PIPELINE INSPECTION-TOOL SPEED ALTERS MFL SIGNALS

D. L. Atherton, C. Jagadish, P. Laursen, V. Storm, F. Ham, B. Scharfenberger
Queen's University
Kingston, Ont.

Detailed measurements show that pipeline in-line inspection-tool speed can cause significant reduction in defect-induced magnetic flux leakage (MFL) signals.

This reduction must be taken into account in any attempt to measure defect penetration to 10% accuracy as opposed to simply detecting anomalies.

It is generally known that the defect-induced signals obtained from MFL tools used for in-line inspection of oil and gas pipelines are sensitive to the velocity of the inspection tool.

It is often thought that this is due to the use of simple inductive sensing coils in which, because the induced voltage is proportional to the rate of change of flux, signals should increase with increasing velocity.

Other experts in this field have reported that with proper design the effect can be virtually eliminated, and they have supported this claim with simple examples of metal-loss defect signals.1

Our experience with magnetic levitation systems2 3 suggests that, for typical pipeline MFL tools, the time constant for the magnetic flux to diffuse through the pipe wall could be comparable with the transit time. Consequently we expected significant changes in the anomalous MFL patterns induced by defects, particularly for farside corrosion pits.

We have therefore undertaken precision experimental measurements of defect-induced magnetic leakage flux distributions generated with an MFL anomaly detector representative of the advanced inspection tools used for pipeline monitoring.

Given here are some initial results from detailed maps of defect-induced MFL patterns made at different relative tool velocities for various defect penetrations.

These show obvious and significant variations with velocity. These results will help balance earlier contrary statements.

EDDY CURRENTS

Since line-pipe steel is a conducting ferromagnetic material, the changing magnetic fluxes occurring in the pipe wall during the passage of an MFL inspection tool generate eddy currents. These retard the diffusion of magnetic flux through the pipe wall.

Simply stated, the question of whether velocity effects are significant depends on the ratio of magnetic diffusion time constant to detector transit time.

The problem can be readily stated because transit time is determined by velocity and detector length, or rather wavelength, while the differential equations governing diffusion are also well known.

Formidable complications arise, however, when solution methods are considered. These are due to the nonlinear, hysteretic behavior of line-pipe steel, to the changing geometry resulting from the relative motion between the defect and detector, and to the need to consider three-dimensional modeling for realistic defects.

The result is that there is very little hope of obtaining analytic solutions without seriously oversimplifying the problem.

The prospects for numerical solutions using, for example, modern finite-element calculation techniques are not much better. The detailed computation of such three-dimensional, nonlinear, transient effects is somewhat beyond the capabilities of both current state-of-the-art computer hardware and software.

Experimental measurements appear to offer the best approach for some time to come.

EXPERIMENTAL METHODS

The precision computer-controlled scanning and data acquisition system previously used has been described.4 This has been upgraded to enable real time dynamic measurements, to allow detailed magnetic measurements of the line pipe being investigated, and to incorporate other novel features.

This system has been used to measure the detailed magnetic leakage fields near corrosion pits simulated by ball-milled blind holes on the far side of the test piece to an MFL detector. The detector uses high-strength neodymium iron boron magnets and is similar to those used in advanced inspection tools.

The field distributions have been measured at different relative velocities. These measurements enable the responses of different sensors such as fluxgates, Hall probes, and induction coils of different sizes and configurations to be calculated easily.

Magnetic flux densities are, of course, vector quantities. Three-component measurements or whole field measurements can be made but are seldom needed because most magnetometers or gra-diometers used as sensing devices respond to only one field component.

This is fortunate because it allows presentation of the needed results as relatively simple contour maps or surfaces. In this case we have chosen to present these for the normal or radial field components because these are often the most useful.

We are able to vary many test parameters because, as indicated, velocity effects also depend on the magnetic wavelength of the disturbance produced by the tool, the magnetic properties of the line pipe, the flux density in the pipe wall, stress, etc.

The thickness of the test piece and the penetration of the pit are, of course, also crucial parameters. The only parameters varied for the results presented here are defect penetration and velocity.

Other parameters have been chosen to be representative of the high flux density advanced MFL tools used in large-diameter transmission lines.

FLUX DIVERSION

Figs. 1 and 2 show an example of a surface profile and contour map of the radial component of the magnetic flux density measured above a 75% penetration simulated far-side corrosion pit, measured at a velocity of 6.8 m/sec (15 mph).

Near the defect, flux is diverted from the pipe wall and then returns to it. This effect is clearly seen in both the surface profile and the map contours.

Figs. 3 and 4 show the corresponding plots with the detector stationary for comparison.

Fig. 5 shows processed versions of similar data for a 35% penetration defect in which we have averaged the signal over a width of 16 mm normal to the direction of motion in order to simulate the track width scanned by a typical MFL sensor.

This also has the effect of smoothing the irregularities in the data caused by local variations in the magnetic properties of the steel wall. The defect signals are very distinct.

Fig. 6 shows the peak-to-peak signal amplitudes of 35%, 55%, and 75% far-side simulated corrosion pits as functions of tool velocity up to about 7 m/sec (16 mph), which is a reasonably typical maximum inspection velocity.

These results show defect-induced magnetic leakage flux densities decreasing substantially with increasing velocity. The effects are most severe for shallow far-side defects.

This is as anticipated qualitatively. The magnitudes of the decreases are large, however, and could not be predicted accurately.

SIGNIFICANT REDUCTION

These detailed measurements show that increasing MFL tool speed can cause significant reduction in defect-induced flux leakages and that the effects depend on the defect geometry. Clearly these must be taken into careful consideration in attempts to measure defect penetration to within 10% of wall thickness.

It should be emphasized further that these effects also depend on tool geometry, in particular sensor position and the effective magnetic properties of the line pipe steel.

These factors are themselves dependent on the line-pressure stress as well as its composition, structure, processing, and magnetic history.

In short, this is a complicated situation requiring detailed understanding of all these effects. Pipeline operators should bear this in mind when interpreting inspection reports.

ACKNOWLEDGMENT

Research for this article was supported by the Natural Sciences & Engineering Research Council of Canada and Pipetronix, a wholly owned subsidiary of TransCanada PipeLines Ltd.

REFERENCES

Shannon, R. W. E., and Jackson, L., "Flux Leakage Testing Applied to Operational Pipelines," Materials Evaluation, Vol. 46, No. 11, pp. 1516-1524, November 1988.
Atherton, D. L., et al., "The Canadian High Speed Magnetically Levitated Vehicle System," Can. Elect. Eng. J., Vol. 3, No. 2, pp. 326, April 1978.
Atherton, D. L., Eastham, A. R., Ooi, B. T., and Jain, O. P., "Forces and Moments for Electrodynamic Levitation Systems - Large Scale Test Results and Theory," IEEE Trans. on Magnetics, Vol. MAG-14, No. 2, pp. 59-68, March 1978.
Atherton, D. L., "Finite Element Calculations and Computer Measurements of Magnetic Flux Leakage Patterns from Pits," British J. Non-Destructive Testing, Vol. 30, No. 3, pp, 159-162, May 1988.