Fracture Control in Carbon Dioxide Pipelines: The Effect of Impurities

Abstract

The fourth report from the Intergovernmental Panel on Climate Change states that “Warming of the climate system is unequivocal...” It further states that there is a “very high confidence that the global average net effect of human activities since 1750 has been one of warming.” One of the proposed technologies that may play a role in the transition to a low-carbon economy is carbon dioxide capture and storage (CCS). The widespread adoption of CCS will require the transportation of the CO2 from where it is captured to where it is to be stored. Pipelines can be expected to play a significant role in the required transportation infrastructure. The transportation of CO2 by long-distance transmission pipeline is established technology; there are examples of CO2 pipelines in USA, Europe and Africa. The required infrastructure for CCS may involve new pipelines and/or the change-of-use of existing pipelines from their current service to CO2 service. Fracture control is concerned with designing a pipeline with a high tolerance to defects introduced during manufacturing, construction and service; and preventing, or minimising the length of, long running fractures. The decompression characteristics of CO2 mean that CO2 pipelines may be more susceptible to long running fractures than hydrocarbon gas pipelines. Long running fractures in CO2 pipelines may be preventable by specifying a line pipe steel toughness that ensures that the ‘arrest pressure’ is greater than the ‘saturation pressure’ or by using mechanical crack arrestors. The preferred choice is control through steel toughness because it assures shorter fracture lengths. The ‘saturation pressure’ depends upon the operating temperature and pressure, and the composition of the fluid. ‘Captured’ CO2 may contain different types or proportions of impurities to ‘reservoir’ CO2. Impurities, such as hydrogen or methane, have a significant effect on the decompression characteristics of CO2, increasing the ‘saturation pressure’. The implication is that the presence of impurities means that a higher toughness is required for fracture arrest compared to that for pure CO2. The effect of impurities on the decompression characteristics of CO2 are investigated through the use of the BWRS equation of state. The results are compared with experimental data in the published literature. The implications for the development of a CCS transportation infrastructure are discussed.