Spectral wave explicit weak-scattering approach for higher harmonic wave loads and ringing response of flexible monopile

This paper presents an accurate potential decomposition strategy for wave–structure interactions, known as the spectral wave explicit weak-scattering approach, where the entire original problem is divided into incident and scattered components. The incident component is evaluated using spectral models in advance; hence, only the scattered component is to be solved via a higher-order boundary element approach. The proposed coupled model is validated using published experimental and computational fluid dynamics results for wave interactions with a vertical cylinder. The results obtained are consistent with wave elevations and wave loads for regular waves and focused wave groups. A finite-element numerical model is applied to measure the severity of structural responses. Fluid–structure interactions are simulated by converting the obtained pressures into equivalent nodal forces, and new nodal velocities are converted into boundary conditions. The ringing response of a bottom-fixed monopile due to moderately steep focused wave groups is discussed based on various focal positions relative to the monopile, incident wave conditions, and damping ratios. The time–frequency characteristics of the ringing phenomenon are successfully revealed via wavelet analyses.