An API 579 Level 3 assessment was performed to determine the stresses in a 2% dent in a 20-inch x 0.406-inch pipeline. The intent was to determine the stress concentration factor (SCF) in the dent with a finite element model using geometry data provided from an in-line inspection caliper run. In addition to the analytically-derived SCF, data were also evaluated from a recent experimental study involving a plain dent subjected to cyclic pressure conditions with a profile comparable to the dent in question. This sample was cycled at a stress range of 70% SMYS and failed after 10,163 cycles had been applied. Using the DOE-B mean fatigue curve, combined with the experimental fatigue life, the resulting SCF factor was derived to be 4.20. This value is within 1% of the calculated FEA-based SCF and served to confirm the technical validity of the SCF. The operator provided historical pressure data covering a 12-month period and a rainflow count analysis was performed on the data. Using this data, along with the API X’ design fatigue curve, the estimated remaining life was determined for the dent in question and conservatively estimated to be 65 years. This paper provides details on the analysis methodology and associated results, discussions on the empirically-derived SCF with its use in validating the analytical SCF, and application of the results to estimate the remaining life of the pipeline system. It is the intent of the authors to provide the pipeline industry with a systemic approach for evaluating dent severity using caliper and operating pressure history data.
Alexander, C., and Jorritsma, E., “A Systematic Approach for Evaluating Dent Severity in a Liquid Transmission Pipeline System,” Proceedings of IPC2010 (Paper No. IPC2010-31538), 8th International Pipeline Conference, September 27 – October 1, 2010, Calgary, Alberta, Canada.