Selection of Materials for High Pressure Environments using Accelerated Life Test Methods

Abstract

Abstract Selection of material for subsea environments is a key part of subsea equipmentdesign. The emerging technologies for HPHT environments require moresophisticated tools for material selection than the traditional qualificationtesting based on safety factors. Accelerated life testing (ALT) is an effective, data driven method to aid inthe selection of materials for such ex-treme environments. By testing atmultiple stress levels and extrapolating the results back to field usageconditions, a time to failure at field conditions can be calculated. Established methods, such as Arrhenius, are generally accepted for hightemper-ature conditions. No such methodology or model exists for subseapressure. This paper addresses the development of such a model by testingmaterials at multiple high pressure levels (10 ksi to 30 ksi) to makepredictions at a lower use level. The accelerated life testing was conducted to evaluate two differentengineering plastics, by testing five different material properties- physicaldimensional change, tensile strength, compressive strength, hardness, andvolumetric resistivity. The data was used to develop and validate anacceleration life prediction model that can be used for material selection incritical high pressure subsea applications. The validation process involvedcomparison with data from component level tests (made from engineering plasticsevaluated) and comparison to field data. This research has, for the first time, provided a model for the pressure-liferelationship used in acceleration models for life testing. The results of theresearch will be useful to any engineering/technology organization attemptingto perform accelerated life testing to select materials for high pressureapplications. The methodology can be used in conjunction with the provenArrhenius methodology to simulate real world HPHT applications. Introduction Materials play a key role in the design of any product or equipment. Selectionof material is one of the primary responsibilities of the mechanical designerand ensures that the product functions as intended for the prescribed designlifetime. Sucess in designing reliable products, while averting long-termfailures, can only be achieved by understanding and evaluating (throughtesting) the potential long term failure modes. To be effective in preventingfailure, it is essential for a designer to have a good working knowledge ofanalytical and/or empirical techniques for predicting failure so that failureduring prescribed design life may be prevented. In order to simulate potentialfailure modes, one must be acquainted with an array of failure modes observedin the field and with the conditions leading to these failures. Theintroduction of new materials and the need for more severe operating conditionsand longer life all serve to demand better material selection techniques [1]. Severe service conditions have required that we study the behavior of materialsmore carefully, to better assess the nature of actual service conditions and tobetter understand all possible modes of long term material failure. Opposingrequirements of longer life time and greater reliability add to this challenge. Accelerated life testing (ALT) is one such technique in predicting failure andis very useful in failure analysis, prediction, and prevention. It must berecognized also that techniques such as ALT must be improved and customized tospecific applications and operating conditions in order to accurately predictand prevent failures.