New Deepwater, High-Pressure GOM Tubular Maximizes Capability, Reduces Cost
A 2-year comprehensive effort to design, test, manufacture, and deploy a new high-pressure completion tubular for Chevron’s deepwater Gulf of Mexico (GOM) operations is presented.
A 2-year comprehensive effort to design, test, manufacture, and deploy a new high-pressure completion tubular (CT) for Chevron’s deepwater Gulf of Mexico (GOM) operations is presented. The completion application expected harsh, aggressive loading modes and high pressures to be encountered. The major challenge was to design, test, and manufacture a subsea-completion string that would provide efficient hydraulics during fracturing operations while ensuring mechanical and pressure integrity.
In 2004, the first built-for-purpose CT incorporating a gas-tight, rotary-shouldered connection was developed and deployed in the GOM. Since that time, rotary-shouldered connections have evolved (this evolution is described in detail in the complete paper). Present-day connections offer improved performance, increased torsional capacity, and improved hydraulics, and have created slimmer profiles. Additionally, running and handling characteristics have been improved, providing faster makeup, reduced trip time, and decreased repair cost because of reduced connection damage. However, the need remained to incorporate these technological advancements and benefits into a second-generation CT.
In March 2002, the operator drilled the Tahiti 1 well in 4,017 ft of water to a total depth of 28,411 ft. The GOM well was located approximately 190 miles southwest of New Orleans in Green Canyon Block 640. Results from the exploratory well indicated the presence of high-quality reservoir sand with a total net pay of more than 400 ft. Following the results of the exploratory well, two appraisal wells were drilled simultaneously in Green Canyon Blocks 596 and 640. The appraisal program verified the operator’s initial estimates of 400 to 500 million bbl of ultimate recoverable oil reserves, one of the most significant net-pay accumulations recorded in the GOM.
Because of the extreme depth and bottomhole pressure of the Tahiti reservoir, the completion and well test required the design of new equipment to successfully perforate, frac pack, and flow test the Tahiti 1 discovery well, including the design and qualification of a perforating and frac-pack CT.
The CT requirements included
- Fishable within the 9⅞-in. tieback, limiting the connection outer diameter (OD) to 7 in.
- An inner diameter (ID) of no less than 4¼ in. to provide the desired treating rate during fracturing operations
- An absolute-internal-working-pressure rating of approximately 29,000 psi to sustain internal pressure and not leak, potentially jeopardizing the frac-pack results
- An absolute-external-working-pressure rating of approximately 24,500 psi to sustain elevated annulus pressures and not collapse or leak
- The ability to withstand multiple makeup and breakout cycles while maintaining sealability at pressure to enable multiple trips into the well for cleanout, displacement, perforation, and fracture operations
- A high-performance metal-to-metal seal that possessed integrity against fluid leak
The basis of the first-generation-CT connection was the first-generation metal-to-metal (MTM) connection. Finite-element analysis (FEA) of the first-generation MTM connection revealed that the extended box counterbore lacked stability under the immense external pressure and compressive loads anticipated at the bottom of the well. Sectional stresses in the box counterbore significantly exceeded the material capabilities, and counterbore collapse was predicted.
To provide additional rigidity and structural stability to the box counterbore of the first-generation CT connection, the counterbore length was shortened from 2¼ to ⅝ in. Adjusting the box counterbore length created an imbalance of forces acting on the internal and external shoulders. The initial clearance, or gap, between the pin nose and the box internal shoulder when the connection is made up “hand-tight” and the makeup position when the external shoulder and thread flanks are touching slightly had to be determined. The proper gap for the first-generation CT was determined through a series of FEA runs.
The new design was then analyzed with FEA against the anticipated Tahiti loads. The connection showed that sectional stress values in the box counterbore still exceeded the 120,000-psi tool-joint-material specified minimum yield strength (SMYS). A decision was made to incorporate 135,000-psi-SMYS tool-joint material for the connection.
Rigorous physical testing verified the design, and the first-generation CT was deployed successfully on the Tahiti 1 well and has become the standard high-performance CT for the operator and other deepwater-GOM operators.
Development of Second-Generation CT
The objective of the new deepwater, high-pressure CT was to maintain or improve the combined-load capability of the first-generation CT while providing the speed of makeup and proven cost savings of the third-generation double-shouldered connection (DSC).
