MFL Tool for In-line Inspection (ILI)

Principle of Magnetic Flux Leakage (MFL)

A magnetic flux leakage (MFL)  inspection is the type of in-line inspection technology in which a magnetic saturation is induced in the pipe wall between two poles of a permanent strong magnet. The pipe wall is magnetised by using this magnet. Anomalies affect the distribution of the magnetic flux in the wall, by which it is used to detect and characterize anomalies accordingly. Any metal-loss in the pipe wall will cause the induced magnetic flux leakage, which can be monitored using a detector and hence metal-loss defects can be identified and sized. MFL can be utilised by both liquid and gas pipelines. MFL principles

 

Axial MFL Tool

Axial MFL Tool  Detection 

Axial MFL Tool

Axial MFL Tool

Depending on the layout of magnets, the permanent magnetic field can either be axial or circumferential. For the circumferential MFL, it is best for detecting and sizing the defects orientated along the pipeline axial direction. For axial MFL, it is best for detecting and sizing the circumferential defects, such as circumferential slotting and grooving. Therefore, the understanding of major flaw type is critical for the MFL tool selection.

MFL Application and Limitation

Circumferential MFL Tool

Circumferential MFL Tool

MFL is considered as a metal loss tool since significant of leakage is required for the detection. NACE SP 0102 has outlined the anomalies and features are likely to be detected by MFL.

Table 1 Guideline of  MFL for pipeline anomalies detection (NACE SP 0102)

Anomaly Imperfection/Defect/Features Standard resolution MFL High resolution MFL
Metal loss External corrosion Detection

Sizing

Detection

Sizing

Internal Corrosion Detection

Sizing

Detection

Sizing

Gouging Detection

Sizing

Detection

Sizing

Crack-Like Anomalies Narrow Axial External Corrosion Detection Detection
Stress Corrosion Cracking No Detection No Detection
Fatigue Cracks No Detection No Detection
Long Seam Cracks, etc. (toe cracks, hook cracks, incomplete fusion, preferential seam corrosion) No Detection No Detection
Circumferential Cracks No Detection Detection

Sizing

Hydrogen-Induced Cracking (HIC) No Detection No Detection
Deformation Sharp Dents Detection Detection
Flat Dents Detection Detection
Buckles Detection Detection
Wrinkles, Ripples Detection Detection
Ovalities No Detection No Detection
Misc.
Components
In-Line Valves and Fittings Detection Detection
Casings (Concentric) Detection Detection
Casings (Eccentric) Detection Detection
Bends Limited detection Limited detection
Branch
Appurtenances/Hot Taps
Detection Detection
Close Metal Objects Detection Detection
Thermite Welds No Detection No Detection
Pipeline Coordinates No Detection Detection
Previous
Repairs
Type A Repair Sleeve Detection Detection
Composite Sleeve Detection Detection
Type B Repair Sleeve Detection Detection
Patches/Half Soles Detection Detection
Puddle Welds Limited detection Limited detection
Misc.
Damage
Laminations Limited detection Limited detection
Inclusions (Lack of Fusion) Limited detection Limited detection
Cold Work No Detection No Detection
Hard Spots No Detection No Detection
Grind Marks Limited detection Limited detection
Strain No Detection No Detection
Girth Weld Anomaly(voids, etc.) Limited detection Detection
Scabs/Slivers/Blisters Limited detection Limited detection

Reference

NACE SP0102 In-Line Inspection of Pipelines

M Orazem, Underground Pipeline Corrosion,  Woodhead Publishing

Lloyd Pirtle, An Update of ILI Tools & Other Industry Technologies for the Oklahoma Gas Association,  August 27th, 2013

In-Line Inspection Tools for Pipelines

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