The rapid expansion of offshore wind energy is increasingly pushing projects into seismically active regions, necessitating monopile foundation designs that account not only for typical environmental loads but also for seismic displacement demands. To accurately assess the dynamic response of an offshore wind turbine (OWT) and design its monopile foundation accordingly, it is essential to model the soil-monopile-superstructure interaction with high fidelity. Three-dimensional (3D) finite element (FE) analyses represent the most advanced numerical approach for capturing this complex behaviour. To address this need, this thesis develops a comprehensive FE framework in PLAXIS 3D that integrates free-field site response and soil-structure interaction (SSI) under both linear elastic and nonlinear elastoplastic soil to evaluate the seismic performance of monopile-supported OWTs.
A structured four-step approach is adopted. Step 1 performs one-dimensional (1D), two-dimensional (2D), and 3D site response analyses (SRA) in linear elastic soil, validating the numerical setup against analytical solutions. Step 2 introduces the pile and superstructure, modelled with linear elastic material, in the same medium to assess the dynamic characteristics and their response under steady-state monoharmonic excitation, allowing verification of the SSI model against benchmarks from the literature. To capture more realistic soil behaviour, the framework then incorporates the SANISAND-MS constitutive model, which accounts for nonlinear cyclic sand response, including strain accumulation and stiffness degradation. In Step 3, SRAs with displacement input motions of varying amplitude are performed under both broadband Ormsby and steady-state monoharmonic excitation to evaluate the influence of nonlinearity on free-field response. Finally, Step 4 couples the nonlinear soil with the structure to examine the fully integrated dynamic characteristics under nonlinear SSI conditions.
Overall, the analyses establish best practice boundary conditions and numerical setups for seismic SRA and SSI modelling in PLAXIS 3D for linear elastic soil (Steps 1 and 2), and quantify the transition from inertial to kinematic dominance in the i response of a flexible pile (Step 2). In the nonlinear domain, the results from Step 3 highlight the critical role of loading type in capturing key features of nonlinear free-field response. More specifically, with steady-state monoharmonic excitation of increasing amplitude, the transition from linear to nonlinear behaviour is captured, along with a shift of predominant frequencies to lower values compared to the linear elastic SRA. In contrast, when the same procedure is applied with the broadband impulse load Ormsby wavelet, post-impulse stiffening effects are revealed, expressed as an upshift in resonance frequency. In Step 4, the dynamic characteristics of the monopile-superstructure system are assessed under nonlinear SSI conditions. However, computational cost prevents the execution of full amplitude sweeps, underscoring the practical trade-off between accuracy and run time in high-fidelity 3D nonlinear SSI modelling. Instead, the 1995 Kobe earthquake record is employed to assess the system’s behaviour under real, multi-harmonic excitation. Altogether, the work demonstrates the potential of advanced 3D FE tools to enhance the seismic design of monopiles.