Time-domain uncoupled analyses for seismic assessment of land-based wind turbines

Seismic analysis plays an important role in the design of land-based and offshore wind turbines in areas at seismic hazard. For seismic assessment, International Standards and Guidelines allow combining two separate analyses, one under wind and another under earthquake only, as alternative to computationally expensive, fully-coupled time-domain simulations. In these uncoupled analyses, the separate earthquake response is generally computed using the standard acceleration response spectrum, upon including an additional damping referred to as aerodynamic damping. By a response-spectrum approach, however, important sources of nonlinearity, such as those related to foundation flexibility, cannot be properly accounted for. Focusing on land-based wind turbines, this paper investigates a time-domain implementation of uncoupled analyses, which may involve a nonlinear foundation model. The case study is a 5 MW baseline wind turbine, resting on a pile foundation modeled by nonlinear springs. For different earthquake records and wind velocities, comparisons with fully-coupled simulations show that the combination of uncoupled analyses implemented in the time domain yields accurate results, provided that an appropriate level of aerodynamic damping is included in the model. Notably, it is seen that such aerodynamic damping level agrees with the one generally recommended for response-spectrum based uncoupled analyses.