In 2016, Stress Corrosion Cracking (SCC) was identified as a threat to the integrity of a 1980s gas transmission pipeline with a factory-applied asphalt enamel coating. Environmental cracking processes are complex, not always well understood, and often difficult to predict. Additionally, the morphology of environmental cracking is highly variable, and there may be many other features present in the pipe body and seam weld which are not a result of environmental cracking but create similar signals in inspection data. These challenges make it difficult to reliably identify the different feature types based on a single source of inspection data. This in turn creates significant uncertainties as to how to best manage the future integrity of a pipeline subject to environmental cracking.
The ROSEN Crack Management Framework provides a reasoned and systematic approach that focusses efforts on the critical issues and locations. All available data is integrated and utilized to fully understand the probable cause and morphology of any SCC, and to evaluate the sizes of these critical defects in order to ensure safe operation. Since not all parts of the pipeline have the same susceptibility to SCC, dividing the pipeline into segments of similar SCC risk is required to help focus the ILI data evaluation effort and optimize future integrity management activity. Effective segmentation requires the integration of extensive pipeline information, including original pipeline design data, pipeline coating data, construction methods, terrain representations such as those from Google Earth™, soil data, and rainfall records. This information is then combined with the inline inspection reports; since SCC may be linked to stress raisers such as dents, areas of corrosion, and locations with bending strain.
With a thorough understanding of the issues and constraints, an appropriate inspection system can be selected. In this case, four technologies were chosen: EMAT-C, MFL-C, Geometry and IMU. The technologies are delivered via three different tools. The pipeline comprises 6 piggable segments, giving a total of 18 inline Inspections. The resulting multiple data sets, including information on crack-like features, weld anomalies, metal loss, coating disbondment, dents, wrinkles, and curvature (bending) were then aligned and evaluated together to aid in the identification of any features of concern.
The performance of the inspection system was proven by pull testing the tools through sample pipes to demonstrate that any significant features could be reliably identified and sized. The pipe sample included several SCC colonies that were found by Magnetic Particle Inspection (MPI). Phased Array UT indicated that the depths of all colonies were less than 0.5 mm. The EMAT data identified one colony with a depth of > 2mm. The depth was confirmed to be > 2 mm by grinding, and the length of the deep section of the colony was 75 mm. An API 579 Level 2 Assessment of this crack colony shows that it would be acceptable for operation at the pipeline design pressure. Therefore confirming that there would be a high-probability of detecting a critical defect which would have to be deeper than 2 mm and longer than 75 mm.
With multiple sections to inspect, the findings from early field verifications were fed back into the data evaluation process to improve the understanding of likely feature types, and to enhance the susceptibility model. After all inspections were complete, and several verification digs were executed to provide additional information, a long-term management plan was developed. The plan was based on a thorough understanding of the potential causes of cracking, the susceptibility to cracking, credible growth rates for different locations, and the overall risk with local population levels considered. The comprehensive long-term plan encompasses a schedule of further excavations, above-ground surveys, repeat inspections in several years, and diligent data management.