Novel Foundations for Offshore Wind Turbines

Prof. Guy Houlsby, Dr Byron Byrne and Dr Chris Martin

This work is sponsored by the DTI and EPSRC, by the following research grant proposals:
Novel Foundations for Offshore Wind Turbines sponsored by EPSRC (Value: £221k)
The Application of Suction Caisson Foundations to Offshore Wind Turbines sponsored by the DTI (Value: £478k)

The Problem

The market for offshore wind farms in the UK is expected to be substantial. The initial sites proposed for such structures are predominantly near shore shallow water sites (less than 15 m water depth). The cost of the foundations for these developments is a significant fraction of the overall installed cost (current estimates suggest between 15% and 40%), and so the development of appropriate designs for the foundations is of importance to the financial viability of such projects. Early developments in offshore wind energy will most probably involve structures fixed to the seabed in shallow water (say 5m - 20m). The hub of a typical large turbine would be located about 60m above water level. The turbine and supporting structure, which would typically be a single column, might have a weight of about 2 - 3 MN. The structure would be subjected to large horizontal forces, due to wind, waves and current. The maximum horizontal force might be of the order of 2 MN, and would result in overturning moments at foundation level of say 50 - 80 MNm. Furthermore these loads would be pseudo-random and repetitive in nature. Foundations must be designed to withstand these forces.

There is of course extensive knowledge and experience of foundation design for offshore structures within the offshore oil and gas industry. However, by comparison with typical oil and gas applications, the offshore wind turbine problem differs in several key aspects:

• The vertical loading is much smaller (by a factor of say 5 to 200), and as a proportion of the vertical loading the horizontal load and overturning moment are much larger. The loading pattern is complicated by the dynamic interaction between wind-induced loading of the turbine rotor and the wave induced loading on the structure.
• The water depth is typically much smaller. Sediment movement and local scour are important phenomena in these coastal waters due to high currents and shallow water wave effects. The performance of foundations in a partially eroded seabed and effective means of scour mitigation therefore become important design issues.
• Multiple installations (say 10 to 100) of relatively economical designs are required, rather than "one-off" foundations for large high-cost structures. Since these may be spread over a large area, less detailed site investigation information may be available.

Therefore, while it will be possible to draw to some extent on existing offshore experience, new design concepts for both the finished structure and for installation procedures will be needed to meet the challenges of the offshore wind industry.

Solving the Problem with Suction Caissons?

For the offshore wind farm application the suction caisson offers the following advantages:

• Suction caissons are simple steel fabrications that can be designed to be lighter and cheaper than the steel required for an equivalent pile foundation. This has been demonstrated in a number of offshore oil and gas projects.
• The installation method is potentially much quicker and simpler than piling. A foundation incorporating suction caisson(s) is a single unit that can be deployed and installed within a matter of hours as a single operation. It is not significantly weather dependent. By comparison, installing driven or socketed piles would involve multiple operations, and possibly delays associated with grout curing. These could take several days to complete and also risk additional weather delays, especially in winter. Complete unit installation offers potential savings on commissioning.
• Suction caissons are expected to be particularly suitable for foundations in the type of seabed sediments found around the coast of the UK. The small number of offshore wind farm developments in Europe to date have been founded in or on rock. The foundation designs used in these developments may not be cost effective in the majority of the UK sites.
• Suction caisson foundations can be flexibly adapted to a variety of structural forms thus enabling designs to be adapted to maximise accessibility by boat and accommodate a variety of water depths and tidal conditions.

At the end of a wind turbine's life, a suction caisson may be removed completely from the seabed, unlike piled foundations.