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QUANTITATIVE RISK ASSESSMENT
Risk can be assessed qualitatively or quantitatively.
Qualitatively, risk is considered proportional to the expected losses which can be caused by an event and to the probability of this event.
The harsher the loss and the more likely the event, the greater the overall risk
Definition
Risk = Severity x Likelihood
Extent of Damage Fatality Injuries Losses Analysis based on design and
modeling equations
Likelihood of event Based of failure frequency of
process components Analysis based on
data
Understanding of risk
What can go wrong?
What are the consequences?
How likely is it to happen?
Quantitative Risk Assessment
What is QRA
Systematic methodology to assess risks associated any installation
Taking into consideration all forms of hazards
Uses design information and historical data to estimate frequency of failure
Uses modelling software to assess consequence
Where/when is QRA needed
CIMAH 1989 part of CIMAH safety report
EQA 1985 a section under EIA
Methodology
Hazard Identification
Frequency Analysis Consequence
Analysis
Risk Estimation and Evaluation
Risk Management
Hazard Identification
Purpose: to identify plausible hazard conditions
Hazard can be from human, situational, chemical, physical, mechanical, external threats
Methods
Check-list, Preliminary Hazard Review, HAZOP etc.
Unstructured brainstorming?
Hazard Identification
Frequency Analysis
Consequence
Analysis
Risk Estimation
and Evaluation
Risk Managem
ent
Frequency Analysis
Sometimes referred to as Hazard Analysis
Purpose: To estimate the likelihood for a hazard scenario to occur
Methods
Event-Tree Analysis
Fault-Tree Analysis
Hazard Identificati
on
Frequency Analysis
Consequence
Analysis
Risk Estimation
and Evaluation
Risk Managem
ent
Consequence Analysis
Purpose: To assess the extent of damage
Typical Hazard
Toxic Release, Fire and Explosion
Modeling of hazard scenario
Toxic Release: Dispersion Model
Fire and explosion: TNT equivalent
Fatality Assessment: Probit Analysis
Nonfatal Consequence: Skin-burn, Property damage
Hazard Identificati
on
Frequency Analysis
Consequence
Analysis
Risk Estimation and Evaluation
Risk Managem
ent
TOXIC RELEASE: DISPERSION MODELS
Dispersion models describe the airborne transport of toxic
materials away from the accident site and into the plant and
community.
After a release, the airborne toxic is carried away by the
wind in a characteristic plume or a puff
The maximum concentration of toxic material occurs at the
release point (which may not be at ground level).
Concentrations downwind are less, due to turbulent mixing
and dispersion of the toxic substance with air.
Plume
Factors Influencing Dispersion
Wind speed
Atmospheric stability
Ground conditions, buildings, water, trees
Height of the release above ground level
Momentum and buoyancy of the initial material released
Wind speed
As the wind speed increases, the plume
becomes longer and narrower; the
substance is carried downwind faster but is
diluted faster by a larger quantity of air.
Atmospheric stability
Atmospheric stability relates to vertical mixing of the air. During the day the air temperature decreases rapidly with height, encouraging vertical motions. At night the temperature decrease is less, resulting in less vertical motion. Sometimes an inversion will occur. During and inversion, the temperature increases with height, resulting in minimal vertical motion. This most often occurs at night as the ground cools rapidly due to thermal radiation. Three stability classes: unstable, neutral, stable
Day & Night Condition
Air temperature as a function of altitude for day and night conditions. The temperature gradient affects the vertical air motion.
Ground conditions
Ground conditions affect the mechanical mixing at the
surface and the wind profile with height. Trees and
buildings increase mixing while lakes and open areas
decrease it.
Effect of ground conditions on vertical wind gradient.
Height of the release above ground level The release height significantly affects ground level
concentrations.
As the release height increases, ground level
concentrations are reduced since the plume must disperse
a greater distance vertically.
Momentum and buoyancy of the initial material released
The buoyancy and momentum of the material released
The initial acceleration and buoyancy of the released material affects the plume character.
EXPLOSION: TNT EQUIVALENT
TNT equivalency is a simple method for equating a known
energy of a combustible fuel to an equivalent mass of TNT.
The approach is based on the assumption that an
exploding fuel mass behaves like exploding TNT on an
equivalent energy basis.
TNT Equivalent
The procedure to estimate the damage associated with an
explosion using the TNT equivalent method is as follows :
1. Determine the total amount of flammable material involved in the
explosion.
