The Magnetic Flux Leakage (MFL) technique is extremely useful for the detection of metal loss features (among others) in carbon steel pipelines carrying oil or gas. However, feature depths reported by an MFL ILI run should not be taken as absolute; the feature dimensions need to be validated using a more direct measurement technique such as ultrasonic testing (UT). This is especially the case for feature depths.
iicorr has recently undertaken analysis of a 20 inch oil line with a wall thickness of 15.9 mm. The pipeline produces some 150,000 barrels of oil a day. Concerns were raised by the operator having conducted three MFL ILI runs in consecutive years, with significant wall thickness loss reported throughout the pipeline. Recently, feature depth verification was carried out on an onshore section of the pipeline, the results of which were passed to iicorr and can be seen in Figure 1.
The graph in Figure 1 indicates that the MFL ILI has
significantly over-estimated the depths of the features. On a cautionary note, the UT inspection will not provide absolute depth values as it is also subject to error. In reality the absolute depth is likely to lie somewhere in between and it should be noted that only the deeper reported features were verified.
Using the results of the validation, a depth correction factor can be evaluated and applied to all the MFL ILI features giving a truer representation of feature depth. The graph in Figure 2 demonstrates the application of the correction factor to reduce the apparent error. The graph in Figure 3 shows the effect of the correction on a typical Fitness for Purpose assessment.
iicorr’s Perth office is the company’s centre of excellence for pipeline integrity activities and is part of a network of integrity management operations with offices around the world. The company specialises in supporting all integrity management activities for upstream and downstream oil and gas operations, and also works in the process, marine and utilities sectors.