- May 4, 2021
- Posted by: Ian Smith
- Category: White Paper
At the heart of a corrosion growth assessment is a comparison between two ILI runs that can highlight changes between them. Those changes can only be caused by two things: corrosion growth or ILI measurement error. Both of those are important but the trick in taking advantage of the CGA process is to find the corrosion growth in amongst the measurement error.
I think it helps to just accept that ILI measurement error is inherent to the process and move towards understanding how it is impacting our ability to find what we are truly after. That is whether there is active corrosion and what is a safe upper bound assumption about a growth rate.
ILI comparisons are normally started at the spreadsheet level and that tells us the differences between the runs but not much about which of the two ILI runs is correct. We have all seen the shrinking metal loss anomaly that was sized smaller than the previous run but that can mean the old ILI was over sizing, or the new ILI was under sizing, or some combination of the two.
A statistical analysis is helpful in identifying trends and can applied to compare based on the anomaly classification. Are the average and peak changes the same for external, internal, repaired, and manufacturing related anomalies? Is the ILI tool performance driving any changes or are they due to corrosion growth?
These questions can only be fully explored with some additional information. The main two sources are a review of the ILI data and anomaly investigation results. By far the easier (and cheaper) of the two is a comparison of the ILI data to look for changes in the underlying ILI signals that are likely to indicate corrosion growth between the inspections. Sometimes the results will surprise you and those surprises are a great way to learn about your pipeline integrity.
Dig programs can be designed to not only validate ILI tool performance but also any findings from the ILI data review such as are ILI signal changes predictive of active corrosion and vice versa.
To sum up, the value of the CGA is often in the comparison in the two runs and in further understanding those differences. ILI data reviews often provide insight that a statistical analysis alone is unable to.