of the study is a unique twin-wall construction system, which enables an integrated approach to the offshore wind turbine construction instead of today's sequential approach, which starts with foundations, moves on to tower erection and then turbine installation.
The Bi-Steel concept uses twin wall panels comprising pairs of steel plates connected by a series of transverse bars. These can be prefabricated into sub-assemblies, possibly comprising a conical foundation base, and telescopic tower sections, to reduce on-site construction time. The units have excellent strength to weight ratios and, once installed, can be filled with concrete or other material to further increase strength. Corus estimates that offshore assembly could be completed in less than 12 hours, which cuts down the hire charges of both expensive offshore barges and heavy lifting equipment.