Paper presented at Conferenza Nazionale sulle Prove non Distruttive Monitoraggio Diagnostica, 11th Congresso Nazionale dell'AIPnD, 13-15 October 2005.
Over recent years, Ultrasonic Guided Waves have been successfully used in the non-destructive, in-situ inspection of pipes.
TWI Ltd is currently researching methods of 'focussing' guided waves at an arbitrary distance along the pipe and circumferential position. Delays are applied to a Teletest transducer tool, facilitating a greater concentration of energy at a given point, and hence a greater reflection from changes in cross sectional area. More sensitivity in the results can thus be obtained.
The procedure also allows a three dimensional plot of the pipe surface to be compiled, showing response amplitude against axial distance along the pipe and angular position. This gives an intuitive representation of the findings. In doing so, more quantitative information about defects can be drawn from test data.
The technique has been proven both in the laboratory and in the field, using simulated and real defects. The field test results corresponded with those of radiography tests conducted on the pipe. Investigations using finite element analyses have also been carried out, thus validating the experimental results.
For some years ultrasonic guided waves have been used for non destructive evaluation of pipelines (see [1-5] for general reading on guided wave theory and recent practice). This technique was originally pioneered by TWI Ltd, using rings of transducers forced onto the pipe by means of an inflatable bladder constrained by a clamp. This system, known as Teletest, uses either the T(0,1) wave mode (for torsional testing) or the L(0,2) wave mode (for longitudinal testing). A completely axisymmetrical wave pulse is excited and travels down the length of the pipe. When the wave front hits a change in cross sectional area at a defect, mode conversion takes place and a portion of the energy is reflected back as a flexural mode. The various orders of flexurals have lobes of energy at differing angular positions as they propagate the length of the pipe.
Research is now being conducted into transmitting different flexural modes in a manner such that the lobes of energy converge at a specified distance along a pipe, at a specified angular position. In doing so, greater energy is concentrated at the focal point, and thus more energy is reflected back to the tool. This not only yields higher signal to noise ratios, but also allows information about a specific point on a pipe to be gained. Sweeping the length and circumference of the pipe in this manner allows a c-scan of the pipe surface to be constructed.
In order to achieve a focus, the Teletest tool needs to be split into segments of transducers, controlled by independent channels - similar to a phased array. Time delays can then be exerted on the firing of each segment. These delays are specific for test frequency, pipe material and geometry (diameter and wall thickness), and focal position (both axial distance from the transducer and angular position around the pipe).
Further to the delays, Teletest is capable of firing the different transducers with different amplitudes. As such, the different segments can be fired with different signal output levels in such a manner so as to reinforce the focussing effect.
Currently, Teletest is divided into either four or eight segments (quadrants or octants respectively), allowing the circumference to be scanned in 45° increments. Studies are underway into the feasibility of further dividing the tool into twelve segments, to allow the focal point to be rotated in 30° increments.
The finite element analysis software Abaqus has been used to verify the technique. Figure 1 clearly shows how a concentration of energy occurs at a given position (top dead centre, in this example) on a pipe. Further modelling work is currently underway to better understand the properties of the focal spot,such as quantifying its size and shape.
Tests in the laboratory on welds and both artificial and natural defects using the focussing technique have been conducted, as have tests in the field on pipes still in service. Both laboratory and field tests have yielded promising results.
Figure 2 shows a c-scan of an 8" schedule 80 pipe with a small saw cut. The saw cut (shown in Figure 3) represents less than 4% cross sectional area of the pipe. It is clear to see the location of the saw cut (between 0° top dead centre and 45°) and the end of the pipe on the plot.
Figure 4 gives a three dimensional plot of a similar 8" schedule 80 pipe. Here, a weld was present 5.3m from the tool. The three dimensional representation clearly shows the location of two symmetrical features - the girth weld and the end of the pipe 8.4m from the tool.
Figure 5 shows a 16" pipe with a 3" branch (representing roughly 5% cross-sectional area loss). The branch can clearly be seen on the plot, as can the location of a girth weld and a flange. This test was conducted while the pipe was still in service.
Note how the symmetrical features in Figure 4 are present at all angles (from 180° to 180°), whereas the defect in Figure 2 and the branch in Figure 5 are shown only at the appropriate angle (0° to 45° in Figure 2 and 0° in Figure 5). The 'shadow' of the branch in Figure 5 causes the lower signals at 0° at the responses from the weld and the flange.
Other field tests have been carried in tandem to radiography inspection. The results from the focussing technique accurately agreed with those of the radiography.
Experimental work is continuing to further investigate the potential of this novel technology, including engineering software to automate the technique, and improved methods of displaying the data.
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