Pipeline flaw detection using shear EMAT and wavelet analysis

An EMAT based PIG for in-line inspection of flaws in a 30 in. natural gas pipeline is presented. The sensor is capable of detecting physical flaws, e.g., SCC, circumferential and axial flaws, and corrosion, in pipe walls of gas pipelines. Using an in-line non-contact ultrasonic sensor (EMAT), flaws are determined in the wall of the pipe that the current MFL technology has problems detecting. One EMAT is used as a transmitter, exciting an ultrasonic impulse into the pipe wall while a second EMAT located a few inches away from the first, is used as a receiving transducer. Since the discrete wavelet transform of the signal will be non-shift invariant, its coefficients cannot be directly used as a pattern recognition feature. However, comparing composite properties of the signal on different scales is useful, because the mode conversion caused by n flaw changes the composite properties of the signal in wavelet space. For the EMAT data, the useful information projects onto five mutually orthogonal wavelet scales. The results of three-dimensional modeling using BEM performed on a horizontal guided wave incident on a flaw are presented.