Understanding Magnetic Flux Leakage Signals From ...

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Sep 27, 2010 - Vijay Babbar*, Jian Dien Chen*, and Chris Alexander**. *Applied Magnetics Group, Department of Physics, Queen's University, Kingston, ...
Proceedings of the 8th International Pipeline Conference IPC2010 September - October 1, 2010, Calgary, Alberta, Canada September27 27-October

IPC2010-31 IPC2010-31668 Understanding Magnetic Flux Leakage Signals from Gouges Lynann Clapham*+, Vijay Babbar*, Jian Dien Chen*, and Chris Alexander** *Applied Magnetics Group, Department of Physics, Queen's University, Kingston, Ontario, Canada Ph. (613) 533-6444, fax (613) 533-6463, email: [email protected]

**Stress Engineering Services Inc., Houston, Texas, U.S.A. +

corresponding author

ABSTRACT 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 2.5 to 12.5mm. 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.

signals from gouges – in particular, gouges having little or no associated dent. Reported in this paper is the initial subset of results from an ongoing study of gouges at Queen’s University. The larger study involves MFL measurements, magnetic modeling and residual stress determination using neutron diffraction techniques. The present paper is concerned primarily with results from initial MFL measurements. The more comprehensive study, including the residual stress measurement results and magnetic modeling of MFL signals from gouges, will be considered in a subsequent publication. 40

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INTRODUCTION Mechanical damage in pipelines is a serious concern. Although mechanical damage is often benign, in some cases the damaged pipe may fail immediately; alternatively it may remain undetected for years, with the region acting as a site for corrosion or cracking and potentially leading to a delayed failure. Intelligent magnetic flux leakage (MFL) tools are the most cost-effective method for in-line inspection, however traditionally they have been optimized for corrosion detection. Mechanical damage generates MFL signals, but as yet these signals are not yet sufficiently well understood to provide for accurate defect characterization. The difficulty in interpreting these signals arises because MFL signals can arise from geometry changes, but also local microstructural and strain variations in the pipe wall. All three of these are often present in the vicinity of mechanical damage. The present study focuses on understanding MFL

c) gouge MFLaxial signal Figure 1: typical MFLaxial results for mechanical damage: a) modelled result for a circular dent, b) “decoupled” experimental results obtained from a gouge study by Battelle3 c) experimental results obtained from work by the authors on a 17mm gouge created by Gaz de France using their pipeline aggression rig (PAR) experimental facility. All measurements were obtained at the pipe inner wall, and the MFL magnetizing field is applied from left to right.

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Displacement Transducers

Earlier work by the Queen’s Applied Magnetics group1,2 used a combination of modeling and experimental work to successfully identify the origins of the key MFL signal features associated with pipeline dents. Figure 1(a) shows a typical modeled MFLaxial component signal for a circular dent. As seen in figure 1(a), the characterizing features of the MFLaxial dent signal are the two positive peaks that lie on either side of the dent rim. These are generally separated by a shallower negative peak as shown here. MFLaxial component signals arising from gouges are less well understood. Figure 1(b) shows a “decoupled” MFLaxial signal obtained from a study by Battelle3. In a subsequent study involving a pull test with a dual-field MFL tool, a similar pattern was obtained4. Because the primary characteristics of the signal shown in Figure 1(b) are its distinct peaks of opposite polarity (shown as red and blue peaks) the authors of references 3 and 4 referred to this as an MFLaxial “dipole” signal. Although these references indicate that these dipole signals are present after ‘decoupling’ MFLaxial signals obtained from a dual-field tool measurements, the authors of the present study have observed similar dipole-type MFLaxial results using standard (single field level) MFL scan tests on axially-oriented gouges5. Such a result is shown in Figure 1(c). The aim of the present work was to examine the characteristics of MFL signals originating from gouges. A systematic study was designed in which MFL signals were obtained from gouges of similar shape but increasing severity. The origin of the “dipole” characteristic associated with the gouge MFLaxial component signals was of particular interest.

