Offshore Foundations: Gravitation + Steel
Most of the existing offshore wind parks use gravitation foundations. A new technology offers a similar method to that of the concrete gravity caisson. Instead of reinforced concrete it uses a cylindrical steel tube placed on a flat steel box on the sea bed.
A steel gravity foundation is considerably lighter than concrete foundations. Although the finished foundation has to have a weight of around 1,000 tonnes, the steel structure will only weigh some 80 to 100 tonnes for water depths between 4 and 10 m. (Another 10 tonnes have to be added for structures in the Baltic Sea, which require pack ice protection).
The relatively low weight allows barges to transport and install many foundations rapidly, using the same fairly lightweight crane used for the erection of the turbines.
The gravity foundations are filled with olivine, a very dense mineral, which gives the foundations sufficient weight to withstand waves and ice pressure.
The base of a foundation of this type will be 14 by 14 m (or a diameter of 15 m for a circular base) for water depths from 4 to 10 m. (Calculation based on a wind turbine with a rotor diameter of 65 m).
The advantage of the steel caisson solution is that the foundation can be made onshore, and may be used on all types of seabed although seabed preparations are required. Silt has to be removed and a smooth horizontal bed of shingles has to be prepared by divers before the foundation can be placed on the site.
The seabed around the base of the foundation will normally have to be protected against erosion by placing boulders or rocks around the edges of the base. This is, of course, also the case for the concrete version of the gravitation foundation. This makes the foundation type relatively costlier in areas with significant erosion.
Costs by Water Depth for Steel Gravitational Foundations
The cost penalty of moving to larger water depths is minimal compared to traditional concrete foundations. The reason is, that the foundation base does not have to increase in size proportion to the water depth to lean against ice pressure or waves.
The cost estimates for this type of foundation is for instance 2,343,000 DKK (= 335,000 USD) for a 1.5 MW machine placed at 8 m water depth in the Baltic Sea (1997 figures). The costs include installation.
The graph shows how the cost varies with water depth. Interestingly, the dimensioning factor (which decides the required strength and weight of the foundation) is not the turbine itself but ice and wave pressure forces.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 10 May 2003