This thesis describes the development of a mathematical model for simulating the loss in the mechanical integrity of pressurised hydrocarbon conveying pipelines under fire attack. The model is based on the resolution of the conservation equations using the Method of Characteristics. It accounts for real fluid behaviour, pipeline mechanical strength, as well as phase and flow dependent transient heat transfer effects and frictional pressure losses. Failure is assumed to occur when any one of the simulated triaxial thermal and pressure stresses in the pipeline wall exceed its ultimate tensile strength. Two types of failure scenarios both involving thermal loading of a pressurised pipeline are modelled and the consequences of failure are elucidated using hypothetical case examples. The first deals with direct jet fire impingement in which a section of the pipeline is assumed to be completely enveloped by the fire. Here the results of the simulations show that the pipeline fails through bulging and buckling due to the prevailing tangential stresses. The efficacy of emergency depressurisation using different diameter relief valves as a means of protecting the pipeline mechanical integrity during fire attack is also quantitatively investigated. The more complicated alternative failure scenario modelled involves the puncture of the pressurised pipeline and the immediate ignition of the escaping high pressure inventory. The impact of the resulting jet fire back radiation on the mechanical integrity of the depressurising pipeline is then modelled. An important precursor to the above is the presentation followed by linking of an appropriate jet flame model based on published literature describing the transient jet fire characteristics to the outflow model. Application of the model to a 1 Omm puncture positioned at the downstream end of a hypothetical 0.5km, 0.395m dia. steel pipeline conveying natural gas at HObara shows that the pipeline fails in the tangential direction some 1070s following the initial release. The size and location of the puncture during unisolated release are found to have a profound effect on delaying or circumventing catastrophic pipeline failure. The former was expected as increasing the puncture diameter results in a more rapid depressurisation rate thus resulting in a faster reduction of the accompanied pressure stresses which contribute to the pipeline failure. The significant effect of the location of the puncture on the fate of the pipeline was however somewhat unexpected. Here it is found that placing the puncture at the downstream end of the pipeline results in a discharge pressure and hence jet flame overall dimensions that are approximately double those compared to mid point puncture. The above is manifested in catastrophic pipeline failure due to the much more severe thermal loading in the case of downstream end puncture. The study concludes by investigating the effect of using different grades of carbon steel on the pressurised pipeline's resistance to withstand thermal loading.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:479186 |
| Date | January 2008 |
| Creators | Abbasi, Muhammad Umar |
| Publisher | University College London (University of London) |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://discovery.ucl.ac.uk/1444004/ |
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