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3.1 INSPECTING BALLAST TANKS
Case study
A VLCC at an Arabian Gulf loading terminal began discharging ballast from
its segregated, double bottom tanks while loading operations were being
started. The day was clear and fine, with a light northerly wind, nil
current and sea. As the first ballast tanks were being emptied a film
of discolouration was sighted on the water surrounding the ship's overboard
discharge. Ballast discharge was
stopped immediately, the discolouration soon dissipated. The balance of
the segregated ballast was required to be pumped ashore, causing delay
to the loading operator. A fine was expected to be imposed by the port
authorities. Case analysis
It is not difficult to examine ballast tanks visually before they are
discharged in port, provided that the ballast tank openings are arranged
so that the surface of the ballast water can be inspected. Ballast contamination
is more difficult to detect at night. To confirm ballast cleanliness,
a small piece of oil absorbent pad should be lowered to the water surface
and allowed to remain in the tank for a short time. When it is retrieved
and examined, any discolouration of the adsorbent pad will indicate
ballast contamination.
When ballast tanks have been inspected, enter the time of the inspection
and list of tanks inspected in the deck logbook.
Visual or adsorbent examination of ballast water is normally possible
for wing or centre ballast tanks. However, it is rarely possible with
double bottom tanks.
Some ports require ballast, segregated or otherwise, to be sampled and
analysed prior to discharge. A few ports (ie. Scapa Flow), require all
ballast, including segregated, to be discharged ashore.
Double bottom tanks must be checked by an alternative method. The best
method is to have used a combustible gas indicator to sample the air
expelled while the tanks were being filled in the discharge port or
after departure. Any expelled hydrocarbon vapours will indicate that
the tank has been contaminated by oil.
3.1.1 Discharging ballast with the oil discharge
monitor
When clean ballast is being discharged in port it is advisable, if possible,
to discharge the ballast through the Oil Discharge Monitor, with the
alarm point set at its lowest level. An alarm and pump shutdown at a
five to ten PPM level will alert the chief officer to the fact that
the ship has contaminated ballast and prevent a pollution incident by
stopping the discharge automatically. The harbour surface and discharge
outlet must be under continuous observation when discharging ballast.
3.1.2 Disposal of contaminated ballast
If contaminated clean ballast is discovered by any means of inspection,
that tank must be handled as dirty ballast. If the ship is in international
waters, outside any prohibited zone, the contaminated ballast may be
decanted at sea within accepted discharge criteria. The last meter of
innage in the tank should be transferred to the slop tank. The tank
can then be cleaned and inspected to determine the cause of the contamination.
If contaminated ballast is discovered in port, then the entire tank
must be discharged ashore before loading cargo. Shore reception facilities
require advance notice of ballast quantity and oil/chemical content
before they will agree to receive discharges from ships.
At the completion of ballast discharge operations, any tank or pipeline
strippings not transferred to the slop tank should be stripped ashore
using the small-diameter line.
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