Validation testing of the subsea equipment designed for HPHT applications using external pressures due to the hydrostatic head can be a challenge. This paper presents the tests performed to validate the design methods proposed in OTC-27891-MS. Use of a seawater hydrostatic head enables 15,000-psi-rated subsea equipment to higher than 15,000-psi applications without the additional costs related to developing new 20,000-psi-rated equipment. The design methods utilize the guidelines in technical report API Technical Report 17TR8 and load cases per API Technical Report 17TR12.
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 the bolts. The strains predicted using the finite element analysis (FEA) methods are then compared to 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 which 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 provide good guidelines on what the typical subsea equipment manufacturers can perform to validate their equipment with similar design considerations.
The paper presents the various practical tests that can be performed to validate the verification analysis utilizing the external pressures due to seawater hydrostatic head. Validation is a necessary part of the design process and can be extremely expensive and nonfeasible for subsea equipment. This paper presents a practical approach for validating the design verification analysis for subsea equipment.