A Hybrid Approach for Turret Deepwater Mooring System

The oil industry is concentrating its efforts and activities on developing the Turret mooring system in deep waters. These waters range from 1,000m – 3,000m and include areas the Gulf of Mexico, Brazil, West of the Africa and North Atlantic/Norwegian Sea. The mooring stiffness of vessel which is moored in the deep sea is softer than the one in shallow water. This causes large non-linear dynamic effects on the moored vessel in the ultimate sea state. The accurate prediction of low frequency motions of FPSOs is essential for safe design of mooring and riser systems. For ultra deep water, the modeling of full-depth mooring system is very difficult since no tank facility is large enough to perform complete system tests with reasonable depth for model scale. In this study, a combination of model test with truncated set-up and computer simulations — the hybrid approach — is suggested. The damping terms on FPSOs, such as wave drift damping, are very important dynamics of moored vessels. The amount of slow drift damping of a FPSO in the still water and regular waves is measured through free decaying tests. By the source and dipole distribution method, the first-order and second-order wave exciting forces are calculated. Quasi computed motions of the low frequency surge of a turret moored FPSO are simulated, using added masses and damping coefficients measured from model tests. The simulated motions are compared with test results.