Effect of bottom unlatch and top breakage mooring lines on the dynamic behavior of a semisubmersible platform system

Mooring disconnections occur frequently in a floating production system (FPS), probably resulting in the instability of structures or even offshore accidents. Therefore, this paper investigates the influence of mooring disconnections on the response of a SEMI system under regular waves and irregular waves. Two failure modes were performed and compared according to the disconnected positions of mooring lines: the top breakage and bottom unlatch. A numerical model of a fully coupled hull/mooring/riser system was established through a self-developed program to obtain the detailed behavior of each component. The results indicated that the platform motion became a 4-period nonlinear response containing a super-harmonic motion when three moorings unlatched. The disconnected moorings produced a large deformation due to the sliding of anchor points. The riser tension exhibited robustness under mooring failure because of its low tension level and the robustness of platform heave. Compared with bottom unlatch conditions, the platform under top breakage conditions showed a larger-amplitude fluctuation and a slightly farther offset particularly three moorings all disconnected. The total mooring force under top breakage conditions would lose more fairlead tension at the failure time, thus inducing a lower mooring force for the platform. •The dynamic behavior of a coupled platform/mooring/riser model was described through a self-developed program.•The response of SEMI system under bottom unlatch conditions was calculated in details.•The motion response and mechanical characteristics of lazy-wave risers in the SEMI system were described.•The dynamic characteristics of the unlatched mooring lines before and after the disconnection moment were reported.