A Comprehensive Approach to Deepwater Marine Riser Management
Abstract: A system for riser management has been developed and used, consisting of: 1) a variable buoyancy system, 2) hardware to hangoff a disconnected marine riser near the keel of a drillship, 3) a heave acceleration prediction capability, 4) instrumentation for monitoring heave acceleration and riser buoyancy, tension, pressure, vibrations, and inclination, and 5) a technique for making hang-off and disconnect decisions based on forecasts and real time vessel motion measurements.
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Summary The principal focus of this paper is to discuss and summarize, from the manufacturer's perspective, the primary milestones in the development of the marine riser system used to drill in record water depths off the U.S. east coast. This riser system is unique in that it used advanced designs, material technology, and quality control to enable safe operation in war depths beyond the capability of conventional drilling riser systems. Experience and research have led to design improvements that are now being incorporated in new riser systems that have the potential of expanding the frontiers to increasingly deeper water. Introduction In early 1982, we received a contract to construct a marine drilling riser capable of drilling in 7,500 ft [2286 m] of water for the Discoverer 7 Seas drillship. To meet the challenge, each step of the development process was evaluated for optimum contribution-from process was evaluated for optimum contribution-from the design and material availability to manufacturing techniques and quality control. The resulting product has met or exceeded the expectations of the end user and is still one of the most advanced drilling risers currently in operation. Fig. 1 covers the major components, from the diverter at the top to the blowout preventer (BOP) stack on the bottom. Riser System Components Riser Joints. The riser joint includes (1) riser coupling (HMF bolted flange connector rated at 1.5 ×10(6) lbf [6.7 ×10(6) N] tensile), (2) principal tube (18 5/8-in. [47.- 3cm] -OD ×0.688-in. [1.748-cm] -wall RMWX-80 pipe), (3) chokelines and kill lines (3-in. [7.6-cm] -ID 4130 steel pipe for 15,000-psig [103.4-MPa] working pressure), and pipe for 15,000-psig [103.4-MPa] working pressure), and (4) auxiliary lines (2 1/2-in. [6.35-cm] nominal schedule 80 stainless-steel pipe for 3,000-psig [20.7-MPa] working pressure). A total of 124 riser joints were contracted-94 to accommodate fixed-buoyancy syntactic foam modules and 30 equipped with adjustable-buoyancy air cans. During the manufacturing process, the riser connector flanges were machined to mate with the actual pipe received. This customized approach eliminated the mismatch problems normally encountered during fabrication. HMF Joint Design. The HMF bolted flange connector was developed specifically for deepwater riser systems. The popular dog-type latch connector was not considered for deeper water because of the high stress concentrations inherent with the design. Standard API-type flanges also result in excessive stresses in the flange-to-pipe weld when subjected to bending moments encountered in marine risers. To reduce stresses in the critical transition area between the relatively thick flange and the thin pipe, the HMF riser connector incorporates a patented elliptical contour (see Fig. 2). This elliptical transition provides a nearly uniform stress distribution between the flange and pipe. The average stress on the surface of the ellipse is almost the same as on the pipe. Finite-element analysis verified acceptable stress levels in both the hang-off and bolted-up conditions. In the hang-off condition, when the marine riser and BOP stack are separated from the wellhead and are suspended in the riser spider, the limiting factor is stresses caused by axial loading. In the bolted-up condition experienced during routine operations, the limiting factor is fatigue life. Riser Flange Bolts and Torque Wrenches. Riser-flange-bolt design and proper preloading are critical elements in the riser system. The primary function of the bolts is to retain the flanges and preload the connection to minimize metal fatigue. Bolts must also be damageresistant and easy to use, to maintain, and to replace. The HMF connection bolt has a 4-threads/in. -stub acme thread form with a special self-cleaning notch, is protected with a corrosion-resistant low-friction coating, and is retained in the riser flange by a retaining ring. The bolts, as designed and manufactured, met their primary objectives. Original strain-gauge testing for bolt primary objectives. Original strain-gauge testing for bolt preload, however, was performed on bolts made with cut preload, however, was performed on bolts made with cut threads rather than rolled threads and mated with stainless-steel nuts. When production bolts with rolled threads were made up with the same torque, some bolt head flanges yielded because of the lower friction coefficient. Although the immediate remedy was a 30% reduction in makeup torque, bolts were redesigned with thicker flanges. The HMF flange connection requires considerable torque to obtain the desired preload. Hydraulic torque wrenches are necessary to generate the torque required for makeup and breakdown. To save time, air impact wrenches are recommended to provide the initial 1,000-lbf-ft [1356-N.m] torque rapidly. SPEDE p. 325
