Response of offshore wind turbine tripod suction bucket foundation under wind and multi-directional seismic loading conditions

Abstract

This study investigates the seismic response of tripod suction bucket foundations supporting offshore wind turbines on clay, considering both isolated seismic loads and combined wind-seismic loading. A holistic 3D finite element model integrates the NREL 5 MW turbine, tower, jacket, foundation, and soil domain. The kinematic hardening constitutive model captures the cyclic clay behaviour under seismic excitation. Super-element techniques significantly enhance computational efficiency by condensing the superstructure. Analyses encompass unidirectional and multidirectional seismic inputs, including the Kobe, Loma Prieta, and Northern California earthquake record, alongside constant wind loads. Results demonstrate that foundation rotation, critical for serviceability, is primarily governed by differential vertical settlement among the suction caissons induced by overturning moments. Horizontal seismic loads cause significant differential settlement, while vertical seismic loads induce uniform settlement. Crucially, the superposition of constant wind load amplifies the permanent settlements and rotations caused by seismic events. Under favourable wind conditions, co-directional horizontal seismic loading most significantly amplifies rotational response. Under critical wind conditions, orthogonal horizontal seismic loading maximizes rotational amplification. The validated model provides insights into the complex soil-structure interaction mechanisms governing the stability of tripod foundations under multi-hazard conditions.