Finite element calculations on the effects of permeability variation on magnetic flux leakage signals

Finite element calculations on the effects of permeability variation on magnetic flux leakage signals D.L. Atherton and W. Czura Magnetic flux leakage anomaly signals from pipeline inspection tools can be obtained from hard spots or other regions of anomalous permeability. The signals obtained from corrosion pits are dependent on pressure and this is attributed to stress-induced permeability changes. The results of two-dimensional finite element calculations of the effects of permeability measurements on such signals suggest that permeability changes by as much as a factor of two may be required to produce these effects.

Keywords: magnetic

flux leakage, calculations, pipeline inspection

We have investigated the effects of permeability variations on the magnetic flux leakage signals obtained from pipeline inspection tools. These use strong permanent magnets to drive the pipe wall to near saturation. Anomalies in the pipe wall, such as corrosion pits, hard spots oi" permeability variations, produce anomalies in the leakage fluxes near the wall which may be detected by sense coils moving with the tool or by Hall probes. Figure l shows a record, made on our rotating-drum test riglq, of the radial component of the leakage flux density measured around the circumference of a 914 m m diameter, 10 m m thick wall section of pipeline containing

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Circumferential distance (cm) Fig. 1 A circumferential scan of the radial component of flux density from a magnetic flux leakage detector made using a Hall probe. Flatbottomed holes milled in the wall simulate defects on the nearside. D i a m e t e r s a n d p e n e t r a t i o n s are as indicated. The wall thickness is 10 mm a n d the flux density in the pipe in the vicinity of the sensor is approximately 1 T

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simulated corrosion pits. Anomaly signals from the corrosion pits (A-D) and welds (W) are seen superimposed on what appears at first to be background noise. The noise is in fact absolutely repeatable from run to run over long periods of time and including demagnetization and magnetization cycles and follows a pattern which varies slowly along the axial length of the pipe. It is therefore attributed to predominantly circumferential permeability variations in the wall of this seam-welded pipe and is in fact detailed information rather than noise. Hard spots in the wall, produced by unusually rapid local water cooling in the pipe mill, imply locally anomalous magnetic as well as mechanical properties and also give rise to magnetic flux leakage signalsl21. Figure 2 shows an example of a residual field magnetic flux leakage signal caused by a hard spot. Line pressure stress causes significant changes in magnetic flux leakage signals and noise levels observed in pipelinesl31, as shown in Figure 3. Signals initially decrease with increasing pressure and then increase until, at high

0308-9126/87/040239-O3 $3.00 © 1987 Butterworth ~ Co (Publishers) ktd NDT International Volume 20 Number 4 August 1987

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identical meshes are used for all the calculations. The only changes made between calculations are in the specification of materials so that results are fully comparable. The outputs shown are profiles of the radial flux density component in the air just inside the pipe in the vicinity of a farside defect. The centrally located farside defect is specified as either air or steel with an anomalous permeability. It is flat-bottomed and has 50% penetration and square cross-section; since the calculations are two-dimensional, it represents a circumferential groove or anomaly in the pipe rather than a pit. Experimentally measured mean magnetization curves have been used to represent the various materials properties. Permeability variations have been modelled by scaling the H field axis of the material property curve, thus giving a permeability ajustment which is independent of flux density.

Pressure (MPa) Fig. 3 Magnetic flux leakage signal-to-noise ratio as a function of internal line pressure. The ratios are normalized with respect to the zero-stress value

pressure, there has been an overall increase, while noise decreases monotonically with increasing pressure. The effects are reversible with pressure and independent of defect penetration and are therefore attributed to overall stress-induced changes in incremental permeability rather than to local stress effects at the defects. While the changes are in general qualitative agreement with our measurements of stress-induced changes in magnetic properties of pipeline steelsl4,51, the effects seem surprisingly large. Since it is not easy to measure these local changes in permeability directly under these conditions, we have made finite element calculations of the effects of permeability changes.

