ABSTRACT

Various oil companies and national standards organizations have developed test protocols aimed at defining elastomeric seal materials that can avoid damage during decompression in high pressure gas duty. This paper reviews four such protocols and discusses their relevance to practical seal selection in this type of aggressive oilfield service. The comparison has shown that each of the standards has strengths and weaknesses, and that in selecting a test standard for a particular application, the end-user must aim to get as close to the final working conditions as possible within the test method. Test environment, housing geometry, seal geometry and cyclic conditions must all be replicated.

Finite element analysis potentially provides a tool for reducing the quantity of costly and time-consuming performance testing which currently has to be carried out to prove seal integrity.

INTRODUCTION

'Explosive decompression' is a commonly used term for the damage caused when a rubber seal, containing absorbed gas, is subjected to rapid decompression from a high pressure. The failure of a seal due to explosive decompression damage can lead to obvious safety and lost production consequences.

The mechanism of damage during decompression is well understood, at least qualitatively. Upon depressurization the gas will attempt to diffuse out of the material. If the rate of depressurization is too fast then it is envisaged that failure may initiate through accumulation of gas at discontinuities that pre-exist within the elastomer as voids, e.g. entrapped air or curing by-products, or rigid inclusions, e.g. dirt or dust.

There are several industry and customer specific test procedures in existence, which are used for validating elastomeric and polymeric seals for explosive decompression service. It is generally understood that technically these procedures often differ from one another in a number of aspects and, hence, the suitability of a particular material proven under one test regime may not automatically guarantee its suitability if tested under another. The ability of any of these tests to predict field performance is also open to question.

Four test specifications have been studied for this paper. Table 1 summarizes the test conditions specified in these standards, which are:

i. NACE TMO192-98 "Evaluating Elastomeric Materials in Carbon Dioxide Decompression

Environments" ~)"

ii. NORSOK M-CR-710 Rev. 1 1994 "Qualification of Non-metallic Sealing Materials and

Manufacturers" (2).

iii. SHELL test procedure, as described by Cox ~3), and referred to herein as Operator Procedure 1;

iv. TOTALFINA SP-TCS-142 Appendix H "Elastomer "O"-Ring Seals Explosion Decompression Type

Testing Procedure ~4~, and referred to herein as Operator Procedure 2;

TEST COMPARISONS

In order to highlight the differences in the four test standards, finite element analysis (FEA) has been carried out on four materials for each of the standards:

Material A - Hydrogenated Nitrile Rubber (HNBR) 90 IRHD, 36% acrylonitrile

Material B - Fluorocarbon (FKM) 90 IRHD (supplier 1)

Material C - Fluorocarbon (FKM) 90 IRHD (supplier 2)

Material D- Nitrile (NBR) 70 1RHD

Materials A, B and C are marketed as decompression resistant grades, while material D is a standard nitrile-based elastomer, known to fail readily under explosive decompression. The FEA model, which has been developed in the course of a joint industry project (JIP), has been shown to be over 80% accurate, on predicting the occurrence of cracks when compared with actual test data for the four materials. The model is applied in two stages. Firstly, stres

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