1 An attained subdivision index A is obtained by the
summation of the partial indices As, Ap and
Al, weighted as shown and calculated for the draughts
ds, dp and dl defined in
regulation 2 in accordance with the following formula:
A = 0.4As + 0.4Ap +
0.2Al
Each partial index is a summation of contributions
from all damage cases taken in consideration, using the following
formula:
A = Σpisi
where:
- i represents each compartment or group of compartments under
consideration,
- p
i
accounts for the probability that only the compartment or group of compartments
under consideration may be flooded, disregarding any horizontal subdivision, as
defined in regulation 7-1,
- s
i
accounts for the probability of survival after flooding the compartment or group
of compartments under consideration, and includes the effect of any horizontal
subdivision, as defined in regulation 7-2.
2 As a minimum, the calculation of A shall be
carried out at the level trim for the deepest subdivision draught ds
and the partial subdivision draught dp. The estimated service trim may
be used for the light service draught dl. If, in any anticipated
service condition within the draught range from ds to
dl, the trim variation in comparison with the calculated trims is
greater than 0.5% of L, one or more additional calculations of A are to be
performed for the same draughts but including sufficient trims to ensure that, for all
intended service conditions, the difference in trim in comparison with the reference
trim used for one calculation will be not more than 0.5% of L. Each additional
calculation of A shall comply with regulation 6.1.
3 When determining the positive righting lever (GZ)
of the residual stability curve in the intermediate and final equilibrium stages of
flooding, the displacement used should be that of the intact loading condition. All
calculations should be done with the ship freely trimming.
4 The summation indicated by the above formula
shall be taken over the ship's subdivision length (L
s)
for all cases of flooding in which a single compartment or two or
more adjacent compartments are involved. In the case of unsymmetrical
arrangements, the calculated A value should be the mean
value obtained from calculations involving both sides. Alternatively,
it should be taken as that corresponding to the side which evidently
gives the least favourable result.
5 Wherever wing compartments are fitted, contribution
to the summation indicated by the formula shall be taken for all cases
of flooding in which wing compartments are involved. Additionally,
cases of simultaneous flooding of a wing compartment or group of compartments
and the adjacent inboard compartment or group of compartments, but
excluding damage of transverse extent greater than one half of the
ship breadth B, may be added. For the purpose of this
regulation, transverse extent is measured inboard from ship's side,
at right angle to the centreline at the level of the deepest subdivision
draught.
6 In the flooding calculations carried out according
to the regulations, only one breach of the hull and only one free
surface need to be assumed. The assumed vertical extent of damage
is to extend from the baseline upwards to any watertight horizontal
subdivision above the waterline or higher. However, if a lesser extent
of damage will give a more severe result, such extent is to be assumed.
7 If pipes, ducts or tunnels are situated within
the assumed extent of damage, arrangements are to be made to ensure
that progressive flooding cannot thereby extend to compartments other
than those assumed flooded. However, the Administration may permit
minor progressive flooding if it is demonstrated that its effects
can be easily controlled and the safety of the ship is not impaired.