Air-Coupled Ultrasonic Arrays for Assessment of Pipe Internal Geometry and Surface Condition

This article explores what useful information can be retrieved from pipeline interiors using an air-coupled ultrasonic array. Experiments are performed using an array, custom array controller, and supporting electronics controlled by a Raspberry Pi 4, mounted on board a crawler robot. A 64-transducer 40-kHz array configuration is selected based on uniformity of imaging amplitude over the circumference of the pipe wall. Testing revealed joints between pipe sections could be imaged at high amplitude, and that angular displacement between sections produced a different response to a properly aligned joint, potentially enabling detection of faulty joints. The surface roughness of some pipes also provides enough backscatter to be imaged, which is useful for detecting regions of corrosion. It was also found that reflections from the pipe wall in the plane of the array allow imaging of the wall shape. This can indicate the presence of junctions, as well as detect ovality to within 1%. These in-plane wall reflections were also found to be a source of low-amplitude coherent noise throughout the imaging domain, which is of similar amplitude to small (< 10 mm) through-holes in the pipe wall.