Worst case credible event

Normal loss prevention practices are to design protection measures for the worst case fire event that can occur at the facility. To interpret this literally would mean that an oil or gas facility is completely on fire or totally destroyed by an explosion. Practical, economical and historical review considerations indicate this rational should be redefined to the Worst Case Credible Event (WCCE) or the as referenced in the insurance industry, the Probable Maximum Loss (PML), that could occur at the facility. 

Several additional factors are important when considering the worst case credible event. 

Much discussion could be presented as the most credible worst case event at the facility. Obviously a multitude of unbelievable events can be postulated (industrial sabotage, insane employees, plane crash impacts, etc.). Only the most realistic and probable events should be considered. In most cases, historical evidence of similar facilities is used as a reference for the worst case events. Alternatively, the effect of the most probable high inventory hydrocarbon release could be postulated. The worst case event should be agreed upon with loss prevention, operational and senior executive management for the facility. The worst case credible event will normally define the highest hazard location(s) for the facility. From these hazards, suitable protection arrangements can be postulated to prevent or mitigate their effects. 

As an aid in determining the severity of vapor cloud explosions, overpressure radius circles are normally plotted on a plot plan from the source of leakage or ignition. Computer applications are available that can easily calculate and plot these on electronic plant design and drafting applications. These overpressure circles can be determined at the levels at which destructive damage may occur to the facility from the worst case credible event (WCCE). 

Most process facility areas and high volume storage areas have been standardized for a minimum supply or availabihty of 4 h of firewater for the worst case credible event. The performance of a risk analysis may reveal the level of firewater protection needs to be more or less than this requirement. Once a detailed design is completed on the facility or verification of existing water demands is done a tabular calculation for firewater requirements can be made. This table can be used to document spray density requirements, duration levels, code requirements, and other features. Table 19.1 provides an example of ways to document such information (see Table 19.2). 

The sizing of piping is based on a hydraulic analysis for the water distribution network for the worst case credible event (WCCE).The main delivery pipe should be sized to provide 150% of the design flow rate. 

Testing of firewater distributions systems is performed to determine if the condition of the system is adequate to support a worst case credible event (WCCE) need for firewater. The condition of the piping, leaks, existence of closed valves or sediment, operability of valves for firewater delivery systems (sprinklers, deluge, hose reels, monitors) should be determined annually, but at a minimum every 5 years (NFPA 25, Chapter 7). 

This same model is used to calculate the maximum credible event (MCE) which is defined as that unintended, unplanned, or accidental adverse occurrence which causes release of agent from an ammunition item, bulk container, or process. It must be realistic with a reasonable probability of occurrence. It is necessary to hypothesize an MCE to enable calculation of the magnitude of a worst case hazard. The hypothetical MCE for any given situation will be based upon the nature and characteristics of the agent involved, ammunition, container, configuration, and... 

The possible loss scenarios, including events originating both internally and externally, should be explored. An assessment should then be made of the controls and protection features provided and their mitiga-tive effect on each loss scenario. Failures of controls and protection features should then be assumed to determine the worst-case and other, more credible, loss scenarios. Finally, the likelihood and degree of risk associated with each loss scenario should be developed. 

Risk has different types, such as individual risk, societal risk, ecological risk, and economic risk. Risk can also have different meanings, such as the maximum thinkable (probable or credible) outcome of an undesirable event, worst-case outcome, and most... 

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