Steel Catenary Risers for Semisubmersible Based Floating Production Systems

Abstract

Abstract The DeepStar production riser committee has investigated the feasibility of using steel catenary risers (SCRs) in water depths of 3000-6000 ft. Using Sonat's George Richardson as the base semi submersible, Deep Star has examined both extreme event response and fatigue life of an SCR made of pipe sections welded end-to-end. Concepts using alternative materials were investigated, This included steel, steel with titanium and titanium catenar risers. The pros and cons of frequency main versus time domain analysis were investigated with a commercial y available analysis package. A second study outlined a definitive analysis procedure which optimized the analysis time requirements. Analyses showed that steel catenary risers are feasible for semisubmersible based floating production systems. For the DeepStar Gulf of Mexico design criteria, alternative materials are not required. The greatest fatigue damage occurs in the touchdown region of the riser. Mild sea states contribute most to fatigue damage near riser touchdown. Wave drift and wind forces provide a significant contribution to touchdown area fatigue damage. Estimated fatigue lives are acceptable. Although the rotations of the upper end of the riser are large relative to an SCR attached to a TLP, the rotation required can probably be accommodated with existing technology. For the case of product export, steel catenary risers provide very cost effective and readily installable deep water riser alternatives. @ Introduction Since April 1993, the DeepStar Production Rise Committee has sought to advance those riser concepts most suitable for use with floating production systems in water depths ranging between 3000 f? and 6000 ft. One DeepStar goal is to develop technology which will enable us to commingle flow from several subsea wells with a manifold, and flow produced fluids two-phase over a distance of up to 60 miles back to a shallow water (-600 R) platform on the shelf. This extended reach concept may be feasible in case produced fluids are light non-problem crudes, however, for the case of the more viscous heavy oils or crudes with strong depositional tendencies, some in-field processing will be required to produce these reserves with today's technology necessitates having a floating production system outfitted with processing equipment. In DeepStar, for the preliminary efforts carried out to date it has been assumed this is a semisubmersible floating production system. With the requirement of floating production, comes the need for deep water risers. The DeepStar Production Riser committee initially conducted a screening effort in which all those riser concepts which did not tit into the DeepStar base case drilling and production scheme were eliminated horn further consideration. Of the 8- 1O concepts reviewed, those considered suitable for further investigation included two hybrid riser concepts, flexible pipe risers, and the steel catenary riser. The hybrid risers studied consisted of rigid steel risers running from the seabed to subsurface steel buoyancy cans, and flexible pipe jumpers running between the steel buoys and the semisubmersible.