Wave Drift Forces and Low Frequency Drift Motions for Mooring Analysis of a Spread Moored FPSO

Abstract

This paper presents a comparison study on wave drift models as well as corresponding low frequency (LF) drift motions for mooring analysis of a spread moored FPSO in moderate seastates with and without current. There are four wave drift force models applied in this study, including two numerical models from potential theory with Newman’s approximation and full quadratic transfer functions (QTFs), two combined models determined from cross bi-spectral (CBS) analysis of model test data and merged with two numerical potential flow wave drift force models at low and high frequencies, respectively. The comparison is performed in terms of both diagonal and off diagonals with constant difference frequency. The equations of motion are solved in the time domain with a fully coupled method, accounting for different wave drift forces models. Both the LF motion response spectra and the time domain series are presented and discussed. The LF damping is adjusted by comparing simulations to the corresponding model test data and tuning the results. The results show the underestimates of wave drift forces as well as the corresponding LF motions by second order potential flow theory, especially in the cases with current. Application of full QTFs can improve the simulation results in terms of phasing of LF responses.