COBAREA: The COpula-BAsed REliability and Availability Modeling Environment

Traditional algorithms for the analysis of fault trees (FT) and reliability block diagrams (RBD) are relying on the assumption that there are no dependencies in the failure and repair behavior and thus independence assumptions simplify the calculation. In practice, however, the components of a system are usually not independent. Prominent examples for inter-component dependencies include failures with a common cause (e.g. due to spatial closeness or a shared design), failure propagation, limited repair resources, failures induced by repair, and overload due to failure. Using traditional evaluation techniques implies neglecting these system properties which may lead to over-optimistic results.As an alternative approach to deal with inter-component dependencies, we present a tool based on copulas. Copulas are a way of specifying joint distributions if only the marginal probabilities are known. In terms of system reliability, this can be interpreted as inferring the system state vector probability from the component state probabilities. What makes copulas a valuable modeling method for large reliability models is the separation of the component distributions (the marginals) and the dependencies. Therefore, copulas can be used with arbitrary fault tree evaluation algorithms.