4.8.1 Where a product is carried
at a temperature below -10°C suitable insulation should be provided
to ensure that the temperature of the hull structure does not fall
below the minimum allowable design temperature given in chapter 6 for the grade of steel concerned, as
detailed in 4.9, when the cargo tanks
are at their design temperature and the ambient temperatures are 5°C
for air and 0°C for seawater. These conditions may generally be
used for world-wide service. However, higher values of the ambient
temperatures may be accepted by the Administration for ships operated
in restricted areas. Conversely, lesser values of the ambient temperatures
may be fixed by the Administration for ships trading occasionally
or regularly to areas in latitudes where such lower temperatures are
expected during the winter months. The ambient temperatures used in
the design should be shown on the International Certificate of Fitness
for the Carriage of Liquefied Gases in Bulk as provided for in 1.5.
4.8.2 Where a complete or partial
secondary barrier is required, calculations should be made with the
assumptions in 4.8.1 to check that
the temperature of the hull structure does not fall below the minimum
allowable design temperature given in chapter
6 for the grade of steel concerned, as detailed in 4.9. The complete or partial secondary barrier
should be assumed to be at the cargo temperature at atmospheric pressure.
4.8.3 Calculations required
by 4.8.1 and 4.8.2 should be made assuming still air and still water, and except
as permitted by 4.8.4, no credit
should be given for means of heating. In the case referred to in 4.8.2, the cooling effect of the rising
boil-off vapour from the leaked cargo should be considered in the
heat transmission studies. For structural members connecting inner
and outer hulls, the mean temperature may be taken for determining
the steel grade.
4.8.4 In all cases referred
to in 4.8.1 and 4.8.2 and for ambient temperature conditions
of 5°C for air and 0°C for seawater, approved means of heating
transverse hull structural material may be used to ensure that the
temperatures of this material do not fall below the minimum allowable
values. If lower ambient temperatures are specified, approved means
of heating may also be used for longitudinal hull structural material,
provided this material remains suitable for the temperature conditions
of 5°C for air and 0°C for seawater without heating. Such
means of heating should comply with the following requirements:
-
.1 sufficient heat should be available to maintain
the hull structure above the minimum allowable temperature in the conditions
referred to in 4.8.1 and 4.8.2;
-
.2 the heating system should be so arranged that, in
the event of a failure in any part of the system, stand-by heating could be
maintained equal to not less than 100% of the theoretical heat load;
-
.3 the heating system should be considered as an
essential auxiliary; and
-
.4 the design and construction of the heating system
should be to the satisfaction of the Administration.
4.8.5 In determining the insulation
thickness, due regard should be paid to the amount of acceptable boil-off
in association with the reliquefaction plant on board, main propulsion
machinery or other temperature control system.