Variable ordering schemes to apply to the binary Decision diagram methodology for event tree sequences assessment

Abstract

Binary Decision Diagram (BDD) methodology is the most recent approach to improve Boolean reliability models assessment. The final size of the BDD, and therefore the ultimate benefits of this technique, is very sensible to the initial variable ordering that has to be fixed prior to conversion. Several variable ordering strategies have been proposed within the literature, all of them focused on the treatment of single Fault Tree models. This paper proposes some extensions of existing variable ordering schemes for the case of combinations of non-disjoints Fault Trees, as is the case in quantifying sequences of Event Trees. These extensions work combining ordering schemes applied to each Fault Tree, but considering whether variables within the domains intersection should be kept together or not. They have been designed specifically to be applied together with an incremental procedure to compute the BDD of the sequence accumulatively and to be used to quantify sequences of Dynamic Event Trees.