Risk CriteriaWhere the Answer Lies

Troy Millen - Principal Safety & Risk EngineerNovember 2016

Why Do We Quantify Risk?

• Compliance with internal policies/standards

• Regulatory requirement• Aids decision making• People like numbers!

But can we trust the numbers that are

generated?

4 Measures of Risk

1 2Individual Risk Per Annum

Location Specific Individual Risk

3 4Potential Loss of Life/Societal Risk

Implied Cost to Avert a Fatality

Individual Risk1.

• Derived values can be compared against real-world measures of risk (e.g. the road toll)

• A key criteria where many organisations will have defined tolerability levels

Individual risk is the probability that, within a defined period of time (or for a defined activity), a person will, sadly, become a fatality.

• Can be reduced by “sharing” the risk across a larger population (but absolute risk stays the same)

• Transient work forces (e.g. drilling) may only have a small percentage of their annual risk accounted for.

Strongly influenced by the amount of time at exposure

Location Specific

Individual Risk2.

Location Specific Individual Risk is the likelihood that a fatality would occur at a particular location, based on continuous exposure over a calendar year.

1 Published exposure criteria available based on land use (e.g. HIPAP 4).

2 Typically represented as contours of constant risk, overlaid on a facility/area plan.

• At low level typical of offsite criteria, the error margin is large.\

• Criteria relies on “average” industry values (e.g. failure rates)

• Key factors of influence include:• Hole size distribution• Release orientation• Weather conditions• Time of exposure (for fatality probits)

• May not accurately reflect the risk of low frequency transient operating modes

Potential Loss of Life/Societal

Risk3.

Represents the aggregation of the individual risk level for all persons exposed to a particular hazard or facility. • Can be displayed as societal (f-N)

risk curves• Facility size impacts results• How do we define the basis for:

• Grouped assets• Pipelines

Implied Cost to Avert a Fatality

4.

Derived from the cost to implement a risk reduction measure, divided by the estimated risk benefit.

Requires a dollar value to be placed on lifeCan be easy to manipulate a “do nothing” outcome

Risk criteria case studies

1. Hiding from RiskProcess risk based on a parts count, and assumes component failures are randomised.Facility risks are low, premised on infrequency of visits.This approach ignores:

• Human influence on component failures• Personnel presence at other facilities

The issue is compounded when assets span multiple regulatory regimes (e.g. offshore platform and onshore plant)

Water treatment utilising chlorine gas is delivered from a bank of one tonne drums.Risk contours were directly impacted by the number of drums online, and the number stored at the facility.Reducing the drum bank/storage reduced LSIR contours.

But frequency of truck delivery (down a steep gravel road) increased!

2. Truckload of Risk

Client standard required that no Major Accident Event (MAE) impaired key safety systems above a target frequency.

• Client standard did not provide a definition of a MAE.

• Initial assessment had MAE (process release) exceeding requirement.

Revision of MAE definition (Jet Fire, Pool Fire, Explosion)

lead to compliance with criteria.

3. Cutting it Fine

Approach

To survive post-event scrutiny, numerical analysis should be used as a tool to inform risk arguments, not considered to provide

the ultimate answer.

Be transparent, and avoid black-box approaches.

Use a combination of Quantitative

AND Qualitative approaches.

Make conservative assumptions, and refine as

required.

Understand the factors driving risk results. Explore sensitivity to key assumptions, and provide justification for their use.Be aware of risk trade-off - look beyond the study scope to avoid risk transfer.

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