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c APPARATUS AND EXPERIMENTAL GOUGING PARAMETERS Ten different gouges were prepared for this study. As indicated in Table 1, these gouges were introduced into 5 different pressurised pipeline sections (pipes 1-5). All 5 pipes were identical, grade X42, 5m long, 12” diameter and having a 9.5mm wall thickness. The apparatus that was used for gouge creation is shown in Figure 2(a) with a description of the gouging process given in the caption. The gouging tool is illustrated (with gouging in progress) in Figure 2(b). A typical gouge is shown in 2(c). Each gouge was approximately 50mm in length. The “gouge depths” shown in the last two columns of Table 1 are measured relative to the original outer pipe surface. Thus a “gouge depth during gouging” of 12.7mm was created by the tool being hydraulically lowered 12.7mm below the original outside wall surface position, and then held in place when the pipe was pulled axially using the chains.

Figure 2: Figure 2(a) shows equipment available at SES for introducing mechanical damage into a pressurised pipe section. The end-capped, pressurized pipe section is secured onto a moving platform, and connected at one end to a hydraulic cylinder via chains and a mounting ring. The gouging tool (shown in Figure 2(b)) is hydraulically lowered into the pipe wall to a selected displacement below the original pipe wall position (indicated in the “Gouging depth during gouging” column in Table 1), then the pipe is pulled to the left, creating a gouge. Figure 2(b) shows the tool geometry and gouging process for a tool displacement of 6.4mm (250mils)1. Figure 2(c) shows the gouge (~50mm long) created through this process.

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The discussion that follows frequently compares inner wall MFL results, necessarily obtained at zero internal pressure, with outer wall MFL results obtained while the pipe was held at an internal pressure corresponding to 50%MAOP. At this point in the work the zero pressure, outer wall measurements were not available, thus those at 50% MAOP internal pressure outer wall measurements were used instead for comparison purposes. Preliminary measurements of the outer wall MFL signals indicated that the outer wall signal features were similar in both cases (zero and 50% MAOP pressures), although they were somewhat more pronounced at zero pressure. Thus, given that the present study is primarily qualitative in nature, the comparisons between zero and 50% MAOP signals are considered to be legitimate.

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commonly used for field measurements, however radial MFL signals are also very useful – since the radially oriented probe has less liftoff than the axial probe (due to the intrinsic nature of the probe geometry). After completion of all of the outside wall measurements, pipe rings were cut in order to be able to make MFL measurement at the inner pipe wall. These measurements were done (obviously) at zero pressure. Note that the designation “50%MAOP” and “100% MAOP” refer to the internal pipe pressure during the gouging process, not during MFL measurements.

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Table 1: A description of the 10 gouges studied in this investigation. Two gouges were introduced into each of 5 pipe sections. Internal pressure was either 50% or 100% MAOP (Maximum Allowable Operating Pressure) during gouge introduction. Pipe #3 was fatigued cycled (the results of this are not discussed in this paper). The final two columns indicated gouge depths before and after tool removal.

MFL RESULTS: PIPE 1, COMPARISON OF GOUGES PRODUCED AT 50% AND 100% MAOP Pipe 1 contained the shallowest defects – in fact there was no significant visible damage to the pipe after the gouging process – only minor-looking scratches. The MFL result, however, tells a different story. Figure 3 shows the MFLradial result measured at the outer wall, for 50% and 100% MAOP gouge introduction pressures. The result for 50% MAOP (Figure 3(a)) suggests that, although there is no visible damage, gouging has created significant strain in the pipe wall which has given rise to a very apparent MFL signal. The MFLradial result for pipe 1 gouging at higher pressure (100% MAOP, figure 3(b)) indicates that the residual strain for this case is lower. The lower level of residual strain is consistent with the understanding that the higher pressures create more constraint – thus making the pipe wall effectively ‘stiffer’ and more resistant to damage. In fact, in this study all of the 100% MAOP gouging produced similar, but smaller, MFL signals compared to 50% MAOP gouging. As such, no further 100% MAOP results will be shown in this paper. Figure 4 shows MFLaxial component data for pipe 1. As mentioned earlier, the axial probe necessarily has a larger liftoff than the radial, therefore the MFLaxial signal tends to be smaller. Little or MFLaxial component signal can be seen in these outer wall measurements. Inner wall MFL results are not shown for these two cases – because there was no observable signal (radial or axial component) at the inner wall. This is very important, since it indicates that significant gouging strain damage on the outside wall is sufficiently localized that it produces no MFL signal on the inside wall surface.

