Carbon dioxide (CO2) pipelines are more susceptible to long running fractures than hydrocarbon gas pipelines because of the decompression characteristics of CO2. The key to understanding this issue is the phase diagram and the liquid-vapour phase boundary. GASDECOM — based on the BWRS equation of state — is a program widely used for calculating the decompression behaviour of mixtures of hydrocarbons. The calculated decompression wave velocity curve is then used in models such as the Battelle Two Curve Model to determine the toughness required to arrest a propagating ductile fracture. GASDECOM is capable of modelling mixtures of hydrocarbons (methane through to hexane), nitrogen and carbon dioxide. It therefore can (and has) been used to investigate the effect of methane and nitrogen on the decompression characteristics of CO2. Pipelines can be expected to play a significant role in the transportation infrastructure required for the successful implementation of carbon capture and storage (CCS). The composition of the carbon dioxide rich stream to be transported in a pipeline depends on the capture technology, e.g. post-combustion, pre-combustion and oxy-fuel. Post-combustion tends to result in an almost pure stream. The other capture technologies produce a less pure stream, containing potentially significant proportions of other components such as hydrogen, nitrogen, oxygen, argon and methane. One of the factors that will constrain the design and operation of a carbon dioxide pipeline is the effect of these other components on the decompression characteristics, and hence the arrest toughness (amongst other issues). Components such as hydrogen, oxygen and argon cannot currently be considered using GASDECOM. Through a study of the underlying algorithms implemented in GASDECOM, it is shown how GASDECOM can be modified to include these additional components relevant to carbon capture and storage. The effect of impurities such as hydrogen on the decompression characteristics is then illustrated, and related back to their effect on the phase diagram and the liquid-vapour phase boundary. The sensitivity of the results to the use of equations of state other than BWRS is also illustrated. Simplifications that follow from the decompression behaviour of carbon dioxide are also highlighted. Finally, the small and large scale experimental studies that are required to validate predictions of the decompression behaviour and the arrest toughness are discussed.

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