TLP Production Risers for 1200-m Water Depth

Abstract

Summary. A number of technically feasible alternative riser systems exist for tension leg platform (TLP) production systems in deep water. These include conventional steel, steel with added buoyancy, titanium, and carbon-fiber composite. The last three of these options provide substantial reductions in required riser tension for a 1200-m [3,935-ft] depth. For a minimum-size TLP, these reduced tensions translate into substantial reductions in fabricated steel weight. Replacing conventional top tensioners with a hard-mount or gimbal arrangement requires a relatively small increase in top tension, which would be balanced out by weight savings on the tensioning equipment. Operational concerns related to pressure and temperature changes must be addressed to ensure feasibility of the hard-mount or gimbal approach. Introduction Cost-effective development of deepwater acreage presents a major challenge in today's offshore oil industry. To meet this challenge, it will be necessary to rely, as much as possible, on proven technology developed progressively as the industry has pushed back water depth frontiers. The TLP, which has now completed its fourth year in the Hutton field, is an example of just such an incorporation of known technology. The TLP provides a stable platform for oil operations. The vertically tensioned mooring system almost totally eliminates vertical motion, allowing the adaptation of conventional fixed-platform drilling and production techniques. At the same time, the buoyancy of the hull and lateral compliance of the mooring system drastically reduce water-depth cost sensitivity (in comparison to fixed structures). The TLP subsystems of most concern in the move to much greater depths are the moorings and risers, which connect the hull to the seafloor. This paper focuses on the extension of TLP production risers to water depths almost an order of magnitude greater than Hutton. A deepwater development scheme based on a minimum-size TLP is explained, and riser system options for TLP's in 1200-m [3,935-ft] depths are reviewed. Gulf of Mexico design criteria are used to develop and to compare several alternative riser concepts. The impact of these concepts on the design of the TLP hull is discussed. Special considerations for riser operations in very deep water are also addressed. TLP Deepwater Development Concept The fundamental mission of a production TLP is to support the Christmas trees for the necessary production and injection wells on a stable deck above the water surface. This allows conventional (fixed-platform) well maintenance and adjustment without subsea intervention or remote-control subsea trees. Fully integrated TLPs (like Hutton) incorporate a number of additional capabilities (e.g., drilling and MI processing), requiring additional deck load and a larger, more costly TLP. For some fields (particularly smaller fields in deeper water), it may be advantageous to reduce the TLP payload to the bare essentials. This would include the risers and trees, a smaller workover rig, only essential process equipment (e.g., manifold and first-stage separation), and a smaller crew quarters. In this type of scheme, most of the processing, together with the export function, would be located on a separate facility that is less weight-sensitive than the TLP. This could be a fixed platform, if shallow water is nearby, or a tanker, if pipeline export is not feasible. Fig. 1 depicts a minimum-size TLP based on the scenario described above. Such a platform would be about one-third the size of the Hutton TLP for a Gulf of Mexico application (about 20 000 vs. 60 000 Mg [20,000 vs. 60,000 tonnes]). The wells would be predrilled by a floater anchored over a template at the TLP location. Some subsea satellite wells could be included to reach the extremities of the reservoir. This minimum-size TLP development concept is intended to enhance the fundamental adaptability of the TLP to a wide range of water depths. If it is to become a workhorse for deepwater development in the Gulf of Mexico and elsewhere, this type of TLP must be extendable from 500 to more than 1000 m [1,640 to 3,280 ft] with relatively little modification. Thus, the riser system must be adapted as water depth increases, preserving the basic concept of surface trees while controlling TLP payload. To illustrate this conceptual design process, consider a minimum-size TLP based on the above scenario with 24 single completion wells. The basic production riser configuration is illustrated in Fig. 2. It is designed to survive a 100-year Gulf of Mexico hurricane in place. Starting at the seabed, a hydraulic connector makes the structural connection with the wellhead. Casing (typically 24.4 cm [9 5/8 in.]) is tied back inside the wellhead with a pressure-tight connection. Inside the casing, production tubing runs continuously from downhole up into the riser.A thick-walled taper joint controls moments at the seabed without a dogleg causing the tubing to kink. At the top end, a conventional tree is fitted to the tubing and casing tieback. The riser is supported by tension applied from the TLP deck. The amount of riser tension necessary and the type of tensioning system are very important parameters in the TLP design. Riser-System Options Within the configuration outlined above, several options exist for the riser system. The most important options center on the choice of material for the outer riser pipe (casing) and the top interface design. The bottom interface also includes design options. but because its effect on the TLP is less dramatic, it becomes more of a detailed design selection. Material Selection. Available material choices for riser pipe include the following: steel, for conventional casing and the baseline system; steel with buoyancy: syntactic foam is placed around the lower riser to distribute and reduce top tension better; titanium, to save weight and top tension, to improve flexibility, and to prevent corrosion; and composite: carbon fiber and fiberglass/epoxy further reduce weight and top tension, while improving flexibility and corrosion resistance. Steel inserts are incorporated into the composite pipe to provide threaded connections between joints. Hybrid designs may also be developed (e.g., composite with conventional steel at the surface to improve fire resistance and conventional steel with titanium taper joints). These would tend to result, however, from detailed rather than conceptual design tradeoffs. SPEPE P. 558^