Methanol-Induced Axial Stress Corrosion Cracking in a Northern Canadian Liquids Pipeline

Abstract

Three in-service leaks on a crude oil pipeline operating in Canada were investigated to identify their metallurgical cause(s). The releases were found to be associated with cracks originating from the internal surface of the pipeline. Further similarities between the releases included: (1) the axial directionality of the cracks, (2) the short crack length, (3) the crack location adjacent to girth welds, (4) the circumferential location of the cracks and (5) the intergranular crack morphologies. A comprehensive metallurgical investigation concluded that the likely crack-initiating mechanism was methanol-induced stress corrosion cracking (SCC). While this SCC mechanism is extremely rare in buried petroleum pipelines, all other plausible causes were considered and eliminated. Methanol-induced SCC, similar to other forms of SCC, requires three contributing factors: (1) a susceptible material, (2) a corrosive environment and (3) sufficient tensile stresses. Although much research has been performed on the effects of ethanol on pipeline steels, published data on the effects of methanol is very scarce. A laboratory research program using slow strain rate (SSR) testing was initiated to determine if pipeline steels are susceptible to methanol-induced SCC and identify the conditions necessary to reproduce it in a laboratory environment. This paper outlines the major findings from this program.