Summary The principal focus of this paper is to discuss and summarize, from the manufacturer's perspective, the primary milestones in the development of the marine riser system used to drill in record water depths off the U.S. east coast. This riser system is unique in that it used advanced designs, material technology, and quality control to enable safe operation in war depths beyond the capability of conventional drilling riser systems. Experience and research have led to design improvements that are now being incorporated in new riser systems that have the potential of expanding the frontiers to increasingly deeper water. Introduction In early 1982, we received a contract to construct a marine drilling riser capable of drilling in 7,500 ft [2286 m] of water for the Discoverer 7 Seas drillship. To meet the challenge, each step of the development process was evaluated for optimum contribution-from process was evaluated for optimum contribution-from the design and material availability to manufacturing techniques and quality control. The resulting product has met or exceeded the expectations of the end user and is still one of the most advanced drilling risers currently in operation. Fig. 1 covers the major components, from the diverter at the top to the blowout preventer (BOP) stack on the bottom. Riser System Components Riser Joints. The riser joint includes (1) riser coupling (HMF bolted flange connector rated at 1.5 ×10(6) lbf [6.7 ×10(6) N] tensile), (2) principal tube (18 5/8-in. [47.- 3cm] -OD ×0.688-in. [1.748-cm] -wall RMWX-80 pipe), (3) chokelines and kill lines (3-in. [7.6-cm] -ID 4130 steel pipe for 15,000-psig [103.4-MPa] working pressure), and pipe for 15,000-psig [103.4-MPa] working pressure), and (4) auxiliary lines (2 1/2-in. [6.35-cm] nominal schedule 80 stainless-steel pipe for 3,000-psig [20.7-MPa] working pressure). A total of 124 riser joints were contracted-94 to accommodate fixed-buoyancy syntactic foam modules and 30 equipped with adjustable-buoyancy air cans. During the manufacturing process, the riser connector flanges were machined to mate with the actual pipe received. This customized approach eliminated the mismatch problems normally encountered during fabrication. HMF Joint Design. The HMF bolted flange connector was developed specifically for deepwater riser systems. The popular dog-type latch connector was not considered for deeper water because of the high stress concentrations inherent with the design. Standard API-type flanges also result in excessive stresses in the flange-to-pipe weld when subjected to bending moments encountered in marine risers. To reduce stresses in the critical transition area between the relatively thick flange and the thin pipe, the HMF riser connector incorporates a patented elliptical contour (see Fig. 2). This elliptical transition provides a nearly uniform stress distribution between the flange and pipe. The average stress on the surface of the ellipse is almost the same as on the pipe. Finite-element analysis verified acceptable stress levels in both the hang-off and bolted-up conditions. In the hang-off condition, when the marine riser and BOP stack are separated from the wellhead and are suspended in the riser spider, the limiting factor is stresses caused by axial loading. In the bolted-up condition experienced during routine operations, the limiting factor is fatigue life. Riser Flange Bolts and Torque Wrenches. Riser-flange-bolt design and proper preloading are critical elements in the riser system. The primary function of the bolts is to retain the flanges and preload the connection to minimize metal fatigue. Bolts must also be damageresistant and easy to use, to maintain, and to replace. The HMF connection bolt has a 4-threads/in. -stub acme thread form with a special self-cleaning notch, is protected with a corrosion-resistant low-friction coating, and is retained in the riser flange by a retaining ring. The bolts, as designed and manufactured, met their primary objectives. Original strain-gauge testing for bolt primary objectives. Original strain-gauge testing for bolt preload, however, was performed on bolts made with cut preload, however, was performed on bolts made with cut threads rather than rolled threads and mated with stainless-steel nuts. When production bolts with rolled threads were made up with the same torque, some bolt head flanges yielded because of the lower friction coefficient. Although the immediate remedy was a 30% reduction in makeup torque, bolts were redesigned with thicker flanges. The HMF flange connection requires considerable torque to obtain the desired preload. Hydraulic torque wrenches are necessary to generate the torque required for makeup and breakdown. To save time, air impact wrenches are recommended to provide the initial 1,000-lbf-ft [1356-N.m] torque rapidly. SPEDE p. 325
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