7.6.1 When a combination carrier is carrying a
cargo other than oil it should be considered as a tanker unless the
requirements in 7.6.8 are complied with.
7.6.2 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 7.6.6 and 7.6.7 are emptied of oil and oil residues, and
cleaned and ventilated to such a degree that the tanks are completely
gas-free and internally inspected. The pump-room, cargo pumps, pipelines,
duct keel and other void spaces are to be checked to ensure that they
are free of oil and hydrocarbon gas.
7.6.3 Where holds are required to carry cargo
other than oil they should be isolated from the inert gas main and
oil cargo pipeline by means of blanks which should remain in position
at all times when cargoes other than oil are being handled or carried.
7.6.4 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 7.6.6 and 7.6.7, cargo pump-rooms, 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:
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.1 there has been no generation of hydrocarbon
gas or leakage of hydrocarbon gas from the slop tanks referred to
in 7.6.6 and 7.6.7.
If concentrations of more than 20% of the lower flammable limit are
detected, the compartments should be ventilated;
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.2 there is no deficiency of oxygen which could
be attributable to leakage of inert gas from another compartment.
7.6.5 As an alternative to 7.6.4,
those cargo tanks which are empty of cargo may be re-inerted in accordance
with 5.1, 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 1% by volume. If such a concentration
is detected the compartments should be purged in accordance with 5.10.
7.6.6 Slops should be contained in a properly
constituted slop tank and should be:
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.1 discharged ashore and the slop tanks cleaned
and ventilated to such a degree that the tanks are completely gas-free
and then inerted; or
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.2 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 7.6.5.
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 7.6.5 and
in addition a report should be forwarded to the Administration.
7.6.7 Slop tanks which have not been discharged
should comply not only with the requirements of 7.6.6, but also with regulation 62.11.2 which
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 these Guidelines. In this
connection reference is made to 7.6.3.
On combination carriers where there are also empty cargo tanks which
are not required to be isolated from the inert gas main then the arrangement
for isolating the slop tanks from these tanks should be such as to:
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.1 prevent the passage of hydrocarbon gas from
the slop tanks to the empty tanks; and
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.2 facilitate monitoring (see regulation 62.16.3.1) of and, if necessary,
topping up of the pressure in slop tanks and in any empty cargo tanks
if the latter are being kept in the inert condition as referred to
in 7.6.5.
A suggested arrangement is shown in figure
19.
In addition, all cargo pipelines to or from
the slop tanks should be blanked off.
7.6.8 Instead of complying with the requirements
in 7.6.2 to 7.6.7 a combination carrier
may be operated as a bulk carrier without having to use its inert
gas system if either:
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.1 it has never carried a cargo of oil; or
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.2 after its last cargo of oil, all its cargo
tanks, including slop tanks, the pump-room, cargo pumps, pipelines,
cofferdams, duct keel and other void spaces are emptied of oil and
oil residues, cleaned and completely gas-free and the tanks and void
spaces internally inspected to that effect. In addition the monitoring
required in 7.6.4 should be continued until
it has been established that generation of hydrocarbon gas has ceased.