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Monitoring the Growth of Hidden Corrosion Discontinuities in a Pipeline with Complex Geometry Using Torsional Mode Ultrasonic Guided Wave

The peak amplitude of the fundamental torsional mode, T(0,1), ultrasonic guided wave, excited and sensed by commercially available equipment using permanently mounted magnetostrictive transducers, was used to monitor the growth of electrochemically induced, external discontinuities in a steel pipeline (216 mm outside diameter). The pipeline was more than 30.5 m long, had multiple welds and two 90° elbows. Over the course of eight months, two discontinuities, hidden from ultrasonic testing (UT) technicians, grew systematically in both area and depth, providing the opportunity to experimentally evaluate the effects of complex geometry on signal characteristics. Corrosion around a weld was also studied. The obtained results were compared with a surface discontinuity that was unaffected by intermediate welds located between the transducer and the discontinuity. It was found that the effects of multiple welds and an elbow decreased the sensitivity for discontinuity growth monitoring by a factor of less than two. This was established by examining the scaling of discontinuity signal peak heights against a weld peak instead of using a conventional distance-amplitude-correction (DAC) curve to account for material attenuation, by comparing the signals from a discontinuity viewed along the forward and the reverse direction, and by allowing the discontinuities to change in all three dimensions in the course of this monitoring effort. This controlled study for discontinuity growth monitoring can be useful to guide efforts on discontinuity growth monitoring of complex pipelines in the field.

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