All gouges were introduced while the pipe sections were internally pressurized. As seen in the table, two pressures were used, corresponding to 50% and 100% of MAOP. In addition, one of the gouged pipe sections (pipe 3) was used for pressure cycling. The results of this study are not included in the present paper. The visible mechanical damage resulting from the gouging process varied considerably. The shallowest gouges, in pipe 1, were not gouges at all – the gouging process left only what appeared to be a scratch on the sample. Pipes 2 – 5 had gouges that resembled that seen in Figure 2(c). Pipes 1-3 displayed no corresponding dents with the gouge damage, however the deepest gouges in pipes 4 and 5 had some associated denting, with the most significant denting being at the tool entry location. No pipes failed during or after the gouging process. MFL measurements were made initially at the outer wall surface, with the internal pipe pressure at a level corresponding to 50% MAOP. The pipe wall was magnetized (to magnetic saturation ~ 1.8T) using a stationary permanent magnet housing mounted on the outer pipe wall. MFL measurements were made by XY scanning a Hall probe over the region of interest. Hall probes were mounted such that measurements were obtained for both the radial and axial MFL signal components. Axial MFL signals are the ones most

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Photographs of the gouges for Pipe 2 and Pipe 4, introduced at pressures of 50% MAOP, are shown in Figure 5. The significant features of each are noted in the photographs. Pipe 2 had a gouging depth of 6.4mm (during the gouging process) and a residual gouge depth of ~2mm (measured at the deepest point – just adjacent to the exfoliation region). Pipe 4 had a gouging depth of 9.5mm, and a residual gouge depth of ~5mm. The gouge in Pipe 2 contained no significant dent. The one in Pipe 4 displayed slight denting, particularly at the tool entry end. MFL results for pipes 2 and 4 are displayed in Figures 6-9, shown adjacent to one another for comparison. These results were obtained from the two gouges shown in Figure 5.

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MFL RESULTS: PIPE 2 AND PIPE 4 , 50% MAOP, COMPARISON OF INNER AND OUTER WALL RESULTS

MFLradial Component results – Figures 6 and 8 The MFLradial component results for Pipe 2 and Pipe 4 are shown in Figure 6 and 8, respectively. Figure 6(a) and 8(a) show results from the outer wall MFL measurement, and 6(b) and 8(b) from the inner wall measurement. The following characteristics are noted: o In all cases there are distinct radial signal indications at the extreme ends of the gouge. On the outer wall these are most significant – much less so on the inner wall. o In addition to the “gouge end” peaks, the inner wall MFLradial signals (Figure 6(b) and 8(b)) display additional features (a halo*) which suggest the presence of residual strain at the sides of the gouges. o The ‘halo’ is more pronounced for the more severe Pipe 4 gouge - Figure 8(a) and 8(b).

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Figure 3. Pipe 1:Outer wall, MFLradial signal for 2.5mm gouge tool depth, for a) 50%MAOP and b) 100% MAOP. After tool release no dent – only a slight scratch – was visible on the outer wall surface. The black arrow represents the gouging direction. The scale on the right hand side of this and all similar diagrams is in gauss.

