Vessel Interface Considerations for Ultra Deepwater Intervention Risers

For ultra-deepwater subsea wells, a riser system is required to conduct completion, intervention/workover and end of life activities. For ultra-deepwater riser systems with high temperature and pressure requirements, the intervention riser system often requires vessel interface optimization to achieve acceptable design response. The upper riser can be configured in several different ways, each with its own benefit from a safety, risk and performance perspective. This paper compares the riser response for various vessel interfaces for ultra-deepwater applications. As discussed above, intervention riser structural response is sensitive to the riser configuration at the vessel interface. For a typical intervention riser, due to ultra-deepwater and high tension requirements, the functional tension load may utilize up to 40% of yield strength thus decreasing the capacity available to accommodate bending and pressure loads. Vessel operators have options to modify the system configuration to improve the strength and fatigue response of the riser. The different vessel interface options include the tension lift frame (TLF) to vessel interface, the top tension application method and the use or otherwise of a surface tree dolly. Upper riser assembly (URA) loads may be optimized by use of rotary wear bushings, a cased wear joint assembly or flexjoints as a part of the stack-up. The various riser-vessel interface options are evaluated and compared in this paper. This paper highlights the riser design challenges for ultra-deepwater applications.