Section 8.2.4 highlights three well explored techniques for the purpose of corrosion detection, each of which have strengths and weaknesses depending on the coverage and the required detection ability. However, of the remaining unexplored space between the three NDE techniques, the coverage gap between the LRGWs and circumferential guided waves is of less interest. This is because a technique which operated over this area (5x103-
13x106mm2) would require guided waves, and therefore, the sensitivity of that technique
could be extrapolated from the current literature and knowledge. A technique which operated over a 10-5000mm2 area, however, would begin to use either higher frequency
guided waves or low frequency bulk waves, which ultimately converge towards each other. The response of such a device could not be extrapolated from existing knowledge, and an in-depth study of its potential would need to be carried out. Another avenue of research would be to expand the area coverage of bulk wave techniques and investigate different transducer apertures or shapes, or spacings between pitch catch transducers.
Chapter 4 made several assumptions regarding the operation of a circumferential guided wave system, thereby allowing a value of detectability close to the theoretical maximum to be calculated. One assumption stated that the transducer was able to excite a broad spectrum of frequencies, allowing for pulses which were suitable for reflection and transmission to be excited. Research into whether a guided wave transducer could be constructed or whether two transducers housed in the same package would be required. In the case of an EMAT, one possibility would be to use the same set of magnets but with
188 different coils. Further developments of the circumferential guided wave signal processing techniques should concentrate on improving monitoring sensitivity using techniques such as baseline subtraction. In reflection, this would mirror the work done for long range guided waves [33]. The circumferential guided wave techniques studied may also have immediate application in specific corrosion management scenarios within the high value/risk areas of the oil and gas industry.
The ability of the waveguide transducer to identify a pit has been demonstrated, however, its ability to track a pit was only briefly touched upon in chapter 7, which requires a more detailed evaluation. The incident pulse is heavily scattered and distorted by the pit, and the simple thickness algorithm used to calculate the TOF from the pit needs to be improved to extract a more accurate value, possibly using a cross-correlation technique similar to that described in section 6.3.2. Furthermore, as acknowledged in chapter 7 the pit identification algorithm only used the three reflections with the highest amplitude in the time trace to decide whether or not a pit was present. It was therefore possible that the pit signal was still a significant and recognisable peak but only the fourth (or lower) highest in amplitude. Therefore, more complex pit identification algorithms need to be written. This would especially benefit the waveguide transducer due its anisotropic beam shape; since it is possible for a pit to lie away from the point directly under the transducer (which will ensure a strong backwall signal) but be close enough to cause a strong pit signal. In that scenario, the backwall signal will have little decay and the pit signal is unlikely to be greater in amplitude than the third backwall reflection.
Transducer shape, which influences the beam shape, was shown to be the main factor in determining pit sensitivity. Chapter 6 focussed on three existing techniques all of which were designed specifically for extracting values of inner pipe wall thickness, not for detecting and tracking pits. Therefore, there is scope for exploring different transducer designs to optimise the tracking of pits. The high aspect ratio of the waveguide was shown to be superior to the unitary aspect ratio of the EMAT, however, if the EMAT width were to be narrower would that improve the detection ability? Could the spacing between the waveguides improve coverage? Also, there is scope for exploring whether it is possible to exploit the mode conversion effect observed in the 0-degree shear wave transducers, and to determine whether there is any extra diagnostic information contained within mode converted signals.
The ability of fast finite element enabled much of the work presented in this thesis and the automated defect meshing and modelling methods developed for this thesis have shown that a great variety of pit sizes, shapes and positions can be modelled. The same meshing method
189 can be applied to simulate even more complex defects as well as multiple defects and morphologies. This is not a pressing concern since there is still more knowledge to gather regarding the effect of a single pit. If these conditions are evaluated, however, the study must be relevant and targeted due to the complexity of evaluating more and more dimensions (e.g. number of pits, spacing between pits, pit elongation etc.).
Finally, much of the long range guided wave research has focussed on sharp edged defects, which are known to have a higher reflection coefficient than smooth defects (such as Hann profile defects). Research should be conducted to quantify this effect on LRGW sensitivity and the probability of detection, thereby allowing a fairer comparison of all three NDE techniques.
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