2. Estimate the explosion efficiency and calculate the equivalent
mass of TNT
TNT
CTNT
E
Hmm
TNT ofexplosion ofenergy theis E
nhydrocarbo of mass theis m
(unitless) efficiencyexplosion empirical theis
kJ/kg. 4686ramcalories/g 1120(mass) TNT of mass equivalent theis m
TNT
TNT
3. Use the scaling law, to estimate the peak side on
overpressure 3/1
TNT
em
rz
1000
100
10
1
0.1
0.01
0.01 0.1 1 10 100
Scaled distance, ze (m/kg1/3)
Sca
led o
verp
ressure
, p
s
4. Estimate the damage for common structures and process
equipment using table guide.
Risk Estimation and Evaluation
Purpose: To assess Risk and Make Safety Judgment
Methods
Individual Risk
Societal Risk
Tolerability Criteria
Hazard Identificati
on
Frequency Analysis
Consequence
Analysis
Risk Estimatio
n and Evaluatio
n
Risk Managem
ent
Two distinct categories of Risks
Voluntary Risks
e.g. driving or riding in an automobile, and working in an industrial facility.
Involuntary Risks
e.g. exposure to lighting, disease, typhoons and persons in residential or recreational areas near the industrial facilities.
Examples of risks associated with activities
Voluntary Involuntary
Activity
Risk fatalities (death) per
person per yr (x10
6)
Activity
Risk fatalities (death) per
person per yr (x10
6)
Smoking (20 cigarettes/day) Motor cycling Car racing Car driving Rock climbing Football
5000
2000 1200 170 40 20
Influenza Leukemia Run over by road vehicle (UK) Run over by road vehicle (USA) Floods (USA) Storms (USA) Lightning (USA) Falling aircraft (USA) Falling aircraft (UK)
200 80 60
50
2.2 0.8 0.1 0.1
0.02
Individual Risk
Individual risk is defined formally (by Institution of Chemical Engineering, UK) as the frequency at which an individual may be expected to sustain a given level of harm from the realization of specified hazards. It is usually taken to be the risk of death, and usually expressed as a risk per year.
group of workers on a facility, or a member of the public, or anything as defined by the QRA.
Location Specific Individual Risk
IRx,y,i is the individual risk at location (x,y) due to event i,
pi is the probability of fatality due to incident i at location (x,y). This is normally determined by FTA
fi is the frequency of incident outcome case i, (per year). This value can be determined using Probit Analysis
iiiyx fpIR ,,
n
iiyxyx IRIR
1,,,
When there are more than one release events, the cumulative risk at location (x,y) is given by equation
Average Individual Risk / Individual Risk Per Annum
The average individual risk is the average of all individual risk estimates over a defined or exposed population. This is useful for example in estimating the average risk of workers in reference with existing population. Average individual risk over exposed population is given by CCPS (1989) as
yxyx
yxyxyx
AV P
PIR
IR
,,
,,,
Here, IRAV is the average individual risk in the exposed population (probability of fatality per year) and P x, y is the number of people at location x, y
Example: LSIR for Ship Explosion at a Proposed Port
1 x 10-5
1 x 10-6
Societal Risk
Societal risk measures the risk to a group of people. It is an estimation of risk in term of both the potential size and likelihood of incidents with multiple consequences.
The risk can be represented by Frequency-Number (F-N) Curve.
Determination of Societal Risk
To calculate the number of fatalities resulting from each incident outcome case, the following equation is used:
Here, Ni is number of fatalities resulting from Incident Outcome case i, pf,i is the probability of fatality and Px,y is the number of population.
The cumulative frequency is then calculated using the following equation:
iiN FF
ifyx
yxi pPN ,,
,
Here, FN is the frequency of all incident outcome cases affecting N or more people, per year and Fi= is the frequency of incident outcome case i per year.
Risk Tolerability and ALARP Concept
There is no such thing as zero risk
All activities involve some risks
The issue is at level should we tolerate
Tolerable Risk
Risk cannot be eliminated entirely.
Every chemical process has a certain amount of risk
associated with it.
At some point in the design stage someone needs to
Each country has it owns tolerability criteria.
One tolerability criteria in the UK is "as low as
reasonable practicable" (ALARP) concept formalized
in 1974 by United Kingdom Health and Safety at Work
Act.