Reducing dead zone for TOFD circumferential scan of pipeline by the adaptive deconvolution method

When the circumferential scan is performed along the outer surface of pipeline by the ultrasonic time-of-flight diffraction (TOFD) technique, the layered dead zone is generated in the near-surface area due to the influence of the pipe curvature and the pulse width of direct longitudinal wave (DLW). In this article, the adaptive deconvolution method is applied to perform deconvolution and autoregressive spectrum extrapolation by selecting the sub-band DLW, whose main frequency is close to that of the overlapped signals, as the reference signal, realizing the extension of effective frequency band and the pulse compression of time-domain signal. Meanwhile, the defect depths are determined according to the tip-diffracted waves in circumferential scan images. Experimental results show that the range of layered dead zone is reduced by about 60% under the condition of 5 MHz central frequency and 87 mm probe center separation (PCS) for the carbon steel pipelines with 100.0 mm outer wall radius and 30.0 mm wall thickness, and with 148.0 mm outer wall radius and 27.0 mm wall thickness by the adaptive deconvolution method. When the distances from defect tips to the ray path of DLW are no less than 4.0 mm, the measurement error of the depths for the defect is within 10.6%. Compared with the conventional spectrum analysis method and the autoregressive spectrum extrapolation method, the adaptive deconvolution method has better performance in reducing dead zone and can accurately quantify the defects being difficult to detect with measurement error within 5.8%. © 2022, Science Press. All right reserved.