Experimental and numerical investigations of monopile ringing in irregular finite-depth water waves

Abstract

In storm conditions, nonlinear wave loads on monopile offshore wind turbines can induce resonantringing-type responses. Efficient, validated methods which capture such events in irregular waves inintermediate or shallow water depth conditions are needed for design. Dedicated experiments andnumerical studies were performed toward this goal. The extensive experimental campaign at 1:48 scalewas carried out for Statoil related to the development of the Dudgeon wind farm, and included botha rigid model and a flexible, pitching-type, single degree-of-freedom model. Twenty 3-hour durationrealizations for 4 sea states and 2 water depths were tested for each model. A high level of repeatabilityin ringing events was observed. Uncertainties in the experimental results were critically examined. Thestochastic variation in the 3-hour maximum bending moment at the sea bed was significantly larger thanthe random variation in repetition tests, and highlighted the need for a good statistical basis in design.Numerical simulations using a beam element model with a modified Morison wave load model and sec-ond order wave kinematics gave reasonable prediction of the ringing response of the flexible model, andof the measured excitation forces on the rigid model in the absence of slamming. The numerical modelwas also used to investigate the sensitivity of the responses with respect to damping and natural period.A simple single degree-of-freedom model was shown to behave similarly to a fully flexible model whenconsidering changes in natural frequency and damping.