Fatigue analysis of catenary offset buoyant riser assembly(COBRA) concept

Abstract

Steel Catenary Riser is the preferred solution to riser system in offshore oil & gas production. They are more structurally reliable, technically simple and cost effective compared to other types of riser. As the offshore oil & gas production move to deep and ultra-deepwater regions, the optimum solution have to be determined to overcome the challenges of steel catenary riser such as its weight and size. Hybrid riser seems to address some of the problems but they have complex bottom assembly that could prove technically challenging. Catenary Offset Buoyant Riser Assembly (COBRA) is a concept conceived to address most of the problem of steel catenary riser in deepwater. It combines the advantages of steel catenary and hybrid risers and is made up of two parts. The bottom part consists of a rigid steel pipe lain in catenary configuration and connected to a sub-surface buoy. A flexible jumper connects it to the floating structure. The flexible jumper and the buoy effectively absorbed the forces acting on the riser and floating structure essentially making the steel catenary undisturbed by the dynamic motions, thus improving its fatigue performance. This project will be focusing on numerically simulating the application of COBRA in a Malaysia deepwater project. The environmental conditions from the Kikeh Field is applied to determine the static, dynamic and wave fatigue analysis of COBRA. The static and dynamic analyses showed that. at the touch-down point, the tension for COBRA is about 50% smaller that for steel catenary riser. The simulation also has shown that COBRA has an excellent fatigue life. Therefore, the COBRA concept is suitable for application in deepwater projects in Malaysia.