Acoustic Wall Thickness Assessment of Large Diameter Mains

Many types of large diameter pipelines tend to degrade gradually, with the pipe walls losing thickness over time before complete failure. It is well established in the literature that as pipe walls lose thickness and strength, they also lose stiffness. This stiffness loss in turn has an impact on the speed at which sound propagates through a pipeline. An understanding of the relationships between wall thickness and the speed of sound, combined with a method of measuring the speed of sound over intervals of a pipeline, make it possible to estimate the remaining wall thickness of stretches of pipeline without the need to de-water or even take the line out of service. This paper outlines the principles and application of acoustic wall thickness measurement in large diameter pipelines. The approach discussed employs multiple sensors, some at fixed locations, and one traveling in a controlled manner through the pipeline, in order to take wall thickness measurements at fairly tight intervals along a large diameter pipeline. Several implementations of this technique are discussed: using correlators to measure the speed of sound over set intervals, using a mobile in-pipe hydrophone, and using a combination of the two techniques, correlating signals from a mobile in-pipe hydrophone and a fixed location sensor.