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When the tank washing operation is complete and the slop tank has been decanted to provide sufficient room to receive pipeline flushings, the ship is nearly ready for loading clean ballast. The final preparation is the flushing of the cargo lines to ensure that only clean water will reach the cleaned tanks when/if they are filled. The lines should have been flushed during the washing operation if clean ballast was contemplated, but a second flush is necessary to be confident that no oil enters the clean ballast tanks. The following procedure, as with all important operations, should be prepared in writing by the chief officer and a record maintained of the dates and times that each step is completed.
To effectively flush the cargo piping, the valves must be set to the alignment which will be used for filling the ballast tanks, but with the tank valves closed and the line to the slop tank open. Fill the line(s) with clean sea water and allow it to set for 30 minutes to float off any light clingage or trapped oil.
The flushing should be done using a pump which is run at moderate speed. First thoroughly flush the line to the slop tank, then any branch lines leading to cargo tanks which have not been washed. If there is any doubt about the compatibility of the next cargo with the previous grade and about the effectiveness of the line flushing at the discharge port, then all cargo lines should be again flushed to their cargo tanks and the tanks re-stripped to the slop tank. Finally flush the risers and deck manifold lines. These should contain little or no oil and may be flushed overboard if the vessel is not in a prohibited zone or any special area designated by MARPOL. If the risers and deck lines are known to be dirty or the ship is within a prohibited zone or special area, then flushing the risers and deck lines can only be done using a hose connected between the manifold being flushed and a manifold connection which carries the flushing water to the slop tank (or another accumulation tank).
When the lines are effectively flushed, the ship is ready to load clean ballast. Clean ballast may be taken on board immediately, or the tanks may have simply been prepared to load clean ballast in the event that heavy weather is encountered.

2.14.1 Filling clean ballast tanks
Clean ballast tanks should be filled while the ship is in clean water (not river water) and carefully inspected while filling. If the tank shows significant accumulations of oil on the surface of the ballast while filling, then stop ballasting and empty and rewash the tank. No sheens or oil should be visible on the surface of the water when the ballast tank is full.
Ballast tanks must be completely filled to the deck to reduce the mount of structure exposed to corrosion in the air space above the ballast water surface.

2.14.2 Discharging dirty ballast
Dirty ballast must be discharged in accordance with the requirements of Regulations 9 and 15 of MARPOL. This means that the last one or two meters of dirty ballast in each tank will be retained on board and transferred to the slop tank. The available capacity of the slop tank must be compared to the amount of dirty ballast to be retained. If there is not enough capacity, then the transfer of dirty ballast residues will need to be done in stages, with the slop tank allowed to settle and then decanted between transfers.
Dirty ballast tanks are discharged overboard down to the desired innage (or until the ODM indicates that the oil content has reached the allowed limit). The remaining dirty ballast is then transferred to slop tank.

2.14.3 Flushing lines for ballast discharge
When all clean ballast tanks are filled and the dirty ballast has been discharged and stripped to the slop tank, then the lines and pumps to be used in discharging the clean ballast must be flushed overboard. The cargo piping valves must be set exactly as they will be when discharging clean ballast at the loading port and an amount of ballast run out of each clean ballast tank using the pump.

2.14.4 Decanting the slop tank
After the slop tank has settled for a sufficient time, it is decanted as described in Section 2.12.4.

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