Syed Quarishy, Technical Safety and Risk Management Consultant


Risk is often defined as a function of the likelihood that a specified undesired event will occur and the severity of the consequences of the event. Risk is derived from the product of likelihood and potential consequence. Risk in general is a measure of potential economic loss or human injury in terms of the probability of the loss or injury occurring and magnitude of the loss or injury if it occurs. Risk thus comprises of two variables: magnitude of consequences and the probability of occurrence.

Quantitative Risk Assessment (QRA) is a dominant analysis tool used for risk management and safety improvement. It provides a rational basis for evaluating process safety with respect to its theoretical and practical limitations. QRA should be used when the available input data are sufficient enough to ensure that valid and robust results will be obtained.

To provide results consistent with good industry practice Risk Management Guidelines are often used to classify risks according to the following classifications:

  • Low (Broadly Acceptable);
  • Tolerable/As Low As Reasonably Practicable (ALARP); and
  • High (Unacceptable).

Quantification of effects of the hazardous event is done by using the “Event Tree” approach in which all the possible outcomes of the hazardous event are considered and the likelihood of each type of end event determined. This step involves the use of consequence modeling to predict both physical phenomena such as dispersion of gas, size and duration of fires, overpressures due to explosions, and the performance of equipment and systems such as availability of a fire & gas detection system, availability of emergency shutdown system, and availability of fire protection system. The end result of this phase of the assessment is a series of “end events”, together with their estimated frequency of occurrence. The estimation of frequencies and probabilities of events in Event Trees need to be based either directly on statistical analysis of historical data, or derived by the use of Fault Trees.

An assumptions register to be used while QRA is being developed. The assumptions register will include a list of assumptions e.g. leak size selection, weather condition selection based on location wind rose, effects on personnel, evacuation route impairment, smoke and toxic gas effects, structural integrity, effectiveness of safety and emergency systems, integrity of Emergency Shut Down/Blow Down.

Risks are calculated in terms of Location Specific Individual Risk (LSIR), Individual Risk (IR) and Societal Risk (F-N curve) as applicable.

LSIR will be used to indicate the cumulative risk at a particular location. LSIR is the risk for a hypothetical individual who is positioned at that location for 24 hours a day 365 days per year. Since in reality people do not remain continuously at one location, LSIR is a notional risk measure. The LSIR value provides a tool by which industries make risk-based decisions regarding the location of proposed developments i.e. Land Use Planning.

A graphical representation of the QRA output which may be in the form of bar charts, histograms and/ or pie charts needs to be included in the QRA report. This representation should clearly summarize the results and identify significant contributions to risk. For an onshore project, the LSIR contours shall be plotted on the Plot Plan. LSIR should also be used in combination with site demographic data to calculate the IR to a typical worker by factoring the exposure time within risk isopleths.

Risk contours are developed using the results from the consequence modeling, likelihood calculation and compilation of offsite population, building population, and weather data. Contours were generated for all sorts of hazards individually as well as combined.

The results of Risk Analysis are often reproduced as Individual and groups risks and are defined as below:

Individual Risk: Measure of the risk to a person within an occupied area or building. This includes the nature of the injury to the individual, the likelihood of the injury occurring, and the time period over which the injury might occur. It is the probability of fatality as a result of accidents at a plant facility, installation or a transport route expressed as a function of the distance from such an activity. It is the frequency at which an individual or an individual within a group may be expected to sustain a given level of harm (typically death) from the realization of specific hazards.

Such a risk actually exists only when a person is permanently at that spot (out of doors). The individual results are based on the occupancy factor for different category of personnel’s at that particular location.

Individual Risk = Location Specific Individual risk × Occupancy factor

Whereas, Location Specific Individual Risk (LSIR) corresponds to the level of harm at a particular location or area.

The exposure of an individual is related to:

  • The likelihood of occurrence of an event involving a release and Ignition of hydrocarbon;
  • The vulnerability of the person to the event; and
  • The proportion of time the person will be exposed to the event (which is termed ‘occupancy’ in the QRA terminology).

Societal Risk:The second definition of risk involves the concept of the summation of risk from events involving many fatalities within specific population groups. This definition is focused on the risk to society rather than to a specific individual and is termed ‘Societal Risk’. It is the measure of risk to a group of people. In relation to the process operations we can identify specific groups of people who work on or live close to the installation; for example communities living or working close to the plant. It is most often expressed in terms of the frequency distribution of multiple casualty events (F­N curve.). The predicted number of fatalities based upon the location and presence probability of the population within the areas potentially impacted by an accidental release scenario. This measure of risk is site specific and highly dependent upon the physical location of people, the fraction of the time they are present and whether they are located indoors or outdoors.

Societal Risk (F-N curve) is primarily used to assess the risk to persons not involved in the process facility activities i.e. third party residing outside the process facility perimeter. Where F-N curve are used to represent risk to group of workers in a facility, the assessment will be limited to events which can lead to multiple fatality of group of workers in a single event e.g. worker groups occupying a muster area, accommodation block, administration building etc. F-N curve shall not be used for assessment of multiple fatalities of maintenance and operations personnel in the field as a part of their normal work activity, who by the nature of their work spend significant amount of time in high risk areas; IR shall be used to assess their risk.


Benchmark Solution has highly qualified Process Safety Engineering team who can support in Quantitative Risk Assessment Projects related Natural Gas, LPG and Petrochemical Plants. By utilizing this robust analysis tool safety improvement and efficient risk management can be established in the Natural Gas, LPG and Petrochemical processing facilities.


If you want to know more about process safety and implement an efficient Process safety management system, please contact us today.

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