3D FE seismic analysis of a monopile-supported offshore wind turbine in a non-liqueable soil deposit

Abstract

This work presents a numerical study on the dynamic response of monopile-supported offshore wind turbines (OWTs) under seismic loading conditions. For this purpose the realistic design ofa 8 MW OWT is considered as ideally located in a layered, non-liqueable site off the coast of Japan. This choice allowed to employ available site characterisation and seismic records from the well-known Japanese KiK-net. Modelling highlights are reported about the development of a dynamic, fully 3D FE model, in which the soil is described as a water-saturated elasto-plastic medium. The well-established SANISAND bounding surface formulation is adopted to reproduce the hydro-mechanical cyclic response of coarse-grained materials at the site considered, with model parameters derived via back-analysis of available seismic records. The seismic performance of the OWT is studied in relation to real seismic input of different intensity, accounting for the effect of combined horizontal and vertical components. Although in a non-liqueable site, pore pressure effects are clearly visible in the simulation results and affect the observed interaction between soil and foundation. The likely coexistence of seismic and SLS wind/wave loading is also considered for completeness. The numerical study leads to some interesting conclusions regarding the interaction between monopile head stiffness and seismic/cyclic soil response, and its impact on the motion and strength mobilisation along the OWT structure. In particular, some fundamental dierences between the dynamics induced by wind/wave loading and seismic input are put in evidence.
More generally, it is also shown how existing analysis approaches developed in the context of earthquake geotechnical engineering prove still suitable to support modern offshore wind developments in seismically active regions.