Abstract
A 22-inch onshore natural gas transmission pipeline experienced an in-service rupture in January of 2018. The rupture was caused by an axial crack that occurred 150 Km (93 miles) downstream of the closest compression station.
While the root cause analysis of the rupture was in progress, excavations of indications reported by a Hard Spot In-Line Inspection (ILI) were undertaken starting approximately 42 km (26 miles) downstream of the rupture. These excavations discovered predominantly circumferential crack colonies, with a number of through-wall cracks being leak.
The root cause analysis concluded that both the axial and circumferential external surface cracks are consistent with hydrogen environment assisted cracking generated by the impressed-current cathodic protection system, operated near the pipe-to-soil recommended potential limit of −1200 mV as measured with respect to a saturated copper/copper sulfate reference electrode (CSE).
Transportadora de Gas del Norte (TGN) implemented a series of preventive and corrective measures that included pressure testing, cathodic protection improvement, coating rehabilitation programs and In-Line Inspection programs among others.
This paper presents the background and approach used in the root cause analysis, summarizes the methodology, results and findings, and discusses their implications in the management of the Pipeline Integrity of the pipeline system operated by TGN.
