Nonlinear Analysis of a 15 MW Monopile Supported Offshore Wind Turbine under Combined Wind and Seismic Loading

The combined wind and seismic response of the IEA 15 MW Offshore Reference Wind Turbine supported by a monopile foundation on sandy soil was investigated with consideration of nonlinear soil structure interaction (SSI) effects. A three-dimensional finite element model is developed in OpenSees, integrating the tower - monopile structural system with a fully coupled solid fluid soil domain represented using the Pressure-Dependent Multi-Yield (PDMY02) constitutive model. Wind loading on the tower and rotor thrust are derived from TurbSim generated velocity fields, while seismic excitation of the 1995 Kobe earthquake ground motion is used. Four soil profiles, comprising uniform and layered deposits with relative densities ranging from 30% to 75%, are studied to evaluate the displacement at tower top and that of the monopile. Analysis results show that wind tends to dominate the tower displacement while earthquakes amplify the base rotation and curvature near the mudline and below it. The results highlight the necessity of multi-hazard, fully coupled time-domain simulations for safe and reliable design of large-scale offshore wind turbines in seismic regions.