Influence of Liquefiable-Layer Thickness on the Seismic Response and Apparent Natural Frequency of a Monopile-Supported DTU 10-MW Offshore Wind Turbine

Monopile-supported offshore wind turbines are increasingly deployed in shallow-to-medium water depths because of their structural simplicity and economic efficiency. However, in seismic regions with liquefiable sandy deposits, earthquake-induced excess pore water pressure can substantially reduce soil stiffness and strength, thereby modifying the dynamic response of the coupled soil–foundation–structure system. This study presents a three-dimensional dynamic finite element analysis of a monopile-supported DTU 10-MW offshore wind turbine using PLAXIS 3D. The liquefiable sand was modeled using the UBC3D-PLM effective-stress constitutive model to investigate the influence of liquefiable-layer thickness on soil stiffness degradation, apparent natural frequency, and structural response. The findings demonstrate that the response is governed not only by liquefiable-layer thickness but also by the coupled interaction between the liquefiable layer, non-liquefiable layer, and the monopile foundation. Increasing the liquefiable-layer thickness increases the tower and monopile lateral displacements and rotations, while slightly reducing acceleration transmission. The thicker softened liquefied layer reduced soil–foundation dynamic coupling and slightly attenuated the transmission of seismic acceleration to the superstructure. Time-frequency analysis revealed a liquefaction-induced reduction in the apparent first natural frequency (AFNF), as evidenced by the lower post-earthquake AFNF, followed by progressive recovery with soil-stiffness recovery. This recovery was progressively delayed as the liquefiable-layer thickness increased.