As a result of the existence of the paleo-channel at the south-east corner of the site area, and the uncertainty of the extent to which it projects into the turbine area, two foundation types are recommended. A drilled and grouted monopile foundation is preferred for turbine locations where the rock is outcropping or subcropping (0.5-1m below seabed). Subject to a detailed geophysical and geotechnical investigation, a concrete gravity foundation will be favoured for turbines located within the paleo-channel or where the overlying sands are deep.
The steel monopile is the most common foundation type used for offshore wind turbines, mainly due to the simplicity of fabrication and installation. In most instances, steel monopiles are driven into the seabed from a jack-up barge using a hydraulic hammer, which is available in various capacities for either operation above or below the water surface. This method is preferred where the seabed is sufficiently soft to allow pile penetration, as it is quicker and more efficient than other alternatives.
The monopile is a uniform diameter steel tube, of height that would be surface penetrating to allow connection to the wind turbine tower via a transition piece. Monopile options for the 3 MW turbines would require a pile diameter of between 5-6.5 m, with toe levels to suit the geology of the seabed. The dimensions of the monopile foundation will be subject to the final design and would vary according to water depth and seabed geology at individual turbine locations.
Gravity Base or Caisson
Gravity base foundations are either a wide steel or concrete base, designed to be placed on the seabed and are sufficiently heavy to stay in place even in extremely rough conditions, i.e. a 50-year storm. Gravity base foundations are typically installed in water depths of 0-30 m. The structure is fitted with an access ladder, platform and other fittings as required such as fenders and one or more J-tubes.