EMAT-Based Inspection of Natural Gas Pipelines for Stress Corrosion Cracks

This paper describes the wavelet-based analysis of electromagnetic acoustic transducer (EMAT) signals for in-line inspection of flaws in a 30-inch-diameter natural gas pipeline. The sensor is capable of detecting physical flaws (e.g., stress corrosion cracks, circumferential and axial flaws, and corrosion) in the walls of gas pipelines. Using an in-line non-contact EMAT transmitter–receiver pair, flaws can be detected in the walls of a pipe that the more commonly used magnetic flux leakage technology has problems detecting. One EMAT is used as a transmitter, exciting an ultrasonic impulse into the pipe wall, while the second EMAT, located a few inches away from the first, is used as a receiving transducer. Using the 58-coefficient Symlet wavelet with 4-level (scale) wavelet decomposition, the EMAT data were analyzed. The first-level detail was not used in any of the algorithms, but the other resulting wavelet levels were used in different combinations to calculate a vectorized feature set for comparing “flaw ” and “no-flaw ” signals. “Entropy, ” “energy,” and a “difference measure ” were the various attributes evaluated to differentiate the “flaw ” and “no-flaw ” signals. The results using this approach have shown promise for EMAT measurements obtained from a translating pipe inspection gauge. There are still issues that need further research: establishing the ability to differentiate flaw types and quantifying the sensitivity of the EMAT measurements