Reliability of the conventional approach for stress/fatigue analysis of pitting corroded pipelines – Development of a safer approach

Abstract

The remaining strength and the fatigue life of externally pitted corroded metallic pipeline are often assessed using an idealization to model the pit morphology, typically semi-ellipsoidal and cuboidal. Such idealized pit geometries are shown herein to underestimate Stress Concentration Factors (SCFs). By implication this leads to significant overestimates of fatigue life for pitting corroded pipes under sustained cyclic loading which could be the reason behind some of the recent unexpected pipeline failures. Further investigation clarified that this can occur only when there is little or no plastic deformation within the pit which is typical for pipelines constructed from brittle and quasi-brittle materials. Therefore, applying the conventional approach that uses simplistic geometries to model corrosion pits is not reliable for stress/fatigue analysis of pipelines with brittle or quasi-brittle materials such as cast iron or high-grade steel. However, for pipelines made of ductile materials, there is a specific internal pressure, termed critical operating pressure herein, below which there is no plastic flow within the pit and simple idealization of pit morphology results in underestimated SCFs. A semi-empirical equation is developed herein to allow this critical operating pressure to be calculated, based on which a protocol is outlined for correct estimation of the SCFs of pitting corroded pipelines. The conclusions of this study are supported by numerical results validated against novel full-scale burst capacity tests with both simplistic and complex-shaped pits.