This study investigated the seismic performance and assessed the seismic fragility of an existing pentapod suction-bucket-supported offshore wind turbine, focusing on the amplification of earthquake ground motions. A simplified suction bucket-soil interaction model with nonlinear spring elements was employed within a finite element framework, linking the suction bucket and soil to hypothetical points on the OWT structures at the mudline. Unlike conventional approaches using bedrock earthquake records, this study utilized free-field surface motions as input, derived from bedrock ground motions through one-dimensional wave theory propagation to estimate soil-layer-induced amplification effects. The validity of the simplified model was confirmed, enabling effective assessment of seismic vulnerability through fragility curves. These curves revealed that the amplification effect increases the vulnerability of the OWT system, raising the probability of exceeding damage limit states such as horizontal displacement of the tower top, tower stress, and horizontal displacement at the mudline during small to moderate earthquakes, while decreasing this likelihood during strong earthquakes. Comparisons between the Full Model and the simplified Spring Model reveal that the simplified model reduces computational time by approximately 75%, with similar seismic response accuracy, making it a valuable tool for rapid seismic assessments. This research contributes to enhancing seismic design practices for suction-bucket-supported offshore wind turbines by employing a minimalist finite element model approach.

This article examines the effects of slope topography, soil non-linearity and soil-structure interaction (SSI) in hilly areas, where severe damage to hill buildings during past earthquakes were observed. Two-dimensional finite element analysis is carried out to simulate seismic response of hill buildings situated on the center of the slopes for three earthquake time histories. The influence of topographic amplification and SSI as a function of frequency of ground motion and site condition are examined. The present study shows significant ground motion amplification near the crest. It was found that the Seismic-Slope Topographic Amplification Factor (S-STAF) indicating the effect of slope on the seismic response, increases with the increase of slope angle and peak ground acceleration. However, S-STAF was increased by a margin as much as 30% when the non-linearity of the soil is considered. The effects of structural irregularity are also investigated by considering two types of buildings, (i) stepback and (ii) stepback and setback. Relative displacement of each story normalized with its height is reported as a drift ratio for two different slopes. The inter-story drift ratio of stepback building is slightly smaller than that of stepback and setback building. The seismic displacement of the slope increases significantly due to the presence of the building. The significant effect of SSI is observed with the increase of slope angle and this effect is much dependent on the earthquake characteristics. Further, period lengthening characteristics, seismic displacement, rocking and stress distribution of the footings of a stepback building on slopes are also investigated.