RISK MANAGEMENT
TO QUANTIFY
In previous issues Risktec introduced us to an effective Risk Management Process before describing the concept of Tolerability of Risk (TOR) and the principle of ALARP (As Low As Reasonably Practicable) as well as various deterministic risk assessment techniques. Here we continue their series of articles on risk management by exploring the use of Quantitative Risk Assessments (QRA).
OR NOT TO QUANTIFY? THAT IS THE QUESTION...
AN INTRODUCTION TO QUANTITATIVE RISK ASSESSMENTS
WHY PERFORM A QRA? For many industries the consequences of an accident are relatively low and the progression from release of a hazard to its ultimate effects is easy to comprehend. In such cases, simple, qualitative (or deterministic) risk assessment techniques are usually more than adequate. The Risk Assessment Matrix (RAM) (see February/ March 2012 issue) and Bow-Tie technique (see December/January 2013 issue) are two commonly used deterministic techniques for assessing and controlling risk.
However, deterministic techniques alone are unlikely to be adequate when assessing facilities, operations etc. which employ complex processes and engineering systems, have high potential consequences and/or have complex hazard progressions. In such scenarios it is appropriate to undertake a Quantitative Risk Assessment (QRA), also commonly referred to as a Probabilistic Safety Assessment (PSA) or Probabilistic Risk Analysis (PRA). Table 1 summarises the pros and cons of deterministic and probabilistic approaches.
WHAT IS A QRA?
If we jump back to our second article on Risk Management (see June/July 2011 issue), we defined risk as being the product of the likelihood of an unwanted event occurring (release of a hazard) and the severity of the resulting consequences;
Risk = Likelihood x Consequence
Whilst QRAs come in a vast array of shapes and sizes, performed using a plethora of tools and techniques, in very simple terms it is a systematic technique whereby appropriate numerical values are applied to ‘likelihood’ and ‘consequence’ allowing the associated risk to be expressed in numerical terms.
*The potential for a single cause to negate a number of safeguards (e.g. power failure causes a loss of control and deactivates the fire detection system).
Table 1: Relative pros and cons of deterministic and probabilistic risk assessment techniques
The results of a QRA are numerical estimates for risk exposure that can cover the full range of consequence effects being considered (e.g. harm to the environment, plant/asset damage, programme delays, cost overruns, etc.) but are most commonly used in a health and safety context to quantify risk to people and society in general, for example by expressing risk as the frequency of fatality per annum.
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