SCR Application for Turret Moored FPSOs in West Africa

Abstract

Turret moored Floating Production Storage and Offloading (FPSO) systems with flexible risers have been successfully used in many deepwater developments in West Africa. However, Steel Catenary Risers (SCR) have not been used on a turret moored FPSO because of concerns with the riser fatigue and compression loading due to the large FPSO motions at the turret location. ExxonMobil has initiated a development program to establish the feasibility of SCRs on turret moored FPSOs. The goal of this work effort is enable the use of SCRs on West African turret-moored FPSOs and thereby expand the riser concept options to bring potential economic advantage for future prospects. Additionally, the SCRs may be needed to comply with local content requirements. The development program consists of identifying the limitations of a conventional SCR design, establishing feasible SCR concepts that meet the strength and fatigue requirements, and pre-qualification of a high fatigue performance welding technology. The program focuses on alternate SCR concepts including SCRs with locally weighted and buoyant sections. This paper presents the SCR concepts and the associated analysis results which demonstrate the elimination of compressive loads in the touch down area and the achievement of adequate weld fatigue performance throughout the riser. This paper further presents the results of the inconel weld pre-qualification program including fatigue tests on large and small diameter pipe to demonstrate enhanced fatigue performance.