3.4 CBT BALLAST RESIDUES AND SLOP TANK
Large quantities of water are often found in crude oil cargoes. Some
of this water is a result of poorly drained ballast piping on CBT ships.
It is important that the CBT bottom lines be thoroughly drained after
CBT discharge is completed. Since the lines cannot be vented by leaving
the ballast tank suction valves open the following procedure is recommended:Complete CBT discharge and close all valves.
Align stripping pump suction valves to take suction on the CBT main suction
Start stripping pump slowly and observe the suction gauge.
When stripping pump has developed a vacuum on the CBT line, open a tank
suction valve on the forewardmost cargo tank connected to the CBT line.
Continue running the stripping pump at slow speed until the stripping
discharge pressure drops or it is otherwise noted that the pump has lost
Close all valves.
The pipeline strappings should never be discharged overboard even
if the stripping pump, associated piping and forwardmost cargo tank
have been thoroughly flushed before the vessel arrived in port.
There are too many opportunities for previously undetected oil residues
to be discharged with the final ballast withdrawn from the ballast tanks
or pipeline. The most prudent procedure is to strip the pipelines ashore,
or to the vessel's slop tank.
Stripping of pipelines is important because the salt water residues
from the pipelines, if left in the cargo tanks, can:Increase tank bottom corrosion.
Seriously affect the quality of certain grades of cargo.
Cause damage to the receivers shore plant equipment.
Distort cargo measurement calculations.
Waste freight earning capacity.
Require special precautions with certain grades of cargo, such as static
accumulators and highly heated cargoes.
Fresh water flushing of some pipelines or tanks may be required after
ballast discharge to eliminate the possibility of salt contamination
of the cargo.
Cargoes where this is advisable include:
Virgin gas oil
3.4.1 Contaminated ballast
If ballast contamination has been discovered, the ballast cannot be
discharged overboard and must be either retained on board or pumped
to the shore facility. Most shore terminals have some facilities for
receiving slop or ballast residues, but this capability depends on the
capacity of the shore holding and processing facilities and problems
such as dealing with residues containing lead (TEL).
Even if clean ballast appears to have no contamination, it should never
be discharged overboard entirely. The last half meter or meter of ballast
innage should be stripped ashore or to the slop tank.
3.4.2 Slop tank gauging
When all ballast operations have been completed, the slop tank
should be carefully gauged and an interface reading taken to determine
quantities of oil and water contained. If the slop tank is to be used
for load on top (LOT), operations, as much water as possible should
be stripped ashore from the bottom of the slop tank, leaving little
or no emulsion under the oil residues. After the tank has been stripped
of water (pumped to shore), it must be gauged again and a 'Certificate
of slops' prepared by the master. The slop tank may be gauged by the
independent petroleum inspector prior to loading.
If the slop tank will not be loaded into, then its contents must be
disposed on in accordance with the provisions of MARPOL 73/78.