Quantitative Risk Assessment | QRA | Risk Assessment | Gaurav Singh Rajput

1. Quantitative Risk
Assessment

2. QRA
2
 Identify incident scenarios & evaluate the risk by defining the
probability of failure, probability of various consequence & potential
impact of those consequences.
 Risk is a function of :-
 Risk = F (s, c, f)
 S = Hypothetical scenario
 C = Estimated Consequence
 F = Estimated Frequency
 CPQRA consists of Risk Analysis, Risk Assessment.

3. Difference – QRA/HAZOP
QRA HAZOP
QRA provides numerical (quantitative)
estimates to understand risk exposure to
people and environment.
QRA provides numerical (qualitative)
estimates to understand risk exposure to
people, Assets and environment.
QRA studies account for potential releases
of hazardous material, their consequences
(e.g., fire, toxic, explosion, etc.) and
estimated frequency of occurrence.
Hazop study accounts for abnormal/
Hypothetical Events and their effects in
downstream as well as upstream
Risk is calculated to people Assets hazard analysed qualitatively
Recommendations of QRA are based on the
mitigation system to reduce the
consequence.
Recommendations are related to Alarms,
trips to keep the system in safe operation
mode.

4. Difference – QRA/HAZOP
QRA HAZOP
Risk is calculated considering Operating
condition, wind speed & direction.
Hazard are analysed using deviation in
parameter line by line or by Equipment.
Risk is calculated based on number of
people present in the facility and
surrounding areas.
Consequence are analysed qualitatively and
not considered the surrounding areas.
Abnormal case are not considered in QRA
study.
Abnormal case are considered in HAZOP
study.

5. General Steps
•5
 Define potential event sequence/potential incidents – Qualitative
analysis (HAZID) or screening level analysis. Most effective method
is Plant sectionalisation.
 Evaluate incident outcomes – consequence analysis
 Estimate frequencies – Fault tree/Generic database for initial event
sequences or event tree to account for mitigation & post release
events.
 Estimate impact on people, environment & property
 Estimate risk – combine consequence & frequency of each event &
summing over all events.

6. General Steps
•6
 Evaluate the risk.
 Identify major sources of risk & determine if there are cost-effective
process or plant modifications which can be implemented to reduce
risk
 If risk is excessive – identify & prioritize risk reduction measures.

7. General Steps
•7

8. Definitions
•8
 Frequency – Number of occurrences per unit time.
 Hazard – chemical or physical condition which has a potential to
cause damage to people, property & Environment
 Incident – Loss of material or energy.
 Event Sequence – A specific unplanned sequence of events
comprising of initiating events & intermediate events that may lead
to an incident
 Initiating event – First event of the event sequence (eg – leak from
pipeline)
 Intermediate event – Event that propagates or mitigates initiating
event (e.g. – operator failure to stop leak from pipeline)

9. Definitions
•9
 Incident outcome – Physical manifestation of incident e.g. – toxic
release, VCE, jet fire, etc.
 Incident outcome case – Quantitative definition of single result of
incident outcome thro specification of sufficient parameters to allow
distinction of this case from all other for the same incident (e.g. – jet
fire results for two or more weather conditions)
 Consequence – Measure of expected effects of an incident
outcome case.
 Likelihood – measure of expected frequency of occurrence
 Probability – likelihood of occurrence over a time interval or
likelihood of occurrence of success or failure

10. Definitions
•10
 Risk Analysis – Development of quantitative estimate of risk based
on Engg. evaluation & mathematical techniques for combining
incident consequences & frequency.
 Risk Assessment – process by which results of risk analysis are
used to make decisions either through relative ranking of risk
reduction strategies or through comparison with risk targets

11. QRA Goals
•11
 To screen or bracket the range of risks for further study: It is often
consequence analysis without frequency part.
 To Evaluate a range of risk reduction measures: Identify major risk
contributors & suggest risk recommendations.
 To prioritize safety investments: Ensure safety investments are
directed to greater risks.
 To estimate employee risk – individual risk
 To estimate public risk – Societal risk
 To meet legal or regulatory authorities
 To assist emergency planning: effect zones to be marked for use in
emergency planning

12. Consequence Analysis
•12
•Release
Source
•Discharge •Dispersion
•Weather •Flash Fire/
Toxic
•Radiation
•Explosion
•Alternative
Inputs
•Release Source:
• Piping Release - Flanges, Valves, Instrument Connections, Piping
corrosion etc.
• Vessel/ Storage Release.

13. Consequence Analysis
•13
•Release
Source
•Discharge •Dispersion
•Weather •Flash Fire/
Toxic
•Radiation
•Explosion
•Alternative
Inputs
• Wind Speed/ Roses • Humidity
• Pasquill Stability • Atm./ Substrate Temperature
• Surface Roughness

14. Consequence Analysis
•14
•Release
Source
•Discharge •Dispersion
•Weather •Flash Fire/
Toxic
•Radiation
•Explosion
•Alternative
Inputs
 Liquid/ Gas/ Two Phase Release
 Release Chemical
•Discharge Calculation from Piping Leak, Catastrophic Rupture, Vent,
Vessel/ Tank leak/Failure

