Function
The foundation provides support for the wind turbine, transferring the loads from the turbine at the tower interface level (typically around 20 m above water level) to the sea bed where the loads are reacted. The foundation also provides the conduit for the electrical cables, as well as access for personnel from vessels.
What it costs*
About £424 million for a 1 GW wind farm of 15 MW turbines using monopiles at 50 m water depth.
Who supplies them
Design: Atkins, Ballast Nedam, COWI, Ramboll, and Wood Thilsted.
Monopiles: CS Wind, Dillinger Hütte, Dajin Heavy Industries, EEW, Haizea Windgroup, SeAH Wind Limited, Sif and Riggs Distler.
Transition pieces: CS Wind, EEW, Smulders and Sif.
Jackets: Eni, Lamprell, SK Oceanplant and Smulders.
Gravity bases: Ballast Nedam. BAM Nuttall, Bilfinger, Bouygues Travaux Publics and Per Aarsleff A/S.
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.
Around 80% of the fixed offshore wind capacity currently being installed, or installed to date, outside of China, has been supported by monopiles driven into the sea bed, with jacket foundations representing approximately 15%. Gravity bases are the least common design and most were deployed at early offshore wind farms in water depths of less than 10 m. However, gravity bases continue to be used sporadically on nearshore projects and have been installed on a commercial scale project at 30 m water depths.
Monopiles require more steel than jackets but they are easier to manufacture and install in volume and they are well suited to the geology of the North and Baltic Seas. 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. However, increased efficiencies and capabilities in monopile manufacture have meant that monopiles continue to be utilised at increasing depth. Monopiles are currently thought to be limited to about 60 m water depths, this depth may continue to increase, especially if novel concepts such as tethered monopiles are utilised.
Jacket designs become cost competitive at around 40 m water depth and are suited to a wider range of ground conditions. As they have been less prominent, serial production has not been optimised in the same way as monopiles, but as projects are installed in deeper waters with harsher ground conditions serialisation is expected to become more efficient. The latticed steel structure of a jacket is secured to the seabed utilising pin piles or suction buckets. It is easier to design a stiffer jacket structure for larger turbines to meet natural frequency requirements, which can offer an edge over monopiles.
For a 15 MW turbine at 55 m water depth, indicative mass for a monopile (including transition piece) is around 3,100 t. For a 15 MW turbine at 55 m water depth, indicative mass of a jacket (including pin piles) is 2,200 t.
In shallower waters and where ground conditions make piling or drilling less feasible or economical, gravity bases have been used successfully. Fécamp is the largest project globally to have utilised gravity bases at 498 MW. This comprised of 71 foundations each with a mass of 5,000 t (pre-ballasting) to support the 7 MW turbines in water depths of 25-30 m.The Blyth Offshore Demonstrator Project used concrete gravity bases in waters of 36-42 m, but at relatively high costt. 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.
In water depths greater than about 60 m, floating solutions may compete with fixed foundation and beyond 100 m they will be the preferred option. Commercial deployment is expected in the late-2020s.