## 4 Case Study
### Results and Discussion  
As shown in Figures 9 and 10, both the preventive and protective SBs can reduce the failure risk of the storage tank to a certain extent. In particular, Y4 (the regular detection of deformation or damage in the storage tank), Y7 (the installation and regular maintenance of safety electrical equipment), Y6 (the regular maintenance of safety alarm system equipment), F1 (fire source detection and alarm devices), and F4 (fire alarm and extinguishing systems) have high \(E\!f\!g_{\text{SB}}\). This is because Y4, Y7, and Y6 are parts of the regular inspection and maintenance of key equipment, which play an important role in identifying gas leakage sources and can stop the incidents in their initial stages. F1 and F4 are responsible for monitoring sources of ignition and extinguishing fires, respectively, playing an obvious role in curbing fire, explosion, and other events with very high consequence severity. Y1 (personnel operation and safety training), Y2 (using a checklist to establish complete and strict specifications and requirements for daily inspection work), Y3 (the regular detection of tank subsidence height and ground soil condition), and Y5 (the regular detection of the complete length of the surface coating) have low \(E\!f\!g_{\text{SB}}\). The reason is that Y1 and Y2 involve personnel operation, which has a high degree of uncertainty, and these two SBs can only limit third-party damage. Furthermore, Y3 mainly controls tank subsidence, having little influence on leakage. Y5 mainly limits external corrosion, which is not promi  
Figure 8: The failure consequence severity under different safety barrier scenarios. Figure 9: Risk indexes of the natural gas storage tank under different safety barrier scenarios. nent for tanks in the dry western region of China. The protective SBs of F2 (storage tank pressure detection) and F3 (gas concentration detection instrument) also have relatively low \(\textit{Eff}_{\text{SB}}\). This is because the F2 and F3 can only detect leakage but not locate it, and they have limited effect on mitigating the hazard of consequences on their own. As a result, they must cooperate with other protective measures to reduce the consequence severity effectively. By comparing the corresponding \(RI\) and \(\textit{Eff}_{\text{SB}}\) of Y1-Y7 (the combination of the preventive SBs) and F1-F4 (the combination of the protective SBs) in Figures 9 and 10, it can be inferred that Y1-Y7 are more effective than F1-F4. Therefore, when configuring SBs, it is not advisable to rely solely on protective SBs to reduce tank risks. Instead, a thorough analysis of the causes of the critical event should be carried out to ensure that preventive SBs are set up correctly to control most risks before critical events occur. Although some barriers have a significant effect when acting alone, most barriers have a low effect. When multiple barriers work together, the risk reduction for storage tanks is very significant. In particular, the risk reduction for storage tanks is most pronounced when all SBs are present, with an \(\textit{Eff}_{\text{SB}}\) of 37.7%. The reason for this is that a single SB may only limit the probability of a single failure path, but there are often multiple failure paths for a complex system. Therefore, only when multiple SBs are in play can the risk of the system be effectively reduced and the safe operation of the storage tank be guaranteed. Furthermore, all the SBs considered in this system were freely combined, and the effectiveness index of each combination was analyzed. The results are shown in Figure 11. Here, \(n\) is the number of SBs considered. The orange markers and labels indicate the combination of SBs with the highest effectiveness index for each combination number scenario as well as the specific effectiveness index value. It can be inferred that the \(\textit{Eff}_{\text{SB}}\) gradually increases as the number of SBs increases.