The Accurate Measurement of Elevated Pressure Gas Permeation through Polymers Based on New Specimen Geometries

Abstract

Knowledge of high pressure (HP) gas permeation rates is essential for designing thermoplastic-based flexible pipes for offshore oil and gas production. An effect which masks the permeation flow associated with normal HP gas permeation test procedures used for testing engineering thermoplastics at MERL and elsewhere has been illustrated herein by laboratory testing at medium pressures. HP permeation tests conventionally use porous steel sinters as supports for test samples, which are now seen to reduce measured rates; hence calculated associated permeation coefficients are diminished correspondingly. These coefficients are used for prediction purposes of permeation rates at other, service-related, conditions, which in turn will be affected by the error arising from sinters during testing. It has been possible to make these observations because situations have been identified where testing of unsupported samples, allowing free escape of permeated gas, could be carried out at medium pressures (∼70 bar). However, another factor also applies; normal thermoplastic samples cut from the walls of flexible pipe lining materials and used for HP permeation work, whether flat or “curvidisc” samples, are invariably sealed with O-rings etc in such a way that edge regions exist – and these will absorb some of the permeating species, thus again affecting measured rates. In quantifying such effects, some finite element analysis modelling has been used to provide valuable information, highlighting deficiencies in the current permeation test procedure, to suggest tentatively the degree of correction required. In summary, for the systems measured (involving plasticised PVDF (pPVDF) and methane at 70 bar), reductions in permeation rate brought about by the sinter support during laboratory testing mean that the permeation coefficient obtained by the normal means of testing and calculation should in reality be increased by a factor of 1.8. Moreover, even for unsupported pPVDF permeation tests, the coefficient should also be increased by another ca two thirds, to allow for a combination of outer edge sample effects and the need to reach absolute equilibrium. Hence the overall correction factor for the coefficient applying when using sinters is apparently nearer three. In a validation exercise which also leads to an improved test procedure, initial work was conducted with a novel sample based on a conical section, an arrangement needing no sinter support and possessing no edge regions; a test at medium pressure shows directly an increase in the coefficient obtained of more than two compared with testing an unsupported curvidisc possessing edge regions. However, considerable sealing difficulties exist; when solved, this geometry has the potential of operating at genuine high pressures, to lead to near-absolute measurements of HP permeation coefficients. In applying a similar approach to the performance of flexible pipes in service – which is much associated with HP gas permeation – a small simulation of armour windings of the type which support their thermoplastic liners has been found also to reduce rates, for analogous reasons. Here, the effect is beneicial to pipe users, as support windings reduce the unsupported permeation rate by 70%. Additionally, an equation giving a single permeation coefficient for a sample (or component) having a temperature gradient occurring across it has been developed and validated by laboratory testing.