Remaining strength assessment of deteriorating energy pipelines

Mondal, Bipul Chandra (2018) Remaining strength assessment of deteriorating energy pipelines. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Pipelines are extensively used as the most economic means of transporting oil and gas. The steel pipelines have been widely used for these applications due to the high strength to weight ratio of the material, resulting in lower material cost. These pipelines are subjected to corrosions during the service life, resulting in the reduction of wall thicknesses. The prediction of the remaining strength of a corroded pipeline is required for fitness-for-purpose assessment. For the prediction of the remaining strength, different models were developed based on simplified results of analysis and/or empirical fits to limited experimental data which are expressed in terms of burst pressure. The established design codes adopt simplified design equations for the burst pressure prediction for corroded pipelines. However, the burst pressures predicted using the simplified equations are not consistent with the burst test results and results obtained from rigorous finite element (FE) analyses. Besides, the pipelines are often subjected to axial force and bending moment. The effects of the axial force and bending moment on the burst pressure are not rationally accounted. In this research, the axial forces and bending moments experience by energy pipelines are first examined considering a case of offshore pipelines. The improved burst pressure models are then developed for pipelines with and without the axial forces and bending moments. The existing models of burst pressures for deteriorated-steel pipelines are investigated to determine the contributing parameters to the burst pressures. The Folias factor and flow stress are identified as the major parameters contributing to the burst pressures of the corroded pipelines. A detailed study, based on FE analysis using Abaqus, has been carried out to develop a new method of defining the Folias factor and to develop an improved model for burst pressure prediction for a corroded pipeline. The finite element analysis is then extended to develop the new interaction rules for the pipelines subjected to multiple patches of the corrosion defects. The FE analysis is used to develop failure loci for burst pressure prediction for pipelines subjected to axial forces and bending moments. Corroded pipelines often suffer from the stress corrosion cracking (SCC) when the pipelines in corrosive environments are subjected to high tensile stresses. The SCC occurs at a stress intensity factor well below the fracture toughness of the material. The effects of the SCC and the crack propagation in the deteriorating pipelines cannot be captured using standard FE modeling techniques. It is proposed to employ fracture mechanics to determine the remaining strengths of pipelines containing corrosion defects or crack-like defects or corrosion with crack-like defects.

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