A floating oil storage system developed by Mitsubishi Heavy Industries, Ltd. (MHI) is being examined as one of the national oil storage projects by the Japan National Oil Corporation, which will be constructed in Aokata Bay of Nakadorishima, one of the Goto Islands in Nagasaki Prefecture. The system consists mainly of seven (7) oil storage barges capable of holding 835, 000 kiloliters of crude oil (about six million kiloliters in total), mooring dolphins and other facilities. Procedure to determine the initial planning is as follows : (1) Principle of how to develop the system was established. (2) A site suitable for construction of the system was selected. (3) Meteorological and sea design conditions of the site were investigated and then estimated for over a recurrence period of 100 years. (4) Necessary technology was analyzed point by point and related experiments were carried out. (5) Subsystems such as oil storage barge, mooring dolphin, safety device, etc. were designed. (6) Finally the initial planning was made. The most important matters at the initial planning are to establish the proper principle at first and to estimate meteorological and sea design conditions of the site.