A New Probabilistic Model for High-pH Stress Corrosion Cracking

Stress corrosion cracking (SCC) poses a threat to integrity of buried pipelines in many parts of the world. In North America there is now a requirement that integrity management plans should address SCC and a direct assessment methodology, SCCDA, for managing the threat due to SCC, is now becoming established. Like general corrosion or fatigue, SCC is a time dependent deterioration process that leads to progressive weakness of the pipe wall eventually causing failure as a leak or rupture, if not managed. There are indeed two known forms of SCC; High-pH and near neutral-pH SCC. The focus of this paper is on High-pH SCC. High-pH SCC involves a number of phases including incubation, initiation, anodic growth, coalescence, mechanical growth and final failure. Factors affecting these processes include temperature, static and cyclic stress, soil conditions, type of coating and level of Cathodic Protection. Some of these factors may vary seasonally. The temporal development of SCC damage is thus both complex and subject to significant uncertainty. The purpose of this paper is to describe a detailed probabilistic model that addresses the various phases of High-pH SCC taking account of uncertainty in the relevant influencing factors. The model determines the likely times to coalescence and to grow to a critical size thus providing a time dependent probability of failure. The model gives a clear indication of which parameters should be managed in order to reduce the likelihood of failure to an acceptable level. The model provides the basis of a powerful decision making tool for the purpose of managing High pH SCC. Consequently, the model can be used in conjunction with relevant in-line inspection data and/or above ground survey data to provide an effective SCC integrity management tool. The model is illustrated through a numerical example and the use of the model as an integrity management tool is clearly demonstrated.