Comparative Performance Of A Composite Drilling Riser In Deep Water

Abstract

ABSTRACT 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.