This paper focuses on the design and feasibility assessment of 18" steel catenary riser (SCR) hosted by deep draft semi-submersible platform for LS17-2 gas field development plan in water depth of 1422m. The SCR is for high pressure and temperature gas export, the material grade of X65 and thick wall of 1.25" are selected. Strength design analysis has been conducted to confirm that the gas export riser meets API RP 2RD stress criteria. The 18" SCR is suspended at the porch of pontoon with a departure angle of 15deg. A Stake coverage of 75% suspend length are installed for VIV suppression. Verification of the SCR in touch down region is also conducted by fatigue analysis (including VIV, VIM fatigue and wave induced fatigue) with a commercial riser software package.
The present study shows that the 18" riser can be designed with sufficient robustness and fatigue life under the design case environmental conditions. It is concluded that the LS17-2 SCR system is feasible, which may not only benefit future study and design of subsea riser system, but also help accelerate the semi-submersible application in South China Sea.
CNOOC is designing a deep-drafted semi-submersible Floating Production Unit (FPU) for the LS17-2 field development. The LS17-2 FPU will be located in the canyon of Qingdongnan basin in the South China Sea. The water depth in this area is approximately 1,422m. This is the first application of deep draft semi-submersible with oil storage capabilities in such record water depth in South China Sea, The FPU is anchored with a polyester spread mooring system, and is initially connected to two drill centers through subsea flowlines and production catenary risers (SCRs). In addition, the processed gas will be exported to the existing export infrastructure through one 18-inch gas export SCR, and the condensate oil will be stored in the cargo tank of FPU and then transported to shore via shutte tanker. In total there are two 12-inch production SCRs, two 10-inch production risers, one 18-inch gas export SCR, one 6-inch MEG injection riser, and one umbilical. The FPU and SCR layout is shown in Figure 1.