Fatigue assessment of an electrical resistance welded oil pipeline

A blowout in a 14 in. diameter, 1/4 in. thick API 5L X46 oil pipeline was due to the sudden propagation of a fracture at the longitudinal Electrical Resistance Welding (ERW) weld. The cracks initiated from small hook crack-type defects on the external surface of the tube, in the highly deformed and hardened central area of the ERW weld. Fatigue tests were carried out to characterize initiation and propagation of fatigue cracks in base and weld metals, in two regions of the pipeline. Specimens were subjected to cyclic stresses similar to those that were produced during the passage of the scraper. The fatigue growth was modeled by integrating experimental results. Predicted fatigue lives of about 20,000 cycles justify fatigue propagation up to failure in weld metal, from initial 2 mm deep defects introduced during manufacturing.