An integrated model for offshore wind turbine monopile in porous seabed under multi-directional seismic excitations

This study proposes an integrated three-dimensional time-domain finite element model for offshore wind turbine (OWT) monopile in porous seabed under multi-directional seismic excitations. Biot's u-p formulation is used to describe the dynamic interaction between the soil skeleton and pore water. Potential-based fluid elements are used for seawater to accurately model fluid-structure interaction (FSI). Moreover, FSI interface elements are applied to the interface of seawater and pile and to the interface of seawater and mud line. To analyze the response of the OWT-monopile-seabed-seawater system to multi-directional seismic excitations, a viscous-spring artificial boundary is implemented in the finite element model. The integrated model was sufficiently verified in terms of the vibration characteristics of OWTs, the displacement and the excess pore water pressure using implemented viscoelastic artificial boundary, and the FSI interaction under both horizontal and vertical excitation. The results indicated that: (1) The hydrodynamic pressure and the excess pore water pressure varied with different positions along the OWT and the pile; (2) Adding the excitation of the EW component ground motion caused the change of the distribution of transient liquefaction with respect to that under the excitation of only NS component ground motion; (3) The vertical ground motion caused a substantial fluctuation of the vertical displacement of the OWT and larger area of transient liquefaction near the surface of the seabed and near the tip of the pile; (4) resonance occurred when the frequency of input motion was approaching to the first natural frequency of OWT, leading to extremely large acceleration and displacement of OWT.