Risk analyses or 'predictive' aspects of comah safety reports guidance for explosives sites

The COMAH Safety Report Process for Predictive Assessment of Explosives Sites

Step 2: Initial Risk Assessment

Expert judgement can be used to estimate the consequences and likelihood of each of the identified MAS's. This can be achieved by using a small group / team of people knowledgeable about the operations, processes, materials etc. and with expertise on risk analysis to carryout a 'high level' analysis of the consequences and likelihood of each of the MAS's. This can be a purely qualitative assessment, although all descriptor terms used will need to be defined, but is more likely to be semi-quantitative with estimates of numbers of people affected. The emphasis here is on professional judgement.

Likelihood is recognised as a potentially difficult aspect. However, there is data available which can help inform the analysis and the frequencies used can be based on expert judgement, accident records and historical experience. Generic 'major accident' rates (ref. J. Haz. Mats A:63 (1998) 107 – 118), where the MA was defined as an explosion which substantially destroyed the building in which it occurred, resulting in projection of debris and/or blast effects at a distance, and which were derived from historical experience for non-remote explosives operations, were given as:-

Accident Rate (per building year)

• Manufacture of RDX: 5 x 10 – 2
• Drying of safety fuse: 2 x 10 – 2
• Drying of nitrocotton: 1 x 10 – 2
• Drying of propellant: 4 x 10 – 3
• Commercial storage of explosives: 1 x 10 – 4
• Military storage of explosives: 1 x 10 – 5

For explosives operations which are carried out remotely, the above reference gives the following:-

• Extruding propellant: 2 x 10 – 1
• Centrifuging ball powder: 2 x 10 – 1
• Mixing pyrotechnic comp'n SR580: 1 x 10 – 1
• Burning of waste explosive: 2 x 10 – 2
• Nitrating glycerine (continuous): 5 x 10 – 3

Since the figures above are based on historical performance, these accident rates are likely to represent the maximum and that better future performance would be expected. Where better performance is claimed in a safety report, it should be supported with a suitable justification. It should also be recognised that these figures are accident rates for MA's as opposed to MAS's; ie there may be several causes of these various events to provide the total accident rate. However, the figures do provide an appropriate baseline for risk analysis of operations of this type.

It should be noted that it is considered poor practice to rely on reduction of risk solely by low exposure times, especially where the low exposure time is not inherent to the overall task.

The safety report should demonstrate how, when evaluating the risks of each identified MAS that consideration was made of:-

• how each scenario can be realised (ie for each individual possible major accident scenario, their likelihood or the conditions under which they occur).
• the events which may play a role in triggering each of these scenarios whether internal or external to the installation.

Any key assumptions made in the analysis such as likelihood of events should be described in the report, especially if such assumptions lead to the elimination of significant events from the eventual representative list of MAS's.