Coherence Weighting Applied to FMC/TFM Data from Austenitic CRA Clad Lined Pipes

Abstract

Lined pipe girth weld regions are challenging to inspect using ultrasonic techniques due to their anisotropic and inhomogeneous characteristics, which lead to noise and acoustic beam distortion. This paper is focused on the application of coherence weighting methods to decrease the background noise level and false indications on total focusing method (TFM) images, encompassing the triple boundary of mechanically lined pipes. Three coherence weighting approaches were used here; the coherence factor (CF), the sign CF (SCF) and the phase CF (PCF). All three were found capable to improve the TFM images, however, the CF and the SCF performed better due to their exponential sensitivity factors, unlike the PCF which has a linear sensitivity factor.