Strain response of offshore wind turbines with pile-bucket versus monopile foundations under wind–wave loading

During service, offshore wind turbines (OWTs) experience complex wind and wave loads, and the foundation configuration affects the magnitude and spatial distribution of structural strain responses. Existing studies have focused on either the structural response of the conventional monopile foundation (MPF) under fluid-dynamic wind–wave loading or the local bearing and deformation behavior of the pile-bucket foundation (PBF) under simplified loading. Using an integrated wind–wave–seabed test system, this study directly compares, for the first time under a unified test framework, scaled OWT models supported by a PBF and an MPF under fluid-dynamic wind–wave loading, to investigate the influence of the foundation configuration on the heightwise strain response of the support structure. Tests are conducted using two 1:200-scale models of a typical 18 MW-class bottom-fixed OWT, representing the PBF and MPF configurations, under wind-only, wave-only, and combined wind–wave loading; comparisons are performed for the monopile and the tower under wind-only and combined loading and for the submerged monopile segment under wave-only loading. Compared with the MPF, the PBF does not change the strain time-history patterns under any loading condition but alters the strain level and its heightwise distribution. In the wind-dominated response, the strain is reduced overall, with the largest reductions occurring near the bucket-affected zone and at the tower base; in the wave-dominated response, the periodic strain decreases in the bucket-affected zone and the adjacent shallowly embedded segment, whereas localized amplification occurs in the deeply embedded segment. The findings clarify the locations and magnitudes of PBF-induced strain reduction and localized amplification, and provide guidance for the region-specific assessment of heightwise responses, the identification of critical response regions, the optimization of monitoring layouts, and the numerical modeling of pile–soil interaction in PBF-supported OWTs.