**1**The Maritime Safety Committee, at its fifty-ninth session, considered the proposal put forward by the United Kingdom, to extend the standard of residual stability known as the 'SOLAS 90' standard, by amending the SOLAS regulation II-1/8 as set out in circular letter No.1470 of 25 October 1990.

**2**After detailed consideration of the matter, the Committee concluded that a different approach should be adopted. This amended approach would be aimed at improving the survivability characteristics of existing ro-ro passenger ships over a period of time of five years.

**3**It was decided further that the time when the ferries are required to be modified should be based on their survivability characteristics such that those having the lowest characteristics are modified first. In order to determine the survivability characteristics of the individual ferries the Committee decided that it should be done on the basis of calculations first presented to the SLF Sub-Committee at its thirty-sixth session and subsequently reviewed by the Committee at its sixtieth session with a view to adopting an amendment to the SOLAS Convention to implement the SOLAS 90 standard by means of an agreed timescale.

**4**In respect of modifications made by the Committee it was considered that the comparison of the attained subdivision 'A' with the 'maximum' attained index 'Amax' as defined in the annex hereto, would provide a more sound and meaningful basis for the consideration of the matter by the SLF Sub-Committee, than the comparison of 'A' with the required subdivision index 'R'. As a consequence, it was decided to delete the reference to the required index 'R' in the calculations to be undertaken.

**5**The Committee urges its Members to carry out calculations in accordance with the annex hereto, as soon as possible, and submit results therefrom, with a copy to the Secretariat, to the United States*, the co-ordinator of this work, by not later than 31 October 1991.

**6**The Committee requests those Members who have already started their calculations, in accordance with the method given in annex 5 to SLF 35/20, to adjust them in order to take account of the changes in the concept as set out in the annex.

* Mr. H. Paul Cojeen

United States Coast Guard (G-MTH)

2100 Second Street, S.W.

Washington, D.C.20593-0001

United States

Telephone: (202)267-2988

Telex: 892427

Facsimile: (202)267-4861

## Annex The attained subdivision index ''A'' for existing RO-RO passenger ships

The calculation procedure to assess the survivability characteristics of existing ro-ro passenger ships when using a simplified method based upon resolution A.265(VIII)

### 1 General comments

**1.1**At the thirty-fifth session of the SLF Sub-Committee, following a proposal by the United States (SLF 35/4/23), it was agreed that a simplified version of resolution A.265(VIII) should be used to assess the survivability characteristics of existing ro-ro passenger ferries (SLF 35/20, paragraphs 4.21, 4.27 to 4.32 and annex 5 refers).

**1.2**The method proposed involves a calculation procedure which contains all the essential probabilistic elements of the full resolution A.265(VIII) method given in the "Regulations on subdivision and stability of passenger ships as an equivalent to Part B of chapter II of the International Convention for the Safety of Life at Sea, 1960".

**1.3**The principal probabilistic elements mentioned in 1.2 are the factors 'a', 'p', 'r' and 's'.

'a' | is a factor which estimates the probability of damage occurring at a particular position in the ship's length; |

'p' | is a factor which estimates the probability of the longitudinal extent of damage; |

'r' | is a factor which estimates the probability of the degree of penetration in from the ship side (this factor is only relevant where longitudinal subdivision is taken into account); and |

's' | is a factor which is a measure of survival probability. |

When s = 0, this means that there is no contribution to the index 'A' for the damage case being considered. When s = 1, this means that all the conditions for survival given by the specified residual stability criteria are fully met. |

**1.4**The factors 'a' and 'p', which refer to the centre of damage and longitudinal extent of damage, are to be taken directly from formulae (III) and (IV) of regulations 6(b) and 6(c) respectively.

**1.5**Where longitudinal subdivision is provided, allowance can be given for this - and it should be noted at this point that such subdivision may be inboard or outboard of the B/5 line. In such cases, the 'r' factor given at formula (X) of regulation 7(b)(ii) is to be used. However, the deterministic requirement of a minimum double bottom height of B/10 of regulation 7(a)(i) is not to be applied.

