Page 256 - CHEF Guide

Likelihood Evaluation

Table 15-1 Common Initiating Event Frequencies
[Adapted from [7, p. 71]

Initiating Event Frequency Range from
Literature (per year)

Basic process control system failure 1 to 10 -2
Pressure regulator failure 1 to 10 -1
-2
Gasket or packing blowout 10 to 10 -6
Cooling water failure 1 to 10 -2
-1
Pump seal failure 10 to 10 -2
Unloading hose failure 1 to 10 -2
-2
Safety valve opens spuriously 10 to 10 -4
-3
Piping leak per 100 m pipe length (20% leak) 10 to 10 -5
-5
Piping leak per 100 m pipe length (full breach) 10 to 10 -6
-2
Large external fire (aggregate causes) 10 to 10 -3
-3
Lightning strike 10 to 10 -4
-2
Third party intervention (impact by vehicle) 10 to 10 -4
-1
Human error (routine procedure, unstressed) 10 to 10 -3
per opportunity

15.8 Human Performance

The reliability and error probability of people involves the ratio between the number of human performance issues and
the number of opportunities for inadequate performance. We will discuss this in more detail in the upcoming discussion on
Procedures and Human Performance.

The Reliability by which tasks are performed depends on:
 Familiarity with the task
 Complexity of the task (including problem diagnosis and decision making)
 Time to complete
 Human – Machine Interface
 Work Environment – stress, fatigue, training, etc.

15.9 Other Factors – Enabling Conditions and Conditional Modifiers

An Enabling Condition that is not a failure, error or a protection layer but makes it possible for an incident sequence
to proceed to a consequence of concern. It consists of a condition or operating phase that does not directly cause the
scenario, but that must be present or active in order for the scenario to proceed to a loss event; expressed as a
dimensionless probability.

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