4.7.1 Where the cargo temperature
at atmospheric pressure is below -10°C, a secondary barrier should
be provided when required by 4.7.3 to
act as a temporary containment for any envisaged leakage of liquid
cargo through the primary barrier.
4.7.2 Where the cargo temperature
at atmospheric pressure is not below -55°C, the hull structure
may act as a secondary barrier. In such a case:
-
.1 the hull material should be suitable for the cargo
temperature at atmospheric pressure as required by 4.9.2; and
-
.2 the design should be such that this temperature
will not result in unacceptable hull stresses.
4.7.3 Secondary barriers in
relation to tank types should normally be provided in accordance with
the following table. For tanks which differ from the basic tank types
as defined in 4.2 the secondary barrier
requirements should be decided by the Administration in each case.
| Cargo temperature at
atmospheric pressure
|
-10°C and
above
|
Below -10°C down to
-55°C
|
Below -55°C
|
| Basic tank type
|
No secondary barrier required
|
Hull may act as secondary
barrier
|
Separate secondary barrier where
required
|
| Integral
|
|
Tank type
not normally allowed1
|
| Membrane
|
|
Complete secondary barrier
|
| Semi-membrane
|
|
Complete
secondary barrier2
|
| Independent
|
|
|
|
Type A
|
|
Complete secondary barrier
|
|
Type B
|
|
Partial secondary barrier
|
|
Type C
|
|
No secondary barrier required
|
| Internal
insulation
|
|
|
|
Type 1
|
|
Complete secondary barrier
|
|
Type 2
|
|
Complete secondary barrier is incorporated
|
| NOTES
1. A complete secondary barrier should
normally be required if cargoes with a temperature at atmospheric
pressure below -10°C are permitted in accordance with 4.2.1.3.
2. In the case of semi-membrane tanks which
comply in all respects with the requirements applicable to type B
independent tanks, except for the manner of support, the Administration
may, after special consideration, accept a partial secondary
barrier.
|
4.7.4 The secondary barrier
should be so designed that:
-
.1 it is capable of containing any envisaged leakage
of liquid cargo for a period of 15 days, unless different requirements apply for
particular voyages, taking into account the load spectrum referred to in 4.3.4.4;
-
.2 it will prevent lowering of the temperature of the
ship structure to an unsafe level in the case of leakage of the primary barrier as
indicated in 4.8.2; and
-
.3 the mechanism of failure for the primary barrier
does not also cause the failure of the secondary barrier and vice versa.
4.7.5 The secondary barrier
should fulfil its functions at a static angle of heel of 30°.
4.7.6.1 Where a partial secondary
barrier is required, its extent should be determined on the basis
of cargo leakage corresponding to the extent of failure resulting
from the load spectrum referred to in 4.3.4.4 after the initial detection of a primary leak. Due account
may be taken of liquid evaporation, rate of leakage, pumping capacity
and other relevant factors. In all cases, however, the inner bottom
adjacent to cargo tanks should be protected against liquid cargo.
4.7.6.2 Clear of the partial
secondary barrier, provision such as a spray shield should be made
to deflect any liquid cargo down into the space between the primary
and secondary barriers and to keep the temperature of the hull structure
to a safe level.
4.7.7 The secondary barrier
should be capable of being periodically checked for its effectiveness,
by means of a pressure/vacuum test, a visual inspection or another
suitable method acceptable to the Administration. The method should
be submitted to the Administration for approval.