Characterisation of the morphology of inclined SCC cracks in Australian gas pipelines.

Abstract

Stress Corrosion Cracking (SCC) in pipeline steel occurs when an aggressive environment and tensile stresses act on a susceptible microstructure. Typically, SCC in gas pipelines tends to travel perpendicular to the hoop stresses in the through-wall direction. Studies conducted on the TransCanada pipeline, where a rupture had occurred, revealed the incidence of SCC cracks, whose crack path deviated at an angle from the normal. These unusual inclined cracks have also been found in an Australian pipeline, resulting in a need for a more comprehensive understanding of inclined SCC. As a result, this study has been undertaken to investigate SCC in Australia, in particular the morphology of inclined SCC and together with the many inclined crack features and crack interactions and anomalies (inclusions) in the pipe steel. This study revealed that 81% of SCC cracks investigated were inclined. The majority of cracks analysed were over 4 mm in length, which corresponded to the calculated critical crack length according to industry guidelines. Inclined cracks morphologically presented with a straight section before they inclined away from the perpendicular direction. The straight section tended to be between 200-900 μm and the inclination angle varied between 30-60°. This inclination angle increased as the crack grew deeper into the pipe wall, resulting in long cracks travelling a considerable distance in the hoop direction (3.8 mm travel for a 51 mm longitudinal surface SCC crack). In two cases (out of 120 cracks), subsurface longitudinal crack travel was observed to be approximately 1.5 mm. In most other cases, no subsurface longitudinal travel was observed. Observed crack interactions did not breach current industry guidelines used for SCC threat assessment. Hence, procedures currently employed for critical crack assessment are still deemed valid and conservative enough for Australian operations.