Using Advanced Probability of Exceedance Methods for Defining Remediation Options

Abstract

The Probability of Exceedance (POE) methodology is used by pipeline operators to manage corrosion. It prioritizes anomalies identified by an in-line inspection (ILI) based on their likelihood of exceeding one or more failure thresholds. Two types of thresholds are commonly used: leak and rupture. The calculations represent the likelihood an anomaly will leak or rupture as a function of time. By assessing the likelihood of such a release as a function of time, an operator can design rational and statistically valid remediation programs. In this study, the likelihood that an anomaly will cause a leak or rupture was based on the ILI dimensions, the ILI system’s accuracies, and the corrosion growth rate at the anomaly location. The ILI dimensions were adjusted by the mean error as determined using ILI-to-field comparison data. The same ILI-to-field data were used to determine the scatter (standard deviation) in reported dimensions for different categories of anomalies. The POE (likelihood) of a leak or rupture was taken as a function of the mean and standard deviation of a corrosion growth rate at the anomaly location. Anomalies were “grown” using mean growth rates that were established by performing ILI run-to-run comparisons. By using accurate growth rates and ILI performance characteristics, realistic estimates were made of the POE of an anomaly, pipe joint, pipe segment, and the overall pipeline itself. This paper discusses improved methods of calculating leak and rupture POEs using realistic assumptions on how anomaly growth occurs as a function of time. Case studies are used to demonstrate the effectiveness of the advanced POE method. The introduction and select portions of this paper are based, in part, on “Advanced Probability of Exceedance Method Overcomes Previous Weaknesses,” by the authors,1Pipeline Pigging and Integrity Management Conference, February 2–4, 2022.