Frequency and time domain motion and mooring analyses for a FPSO operating in deep water

Abstract

An investigation on the motion responses of a Floating Production, Storage and Offloading (FPSO) vessel moored in irregular waves has been carried out based on both frequency- and time-domain approaches. In the frequency-domain approach a three-dimensional panel method was employed in order to calculate the first-order hydrodynamic forces and moments such as added masses, potential damping and wave excitation forces and moments and of the resulting the first-order motions and mean second-order forces and moments on the vessel in six degrees of freedom behaviour. A spectral analysis was carried out in order to estimate both the significant and the extreme values of the first-order motions. Additionally Pinkster’s approximation was used to find the mean-square values of slow drift motions, in order to calculate wave-induced extreme excursions and the resulting tensions on the mooring lines of the vessel. Two different methods were used in the time-domain approach for undertaking a mooring analysis. One method used a fast practical time-domain technique that calculates the first-order motion responses in random waves based on the frequency-domain response amplitudes and simulated seas, and also solves the uncoupled second-order motion responses of the FPSO induced by second-order forces, based on Newman’s approximation in irregular seas. The other method is by solving six coupled equations of motion based retardation functions transformed from potential damping for the FPSO and induced by the first-order and second-order wave excitations in random seas. The results of the wave-induced extreme excursions and the mooring line tensions obtained by means of the frequency- and time-domain methods are compared and discussed. As the selected FPSO is operating in deep water, the effect of the mooring line inertia may be significant. The equations of motion of line dynamics were formulated and numerically solved to investigate the importance of line dynamics for deep water mooring. Comparisons between the results of the line tensions both with and without the effects of line dynamics are made and discussed.