SCC is the most insidious, and often the most unexpected form of pipeline failure that can involve no metal loss and must not be confused with wall thinning rupture. Whilst there is a fair understanding of the types and causes of SCC, the prediction and location of SCC cracks on Operational Pipelines is not so well developed. However, the location and removal of SCC Cracks before the pipeline bursts uncontrollably is vital for safe Operations.

Looking for Stress Corrosion Cracks

SCC can occur in both Gas and Liquid pipelines but is more common and catastrophic in Gas Pipelines. PIM Ltd offer a proven technique of locating external pipeline SCC through a combination of high quality data gathering and detailed analysis of combined data sets from a variety of pipeline parameters and survey techniques.

This approach is proven to be 40 to 50% accurate at locating Carbonate/Bicarbonate SCC, an accuracy unmatched by others. In other words if you dig ten holes to expose a pipeline at predicted locations four or even five of them of them should have pipe showing SCC. Such studies do not stop there, an analysis of the Operating Conditions of the pipeline allows recommendations to be made to reduce the risk of SCC recurring or developing.

Types and Causes of Stress Corrosion Cracking.

The two common forms of SCC are Low pH and Carbonate /Bicarbonate. Both forms require a coating fault exposing the steel pipe to the local soil environment where conditions conducive to SCC must exist. SCC has been observed on pipelines coated with Tape, Coal Tar and Bitumen. All pipeline steels are susceptible but most failures to date have been on X52 Pipe, which together with the above coatings have been the most widely used pipe and coatings of sufficient age to show SCC.

External Conditions Needed to Develop Carbonate/Bicarbonate SCC.

1. Stress levels in the pipe generally greater than 45% SMYS, and slow cycling of that stress.
2. A coating fault exposing steel to the soil.
3. Polarised potentials on the steel surface at coating fault typically between -600 to -720 mV wrt Cu/CuSO4 half-cell as result of applied CP to pipelines with poor coating, often disbonded.
4. A local soil environment containing Carbonates and Bicarbonates, by-products of the Cathodic reaction on the exposed steel pipe surface.
5. Elevated temperature from hot fluid being transported through the pipe, as crack growth is thermally controlled.
6. Crack Morphology -- Intergranular