Improved Life Prediction Techniques for Offshore Marine .Elastomeric Sealing Elements

Abstract

ABSTRACT Predicting the anticipated life of static elastomeric sealing elements used in subsea drilling and production equipment is critical to the economic success of these projects. This paper reviews a technique of verifying the long-term performance of elastomers used for critical static sealing elements. The method is based on the Arrhenius aging technique used in the nuclear and other industries to predict long-term material life using short-term testing. INTRODUCTION Elastomeric or rubber sealing elements have been used extensively in the energy sector to provide gas and liquid tight seals between mating metal parts. Recent developments have exposed elastomeric sealing elements used in subsea drilling and production to severe thermal and chemical environments. The performance of some of these seals has been less than satisfactory. This has led to the use of metal to metal sealing elements which tend to be costly and susceptible to installation damage. Elastomeric sealing elements tend to be less costly than metal to metal sealing elements and less susceptible to installation damage. Traditional methods of evaluating compatibility and selecting elastomers for sealing elements do not address the long-term effect of the chemical and thermal environment encountered. For this reason, many short-term failures have been experienced, even when using exotic elastomer sealing elements in severe environments. Traditional elastomeric material evaluation for typical oil field service involves free state immersions testing of samples in the specified or simulated liquid or gas environment for a short period of time (1, 3, 7, 14 days or longer). Applicable ASTM specifications for immersion testing are shown in reference 1. After immersion, the samples (ASTM dumbbell specimens or O-rings) are subjected to stress/strain (tensile) testing, and measurements of volumetric swell, weight changes, hardness changes. In free state immersion testing, 100% of the sample surface area is exposed to the environment. None of the data obtained in the above tests directly relates to an elastomer's ability to function as a sealing element in the specified chemical environment. The testing does not relate to long-term changes in the elastomer sealing characteristic caused by the chemical environment. Life prediction testing, based on the Arrhenius aging technique, tests a sealing element under the influence of the intended chemical environment and uses temperature to accelerate the thermo-chemical degradation of the elastomer. The Arrhenius aging technique is based on the observation that for every 10°C (18°F) increase in temperature the first order chemical reaction rate doubles. The converse is also true. Therefore, short-term testing (in the order of weeks or months) of an elastomeric sealing element at elevated temperature in a specific chemical environment can be used to approximate longterm effects at lower temperatures. References 2, 3, and 4 offer more detailed discussion of the theory and practices of life prediction testing. The testing described in this paper was conducted to verify the performance potential of elastomeric sealing elements planned for use in a North Sea gas injection project.