Periodic Re-Verification Intervals for High-Consequence Areas

Executive Summary:
Factors that are potential threats to the integrity of natural gas transmission pipelines were evaluated to determine the interval at which the integrity of the pipeline must be re-verified to ensure safe operation in high-consequence areas. Corrosion is the primary factor controlling the re-verification interval. The time interval for integrity to potentially be reduced to a level that requires re-verification was determined in terms of stress and wall thickness as a function of design factor. Worst-case corrosion kinetics derived from the Office of Pipeline Safety (OPS) incident database indicated the re-verification interval for Class 3 and 4 designs was 16 years to avoid leaks or ruptures. For Class 1 designs the results indicated an interval of 11.4 years. If average corrosion kinetics were adopted instead of worst-case rates, the re-verification intervals would be about three times longer. Fatigue and stress-corrosion cracking were indicated to be unlikely threats in Class 3 and 4 designs. A prescriptive framework was found inappropriate for features like weld-seam defects and hard spots, as well as for corrosion driven by locally unique factors.