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PART VII
COMBINATION CARRIERS
The basic principles of inerting are exactly the same for a combination
carrier as for a tanker; however, differences in the design and operation of
these vessels make necessary the relevant considerations outlined below.
Slack Holds
48. It is particularly important for combination carriers to have
their holds inerted because, whenever a hold in an OBO carrier (which
could extend the full breadth of the ship) is partially filled with
clean or oily ballast, water agitation of this ballast can occur at
small angles of roll, resulting in the generation of static electricity;
the agitation, sometimes referred to as ‘sloshing’, can happen
whenever the ullage above the liquid level of the hold is more than 10
per cent of the depth of the hold, measured from the underside of the
deck (see Figure 18 for remedy condition).
![Figure 18](/web/20071224085224im_/http://www.tc.gc.ca/MarineSafety/TP/Tp4295/images/fig-18.gif)
Figure 18 Cargo tank ullage effect on generation of static electricity
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Leakage
49. To ensure that leakage of tank gas, particularly through the
hatch centreline joints, is eliminated or minimized, it is essential
that the hatch covers be inspected frequently to determine the state of
their seals, their alignment, etc; when the hatch covers have been
opened, particularly after the ship has been carrying a dry bulk cargo,
the seals and trackways should be inspected and cleaned to remove any
foreign matter.
Ballast and Void Spaces
50. The cargo holds of combination carriers are adjacent to ballast
and void spaces; leakages may occur in pipelines or ducts in these
spaces, or from a fracture in the boundary plating; oil, inert gas and
hydrocarbon gas may leak into the ballast and void spaces; consequently
gas pockets may form and cause difficulty with gas-freeing because of
the considerable steelwork, acting as stiffening, that is characteristic
of these spaces; personnel should be alerted to this hazard.
Inert Gas Distribution System
51. Due to the special construction of combination carriers, the vent
line from the cargo hatchway coaming is situated very close to the level
of the cargo surface; in many cases, the inert gas main line passing
along the main deck may be below the oil level in the hold; during rough
weather, oil or water may enter these lines, completely blocking the
opening, and thus prevent an adequate supply of inert gas during either
tank cleaning or discharge; vent lines should therefore have drains
fitted at their lowest point, which should always be checked before any
operation takes place within the cargo hold.
Application when Carrying Oil
52. When combination carriers are engaged exclusively in the carriage
of oil, the inert gas system should be utilized in the manner described
in Part (V).
Application when Carrying Cargoes Other than Oil
53. (1) A combination carrier transporting a cargo other than oil
should be considered a tanker, unless the requirements in subsection
53(8) are complied with.
(2) When cargoes other than oil are intended to be carried, it is
essential that all holds/cargo tanks other than slop tanks referred to
in subsection 53(6) and subsection 53(7) be emptied of oil and oil
residues, cleaned and ventilated to such a degree that the tanks are
completely gas free and internally inspected; the pumproom, cargo pumps,
pipelines, duct keel and other void spaces are to be checked to ensure
that they are free of oil and hydrocarbon gas.
(3) Holds required to carry cargo other than oil should be isolated
from the inert gas main and oil cargo pipeline by means of blanks; these
blanks should remain in position at all times when cargoes other than
oil are being handled or carried.
(4) During the loading and discharging of solid cargoes, and
throughout the intervening periods, all holds/cargo tanks [other than
the slop tanks referred to in subsection 53(6) and subsection 53(7)],
cargo pumprooms, cofferdams, duct keels and other adjacent void spaces
should be kept in a gas-free condition and checked periodically at
intervals of not more than two days to ensure that:
- no hydrocarbon gas has been generated or leaked from slop
tanks referred to in subsection 53(6) and subsection 53(7); if
concentrations of more than 20 per cent of the lower flammable limit
are detected, the compartments concerned should be ventilated;
- no oxygen deficiency exists that could be attributable to
leakage of inert gas from another compartment.
(5) As an alternative to subsection 53(4), empty cargo tanks may be
re-inerted in accordance with section 30, provided they are subsequently
maintained in the inert condition and at a minimum pressure of 100 mm
water gauge at all times, and provided that they are checked
periodically, at intervals of not more than two days, to ensure that any
generation of hydrocarbon gas does not exceed one per cent by volume; if
such a concentration is detected, the compartments concerned should be
purged in accordance with section 39.
(6) Slops should be contained in a properly constituted slop tank and
should be
- discharged ashore, and the slop tanks cleaned and ventilated
to such a degree that they are completely gas free and then inerted;
or
- retained on board for not more than one voyage when, unless
the vessel reverts to carrying oil, the slop tank should be treated
as in subsection 53(5).
If slops are retained on board for more than one voyage because
reception facilities for oily residues are not available, the slop tank
should be treated as in subsection 53(5) and, in addition, a report be
forwarded to the Board.
(7) Slop tanks that have not been discharged should comply not only
with the requirements of subsection 53(6), but also with the Fire
Protection, Detection and Extinguishing Equipment Regulations,
subsection 11(3), Schedule VII; the latter requires that they be
isolated from other tanks by blank flanges, which will remain in
position at all times when cargoes other than oil are being carried,
except as provided for in this Standard; reference is also made to
subsection 53(3); on combination carriers where there are also empty
cargo tanks that are not required to be isolated from the inert gas
main, the arrangement for isolating the slop tanks from these tanks
should be such as to:
- prevent the passage of hydrocarbon gas from the slop tanks to
the empty tanks; and
- facilitate monitoring of the pressure in slop tanks and in
any empty cargo tanks, and, if necessary topping it up, if the
latter are being kept in the inert condition as referred to in
subsection 53(5)
Figure 19 shows suggested arrangement.
![Figure 19](/web/20071224085224im_/http://www.tc.gc.ca/MarineSafety/TP/Tp4295/images/fig-19.gif)
Figure 19 Inert gas by-pass arrangement for topping up cargo tanks
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in addition, all cargo pipelines to or from the slop tanks should be
blanked off.
(8) Instead of complying with the requirements in subsections 53(2)
to 53(7), a combination carrier may operate as a bulk carrier without
having to use its inert gas system if either
- it has never carried a cargo of oil; or
- after its last cargo of oil, all its cargo tanks, including
slop tanks, the pumproom, cargo pumps, pipelines, cofferdams, duct
keel and other void spaces are emptied of oil and oil residues,
cleaned and completely gas-freed, and the tanks and void spaces
internally inspected to that effect; in addition, the monitoring
required in subsection 53(4) should be continued until it has been
established that generation of hydrocarbon gas has ceased.
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