A leak 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. This pre-existing defect had not been evolving over time. Nonetheless, because of the stress particular condition to which the gas pipeline was subject during the recoating work, the defect destabilized and progressed to failure.
In order to reduce the risk of occurrence of similar failures, a total of 27,000 girth welds were individually analyzed, corresponding to a length of 191 kilometers (118 miles) of the same gas pipeline section. The analysis included evaluation of internal inspection runs and girth welds gammagraphs available from the recoating work. Hence, not only was the data analyzed individually, but a comparison of the different sources of information to find a correlation was also performed.
Features 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. Severity of anomalies was categorized on four levels. A set of the highest severe crack like anomalies with potential failure were selected for direct assessment.
The complete process involved the assessment of 399 anomalies in total, revealing cracks on 79 girth welds. Metal reinforcements were installed as permanent repair on 27 girth welds according to repair criteria, and direct assessment findings were used to correlate internal inspection signals with features.
This paper discusses the results of the analysis, the field findings, and the actions taken.