High-performance materials support optimal metal end cap seals
Wellhead and tubing hanger sealing materials are subjected to an array of challenges, from high temperatures to corrosive fluids, and especially, high pressures. That’s one reason metal end cap (MEC) seals traditionally have been designed by wellhead manufacturers and produced to specification by sealing experts.
It seems logical that the most robust, capable MEC seals would be created by those with the most sealing experience in a wide variety of situations. A new MEC design combines high-performance materials with a state-of-the-art design, creating an optimal sealing solution.
The key to a superior MEC seal begins with a high-performance elastomer that is chemically bonded to metal C-rings. The elastomer must not only meet industry standards that include ISO 23936 (formerly NORSOK) and ISO 10423 (formerly API-6A), but also provide a high level of resistance to extrusion, explosive decompression, and downhole fluids. Additionally, it must simultaneously withstand temperatures up to 350°F and pressures up to 15,000 psi.
One key failure mode is elastomer destruction by rapid gas decompression (RGD). Elastomers used in wellheads and various tubing sections are tested by being subjected to high-pressure gas molecules, such as carbon dioxide, being forced into them. The molecules then force their way out of the elastomer matrix when the pressure is released, potentially causing blisters and cracks within the elastomer that can propagate to the elastomer exterior. Thus, MEC seals should be manufactured from an elastomer that resists the majority of gas molecules trying to enter into its matrix.
Most industry RGD specifications assign subjective grades of zero to five to elastomers. The typical test specimen for an RGD test is an O-ring. After an RGD test, the O-ring specimen is usually cut into four equal pieces so the interior can be visually examined along with the exterior.
Zero is the best rating and means no blemishes or imperfections are noticed within or on the elastomer surface. Grade one means no more than four internal cracks are seen, with each shorter than 50% of the O-ring cross section. Two means less than six internal cracks are seen, each less than 50% of the O-ring cross section, and three means less than nine internal cracks, with two of the cracks allowed up to 80% of the O-ring cross section. Grades of four and five are typically considered unacceptable and based on cracks extending to the O-ring exterior and the O-ring being split or fragmented.
In addition to receiving a preferred grade of zero or one during the RGD test, the elastomer in an MEC seal must be highly resistant to downhole fluids such as hydrogen sulfide, drilling fluids, completion fluids, and the crude oil itself.
Industry specifications, such as ISO 23936 and ISO 10423, also dictate specific chemical formulations that a seal material must survive in an immersion test. The seal material specimen is immersed in the chemical formulation for specified time periods, temperatures, and pressures. Once removed from the immersion test, the specimens are tested, and results compared to industry-specified failure criteria. Any specimens exhibiting properties within the failure criteria are considered compliant, and those outside it are unacceptable.
It should be noted that even though a seal material is demonstrated to be compliant to an industry specification, it does not mean the seal material is wholly acceptable in an application. Additional testing to specific application requirements is needed.
Finite element analysis
Once MEC elastomer materials are tested to industry specifications and shown to be compliant, design work can begin on the elastomer and metal end caps. MEC seals are large diameter, and testing in lab fixtures is complex and expensive.
Finite element analysis (FEA) can therefore support product proving. MEC seal designs can be virtually modeled using FEA under a myriad of conditions to predict seal behavior before spending significant resources on lab fixtures. The FEA results guide engineers to optimize the hardware and seal design as a system, to mitigate potential failure modes. The FEA results also can graphically demonstrate what forces and stress the seal encounters under arduous conditions. Having this knowledge allows engineers to have the highest confidence possible before spending resources on lab testing.
At this point it should be eminently clear to the oil & gas industry that equipment and seal manufacturers have collaborated to analyze and engineer the best MEC product possible. MEC seals are expected to have a multi-year life span, and proper functioning is critical to ensure well integrity and overall safety.
Eric Bucci is oil & gas segment manager, Trelleborg Sealing Solutions