More progress - the overview.

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

Where:

• ·         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

1. 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.
2. Release frequencies are defined on a separate sheet and referenced via lookup during calculations.
3. For each event the model walks through each release (hole) size required for both Impinged and Unimpinged states.
4. 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.
5. 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. 
6. When the QRA is run, the program fills in the data for each event and calculates the branch probabilities and outcome frequencies. 
7. 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.

Voila....