Establishing Corrosion Growth Rates Based on MFL-C vs. MFL In-Line Inspection Run-to-Run Comparisons

Performing in-line inspection (ILI) run-to-run comparisons is a vital component of any pipeline integrity management program. A primary objective of ILI run-to-run comparisons is to establish corrosion growth rates along the length of a given pipeline segment that are defensible, justifiable, and realistic without being overly conservative. Establishing realistic corrosion growth rates based on ILI data can be challenging, especially when considering different technologies are sometimes employed between the subsequent ILI tool runs. One example of this is a circumferential magnetic flux leakage (MFL-C) ILI survey followed by an axial magnetic flux leakage (MFL) ILI survey. While the two technologies are similar in that both can detect and characterize metal loss anomalies, they differ in the direction of the applied magnetic field. As a result, the tools may detect and characterize (i.e., size) the anomalies differently depending on the shapes and orientations of the anomalies. This paper explores the differences observed in the detection and sizing of different anomaly types by the two technologies, how these differences contribute to calculated corrosion growth rates, and what considerations should be made when comparing MFL and MFL-C ILI data for the purpose of determining corrosion growth rates.