Critical assessment of non-linear hydrodynamic load models for a fully flexible monopile offshore wind turbine

This paper presents a comparison between experimental data of a model-scale 4 MW monopile offshore wind turbine subjected to extreme irregular sea states in finite water and the numerical models suggested in offshore wind energy standards to assess ULS conditions. The model is fully flexible with its 1st and 2nd eigenfrequencies and 1st mode shape tuned to fit those of the full-scale turbine. The measured and simulated bending moments at the sea bottom are decomposed around the eigenfrequencies of the structure, and the Morison equation with stream function wave kinematics is found to trigger transient 1st mode response (so-called ringing response). The amplitude of the simulated 1st mode response is proportional to the incoming wave steepness; such a relationship is not observed experimentally. Similarly, 2nd mode response is triggered by Wienke's slamming model, but generally does not match the experimental data. Although the numerical models from the design standards (Morison's equation with stream function kinematics, plus a slamming model) can give conservative estimates of the extreme responses, the models miss the balance between 1st and 2nd mode responses. The simplification of the physics in the numerical models can thus lead to inaccuracies in response prediction, such as the stress distribution along the monopile.