The Use of Guided Waves for Rapid Screening of Pipework

The safe operation of petrochemical plant requires screening of the pipework to ensure that there are no unacceptable levels of corrosion. Unfortunately, each plant has many thousands of metres of pipe, much of which is insulated or inaccessible. Conventional methods such as visual inspection and ultrasonic thickness gauging require access to each point of the pipe which is time consuming and very expensive to achieve. Extensional or torsional ultrasonic guided waves in the pipe wall provide an attractive solution to this problem because they can be excited at one location on the pipe and will propagate many metres along the pipe, returning echoes indicating the presence of corrosion or other pipe features. Guided Ultrasonics Ltd have now commercialised the technique and this paper describes the basis of the method, together with examples of practical test results and typical application areas. Introduction: The safe operation of petrochemical plant requires screening of the pipework to ensure that there are no unacceptable levels of corrosion. Since a significant proportion of industrial pipelines are insulated, this means that even external corrosion cannot readily be detected without the removal of the insulation, which in most cases is prohibitively expensive. A quick, reliable method for the detection of corrosion under insulation (CUI) which does not involve removal of all the insulation is therefore required. The problem is even more severe in cases such as road crossings where the pipe is underground (often in a sleeve) for a limited distance; excavation of the pipe for visual or conventional ultrasonic inspection is extremely expensive so a technique to address this problem is particularly beneficial. The use of cylindrical guided waves propagating along the pipe wall is a very attractive solution to this problem since they can propagate a long distance under insulation and may be excited and received using transducers positioned at a location where a small section of insulation has been removed. There has been a considerable amount of work on the use of guided waves for the inspection of pipes and tubes, most of which has been on small (typically 1 inch) diameter heat exchanger tubing [1-4]. The authors have developed a guided wave technique designed for the screening of long lengths (>10m) of pipes for corrosion. It seeks to detect corrosion defects removing of the order of 5-10% of the cross sectional area of the pipe at a particular axial location. It was originally developed for use on pipes in the 2-24 inch diameter range, though it can be used on both smaller and larger pipes; there have been recent applications to 36, 48 and 52 inch lines. This paper discusses the basis of the technique and presents recent practical results from different sites; it concludes with a review of typical application areas. Modes and Excitation: Only two ultrasonic waves exist in a bulk solid material (compression and shear); in contrast there are many guided wave modes in plates and pipes and they are in general dispersive (their velocity is a function of frequency). There are about 50 modes below 100 kHz and in order to obtain signals that can reliably be interpreted, it is essential that only one of them be excited. In most guided wave testing, the sensitivity of the test is a function of the signal to coherent noise ratio, the coherent noise being caused by the excitation of unwanted modes. This coherent noise cannot be removed by averaging, whereas if low signal levels cause a poor signal to random noise ratio, significant improvements can be obtained by averaging. The most attractive modes to use are those which have a mode shape which has uniform stress over the whole cross section of the pipe. This means that there will be equal sensitivity to cross section loss at any location through the wall thickness or round the circumference. Modes with a simple mode shape are also easier to excite in a pure form which is important in controlling coherent noise. The two modes which meet these criteria are the L(0,2) and T(0,1) modes; these are essentially extensional and torsional modes respectively. Both modes have the additional advantage of being non-dispersive over a wide frequency band, i.e. their velocities are constant with frequency which means that all frequency components of the input signal travel at the same velocity. This means that the input signal retains its shape as it propagates along the pipe, whereas a dispersive signal would spread in time as it propagates along the pipe, the maximum amplitude reducing and the signal duration increasing. Dispersion therefore reduces the signal to noise ratio and makes the spatial resolution poorer. These issues are discussed further by Alleyne and Cawley [5]; a technique for dispersion compensation is described by Wilcox et al [6]. Alleyne and Cawley [7] reported the development of a dry coupled piezoelectric transducer system for the excitation of the axially symmetric L(0,m) modes in pipes. It comprises a ring of piezoelectric elements which are clamped individually to the pipe surface; no coupling fluid is required at the low ultrasonic frequencies used here. Initial site trials of the technique carried out in the research phase in the mid 1990s used the L(0,2) mode at frequencies around 70 kHz and have been reported previously [8, 9]. Propagation distances approaching 50 m were obtained and by using multiple rings of transducers it was shown to be possible to obtain uni-directional propagation. The field trials reported in [8, 9] employed two rings of transducers in order to excite the L(0,2) mode in a single direction. However, there is a second axially symmetric mode with particle displacements primarily in the axial and radial directions, L(0,1). This mode is also excited by the two ring system. The presence of reflections of this mode can make interpretation of the results less reliable so it is desirable to remove it. It is possible to suppress the L(0,1) mode by adding further rings of transducers. The original commercial implementation of the system marketed by Plant Integrity Ltd [10] uses three rings, but the Guided Ultrasonics Ltd Wavemaker