Simplified methods for seismic design of monopile-supported offshore wind turbines

With the increasing number of offshore wind farms being designed and constructed in highly seismic regions, there is a need for cost-effective seismic analysis methods that are simple to implement and yet provide reasonably accurate results. Currently, performing time history analyses of full three-dimensional (3D) dynamic soil-structure interaction (DSSI) models of monopile-supported wind turbines at hundreds of locations is computationally expensive and is not economical for routine design. Hence, this paper compares the monopile response from 3D DSSI analysis, developed within LS-DYNA environment, with simpler beam-on-nonlinear-Winkler springs model formulated within the substructure approach framework. Two different methodologies for the soil springs are examined: (1) linear elastic springs with viscous damping; and (2) linear elastic-perfectly plastic springs with hysteretic behavior. Results of the simplified models are then compared with the 3D DSSI results in terms of fundamental period, shear forces and bending moments along the monopile and the superstructure, and indicative run times. The importance of damping will be discussed in relation to the severity of soil straining and earthquake excitation.