Crack Growth Behavior of Pipeline Steels under Variable Pressure Fluctuations in a Near-Neutral pH Environment

Abstract

This research aims at understanding the crack propagation behavior of pipeline steels under variable pressure fluctuations in a near-neutral pH environment. Although pressure fluctuations have been recognized as the driving force for crack growth for more than two decades, the mechanisms governing their effects on crack growth have yet to be determined. Existing crack growth models that consider the effect of pressure fluctuations are established primarily by assuming a fatigue loading with constant stress amplitudes. This often yields a predicted life that well exceeds the service lifetime observed in the field. We believe that the significant discrepancies between laboratory findings and field observations could be bridged by modeling crack growth through a consideration of variable amplitude fatigue loadings, which are the conditions in the field. This investigation has focused on the evaluation of a variable amplitude fatigue waveform consisting of cycles with higher R ratios (minimum stress/maximum stress), termed as minor cycles, and periodic underload cycles with lower R ratios. Such a waveform is typical for the pressure fluctuations in pipeline discharge sections, which was performed both in air and in near-neutral pH solution. Compared with the corresponding constant amplitude loadings, the variable amplitude waveform significantly enhanced fatigue crack growth. The waveform was varied to study the effects of various contributing factors, including loading frequency, maximum stress intensity factor, number of minor cycles, and amplitudes of underload and minor cycles. By comparing results in near-neutral pH solution with that in air, the role of hydrogen and corrosion in crack growth was characterized and clearly defined. The obtained results agree with various failure scenarios found in the field, including the high frequency of failures found at the discharge sections of pipelines, and the different crack growth characteristics between oil pipelines and gas pipelines. Based on these findings, practical strategies were proposed for avoiding accelerated crack growth.