**1.6**In the case of the 's' factor, however, the formula for 's' is to be that which was first proposed by the USSR in SLF 35/4/9, and is reproduced in page 3, annex 5, of SLF 35/20. The use of this formula ensures that in all cases where all the SOLAS 90 criteria are met, the 's' factor is equal to 1.

**1.7**Further simplification has been introduced by specifying that the calculation of 's' is to be confined to the deepest subdivision draught, rather than the three draughts d1, d2 and d3 and corresponding s1, s2 and s3 values given in regulation 6(d)(ii) of the full resolution A.265(VIII) method.

**1.8**Finally, to limit as far as possible the number of damage stability calculations which need to be carried out and also to standardize the calculation procedure, regulation 6(a)(ii), should be applied only as far as the words "... the summation is also taken for all possible pairs of adjacent compartments." The remaining wording of regulation 6(b)(ii) should be ignored for the purposes of this simplified method. 2 The calculation procedure 2.1 Establish the following principal parameters: .1 the subdivision length, Ls Regulation 1(b) .2 the subdivision breadth, B1 Regulation 1(d)(i) .3 the subdivision breadth, B2 Regulation 1(d)(ii) .4 the deepest subdivision draught, ds Regulation l(a)(ii) .5

### 2 The calculation procedure

**2.1**Establish the following principal parameters:

.1 the subdivision length, Ls | Regulation 1(b) |

.2 the subdivision breadth, B1 | Regulation 1(d)(i) |

.3 the subdivision breadth, B2 | Regulation 1(d)(ii) |

.4 the deepest subdivision draught, ds | Regulation l(a)(ii) |

.5 the number of main compartments | |

.6 the ship's maximum operational KG at the deepest subdivision draught. |

**2.2**For each of the main compartments establish the following:

.1 the values X1, X2 | Regulation 6(b) |

.2 the corresponding ksi1, ksi2 and ksi12 values | Regulation 6(b) |

.3 using the values obtained from .1 and .2, calculate: | |

'a' - see regulation 6(b), formula (III) | |

'p' - see regulation 6(c), formula (IV) | |

'r' - see regulation 7(b)(ii), formula (X) | |

(where longitudinal subdivision is concerned). |

**2.3**The calculation of the 's' factor is by the use of the formula given in page 3, annex 5, of SLF 35/20. The formula is:

s = c x 2.58 x

^{4}√ GZmax x Range x Area

When the criteria for compliance with the requirements of regulation II-1 of the 1974 SOLAS Convention, as amended, are fully met, then s = 1 is to be assumed. The 's' factor is only to be calculated for the deepest subdivision draught (ds), rather than for the three draughts specified for the full resolution A.265(VIII) method. The deepest subdivision draught in this instance is the subdivision draught appropriate to the vessel.

**2.4**The damage stability results which are used to obtain the residual stability characteristics, that is, Gzmax, range and area under the curve, are to be based on the ship's maximum operational KG at the deepest subdivision draught. Level trim is to be assumed.

**2.5**A tabular summary of 'a', 'p', 'r', 's' should now be made for all the main compartments. The product a×(pr)×s is to be calculated for each damage case to obtain the contribution to the index 'A' (say, delta A>0). A summation of the 'delta A' values is then made to obtain the contribution to the 'A' value from the single compartments alone.

**2.6**The procedure outlined above is now performed for all cases involving the assumed flooding of two adjacent compartments.

**2.7**If the vessel is not fully compliant with the required residual stability standard, then at least one of the damage cases appropriate to the subdivision standard will have an 's' value which is less than 1, i.e:

.1 for a two-compartment vessel, at least one two-compartment damage case will have s<_1 br="br" xmlns="http://standaarden.overheid.nl/puc/terms/" xmlns:puc="http://standaarden.overheid.nl/puc/terms/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xhtml="http://www.w3.org/1999/xhtml">.2 for a one-compartment vessel, at least one one-compartment damage case will have s<_1. br="br">