Revised Corrosion Management With Reliability Based Excavation Criteria

For decades, TransCanada Pipelines has used inline inspection (ILI) to manage the threat of corrosion on gas pipelines. Immediate integrity was addressed using rupture pressure ratio and leak criteria, and future integrity was addressed using a growth assessment. However, a review of excavations based on predictions of defect growth shows that few excavations actually lead to repairs. This study investigated the areas of undue conservatism in both the integrity assessments and the excavation criteria. All aspects of the immediate and future integrity assessment based on ILI were examined. All relevant uncertainties were accounted for in calculating the reliability of the pipeline. A new basis of defining excavation criterion was established based on a reliability assessment and calibration. This criterion was validated against previous ILI based excavations which reveal the features that actually required repair. The criterion was also compared to reliability based criteria recommended in Annex O of the CSA Z662 which gives guidelines for Reliability Based Design and Assessment. This paper addresses the practical limitation of data and presents methods for extracting best information from available data. Case studies that demonstrate the application of the revised assessment method and criterion are also discussed.