Failure analysis of a crude oil pipeline

The transversal cracking of a seamed API 5L X46 steel tube belonging to a crude oil pipeline was investigated. The main cracking nucleated in the internal surface of the tube, at the boundary between the heat-affected zone (HAZ) and the weld metal, propagating in a stable mode along the radial and longitudinal directions. Stress raisers, such as welding defects and corrosion pits, were associated to the cracking nucleation. The internal surface of the tube and the cracking surfaces presented a deposit layer, which was rich in Fe, O and S. Diffractometry on the internal identified the presence of a multi-layered corrosion deposit, formed by iron oxide (Fe2O3 and Fe3O4) and iron sulphides, such as pyrrhotite, mackinawite and pyrite, indicating the action of a H2S corrosion assisted mechanism. The crack propagation path did not depend on the welding macrostructure, growing perpendicular to both the internal surface and main tensile stresses. Crack propagation was, however, microstructure sensitive, with a more intense branching occurring inside the base metal rather than the HAZ region. Both regions presented cracking (blistering) of the sulphide/matrix interface and microfractographic examination indicated the action of a ductile fracture mechanism linking the H2 blisters, reinforcing the idea that atomic hydrogen association rather than hydrogen embrittlement was the active mechanism during the cracking of the pipeline. These observations indicated that failure of the pipeline occurred by a stress-oriented hydrogen-induced cracking (SOHIC) mechanism.