Coupled Dynamic Analysis of Deepwater Semi-submersible with Spread Mooring System

Exploration and production in deepwaters are dominated by compliant offshore structural systems due to the advantages that arise from their geometry and construction practices. Semi-submersibles are a class of floating offshore structures, which are widely preferred for deep and ultra-deepwater applications due to their better stability characteristics and lesser sensitivity to the harsh ocean environment. A semi-submersible is positioned-restrained using the spread mooring system with either a steel catenary geometry or taut-mooring. The present study highlights dynamic response analysis of a semi-submersible with the spread mooring system, and its fatigue life under cyclic environmental loads is estimated. Lateral loads that arise from waves, wind, and current cause dynamic tension variations in the moorings, influencing their fatigue life significantly, and it is observed that fatigue life of catenary mooring lines is higher than that of taut mooring lines. Lateral loads under different directions are considered to exhibit the influence of wave directionality on the semi-submersible response. The nonlinear coupled dynamic analysis between the semi-submersible and spread mooring system is carried out using commercially available tool ANSYS AQWA, and fatigue life of the mooring system is evaluated based on the S–N curve approach.