Response of offshore wind turbine monopile-liquefiable seabed-seawater coupled system to vertical and horizontal seismic excitations

A comprehensive understanding of the underlying mechanisms governing offshore wind turbine (OWT) response in liquefiable seabeds during seismic events remains incomplete, particularly with respect to vertical ground motion components and tripartite seawater-structure-soil interaction dynamics. A 3D integrated monopile-seabed-seawater model for a monitored 6.45-MW OWT is developed, featuring acoustic elements for seawater hydrodynamics, advanced constitutive models for liquefaction and material damping, and coupled vertical-horizontal seismic input. The dynamic characteristic of the OWT is rigorously validated through ​continuous vibration acceleration monitoring. Key findings reveal that: (1) Increasing monopile embedment depth in non-liquefied soil layers effectively mitigates liquefaction-induced residual lateral displacements; (2) Monopile vertical displacement amplitude depends critically on both seismic input frequency and the phase difference between horizontal/vertical peak ground accelerations; (3) Seawater presence restrains monopile displacements, while neglecting seabed-seawater coupling leads to underestimated excess pore water pressure (EPWP) buildup in shallow seabed layers during seismic events; (4) Vertical seismic motion dominates seawater’s influence on OWT seismic response by coupling with hydrodynamic pressure to amplify the EPWP changes in liquefiable seabeds. The results demonstrate that comprehensive consideration of coupled vertical seismic excitation and fluid-soil-structure interaction is essential for accurate liquefaction assessment of OWT foundations.