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An event or a condition that on its own or in combination with other events or conditions can trigger an initiating event or enable an accident scenario to develop further toward an accident.

Enabling events and conditions are events and conditions that contribute to instigate the initiating event and to drive the accident scenario forward toward harm of an asset. Sometimes, it may be difficult to distinguish clearly between events that are in the accident scenario sequence and enabling events, but as a general rule, all events that are not on the “main path” toward the accident scenario end event are enabling events. It may seem unnecessary to distinguish between these two, but for the purpose of managing risk it may be quite important. If an initiating event or another event occurs that is defined as being part of the accident scenario, this means that the situation has moved one step closer to becoming an accident. Enabling events (and conditions) only change the probability that an event in the sequence occurs. In an earlier example, “Gas leak from flange A” was used as an initiating event. An enabling event could be “impact on flange” and an enabling condition could be “corrosion” because both increase the probability of failure of the flange.

Table 2.4 lists some hazards, enabling events and conditions, and initiating events to help clarify the concepts and illustrate the differences between them.

Table 2.4 Hazards, enabling events and conditions, and initiating events.

Hazard	Enabling event/condition	Initiating event
A car on top of a hill	Handbrake is not on	Car starts rolling
Propane gas under pressure	Corrosion in tank	Gas is released
Water in a hydroelectric power dam	Extreme rain	Water flows over top of dam
A large crowd in a confined space	Excitement in crowd	Panic breaks out
Tension between tectonic plates	Build up over long period	Earthquake
Pressure differences in the atmosphere	Increasing pressure difference	Storm
Tension in an offshore structure	Crack growth in structure due to fatigue	Failure of a structural member