Measurement of flaws to a precision of a few percent in a pipeline would allow pipeline engineers to accurately calculate the remaining pipe strength and to determine what mitigating actions to take without investigative digs at pipeline flaw locations. Magnetic flux leakage, the most often used technique for pipeline inspection, will never reach that precision. Ultrasonic inspection is precise enough but traditionally requires a couplant of some kind between the transducer and the pipe wall. For liquid product pipelines, ultrasonic inspection is already commercially available for both remaining wall thickness measurement and crack depth measurement. Since it is undesirable to introduce liquids into a gas transmission pipeline, we have developed a gas coupled ultrasonic inspection method that does not require a couplant in pressurized pipe. Although the signals are small, with improved transducers and low noise electronics enough of a signal comes back to the ultrasonic transducer to accurately measure flaw size.
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