Effect of Coupling of Mooring Lines and Risers on the Design Values for a Turret Moored FPSO in Deep Water of the Gulf of Mexico
In the framework of the DeepStar CTR 4401A "Theme structures" coupled computations were carded out on a turret moored tanker based FPSO in 3,000 ft, 6000 ft and 10, 000 fi water depth for the Gulf of Mexico. The FPSO was exposed to both Hurricane and Loop-current conditions. The size of the FPSO corresponds to a 200 kDWT tanker moored by means of 12 lines. For the production, water injection, oil and gas transport, the turret was provided of a 13 SCR system. The mooring system in 3,000 ft exists of a chain-jacketed spiral strandground chain, while the mooring system in 6000 ft was both chainpolyester-ground chain and chain-spring buoys-jacketed spiral strandground chain and for 10,000 ft water depth it consists of a chainpolyester-ground chain combined line. Considering the design values the question may arise on the value of the coupled analysis and the effects of the Cd-values on the dynamics on the mooring legs and risers at large water depth. To understand the questions a fully coupled mathematical model was developed. Using the DeepStar FPSO system a new set of computations in the time-domain has been carried out. By changing systematically the Cd resistance coefficients on the risers and the mooring lines the effect of the coupling on the global motions and the mooring forces can be distinguished. The computations were applied for the system in 3,000 ft water depth exposed to the Hurricane and Loop-current condition and in 10,000 ft water depth during Hurricane condition. The results show that the complete coupling has to be taken into account to obtain realistic design values.