Risk Calculation Model The model is an analytical model used to calculate the risks

associated with hazardous materials developed following QRA procedure given by several authors.

The MPACT (population impact of toxic and flammable effects) model implemented in the SAFETI DNV software is a mathematical model that calculates a number of results relating to the impact on the land and population potentially affected by the various toxic and/or flammable events that are defined as inputs by the user.

The accident consequences of hazardous materials (i.e. fire and explosion outcomes) in terms of their effect zones (distances), as one of the model inputs were calculated using PHAST.

The results are presented as F-N curve for societal risk and individual risk profiles.

Risk calculation model environment modified form

Risk Measures for Loss of Life

Risks to people may be expressed in two main forms:

1. Individual risk – the risk experienced by an individual person

2. Societal (or group) risk – the risk experienced by the whole group of people exposed to the hazard. The term societal is used to encompass both public and worker risk.

INDIVIDUAL RISK Individual Risk (IR) is the frequency at which an individual may be expected

to sustain a given level of harm from the realization of specified hazards. IR measures are single numbers or a set of risk estimates for various

individuals or geographic locations. IR is calculated by identifying all sources of fatality risk to a given individual,

deriving the contribution from each source and then summing these to give the overall risk.

For typical oil, gas and petrochemical workers the primary sources of risk are:

Occupational, e.g. slips and falls, drowning Transport, e.g. road traffic accidents, air transport accidents Hydrocarbon related, e.g. loss of containment leading to toxic releases, fires

or explosions IR is the risk that an individual would face from a facility if they remained

fixed at one spot 24 hours a day 365 days per year. Its value is a frequency of fatality, usually chances per million per year, and it is displayed as a 2 dimensional plot over a locality plan as contours of iso-risk.

Simplified Individual Risk Plot (numbers are fatality frequency per year)

Example of an individual risk, i.e. risk profile

Workers Members of Public

Maximum tolerable criterion

10-3 per year 10-4 per year

Broadly acceptable criterion

10-6 per year 10-6 per year

Individual Risk Criteria according to UK HSE

Total Individual Risk Action Indicated

> 1.0 x 10-3 Risk mitigation or further risk assessment is required

1.0 x 10-3 to 1.0 x 10-5 Risk reduction should be considered

<1.0 x 10-5 Further risk or assessment reduction need not be considered

Individual Risk Criteria by API RP 752

SOCIETAL RISK Societal Risk is the relationship between the frequency and

the number of people suffering from a specified level of harm in a given population from the realization of specified hazards.

Societal risk is used in quantified risk assessment (QRA) studies and is depicted on a cumulative graph called an F/N curve

The concept of societal risk is illustrated in figure. Situations A and B have equal individuals risk levels (IR and IR’) but B has a larger societal risk (SR) because more people are exposed (Jonkman 2003)

FN-Diagram

A common form of presenting risk tolerability criteria for societal risk is on an FN-diagram, where two criteria lines divide the space into three regions – where risk is intolerable, where it is broadly acceptable and where it requires further assessment and risk reduction as far as is reasonably practicable

ALARP• The ALARP (as low as reasonably practicable) technique is

one method used in the establishment of risk criteria. The general framework for this technique is represented by the three tier system. The risk level boundaries vary along with the number of people at risk. The values vary depending on the industry sector, safety philosophy implemented by the industry and the practically of applying risk reduction techniques.

• The ALARP principle is applied to the intermediate region between the unacceptable and acceptable bounds where further individual and societal risk reductions are required to achieve a level considered “as low as reasonably practicable”.

• In reality, the risks associated with most facilities lie in the middle band of the risk tolerability framework – the ALARP region.

A Framework for Risk Criteria• The most common and flexible

framework used for risk criteria divides risks into three bands (HSE 2001):

• An unacceptable region, where risks are intolerable except in extraordinary circumstances, and risk reduction measures are essential.

• A middle band, or ALARP region, where risk reduction measures are desirable, but may not be implemented if their cost is disproportionate to the benefit achieved.

• A broadly acceptable region, where no further risk reduction measures are needed.

ALARP Process

ALARP Tools

Tools are Available to Help Demonstrate ALARP

International Risk Acceptance Criteria

UK HSE (COMAH)

NSW (Netherland)

Hong Kong Societal Risk

Victorian Societal Risk Criteria used in Australia and New Zealand

Individual Risk Criteria by API RP 752

United Kingdom HSE FN Criteria

NSW (Netherland) F-N Curve

Hong Kong Societal Risk Guidelines (risk to public only)

Victorian Societal Risk Criteria used in Australia and New Zealand