Risk Evaluation and Risk Ranking

In order to be able to differentiate between the risks from various scenarios, we need a technique that is relatively easy to use, reasonably reproducible, and giving a high level of differentiation. For most situations, full Quantified Risk Assessment (QRA) is not an option, but a risk index approach can meet the bill. In order to give easy of decision making, a logarithmic scale is much more helpful than a linear scale.

The first factor we need to estimate and differentiate is the potential outcome of the event. In this estimation, we will normally choose the most likely outcome, as if we choose the most serious outcome, it is amazing how many times the possibility of fatality seems to exist (even from trips and falls). The most important thing to recognise is that the frequency we estimate should be that of the outcome we choose, not of all outcomes! Once again, since we are not only interested in injury events, we need to be able to differentiate outcomes from damage and loss events, and relate these to injury events. This, in effect, requires us to put a value on an injury or fatality. Table 1 suggests a value for a life of £1 million or more. This stands up reasonably well compared to some recent court judgements.

Table 1: Severity Index

The table includes only whole numbers, but the missing odd numbers may be used for intermediate values. Decimal numbers would imply too much accuracy for estimation, but can be used if averaging the judgement of several members of the team.

The second factor to be estimated is the scenario frequency, which expresses how likely it is to occur, and give the outcome we have selected. Although we should try to avoid mathematics, it is helpful to decision making to have some understanding of number when attempting to differentiate between different repeat frequencies for a particular scenario. Table 2 gives an index for frequency ranging from 1 to 10, which will cover most of the frequencies of interest for our simple risk assessment. Lower repeat frequencies than 1 in 200 years are too difficult to differentiate by judgement, so the scale stops there, and quantified techniques would be needed.

Table 2: Frequency Index

It should be noted that the words used for describing the frequency steps are only included to demonstrate how difficult it is to differentiate with words alone! Note that this scale is not truly logarithmic, but is much easier to use than a linear scale.

Use of these tables is far more accurate if a team is involved, rather than a single individual, and this team is best chosen to include those with operations and safety experience, and knowledge of past accidents. Use of the IChemE Incident Database [5] could be considered.

In order to calculate the risk index, the two separate indices are summed, as one would for logarithms. This gives an index from 2 to 20, allowing good differentiation. Although this process is for purposes of comparison, and decision making should not be based just on the numbers, many users consider that an index of 10 may be acceptable at the first pass without immediate measures to reduce the risk, whereas higher values would trigger risk reduction.

With our Risk Ranking Technique we can now simply list the scenarios in order of risk index, with the highest first.