Ultrasonic thickness structural health monitoring of steel pipe for internal corrosion

Abstract

<p>The naphthenic acid corrosion that can occur in oil refinery process plants at high temperature (400à  à °C) due to the corrosive nature of certain crude oils during the refining process can be difficult to predict. Therefore, the development of online ultrasonic thickness (UT) structural health monitoring (SHM) technology for high temperature internal pitting corrosion of steel pipe is of interest. A sensor produced by the sol-gel ceramic fabrication process has the potential to be deployed to monitor such pitting corrosion, and to help investigate the mechanisms causing such corrosion. This thick-film transducer is first characterized using an electric circuit model. The propagating elastic waves generated by the transducer are then experimentally characterized using the dynamic photoelastic visualization method and images of the wave-field are compared with semi-analytical modeling results. Next, the classic elastic wave scattering theory for an embedded spherical cavity is reviewed, results are compared with a newer scattering theory from the seismology community, that has been applied to a hemispherical pit geometry. This hemispherical pit theory is extended so as to describe ultrasonic Non-Destructive Evaluation (NDE) applications, for pitting corrosion, with the derivation of a far-field scattering amplitude term. Data from this new scattering theory is compared with experimental results by applying principals from the Thompson-Gray measurement model. The initial model validation provides the basis for a possible new hemispherical pit geometric reference standard for ultrasonic NDE corrosion applications. Next, UT SHM measurement accuracy, precision, and reliability are described with a new weighted censored relative likelihood methodology to consider the propagation of asymmetric uncertainty in quantifying thickness measurement error. This new statistical method is experimentally demonstrated and applied to thickness measurement data obtained in pulse-echo and pitch-catch configurations for various time-of-flight thickness calculation methods. Finally, the plastic behavior of a corroded steel pipe is modeled with analytical and finite element methods to generate prognosis information.</p>