Atti, O.F.; (2007) Modelling outflow and low temperature induced crack propagation in pressurised pipelines. Doctoral thesis , University of London.

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Abstract

This thesis describes the development and refinement of a rigorous mathematical model for simulating outflow following the failure of pressurised single pipelines and pipeline networks containing multi-component hydrocarbon mixtures. The same model is then extended to simulate the progression of a defect in the pipeline into a running fracture. The outflow model is based on homogeneous equilibrium assumption with the conservation equations resolved using the Method of Characteristics (MOC). The model addresses some of the shortcomings associated with the incorrect posing of boundary conditions reported in earlier works. Both types of failures including pipeline puncture as well as full-bore rupture are modelled. Typical pressurised pipeline inventories include permanent gases and liquids, condensable gases, flashing liquids as well as two-phase liquid gas mixtures. Model validation is performed against the Isle of Grain field data as well as those logged during the Piper Alpha tragedy. In cases where real data are not available, a mass conservation index is determined to assess the accuracy of the numerical simulation. In most cases, good agreement between the simulated and field data along with reasonable mass conservation indices (close to unity) are observed. A significant aspect of the work involves the development of methodologies for reducing the computation run time. This has involved the use of various numerical grid discretisation schemes such as simple and nested grids as well as the development of a quadratic interpolation scheme. Investigations using different types of pipeline inventories show that the nested grid system is primarily effective in reducing the computation run time when used in conjunction with long pipelines (ca.> 0 km) containing gases. For other cases, the use of the simple grid system is recommended. The interpolation scheme involves the construction of a database encompassing a pre defined fluid pressure/enthalpy range. This method is found to be universally effective in reducing the computational run time by as much as 80 % for all types of inventories without a loss in accuracy. This computational run time saving is made in comparison to when actual flash calculations are made. The crack propagation model invokes fracture mechanics principles and accounts for the important processes taking place during depressurisation including the thermal, and pressure stresses in the pipe wall to simulate the progression of a simple defect into a running fracture. The application of the model to an isolated exposed pipeline, where the released inventory freely moves away from the discharge plane shows rapid localised cooling of the pipe wall to temperatures well below its ductile to brittle transition temperature. The resulting drop in the fracture toughness coupled with the pressure stresses at the defect plane suggests that catastrophic pipeline failure through a running fracture can arise. In the case of buried pipelines, such effects are found to be even more pronounced due to the additional thermal stresses in the pipe wall. The latter is brought about as a result of the cooling the pipe wall by the confined escaping gas. The above study for the first time quantitatively highlights the importance of taking into account the expansion induced cooling effects as a credible failure scenario when undertaking safety assessment of pressurised pipelines.

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