Frequencies and probabilities

• Definition: Corresponds to the frequency with which an engineered system or component fails.

• Types: The base frequency can be expressed in:

  Stationary equipment: Events per year (/year).

  Transport equipment: Events per kilometer-year (/km.year).

Frequency correction factor

• Definition: Adjusts the scenario’s frequency to account for deviations from standardized conditions due to risk reduction measures or specific circumstances.

• Usage: Using a correction factor instead of adjusting the base frequency makes adjustments more traceable.

  This value is used for stationary equipment. For transport equipment, the correction factor can be modified per location, leading to an additional column in the route definition.

Frequency equally distributed day/night

• Definition: Assumes scenario frequency is equally distributed over day and night according to meteorological conditions.

• Usage: Some activities (e.g., loading, unloading) might be planned to occur during the day or night.

  Set this setting to "No" if the scenario has a preference for daytime or nighttime activities, allowing for specific distribution settings.

Fraction frequency in daytime hours

• Definition: The base frequency covers both daytime and nighttime conditions. Users can specify activities to occur only during daytime (fraction daytime = 100%), only at nighttime (fraction = 0%), or any other ratio.

• Usage: This input is only available if "Frequency Equally Distributed Day/Night" is set to "No".

Chance of direct ignition

• Definition: Corresponds to the probability of an immediate ignition event.

• Usage: Relevant for scenarios with multiple phenomena (e.g., pool fire and vapour cloud explosion, jet fire and vapour cloud explosion). The default value is 0.8 but can be adjusted based on chemical type (flammability) or release rate. Use guidelines such as those from Dutch BEVI, which provide values depending on the release rate and substance flammability.

Chance of delayed ignition

• Definition: Corresponds to the probability that a delayed ignition event (e.g., flash fire and/or vapour cloud explosion) will occur.

• Usage: The sum of direct and delayed ignition probabilities does not have to equal one, because the chance of no ignition also exists: (1 - Chance direct ignition + Chance delayed ignition) = Chance no ignition.

BLEVE fraction

• Definition: The probability of a BLEVE (Boiling Liquid Expanding Vapor Explosion) event occurring.

• Usage: Relevant only to instantaneous two-phase releases of flammable materials and influences scenarios with multiple phenomena

Fraction with explosion phenomena

• Definition: Describes the fraction of vapour cloud explosions (delayed ignitions) that will have overpressure effects.

• Usage: It can be calculated by connecting a Receiver - Congestion area to a consequence model in EFFECTS.

• Calculation: Different from other QRA tools, RISKCURVES assumes that all delayed ignitions lead to a flash fire, with only a portion including overpressure effects (if congestion occurs).