Failure Investigation of Hydrotest Rupture on a Natural Gas Pipeline

A land-based natural gas pipeline, API 5L Grade B pipe with an electric resistance weld (ERW), failed during hydrotesting inspection along the ERW seam, which led to further investigation. Linear indications, indicative of surface cracks, were observed on the outer diameter of the pipe below the fracture surface, made visible by fluorescent light. Using transmission electron microscopy, it was evidenced that a crack may have initiated within the martensite/austenite islands and propagated through the ferrite bands seen beneath the fracture surface in the heat-affected zone. It was also evidenced by scanning electron microscopy (SEM) that the primary fracture mode consisted of high volume fractions of transgranular cleavage and low volume fractions of transgranular microvoid coalescence. Also evidenced in SEM, were regions of non-fracture evidence indicative of inadequate fusion within the ERW seam.