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.
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.
A 3D partially non-linear transient filly-coupled riser analysis method is evaluated which uses modal superposition of independently extracted lateral and axial modes. Many lateral modes are combined with a lesser number axial modes to minimize adverse time step requirements typically induced by axial flexibility in direct time integration of beam-column elements. The reduced computer time option enables much faster parametric analysis of hang-off, as well as other connected drilling environments normally examined. Axial- Iateral coupling is explicitly enforced and, resonance fidelity is preserved when excitation is near or coincident with axial natural periods. Reasonable correlation is shown with envelopes of test case dynamic responses published by API. Applicability of the method is limited by linearity assumptions indigenous to modal representation of dynamic deflections relative to a mean deflected shape. Sensitivities of incipient buckling during hang-off to axial damping and stiffness aredescribed for an example 6000 ft. deep composite drilling riser system. Introduction Numerical simulation of advanced composite drilling riser structural response in deeper depth environments will potentiality require increasing inclusion of axial-lateral dynamic coupling to support development of design requirements. This coupling is most significant to simulation of hang-off environments. Advanced composite materials have attracted serious attention due to their favorable strength to weight ratios so important to deeper depth drilling riser systems.12 However, composite riser joints must be designed with sensitivity to axial dynamics that are potentially more adverse than predicted for steel joints. The substantial by lower axial stiffness of composite drilling riser joints causes these systems to have axial natural periods closer to wave excitation periods than corresponding steel systems. Fig, 1 describes natural lateral and axial periods for an example 6000 ft. deep composite drilling riser system which is rigidly (axially) connected to a drilling ship in a hang-off condition. Lateral (end-constrained) modes 6 through 19 exist within the dominant range of the wave spectrum. However, the fundamental (top-constrained) axial period is below the wave excitation range and minor axial excitation should be expected. Fig. 2. shows the effect of compliance added at the ship interface, causing the natural periods to increase into the wave excitation range. The same trend may be caused by a multitude of reasons including composite material or lay-up variations, emergency hang-off, heavier joints or increase in drilling depth. We are here concerned with making preliminary assessments of potentially adverse dynamic interactions when the fundamental axial periods migrate into the wave excitation range, potentially inducing significant hang-off loads3. When this interaction or coupling is known or shown to be insignificant, superposition of responses computed from independent lateral and axial dynamicanalyses is appropriate and efficient. However, if axial-lateral interactions cannot be discounted, filly coupled analyses become appropriate, even if only to confirm that coupling effects are insignificant.
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