Most leaks or ruptures due to stress-corrosion cracking (SCC) originate at clusters of cracks that are oriented in the longitudinal, or axial, direction of the pipe. This mode of failure has been under investigation since 1965, when high-pH SCC was first discovered, and near-neutral-pH SCC (sometime called low-pH SCC) has been studied intensely since it was discovered in 1986. As a result, both forms of SCC are reasonably well understood in terms of causes and mitigation. However, there have been a comparatively small number of failures due to stress-corrosion cracks that were oriented in the circumferential direction, and methods to manage this form of SCC are not so well established.
Therefore, this project is aimed to examine records of cases where circumferential stress-corrosion cracking (CSCC) was discovered, in an attempt to learn more about the causes and to consider possible ways to manage the problem. The results will provide important guidance to pipeline companies for locating and treating CSCC and to in-line-inspection (ILI) vendors for developing and evaluating tools for locating, identifying, and sizing CSCC.
While much more rare than axial stress-corrosion cracking (SCC), circumferential SCC (CSCC) has been observed in Canada, the United States, and two European countries. Because the orientation of stress-corrosion cracks invariably is perpendicular to the maximum tensile stress, the axial stresses at the locations of the cracks must have been greater than the hoop stress. The Poisson effect and thermal effects can account for about half of the axial stresses. Evidence from the field suggests that there are three probable sources of additional axial stresses that can promote CSCC: residual stresses in bent pipe, axial stresses caused by movement of unstable soil on slopes, and residual stresses opposite rock dents.
CSCC can be managed by one or a combination of the following procedures: direct assessment (DA), in-line inspection (ILI), or hydrostatic testing. Guidance for selection of sites for DA is derived from industry experience, which was determined from responses to a questionnaire and from published reports. The capabilities of ILI to detect circumferential stress-corrosion cracks or the conditions that promote them are summarized. A method for calculating the size of circumferential flaws that can cause ruptures is presented and compared with service experience. That information can provide useful guidance for ILI requirements and decisions about which flaws need to be removed immediately. The benefits and limitations of hydrostatic testing also are described. PRCI Members can access the draft Final Report on PRIME.