As exploration for hydrocarbons moves into progressively deeper offshore waters worldwide, life cycle costs are becoming an increasingly more significant consideration in the economics of deepwater projects. The high strength-to-weight ratio, corrosion resistance, fatigue tolerance and design flexibility of advanced composite structures can reduce deepwater project life cycle costs. An industry/university team, which includes ABB Vetco Gray, Deepstar, Reading & Bates/Falcon, Northrop Grumman Marine Systems, Hexcel Carbon Fibers, and Offshore Technology Research Center, with cooperation from the National Institute of Standards and Technology(N1ST) Advanced Technology Program(ATP), has designed, developed and fabricated advanced composite deepwater drilling riser prototype joints and choke and kill line prototypes for full scale testing and evaluation. This paper describes the project plan, the development and fabrication of the full scale prototypes, and focuses on the results of recently completed testing of the full scale prototypes. In addition, the plans for the remainder of the project are presented. Background The benefits of advanced composite structures have been acknowledged by the offshore industry for a long time, however the recent emphasis on deep water tracts worldwide has magnified the potential benefits. Stationary seabed supported jacket type structures have been used in shallow water projects, but in deeper water projects, floating structures anchored to the sea bed are the preferred platform. Steel is the common material for the riser and mooring systems, but the weight, fatigue characteristics and corrosion properties are becoming more significant life cycle cost drivers. In addition, steel drilling riser systems tend to limit the depth capability of mobile drilling platforms as a result of their variable deck weight capability. Figure 1 shows a mobile offshore drilling unit with a deployed drilling riser and the details of an advanced composite individual drilling riser joint conceptual design. Development work on lightweight advanced composite drilling riser systems was started in April 1995 by an industry/university team. References 1 and 2 describe the project and review the initial results achieved. Key aspects of the work included a definition of the requirements for a 6000 ft. water depth Gulf of Mexico system with input by DeepStar as shown in Figure 2. Using these requirements, individual from riser joint performance requirements were developed. Advanced composite drilling riser joints were designed and fabricated at Northrop Grumman Marine Systems as shown in Figure 3. In addition, choke and kill line prototypes were fabricated, Figure 4. The first full scale prototype test to be conducted was an external pressure test which was designed to assess the collapse pressure capability of the riser main body structure. The University of Texas part of the team designed and built a specially configured test fixture which eliminated the end loads on the specimen during test and thus just applied acircumferential compressive load. The test specimen and the installed fixture are shown in Figure 5. The assembly was installed into the hyperbaric test chamber at H. 0. Mohr in Houston, Texas as shown in Figure 6. The collapse pressure test was conducted and the prototype collapsed at 3 120 psi. The post test result is shown in Figure 7.
Cayright 1997. Offshore Technology Confaenm This papa was prspored f a prassntatlon at the 1997 O m h m Technology Confaenm hdd inHomton. Texas. 5-8 May 1997.
A composite drilling riser is technically and commercially feasible in 3,000 feet of water subjected to the Gulf of Mexico environment. Comparison to a similar steel drilling riser configuration shows that the composite riser requires 72 percent Iess foam weight and reduces total deck weight by 739 kips. Performance comparison shows that the steel riser must be disconnected for a 20-year storm. The composite riser can remain connected even for the 1OO-year storm. INTRODUCTION Top tension, deck weight and buoyancy requirements of drilling riser systems are very sensitive to water depth. The cost for these requirements increases with water depth. Deep water drilling and exploration is limited by the vessel's deck capacity and the weight and foam requirements of the steel drilling riser technology. Alternative technologies deserve serious considerations if they reduce the riser weight, foam requirements and improve the down time during operation. The objective of this work is to compare weight, cost and operational advantages of a steel and composite drilling riser systems in 3,000 feet of water subjected to the Gulf of Mexico environment. This paper advances the state of the art of composite drilling risers by addressing the problems related to composite to metal end connection and internal wear. APPROACH The design of the two riser systems used the same vessel, water depth and environment; therefore, the make ups of the steel and composite riser systems are similar, but analyses determined that the composite riser does not require a lower flex joint. Several frequency domain analyses determined the riser performance for storms of different severity and four operating conditions defined by API RP 2Q. The storm severity for each riser was increased until the reduced safety factor for bottom angle and or extreme stress at any location along the riser reached the value of 1 (1.5 for the composite riser). A plot of the reduced combined axial extreme stress versus significant wave height determines the limits of the weather window for riser operation. Deterministic and spectral analyses determined the fatigue life for both risers. The fatigue life analysis follows recommendations of API W 16Q (in draft form). The steel riser uses the API RP 2A's X allowable S-N curve, The composite riser uses the SN curve recommended by Curtis3 for fibrous composite materials. Discussion of cost comparison for the two risers includes the impact on cost of the down time during operation. SYSTEM CONFIGURATION The riser configuration reflects a typical deep water system offered in the market place for the Gulf of Mexico in 3,000 feet of water. The system consists of a BOP stack LMRP, lower flex joint (steel riser only), pup joint, riser joints with and without foam modules, telescopic joint and diverter flex joint. The riser joint includes choke and kill lines, booster line and hydraulic lines, The choke and kill lines are 5-inch OD designed for 15,000 psi WI?. The booster line is 5-inch OD designed for 5,000 psi W. The hydraulic lines are 2.25-inch OD.
This paper was selected for presentaijon by the OTC Program Committee following revi_ cA informBlion contained in an abstrad submitted by the authors. Contents cA the paper, as presented, have not been revi~by the Offshore Technology Conference and are subject to CllITllClion by the authors. The material, as presented, does not necessarily reflect any posnion cA the Offshore Technology COnference or its officers. Electronic reproducijon, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reprodUce in print is restrided to an abstract of no more than 300 words; Illustrations may not be copied. The abstrad must contain conspicuous acknowiedgment of where and by whom the paper was presented. AbstractWith deepwater drilling becoming an area of increased interest, the National Institute of Standards and Technology (NISn sponsored composite drilling riser program objectives were extended in 1997 to include a comparative analysis of steel and advanced composite drilling risers for drilling in water depths down to 12,500 ft. A water depth of 12,500 ft was established as the maximum water depth thought to be of interest worldwide. Using requirements established by DeepStar and Reading & Bates, Northrop Grumman Marine Systems and ABB Vetco Gray synthesized a steel drilling riser system and an advanced composite drilling riser system such that the two drilling risers could be compared with regard to key system characteristics such as top tension required, buoyancy required, total system weight and stacked volume. This paper presents preliminary results of the comparative study which identified a weight savings of more than 50% and a volume reduction of 33% as benefits gained by the use of advanced composite materials for fabrication of the riser tubulars. Reduced buoyancy requirements are shown to be a major contributor to weight and volume savings, and also to reduced hydrodynamic drag forces. The comparative analysis performed included dynamic analysis in the hang-off modes using the Northrop Grumman developed Modal Dynamic 189Riser Analysis (MODRAN) program. MODRAN allowed comprehensive 3D analyses of coupled transverse and axial dynamics during hang-off for broad ranges of conditions. The differences (and similarities) in hang-off response are described.
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