Offshore/subsea systems

Cost-Effective Validation of HP/HT Subsea Equipment Using Seawater Hydrostatic Head

The complete paper presents a practical approach for validating design-verification analysis for subsea equipment, using a representative pressure valve block to correlate finite-element analysis (FEA) predictions for strain changes with actual measured changes.

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Validation testing of subsea equipment designed for high pressure/high temperature (HP/HT) applications is necessary but can be extremely expensive and infeasible. The complete paper presents a practical approach for validating design-verification analysis for subsea equipment, using a representative pressure valve block to correlate finite-element analysis (FEA) predictions for strain changes with actual measured changes. The design methods use the guidelines in technical report API 17TR8, and load cases per API 17TR12.

Introduction

Current editions of API standards covered in API Subcommittee 17 (API SC 17) for subsea production equipment are geared toward designing the equipment for its absolute internal working pressures. Also, on the basis of previous regulatory requirements for the offshore industry, until 2014 it was not advised to take advantage of the external seawater hydrostatic head and other external pressures to design certain types of subsea equipment covered under API SC 17. Thus, equipment per its corresponding API standards was designed to have an absolute rated working pressure always greater than the well shut-in tubing pressures.

Three API technical reports published in 2014 and regulatory guidelines for the Gulf of Mexico published in 2017–2018 provide design guidelines for depth-adjusting subsea equipment specific to its working seawater depths, and detailed verification and validation guidelines for designing HP/HT equipment—that is, with absolute internal pressure rating exceeding 15,000 psi and temperatures exceeding 350°F. These guidelines can be used to develop subsea equipment rated to working pressures greater than 15,000 psi. The complete paper details the step-by-step methodology of combining the API 17TR12 and API 17TR8 design verification guidelines to depth-adjust typical existing 15,000-psi-rated subsea equipment for HP/HT use.

Validation tests for external pressures for entire equipment assemblies can quickly become impractical and infeasible for most of the equipment covered in API SC 17 because of the sizes and complexity of the equipment. The authors believe such testing is not entirely necessary because testing for external pressure of critical and representative components, which are pressure-containing and -controlling, can be sufficient to validate the applicability of the external pressures for the equipment. Furthermore, they contend that for larger components for which hyperbaric chamber testing might not be feasible, comprehensive FEA can be used to validate stresses and deflections as long as the FEA methods and results are appropriately validated. The complete paper presents the various tests that were performed covering typical subsea equipment and are deemed sufficient to validate the verification methods used to depth-adjust using external pressures.

The actual tests performed were part of an HP/HT development for the completion and workover riser (CWOR) system for a potential application that required depth-adjusted working pressure higher than 15,000 psi. The work performed was to validate the design and verification of the equipment manufactured by the services provider and cannot be applied to other manufacturers or engineering houses.

Validation Methods

API 17TR8 introduces the terminology of PR3 and PR4 levels of testing that are specific to HP/HT equipment. API 17TR12 recommends some additional testing to cover the effects of external pressures on the equipment. However, validation of these external pressure effects on all pressure-containing blocks of the equipment can quickly become infeasible if they are tested in hyperbaric chambers for external pressure validations. Additionally, these load cases might not even be the worst load cases when compared with absolute pressure load cases. The complete paper details the minimum additional, but practical, validation tests that can be conducted to satisfy API requirements. Three primary validation tests are presented—one to validate the pressure-containing equipment, one to validate the pressure-controlling equipment, and one to validate the equipment subjected to trapped-air voids.

To validate the pressure-containing equipment, a 20,000-psi valve block was pressure-tested to internal pressure up to 25,000 psi, with application of 5,000-psi external pressure simulating 10,000-ft applications. The valve block was strain-gauged at multiple locations including the body and bolts. The strains predicted using FEA methods were then compared with the strains evaluated from the tests.

For the pressure-controlling equipment, a 15,000-psi valve was tested to 17,000-psi upstream pressure and 2,000-psi downstream pressure across the gate of the valve assembly, with 2,000-psi external pressure, for various operational load cases to monitor the effects on performance of the gate valve and the actuator mechanism. The final validation test was performed for stem seals of the gate valve assembly, which are exposed to trapped-air voids. These are tested separately to their absolute working pressures higher than 15,000 psi per the API 6A Annex F test regime.

The tests for the pressure-containing equipment showed that the ­actual strains in the valve block and bolts correlated well with the FEA. For the pressure-controlling equipment, various upstream and downstream pressure combinations and functions were tested and showed that the effect is minimal on the actual performance on the gate valve and the actuator, and that the ­pressure-controlling equipment can handle the various expected differential pressure load cases. The stem seal test increased their absolute working pressure rating. These types of tests are guidelines for what the typical subsea equipment manufacturers may use to validate their equipment with similar design considerations.

The complete paper presents the various practical tests that can be performed to validate the verification analysis using the external pressures resulting from seawater hydrostatic head.

Testing for pressure-containing components was conducted in a hyperbaric chamber on a representative gate valve block that shared similar geometry and stress profiles with the pressure valve blocks in a specific CWOR system (Fig. 1). Simultaneous internal and external pressures were applied to cover all applicable load cases.

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Fig. 1—Sealed hyperbaric chamber.

 

Testing for pressure-controlling components focused on validating unchanged operational performance of these components after an upstream/downstream pressure differential equal to rated working pressure, with an upstream pressure of at least depth-adjusted working pressure, was applied across the closed gate. These test pressures were seen in combination with external-pressure matching levels experienced at the specified working depth. Because the tests performed on components exposed to trapped-air voids do not consider effects of external pressure, they are not described in the paper.

Conclusions

  • Using a seawater hydrostatic head enables 15,000-psi-rated subsea equipment validation to higher-than-15,000-psi applications without the additional costs related to developing new 20,000-psi-rated equipment while validating the FEA methods recommended in API 17TR12.
  • Confirming FEA predictions allows using FEA solely to confirm acceptability for big pressure valve blocks that are infeasible for hyperbaric-chamber tests.
  • Both FEA verification and a series of tests with upstream and downstream pressure differentials and blowdown cycles on a representative valve assembly proves unchanged performance for additional load cases on pressure-controlling equipment. Observing the correlation can avoid similar testing on remaining valve assemblies of the same family.

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper OTC 29413, “Validation of Cost-Effective Design Methods Using Hydrostatic Head for High-Pressure/High-Temperature Applications,” by Parth Pathak and Nicholas Katsounas, OneSubsea, prepared for the 2019 Offshore Technology Conference, Houston, 6–9 May. The paper has not been peer reviewed. Copyright 2019 Offshore Technology Conference. Reproduced by permission.