With increasing reliance on In-Line Inspection (ILI) technologies providing integrity assurance information to pipeline operators, data interpretation is key to ensuring quality integrity assessment activities. The Magnetic Flux Leakage (MFL) technique is extremely useful for detection of metal loss features (among others) in carbon steel pipelines carrying oil or gas. The technique uses disturbances in an induced magnetic field in the pipe wall to detect and size features. Detection and sizing is carried out by automatic or manual interpretation of field disturbances. Features are reported based on specifications defined by the Pipeline Operator’s Forum unless the operator requests otherwise.

As with any measurement technique, assumptions are made regarding the accuracy of the technique, usually specified in terms of Probability of Detection (POD) and Accuracy with an associated confidence level. Feature depth probability of detection is quoted as a percentage of the wall thickness at 90 per cent probability. This means that ILI may not reliably detect features with depths less than this percentage. Measurement accuracy for sizing is quoted in terms of a percentage confidence, with dimensions provided defining the limits of the confidence distribution as shown in Figure 1.

These tolerances can have significant effects on assessment of features for integrity assurance purposes. POD threshold can significantly affect the length and width of reported features when compared to verified data obtained following dig-up or similar verification. Accurate sizing is extremely important for any assessment criteria such as: AS 2885.3, ASME B31G, Modified B31G, BS 7910 and DNV-RP-F101. The same is valid for depth sizing, as the 80 per cent confidence boundary excludes the top and bottom 10 per cent of features - the top 10 per cent being the deepest features, and therefore most likely to affect pipeline integrity.

Verification inspection is the most cost effective method of providing confirmation data of feature dimensions for onshore pipelines. iicorr have found that correction factors based on verification inspection results have significant implications on both shallow and deeper features in terms of pipeline integrity.

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The method which the ILI contractor uses to ‘cluster’ features can also have a significant effect on the results of a fitness for purpose assessment. Clustering is the process by which feature interaction is considered. Where interaction is present, features are combined to be treated as a single feature with dimensions modified by the relevant clustering rules.

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 & gas operations, and also works in the process, marine and utilities sectors.