Coupled vs. uncoupled analyses for seismic assessment of offshore wind turbines

Seismic analysis of offshore horizontal-axis wind turbines is inherently non-linear, with interactions among aerodynamic, hydrodynamic and seismic responses to be accounted for by time-consuming fully coupled time-domain simulations, for which only dedicated software packages can be used. For seismic assessment of land-based turbines, existing standards and guidelines allow uncoupled analyses, where aerodynamic and seismic responses are separately computed on a structural model with an appropriate additional damping, named aerodynamic damping, and then linearly combined. Although similar approaches are permitted also for offshore wind turbines, to date no specific recommendations have been released on crucial issues, such as how to implement the uncoupled analyses and which aerodynamic damping has to be used. This paper investigates these issues, considering a 5 MW turbine mounted on a bottom-fixed tripod as a study case.