DESIGN BASED ON DUCTILE-BRITTLE TRANSITION TEMPERATURE FOR API 5L X65 STEEL USED FOR DENSE CO2 TRANSPORT G Pluvinage*1,J. Capelle2 and Z. Azari 1.Fiabilité Mécanique. Conseils Silly sur Nied France 2. LABPS. ENIM Metz France * Corresponding author : pluvinage@cegetel.net Abstract. Safe and reliable transport of dense CO2 by pipes needs a careful choice of the constitutive pipe materials to prevent brittle crack propagation after ductile or brittle failure initiation. This unexpected phenomenon can occur after failure or leak promoted by external interferences. In this case, the rapid decompression of dense CO2 into gas leads to a very low local temperature of about -80°C. To prevent risk of brittle fracture initiation and propagation, the material must remain ductile at this temperature. In other terms, its ductile-brittle transition temperature (DBTT) has to be lower than -80°C minus a margin. It is admitted that the DBTT is not a material characteristic but depends on specimen geometry, loading rate and loading mode, i.e. on constraints. A loss in constraint leads to a lower brittle-ductile transition temperature. In order to select a steel for transportation of dense CO2, transition temperatures Tt (from tensile test), TK27 and TK50 (from Charpy test) and TK100 (from fracture mechanics test) have been determined on an API 5L X65 pipeline steel. These transition temperatures have been reported versus a constraint parameter, e.g. T-stress, in a master curve. Differences between different brittle-ductile transition temperatures and temperature corresponding to T-stress acting in a pipe submitted to internal pressure on the master curve, give an estimation of the conservatism of the chosen reference transition temperature. Key words: transition temperature, pipe steel API 5L X65, constraint, T-stress, CO2 transportation 1. INTRODUCTION According to temperature and pressure, CO2 is present in 3 distinct states. CO2 is in a supercritical phase with temperatures higher than 31.1°C and pressures higher than 7.38 MPa (values of the critical point). For conditions of temperature and pressure lower than these values, CO2 will be in a gas, liquid or solid state. Beyond its critical point, carbon dioxide enters a phase called supercritical. Dense CO2 transport is mainly performed by pipeline. Only in the United States, the existing national CO2 pipeline infrastructure dedicated primarily to deliver CO2 for enhanced oil recovery (EOR) comprises 3900 miles, and an extended national CO2 pipeline system is forecasted with the implementation of carbon dioxide capture and storage (CCS)-derived emission reductions. The entire system could be comprised between 11,000 and 23,000 additional miles dedicated CO2 pipeline before 2050 and dependent upon the hypothetical climate stabilization policies adopted [1]. Transport of CO2 in dense state presents a high potential for auto-refrigeration due to depressurisation, either during operations or due to equipment failure.
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