P. F. Schoeffl, R.W. Lang
Papers # 2014 Chicago
To accommodate the recent industrial interests to utilize PE-HD pipes for oil field applications (e.g. injection water pipes, flow lines), this paper introduces a “fully integrated test concept” to allow a comprehensive pipe lifetime prediction under severe environmental conditions. Moreover, experimental set-ups and test procedures were designed and implemented to characterize material behavior under superimposed mechanical-environmental loading at the specimen level (5A and CRB) and at the component level (pipe testing) covering failure analysis in all three of the main failure regimes (Region I to Region III) [1-3] in order to allow for a 25 year lifetime prediction of PE pipes in oilfield service.
PE 100 and PEXc pipe materials were selected as model materials. Mechanical-environmental tests were conducted at two test temperatures (35 °C and 60 °C) in liquid hydrocarbons (LHC; 90/10 w-% i-octane/ toluene) and in deionized water as a reference environment. Pipe testing under liquid hydrocarbon pressurization (including results of 10 000 hours test duration) revealed a reduction in long-term hydrostatic strength (LTHS) by approx. 50 % compared to water pressurization. With respect to changes in slow crack growth (SCG) behavior, cyclic tests with cracked round bar specimen (CRB; conducted for more than 10 million failure cycles under full media immersion) revealed, that at low stress intensity factors (SIF) the LHC saturated samples indicate a superior SCG resistance compared to samples tested in water. Moreover, the observed failure mode of LHC saturated CRB specimens was designated as cyclic shear yielding, in contrary to CRB tests in water, where crack-tip crazing was observed as the dominant failure mechanism. To account for material aging and degradation, long-term exposure tests up to two years were conducted and subsequently the oxidation onset temperature (Tox) was determined and supplemented by size exclusion chromatography (SEC) results.
