Scour Evaluation of Offshore Foundations by using Ultrasonic Reflection Images and Natural Frequency Variation

Abstract Local loss of geomaterials, which reduces the stability of the structures, around foundation of offshore structure occurs due tothe interaction between water and ground. The objective of this study is to evaluate scouring in offshore foundation structures byusing ultrasonic reflection method and natural frequency analyses. Sand-cement mixture is prepared to model the ground, which isencountered the scour depth and the scour shape. Ultrasonic wave and natural frequency are measured according to the scourdepth. Ultrasonic transducers are used to measure the ultrasonic wave, and strain guages are used to measure the natural fre-quency. Ultrasonic transducers are fixed on horizontal movement system and rotational movement system for the change of loca-tion. Strain guages are attached on upper part of model monopile. Ultrasonic reflection image and strain responses are monitoredin laboratory experiment. Maximum scour depth is estimated by natural frequency, and the characteristics of the scour shape areestimated by ultrasonic reflection image. This study suggests that the ultrasonic scanning and natural frequency analyses may beeffectively used to monitor the characteristics of the scour depth and shape. Key words : Monitoring, Monopile, Natural frequency, Ultrasonic imaging, Scour