Failure Investigation of a Defective Weldment of an Oil Product Transmission Pipeline

An API 5L X52 oil product transmission pipeline experienced a sudden failure at a portion which had been buried in the bed of a river crossing after 33 years of service. Results showed that mechanical properties of the weldment had been reduced due to weld defects. The defects had been formed during construction welding and installation of the pipeline in the bed of the river crossing. No failure had occurred until this portion of the buried pipeline came out of its place in the bed of the river crossing. Under this condition, defective weldment (due to hydrogen stepwise cracking, incomplete fusion, porosity formation, and entrapped slag inclusion) was not capable of enduring stress imposed by the strong river stream and consequently failed. Metallographic and SEM studies as well as stress analysis were used for pipeline fracture investigation.