Seal Qualification and Single-Trip Lockdown Sleeve Reduce Risk
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Traditionally, a wellhead system requires a drilling lockdown sleeve (LDS) during drilling operations to ensure that the seal assembly is rigid and prevents dynamic movement. By eliminating the LDS, however, the operator will save multiple trips into the well. Because of a seal-assembly design outlined in the complete paper, development of a single-trip LDS was possible for wellhead systems requiring extremely high lockdown requirements, allowing the user to save trips required by legacy designs.
New Subsea Challenges
In high-pressure/high-temperature subsea wells, annular pressure buildup, caused by thermal expansion of fluid, endangers well integrity. This phenomenon is caused by the increase of pressure during the production phase, temperature differential during the drilling phase, and fluid parameters. This type of annuli pressure buildup causes uplift as high as 3,200,000 lbf. The number of times the system needs to be shut down could be as many as eight per year for a 25-year well life; the life of the field during the production phase, therefore, could see as many as 200 total cycles.
Saving Rig Time by Implementing New Technologies
The technologies presented in the complete paper are the single-trip LDS and the dynamic casing hanger seal assembly. The single-trip LDS is used if an extremely high lockdown capacity is required. When this LDS is used, the need to run a lead impression tool (LIT) before running the lockdown sleeve is eliminated. Thus, an extra tool on the rig floor is eliminated, as well as a trip into the well. By eliminating the LIT run, between 5 and 10 hours could be saved, depending on water depth. Similar to the way in which LDS saves operators both operating expense and capital expenditure, the dynamic casing hanger seal assembly does as well. The dynamic casing hanger seal assembly has a higher lockdown capacity than the industry-standard seal assembly, which allows this seal to be run downhole without installation of an LDS. Because of this design, running the casing hanger seal assembly could save between three and five trips into the well, reducing overall risk.
Casing Hanger Seal Assembly: Design Challenges. The industry has struggled with developing a casing hanger seal assembly lockdown mechanism that can withstand more than 1,000,000 lbf. Thus, it has become accepted that, if a lockdown requirement exceeds 1,000,000 lbf, an LDS will be installed to achieve this higher lockdown rating. This goal, however, is complicated by the limited space manufacturers must modify within the load path. Therefore, the lockdown rating may not be limited by the lock ring itself but by something else within the design, such as threads or body stresses.
Because a production well might see as many as 200 cycles over its life, another hurdle is the industry’s perception of how dynamic metal seals degrade after each cycle. The design criterion for creating a casing hanger seal assembly with a proven dynamic metal-to-metal seal with 200 cycles performed was met by redesigning the sealing mechanism on the assembly. Once the dynamic seal portion of the casing hanger seal assembly was proved, the lockdown mechanism was designed.
To achieve 2,000,000-lbf lockdown capacity, a total redesign of the upper body of the seal assembly and locking grooves was performed. The upper body was designed around the goals of moving the load path and strengthening the system.
Single-Trip LDS: Design Challenges. The single-trip LDS is installed with weight set to energize the lock ring and right-hand torque set to eliminate any gaps within the locking mechanism, thus avoiding the relative upward movement between the casing hanger and high-pressure housing (Fig. 1). The LDS design is intended to be installed in one trip, in addition to other design requirements such as being set in open water, passing through the drilling riser and blowout preventer, maintaining existing capacities, landing out on top of the casing hanger, and opening the lock ring without the need for an LIT. These issues must be overcome during the design phase to ensure the reduction of nonproductive time during the installation phase.
Casing Hanger and Seal-Assembly Development. Testing completed in 2017 combined successfully the validation of a nominally sized 18.75-in. casing hanger seal element with the validation of the structural components included in the design of the seal. The completion of this single test has shown that the casing hanger seal assembly is fully qualified to the claimed pressure and temperature ratings as well as the casing hanger lockdown rating in excess of API standards. The ratings for which this seal meets the validation standards include the following:
- 20,000 psi from above
- 15,000 psi from below
- 35 to 350°F
- 2,000,000-lbf lockdown at all pressures
- 200 extended life cycles
The fixturing for this test allowed the casing hanger and seal assembly to react within the same geometric degrees of freedom required for operation within an actual field installation. No additional structural members were put in place to fix the seal into the annulus between the test fixture housing and the test fixture casing hanger. External load was applied to the test article casing hanger while the maximum rated pressure was applied below the seal element. This simulated all scenarios in which the maximum pressures, temperatures, and loads might be applied. This method of testing combined the two critical aspects of seal qualification, which typically are tested independently, and qualified them simultaneously across the entire temperature envelope for which the seal assembly is rated. By combining both critical qualification tests and by using the complete seal assembly within the test, every minute detail of the seal design was incorporated into proving the seal’s ratings. Structural stability within the seal assembly was verified while the seal was under both maximum load and pressure and at both ends of the operable temperature range. As the lockdown feature of the seal was tested, pressure was maintained below the seal assembly. This verified that the lockdown rating of the seal is completely decoupled from the applied pressure below the seal.
Testing has also proved that metal-to-metal seals can be developed and effectively perform to their full pressure and temperature ratings while being allowed to move within the geometric range of motion allotted by dimensional tolerance and clearance features within the wellhead system. The results from this testing disprove former notions that these types of metal-to-metal seals must be held completely static in order to maintain the isolation of annular pressure.
Single-Trip LDS Development. Testing was performed successfully on the nominally sized single-trip first-position and second-position casing hanger LDS with the validation of the structural components. The completion of this test has shown that the single-trip LDS was fully qualified to the claimed load ratings in excess of the standards set forth in API 17D, 2nd Edition. The ratings for which this single-trip LDS meet the validation standards include an ambient temperature of 60–90°F and 2,000,000-lbf lockdown.
The fixturing and parts for this test allowed for the casing hanger and LDS to react within the same boundary conditions required for operation in an actual field installation. No additional structural members were put in place to fix the casing hanger at the high-pressure housing. External load was applied to the test-article casing hanger while the maximum rated load was applied from below. This simulated all scenarios in which the maximum loads may be applied. This method of testing combined the two critical aspects of the LDS qualification, which are typically tested independently, and qualified them simultaneously across the ambient temperature envelope for which the LDS is rated. As the lockdown feature of the LDS was tested, load was maintained below the LDS assembly. The lockdown capacity of this LDS design will provide the full 2,000,000 lbf of lockdown up to the rated well loads.
Seal Qualification and Single-Trip Lockdown Sleeve Reduce Risk
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