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Hazardex 2018 Conference - 7. Risk tolerability targets; misconceived, misunderstood and misapplied

30 January 2018

Carolyn Nicholls - Consultant, RAS Ltd. 
Weds 15.30 – 16.10: Main conference room

Following on from a quantification (or semi-quantification) of risk, the next step in any assessment is to compare the result to a set of risk tolerability criteria. Three regions of risk are usually defined, an unacceptable region and a broadly acceptable region bordering a region of tolerable risk. The tolerability of this middle region is dependent on those risks being As Low As Reasonably Practicable (ALARP).

The 2001 HSE publication ‘Reducing Risks, Protecting People - HSE’s decision-making process’ (R2P2) is widely used in industry to set these region boundaries. R2P2 clearly defines the tolerability boundaries for individual risk, and gives guidance regarding societal (or group) risk tolerability.

How these criteria have been applied across the industry varies dramatically, particularly in setting targets for Layer of Protection Analysis (LOPA). LOPA is often used to identify the need for, and define the required integrity of, a Safety Instrumented System (SIS). In this context, it is easy to see how setting the wrong targets for a risk assessment can result in serious consequences.

For example, setting a risk target that is too lenient could result in a process that is not adequately protected, and setting a risk target that is too onerous could result in the requirement to needlessly install a high integrity safety system at a significant cost. To set appropriate risk targets, the intricacies between types of risk need to be well understood, and applied correctly for the given situation. Our experience is that this is often misunderstood and misapplied, people take simple rules and apply them in the wrong context.

This paper considers the differences between individual risk and societal (group) risk, and how these are often confused and misapplied. Differences between scenario risk and whole site risk, and how these should be considered in different ways depending on the type of risk and the type of study are then discussed. Risk targets are proposed which may be used in LOPA, including a demonstration of how these were developed and how they meet industry and regulatory standards. How these targets can be used and adapted into existing or new facilities, and how they may interact with existing corporate risk criteria and matrices is also considered. 


A director of RAS Limited, Carolyn Nicholls is a process safety specialist by training, and has been in the risk and hazard management industry for over ten years.

Carolyn leads the RAS team of risk and hazard management consultants and has been instrumental in creating the company’s assessment methodologies. Carolyn has experience of working with a number of UK COMAH sites to develop safety reports and provide support in all aspects of risk management, in particular developing emergency response plans, evaluating the economic impact of safety improvements and reviewing operational controls. 

Her particular area of interest is the demonstration of ALARP, an often misunderstood concept.



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