Application of Morison Equation in Irregular Wave Trains With High Frequency Waves

Abstract

Most numerical models for the analysis of offshore wind platforms are based on one of two different approaches, depending on how waves forces are applied to the structure: 1) the potential flow theory, and 2) the Morison equation. Potential flow theory allows to compute the wave forces more accurately when diffraction is relevant. Otherwise, this kind of models assume a fixed position of the floating platform when computing the wave forces. Additionally, second-order effects, as the position and the spin of the structure relative to the incident wave can only be taken into account if second order potential flow is considered. On the other hand, Morison equation can apply the wave forces on a structure based on its spin and position which can be assessed at each time step, but is prone to overestimate the waves forces at the frequencies where diffraction is relevant. In this paper, a modification of the implementation of the Morison equation is presented. This modification allows to reduce the forces in the diffraction frequency range based on the real response from MacCamy and Fuchs’s diffraction theory for cylinders. The implementation can be applied using a frequency-dependent coefficient of added mass, or modifying the amplitudes of the incident waves in the diffraction frequency range in a way that the accelerations derived from the regular wave theory used for the Froude-Krylov wave force computation in Morison equation are equivalent to those computed in the diffraction theory. The implementation is tested in the FloawDyn code, developed at the UPC, and FAST from NREL.