FOUNDATIONS
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.
Drilled Monopile
Foundation
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
Foundation
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.