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MFLaxial Component results – Figures 7 and 9 The MFLaxial component results for Pipe 2 and Pipe 4 are shown in Figure 7 and 9 respectively. Figure 7(a) and 9(a) show results from the outer wall MFL measurements, and Figure 7(b) and 9(b) from the inner wall measurements. The following characteristics are noted: o For the outer wall MFLaxial component signals, the exfoliation peak signal is very large. For the Pipe 2 gouge (Figure 7(a)) there appears to be no outer wall MFL signal from the tool entry end of the gouge, however a peak is present at this position for the deeper gouge in Pipe 4 (Figure 9(a)). o A ‘halo’ appears to be present in the outer wall signal for the more severe gouge in Pipe 4 (Figure 9(a)). o In the inner wall MFLaxial result for the Pipe 2 gouge (Figure 7(b)), there is little or no indication of the massive ‘exfoliation peak’ seen in the outer wall signal of Figure 7(a). However, there is a notable peak in Figure 7(b), that appears to be associated with the tool entry end, rather than the exfoliation end.

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Figure 4 Pipe 1: Outer wall, MFLaxial signal for 2.5mm gouge tool depth, a) 50%MAOP and b) 100% MAOP.

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the term “halo” is used here to refer to a partial or full circular ring feature in MFL signal patterns.

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Pipe 2, gouge introduced at 50%MAOP. Gouge depth during gouging 6.4mm. Final gouge depth ~2mm at deepest point. No denting noted

Figure 9(b) - the inner wall MFLaxial component signal for the Pipe 4 gouge - is very interesting. The ‘entry end’ MFL peak (also seen in figure 7(b)) is now very pronounced. Additionally, there is also a significant peak associated with the exfoliation end of the gouge. Finally, the two peaks seen in figure 9(b) are opposite in polarity, and, in fact, exactly reflect the “dipole” characteristics observed in MFLaxial component results from earlier studies (such as those shown in Figure 1(b) and 1(c).

This will be discussed further in the section that follows.

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Pipe 4, gouge introduced at 50%MAOP. Gouge depth during gouging 9.5mm. Final gouge depth ~5mm at deepest point. Slight denting noted, most significant at the tool entry end.

Figure 5 Photographs of the gouges produced in Pipes 2 and 4, both at introduced at pressures corresponding to 50% MAOP.

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Figure 8. Pipe 4: MFLradial signal for 9.5mm gouge tool depth, for 50%MAOP introduction pressure. a) outer wall signal, and b) inner wall signal.

Figure 6. Pipe 2: MFLradial signal for 6.4mm gouge tool depth, for 50%MAOP introduction pressure. a) outer wall signal, and b) inner wall signal.

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Figure 9. Pipe 4: MFLaxial signal for 9.5mm gouge tool depth (before release), for 50%MAOP introduction pressure.

Figure 7. Pipe 2: MFLaxial signal for 6.4mm gouge tool depth, for 50%MAOP introduction pressure.

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DISCUSSION ON THE ORIGIN OF THE MFL “GOUGE DIPOLE” As discussed earlier, one of the specific goals of this work was to examine the origin of the “MFLaxial dipole” that is generally associated with pipeline gouges. The MFL signal apparent in Figure 9(b) appears to be consistent with this type of dipole feature. A hypothesis for the origin of this dipole feature follows. As part of the present study, preliminary stress modeling work was commissioned to determine if information could be gleaned regarding strain patterns around gouges. Such work is very difficult, and the results of our stress modeling study were not conclusive, but they suggested the following: 1. At the tool entry end (the initial impact zone), there is significant compressive stress “at impact”. This causes an anomalous strain field that extends through to the inside wall. This suggestion is supported experimentally by results from the present gouge study, in which the most significant inner wall deformation was observed underneath the tool entry location. 2. After impact, as the tool is scraped along the pipe wall, significant shearing occurs. However, despite being very severe, the residual strain associated with this shearing is localized in the immediate vicinity of the gouge.

