History of Pressure Fluctuations Related to Severity of Near-Neutral pH SCC

Abstract

This paper describes a small exploratory study into the feasibility of predicting where near-neutral-pH stress-corrosion cracking (NN-pH SCC) would be more likely to occur based upon the history of pressure fluctuations in a pipeline. Such a correlation would be very valuable for prioritizing hydrostatic testing, in-line inspection (ILI), and direct assessment in addition to possibly suggesting a way to minimize the occurrence of SCC. The recent development of ILI tools that can find stress-corrosion cracks coupled with computerized SCADA and GIS systems provides an opportunity to make accurate assessments of the severity of SCC in a pipeline segment and to characterize the history of that segment. Previous work had shown the importance of strain rate and pressure fluctuations in promoting SCC. On that basis, Enbridge Pipeline, Inc. initiated a review of their system, which indicated that there appeared to be a strong correlation between SCC severity and magnitude and frequency of pressure fluctuations. Data from two other pipeline companies — one liquid and one gas — was obtained to see if the correlation was more broadly valid. It was determined that the correlation utilized by Enbridge was biased towards fatigue-type considerations. This suggests that the cracking on pipelines that demonstrate this correlation may involve a corrosion-fatigue mechanism in addition to, or instead of, traditional environment-sensitive SCC. Analysis of data from the other two companies was ambiguous, primarily because the available SCADA data covered only 1.25 years of recent operation. It also is possible that some effects of pressure fluctuations might have been masked by environmental effects and steel susceptibility. Consideration of such factors in addition to the inclusion of information on coating, pipe manufacturer, and geological features, would be expected to produce an even higher level of predictability.

Further study on crack growth model of buried pipelines exposed to concentrated carbonate–bicarbonate solution

“…Both amplitudes and frequencies of the pressure fluctuations are low in average. In contrast, cyclic loading of pipelines transporting liquid products has relatively low stress ratio and higher frequency liquid lines, because liquid is uncompressible [11,23]. Thus, fatigue may be also a damage component in crack development in operation of liquid line [11].…”

“…Typical record of frequency-stress ratio combinations a liquid line (the numbers are the times of appearance)[23].…”