Performance change of a semi-submersible production platform system with broken mooring line or riser

Abstract

Under high sea states, positioning mooring lines or production risers could be led to failure. Such an accident will change the watching circle of the platform or even cause production to shut down. The present paper is aimed at investigating the transient/unsteady response of a semi-submersible platform, tension variation of the remaining lines, and evaluate the risk of risers clashing after mooring lines or risers failure. The fully coupled time-domain analysis is set up for the whole system which including a semi-submersible platform, 12 mooring lines, and 22 production risers. One or several mooring lines and risers are intentionally disconnected from the fairleads at a certain time. Combined environmental loads are selected from irregular waves and the steady current varying from very rough to phenomenal sea conditions. Hydrodynamic response of the semi-submersible with regarding first-order and second-order wave forces is calculated with the combination of three-dimension radiation/diffraction theories and Morison's equation. Nonlinear time-domain finite element methods are employed to analyze the behavior of mooring lines and risers. Results indicated that the platform has a large horizontal offset with the lost positioning forces. This will lead to the successive failure of the neighboring objects. The tension of the rest lines is also increased accordingly. Remaining lines that are closer to the failed lines will have larger tension increase to compensate. A practical methodology for investigating the risk of risers collision is also provided in this paper.