Periodic inspection and diagnosis plays a crucial role in the series of maintenance works of port structures. In the general periodic inspection of concrete superstructures of an open-type wharf, the appearance of the bottom surfaces of concrete beams and slabs are visually observed by divers and/or inspectors with a small boat. However, conducting the visual inspection is greatly influenced by tidal and wave conditions and utilization situation of the facility such as mooring and/or cargo handling, and, developments of ever safer and ever more efficient inspection methods are coveted.
　In order to ensure the safety and efficiency of the inspection work, the authors developed a test model of semi-submergible type ROV equipped with upward cameras. The developed ROV has measurement devices of position and motion of the vehicle under the superstructures, where GPS signal is not available. The acoustic positioning system and the applied motion estimation with the optical flow worked as the tele-operation assistance features for operator. The position data was automatically associated with obtained pictures, and, it worked as management feature of inspection position. The utility of these operational assistance features were verified through field experiments of the visual inspection.