Feasibility Of Top-Tensioned Dual Casing Production Riser Systems In Ultra-Deepwater

ABSTRACT This paper presents a feasibility study for design of a spar with Top-Tensioned Production Risers (TTRs) in 10,000 ft water depth. The study was sponsored by DeepStar as part of a series of evaluations of floaters and riser systems for ultradeep water applications. A dual casing riser configuration is preferred by many operators for easy well intervention and completion, but conventional technologies used in shallower waters have been shown to be too heavy to operate in 10,000 ft of water. A conventional strength riser pipe would not be able to withstand the stress induced by its own weight, plus additional tension and bending. It is also difficult to design a practical tensioner system to support such a heavy riser system. The present study shows that by using high strength pipe and various weight reduction schemes, a dual casing riser system can be designed with adequate strength, sufficient fatigue life, seabed and wellbay layouts that prevent clashing, and reasonable strokes. Both buoyancy can tensioners and hydraulic tensioners can be sized to support the risers. Moreover, the spar and mooring system designed to support the risers and the associated topsides are comparable in size to existing spar hulls and moorings. INTRODUCTION Offshore production is rapidly moving into ultra deep waters with drilling presently occurring in 10,000 ft. [1-6, 9]. In recent years spar floaters have become a popular option for the development of deepwater fields. Current hull and mooring technologies can be extended to ultra-deep water, but design of top-tensioned risers has been identified as a key challenge for extending application of spars to 10,000 ft water depth (WD). A dual casing riser system configuration was investigated in this study. Although this configuration is preferred for advantages on overall-system feasibility, risk assessment, and life-cycle cost [4], a conventional dual casing riser system would be too heavy to operate in 10,000 ft WD. A conventional riser pipe is not be able to withstand the stress induced by their own weight, plus additional tension and bending. It would also be difficult to design a practical tensioner system to support such a heavy riser system. Several proven and emerging technologies can be used to reduce the riser weight to a manageable level:use high strength pipe and threaded & coupled (T&C) connectorsuse different pipe wall thicknesses at various water depths,attach foam buoyancy along the riser,use composite outer riser joints, anduse hoop-reinforced-steel inner riser joints. This paper presents challenges of designing a dual casing TTR riser system for a spar in 10,000 ft water depth. Using proven and emerging riser technologies, various riser systems are configured and the following aspects of riser design issues are investigated.size the risers using the recommended weight reduction schemes;develop details for the risers for the various combinations of outer and inner casing types;develop required tensioning systems;investigate riser layout, riser clashing problems, VIV and motion induced fatigue; anddesign a spar and mooring system to support the risers.