Appendix 4 - Initial Ranking of Accident Scenarios
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Statutory Documents - IMO Publications and Documents - Circulars - Maritime Safety Committee-Marine Environment Protection Committee Circulars - MSC-MEPC.2 Circulars - MSC-MEPC.2/Circular.12/Rev.2 – Revised Guidelines for Formal Safety Assessment (FSA) for Use in the IMO Rule-Making Process – (9 April 2018) - Appendix 4 - Initial Ranking of Accident Scenarios

Appendix 4 - Initial Ranking of Accident Scenarios

 1 At the end of step 1, hazards are to be prioritized and scenarios ranked. Scenarios are typically the sequence of events from the initiating event up to the consequence, through the intermediate stages of the scenario development.

2 To facilitate the ranking and validation of ranking, it is generally recommended to define consequence and probability indices on a logarithmic scale. A risk index may therefore be established by adding the probability/frequency and consequence indices. By deciding to use a logarithmic scale, the Risk Index for ranking purposes of an event rated "remote" (FI=3) with severity "Significant" (SI=2) would be RI=5.

  • Risk = Probability x Consequence
    Log (Risk) = log (Probability) + log (Consequence)

3 The following table gives an example of a logarithmic severity index, scaled for a maritime safety issue. Consideration of environmental issues or of passenger vessels may require additional or different categories.

Severity index
SI SEVERITY EFFECTS ON HUMAN SAFETY EFFECTS ON SHIP S (Equivalent fatalities)
1 Minor Single or minor injuries Local equipment damage 0.01
2 Significant Multiple or severe injuries Non-severe ship damage 0.1
3 Severe Single fatality or multiple severe injuries Severe damage 1
4 Catastrophic Multiple fatalities Total loss 10

4 The following table gives an example of a logarithmic probability/frequency index.

Frequency index
FI FREQUENCY DEFINITION F (per ship year)
7 Frequent Likely to occur once per month on one ship 10
5 Reasonably probable Likely to occur once per year in a fleet of 10 ships, i.e. likely to occur a few times during the ship's life 0.1
3 Remote Likely to occur once per year in a fleet of 1,000 ships, i.e. likely to occur in the total life of several similar ships 10-3
1 Extremely remote Likely to occur once in the lifetime (20 years) of a world fleet of 5,000 ships 10-5

5 The following table gives an example of a risk matrix based on the tables above.

Risk Index (RI)
FI FREQUENCY SEVERITY (SI)
1 2 3 4
Minor Significant Severe Catastrophic
7 Frequent 8 9 10 11
6   7 8 9 10
5 Reasonably probable 6 7 8 9
4   5 6 7 8
3 Remote 4 5 6 7
2   3 4 5 6
1 Extremely remote 2 3 4 5

6 In case of FSA on prevention of oil spill from ships, the following severity index can be used.

Severity Index
SI SEVERITY DEFINITION
1 Category 1 Oil spill size < 1 tonne
2 Category 2 Oil spill size between 1-10 tonnes
3 Category 3 Oil spill size between 10-100 tonnes
4 Category 4 Oil spill size between 100-1,000 tonnes
5 Category 5 Oil spill size between 1,000-10,000 tonnes
6 Category 6 Oil spill size >10,000 tonnes
Parent topic: MSC-MEPC.2/Circular.12/Rev.2 – Revised Guidelines for Formal Safety Assessment (FSA) for Use in the IMO Rule-Making Process – (9 April 2018)
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