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.