Estimating Frequency

In general, probabilities of a hazardous event occurring during a given time interval can be derived from the probabilities (frequencies) of each of the contributory events whose occurrence, individually or in conjunction with other events, could lead to the occurrence of the hazardous event. A logical relationship between hazardous events and the corresponding contributory events are conveniently represented as a 'fault tree'. However, in some cases the tree is trivial due to the frequency being dominated by a single contributory cause, or due to the availability of statistical data on the frequency of the hazardous event itself, rather than only on the contributory events [158]. For example, onboard hydrogen storages in traffic systems or hydrogen transportation are more likely to occur because of traffic accidents than through the system malfunction. On the other hand, a large cryogenic storage vessel (e.g. LH2 bulk storage) relies for integrity not only on the quality of its construction but also on the reliability of its pressure control system and protective devices. Therefore, a dual approach to the frequency estimation was used in the study. Firstly, fault tree analyses were carried out on a larger containment system where safety depends on the reliability of a large number of components. It includes stationary hydrogen storages (both liquid and gaseous). Secondly, failure rate data are used for certain discrete events for which adequate statistics exist, or for which system reliability considerations is not the main cause of failure. It includes hydrogen onboard storage and hydrogen transportation.

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