Session: 04-04-03 Rigid Pipelines III

Submission Number: 182055

Challenge of Design of Steel Catenary Riser for Deepdraft Semi 

The free hanging SCR has been widely used in Deepwater oil and gas field development, due to its economic efficiency and ease of installation.However, SCR is sensitivity to strength and fatigue in harsh environmental condition and periodic floater motions, especially for the large diameter gas export riser.LingShui 17-2 gas field development project is the first attempt for CNOOC to utilize DeepDraft SEMI and Steel Catenary Riser (SCR) system at 1500m water depth. In the process of determining the gas field development scheme, feasibility study of different floating platforms and riser types is carried out, and measures such as platform scale optimization, route orientation optimization and environmental parameter refinement research are taken. This paper describes the challenges encountered during the FEED design of the SCR system.

Total six steel risers are involved in LingShui 17-2 development project, two 10inch production risers, two 12inch production risers, one 6inch MEG riser and one 18inch gas export riser, all of which are of free-hanging SCR configuration.The SCRs are hung outside the pontoon of DeepDraft SEMI via Flex-joint, with the top hang-off angle varies from 10deg to 15deg.The hang-off orientation and positions are detailed assessed to achieve the best fatigue performance and the optimal route of subsea flowline.The SCRs are designed according to API codes, with the material of API X65 steel. Strakes are used to suppress the Vortex Induced Vibration (VIV), and the coverage zone is about 75% of the catenary arc length.For the 18inch gas export SCR, monitor system is utilized to monitor the fatigue response at the hang-off and touch down zone (TDZ), track low frequency vessel induced motion, both standalone and hardwired strategy are studied and compared. The SCRs will be installed using pre-lay and hook-up procedure, and the feasibility of S-Lay method is detailed evaluated.

Main factors affect the SCR design are the harsh environmental condition in South China Sea and the relative high vertical motion velocity of the floater, though the LingShui 17-2 DeepDraft SEMI has shown an excellent motion behavior compared with conventional Semi-submersible production platform.Compression at the TDZ under 100-year Typhoon is unacceptable in the design, thus inducing the large wall thickness with D/t<10. The DeepDraft concept enables the SCR to achieve a fatigue life of 30 years.However, motion and VIM fatigue is still a big challenge during the SCR design. VIM fatigue contributes more than 60% of total fatigue for the 18inch gas export SCR and the 12inch production SCR.

Work conducted during FEED phase show that SCR is feasible for LingShui17-2 development.

Presenting Author: Yi Liu CNOOC Research Institute

Presenting Author Biography: Senior Riser engineer of CNOOC Research Insititute.more than 10yrs experience in Deepwater engineering projects

Authors:

Yi Liu CNOOC Research Institute
Lusheng Jia CNOOC Research Insititute
Jun Huang CNOOC Research Insititute
Zijun Song CNOOC Research Insititute.