Dynamic responses of monopile wind turbine with coupled spring boundaries in bed rock-soil-seawater offshore site under offshore ground motions

Recently, the synthesis of spatially varying offshore ground motions received great attention. Meanwhile, the limited investigations of the dynamic characteristics of offshore wind turbines (OWT) under offshore earthquakes are performed. In the study, the nonlinear wave propagation theories are applied to derived the transfer functions of offshore shear seismic waves, e.g. SH and SV seismic waves. For the compressional P seismic wave at the typical bed rock-soil-seater offshore site, the proposed C-Q model with the soil saturation by seawater is applied in the derivation. The linearized method for the OWT pile foundation is established based on nonlinear p-y curve, Winkel elastic beam model and empirical formulas. Then the equivalent pile foundation model of the linearized stiffness matrix coupling with the degree of freedoms (DOFs) of pile head node at the mudline is recommended. Further, the governing equations of the OWT substructures under environmental and seismic loads are updated. Based on the derivations and modifications, modules for seafloor ground motion synthesis and equivalent coupled elastic boundaries of pile-soil interaction (PSI) are developed and added into FAST v8, then the fully coupled analysis model of OWTs with coupled spring boundaries under wind, wave, and earthquake loads is established. In addition, the synthesized ground motions at onshore and offshore sites are compared. The variations in soil saturation influenced by the seawater are proven to be a dominant factor in the synthesis of offshore ground motions. Moreover, based on the updated fully coupled analysis model, the dynamic characteristics and coupling mechanisms of the OWT under spatial ground motions are illustrated. Consequently, the influence of the seawater and soil saturation on the linearized pile foundation stiffness, natural modes, and structural responses of the OWT is proved sequentially. Following the comparisons of the WT dynamic responses, it can be seen the generally decreasing of the responses at the offshore site compared with onshore site. For the base shear and bending moment at the mudline, the high order frequencies shall be significantly activated at the onshore site, owing to the abundant energy distribution of P and SV seismic waves under such site conditions.