C
APPENDIX
Probability, Severity, Risk, and
Risk-Acceptance Tables
Table C.1 Typical probability levels and descriptions
Level Probability (likelihood) descriptions
1
2
3
4
5
Frequency: 0.0 to 1 106 (never to 1 in 1,000,000 years).
Scenario: Should not occur in the life of the process, and there is no
historical industry experience to suggest it will occur.
Layers of protection: Four or more independent highly reliable safeguards
are in place, failure of three safeguards would not initiate an unwanted
event.
Frequency: 1 106 to 1 104 (1 in 1,000,000 years to 1 in 10,000
years).
Scenario: Similar events are unlikely to occur but have historically occurred
in this type of process somewhere within the industry.
Layers of protection: Three independent highly reliable safeguards are in place,
failure of two safeguards would not initiate an unwanted event.
Frequency: 1 104 to 1 103 (1 in 10,000 years to 1 in 1,000 years).
Scenario: This particular scenario is likely to occur somewhere in the
industry during the life of this general type of process.
Layers of protection: Two independent highly reliable safeguards are in place,
failure of one safeguard would not initiate an unwanted event.
Frequency: 1 103 to 1 102 (1 in 1,000 years to 1 in 100 years).
Scenario: This particular scenario will almost certainly occur somewhere
in the industry during the life of this specific type of process (but not
necessarily at this location).
Layers of protection: Single layer of safeguard and operator interface are in
place to prevent unwanted events.
Frequency: 1.0 to 1 102 (always to than 1 in 100 years).
Scenario: This particular scenario has occurred somewhere in the industry
in this specific process or is likely to occur at this location during the
life of this facility.
Layers of protection: Procedures or operator interface relied upon to prevent
unwanted events.
Safety and Security for the Process Industries
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Safety and Security for the Process Industries
Table C.2 Typical severity levels and descriptions
Level Severity (consequence) descriptions
1
●
●
●
●
●
2
●
●
●
●
●
●
3
●
●
●
●
●
●
4
●
●
●
●
●
●
5
●
●
●
●
●
●
Minor onsite injuries (first aid and nondisabling, reportable injuries).
Property damage less than base level amount*.
Minor environmental impact (no remediation).
Loss of production less than base level amount*.
No offsite impact or damage. No public concern or media interest.
Serious onsite injuries (temporary disabling worker injuries).
Property damage from 1 to 20 times base level.
Moderate environmental impact (cleanup or remediation in less than 1
week and no lasting impact on food chain, terrestrial or aquatic life).
Loss of production from 1 to 20 times base level.
Minor offsite impact (public nuisance—noise, smoke, odor, traffic).
Potential adverse public reaction. Some media awareness.
Permanent disabling onsite injuries or possible fatality.
Property damage from 20 to 50 times base level.
Significant environmental impact (cleanup or remediation less than 1
month and minor impact on food chain, terrestrial or aquatic life).
Loss of production from 20 to 50 times base level.
Moderate offsite impact limited to property damage, minor health effects
to the public or first aid injuries.
Adverse public reaction. Local media concern.
Onsite fatality or less than four permanent disabling worker injuries.
Property damage from 50 to 200 times base level.
Serious environmental impact (cleanup or remediation requires 3–6
months and moderate impact on food chain, terrestrial and/or aquatic life).
Loss of production from 50 to 200 times base level.
Significant offsite impact property damage, short-term health effects to
the public or temporary disabling injuries.
Significant public concern or reaction. National media concern.
Multiple onsite fatalities or four or more permanent disabling onsite injuries.
Property damage greater than 200 times base level.
Extensive environmental impact (cleanup or remediation exceeding 6
months, significant loss of terrestrial and aquatic life or damage to food
chain uncertain).
Loss of production greater than 200 times base level.
Severe offsite impact property damage, offsite fatality, long-term health
effect, or disabling injuries.
Severe adverse public reaction threatening facility continued operations.
International media concern.
Levels of severity may especially differ at foreign locations, based on the society and cultural
acceptance of hazards.
*Base level amount determined by insurance coverage and financial impact acceptable to senior
management.
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Probability, Severity, Risk, and Risk-Acceptance Tables
Table C.3 Suggested risk assessment matrix (RAM)
5
C
C
D
E
E
4
B
C
C
D
E
Probability 3
B
B
C
D
D
2
A
B
C
C
D
1
A
A
B
C
D
1
2
3
4
Consequence
5
Table C.4 Risk response—actions and responsibilities
Risk response
a. No further action or safety studies required. Individual personnel judgment
required for operation to occur.
b. Document process safety and security studies, hazard- and risk-reducing
measures. Consider feasibility and cost/benefit of additional risk-reducing
measures. Supervision approval required for operation.
c. Document process safety studies, evaluate feasibility of additional risk-reducing
features, and implement if worker and offsite exposure can be reduced to a
lower level. Operating group’s approval is required for operation.
d. Document process safety studies, hazards- and risk-reducing measures. Identify
additional risk-reducing measures and implement if worker and offsite
exposure can be reduced to a lower level. A quantitative risk analysis is required
to assess hazards. Divisional management (company*) approval is required for
operation.
e. Additional process safety studies and risk-reducing measures are mandatory to
achieve lower risk. Corporate (parent company*) senior management approval
is required for operation.
*Large multinational companies usually create “in country” companies for financial and legal reasons.
In this particular risk-ranking matrix, the risk level is not inversely equal,
i.e., C4 and P1 do not carry the same risk as P4 and C1. Generally it is
considered that the risk is higher when the consequences are more severe
rather than when frequency is greater.
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