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Burst Pressure of Pipeline with Longitudinal Crack in Dent Defects Using Extended Finite Element Method
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- Author / Creator
- Allan Okodi
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Pipelines are exposed to mechanical damage due to external interference while in service. The damages can be in form of dents, wrinkles, gouges, or cracks. In some instances, two or more of the damages may occur at the same location on the pipe and act as one defect. One of the combined defects in pipeline comprises of a crack inside a dent, commonly referred to as dent-crack defect. Field observations show pipeline with dent-crack defects fail by leakage or rupture at low pressure, requiring pipelines to be shut down, which is a highly consequential choice in pipeline operations. Very little is known about how the constituting dents and cracks interact as they degrade integrity of the pipeline, partly because little research has been conducted on the effect of dent-cracks in pipeline. Currently, there are no predictive analytical models accepted in the pipeline industry to assess integrity of pipelines with dent-crack defects. The assessments could be done experimentally and by using traditional finite element method, which are both very expensive and time-consuming methods. The traditional finite element method works well for problems in a continuum, but are not suited for modeling discontinuities like cracks, especially where the discontinuity is not static, like a propagating crack. The extended finite element method (XFEM) is specifically designed for modeling crack propagation. It is less tedious to use than the traditional finite element method because it does not require re-meshing the body each time the crack propagates. However, its effectiveness for assessment of crack propagation in pipelines has not been widely investigated. This thesis presents the results, discussion, conclusions, and recommendations of a numerical evaluation of the effect of dent-crack defects on integrity of pipeline, conducted using the extended finite element method implemented in the finite element analysis software, Abaqus. The first part of the evaluation focused on establishing the effectiveness of the XFEM methodology implemented in Abaqus for analysis of crack propagation and prediction of burst pressure of pipeline with plain longitudinal cracks. Models of specimens of longitudinally cracked API X60 pipeline with known experimentally determined burst pressure were developed in Abaqus, allowing for XFEM crack propagation, and used to predict burst pressure of the pipeline. In addition, the burst pressures were predicted using the modified NG-18 equation, CorLAS, and Failure Assessment Diagrams (FAD), which are industry accepted analytical methods for assessment of burst pressure. The predictions of different methods were compared with burst test results to assess effectiveness of the XFEM models. The comparisons showed that well-calibrated XFEM models can accurately predict burst pressure of cracked pipelines. In the second part of the study, models of specimens of API X70 grade of pipeline having longitudinal cracks inside rectangular dents were calibrated and validated using burst test data obtained in literature and used in a parametric analysis of pipeline with various sizes of dent-crack defects. The effect of varying crack length, crack depth, dent depth, denting pressure, location of cracks inside dents, and dent restraint condition, on burst pressure of pipeline with dent-crack defects were evaluated in order to establish the relative impact of the various parameters on integrity of the pipeline. The results showed that crack depth and location of crack inside the dent are the most severe parameters for pipeline integrity. The affected pipeline could have significant remaining burst strength depending on the sizes of the defects and the pipe could be retained in service upon an engineering assessment.
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- Graduation date
- Spring 2021
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.