Improvisation of deepwater weight distributed steel catenary riser
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Nowadays, oil and gas sources are found in deeper water depths and in more hostile environments. This results in the need for more advance technologies. Riser system is a key element in providing safety. Riser failure results in spillage or pollution and could endanger lives. Hence, it is important to establish a high degree of reliability for riser design. Steel catenary risers (SCRs) have been a preferred riser solution for deep-water field developments due to its simple engineering concept, cost effective, flexibility in using different host platform and flexibility in geographical and environmental conditions. Flexible riser, on the other hand, is limited by technical and economical reasons when it comes to deep water field. Larger diameter is required in deep water to increase collapse resistance due to high hydrostatic pressure. Consequently, increase in cost and limit the option of host platform. Alternatively, Hybrid riser is a robust design for deepwater and harsh environments. It is insensitive to motion induced fatigue. However, hybrid riser is considered to be an expensive solution because it comprises a number of complex components (buoyancy can, riser bundle, flex joint, etc). A number of SCRs have been installed worldwide over the past years and more to come in the future oil and gas explorations. However, there is no SCR that has been installed in deepwater with harsh environments to date. It is mainly because SCRs in harsh environments experience a great challenge due to large motions from host platform such as semi-submersibles and FPSOs. Therefore, significant design effort is required to prove that the SCRs could safely withstand environmental loads in harsh environments and the effects of deep water. The study investigates the feasibility of 10 inch production SCR for Offshore Norway in a 1000m water depth with SCR attached to a semi-submersible vessel. Conventional SCR was analyzed and found difficulty in meeting strength design criteria at the touch down point (TDP) and at the riser hang off location. From previous industry work, the weight variation along the riser length has demonstrated a remarkable improvement to SCR response, particularly at TDP. This study concentrates on fundamental aspects related to improvement from conventional SCR to weight distributed SCR. A number of insightful sensitivity analyses were performed in order to understand the correlation between the peak response and some fundamental parameters such as displacement, velocity and acceleration. Feasibility enhancement of present weight distributed SCR concept was also studied to provide more applicable SCR configuration solution. The study addresses global design considerations including analysis of strength and fatigue. Deepwater SCR Installation scheme was also discussed. The study concludes that there is significant improvement in SCR response from conventional SCR to weight distributed SCR concept. It also proves that even though the design of SCR in harsh environments and deep water is technically challenging, innovative solutions can be developed.
Master's thesis in Offshore technology : subsea technology