## 4 Case Study
### Construction of the Bow-Tie Model for Describing the Failure of Storage Tank  
Natural gas is a highly flammable gas, so in the case of an ignition source, any large amount of natural gas leakage may lead to fire and explosion; in other words, the leakage of the natural gas in the storage tank is the direct cause and key link to serious safety accidents. Therefore, natural gas leakage was set as the critical event in the BT model. Through a field survey, it was found that fire, explosion, personnel suffocation, and safe dispersion were the main failure outcomes caused by natural gas leakage. Furthermore, thermal radiation and shock waves generated by fire and explosion can cause serious damage to equipment and personnel, and the high concentration of natural gas can lead to oxygen deprivation leading to personnel suffocation, while gas dispersion cannot cause harm to personnel but can cause gas loss. Based on this, the BT model of the natural gas storage tank with SBs was established, as shown in Figure 5 and Table 6. This model had seven preventive SBs and four protective SBs, as shown in Table 7. Y1-Y7 were designated as the preventive SBs, mainly involving personnel training, daily inspection, and equipment maintenance, while F1-F4 were considered the protective SBs, mainly involving a series of protective measures after natural gas leakage. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Symbol** & **Content** & **Symbol** & **Content** \\ \hline X1 & Geological factors & M3 & Equipment reasons \\ X2 & Environmental factors & M4 & Natural factors \\ X3 & Failure to standardize operation & M5 & Non-natural factors \\  & according to requirements & & \\ X4 & Daily inspections do not meet the & M6 & Main equipment \\ X5 & Mechanical breakdown & M7 & Others \\ X6 & Installation failure & M8 & Design reasons \\ X7 & Design specification failure & M9 & Processing problem \\ X8 & Unreasonable layout & M10 & Material reasons \\ X9 & Foundation subsidence or displacement & M11 & Instrument problem \\ X10 & Weld defect & M12 & Corrosion \\ X11 & Failure to finish processing as required & M13 & External corrosion \\ X12 & Deformation/breakage & EV1 & Generate an open flame \\ X13 & Internal corrosion occurred & EV2 & Forming steam clouds \\ X14 & Damage of coating & EV3 & Limited space \\ X15 & External environment meets corrosion & EV4 & Delayed ignition \\ X16 & Failure of other fittings & CO1 & Jet fire \\ X17 & No alarm/indicator light & CO2 & Vapor cloud explosion \\ X18 & Other failures in the instrument system & CO3 & Poisoning \\ X19 & Instrument system open circuit & CO4 & Flash fire \\ M1 & Natural gas leakage & CO5 & Natural gas dispersion \\ M2 & Third-party sabotage & CO6 & Deflagration \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The description of the events in the fault tree of the natural gas storage tank. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Symbol** & **Description** & **Safety Barrier** \\ \hline Y1 & Personnel operation and safety training & \\ Y2 & Using a checklist to establish complete and strict specifications & \\  & and requirements for daily inspection work & \\ Y3 & Regular detection of tank subsidence height and & \\  & ground soil condition & \\ Y4 & Regular detection of deformation or damage in the storage tank & \\ Y5 & Regular detection of the complete length of the surface coating & \\ Y6 & Regular maintenance of safety alarm system equipment & \\ Y7 & Installation and regular maintenance of & \\  & safety electrical equipment & \\ F1 & Fire source detection and alarm device & \\ F2 & Storage tank pressure detection & \\ F3 & Gas concentration detection instrument & \\ F4 & Fire alarm and extinguishing system & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The description of safety barriers of the natural gas storage tank.