Girth weld integrity evaluation on a vintage gas pipeline

摘要

ALEAK ON A GIRTH WELD was detected on a 609.6-mm (24-in) diameter transmission gas pipeline while recoating works were being performed. Failure analysis determined that the leak was caused by a pre-existing defect in the girth weld which had not been evolving over time. Nonetheless, because of the particular stress condition to which the gas pipeline was subject during the recoating work, the defect destabilized and progressed to failure.rnIn order to reduce the risk of occurrence of similar failures, a total of 27,000 girth welds were individually analysed, corresponding to a length of 191 kms (118 miles) of the same gas pipeline section.The analysis included evaluation of internal-inspection runs and girth-weld gammagraphs available from the recoating work. Hence, not only were the data analysed individually, but a comparison of the different sources of information to find a correlation was also performed.rnFeatures detected in the analysis were classified and prioritized base on comparison with the failed girth-weld defect. Also, correspondence with crack-like indications was considered.The severity of the anomalies was categorized on four levels.A set of the most severe crack-like anomalies with potential for failure were selected for direct assessment.rnThe complete process involved the assessment of 399 anomalies in total, revealing cracks on 79 girth welds. Metal reinforcements were installed as permanent repairs on 27 girth welds according to repair criteria, and direct-assessment findings were used to correlate internal inspection signals with features.rnThis paper discusses the results of the analysis, the field findings, and the actions taken.