CFD modelling of outflow and ductile fracture propagation in pressurised pipelines.
Doctoral thesis, UCL (University College London).
This thesis describes the fundamental extension, development and testing of a mathematical model for predicting the transient outflow following the failure of pressurised pipelines. The above encompasses improvements to the theoretical basis and numerical stability, reduction in the computational runtime and the modelling of fracture propagation with particular reference to CO2 pipelines. The basic model utilises the homogeneous equilibrium model (HEM), where the constituent phases in two-phase mixtures are assumed to be in thermodynamic and mechanical equilibrium. The resultant system of conservation equations are solved numerically using the Method of Characteristics (MOC) coupled with a suitable Equation of State to account for multi-component hydrocarbon mixtures. The first part of the study involves the implementation of the Finite Volume Method (FVM) as an alternative to the MOC. In the case of gas and two-phase hydrocarbon pipeline ruptures, both models are found to be in excellent accord producing good agreement with the published field data. As compared to the MOC, the FVM shows considerable promise given its significantly shorter computation runtime and its ability to handle non-equilibrium or heterogeneous flows. The development, testing and validation of a Dynamic Boundary Fracture Model (DBFM) coupling the fluid decompression model with a widely used fracture model based on the Drop Weight Tear Test technique is presented next. The application of the DBFM to an hypothetical but realistic CO2 pipeline reveals the profound impacts of the line temperature and types of impurities present in the CO2 stream on the pipeline’s propensity to fracture propagation. It is found that the pure CO2 and the postcombustion pipelines exhibit very similar and highly temperature dependent propensity to fracture propagation. An increase in the line temperature from 20 – 30 oC results in the transition from a relatively short to a long running propagating facture. The situation becomes progressively worse in moving from the pre-combustion to the oxy-fuel stream. In the latter case, long running ductile fractures are observed at all the temperatures under consideration. All of the above findings are successfully explained by examining the fluid depressurisation trajectories during fracture propagation relative to the phase equilibrium envelopes. Finally, two of the main shortcomings associated with previous work in the modelling of pipeline ruptures are addressed. The first deals with the inability of Oke’s (2004) steady state model to handle non-isothermal flow conditions prior to rupture by accounting for both heat transfer and friction. The second removes the rupture plane instabilities encountered in Atti’s (2006) model when simulating outflow following the rupture of ultra high pressure pipelines. Excellent agreement between the new nonisothermal model predictions and the published data for real pipelines is observed.
|Title:||CFD modelling of outflow and ductile fracture propagation in pressurised pipelines|
|Open access status:||An open access version is available from UCL Discovery|
|UCL classification:||UCL > School of BEAMS > Faculty of Engineering Science > Chemical Engineering|
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