4 risk estimation

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Risk Estimation



Two distinct categories of Risies

• 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(x106)

Activity

Risk fatalities(death) per

person per yr(x106)

Smoking (20

cigarettes/day)Motor cyclingCar racingCar drivingRock climbingFootball

5000

2000

1200

170

40

20

Influenza

LeukemiaRun over by roadvehicle (UK)Run over by roadvehicle (USA)Floods (USA)Storms (USA)

Lightning (USA)Falling aircraft(USA)Falling aircraft(UK)

200

8060

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 specifiedhazards. It is usually taken to be the risk of death,

and usually expressed as a risk per year.

• The term ‘individual’ may be a member of a certain

group of workers on a facility, or a member of the

public, or anything as defined by the QRA.



Location Specific Individual Risk

• IR x,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

• f i is the frequency of incident outcome case i , (per year). This

value can be determined using Probit Analysis

i i i y x f p IR ,,

n

i i y x y x IR IR

1

,,,

When there are more than one release events, thecumulative 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 individualrisk 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

y x y x

y x y x y x

AV

P

P IR

IR

,

,

,

,,

Here, IR AV 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 Proposed Chemical

Plant



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 eachincident outcome case, the following equation is used:

Here, Ni is number of fatalities resulting from IncidentOutcome case i , p f,i is the probability of fatality and P x,y is the

number of population.

• The cumulative frequency is then calculated using the

following equation:

i

i N F F

i f y x

y x i p P N ,

,

,

Here, F N is the frequency of all incident outcome cases affecting N or more

people, per year and F i = is the frequency of incident outcome case i peryear.

E l Th di S i l



Example: The corresponding Societal

Risk

1 10 100 1000 10000 1x10-7

1x10-3

1x10-4

1x10-5

1x10-6

ALARP

Region

Intolerable

Region

Broadly AcceptableRegion

Fatalities (N) F r e q u e n c y (

F ) o n o r m o r e F a t

a l i t i e s ( p e r y e a r )



Risk Tolerability

and ALARPConcept



In life, there is always some risks…

• There is no such thing as zero risk

• All activities involve some risks

•The issue is at level should we toleratethese risks…



• 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

decide if the risks are “tolerable".

• One tolerability criteria in the UK is "as low as

reasonable practicable" (ALARP) concept formalized

in 1974 by United Kingdom Health and Safety at WorkAct.

• Serious consideration must be made to decide on

tolerability based on ALARP

Tolerable Risk



INTOLERABLE LEVEL

(Risk cannot be justified

on any ground)

TOLERABLE only if risk reduction

is impracticable or if its cost is

grossly disproportionate to theimprovement gained

TOLERABLE if cost of reduction

would exceed the improvement

gained

BROADLY

ACCEPTABLE

REGION

ALARP Criteria

THE ALARP REGION(Risk is undertaken if benefited

is desired)



Tolerability Criteria in Malaysia

• LSIR is used as a measure of individual risk

– This means that the risk is not influenced by

population

•The Upper limits for LSIR are as follows – For residential receptors : 1 X 10-6 fatality per year

– For industrial receptors : 1 X 10-5 fatality per year

– For workers on site: Voluntary risk (1 X 10-3 fatalityper year). This is considered maximum in UK for

offshore industry.



• This framework is represented as a three-tier system asshown in figure. It consists of several elements :

(1) Upper-bound on individual (and possibly, societal) risklevels, beyond which risks unacceptable. In UK, theguideline and criteria are spelled out in R2P2 (reducing

Risk Protecting People) document. (refer to www.hse.gov.uk)

(2) Lower-bound on individual (and possibly, societal) risklevels, below which risks are deemed not to warrantregulatory concern.

(3) intermediate region between (1) and (2) above, wherefurther individual and societal risk reductions are requiredto achieve a level deemed "as low as reasonably

practicable (ALARP)".

Tolerability Criteria (UK)



Tolerability Criteria (UK)

Dotted line –

general public

Solid line - workers



Tolerability criteria (Netherland)

1. Risk to public cannot bemore than 1X 10-6 fpy

2. Fatality cannot be more

than 10 at risk 1X 10

-5

fpy3. Slope -2

General public only



Tolerability Criteria (Australia)



Tolerability Criteria (Canada)

Major industrial accident council of Canada (MIACC)

recommends the above Individual risks level

4 risk estimation

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