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5.40 PREPARATIONS FOR BALLASTING
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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