Improved SCR Design for Dynamic Vessel Applications

Abstract

Design of Steel Catenary Risers (SCR) has seen increasing challenges in deep water applications due to higher pressures, vessels with more dynamic motions and severe weather conditions. In the Gulf of Mexico, the SCR is sometimes designed with little margin due to harsh environment. In West Africa, SCRs may be infeasible with turret moored Floating Production Storage and Offloading (FPSO) System, and are often limited to a narrow hang-off range near the Center of Gravity (CoG) with spread moored FPSO. Freestanding hybrid risers are feasible solutions for many fields in West Africa. However, the freestanding riser cost is generally considered much higher than that of an SCR. Hence, there is a need to improve SCR performance in dynamic vessel applications. This paper presents the development of Shaped SCR configuration that has significantly improved performance and minimal cost increase compared to an SCR. Compared with a conventional Lazy Wave SCR, the riser configuration has increased installation flexibility and decreased cost as it is installable via J-lay or S-lay. The configuration is developed and assessed for a spread moored FPSO in West Africa. The strength and fatigue response is compared with those of a simple SCR and Lazy Wave SCR. The key considerations in developing the configurations are discussed. This paper also presents the use of upset ends to improve riser fatigue performance. This is achieved due to reduced bending and tension stress ranges resulting from thicker pipe ends, as well as to reduced Stress Concentration Factors (SCF) resulting from forging process.