The NORSOK M-710 specification underlines the need for oilfield equipment OEMs to achieve higher levels of quality and compatibility for non-metallic seals used in oilfield equipment operating anywhere in the world. What started with mainly Norwegian or European requests for NORSOK M-710 approved materials has now extended across the globe, making NORSOK into an internationally recognized standard.
NORSOK M-710 – Qualification of non-metallic sealing materials and manufacturers (Rev. 2, Oct. 2001) – requires that all sub- components of oilfield equipment must be approved to stated specifications. Specifically, individual seal materials are rigorously tested and approved based on numerous criteria such as Rapid Gas Decompression (RGD), sour and sweet gas aging, compression set tests, and material property tests.
The testing is done to ensure adequate safety, value adding and cost effectiveness for existing and future petroleum industry developments. The standard defines the requirements for critical non-metallic (polymer) sealing, seat and back up materials for permanent use subsea, including well completion, trees, control systems, wellheads and valves.
The standard also applies to topside valves in critical gas systems. One of the main differences from previous approvals is that all individual components within an assembly must meet and be approved to NORSOK standards, rather than just the complete assembly.
What is Rapid Gas Decompression (RGD)?
Gas or gas mixtures in contact with elastomer surfaces can be absorbed under pressure. At high pressure absorbed gas is in a compressed state. When external pressure is reduced, either rapidly or over a relatively short period of time, the compressed gas rapidly expands within the elastomer. This can lead to crack initiation within the body of the elastomer. These cracks propagate internally and can cause void formation or cracks to appear on the elastomer surface that significantly reduce seal integrity.
Rapid Gas Decompression (RGD) Testing
Three variables are tested: medium, temperature and pressure. Three media options are available (see Table 1); the media selection being dependent on proposed well conditions. Temperatures can be tested at +212°F, +302°F or +392°F / +100°C, +150°C or +200°C.
The material is heated to this temperature progressively and held for 10 minutes. After this, pressures of 2,175 psi, 2,900 psi or 4,351 psi / 15 MPa, 20 MPa, 30 MPa are applied. Exposure to these conditions must be for at least 72 hours. The sample is then depressurized at a rate of 290-580 psi/min / 2-4 MPa/min, and cooling is only allowed following this. 10 cycles of pressurization and depressurization are applied to the sample, holding pressure for 23-24 hours per cycle.
The sample is then analyzed by microscopy and a rating is applied, summarizing the observed damage to the test sample in terms of number and length of internal cracks visible when the seal is cut into quarters.