Response-spectrum uncoupled analyses for seismic assessment of offshore wind turbines

According to International Standards and Guidelines, the seismic assessment of offshore wind turbines in seismically-active areas may be performed by combining two uncoupled analyses under wind-wave and earthquake loads, respectively. Typically, the separate earthquake response is calculated by a response-spectrum approach and, for this purpose, structural models of various degrees of complexity may be used. Although response-spectrum uncoupled analyses are currently allowed as alternative to time-consuming fully-coupled simulations, for which dedicated software packages are required, to date no specific studies have been presented on whether accuracy may vary depending on key factors as structural modelling, criteria to calculate wind-wave and earthquake responses, and other relevant issues as the selected support structure, the considered environmental states and earthquake records. This paper will investigate different potential implementations of response-spectrum uncoupled analyses for offshore wind turbines, using various structural models and criteria to calculate the wind-wave and earthquake responses. The case study is a 5-MW wind turbine on two support structures in intermediate waters, under a variety of wind-wave states and real earthquake records. Numerical results show that response-spectrum uncoupled analyses may provide non-conservative results, for every structural model adopted and criteria to calculate wind-wave and earthquake responses. This is evidence that appropriate safety factors should be assumed when implementing response-spectrum uncoupled analyses allowed by International Standards and Guidelines.