Magnetic flux leakage (MFL) detection is one of the most popular methods of pipeline inspection. It is a nondestructive testing technique which uses magnetic sensitive sensors to detect the magnetic leakage field of defects on both the internal and external surfaces of pipelines. This paper introduces the main principles, measurement and processing of MFL data. As the key point of a quantitative analysis of MFL detection, the identification of the leakage magnetic signal is also discussed. In addition, the advantages and disadvantages of different identification methods are analyzed. Then the paper briefly introduces the expert systems used. At the end of this paper, future developments in pipeline MFL detection are predicted.

https://www.mdpi.com/1424-8220/15/12/29845/pdf

Theory and Application of Magnetic Flux Leakage Pipeline Detection

Shearography and thermography are optical techniques, both proven to be valuable tools for material nondestructive evaluation. Papers on these topics, however, are scattered and mainly appeared in optical journals. For the convenience of the materials community, this paper aims to present a comprehensive review of shearography and active thermography and their applications in nondestructive evaluation of materials. Both techniques enjoy the merits of full-field, non-contact and allowing speedy detection of material defects in metal, non-metal as well as composites materials. However, they are fundamentally different in flaw detection mechanisms. Shearography measures materials’ mechanical response to stresses, whereas active thermography measures material's heat-transfer response to an instantaneous thermal excitation. A comparison of the advantages and limitations of two techniques for nondestructive evaluation will also be presented.

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Review and comparison of shearography and active thermography for nondestructive evaluation

A review of three magnetic NDT technologies

Ship-shaped offshore units are some of the more economical systems for the development of offshore oil and gas, and are often preferred in marginal fields These systems are especially attractive to develop oil and gas fields in deep and ultra-deep water areas and remote locations away from existing pipeline infrastructures Recently, the ship-shaped offshore units have been applied to near shore oil and gas terminals This 2007 text is an ideal reference on the technologies for design, building and operation of ship-shaped offshore units, within inevitable space requirements The book includes a range of topics, from the initial contracting strategy to decommissioning and the removal of the units concerned Coverage includes both fundamental theory and principles of the individual technologies This book will be useful to students who will be approaching the subject for the first time as well as designers working on the engineering for ship-shaped offshore installations

Ship-Shaped Offshore Installations : Design, Building, and Operation

High-speed non-destructive inspection (NDI) systems using magnetic flux leakage method (MFL) is in great demand in online metal inspection and defect characterisation, especially in pipeline and rail track inspection. For MFL systems at high speed measurement, in addition to magnetic flux, eddy currents exist in metal specimen because of the relative movement between the probe and specimen. These currents alter the profile of electromagnetic field, which increases difficulty in signal interpretation and defect characterization. In this paper, eddy currents generated by high speed movement and their characterisation in high-speed MFL inspection systems were investigated by using numerical simulations. Besides, the MFL signals from high speed MFL measurement against defect depth were examined.

Numerical simulation on magnetic flux leakage evaluation at high speed

Simulated defects of different shapes and sizes were created in a section of API X70 steel line pipe and were investigated using a residual magnetic flux leakage (MFL) technique. The MFL patterns reflected the actual shape and size of the defects, although there was a slight shift in their position. The defect features were apparent even at high stresses of 220 MPa when the samples were magnetized at those particular stresses. However, unlike the active flux technique, the residual MFL needs a sensitive flux detector to detect the comparatively weaker flux signals.

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Residual Magnetic Flux Leakage: A Possible Tool for Studying Pipeline Defects

We have used three-dimensional (3D) magnetic finite element analysis (FEA) to simulate the MFL signal from a circular dent geometry with associated residual stresses. Strain distribution information around the dent was obtained from an earlier work using finite element structural modeling. In the magnetic FEA dent model, the localized residual stresses were simulated by assigning appropriate values of magnetic anisotropy to the relevant magnetic regions. The simulated flux leakage patterns were found to be in good agreement with the experimentally observed MFL patterns associated with dent shape as well as with the stress effects from the dent.