15. Consequence Analysis
•15
•Release
Source
•Discharge •Dispersion
•Weather •Flash Fire/
Toxic
•Radiation
•Explosion
•Alternative
Inputs
• Aerosol Formation • Cloud Dispersion
• Pool Formation • Pool Vaporization

16. Consequence Analysis
•16
•Release
Source
•Discharge •Dispersion
•Weather •Flash Fire/
Toxic
•Radiation
•Explosion
•Alternative
Inputs
Possible Flammable outcomes
• BLEVE and Fireballs • Radiation Levels
• Jet Fire • Probit values or Thermal Load in the
Cloud
• Pool Fire

17. Consequence Analysis
•17
•Release
Source
•Discharge •Dispersion
•Weather •Flash Fire/
Toxic
•Radiation
•Explosion
•Alternative
Inputs
Vapour Cloud Explosion
• TNT
• TNO Multi-Energy
• Baker Strelow
BLEVE Blast

18. Damage Criteria
•18
 Thermal Radiation
 Hazard due to Jet fire and Pool fire will be measured in terms
of Radiation intensity.

19. Damage Criteria
•19
 Explosion Hazards
 Hazard due to Explosion and Pressurized Vessel Burst will be
measured in terms of shock-waves (Over Pressure).

20. Damage Criteria
•20
 Dispersion Hazard
 Dispersion of the flammable vapours of LFL and Half LFL
concentration.
 Dispersion of Toxic gases of ERPG/ IDLH concentration.

21. Frequency Estimation
•21
 Fault Tree Analysis
 Available accident data like CPR 18E – Purple book, OGP, CMPT,
OREDA, etc.
 Event Tree Analysis

22. Probit Equation
•22
 Method of assessing effect of consequences of outcome event on
the Individual.
• P = A + B X Ln(C^n*t)
 Where A, B and n are constant and varies as per first degree burn,
second degree burn or Lethality.
 In case of Toxic release each toxic chemical having different
values depends on its toxicity.

23. Risk
•23
 Individual Risk Per Annum (IRPA)
 Societal Risk
 Location Specific Risk (LSIR)
 Potential Loss of Life (PLL)

24. Individual Risk Per Annum (IRPA)
•24
 The Individual risk represents frequency of an Individual Dying
due to LOCs events. The Individual is assumed to be unprotected
and to be present during the total exposure time.
 It is represented as constant risk lines on the topographic map

25. Societal Risk
•25
 Societal risk represents the frequency of having an accident with
N or more people will killed simultaneously.
 People involved are assumed to have some protection.
 Social Risk is presented as an Frequency-Number of death (F-N)
Curve

26. Risk Criteria
•26
Unacceptable region
(10-4 Per annum)
Risk cannot be justified
The ALARP or tolerability
Region (risk is undertaken
Only if a benefit is
Desired)
Tolerable only if further risk
reduction is impractical, or the cost
is not proportionate to the benefit
gained
Broadly acceptable
Region (10-6 Per annum)
10-6 Per annum
Negligible risk
Risk closer to the unacceptable region merit a closer examination of potential risk
reduction measures

27. As low as reasonably practicable
(ALARP)
•27
 The residual risks are not unduly high and kept as low as
reasonably practicable.

28. Residual Risk

29. Limitations
•29
Cause of limitation Implication to CPQRA Remedies
Incomplete or
inadequate
enumeration of
incidents
Underestimate risk for a
representative set or
expansive list of
incidents
Require proper documentation.
Involve experienced CPQRA
practitioners.
Apply alternative enumeration
techniques
Peer review/quality control.
Review by facility design and
operations personnel.
Improper selection
of incidents
Underestimate risk for
all incident groupings
Involve experienced CPQRA
practitioners.
Apply alternative enumeration
techniques
Peer review/quality control.
Review by facility design and
operations personnel.

30. Limitations
•30
Cause of limitation Implication to CPQRA Remedies
Unavailability of
required data
Possibility of systematic
bias
Uncertainty in
consequences,
frequencies, or risk
estimates
Incorrect prioritization
of major risk
contributors
Secure additional resources for
data acquisition.
Expert review/judgment.
Ensure that knowledgeable people
are involved in assessing available
data.
Check results against other models
or historical incident records;
evaluate sensitivities.

31. Limitations
•31
Cause of limitation Implication to CPQRA Remedies
Unavailability of
required data
Possibility of systematic
bias
Uncertainty in
consequences,
frequencies, or risk
estimates
Incorrect prioritization
of major risk
contributors
Secure additional resources for
data acquisition.
Expert review/judgment.
Ensure that knowledgeable people
are involved in assessing available
data.
Check results against other models
or historical incident records;
evaluate sensitivities.

32. Limitations
•32
Cause of limitation Implication to CPQRA Remedies
Consequence or
frequency model
assumptions/validity
Similar in effect to data
limitations
Ensure appropriate peer review
Check results against other models or
historical incident records
Ensure that models are applied
within the range intended by model
developers
Ensure that mathematical or
numerical approximations that may
be used for convenience do not
compromise results
Use, if feasible, different models
(e.g., a more conservative and a more
optimistic model) to establish the
impact of this type of uncertainty

33. Limitations
•33
Cause of limitation Implication to CPQRA Remedies
Skills unavailable Incorrect preparation
and analysis
Improper interpretation
of
Results
Amend scope of work
Acquire expertise through training
programs, new personnel, or
consultants

34. 34
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