Now started to look into the 'onshore' QRA model - which is a little better structured. For each fire scenario the code undertakes the following calculation flow;
The initiating event sheet provides the event list to be processed by the application. The naming convention for initiating events is: Area/Unit_Inventory
- · area is the primary area designator (eg site),
- · unit specifies the location of the release (eg a fire zone, module/unit or equipment name)
- · inventory is the inventory name which can result in a hazardous scenario, e.g. valves, flanges
- The initiating events (hazardous release scenarios) are based on releases from inventories. A number of hole sizes are defined (typically small, medium, large and full bore) that relate to the event such that the hole size determines the release frequency.
- Release frequencies are defined on a separate sheet and referenced via lookup during calculations.
- For each event the model walks through each release (hole) size required for both Impinged and Unimpinged states.
- Within this nested loop structure the program calculates the branch probability and outcome frequencies for the event tree and copied the results to the Event Tree sheet. The calculations use a combination of the pre-loaded data acquired during program start plus lookup data from worksheets using offsets dependant on the event.
- The risk assessment methodology analyses each of the identified initiating hazardous events (hydrocarbon releases) that can lead to potential jet fires, pool fires, flash fires, explosions or (unignited) toxic dispersion.
- When the QRA is run, the program fills in the data for each event and calculates the branch probabilities and outcome frequencies.
- The branch probabilities are calculated by a user defined maco (function) embedded in the event tree sheet. Once the main process loop has copied data to the tree sheet an Excel re-calculate is invoked to display the results.