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If the ship is a CBT vessel, additional flushing water may be needed in the CBT tanks while the ship is discharging. This additional water reduces the potential for running short of flushing water while preparing the CBT pipelines for ballasting. If additional flushing water is added overall (by hoses emptying into the upper part of the tank), the tank must be tested for flammable gas mixture before filling begins.
CBT and SBT tanks should be carefully examined for signs of cargo contamination before filling. A hydrocarbon sensor (combustible gas indicator) can be used to check for hydrocarbon vapours in the tank. An extended sampling hose must be used to check the tank atmosphere as close as possible to the tank bottom.
If ballast is to be loaded into non-gas free tanks, then the agreement of the terminal must be obtained and all checks and precautions applicable to loading volatile cargo must be applied to the ballasting operation. The expelled tank atmosphere should be vented through the permanent vent lines and preferably to a vent riser/mast, equipped with a suitable flame screen.
The quantity of ballast to be taken is at the master's discretion. While it is commercially beneficial to minimise ballast quantities and ballasting time after cargo discharge is complete, the minimum ballast taken must consider:
Complete propeller immersion.
Air draft limits passing overhead cables and bridges.
Hull bending and shear stresses.
Weather conditions
Manoeuvring capabilities.
Bow thruster immersion.

Recommendations are for the ship to have on board 25% to 30% of its deadweight before departing the berth. The 25% figure is only applicable to optimum manoeuvring conditions. Additional ballast may be loaded during the transit from the berth to sea and during the initial sea passage. Never jeopardise safety for a quick turnaround!

5.40.1 Ballast lines in cargo tanks - precautions
Some tanker designs have the SBT filling and suction lines passing through cargo oil tanks. These lines in the tank bottom are subject to bending stresses and corrosion and may occasionally lose their integrity. These leaking SBT lines will fill with oil during the loaded passage and contaminate the SBT tank is the leakage is not detected prior to starting ballast.
The following precautions are recommended to prevent SBT tank contamination by leaking SBT piping:
Leave the SBT pipelines filled with water during the loaded passage. This will minimise any contamination if the lines leak.
Implement a regular leak testing programme for SBT piping, to be conducted during the ballast passage. The tests and inspections recommended in Section 2.16.4 should be implemented.
Before filling SBT tanks at the discharge port, flush lines through to the ballast tanks and then test the tank bottom atmosphere for hydrocarbon vapour. Detection of leakage before the tank is filled minimises the amount of tank washing that will be required to return it to a clean ballast condition.

Case studies
1 A crude oil tanker ballasting at Brisbane overflowed the No.3 double bottom ballast tank. The overflow from the ballast tank
deck vent was sighted by the deck watch keepers who notified the cargo watch officer in the CCR. Before the ballast stream could be diverted to another tank, three barrels of Arabian Heavy Crude oil had been ejected onto the deck and 1/2 barrel escaped over the side into the sea. Subsequent investigation discovered a leak between the No.6 cargo tank/hold and the No.3 double bottom tank.
2 A ship ballasting at Aqaba was advised by harbour personnel that it was spilling oil. The source of the oil was discovered to be the air vent from No.3 starboard ballast tank. Investigation revealed that the air vent line passed through the No.3 starboard HFO tank where a 30 mm hole was found in the ballast tank air pipe. The level of the ballast tank was reduced to a safe level to prevent further pollution.
3 A double hulled tanker discharging at Saint John, Canada was ballasting when the No.5 wing tank overflowed. Light crude oil was found to have entered the sea from the ballast overflow, causing a minor pollution incident. The ballast tank was known to be subject to contamination from the adjacent cargo tanks and the master had posted standing orders advising that the ballast tank was not to be filled to overflowing.

Case analyses
Little analysis is needed to discover the cause of the three pollution incidents above. In each case, the ballast tank was unnecessarily or negligently overflowed in port. Given the age and condition of many tankers, overflowing a ballast tank in port is closely related to playing Russian Roulette. If SBT or CBT tanks are routinely permitted to overflow in port, sooner or later a pollution will result. The only positive prevention measure against such incidents is to top off ballast tanks as a cargo tank would be topped off! To minimise underdeck corrosion in ballast tanks with internal cathodic protection, it is advisable to press up the ballast tanks until the water is in contact with all underdeck surfaces, but this should not be done until the ship is well offshore. Double bottom tanks, which are difficult to check for contamination before filling, should be filled by gravitation (running in from the sea without use of the ballast pump).

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