Safety reports should consider in detail the risks associated with a sub-set of all the MAS’s for the site, which is known as the representative set of MAS’s. This makes the subsequent risk analysis more manageable. This representative set of MAS’s must be ‘suitable and sufficient’ and should include:-
Depending on the complexity of the site, the number of accidents in the representative set will vary. Typically for relatively simple storage operations there might be several MAS’s. For more complex processing operations there might be in excess of 20 MAS’s.
Using the results of the initial risk assessment which should have been carried out without precautions in place i.e. a recognition that they can fail, a matrix showing the relationship between severity and likelihoods of all the MAS’s for the site can be constructed and populated.
A matrix, as illustrated below, typically comprises a rectangle / square divided into a number of boxes, with each box representing a different underlying risk level. In this example, both qualitative and quantitative descriptors are shown for the likelihood, whilst the severity is represented by numbers of fatalities.
All the site’s individual MAS’s can be plotted on this matrix. From such a populated matrix, it is then straightforward to identify the "worst-case" scenarios and also the safety critical events. The safety critical events are those that dominate the risk at different distances and effectively are the most frequent events in each consequence band. The populated matrix below illustrates the "worst-case" scenarios and scenarios selected as safety critical events.
In addition to the worst-case scenarios and safety critical events, a range of other events should be included in the representative set. These events should include other hazards, substances, processes, etc. and also any other significant events from the risk matrix (e.g. in the populated matrix above, this would include some of the events in the 6-10 fatality / unlikely frequency box e.g. 17 and 18).
A secondary aim of the matrix is to inform the proportionality of the site operations as a whole. The more MAS’s approaching or in the red or uncomfortably high zone, the higher the proportionality and hence the greater the degree of rigour and associated quantification will be required in the detailed assessment (see Step 4). It should be noted that subsequently on detailed assessment, the actual risks might be shown to be significantly reduced either by revised frequencies, which are demonstrated to be lower than was initially judged, or by accounting for systems which reduce the consequence.
A consequence of the explosives licensing regime (which utilises quantity distances), is that the potential numbers of fatalities are generally low (as represented in the matrix above). For sites with other hazards e.g. toxic substances, a matrix such as that in the SPC/Permissioning/12 may be more useful.