The design and analysis of a cost effective multi flowpath production riser for deep and ultra deep water floating production systems is presented. The design is based on a bundled concept using beach fabrication and installation by near surface tow. The study has been funded by the Deepstar project and addresses water depths between 1300 and 2200m in the Gulf of Mexico. The riser is designed to interface with a semi submersible production vessel but may also be used with an FPSO with minor modifications.
The design approach minimisis the riser steel weight and buoyancy requirements and achieves an optimum dynamic response through an efficient structural design and buoyancy distribution. This maximises riser flexibility and minimises hydrodynamic wave loading.
The definition of 'Deep water' changes every year. Operators are currently developing reservoirs in water depths up to 1500m and have drilling campaigns planned for depths beyond 2000m. Production technology for such water depths is likely to center on semi submersible and ship shaped floating production systems which are well established in shallower water depths.
The riser systems, used to transport production fluids between the seabed and vessel, will become an increasingly important aspect in deep water developments. Risers are dynamic systems which operate at high pressures and temperatures often with corrosive fluids. Consequently, risers are technically complex and the materials and methods of manufacture and make them costly. These issues are compounded as water depths increase due to higher loads and lengths involved making riser system selection and optimization even more complex. Deep water developments to date have largely extended shallow water, flexible riser, technology through the development and application of new materials and manufacturing techniques. However, extending a successful shallow water solution to deep water is not necessarily the most economic or technically preferable approach. Whilst in shallow water high pipe flexibility is required to accommodate vessel motions, in deep water stiffer pipe may be considered due to the beneficial effect of water depth on riser system compliancy.
In recent years new riser arrangements have been conceived to meet the challenge of deep water, offering significant commercial and technical advantages over conventional riser systems. These new riser systems utilize steel pipe which has a relatively low cost compared to flexible pipe. The importance of these new riser systems is significant. Commercially, they provide an alternative to the flexible riser but more importantly, in many cases, they provide a technical solution where no feasible solution exists with flexible pipe.
The two most promising riser concepts are the hybrid riser [1,4] and the steel catenary [2,5]. Both concepts can be configured in a number of ways depending on application and even combined as in the case of the Tension Leg Catenary (TLC).