Design of Foundations and Anchors in Rock for Offshore Wind Energy Systems

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Offshore wind farms typically comprise arrays of fixed-base structures supported by piled foundations embedded in sand or clay soils. Future planned developments in European coastal waters include sites where the bedrock is close to the seabed surface. At such sites, piled foundations are installed by drilling and grouting. For deep water sites, future developments involving floating offshore wind systems are envisaged; floating wind systems typically require anchors to moor the system to the seabed.Much of the recent offshore wind farm development in European waters has exploited sites where the soils (sand and/or clay) are suitable for piled foundations to be installed by percussion methods. Over the last decade or so, considerable experience has been accumulated on the design and installation of foundations for sites of this type. There is much less accumulated knowledge on the design of foundations/anchors for sites where the bedrock is close to the seabed surface; such sites are therefore often considered to be relatively unattractive. New design procedures are therefore needed to facilitate the further expansion of offshore wind generation to sites where installation of foundations/anchors into rock is required. The current project will contribute directly to this need.The design of foundations and anchors for offshore wind turbine structures requires a range of design cases to be addressed. Procedures are needed to ensure that the stiffness of the foundation/anchor system lies within appropriate bounds to ensure that the dynamic performance of the overall structure is adequate. It is also necessary to ensure that the strength of the foundation/anchor is sufficient to withstand foreseeable storm loading conditions. Considerations are needed on the risk of fatigue failures within the steel supporting structure and on ensuring that accumulated foundation deformations (due to prevailing wind and wave conditions) are kept within defined bounds. The current project will deliver design procedures that will support the specification of structures and foundations that satisfy these design constraints. In the initial stages of the research the project will involve the development of 3D finite element models of the performance of anchors and piles embedded in rock. Studies will be conducted on appropriate constitutive models to represent the behaviour of the rock and grout. For practical design activities, 3D finite element analysis has the disadvantage that the calculations often require lengthy run times. New alternative design models will therefore be developed that facilitate rapid design calculations. These models will be based on a one dimensional (1D) finite element framework in which the pile or anchor is modelled as an embedded beam. The 1D model will be calibrated using a limited number of more detailed finite element analyses. Initial work will focus on monotonic loading, but the project may also consider cyclic loading effects.This project falls within the EPSRC 'ground engineering' and 'structural engineering' research areas. The research is also related to the 'wind power' research area.The project is being conducted in collaboration with Geowynd (a specialist engineering consultancy).