The use of acoustic guided waves is a promising way to screen corrosion and general wall thinning in elongated test objects such as pipelines. The capacity to assess structural integrity several meters at a time in an elongated structure offers opportunities to decrease inspection costs by pinpointing specific areas of concern. Acoustic guided waves enable a more thorough evaluation of structures with respect to conventional NDT/NDI means spot checks based on historical knowledge or random sampling, which can lead to an increase in overall structure safety, for instance.
Amongst the many possible inspection instruments enabling guided wave inspection, one is a portable probe assembly that may be reused at multiple axial positions along a pipe structure. When properly mounted on a pipe to be inspected, a probe assembly of the inspection instrument is fixed along the circumference of the pipe with acoustically sensitive transducers uniformly distributed along the circumference of the probe assembly, at a plurality of circumferential positions, and facing the exterior of the test object, see U.S. Pat. No. 7,997,139 B2 for instance.
When using such a probe assembly, the acoustic transducers (e.g. piezoelectric transducers) can be dry coupled with the test object by applying a normal force against the exterior of the test object. This may be achieved, for instance, by pneumatic or hydraulic means (e.g. inflating a bladder behind the transducers), or applying a tensile force along the probe assembly.
A satisfactory guided wave inspection instrument should rely on the circumferentially uniform correspondence between the circulating acoustic wave in the pipe structure and the detected wave perceived by the transducers. In other words, the amplitude level detected by the transducers, or groups of transducers, should be proportional in a constant way over the complete circumference of the test object to the circulating acoustic wave amplitude. However, the coupling between the transducers of such inspection instruments and the elongated structure is often non-uniform.
Although existing inspection methods, inspection instruments, and systems involving guided-wave have been satisfactory to a certain degree, there remained room for improvement, test particularly in terms of providing guided wave inspection instruments which avoid the problem of non-uniform coupling between the transducers, or groups of transducers, and an elongated and/or cylindrical test object. Such guided wave inspection instruments could be useful especially given that the non-uniformity may change from one installation to another due to the changing conditions of the surface of the elongated test objects and on the individual transducer positioning relative to the surface of the elongated test object.
Other inventions address the problem of the non-uniform sensitivity of acoustic transducers. For instance, U.S. Pat. No. 8,002,704 addresses the problem of assessing a contact level between an array of transducers and a body through a frequency-based analysis together with a phased array beam. However, this method is not used for the calibration of the guided wave inspection instrument since the beam is formed of free propagating wave packets.
Other publications, such as U.S. Pat. No. 9,577,629 and US 2013/0194891, use averaging of inspection data (different observables may be defined) to provide per transducer element characteristic values. However, these values are compared from element to element to detect significant deviations from a standard acceptability criterion. Elements outside the acceptability range are deemed faulty and their sensitivity deviation may be compensated to equalize the array sensitivity level.