A nonlinear potential flow model for higher-harmonic wave loads and ringing response of a monopile

Abstract

This paper presents a numerical investigation on the high-frequency wave loads and ringing responses of offshore wind turbine foundations exposed to steep waves. A time-domain nonlinear potential flow model is developed based on the weak-scatterer approximation. On the free surface, an arbitrary Lagrangian-Eulerian (ALE) approach is adopted in the time marching process. The higher-order boundary element method (HOBEM) is employed to solve the corresponding boundary value problem at each time step. The proposed model is verified by a series of nonlinear wave diffraction problems in regular waves. It appears that in terms of the wave run-up and higher-harmonic wave forces, the present results are all in satisfactory agreement with the published experimental results and other theoretical or numerical solutions. The ringing response of a bottom-hinged monopile exposed to moderately steep focused wave groups is subsequently investigated for several different wave conditions. The effects of peak frequency, input wave amplitude and damping ratio on the ringing response are discussed.