Study on the Water-exchange Performance of Buoyancy-control Type Ballast-free Ship by Model Tests at Sea

To cope with the problems in the marine ecosystem caused by the ballast water from ships, the International Maritime Organization (IMO) adopted the International Convention for the Control and Management of Ships' Ballast Water and Sediments in 2004. In response to this legislation, ballast water treatment systems are being developed worldwide. The ballast-free ship concept proposed by Yokohama National University is aimed at solving this ballast water problem from a completely different standpoint. In the proposed system, seawater continuously passes through the buoyancy control tank to keep the contents of the water inside the tank the same as that outside the ship. A series of studies on this new concept has been carried out by the authors, including the analysis of seawater circulating performance inside the buoyancy control tanks, an examination of the increase in resistance caused by adding small appendages such as intake and exit gates on the ship bottom, a study of structural integrity of the system, and related work. In the present study, the authors used a scale model of an actual double bottom tank and carried out an experiment to examine the water exchange performance of the proposed system on the open sea. In the model experiment, the water inside the tank was set as fresh water initially, and as time passed, the water inside the tank changed to the seawater outside the tank. The water locality ratio inside the tank was evaluated based on the salinity change of the water inside the tank and the change of the numbers of phytoplankton cells inside the tank. Numerical results and experimental results were compared to examine the performance of the proposed ballast-free system.