On November 1, 2007, a liquid propane pipeline ruptured near Carmichael, Mississippi. Several pipeline industry experts collaboratively concluded the likely origin of the failure was a defect in the longitudinal electric resistance welded (ERW) seam.[1] These experts also noted that a seam-integrity assessment did not prevent the failure. Following the National Transportation Safety Board’s (NTSB’s) public report, they issued Recommendation P-09-1, which called upon the Pipeline and Hazardous Materials Safety Administration (PHMSA) to conduct a comprehensive study to identify actions that can be used by operators to eliminate catastrophic longitudinal seam failures in pipelines, and indicated the required scope. NTSB directed that PHMSA conduct a comprehensive study of ERW pipe properties and the means to assure that they do not fail in service. Battelle contracted Kiefner and Associates, Inc. (KAI) and Det Norse Veritas (U.S.A.), Inc. (DNV) with the objective to assist PHMSA in favorably closing NTSB Recommendation P-09-1.
One of the tasks performed by DNV was to identify the best method(s) to characterize the toughness properties of ERW seams.[2] The objectives of the task were met by performing 1) a literature search to identify current and new practices for characterizing seam weld properties and 2) Charpy V-notch (CVN) impact testing. The findings from the literature search support the use of the Charpy test for the assessment of the toughness of line pipe steels in general, and the ERW weld seams in particular.
CVN testing was performed on specimens 1) where the notch varied in circumferential location from the bond line and 2) on bond line specimens, at and away from seam weld features/defects. The results indicated a significant decrease in the Charpy energy for non-post weld heat-treated (PWHT) pipe with decreasing distance from the bond line. Surprisingly, the Charpy energies (upper shelf) at the bond line were higher adjacent to the five confirmed (lack of fusion) LOF defects compared to away from the defects.
Failure pressure calculations using CorLAS™ on various (direct current) DC ERW failures, where the pipe dimensions, tensile properties, and flaw geometry were known, revealed that very low Charpy energies (<1.4 J, [1 ft·lb] back-calculated) are needed to cause failure. While the data are very limited in this study, they do not support the notion that CVN tests of the bond line can be used in integrity assessments of bond line defects. This paper will outline some basic steps to be performed to establish a range of bond line Charpy energies.
