LINE STRESSES AFFECT MFL DEFECT INDICATIONS

David L. Atherton, Ajay Dhar, Christian Hauge
Queen's University
Kingston, Ontario

Poul Laursen
Pipetronix
Toronto

Pipeline-defect measurements by high-resolution magnetic flux leakage (MFL) in-line inspection tools are influenced by bending and line-pressure stresses.

Tests conducted on a single 36-in. sample with a machined defect indicate that much further research into this phenomenon is needed to reveal methods for correcting for stress effects.

The limited, preliminary tests reported here were conducted at Queen's University, Kingston, Ont., and were supported by the Canadian Natural Sciences & Engineering Research Council and by Pipetronix, Toronto.

SHIFT IN MONITORING

Demand for pipeline monitoring is shifting from simple defect detection and classification to the accurate measurement of defects. Fracture mechanics principles can then be used to calculate maximum allowable operating pressures.

Comparison of the results from periodic regular inspection records can also be used to check for actively growing defects and to monitor the effectiveness of corrective action and protective measures.

Excellent high-resolution MFL inspection tools, such as the one shown in Fig. 1, are now widely available for in-line inspection of oil and gas pipelines.

Obtaining highly accurate, quantitative estimates of defect severity, even with the use of high-resolution detectors, remains difficult, however. Modern high resolution MFL inspection tools can give very detailed maps of defect-induced anomalous magnetic leakage flux patterns, particularly from interior and exterior corrosion. These MFL patterns vary with operating parameters such as tool speed (OGJ, Jan. 29, 1990, p. 84) and stress.

Local stress anomalies, bending stress, and line pressure stress all influence the defect-induced MFL patterns (OGJ, Oct. 27, 1986, p. 86). These factors must either be controlled or proper allowance made for them.

This is particularly important when inspection tools are calibrated by pulling them through a short test section of open line with known defects and then used to monitor in-service lines. Many lines are now fitted with launchers and traps to allow in-service inspection without the expense of depressurizing and the lost product throughput during inspection.

EXPERIMENTS, RESULTS

Shown here are the results of precision field mapping of MFL patterns above a 12-mm diameter, 50% penetration, ball-milled, round-bottomed blind hole which simulates a corrosion pit on the detector side of a 9-mm (0.355 in.) W.T., 910-mm (36 in.) OD pipe wall.

For these tests, bending stress was used so that both tensile and compressive surface stresses could be generated. Other tests using simulated line pressure stress were also run.

A typical MFL anomaly detector was used to magnetize the pipe, and a high-resolution Hall probe was used to scan the surface of the pipe to measure the MFL pattern.1

The results were processed to obtain the anomalous defect-induced MFL pattern by subtracting the corresponding defect-free pattern. The measurements shown here are for radial field components.

Fig. 2 shows the radial components of the defect-induced anomalous MFL patterns above the same 12-mm diameter, ball-milled, round-bottomed, 50% penetration, near-side pit under no stress, 300-MPa (approximately 43,500 psi) tensile, and 300-MPa compressive surface stresses, respectively. A small amount of the magnetic flux approaching the defect is diverted into the air near the pit and subsequently returns to the pipe wall. This gives the characteristic sinusoidal pattern discernible in all these results.

In this case, both tensile and compressive stress reduced the magnitude of the MFL significantly, although the actual patterns are different. It is perhaps somewhat surprising that both tension and compression give reductions in signal amplitude, but signals increasing with stress have also been measured.

In fact, the results depend on the anomaly detector and test conditions and also on the magnetic properties of the particular sample pipe joint under test.

Fig. 3 shows the amplitude of the MFL pattern, integrated so as to synthesize the response from a typical high-resolution sensor, as a function of stress for a cycle of tensile and compressive stress to 300 MPa. It will be noted that the results are repeatable and nearly reversible with little indication of significant hysteresis.

CARE NECESSARY

The examples given here show that the effects of stress on MFL signals are large and complex.

It is impossible here fully to explain these effects since a detailed understanding is needed of the effects of stress on the magnetic properties of line pipe steels.

This understanding depends on research which has already been in progress for more than a decade.

It is, however, clear that the results of high-resolution tools cannot be used directly to obtain reliable high-accuracy measurements of corrosion defect geometries. Considerable care is needed for accurate interpretations of high-resolution MFL responses which are used to ensure pipeline integrity and reliable operation. But there are several obvious, simple initial suggestions worth consideration: Line pressure should, of course, be recorded any time a high-resolution MFL tool is used with the objective of accurately determining defect sizes.

Open line-pull test calibrations against known test defects must be adjusted if the tool is subsequently used in a pressurized line. Unless the monitoring personnel understand the complex effects of line-pressure stress, several (probably large) excavations of the pipeline should be made to measure detected defects.

Interpretation of the effects of stress on MFL Signals depends on a detailed understanding of the magnetic properties of line-pipe steels and these properties' variations with stress. There is as yet inadequate information in the literature on this important science.

Further fundamental research is highly desirable. One of the objectives of such research should be to determine how to correct for stress effects.

Another valuable outcome of the research on the effects of stress on the magnetic properties of pipeline steels is learning which conditions to control in order to obtain repeatable results.

A longer term goal should be to consider the suitability of line-pipe steels for inspection. In addition to being magnetic, the ideal material for MFL inspection should have uniform, isotropic magnetic properties which are independent of stress or other pipeline conditions and have low hysteresis and high electrical resistance.

Failing this, their magnetic behaviors should be well understood. Real materials, however, are far from being ideal, but some are worse than others. Variations in magnetic properties and behavior between different pipes and within joints of pipe are major problems.

Inspectability, therefore, should be as important a criterion in the selection of line pipe as weldability, strength, toughness, and corrosion resistance. In fact, some of the more progressive pipeline operators consider inspectability when selecting materials for new construction.

REFERENCE

Atherton, D.L., "Finite Element Calculations and Computer Measurements of Magnetic Flux Leakage Patterns from Pits," British Journal of Non-Destructive Testing, Vol. 30, No. 3, January 1988, pp. 159-162.