Chapter 6 In-service corrosion patches, calibration defects and welds
6.3 Multiple variable sized defects
In addition to the pipe support area defects, samples with a range of defects around the pipe’s circumference were tested, these defects had different axial positions but some of them overlapped with one another in the axial direction. An example of
such a sample is presented here with a circumference of 861 mm, a nominal wall thickness of 6.4 mm, and an approximately 1 mm thick coating of paint on the external surface (Sample 4 in Table 5.1). The sample had 12 real corrosion patch style defects of varying size, shape, depth and circumferential positioning that had developed in-service. The defects had a range of levels of corrosion, from initial corrosion after a coating breach, to extreme corrosion levels with corrosion products visible as scabs on the outer pipe surface. This is illustrated in the images provided of the defects, where Figure 6.9a shows the initial defect that can only be identified by a small hole in the coating, to a corrosion patch that has expanded underneath the coating in Figure 6.9b, to a severe corrosion patch where considerable pipe wall material has been removed from the sample resulting in a large irregular defect in Figure 6.9c.
(a) (b)
(c)
Figure 6.9: Examples of the range of defects present in the multiple defect sample, from an initial small defect in a), to a larger corrosion patch with the corrosion product in place in b), to a more severe corrosion patch in c).
The approximate sizing and positioning of the defects on the sample surface are shown in Figure 6.10 along with the positioning of the transmitter and receiver probes on the pipe surface.
The circumferential extent of the sample, probe separation (200 mm) and the wavelength of the probe used on this sample allows the CSH0 and CSH1 modes for the short and long paths to be clearly temporally separated from each other,
which gives more clarity to the defect indications. This sample shows how defects can be detected in the short and long path of the circumferentially travelling guided waves, with the defects labelled at 4,5,6,7,8,10 and 11 in Figure 6.10 and Figure 6.11 affecting the short path signal between the transducers, whereas defects 1,2,3,9 and 12 can be said to be affecting the long path of the waves.
Figure 6.10: Schematic diagram of the ’unwrapped’ pipe to illustrate the axial and circumferential positioning of the defects in the 864 mm circumference sample. The different features are to scale but the shape of the defects is not intended to be representative. The circumferential extent of the defect at a is unknown.
This also illustrates one of the weaknesses of the technique when real de- fects are encountered, as a closer inspection of the defect positioning shows that the defects at 1 and 2 overlap with each other in the axial direction but have different po- sitions circumferentially. Both are in the long path of the signal at different positions circumferentially, but as the indicative defect signals are due to transmission through both areas, they cannot be differentiated from each other. These defects would have a chance of being distinguished using the reflections in the smaller samples, but the reflected signals that are caused by the defects in this longer circumference case are too low amplitude to be used in such a way here. This is in contrast to the defects at 3 and 4 and the defects 9 and 10 that overlap with each other axially but their effect on the signals can be separated since 4 and 10 affect the short path signal and 3 and 9 affect the long path signal.
The scan shown in Figure 6.11 illustrates the difficulty that was stated earlier in having a defect develop directly underneath the path of the transducer on the surface of the sample. The indications at 4, 8 and 11 show that signal is removed due to the defect being situated underneath the receiving transducer, increasing its lift off. This effect is smaller at defect 8, as the defect scab has a lower profile and hence the lift off is reduced, but the defects at 4 and 11 increase the transducer stand off to a larger extent and remove the signal almost entirely. This effect of the removal of any signal is dominated by the change in lift off, instead of the depth of the defect. This results in the signals in this area providing inconclusive information
Figure 6.11: Scan of a long axial extent sample with multiple in-service defects spread around the pipe circumference. An unmarked defect in the short path (no measurement data available) is labelled a, with various in-service corrosion patch defects in the short path at 4,5,6,7,8,10 and 11. Defects situated in the long path are labelled 1,2,3,9 and 12. The removal of signal by the change of lift off from the sample is seen at defects 4, 8 and 11.
about the severity of the defect, but allowing the area to be highlighted for further analysis.
The axial sizing method used previously becomes more difficult in this in- stance, as some of the defects overlap in their axial extent and are in the same path between the transducers. The change of lift off caused by the defects being under- neath the transducers also makes it difficult to probe for defects in the long path at these points, as no energy is available for such signals. This leads to varying accu- racy of the axial sizing of the defects, which is achieved by comparing the image of the scan amplitudes and the amplitude of the RMS of the CSH0 and CSH1 modes, taking into account that the axial step here is 5 mm. The results for the axial sizing along with the measured extent on the sample are presented in Table 6.1.
and by tracking the amplitude of the CSH1 mode for the affected mode, the axial position and size of the defect can be estimated and the circumferential position localised to being in the short or long path between the transducers. In this case the defects can be seen and their level of corrosion assessed, with the defects labelled 3, 5, 6, 7, 8 and 10 having a small axial and circumferential extent, with small depths, whereas defects 1, 2, 4, 9, 11 and 12 are large corrosion patches, with and without the corrosion scabs still in place.
However, the different defects can be characterised in terms of their severity without having this prior knowledge of the defects, by using knowledge of the dis- persion curves for the sample and how the different modes interact with defects as considered in Chapter 5. The defects at 3, 5 and 6 show through propagating CSH0 modes that increase in velocity, arriving earlier than the clear area CSH0 mode. This is due to the decrease in thickness at the defect area, causing the frequency thickness product at that point in the sample to decrease, resulting in the operation of the mode at a higher velocity section of the CSH0 mode on the dispersion curve. The defect at 5 then can be seen to have a CSH1 mode that arrives later than in the clear area, which suggests that the defect in this area has a depth that is above the cut off thickness of the CSH1 mode, as a mode converted waveform is not seen in the A-scan data. The defect at 6 experiences the same behaviour as defect 5 in that the CSH1 mode slows in the defect area, suggesting a defect of a depth that is not as extreme as would be necessary to reach the cut off thickness of the CSH1 mode as there is no mode converted wave detected.
