Design Procedures for Marine Renewable Energy Foundations

The Geotechnical Sub-Committee of the ASCE COPRI Marine Renewable Energy Committee is preparing a general guide for foundation design. The content of this paper is a brief summary of what would be included in the guide document. Existing foundation concepts include gravity bases, monopiles, jackets/tripods and more recently, floating turbines tethered to the seabed with anchor lines. At shallow water sites with suitable soils, gravity bases have proven to be successful. Monopiles, consist of a single large diameter steel driven pile, have proven to be an efficient solution in water depths up to 35m and have formed 75% of existing turbine foundations worldwide. These piles resist lateral wind and wave loadings through cantilever action. From 35m to 60m water depths, jacket structures have been used to support the wind turbine super structure. The jacket consists of a steel lattice frame founded on piles under the legs of the structure. For deeper deep waters, floating turbines moored by mooring lines attached to suction anchors or driven pile anchors may be suitable. Included in the proposed guide are procedures recommended for computing the axial and lateral capacity of driven and suction piles, the installation of driven and suction piles, and the bearing capacity and settlement of gravity base structures. Unlike offshore foundation of oil installation which are governed by soil capacity, foundations of wind turbines tend to be governed by the lateral and rotational foundation stiffness, which controls the dynamic response of the turbinetower-foundation system.