Visual investigation of underwater structures by the AUV and sea trials

To investigate underwater structures such as piles and caissons in harbors, and risers and jackets of deep-sea oilfields by taking visual images, Autonomous Underwater Vehicles (AUV\´s) have advantages that they have no umbilical cable which may get entangled in the structures and surrounding obstacles. The authors have proposed a navigation method for AUV\´s to observe such objects using a laser ranging system that consists of TV camera and laser pointing devices in previous papers. This ranging system provides precise distance to the target object so that the vehicle can trace the shape of the target object keeping constant distance and attitude. The method was implemented in the testbed AUV "Tri-Dog 1", and proved experimentally through tank tests in 2001. Following the success of the tank tests, the authors have carried out sea trials in 2002 at the port of Kamaishi, Japan, which has huge breakwaters. This paper describes the result of the sea trials and our perspectives. Based on the proposed system, AUV\´s will be able to carry out autonomous investigation of underwater structures in the near future.