¶ Event Tree
Figure VII-5 gives the representation for the structure of an event tree. This structure makes it possible to walk along the sequences, but not to construct the Boolean formulae associated with each sequence. To do so, we need to fill the structure with instructions. Instructions are actually used for two main purposes:
- To collect formulae or stochastic expressions
- To define flavors of fault trees and probability distributions, i.e., to set values of house events and flag parameters
The collection of a top event consists in a Boolean product of the formula associated with the sequence and a copy of the fault tree rooted with the top event. In the Model Exchange Format, the operation is performed by means of the instruction “collect-formula”. The collection of an expression multiplies the current probability of the sequence by the value of this expression. In the Model Exchange Format, the operation is performed by means of the instruction “collect-expression”.
To give flavors to fault trees, i.e., to change the values of gates, house events, basic events, and parameters, the Model Exchange Format introduces the four corresponding instruction: “set-gate”, “set-house-event”, “set-basic-event”, and “set-parameter”.
Sequences are walked from left to right. Therefore, when a value of an element is changed, this change applies on the current environment and propagates to the right. This default behavior can be changed by using the flag “direction”, which can take either the value “forward” (the default), “backward” or “both”. This feature should be handled with much care.
The flavor given to fault trees, as well as what is collected, may depend on the initial event and the current state of functional events. To do so, the Model Exchange Format provides an if-then-else instruction (the “else” part is optional) and the two expressions “test-initiating-event” and “test-functional-event”. These two instructions have been introduced in the JSON representation below. Since the then- and else-branches of the “if-then-else” may contain several instructions, the Model Exchange Format introduces the notion of block of instructions.
{
"test-initiating-event": {
"name": ""
},
"test-functional-event": {
"name": "",
"state": ""
}
}
Finally, some models require linking event trees. A special instruction “event-tree” is introduced for this purpose. It should be used only in sequence definitions, i.e., in end-state.
It is sometimes the case that the same values of house events and parameter flags are used at several places. Such a configuration is called a split-fraction in the event tree linking approach. The Model Exchange Format refers it as a rule, for it is a sequence of instructions.