Experimentally-calibrated numerical investigation of soil stiffness, permeability, bucket size, and damping effects on seismic response of offshore wind turbine foundations

A nonlinear finite element (FE) model calibrated using shake table testing data is used to simulate seismic response of utility-scale offshore wind turbine (OWT) bucket foundation. A parametric study is conducted to investigate influence of i) soil stiffness, ii) soil permeability, iii) bucket size, and iv) structural viscous damping on the system response, with special attention to foundation rotation, permanent deformation and pore pressure build-up characteristics. The results suggest that: i) the near field pore water pressure build-up and bucket rotation are correlated, ii) lower soil permeability results in larger rotation due to the higher near field pore water pressure build-up, iii) permanent bucket rotation reaches a maximum and a minimum at the lower and upper bounds of soil permeability, respectively, corresponding to fully drained and undrained conditions, iv) the difference in permanent bucket rotation between undrained and drained conditions tends to decrease with increasing soil stiffness, v) within the range of investigated scenarios, a linear correlation can be expressed between the OWT fundamental frequency and the permanent bucket rotation, with diminishing effect of soil stiffness at the larger bucket sizes (i.e., stiffer foundation). In addition, it was noted that viscous damping at higher frequencies can have a significant impact on bucket moment-rotation response. The findings provide a valuable extension to existing seismic design tools and contribute towards developing performance-based seismic design guidelines for bucket foundation OWT. • An experimentally calibrated nonlinear FE model using shake table data evaluates seismic response of OWT bucket foundations. • The study quantifies coupled effects of soil stiffness, permeability, bucket geometry, and damping on seismic performance. • Permeability-dependent bounds on permanent rotation are established for drained and undrained conditions. • Increased soil stiffness is shown to reduce drained–undrained rotation disparity. • A linear relationship is observed between the investigated range of OWT fundamental frequency and permanent bucket rotation.