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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Zhou, Wenxing

Abstract

Central to the design and integrity assessment of oil and gas transmission pipelines is to accurately evaluate their pressure containment capacities, i.e. burst capacities. Corrosion defects threaten the structural integrity of pipelines as they cause thinning of the pipe wall and therefore reduce the burst capacity. Corroded in-service pipelines may be subjected to longitudinal compression resulting from, for example, ground movement or formation of free spans, in addition to internal pressures. The main objective of the research reported in this thesis is to facilitate Fitness-For-Service (FFS) assessment of corroded pipelines.

The first study investigates the conservatism associated with the rectangular and semi-ellipsoidal idealizations of corrosion defects of naturally-occurring corrosion defects by finite element analysis (FEA). The semi-ellipsoidal idealization of naturally-occurring corrosion defects in FEA is found to lead to more accurate predictions of the burst capacity than the rectangular idealization for defects that are less than 70% through the pipe wall thickness. The FEA results conducted with the semi-ellipsoidal-shaped defects indicate that the burst capacity in general increases as the defect width increases if the defect depth and length remain the same. The defect width effect is marked for deep, relatively short defects, and should therefore be taken into account accordingly in the empirical or semi-empirical burst capacity models.

The second study proposes a new burst capacity model for corroded pipelines based on extensive parametric three-dimensional (3D) elasto-plastic FEA validated by full-scale burst tests. Based on the well-known NG-18 equation, the proposed model takes into account the beneficial effect of the defect width on the burst capacity and employs a new Folias factor that depends on both the defect depth and length. The flow stress in the proposed model is defined as a function of the strain hardening exponent and ultimate tensile strength of the pipe steel based on the analytical solution of the burst capacity of defect-free pipes. The accuracy of the proposed model is validated using extensive parametric FEA and shown to be higher than existing burst capacity models.

The third study investigates the burst capacity of corroded pipelines under combined internal pressure and longitudinal compression based on extensive parametric 3D elastic-plastic FEA. It is observed that the longitudinal compressive stress can markedly reduce the burst capacity of corroded pipelines. The adverse effect of the compressive stress on the burst capacity is the strongest for wide, relatively shallow defects, and relatively insensitive to the defect length. Based on the parametric FEA results, an artificial neural network (ANN) model is developed in the open-source platform PYTHON to predict the burst capacity of pipelines under internal pressure only or combined loads. The ANN model is validated using FEA and full-scale burst tests conducted by DNV and the results indicate good accuracy of the ANN model.

The fourth study develops a new semi-empirical burst capacity model for corroded oil and gas pipelines under combined internal pressure and longitudinal compression. The proposed model evaluates the burst capacity of a corroded pipeline under combined loads as the burst capacity of the pipeline under internal pressure only, which is proposed in the second study, multiplied by a correction factor to account for the effect of the longitudinal compression. Extensive parametric elastoplastic FEA are carried out, the results of which are used as the basis to develop the correction factor as a function of the corrosion defect sizes and magnitude of the longitudinal compressive stress. The proposed model is validated by a large set of parametric FEA and full-scale burst tests reported in the literature, and is shown to provide marked improvements over two existing models, the DNV and RPA-PLLC models, for corroded pipelines under combined loads.

The fifth study investigates the interaction effect on the burst capacity of oil and gas pipelines containing closely-spaced corrosion defects under combined internal pressure and longitudinal compression by carrying out extensive parametric 3D elasto-plastic finite element analyses. The analysis results reveal that the interaction effects under combined loads are different from the interaction effects under internal pressure only. The interaction between circumferentially-aligned defects under combined loads is significant: the burst capacity corresponding to the two-defect case can be markedly lower than that corresponding to the single-defect case. On the other hand, the interaction between longitudinally-aligned defects under combined loads is negligible due to the so-called shielding effect.

Summary for Lay Audience

Steel pipelines are widely considered the most efficient and safest mode of transmitting and distributing large quantity of hydrocarbon products (e.g., crude oil, natural gas and various petroleum products). Canada has more than 840,000 kilometres (km) of transmission, gathering and distribution pipelines with most provinces having significant pipeline infrastructure. Of this amount, about 73,000 km are federally regulated pipelines which are primarily transmission pipelines. According to the National Conference of State Legislatures, the United States maintains about 2 million miles of natural gas distribution mains and pipelines, 321,000 miles of gas transmission and gathering pipelines, 175,000 miles hazardous liquid pipelines. Failures of pipelines, albeit infrequent, will cause undesirable impacts on economies, environment and the living conditions of residents. The metal-loss corrosion is one of the most common threats to the structural integrity of pipelines. Based on the Pipeline and Hazardous Material Safety Administration (PHMSA) database, the incidents on onshore gas transmission pipelines from 2002 to 2013 indicates that corrosion is responsible for 32.1% of all incidents. This research will improve the accuracy of fitness-for-service assessments of corroded pipelines in practice including the combined loading condition and provide practical recommendations for the defect interaction rules under combined loads.

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