Defect Measured Scan Comments Axial length Axial length
(mm) (mm)
a - 78 Unmarked, no measured length available.
1 + 2 420 418 Overlapping axially, with all
CSH1 mode removed.
3 50 58 Small defect seen on surface,
testing hampered by the effect of defect 4.
4 150 155/263 Large defect with corrosion
product in place. Extent on scan gives 155 mm but effect on the CSH1 amplitude extends much further due to general pipe condition.
5 + 6 140 138 Small defects too close to
be measured separately using the amplitude of the CSH1 mode, however combined extent is as expected.
7 50 58 Small defect on the sample surface
that extends further under the coating.
8 70 63 Small defect on the surface with
larger corrosion patch under coating.
9 370 433 Extreme defect, adjusted amplitude
scan gives 373 mm but CSH1 amplitude scan overestimates to 433 due to pipe condition.
10 25 30 Small axial extent, small depth
defect, only visible due to slight changes in the arrival time of the CSH0 and CSH1 modes in the scan.
11 220 223 Large corrosion patch with change
of lift off, axial size matches, but only gives an idea of how far the surface is uneven for with the defect present. 9 + 11 + 12 760 778 Part of defect 12 overlaps
with defect 11 where the signal is removed, combined extent of 9,11 and 12 is correct.
Table 6.1: Summary of the measured (with a tape measure) axial length of defects and the approximated length from scan data.
However, the amplitude of the slowed CSH1 mode at defect 6 is much lower than the amplitude seen at defect 5, which suggests that there is considerable reflec- tion and scattering of the CSH1 mode when it interacts with the defect. Character- isation of the defect seen at 3 is difficult as it overlaps with defect 4 axially, which as has been mentioned removes some of the signal from the received signal due to a change in lift off of the probes. However, it shows the CSH0 mode being able to propagate into the defect area as mentioned and then an almost complete removal of the CSH1 mode from the signal suggesting that there is considerable reflection and scattering of this mode when it interacts with the defect.
The defect labelled at 7 exhibits the same behaviour as the defect at 6, suggesting it has a depth that is above the cut off thickness of the CSH1 mode, but causes a large degree of reflection and scattering of the CSH1 mode such that the through propagating mode has a low amplitude. The defect labelled at 8 removes the signal from the receiver due to the increase in stand off of the transducers, which makes it difficult to obtain any quantitative information about the defect. The transducers could be realigned to straddle the defect such that the lift off returns to the normal levels for the rest of the sample in order to gain information about the defect, but due to the rig not being used here, this type of scan was not recorded. From the visual appearance of the defect, it is suggested that it would give similar results to that seen in defect 5. The defect labelled at 10 is an area of corrosion that has a small axial and circumferential size, with a small amount of wall thinning occurring. It is only obvious with careful inspection of the amplitude profile and the encoded waveform amplitude data, so it would be difficult to detect the presence of the defect if it had not been found in the visual survey of the sample.
The more extreme defects that are seen at 1, 2, 4, 9, 11 and 12 can be differentiated from the less extreme defects seen as they affect both the CSH0 and CSH1 modes to a larger extent. In the case of defects 1, 2, and 9, the CSH1 mode is entirely removed from where it would be expected to be detected if there was no defect. This is not accompanied by a mode converted waveform, as would be expected from the other defects that have been investigated, suggesting that the energy of the CSH1 mode cannot propagate as the CSH1 mode through the defect but instead is reflected and scattered when it interacts with the defect due to the depth of the defect and its irregular structure. The more extreme nature of the defect here is confirmed by the fact that the CSH0 mode that should propagate through the defect area is either entirely removed in the case of defect 9, or has a very low amplitude in the case of defects 1 and 2. This suggests that the depth of the defect is extreme, as it was observed in Chapter 5 that the reflection of the
CSH0 mode increases with the depth of the defect, and this is the predominant mechanism by which the expected amplitude of the CSH0 signal would decrease. It also suggests the structure of the defect is irregular, as the reflected energy from both the CSH0 and CSH1 modes is not received, suggesting that it is scattered and does not return to the receive transducer as a coherent waveform.
The defects at 4 and 11 exhibit a removal of signal due to the increase in stand off of the probes, so quantitative information about the depth of the defect is not available, however, a visual inspection of the defects would suggest that the behaviour would be similar to that seen in defects 1, 2 and 9 if a scan at a different angle was carried out. The defect at 12 is partially overlapping with the area of defect 11 where the stand off of the probes is changed, which makes the assessment of the defect difficult. The section of the defect that can be seen however, shows the removal of the CSH1 mode, with a CSH0 mode that increases in velocity but with a smaller amplitude than in the clear area of the sample. This suggests that the defect is extreme enough to cause a large amount of reflection of the CSH0 mode and the CSH1 mode, but still allows for some propagation of the CSH0 mode through the defect area, suggesting it is less extreme than defects 1, 2 and 9.
In summary, the application of this method to detect in-service corrosion patches is very effective as a screening technique, showing the defects clearly as a change in the received signals. The method works best as the axial size of the defect is increased and where the depth of the defect is increased or the abruptness of the defect is extreme, such that the most difficult defects to detect are those that are of a small axial size and have a small defect depth. The removal of the energy of a mode completely suggests a complex defect with a rough profile and therefore usually a more severe defect, with the removal of the CSH0 mode from the signal suggesting a very severe defect. Any defect that causes the mode conversion of the CSH1 mode or total reflection of this mode is severe enough to warrant further investigation.