In most jurisdictions, one of the key elements in the design and operation of transportation pipelines carrying flammable or toxic gases or hazardous liquids is fracture control. CO2 is explicitly categorized in many national standards and regulations as a fluid that requires measures to be taken to control fracture initiation and propagation. Materials for use in CO2 pipelines can be specified to prevent fracture initiation and brittle fracture propagation in the same way as for gas pipelines, as is discussed in Chapter 7. It is the control of ductile fracture propagation that presents the greatest challenge. Because compressible fluids like CO2 do not expand to atmospheric pressure quasi-instantaneously, as conventional liquids do, a high pressure can be maintained in the plane of a propagating ductile fracture. In order to specify materials that can prevent long ductile fracture propagation, it is thus necessary to understand the decompression behavior of the expanding fluid, so that the local pressure that is driving the fracture can be calculated. Supercritical CO2 is known to be particularly challenging from the point of view of fracture control, since its decompression characteristics are such that a high pressure can be maintained at the crack front for a long time.

The determination of the decompression behavior requires accurate knowledge of the thermodynamic characteristics of the fluid (particularly the speed of sound and the phase boundary). The thermodynamic characteristics of pure CO2 have been well understood for many years, but systematic study of impure CO2 began more recently, as a result of growing interest in transportation from anthropogenic sources for the purpose of carbon capture and storage (CCS). Quantitative and validated understanding is still far from complete. However, the methods that need to be applied to determine decompression wave speed are well understood. This chapter will describe the background to the analysis methods and illustrate their use by examining the decompression behavior predicted by two different, widely used equations of state (EOS) for binary mixtures of CO2 with a range of impurity species that might be present in carbon capture streams resulting from the use of different capture technologies. For each of the cases examined, the important aspects of the predicted decompression wave characteristics are reviewed, in particular initial speed of sound and saturation pressure (the pressure corresponding to phase boundary crossing during decompression). These are particularly significant in the determination of required material characteristics for fracture arrest.