In 2012, NDT Global performed an ultrasonic metal loss inspection for a US-based operator on a pipeline that is over 60 km (37 mi) long, with 8" in diameter, that had known external corrosion. NDT Global re-examined the line in 2015 with its ultrasonic metal loss service.
While the first inspection run identified the features, it did not have the highest-resolution scanning grid to get an accurate sizing of such small pitting corrosion. The limitations on the depth sizing accuracy in the first report posed a challenge for the operator’s integrity analysis team. With the operator facing the alternative of an additional, expensive dig program, a new inspection solution was required to give a more accurate measurement of these pittings and pinholes.
The project team originally planned the inspection without it specifically configured to identify small diameter pittings and pinholes. They performed the inspection by utilizing what was the standard UM technology with a 3.0 mm (0.12 in) axial sampling rate. After receipt of the final inspection report, the operator began a rehabilitation campaign which included verification digs for over 51 features over a two-year period, to validate the accuracy of the ultrasonic metal loss inspection. The digs showed the successful identification of all features. Upon further review of the non-destructive examination (NDE) data, analysts identified many features outside of the performance specifications. Several of these features were under-called, the majority of which were below what was at the standard detection and sizing thresholds in the industry at the time. Due to its axial sampling rate, this older tool frequently failed to detect some small pittings within corrosion areas, did not hit the deepest point of the feature and as a result under-called them. Without the correct feature depth, the operator could not plan the appropriate remediation. To properly prioritize repairs and other actions, the operator needed to be confident of the feature sizing accuracy.