Fitness-for-Service of Unconstrained Shallow Dents

Abstract

Current federal regulations in the U.S. require excavation of plain dents identified through in line inspection surveys based primarily on depth. Industry experience, and previous research, has shown that the depth of the dent, alone, is not sufficient to assess dent severity and that releases could occur at dents below the excavation threshold (Dawson, 2006). Canada’s National Energy Board released a safety advisory on June 18, 2010, to all companies under their jurisdiction regarding two incidents involving shallow dents. The safety advisory stated that all integrity management programs should be reviewed and updated where appropriate to address the threat posed by shallow dents. Similar incidents have raised awareness in the United States and elsewhere around the world. This paper focuses on an extensive multi-year effort to analyze the fitness for service of unconstrained shallow dents on multiple pipeline systems. Fatigue and strain analyses were performed to determine the serviceability and estimate the remaining service life. The dents in this study included both topside dents and bottomside dents that were previously evaluated through excavation to be unconstrained. Results of the fatigue and strain assessments are presented, along with field results of dents that were chosen for excavation. Comparison of the fitness for service results and subsequent excavation findings were performed to improve an ongoing campaign to prioritize several hundred in-service unconstrained dents. Maximum strain levels of the dents were calculated based on the geometry of the dent as determined by radial sensor measurements from in line inspection surveys. The results of the in-line inspection and field measurement comparisons were analyzed to determine the accuracy and possible adjustments of strain assessments for the ongoing fitness-for-service program.