Qualitative risk estimation

The classification of a hazard is made based on expert judgment.

In practice, this can be undertaken via the collective opinion of the attendees at a hazard identification workshop.

For a straightforward hazard:

• Identify the causes of the hazard and document them as a table or short explanation;

• Identify the possible consequences of the hazard and the factors that affect those consequences, and document as a table or short explanation;

• Identify the existing safety measures that control the hazard;

• Identify the practical additional safety measures that might be implemented to control the hazard further.

Semi-quantitative estimation

• Used when some data is available, or a good degree of judgment can be applied to estimates of the frequency and consequences of each accident;

• Typically uses a 5 x 5 matrix with the frequency and consequence rankings broadly separated by a fixed factor, but this does not have to be the case;

• The size of the matrix and the factor difference in frequency and consequence rankings should suit a particular stakeholder’s operation.

The main advantages of using a risk matrix:

• It is an easily understandable representation of relative risk levels;

• A tie can be applied relatively quickly;

• It is readily understandable by those whose inputs and opinions are needed to apply it;

• It enables the combination of frequency and consequences to be represented in an intuitive visual way.

The explicit risk estimation involves:

• Identifying the possible causes of the hazard and estimating the likelihood of each cause resulting in an accident;

• Identifying the possible consequences of the hazard and assessing their severity.

Tables

Likelihood

Ranking	Category	Description
5	Frequent or almost certain	Event occurs many times during the period of the project (or life of the facility).
4	Likely	Event likely to occur once or more during the period of the project (or life of the facility).
3	Possible	Event could occur during the period of the project (or life of the facility).
2	Improbable	Event is unlikely to occur, but it is possible. Means an occurrence of failure at a frequency less than or equal to $10^{-7}$ per operating hour.
1	Higly improbable	May occur only in exceptional circumstances. Means an occurrence of failure at a frequency less than or equal to $10^{-9}$ per operating hour.

Consequence

Ranking	Category	Description
5	Catastrophic	A “catastrophic accident” means an accident typically affecting a large number of people and resulting in multiple fatalities and/or major damage to the environment.
4	Critical	A “critical accident” means an accident typically affecting a very small number of people and resulting in at least one fatality.
3	Moderate	Small injuries requiring medical treatment and some lost time.
2	Minor	Minor injuries, first aid only required.
1	Insignificant	No injuries or negligible social cultural impacts.

Frequency/Consequence

Frequency/Consequence	1 - Insignificant	2 - Minor	3 - Moderate	4 - Critical	5 - Catastrophic
5 - Frequent or almost certain	6	7	8	9	10
4 - Likely	5	6	7	8	9
3 - Possible	4	5	6	7	8
2- Improbable	3	4	5	6	7
1 - Highly improbable	2	3	4	5	6

8 - 10	Intolerable	Risks considered Intolerable shall be eliminated.
7	Undesirable	Accepted only when risk reduction is impracticable and with the agreement of the Safety Regulatory Authority (SFA), as appropriate. Action plans must be developed with clear assignment of individual responsibilities and timeframes.
5 - 6	Tolerable	Acceptable with adequate control and with the agreement of the SFA. Risk requires specific ongoing monitoring and review, to ensure level of risk does not increase. Otherwise manage by routine procedures.
2 - 4	Negligible	Acceptable with/without the agreement of the Safety Regulatory Authority. Risk can be accepted or ignored. Manage by routine procedures, however unlikely to need specific application of resources.

Calibrating the risk matrix

Risk classification matrices are used for ranking and comparing the risk of different hazards.

The matrix must be calibrated so that relative risk of different hazard is preserved across the various risk classifications.

• Achieved by having the same factor difference (a factor of 5) between each frequency and consequence category.

	Once in	No / year
Frequent or almost certain	12 days	31.25	5
Likely	2 months	6.25	4
Possible	9 months	1.25	3
Improbable	4 years	0.25	2
Highly improbable	20 years	0.25	1