Zone 1: Initial impact zone Significant stress present at inner pipe wall – MFLaxial signal at inner wall due to stress (primarily)

Zone 2: Central tear zone High, localized stress near gouge at outer wall surface – little MFLaxial signal at inner wall

Zone 3: Final deposit/ exfoliation zone – large abrupt edge and deposited metal creates MFLaxial signal seen on inner surface

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Point 1 above suggests that the most significant through wall strain field may be underneath the initial impact location. Because the strain extends through to the inner pipe wall, this strain it will likely influence the MFL signal at this position. Point 2 suggests that the shearing along the “middle” of the gouge creates a very severe strain field; however this strain is highly localized at the outer pipe wall. The results of the present study – in particular the results from Pipe 1 - suggest that these severe outer wall residual strains will likely not influence an MFL measurement taken at the inner wall. Thus there will be little MFL signal from the central gouge region.

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Figure 10: The 50%MAOP gouge in Pipe 4, accompanied by the corresponding “MFL axial component dipole” signal below. Above are annotations clarifying the positions of the three zones that give rise to the MFL signal.

Figure 10 presents a hypothetical descriptive model of a gouge and corresponding MFLaxial signal, suggesting the presence of three regions:

CONCLUSION This paper describes the initial results of a study aimed at understanding the MFL signals produced by axial gouges in pipelines, and in particular to examine the origin of the “dipole” MFLaxial signal reported to be associated with gouges. The results of the study suggest that much of the severe deformation and residual stress associated with a gouge is too localized at the outer wall to be detected using a MFL tool measuring from the inside of the pipe. A model is suggested that accounts for the MFLaxial dipole signal as a result of three distinct zones – 1) the tool entry/impact zone, where stresses extend sufficiently through-wall to be ‘seen’ by the MFL tool 2) the central gouge zone, where intense shearing deformation is localized at the outer wall, and is thus not detected with an in-line tool, and 3) an exfoliation region, where the MFL signal results from geometry effects.

ZONE 1: In this region the impact at the tool entry zone creates residual stresses that extend to the inner wall surface. This leads to a negative peak in the inner wall MFLaxial signal. ZONE 2: This corresponds to the central ‘tear’ or ‘shear’ zone of the gouge. Here the deformation and residual strain are extremely high, but localized close to the gouge. Because of this localization near the outer wall, these stresses have little influence on the inner wall MFLaxial signal, thus there is no MFL indication in the centre gouge region. ZONE 3: This signal results from a combination of the abrupt edge that accompanies “tool pull out” as well as the exfoliated metal left behind. Thus the inner wall MFLaxial signal here is associated primarily with geometry, rather than stress effects.

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REFERENCES Clapham, L., Babbar, V., Rubinshteyn, A., 2006, 1. “Understanding Magnetic Flux Leakage Signals from Dents”, International Pipeline Conference 2006, Calgary, Alberta. 2. Clapham, L., Babbar, K. Marble, M.Zarea 2008, “Modelling MFL Signals from Dents”, International Pipeline Conference 2008, Calgary, Alberta. 3. P. Massopust, C. Torres, A. Dean, “Improving In-Line Inspection for Mechanical Damage in Natural Gas Pipelines” GRI Contract No. 5096-270-3698, Prepared for the Corrosion and Inspection Technical Committee of the Pipeline Research Council International Inc. 4. A. Rubinshteyn, S. Paeper, B. Nestleroth, 2008, “Testing of a Dual Field MFL Inspection Tool for Detecting and Characterizing Mechanical Damage Features”, International Pipeline Conference, 2008, Calgary Alberta. 5. L. Clapham, V. Babbar, K. Marble, and P. Weyman, Understanding MFL Signals from Mechanical Damage in Pipelines- Phase II, Final Report Contract DTPJ56-05-T-0001 for the US DOT Pipeline Hazardous Materials Safety Administration.

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