Seismic performance of monopile-supported offshore wind turbines in operation under near-field and far-field ground motions considering soil-structure interaction

This study experimentally investigates the seismic behaviour of monopile-supported wind turbines (MWTs) under near-field (Chi-Chi) and far-field (Superstition) ground motions, incorporating operational conditions and soil-structure interaction (SSI) effect. Experiments involve three peak ground acceleration (PGA) levels (0.14 g, 0.40 g, and 0.8 g) and three operational states (corresponding wind speeds: 0 m/s, 4.6 m/s, and 8.5 m/s). The results demonstrate that near-field excitations lead to increases of 85 % in nacelle displacement, 11 % in nacelle acceleration, 42 % in base bending moment and 73 % in pile head rotation angle compared to far-field motions under extreme wind and seismic loads. Time-frequency analysis of nacelle response shows that the SSI delays pile-soil interaction and activates multiple modes. Compared to rigid foundation (Ma et al., 2025), SSI enhances seismic energy dissipation, particularly by filtering high-frequency components. As a result, acceleration amplification factors (APF) remain within 1-2 for wind turbines with SSI foundation, whereas they can reach up to 5 for rigid foundations. However, SSI foundations show greater tilt and displacement, revealing a trade-off between reduced nacelle acceleration and increased deformation. Although wind loading is not the dominant driver during earthquakes, it amplifies peak responses, highlighting the need to consider seismic characteristics, SSI, and turbine operation in design.