The detection of pipeline SCC flaws using the ACFM technique

Whilst pigging techniques are being developed to locate and size Stress Corrosion Cracking (SCC) in pipelines, the industry still relies heavily on manual inspection ofthe pipe at discrete locations to identify cracking. This normally entails excavating sections of the pipeline, removing the protective coatings and inspecting with magnetic particle testing to locate and assess the SCC area, then using incremental grinding to determine the depth. Alternating Current Field Measurement (ACFM) testing is a non-contacting electromagnetic technique normally used for detecting and sizing flaws in welds but has also been found to be able to detect clusters of SCC through coatings. The technique can operate through coatings and provide an estimate of defect depth. Sample sections recovered from pipelines containing SCC were examined using an ACFMT array probe and provided good correlation in identifying and localising the SCC indications. The equipment was then taken out to dig sites in Canada where it was used alongside MT on exposed sections of pipeline both through coating and on bare metal. The correlation with MT results was excellent and the equipment proved robust in field use. To date, models for crack depth sizing on SCC are in their infancy and more work is planned to evaluate the depth sizing predicted by ACFM.