TABLE OF CONTENTS A 'one-directional' time development is often assumed when constructing an event tree. For a gas leak this typically follows the sequence:
Leak
Gas detection
Isolation
Ignition (potential)
Fire detection
Fire fighting
Secondary loss of containment.
In actuality the scenario development is seldom so simple if the scenario is completely without control. Very often there will be loops, where secondary leaks, explosions and escalation of the fire occur. In practice this cannot be integrated into the event tree.
Cause-consequence analysis is another form of event tree which has the ability to show time delays between steps, and to some extent couplings or combinations. The time sequence is still assumed to be 'one-directional', however. The big advantage of the event tree method, on the other hand, is the ease in communicating the assumed accident sequence to non-analysts.
The event trees usually used in QRA are considered as 'static', in the sense that the logic of the tree, its couplings etc. are fixed by the analyst prior to conducting the actual analysis. The alternative to the static event tree is the dynamic event tree, which can be programmed to alter its logic and construction to reflect the modelled development of an accident. Commercially, there is only one package available for modelling of such dynamic trees, namely PLATO , developed by Environmental Resources Management (formerly Four Elements Ltd.), London. (Morris, Miles and Cooper, 1994). PLATO is said to be a simulator for...
