1.1 CBT operation requires full appreciation of
the procedures and thorough understanding of what is going on at all
times. Frequent checks are essential to ensure that contamination
or pollution does not occur. These checks include visual observation
of the ballast tanks, the piping and valve system, etc. and a general
awareness of the possibilities of leakages that may permit oil to
enter the ballast system. The items in the procedure which emphasize
safety must therefore be followed in full.
1.2 The cargo discharging and ballast handling
sequence should be clearly illustrated in a programme worked out by
the key personnel involved. The programme should clearly identify
the responsibility of all persons involved and should specify in a
time-related manner essential facts, such as:
- the procedure and the timing for taking the necessary quantity
of piping flush water into the ballast tanks;
- the cargo discharging sequence, specifying pumps and lines to
be used at each point in the sequence and the projected timetable;
- the procedure and the timing for flushing the ballast piping and
taking on ballast: if this is done before the cargo is fully discharged
great attention must be paid to the details of the programme so that
safe segregation is maintained;
- the procedure for stripping and draining of the cargo tanks, and
particularly the cargo pipes, after completion of the discharge.
1.3 The above cargo discharge and ballast handling
programme together with the checklist (see section 5 of Manual) and
other relevant information should be displayed in the cargo control
room or other appropriate place easily accessible to all persons involved.
1.4 During any part of the CBT operation, when
quick and reliable communication is required between activities taking
place some distance apart, adequate and approved portable radio equipment
should be available.
1.5 The tanker should be loaded bearing in mind
the requirements of a cargo discharge sequence compatible with the
CBT operation.
1.6 When loading cargo that will be unloaded
at two or more discharge ports it is important to ensure that this
can be done following the CBT procedures and while maintaining safe
trim and stress levels.
1.7 Prior to arrival at loading port, the clean
ballast may be reduced to berthing condition, using a pump and pipe
that have been cleaned or kept clean during the ballast voyage.
1.8 The ballast may be discharged prior to loading
or after some cargo has been loaded, depending on the ship and terminal
limiting requirements. If cargo has to be loaded prior to deballasting,
this must only be done through piping that can be kept separated from
the dedicated clean ballast tanks and associated piping.
1.9 Ballast water carried in dedicated clean ballast
tanks should be visually checked prior to and during discharge for
presence of any oil. If any doubt about the cleanliness of the water
during discharge exists, the discharge should be stopped and the remainder
retained in the ballast tank or transferred to the slop tank for later
discharge to the reception facility or at sea in compliance with Regulation
9 of Annex I of MARPOL 73/78.
1.10 Normal line flushing must be carried out
by drawing water form the dedicated clean ballast tanks and discharging
to a slop tank. Flush water must never be discharged into a dedicated
clean ballast tank. The related method for making flush water available
to the ballast tanks for line flushing depends on the piping system
and influences the entire CBT procedure for the tanker.
1.11 If the tanker has a separate pump and deck
line for water washing of cargo tanks, isolated from the cargo system,
this may be used for transferring the necessary amount of flush water
to the ballast tanks at any time. Tank washing hoses are used as required
between the deck line and the individual tanks. This method can be
adopted by most tankers (which have not been built to use fixed, high
capacity washing machines) and it offers the most flexible and convenient
arrangement. Alternatively, the fire pump and the fire main and hoses
or other deck service lines may be used for this purpose, subject
to approval by the Administration of the use of the fire-fighting
system. The operation can normally be carried out completely independently
of the cargo tank activities. The amount of water transferred to the
ballast tanks prior to line flushing should always be at least ten
times the volume of the pipes to be flushed. There is, however, no
disadvantage in taking in more: what is not used for line flushing
simply remains in the ballast tanks, becoming part of the departure
ballast.
1.12 In vessels which do not have a water supply
system suitable for transfer of flush water over deck, flush water
must be transferred to the ballast tanks through the cargo piping
during the operational stages when the piping is clean. This means
that a quantity of flush water has to be left in the ballast tanks
at deballasting and subsequently is used for line flushing after the
loading of cargo. Similarly a quantity of flush water has to be taken
in through the clean piping towards the end of the loaded voyage in
order to be used for line flushing after the cargo discharge. A part
of the cargo piping thus has to remain flushed and clean during the
loaded passage. This procedure requires more control and more careful
planning than that described in paragraph 1.11 above which is therefore the procedure to be followed whenever
possible.
1.13 For tankers in short haul trades operating
on the principle outlined in 1.12 above
it may be preferable to leave the necessary quantity of flush water
in the ballast tanks throughout the loaded passage. In this case some
part of the ballast water is simply left in the tanks at deballasting
and used for line flushing after the following cargo discharge. Since
a CBT tanker always has unused deadweight capacity, this small dead-freight
will normally be of no importance. The effect on tank corrosion should
be considered and necessary preventive measures taken.
1.14 When carrying out pipe flushing, water must
always be routed from a ballast tank to a slop tank. In addition,
in tankers where the piping can be connected as a ring line it is
possible to pump water from the sea or from a slop tank through the
pipe system and back to a slop tank but water which has passed through
an oily pipe section must never be passed to a dedicated clean ballast
tank. The amount of pipe flushing water used should be at least ten
times the volume of the piping system to be flushed.
1.15 When flushing from ballast tanks containing
the prescribed minimum amount of flush water it is important that
the line has first been thoroughly drained of oil. Flushing should
normally start from the tank farthest from the pump. After the line
has been primed and suction established, the pumping is stopped and
the valves are closed for a period of about half an hour to allow
the oil from the pipe walls to separate out. Pumping is then resumed
at a moderate speed, with throttling of the output at the pump delivery
side.
1.16 When a ring line can be arranged, water is
first drawn from the individual tanks to clear the branch pipes and
as the tanks become empty and are closed off from the main line, the
inlet to the ring is opened from a slop tank to permit continued closed
loop flushing. Towards the end, the inlet should be switched to clean
seawater.
1.17 Towards the end of the loaded passage, a
normal amount of pipe flushing water is transferred to the CBT tanks,
either by use of a clean deck line and related pump or via the cargo
piping that has been left clean during the voyage. During short voyages
and provided the CBT tanks have adequate corrosion protection, the
pipe flushing water left in the tanks prior to loading may be retained
in the tanks. This procedure eliminates the need for flushing the
cargo piping during the loaded passage. In either alternative, the
tanker should have the nominal quantity of pipe flushing water in
the CBT tanks when entering the discharge port.
1.18 Before any CBT tank is ballasted a check
should be made for any presence of hydrocarbon gases as a result of
leakage. This check is also important from the safety point of view.
Ballast water must never be loaded into a ballast tank containing
hydrocarbon gases in a higher concentration than 20 per cent of the
lower explosive limit in such a way that splashing or free fall of
water can occur.
1.19 During unloading, the tanks which are served
by the pump and piping used for ballast handling should be discharged
first. At the point when ballast has to be taken on, the affected
piping is first flushed using the procedure in paragraphs 1.14-1.16 whereupon the CBT tanks are
ballasted to the extent required. Thereafter the ballast handling
piping should be kept clean to permit any additional handling of ballast
that may be necessary.
1.20 During the ballast voyage the piping is kept
clean as a preparation for deballasting. Should any part become contaminated,
for instance by its use during tank cleaning of cargo tanks, it is
again flushed using ballast water, whereupon the ballast quantity
may again be adjusted.