Function
The foundation provides support for the wind turbine, transferring the loads from the turbine at the tower interface level (typically around 20m above water level) to the seabed where the loads are reacted. The foundation also provides the conduit for the electrical cables, as well as access for personnel from vessels.
Who is involved
Fixed foundations are manufactured by specialist suppliers contracted by the developer.
Key facts
Foundation design is a complex engineering task. Design requirements include gravity load, thrust, and associated overturning moment, natural frequency, fatigue strength, verticality (over time), personnel access, cable entry and support. Design needs to take account of both wind and wave loading and, in some circumstances, must consider other environmental conditions such as earthquakes, typhoons and sea ice. Over 80% of offshore wind capacity installed to date has been supported by monopiles driven into the seabed, with jacket (and other space frame) foundations representing approximately 15%. Gravity bases are the least common design.
Foundations are made from either concrete or steel. Concrete material prices generally are less volatile than steel, meaning that when steel prices are high concrete is more attractive. In some regions, they can also offer higher levels of local content. The use of suction buckets for attaching the foundation to the seabed is being explored and has been commercially deployed in only a handful of offshore wind farms. These can be used with either jacket structures, or with monopiles (a “mono-bucket”). The main advantage is avoiding the loads associated with driving the monopile or pin piles (for jackets) into the seabed. This both reduces noise for sea creatures and allows foundations to be installed completely assembled with all secondary steelworks. In water depths greater than about 60m, floating solutions are expected.
Monopiles
Monopiles require more steel than jackets, but they are easier to manufacture and install in volume. For larger monopiles, a key design driver is their stiffness, as the natural frequency of the complete wind turbine structure needs to be kept between blade passing frequencies over a range of wind speeds and above wave loading frequencies in order to minimise dynamic magnification and control fatigue loading. For larger turbines and in deeper water, the cost of monopiles rises substantially. For the upcoming generation of 15 MW turbines, a large (XXL) model of monopile is required.
Jackets
At around 35m water depth, jacket designs become cost competitive. It is easier to design a stiffer jacket structure for turbines of 10 MW and above in order to meet natural frequency requirements, giving such structures the edge over monopiles. Jackets can also be used in a wider range of ground conditions, where the ground is too hard or too soft to suit monopiles. In shallow waters and benign ground conditions, gravity bases have been used successfully.
