Fatigue assessment of a double submerged arc welded gas pipeline

An uncommon blow out in a 24″ diameter, 7 mm thick API 5L X52 gas pipeline was due to fracture at the longitudinal double submerged arc weld (DSAW). Fatigue cracks were found to have initiated from a large embedded weld defect of lack of fusion, resulting from severe geometrical mismatch between inner and outer weld passes. Few large pressure cycles had been recorded during the 45 year service life of the pipeline. Fatigue tests were carried out to characterize propagation of fatigue cracks in weld metal, it was found that a large Paris exponent made the few large amplitude cycles most contributing to crack propagation. Crack growth path and striation patterns were studied. Fatigue growth was modelled by integrating experimental results and by extrapolating striation spacing in the fracture surface of the failed pipe. It was found that in the early life of the pipeline many more large pressure cycles than expected had occurred. Good correspondence between predicted and actual fatigue lives was in this way obtained.