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What is Quantitative Risk Assessment? How is the QRA conducted?

What is Quantitative Risk Assessment? How is the QRA conducted?
Quantitative Risk Assessment

What is Quantitative Risk Assessment? How is the QRA conducted?

What is Quantitative Risk Assessment?

Quantitative Risk Assessment is a data-based measure of the amount or quantity of the overall effect of the risk to estimate the likelihood and consequences of hazardous events which ultimately helps determine the results quantitatively as a risk for people, the environment and / or property. It also assesses the robustness of existing safety measures and the validity of quantitative results, by identifying critical assumptions and elements of risk management.

 

What is the Objective of QRA study?

Basically, there are two main objectives of Quantitative Risk Assessment

To estimate likelihood and consequences of hazardous events in specific quantity (quantitatively).

To envisage the risk to people’s life, injury,  environment damage and effect on structures (Industrial installations, critical structures and around the building)

 

Why to conduct Quantitative Risk Assessment?

For robust, safe and cost-effective decision making and to establish an estimate of technical risks for the entire life cycle of the installation, performing a Quantitative Risk Assessment is very important.

Risk assessment is the systematic process of identifying hazards, analyzing the risk and evaluating how existing safety control measures adequate and effective or maintained at appropriate levels. This not only helps prevent and protect life, the environment and property, but also helps meet regulatory and corporate criteria.

It has also been found that industries and corporates conduct QRA to demonstrate acceptable levels of risk when approving large industrial facilities or when making significant changes to operations.

Typically, a Quantitative Risk Assessment (QRA) study is required for hazardous processes that handle highly hazardous chemicals or the process involving high pressure equipment, machinery, pipelines or the storage of hazardous chemicals including flammable gas and oil.

The QRA study helps to improve the decision-making process by estimating the risk of credible emergency scenarios that contribute the most to the overall risk and thus helps to meet an acceptability criterion to demonstrate that risks at ALARP level.

 

What are the basic inputs required for QRA?

Since the name “Quantitative Risk Assessment” and the result will be in a specific quantity, a lot of data is needed as input for this QRA study.

Here is the minimum list of inputs, which may vary from industry to industry and from installations to facility.

  1. Details of facility and general processes information.
  2. Geographical image and details of nearby vicinity.
  3. Data sheet or SDS of chemicals
  4. Details of storage quantity of chemical, solvent, gases, fuel, etc. 
  5. Type of Storage such as bullet, sphere, vertical tank etc.
  6. Existing safety control system and fire protection Systems
  7. Facility and nearby society/villages Population Data
  8. Weather Data  & Release Size
  9. Operating Parameters 
  10. Emergency shutdown philosophy

 

What will be output of QRA?

 After assessing the risk and dispersing modeling, here are the list of QRA output

  1. Estimated / Expected Fatalities (individual, group and social risk)
  2.  Pressure Wave affects to installations and surrounding
  3.  Toxic Release in distance and exposure intensity
  4.  Radiation Energy of Fire and explosion
  5.  Different Couture  in Map
  6. Predicted economic losses
  7. Predicted environmental impacts
  8. Risk to specific main safety functions
  9. Effects of uncertainties and assumptions.

 

How QRA study gives quantitative results?

After collecting all relevant data as input, software such as PHAST is used to estimate the quantity by using the inbuilt library of gases, chemicals, mixtures etc.

  1. It gives quantitative result in
  2. Toxic Dispersion vs Distance
  3. Toxic Dispersion Contour 
  4. Jet Fire
  5. Pool fire  Radiation Vs Distance
  6. Pool fire  Radiation Vs Distance
  7. Explosions vs Distance

 

What are the sources of emergency scenarios for QRA study?

In QRA study, credible scenarios are selected from process hazardous equipment such as reactors, gas compressors, pipelines and other equipment, Storage Tanks of flammable and combustible substances and processes handling toxic chemicals.

 

What are the benefits of QRA study?

 Here are few important benefits of QRA study;

  1. Help management to decide the acceptable risk level
  2. Help management to decide and develop Emergency Management Plan (On-site and Off-site)
  3. Help to identify the escape routes 
  4. Helps to decide the location of vital installations including location of Control Room, fire pump house and master assembly point.
  5. Fire Fighting and other essential building locations 
  6. Environmental issues identification
  7. Help to develop the emergency shutdown procedures and other mitigation plans.
  8. Help to estimate the insurance premium or transfer of Risk
  9. Help to estimate the capital expenditure on Safety 
  10. Helps to develop and review of standard operating procedures (SOP)
  11. Help to decide the blast proof wall requirement or Fire or Explosion Proof Walls
  12. Help to decide the inter distance or installation or facility separation.
  13. It provides information on operational restrictions and design requirements
  14. It identifies and assesses prevention and mitigation measures
  15. It helps to identify the need and function / requirements of safety barriers.

 

What are the limitations of the QRA study?

As in Quantitative Risk Assessment study, large amount of data is processed, mostly peoples use two-dimensional ellipses to represent danger zones, such as the area around an explosion, which has 10% of chances of death. Likewise, a pragmatic approach is used to simplify the dissemination results.

Typically, flat terrain and a clear world are used to determine the behavior of a scattering cloud and / or spray puddle. This poses problems when the effects of uneven terrain or the complex geometry of processing plants undoubtedly affects the behavior of a scattering cloud.

Although they have limitations, the 2D danger zone and the simplified approach to 3D scattering modeling allow handling large volumes of hazard results with known assumptions to aid in decision making. The compensation changes as the processing power of the computer increases.

Modeling the consequences of hazardous events in true 3D form may require a different approach, for example using a fluid dynamics calculation method to study cloud scattering over mountainous terrain. Creating CFD models requires a much larger investment of time on the part of the modeling analyst which may not be justified in all cases.

A major limitation of the Quantitative Risk Assessment in ​​safety is that it focuses primarily on the loss of containment of hazardous fluids and what happens when they are released. This makes Quantitative Risk Assessment impractical in hazardous industries that do not focus on containment of fluids but are still prone to catastrophic events.


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