Experimental Evaluation of the Metal-to-Metal Seal Design for High-Pressure Flexible Pipes

Abstract

Abstract Development of unbonded flexible pipes for high pressure applications posesseveral challenges, where the design of end fittings with effective barrierseal systems is a key issue. To qualify, the end fitting seal system mustoutperform the strength of the pipe in destructive tests up to and includingburst. For prospective designs, this may involve bore pressures greater than30000 psi (2270 bar). The chosen seal design must also allow for deformationsof the polymeric and metallic components, and should maintain sufficientcontact pressure at the seal interfaces during short-term ultra high pressuresapplied during qualification/factory acceptance tests, and the long-termrelatively low pressure fluctuations exerted in the service regime. In order to maintain an effective seal, the contact pressure at the sealinterface must be kept sufficiently higher than that of the liquid/gas pressureinside the pipe. Conventional metal-to-metal seals used in end fittings areconsidered to be pressure energized with sufficient initial sealing actioncreated during the manufacturing of the pipe. The performance of metal-to metalseals is commonly evaluated and optimised using finite element analyses (FEA), though the results of such analyses are not thoroughly evaluated using theactual measurements of contact pressure. Without such measurements, it isimpossible to gauge the accuracy of FE predictions, nor evaluate theperformance of seals without full-scale tests. The work presented in this paper describes an ultrasonic technique, developedto measure contact pressures at the metal-to metal interface of the end fittingbarrier seal. Two tests have been performed using a full-scale end fittingassembly and the contact pressure measured using three different ultrasonicprobes, during the swaging of the seal ring. The results of the ultrasonicmeasurements are compared with FE predictions. The results clearly show thatthe externally-placed ultrasonic probes are capable of measuring contactpressure at the metal-to-metal interface of the end fitting barrier seal. Asimilar trend in contact pressure behaviour is shown by both the FEA andmeasurements. Introduction Global growth in oil and gas exploration in deep water demands the developmentof long unbonded flexible pipes that can sustain high internal pressures in therange of 30000 psi (2270 bar). Due to the bulky nature of these pipes, and withadditional constraints in reeling and transportation, it is necessary tomanufacture the pipes in manageable lengths, and several pipes that areconnected via end fittings, is the common way of constructing the completeservice pipe structure. One of the key requirements in the design of the endfitting is to achieve an effective seal on the polymeric barrier which isdesigned to contain the transporting medium. The barrier needs to be sealedagainst the end fitting body, which is commonly made of steel and isconventionally achieved by swaging a specially designed metallic seal ringbetween the two components. The consequent stiff contacts created by the sealring with the end fitting body and the polymer barrier makes these twointerfaces impenetrable to liquid or gas. However, achieving sufficient contactpressure at the metal-to-metal interface, between the seal ring and the endfitting body, is critical to ensure that an efficient seal is achieved and willperform effectively at the required pressure range.