Fitness for service assessment of corroded pipelines

Balakrishnan, Ram Kumar (2005) Fitness for service assessment of corroded pipelines. Masters thesis, Memorial University of Newfoundland.

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Abstract

Pipelines provide the safe and economic means of transporting oil and gas. Ageing of these pipelines leads to the gradual loss of pipe strength and degradation of performance, because of the development of corrosion defects. Assessment of the corroded pipeline for fitness for service purposes remains as a critical activity of the transmission pipeline integrity management program. Several Level 2 assessment methods have been developed so far to evaluate the remaining strength of corroded pipelines. Most of these methods are based on a semiempirical fracture mechanics approach. Although the ASME B31G criterion for the evaluation of corroded pipelines seems to be adequate for design, it is known to be conservative. The use of high toughness pipeline materials with good post yield characteristics has enabled the application of limit load estimation techniques based on net section collapse criterion for the evaluation of corroded pipelines. -- This thesis discusses the application of an improved lower bound limit load estimation technique that is based on variational concepts in plasticity, obtained by invoking the concept of integral mean of yield criterion as it relates to the integrity assessment of corroded pipelines. Decay lengths derived using classical shell theory have been used to define the kinematically active reference volume. The reference volume approach overcomes the limitations posed by most of the current evaluation procedures with respect to the effect of circumferential extent of corrosion. The limit pressure and the remaining strength factor of pipelines, with both external and internal corrosion sites, subjected to internal pressure loading have been estimated. The results obtained have been found to be in good agreement with three-dimensional inelastic finite element analysis. The results of this study has shown that the variational method provides an improved assessment of the effect of corrosion damage on the integrity of the pipeline in terms of remaining strength factor (RSF). This method has also yielded a better understanding of the behavior and consequence of damage than the ASME B31 G criterion. An improved estimation of the limit pressures have been obtained in most cases.

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