4.6 Attained Subdivision Index A:

 (1) The attained subdivision index A is obtained by the summation of the partial indices A_s, A_p and A_l, (weighted as shown) calculated for the draughts d_s, d_p and d_l defined in section 1.3 in accordance with the following formula-

A=0.4A_s+0.4A_p+0.2A_i

(2) Each partial index is a summation of contributions from all damage cases taken in consideration, using the following formula-

A=Σp_is_i

• 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 section 4.7; and

• 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 section 4.8.

(3) In the calculation of A-

• (a) the level trim shall be used for the deepest subdivision draught and the partial subdivision draught;

• (b) the actual service trim shall be used for the light service draught; and

• (c) if in any service condition, the trim variation in comparison with the calculated trim is greater than 0.5% of L_s, one or more additional calculations of A are to be submitted for the same draughts but different trims so that, for all service conditions, the difference in trim in comparison with the reference trim used for one calculation will be less than 0.5% of L_s.

(4) When determining the positive righting lever (GZ) of the residual stability curve, the displacement used should be that of the intact condition; that is, the constant displacement method of calculation should be used.

(5) 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.

(6) Wherever wing compartments are fitted-

• (a) contribution to the summation indicated by the formula shall be taken for all cases of flooding in which wing compartments are involved;

• (b) 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; and

• (c) 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.

(7) In the flooding calculations carried out according to the regulations-

• (a) only one breach of the hull and only one free surface need to be assumed; and

• (b) the assumed vertical extent of damage is to extend from the baseline upwards to any watertight horizontal subdivision above the waterline or higher.

provided however, if a lesser extent of damage will give a more severe result, such extent is to be assumed.

(8) 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.