All pipelines are susceptible to the possibility of corrosion damage. Corrosion is a time dependent process that leads to localised gradual thinning of the pipeline wall and, if allowed to continue, will eventually cause failure of the pipewall.

Due to the progressive nature of corrosion, the likelihood of failure increases with time.

One means of mitigating the likelihood of such failures is to perform an in-line inspection using a metal loss detection tool. The advantage of this type of inspection is that it provides details on the size and location of metal loss features. Any features found that exceed the repair criteria will be repaired, thus reducing the likelihood of failure immediately after the inspection and repairs to a negligible level. The ILI does not, however, provide details on whether or not a feature is growing.

One way of determining if a feature is growing is to perform a comparison with the previous ILI, analysis of operating conditions between the two inspections, along with overlaying above ground survey inspections that have been performed between the two inspections.

The comparison of the ILIs will allow the Operator to pinpoint areas of growth, new areas of corrosion, and also areas of potential third party activity. An analysis of the operating conditions will provide information on why internal metal loss features may have appeared or are growing. The above ground surveys will provide information on how long this growth has been occurring for external metal loss features.

The general approach adopted by AFAA for performing an ILI run comparison comprises two stages:

Stage 1 involves collection and analysis of the ILIs that are to be compared, along with any additional data that may provide explanations for differences between the two ILIs.

Stage 2 involves firstly aligning the data between the two ILIs to allow a feature comparison to be made. It then involves looking at each aligned spool and detailing the features from the initial ILI to the most recent ILI to find features that match, features that do not match with the most recent ILI, and also new features on the pipeline which did not appear in the initial ILI. This will allow the amount of growth between the two runs to be calculated.

AFAA have developed very sophisticated in-house software for undertaking ILI Run Comparison studies, it has been subjected to extensive validation and verification processes.