Accurate modelling of ultimate or maximum hydrodynamic loads is crucial for the design of offshore wind turbines and ensure the survivability for its design life time. High fidelity direct Fluid-Structure Interaction models can provide accurate results but are too computationally expensive to deploy for the many environmental loading conditions that need to be evaluated during the design phase. Indirect numerical methods that separately obtain the wave kinematics and subsequently use those to calculate the hydrodynamic loads are widely used in the industry. Often extended linear wave theory is deployed to obtain the kinematics. However, for storm wave conditions and breaking wave events, this theory breaks down and requires engineering solutions such as separate slamming models to provide conservative force estimations. Fully nonlinear wave kinematics might directly represent steep and breaking waves, omitting the requirement of slamming models.
This study evaluates the performance of the non-hydrostatic wave model SWASH in simulating fully nonlinear wave kinematics that are subsequently used to obtain the hydrodynamic loads with the Morison equation. The project focuses on the extreme events of a typical 50 year return period storm in the North Sea. Deterministic comparison of the time series of the hydrodynamic loads of the nonlinear model with available experimental data and a linear model, that served as a benchmark representing the industry method, showed mixed results. Several large overshoots were observed in the nonlinear results for non extreme events, which were not present in the experimental data. Load estimates for extreme events were of mixed accuracy, both over and under estimations of the hydrodynamic load magnitudes were observed. The study concludes that while SWASH offers valuable insights into nonlinear wave dynamics, further refinement is needed to improve its reliability in load predictions.
Future research should initially focus on refining the implementation of SWASH, tackling the large overshoots by including a wave breaking turbulence model.