Non-contacting laser-ultrasonic fouling detection on steel pipes

In many industrial processes, accumulation of fouling can lead to decreased production efficiency by weakening the flow in pipes or causing additional friction on the ships' hulls. To detect the fouled areas for descaling, ultrasonic guided waves (UGWs) can be utilized. Usually, this is carried out by coupling phased array collars of contact transducers onto the pipe. This can cause problems if the coupling changes over time, the temperature of the pipe is too high or the sensors need to be relocated. Here, we demonstrate how fouling can be detected without contact sensors, by using a pulse laser and a laser Doppler vibrometer. Furthermore, by employing broadband laser excitation, we are able to define the fouling attenuation coefficient and investigate the frequency dependencies of fouling-induced attenuation.