The steel port structures, such as the sheet-pile quay wall and the pipe-pile pier, are exposed to a heavily corrosive environment, and in many cases corrosion proceedsmore rapidly than initially anticipated. For this reason, the thickness of steel structures is periodically measured to provide decision-making input for repair operations conducted to preserve the structural soundness of port facilities.
　The methods in use today for measuring the thickness of steel pier supports generally utilize ultrasonic thickness gauges. The ultrasonic transducer is brought into contact to the surface of the structure to be measured. However, because seashells and marine life also attach to port structures, all attached objects must be removed before taking measurements, with the ultrasonic transducer attached and the thickness measured only after the measurement surface is scraped. This scraping step consumes the majority of the time required for the entire measurement operation, resulting in poor measurement efficiency. Because measuring the thickness of the entire structure is difficult with the methods in use today, sample measurements at representative points are used to characterize the thickness of structures.
　For these reasons, we have developed a non-contact thickness gauging equipment for underwater steel structures such as steel pipe piles of the pier. Efficiency of non-contact measurement is better than that of conventional contact measurement for inspection work. The measurement principle is as follows. We use the convergence transducer of large diameter in order to focus ultrasonic waves on the inside the steel plates. We measure the time intervals of multiple reflections from the steel plate,
and calculate the thickness using the measured values. Sufficient results were obtained in tank test, and field experiments.