PURPOSE:To prevent the pollution of sea area by providing a ballast tank in a cylindrical bottomed concrete caisson through a sealing member in a vertically movable manner in such a way as to avoid the contact of oil with ballast and water. CONSTITUTION:A ballast tank 17 is set inside a double-walled concrete caisson 11 with a bottom slab 13 and hermetically closed by a sealing member 26. The ballast tank 17 is brought down to the bottom slab 13 through a roller 18, water is charged to an isolated chamber 16 and the ballast tank 17, the caisson 11 is towed with an appropriate draft, and water is further charged to them to settle the caisson. Oil produced on a deck 23 is sent to an oil storage portion 19 by a pump 21, and also at the same time ballast and water A in the ballast tank 17 are discharged without contact with the oil O. Also, the evaporation of the oil O is suppressed by the sealing member 26, and the burden of loads to the bottom slab 13 can be kept constant by adjusting the ballast and water A.
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