Magnetic Flux Leakage (MFL) pipe inspection devices ("pigs") measure wall loss in a gas pipeline, but not the remaining wall thickness that determines the remaining strength. In addition, the precision of MFL tools is limited to 10% of the wall thickness, and the technology cannot find cracks, including stress corrosion cracking. For pipeline operators, the 10% limit means extra digs are required to ensure that all severe corrosion has been found. Another drawback is that this inspection technique offers little ability to monitor corrosion growth rates to determine where mitigation is effective and where it needs to be improved. Measuring pipe wall loss means that the pipeline operator must base repairs on the minimum expected wall thickness rather than the actual measured amount of remaining metal in the pipe wall. Ultrasonic inspection has found cracks and measures the remaining wall thickness with a precision of a few percent. However, ultrasonic inspection currently requires putting a liquid couplant in a gas pipeline. Transducers specialized for inspection in high-pressure gas and specialized inspection methods can eliminate the need for a liquid couplant, bringing the advantages of ultrasonic inspection to gas pipelines.
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