Frequency Estimation Results

This section describes the estimated frequency results of the accident scenarios considered for the study objects. Two approaches were carried out to estimate expected frequencies of the study objects (section 4.4.1), i.e. using fault-tree approach and direct use of the base failure data. The first approach (fault tree analysis) was performed to the four hydrogen objects, namely the GH2 storage at production, the LH2 storage at the depot, the LH2 storage at the filling station and the LH2 storage at the FC-CHP plant. Fault tree analysis for the plants ("Top events") is fully reproduced in Appendix G. It includes fault tree diagrams, reliability data, and summary of the results. Assuming a lognormal distribution for the basic event failure rate data, the expected frequencies of the Top events have been computed using the FTA-based analytical approach (described in section 4.4.2). The results are presented as the mean value as well as the 5th percentiles, 50th percentiles, 95th percentiles, and the 95% uncertainty factor (K-95).

The second approach was performed using equations and historical data described in the Section 4.4.3. The expected frequency for the onboard LH2 storage in vehicle and LH2 truck were estimated from German traffic accident rates. Meanwhile, the frequency for the GH2 pipeline was estimated from the base failure data of gas pipeline for Europe.

Frequencies of the accident outcomes may occur following the accident scenarios are calculated by using event tree analysis (ETA). It performed by assigning conditional probabilities to all of the branches of the event trees of Fig. 4.5 (for LH2) and Fig. 4.6 (for GH2) and multiplying them by the expected frequency of each initiating events (top event). The probabilities for both event trees are given in Table 4-9.

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