Pipe Screening System uses four rings which gives improved suppression of this unwanted mode. The use of three or four rings adds to the cost of the system and also to the mass, which becomes significant when larger pipe sizes are being tested. T(0,1) is the only axially symmetric torsional mode in the frequency range of interest, so axially symmetric torsional excitation will only excite the T(0,1) mode. This means that only two rings of transducers are required in order to obtain single mode, unidirectional excitation. The torsional mode also has the advantage of being nondispersive across the whole frequency range. Torsional forcing can be achieved by simply rotating the same transducers used for the L(0,2) mode through 90 so that they apply force in the circumferential, rather than axial direction; this is implemented in the Guided Ultrasonics Ltd Wavemaker Pipe Screening System. The torsional mode also has the advantage that, in contrast to the L(0,2) mode, it does not involve radial displacement of the pipe wall. Therefore its propagation characteristics are not affected by the presence of liquid in the pipe so in-service inspection of lines carrying a liquid is straightforward. A further advantage of the torsional mode is that it will detect longitudinal cracks, whereas the longitudinal modes are essentially insensitive to thin defects parallel to the pipe axis. However, a disadvantage of this sensitivity to axial features is that the torsional mode reflects relatively strongly from support brackets that are welded axially along the pipe. Large reflections from these features reduce the range of the test and also make it more difficult to detect corrosion at the brackets. This problem is most severe in small diameter pipes. In this relatively unusual case, the longitudinal mode may be preferable. In practice, the more convenient torsional mode is most commonly used, but in occasional special applications the longitudinal mode is employed. Conversion of the system between the two modes is straightforward. If an axially symmetric mode (such as the extensional and torsional modes used in the commercial system) is incident on an axially symmetric feature in the pipe such as a flange, square end or uniform weld, then only axially symmetric modes are reflected. However, if the feature is non axially symmetric such as a corrosion patch, some non axially symmetric waves will be generated. These propagate back to the transducer rings and can be detected. The extent of mode conversion to non axially symmetric modes is therefore a key element in the defect identification strategy. Commercial Instrument: The Guided Ultrasonics Ltd Wavemaker Pipe Screening System instrument and transducer assembly for an 8 inch pipe are shown on site in Fig 1. The instrument is battery operated and is connected to the rings by a flexible cable. The test is controlled by a portable PC that is connected to the instrument by an umbilical cable. In some cases it is convenient for the operator of the PC to be adjacent to the test location, but on other occasions it is better for the computer and operator to be in a van that can be up to 50m from the test location. Solid rings of the type shown in Fig 1 are manufactured for pipe diameters up to 8 inch, but above this they become bulky so a flexible, pneumatic clamping arrangement is used; an example system is shown deployed offshore in Fig 2. The assemblies shown in Figs 1 and 2 are for the torsional mode so each contains two rings of transducers so that unidirectional excitation and reception can be obtained, as discussed above. No surface preparation is usually required. Fig 3 shows typical reflections from symmetric and asymmetric features; the increase in the mode converted signal can clearly be seen in the asymmetric case and this is a key element of the defect identification scheme. Fig 4 shows an example report generated by the Wavemaker WavePro software for an epoxy painted, 4 inch pipe at a test position adjacent to a road crossing. The test range extends over more than 20m on either side of the rings which are located in the middle of the plot. The software identifies welds and computes a distance-amplitude correction (DAC) curve for the welds. It then calculates the defect call level by comparison with the weld echo level and the calculated output amplitude, knowing that an average site weld is a -14 dB reflector. The echo identified as +F2 is the only one where the red (mode converted) signal is significant compared to the black (reflection of incident mode) signal and this indicates possible corrosion at the entry point to a road crossing. Results: Fig 5 shows an example of localised corrosion in a 3 inch pipe at the positions marked +F1 and -F1. These defects occurred at the location of pipe supports and were therefore difficult to detect visually. In each case, the defect is characterised by a strong mode converted (red) component. In the absence of corrosion, these simple supports would give minimal reflection. The features marked +F2 and -F2 are 1D bends (i.e. bends whose radius equals the pipe diameter) and the feature +F3 is a further bend. The bends are characterised by reflections from the welds placed immediately before and after the bend. In each case, the reflection from the second weld has a significant non-axisymmetric (red) component. This is due to mode conversion produced by the bend itself, rather than at the weld; the signal transmitted past a bend contains both the original mode and mode converted components. These issues are discussed by Demma et al [11]. An example of defects at welds is shown in Fig 6. This test was on an 8 inch pipe that had been used for carrying acid. There is a significant non-axisymmetric (red) component associated with the weld reflections, particularly those marked +F1, -F1 and -F3. This was due to internal erosion at the welds. There is also evidence of defects below the call level at the locations marked -F2, F4, -F6 and -F8; the +F2 reflector is a flange. A clear signal is obtained from weld -F9 about 50m from the transducer rings. Fig 1. Solid transducer ring for 8 inch pipe and battery powered test instrument on site. Fig 2. Flexible transducer ring deployed offshore.