Seismic response of large offshore wind turbines on monopile foundations including dynamic soil-structure interaction

This paper presents the results of a study on the dynamic and seismic response of the support structures of three reference Offshore Wind Turbines (OWT) of increasing rated power, founded to the seabed through monopile foundations. Thus, the structural behaviour of the NREL 5 MW, IEA Wind 10 MW and IEA Wind 15 MW reference OWTs under seismic input is analysed. To do so, a model based on the aero-hydro-servoelastic OpenFAST open-software code, modified to include dynamic Soil-Structure Interaction (SSI) and input ground motion, is employed. Dynamic SSI phenomena are incorporated through lumped parameter models fitted to the impedances computed using an advanced boundary elements-finite elements model of the soil- foundation system in which the monopile is discretized as a steel pipe buried in the unbounded seabed. The fore-aft and side-to-side responses of the systems are computed under power production, parked and emergency shutdown operating conditions considering different earthquakes and arrival times. It is found that even low and moderate intensity earthquakes can produce significant increases in the structural demands of large OWTs. There exists a clear tendency for SSI to be beneficial when the size of the OWT increases.