Assessment and Management of SCC in a Liquid Pipeline: Case Study

Stress corrosion cracking (SCC) is a major concern for most gas and oil pipeline operators. Extensive efforts continue to be made to develop strategies for a better management of the problem. The quantification of the life cycle and risk of SCC rupture for a post inspection integrity assessment requires knowledge of (1) Quality of Inspection, namely POD (Probability of Detection), POI (Probability of Identification) and Actual Sizing Tolerance (2) Material and Metallurgy; (3) Appropriate Assessment Methods; and (4) Crack Growth Rates. Previous experience gained from the crack detection inspections showed POD and POI for deep cracks are generally high, with sizing limited up to 40% wt. The uncertainty in sizing for shallow cracks is usually higher, and may not meet the specified tolerance at a specified certainty and confidence level. POD, POI and sizing accuracy can be affected by the inspection operation including speed, other defects and geometry irregularity along the pipeline. Therefore, the qualification of the tool inspection performance directly impacts on the reliability of the assessment and serves as the basis for a reliable assessment. In this paper, an approach for managing high pH SCC in a 30^″ × 340 Km oil pipeline in Mexico is presented. The approach consists of a comprehensive verification excavation plan, a strict in-ditch NDT investigation protocol, statistical models for POD, POI and sizing tolerance analyses, and an appropriate assessment methodology supported by the material testing program. With all the results obtained from application of the approach, the integrity management strategies are developed. An action plan for future integrity is established and being refined from time-to-time prior to next inspection.