Hydrogen assisted cracking driven by cathodic protection operated at near -1200 mV CSE - an onshore natural gas pipeline failure

A 22-inch onshore natural gas transmission pipeline in operation since 1960 experienced an in-service rupture in January of 2018. The rupture was caused by an axial crack located 150 km (93 miles) downstream of the closest compression station. While the root cause analysis of the rupture was being carried out, excavations of indications reported by a Hard Spot In-Line Inspection approximately 42 km (26 miles) downstream of the rupture found leaks from predominantly circumferential crack colonies mixed with spiderweb ones not associated with hard spots.The conclusion from this investigation is that the cracking mechanism for the in-service rupture and leaks is mainly hydrogen environment assisted cracking (HAC), with the hydrogen being generated by the cathodic protection. The root cause for the failures is the combination of an aged and defective coating and the cathodic protection (CP) system operated for decades at near the pipe-to-soil potential limit of - 1200 mV CSE.An extensive literature search indicates that this investigation is the first fully documented case of an onshore transmission pipeline failure due to HAC driven by CP, with the cracks occurring in the pipe body of X52 steel, away from welds and not related with hard spots. The paper summarizes the approach used for the root cause analysis and the findings, where the implications in the management of the pipeline integrity of the pipeline system are discussed. The pipeline operator implemented a series of preventive and corrective measures, which included pressure testing, cathodic protection improvement, coating rehabilitation programs and In-Line Inspection programs.