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CBT ballast is normally discharged after the tanker is safely berthed and before cargo loading operations commence. The ballast piping should have been prepared according to the ship's operating procedure, an example of which is provided in section 2.23 of this manual.
The oil discharge monitor must be functioning and the system ready in all respects to monitor the ballast discharge. All cargo and ballast tanks, including slop tanks, must be open to the inert gas main, with tank IGS isolating valves locked or lashed open. All cargo and slop tank gauging openings must be closed. The inert gas main must be isolated from open mast risers and high velocity vents. The IGS deck isolating valve must be open and the IGS plant producing gas with oxygen content of 5% or less.

Following the guidelines of the IMO publication Dedicated clean ballast tanks, (1982 edition) (see section 2.2.3 of this handbook), the clean ballast discharge procedure would be:
1 Open valves for discharging ballast.
     No.1 cargo main valves nos. 40, 41, 59, 60, 104, 144 and 123.
     No.1 discharge main valves nos. 155, 156 and 127.
     No.2 dedicated clean ballast tank nos. 36, 37.
     Starboard sea suction valve no. 115.
2 Start the No.l pump and begin discharging ballast overboard. Observe the indications of the oil content monitor and the appearance of the water surface near the overboard discharge.
3 Discharge the No.2 clean ballast tank until the tank innage is 0.5 meter; then close valves nos 36 and 37. (This retains enough water in No.2 centre tank to flush the branch pipes when the lines are being cleaned later.)
4 Discharge the No.4 dedicated clean ballast tank until the tank innage is 1.5 meters, then stop the pump and close valves nos. 58 and 67.
5 Drain all ballast piping to the port slop tank.
6 Close all valves.

During the discharge of ballast, if there is any doubt regarding the cleanliness of the water, the overboard discharge must be stopped and the remainder of the ballast retained on board or discharged ashore.

3.2.1 Simultaneous loading and CBT ballast discharge
Occasionally, trim or freeboard restrictions prohibit the complete discharge of CBT before cargo loading begins. CBT ballast may be discharged during the loading of cargo either simultaneously or by interrupting the loading. There must be effective two-valve separation between the cargo piping and the clean ballast system. The master is responsible for ensuring that the separation valves are leak tight.
Simultaneous loading and de-ballasting can also be safely conducted where the cargo tanks are fitted with individual cargo loading lines, not connected to the discharge piping.
Until sufficient clean ballast has been discharged to complete loading operations, the CBT system piping must be kept clean.
Simultaneous cargo loading and ballast discharge reduces both the port time of the vessel. It also reduces the total volume of hydrocarbon emissions produced by the loading operation, since the atmosphere displaced by cargo entering the cargo tanks is transferred through the vent system to the ballast tanks as ballast is discharged.
However, the reduction in air pollution is achieved at some increase in risk of sea pollution. For the owner's protection, it is always better to conduct the de-ballasting and the loading operations one at a time, when possible.

3.2.2 Alternate de-ballasting and loading
If company or terminal policy prohibits simultaneous de-ballasting and cargo loading, then the vessel may load part of the cargo, stop loading, de-ballast and then complete cargo loading. While the vessel is loading, the ballast tank levels must be carefully monitored and while it is de-ballasting the cargo tank levels must be closely monitored.

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