UC San Diego

Bucket foundations have proven to be an efficient cost-effective option for a wide range of offshore applications. Motivated by the strong growth in offshore wind energy, this dissertation aims to investigate the seismic response of bucket foundations. A scaled soil-structure model of a 3.45 Megawatt (MW) utility-scale Offshore Wind Turbine (OWT) was tested on medium-size 1-g shake table. A preliminary testing phase resulted in developing new approaches for: i) tracking shear wave velocity variation within the ground during seismic excitation, and ii) generating accumulated ground deformation via asymmetric base shaking. Thereafter, the OWT model was subjected to a series of harmonic and earthquake-like excitations. From the collected data, key features of the overall system response were gleaned. Utilizing the experimental data, a nonlinear finite element (FE) model was calibrated to simulate the observed seismic response. The developed FE model was then extended to investigate the seismic behavior of a representative utility-scale OWT in sands under earthquake motions, combined with operational wind load effects. A parametric study is conducted to investigate influence of: i) soil stiffness and strength, ii) ground motion characteristics, iii) soil permeability, iv) bucket size, and v) damping on the OWT seismic response. The results suggest that: i) the near field pore water pressure build-up and bucket rotation are correlated, ii) ground motion characteristics have direct impact on the permanent bucket rotation, iii) lower soil permeability results in larger rotation due to the higher near field pore water pressure build-up, iv) permanent bucket rotation tends to reach a maximum and a minimum at the lower and upper bounds of soil permeability, respectively, reflecting the fully drained and undrained conditions, v) the difference in permanent bucket rotation between undrained and drained conditions tends to decrease with increasing soil stiffness, vi) Within the range of investigated scenarios, an essentially linear correlation was noted between the OWT fundamental frequency and the permanent bucket rotation, and vii) viscous damping at higher frequencies might have a significant effect on the bucket moment rotation response. As such, the gained insights provide guidance for seismic design of bucket foundation offshore wind turbines.