http://www.physics.queensu.ca/~lynann/references/research29.pdf

Mechanical damage detection using magnetic flux leakage tools: modeling the effect of dent geometry and stresses

We used three-dimensional (3-D) magnetic finite-element analysis (FEA) to simulate the effect of localized residual stresses on magnetic flux leakage (MFL) signals from a steel plate in the absence of a geometrical defect. We derived the local residual stress patterns from finite-element structural modeling of simulated dents. The magnetic FEA model simulates these localized residual stresses by assigning appropriate directional permeability values to the magnetically anisotropic materials. Considering the necessary simplifications required for magnetic FEA modeling, the simulated MFL patterns are in good agreement with the experimentally observed patterns associated with the stresses around a dent.

/pdf/mechanical-damage-detection-with-magnetic-flux-leakage-tools-23fjaxpvmu.pdf

Mechanical damage detection with magnetic flux leakage tools: modeling the effect of localized residual stresses

Since magnetism is strongly stress dependent, Magnetic Flux Leakage (MFL) inspection tools have the potential to locate and characterize mechanical damage in pipelines. However, MFL application to mechanical damage detection faces hurdles which make signal interpretation problematic: 1) the MFL signal is a superposition of geometrical and stress effects, 2) the stress distribution around a mechanically damaged region is very complex, consisting of plastic deformation and residual (elastic) stresses, 3) the effect of stress on magnetic behaviour is not well understood. This paper summarizes recent results of experimental and modeling studies of MFL signals resulting from mechanical damage. In experimental studies, mechanical damage was simulated using a tool and die press to produce dents of varying depths in plate samples. Radial component MFL measurements were made before and after selective stress-relieving heat treatments. These annealing treatments enabled the stress and geometry components of the MFL signal to be separated. Geometry and stress effects generate separate MFL peaks — the geometry effects lead to central peak regions while the stress effects produce ‘shoulder’ peaks. In general the geometry peaks tend to scale with depth, while the shoulder peaks remain approximately constant. This implies that deep dents will display a ‘geometry’ signature while shallow or rerounded dents will have a stress signature. Finally, the influence of other parameters such as flux density and topside/bottomside inspection was also quantified. In the finite element analysis work, stress was incorporated by modifying the magnetic permeability in the residual stress regions of the modeled dent. Both stress and geometry contributions to the MFL signal were examined separately. Despite using a number of simplifying assumptions, the modeled results matched the experimental results very closely, and were used to aid in interpretation of the MFL signals.Copyright © 2004 by ASME

/pdf/detection-of-mechanical-damage-using-the-magnetic-flux-148b41zrf6.pdf

Detection of Mechanical Damage Using the Magnetic Flux Leakage Technique

The Magnetic Flux Leakage (MFL) technique is sensitive both to pipe wall geometry and pipe wall strain, therefore MFL inspection tools have the potential to locate and characterize mechanical damage in pipelines The present work is the first stage of a study focused on developing an understanding of how MFL signals arise from pipeline gouges A defect set of 10 gouges were introduced into sections of 12″ diameter, 5m long, end capped and pressurized X60 grade pipe sections The gouging tool displacement ranged (before tool removal) between 25 to 125mm Gouges were approximately 50mm in length The shallowest indentation created only a very slight scratch on the pipe surface, the deepest created a very significant gouge All gouges were axially oriented Experimental MFL measurements were made on the external pipe wall surface (pressurized) as well as the internal surface (unpressurized) The early results of the experimental MFL studies, and a hypothesis for the origin of the MFLaxial signal “dipole” are discussed in this paper© 2010 ASME

Vijay Babbar

Papers

Residual Magnetic Flux Leakage: A Possible Tool for Studying Pipeline Defects

Mechanical damage detection using magnetic flux leakage tools: modeling the effect of dent geometry and stresses

Mechanical damage detection with magnetic flux leakage tools: modeling the effect of localized residual stresses

Detection of Mechanical Damage Using the Magnetic Flux Leakage Technique

Understanding Magnetic Flux Leakage Signals From Gouges