Case Study for Conducting QRA for Gantry and Vapour Recovery Unit

A valuable management tool in assessing the overall safety performance of the Chemical Process Industry is Risk Analysis. This techniques provide advanced quantitative means to supplement other hazard identification, analysis, assessment, control and management methods to identify the potential for such incidents and to evaluate control strategies. Risk Analysis techniques provide advanced quantitative means to supplement other hazard identification, analysis, assessment, control and management methods to identify the potential for such incidents and to evaluate control strategies.


  • Conducting Risk Analysis study for Gantry & Vapour Recovery Unit.
  • Calculating Societal Risk for surrounding population.
  • Calculating Individual Risk of various places in the facility and comparing with Risk Acceptance criteria.



Based on consideration of factors such as the physical & chemical properties of the fluids being handled, the arrangement of equipment, operation & maintenance procedures and process conditions, external hazards such as third party interference, extreme environmental conditions, aircraft / helicopter crash should also be considered.

Factors for Identification of Hazards:

Operating Parameters, Inventory and Range of Incidents – Catastrophic Failure of container, Large holes, Small holes & Leak at fittings.


As per CPR 18 E guidelines & Reference Manual BEVI Risk Assessments Version 3.2 only the Loss of Containment (LOC) which is basically the release scenarios contributing to the individual and/ or societal risk are included in the QRA. LOCs of the installation are included only if the following conditions are fulfilled:

  • Frequency of occurrence is equal to or greater than 10-8
  • Lethal damage (1% probability) occurs outside the establishment’s boundary or the transport route.


In consequence analysis, several calculation models is made use to estimate the physical effects of an accident (spill of hazardous material) and to predict the damage (lethality, injury, material destruction) of the effects. The calculations can roughly be divided in three major groups:

  1. Determination of the source strength parameters
  2. Determination of the consequential effects
  3. Determination of the damage or damage distances

Types of Outcome Events

  • Jet fires
  • Pool fires
  • Flash Fire (FF)
  • Overpressure Explosion 


Frequency Analysis

To calculate the risk associated with a LOC scenario, it is necessary to estimate the failure frequency. The frequency of occurrence of such an event is based on the probability of the LOC scenario and the presence of constraints that influence the development of the event.

Base Failure Frequency

The failure frequencies of full release cases are considered since the release is consistent with flow through the defined hole, beginning at the normal operating pressure, and continuing until controlled by emergency shutdown or inventory exhaustion.

Total Failure Frequency

The total failure frequencies for isolatable section are calculated by combining the base failure frequency obtained from the international database (CPR18E – Committee for Prevention of Disasters, Netherlands (Edition: PGS 3, 2005) and Modification factors.

Ignition Probabilities

For gas/ oil releases from the gas/ oil handling system, where a large percentage of rupture events may be due to third party damage, a relatively high probability of immediate ignition is generally used.

Delayed ignition probabilities can also be determined as a function of the cloud area or the location.


The assessment of risks is based on the consequences and likelihood. Likelihood assessment is the methodology used to estimate the frequency or probability of occurrence of an incident.

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

Individual Risk

IR is the probability of death occurring as a result of accidents at a plant, 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.

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’. 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.


This involves identifying opportunities to reduce the likelihood and/or consequence of an accident.

Risk-reduction measures include those to prevent incidents (i.e. reduce the likelihood of occurrence) to control incidents (i.e. limit the extent & duration of a hazardous event) and to mitigate the effects (i.e. reduce the consequences). Preventive measures, such as using inherently safer designs and ensuring asset integrity, should be used wherever practicable. In many cases, the measures to control and mitigate hazards and risks are simple and obvious and involve modifications to conform to standard practice. The general hierarchy of risk reducing measures is:

  • Prevention (by distance or design)
  • Detection (e.g. fire & gas, Leak detection)
  • Control (e.g. emergency shutdown & controlled depressurization)
  • Mitigation (e.g. firefighting and passive fire protection)
  • Emergency response (in case safety barriers fail)