Methanol-induced Internal Stress Corrosion Cracking in a Northern Petroleum Pipeline

Chebaro, Mohamed R. (Enbridge Pipelines Inc.) | Padgett, Barbara N. (Det Norske Veritas (U.S.A.), Inc.) | Beavers, John A. (Det Norske Veritas (U.S.A.), Inc.) | Norfleet, David M. (Det Norske Veritas (U.S.A.), Inc.) | Ironside, Scott D. (Enbridge Pipelines Inc.)



In early 2012, a metallurgical investigation was performed on a section of pipe that leaked in service. The pipeline transports crude oil and is located in Northern Canada. The investigation concluded that internal stress corrosion cracking (I-SCC) was the primary mechanism by which a crack initiated and propagated 96% through-wall, following an intergranular path. The remaining 4% of the wall thickness failed due to fatigue crack propagation. While I-SCC is extremely rare in underground petroleum pipelines, all other plausible cracking mechanisms were eliminated through a detailed metallurgical analysis. Methanol was suspected to be the agent responsible for the I-SCC . Following the completion of the investigation, a laboratory research program was initiated to confirm the mechanism of methanol-induced I-SCC using slow strain rate (SSR) testing under simulated environmental and physical conditions. In addition to the SSR testing, the circumferential, radial and axial distributions of residual stresses along the girth welds of two pipe sections removed from the pipeline were quantified. Residual stresses in the vicinity of the girth welds were found to be a contributing factor to the cracking identified in the investigation. This paper provides an overview of the comprehensive laboratory research program and details the efforts to replicate the atypical cracking mechanism in controlled methanol environments.