Risk EstimationRisk 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 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 RiskIndividual 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.

• 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

• 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 Example: LSIR for Ship Explosion at a Proposed PortProposed 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:

i

iN 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.

Example: The corresponding Societal Example: The corresponding Societal RiskRisk

1 10 100 1000 100001x10-7

1x10-3

1x10-4

1x10-5

1x10-6

ALARPRegion

IntolerableRegion

Broadly Acceptable Region

Fatalities (N)

Risk Tolerability and

ALARP Concept

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 tolerate these risks…

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• 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".

• 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.

Tolerable RiskTolerable Risk

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INTOLERABLE LEVEL (Risk cannot be justified on any ground)

TOLERABLE only if risk reduction is impracticable or if its cost is grossly disproportionate to the improvement 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 MalaysiaTolerability 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 fatality per year). This is considered maximum in UK for offshore industry.

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

(1) Upper-bound on individual (and possibly, societal) risk levels, beyond which risks unacceptable. In UK, the guideline 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) risk levels, below which risks are deemed not to warrant regulatory concern.

(3) intermediate region between (1) and (2) above, where further individual and societal risk reductions are required to achieve a level deemed "as low as reasonably practicable (ALARP)".

Tolerability Criteria (UK)Tolerability Criteria (UK)

Tolerability Criteria (UK)Tolerability Criteria (UK)

Dotted line – general public

Solid line - workers

Tolerability criteria (Netherland)

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

2. Fatality cannot be more than 10 at risk 1X 10-5 fpy

3. Slope -2

General public only

Tolerability Criteria (Australia)Tolerability Criteria (Australia)

Tolerability Criteria (Canada)Tolerability Criteria (Canada)

Major industrial accident council of Canada (MIACC) recommends the above Individual risks level