Fully coupled 3D analysis of the influence of intrinsic soil properties on cyclic monopile response in sandy seabeds

This study investigates the cyclic lateral response of offshore wind turbine monopiles in saturated sandy seabeds using a fully coupled 3D finite element framework in ANSYS. The analysis employs a hypoplastic constitutive model with intergranular strain, incorporating a dynamic permeability update via the Kozeny–Carman relationship to directly couple fluid flow with localized porosity changes during loading. Four reference sands were evaluated over 12 continuous loading cycles under identical initial relative densities, permeabilities, and loading boundaries. Results indicate that pore water pressure (PWP) accumulation concentrates primarily in the upper 10–15 m of the soil profile, a spatial distribution strictly governed by the monopile’s semi-rigid flexural response (L/D≈5.7). This localized PWP directly dictates effective stress degradation and cumulative lateral displacements. Stiff sands characterized by high granular hardness (e.g., Fontainebleau, hs=4800 MPa) resist volumetric contraction, minimizing PWP generation and maintaining a higher ultimate lateral capacity of 20.83 MN. In contrast, highly compressible sands with low granular hardness (e.g., Hochstetten, hs=1000 MPa) experience rapid PWP buildup, which accelerates lateral deformations and reduces the ultimate capacity to 15.87 MN. The findings quantitatively demonstrate how site-specific macroscopic constitutive properties govern cyclic degradation, emphasizing the critical necessity of fully coupled hydro-mechanical modeling for offshore foundation design.