Lazy Wave SCR On Turret Moored FPSO

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 face challenges in feasibility 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. Flexible risers generally have better dynamic response, but are often limited in size, pressure, and price. Hence, there is a need to improve SCR performance in dynamic vessel applications. Lazy Wave SCR has attracted more attention in recent years due to its good motion isolation effect between TDP and hang off. This paper presents the assessment of a Lazy Wave SCR attached to a turret moored FPSO in the West Africa environment. The traditional Lazy Wave SCR and the so called Shaped SCR are both evaluated in terms of extreme strength and first order motion fatigue performance. The results are compared with simple steel catenary riser. The strength assessment is performed by first using a load case screening approach based on extreme porch downward axial velocity. This avoided running dozens of 12-hour whole riser simulations. The worst load case is identified and only one whole riser simulation is performed for a short period of time. The results indicate that traditional Lazy Wave SCR show sufficient fatigue life provided the vessel headings are controlled within a range of angles. More measures are needed for the Shaped SCRs to have enough fatigue life.