On time-domain uncoupled analyses for offshore wind turbines under seismic loads

For seismic assessment of wind turbines in seismically-active areas, International Standards and Guidelines allow the combination of two uncoupled analyses under environmental and earthquake loads, respectively. The separate earthquake response is generally computed including an additional aerodynamic damping in the structural model. Although some work has been done to estimate the effectiveness of uncoupled analyses for land-based wind turbines, and determine appropriate levels of aerodynamic damping, to date no similar studies have been carried out for offshore wind turbines. This paper assesses the accuracy of different time-domain implementations of uncoupled analyses for offshore wind turbines, and investigates pertinent levels of aerodynamic damping. The case study is a 5-MW wind turbine, resting on a tripod in intermediate waters.