Modelling of gas decompression process for CO2 transmission pipeline

The decompression behaviour of CO2 pipelines must be determined accurately in order to estimate the proper pipe toughness for fracture arrest. Anthropogenic CO2 may contain impurities that can modify the fluid decompression characteristics quite significantly. In this thesis, a simulation study of the decompression behaviour of CO2 based mixtures is presented. The current research is aimed at developing a new multi-dimensional gas decompression model using the Computational Fluid Dynamics (CFD) software ANSYS Fluent. The thermodynamic properties of CO2 mixtures must be determined by using an accurate equation of state (EOS). A comparative study between some of widely used equations of state for gas pipelines is conducted. The wide range GERG-2008 EOS is accordingly adopted to provide the thermodynamic properties of CO2 mixtures. Factors that affect the behaviour of decompression wave speed and the arrest toughness such as operating conditions, fluid compositions and the actual pipe deformation are investigated. The simulation results are validated against measured data obtained from ‘shock tube’ and ‘full-scale burst’ tests. The thesis presents a novel technique to implement a real-gas EOS into ANSYS Fluent. The technique can be used to implement any future developed EOS. For the purpose of validation, the model is employed to simulate the decompression of single-phase flow and two-phase flow for some fluids in 2D geometry. The results prove the capability of the model in dealing with different gas mixtures and multiphase flow. The influence of wall boundary on the behaviour of some properties and its impact on the local and average decompression wave speed is discussed. It was found that the local decompression wave speed will always differ from that obtained from pressure-time traces especially at the later stages of the decompression. This difference could be neglected as far as fracture propagation control is concerned, but for longer pipelines and smaller diameters pipelines it may become influential