The first-generation CT benefits from the refinement and optimization of the third-generation DSC design. The design requirements were met through the combination of high-strength material, proven radial MTM seal technology, and the double-start thread that reduced the number of turns necessary to assemble the connection by 50%. Other design parameters of the second-generation CT, such as dual-radius thread form that enhances connection fatigue performance and optimized taper, pitch, and thread height, are identical to those found on the third-generation DSC. The verification of the second-generation CT design involved FEA and a rigorous physical test program.
Higher tool-joint yield strength allowed the optimization of the connection’s taper and thread height to increase stabbing depth, which reduced the number of turns from stab to makeup as compared with the first-generation CT. The deeper stab depth provides increased stability as the pin connection is first rotated, which reduces the chances of cross-threading. Additionally, deeper stab reduces the likelihood of creating pinch points, increasing operator safety.
A comprehensive FEA (detailed in the complete paper) was conducted to optimize the design and evaluate the performance of the second-generation-CT connection. On the basis of the FEA results, the 7-in.-OD×4¼-in.-ID second-generation-CT connection displayed acceptable stress profiles for all the service load points.
Connection Testing. Comprehensive full-scale physical testing of the second-generation-CT-connection design was necessary to verify performance for the operating conditions expected in high-pressure/high-temperature (HP/HT) offshore wells. The testing program consisted of two phases: (1) galling-resistance (make-and-break) testing and (2) structural/sealability testing. The make-and-break testing subjected the candidate connection samples to multiple makeup and breakout cycles to assess the galling resistance of the connection. The structural/sealability testing subjected the same connection samples to a range of combined loading conditions, including axial tension and compression loads, simultaneous internal and external pressures, elevated temperature, and bending loads. Preparations for both make-and-break testing and structural/sealability tests are provided in the complete paper.
Connection Samples. Three second-generation-CT-connection samples were machined from 7½-in.-diameter 4130M bar-stock material that was quenched and tempered to achieve minimum yield strength of 135,000 psi. Each of the three samples was machined to meet specific critical tolerance configurations. An additional sample was machined from the same material for each tolerance configuration (three spare samples in total) in case one or more of the original samples were damaged at any point in the testing process.
Structural/Sealability-Test-Program Load Points. The structural/sealability-test-program load points were designed to replicate various load combinations to which the connections could be subjected during installation and operation in offshore HP/HT applications. The load points for this program subjected the connection samples to net axial loads between 1,100 kips in tension and 215 kips in compression; temperature up to 220°F; bending loads to create a 3°/100 ft dogleg severity; and internal and external pressures up to 29,920 and 25,279 psi, respectively. The three load points (5.1, 6.1, and 7.1) were used to compensate for the axial tensile load as a result of the capped-end pressure effect created by the external geometry of the samples when they were subjected to external pressure.
All of the load points where external pressure was applied to the connection samples had internal pressure applied simultaneously. Applying both internal and external pressure at the same time was critical in the overall evaluation of the second-generation-CT-connection performance. If only the net differential pressure had been applied, only the sealability characteristics of the radial MTM seal would have been evaluated. By applying internal and external pressures simultaneously, the test program not only determined the sealability characteristics, but also further established whether the connection was capable of withstanding the combined axial, radial, and hoop stresses representative of those in the eventual expected operating conditions for this connection design.
Structural/Sealability-Test Results. The structural/sealability tests were performed between 8 March and 20 April of 2013. The samples were tested individually, with Sample 3 being the first one tested, followed by Sample 1 and then Sample 2. Each sample was assembled with the test fixtures and external vessel making up the test apparatus and then installed in a load frame with an axial-load capacity of 3,400 kips in both tension and compression.
Because the test apparatus was constructed in such a way that no leak path was possible from the interior to the exterior pressure chambers, a pressure change between the inner bore of the sample and the external vessel could be attributed only to leakage across the sample connection. As such, connection-sample-sealability performance was assessed by monitoring the pressure differential across the connection. None of the three samples tested showed any signs of leakage across their connections under a net external or a net internal pressure differential.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 173034, “New Deepwater, High-Pressure Completion Tubular for Gulf of Mexico Maximizes Combined-Load Capability While Incorporating Proven Cost-Saving Features,” by T. Anderson, C. Fontenot, and R. Davey, Chevron; J. Dugas and B. White, Quail Tools; K.A. Hamilton, C-FER Technologies; and P.S. Beauchamp, L.C. Karlapalem, A. Muradov, and J. N. Brock, NOV Grant Prideco, prepared for the 2015 SPE/IADC Drilling Conference and Exhibition, London, 17–19 March. The paper has not been peer reviewed.