Results Figure 5 shows calculated axial profiles of the radial magnetic flux density component near the surface of the pipe wall in the vicinity of the defect. Calculations for various defects are compared by specifying the defect

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We have used the Infolytical61 M A G N E T finite element software package for these calculations. The finite element calculation technique has been particularly well described recently by Lowther and Silvester 171. An outline of the permanent magnet magnetic flux leakage detector analysed is shown in Figure 4. The calculations are for a two-dimensional model only, but about 4000 mesh points are used. The same model and

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Circumferential distance (m) Fig. 5 Calculated axial profiles of the radial magnetic flux density component near the surface of the pipe wall in the vicinity of a defect. The defect is specified successively as air, no defect, pipe wall with the permeability increased then decreased by 10% and pipe wall with the permeability increased then decreased by a factor of two 60 /~------.~

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Fig. 6 Calculated axial profiles of the radial magnetic flux density component near the surface of the pipe wall in the vicinity of a defect. The pipe wall permeability is specified successively as normal, increased then decreased by 10% and 20% and increased then decreased by a factor of

two

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reversible incremental permeability to see if this is significantly greater, although this seems unlikely.

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Normalized pipe permeability Fig. 7 Defect signal peak-to-peak amplitude as a percentage normal pipe wall permeability. Signal permeability are normalized with respect to calculated and permeability with experimentally measured magnetic

function of the amplitude and signal amplitude properties

successively as air, no defect, pipe wall with the permeability increased then decreased by 10% and pipe wall with the permeability increased then decreased by a factor of two. The permeability changes are modelled by scaling t h e H axis of the pipe wall magnetization curve. It is clear that large changes in permeability are needed to give signals approaching those due to corrosion pits or rather grooves in this two-dimensional case. This implies that the permeability variations required to give hard spot signals or typical noise levels are also large. Figure 6 shows calculated axial profiles of the radial magnetic flux density component near the surface of the pipe wall in the vicinity of the defect. Calculations made using various pipe wall permeabilities are shown, and again it is clear that relatively large changes in pipe wall permeability are needed to change the amplitude of the defect-induced magnetic flux leakage signal significantly. This is further shown by Figure 7, which shows the normalized amplitude of the defect signal as a function of the percentage normal pipe wall permeability.

Conclusions Typical maximum stress-induced changes in permeability that we have measured for these pipeline steelsl41 at stress levels of 200 MPa have been in the 10%-20% range. which seems too low to account for the observed effect of line pressure stress on pipeline magnetic flux leakage signals. We are also investigating the effect of stress on

Several possible calculation difficulties have also been suggested as being possible reasons for the apparently rather large changes in permeability needed to explain our measurements. One suggestion is that the calculations are only two-dimensional whereas corrosion pit signals are generated in a three-dimensional system; however, we have found that the permeability fluctuations giving rise to noise, or rather the noise itself, run in well correlated axial stripes and are therefore well modelled by twodimensional calculations. A further possibility is that the saturation region of the magnetization curve of the pipeline steel is not well modelled in the calculations. This was checked by reducing the magnet M M F until the pipe wall flux density was reduced by a factor of three in order to be certain of avoiding any problems arising from extrapolations of measured magnetization curves. The results obtained were essentially similar. We conclude from our calculations that surprisingly large variations in permeability are needed to explain our experimental results. Since permeability measurements suggest relatively small changes, there may be other factors involved also.

Acknowledgement This research is supported by the National Sciences and Engineering Research Council of Canada. References

1 Atherton,D.L. and Welbourn, C. 'A rotating drum test rig for the development of pipeline monitoring tools" Can Soc NDT J 6 8 (1985) pp 50-56 2 Athertan,D.L 'Magnetic detection of pipeline corrosion"Proc Int Congr on Metallic Corrosior~ Toronto National Research Council of Canada, Ottawa, 4 (1984) pp 151-156 3 Atherton, D.L 'The effectof line pressure on the performance of magnetic inspection tools for pipelines' Oil & GasJ 84 43 (1986) pp 86-89 4 Dobranksi,L.G. Jiles, D.C. and Atherton, D.L. 'Dependence of the anhysteretic magnetization on uniaxial stress in steel'JAppl Phys 57 1 (1985) pp 4229-4231 5 Atherton D.L. and Szpunar, J.A. 'Effect of stress on magnetization and magnetostriction in pipeline steel" IEEE Trans Magn MAG-22 5 (1986) pp 514-516 6 lnfolyticaCorp 1500 Stanley St, Suite 430, Montreal H3A 1R3, Canada 7 Lowther, D.A. and Silvester, P.P. Computer Aided Design in Magnetics Springer, New York (1986)

Authors The authors are in the Department of Physics, Queen's University, Kingston, Ontario K7L 3N6, Canada.

Paper received 24 November 1986. Revised 24 February 1987

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