Modelling brittle fracture propagation in the next generation CO2 pipelines

• Main Author: Zhang, P.
• Published: University College London (University of London) 2014
• Subjects: 660
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.631825

The development and testing of a fluid-structure interaction model for simulating the transition of an initial through-wall defect in pressurised CO2 transmission pipelines employed as part of the carbon capture and storage chain into running brittle fractures is presented. The model accounts for all the important processes governing the fracture propagation process including the fluid/wall heat transfer, the resulting localised pressure stresses in the pipe wall as well as the initial defect geometry. Real fluid behaviour is considered using the modified Peng Robinson equation of state. Hypothetical but nevertheless realistic failure scenarios involving the transportation of gas and dense phase CO2 using existing natural gas steel pipelines are simulated using the model. The impacts of the pipe wall thickness, Ductile-Brittle-Transition Temperature (DBTT), initial defect geometry, feed temperature, stream impurities, surrounding backfill as well as flow isolation on brittle fracture propagation behaviour are investigated. In all circumstances, the initial defect geometry in the pipeline is shown to have a major impact on the pipeline’s propensity to brittle fracture propagation. For example, in the case of an initial through-wall defect in the form of a circular puncture where there is no stress concentration, fracture propagation is highly unlikely. The opposite applies to an elliptical through-wall defect embodying a hairline crack extending from its side. Furthermore, gas-phase CO2 pipelines are more prone to brittle fracture failures as compared to dense-phase CO2 pipelines despite the higher starting pressure. This is due to the higher degree of expansion-induced cooling for gaseous CO2. The emergency isolation of the initial flow in the pipeline following the formation of the initial defect promotes brittle fracture. For the ranges tested, typical CO2 stream impurities are shown to have negligible impact on brittle fracture behaviour. Puncture in a buried pipeline where there is no blowout of the surrounding soil is more likely to lead to brittle facture propagation as compared to that for an exposed pipeline. This is due to the secondary cooling of the pipe wall by the surrounding soil cooled by the escaping gas.