Fluid-soil-structure interaction under oblique seismic incidence: Mechanisms governing offshore wind turbine foundation demand

Offshore wind turbines (OWTs) are located in marine environments where seawater, soft sediments and a stratified seabed modify incident seismic waves through reflection, refraction and conversion between P- and SV-wave (P/SV) modes, changing the amplitude, frequency content, and polarization of ground motion. Their combined effect on OWT seismic response is not well understood. This paper develops a high-fidelity OpenSeesPy model of a monopile-supported OWT subjected to obliquely incident P and SV waves, incorporating a poroelastic seabed, fluid-structure interaction, PML absorbing boundaries, and real offshore records from the Seafloor Earthquake Measurement System (SEMS). Three issues are examined: (1) P/SV dominance and SV-to-P conversion at sediment interfaces; (2) the influence of oblique incidence (0°, 30°, 60°, 90°); and (3) the effects of seawater depth and soft-silt thickness. The results show that impedance contrasts at the water-silt-sand-bedrock interfaces control wave partitioning, with SV-to-P conversion increasing vertical tower-top motion; oblique incidence raises horizontal demand, the SV-wave response peaking at 30° from critical-angle refraction; and a 10 m soft-silt layer minimizes amplification while a 30 m layer maximizes it. Increasing water depth causes the SV-wave governing location to switch from the tower top to the foundation: tower-top amplification falls by up to 77% while foundation amplification rises to 3.35 times the tower-top value at 45 m depth, owing to reflection and energy trapping at the water-seafloor interface. These findings give a physics-based understanding of subsea wave propagation and highlight the need to account for site-specific stratigraphy and multidirectional wavefields in offshore seismic hazard assessment and design of monopile-supported OWTs.