## 3 Case Research  
### Fault Tree Modeling of Ethane Tank Leakage Risk  
To confirm the validity of the ethane storage tank leakage risk evaluation framework proposed in this paper, we use a cluster of ethane storage tanks in Xinjiang Province as a real case study. The ethane storage tank is designed with a maximum pressure of 0.4 MPa and a design temperature of \(-\)196 \({}^{\circ}\)C, with a capacity of 1500 m\({}^{3}\). The self-pressurizer for the ethane storage tank is designed to operate at a pressure of 1.6 MPa. The storage and loading facility is composed of the ethane storage tank, ethane loading pump, loading arm, BOG preheater, and BOG compressor, collectively forming an integrated system for the storage and loading of ethane.  
Through the field research that was conducted in order to obtain the related ethane storage tank leakage risk statistical report, which was then combined with the related literature and expert interviews, the typical risk events in the process of ethane storage tank leakage were determined; 47 risk factors are summarized and shown in Table 3. These risk factors include 32 basic events and 15 intermediate events; the top event is T (ethane storage tank leakage risk).  
Based on the risk factors associated with ethane storage tank leakage, the logical relationships among these risk factors were systematically sorted out. Using X1, X2, \(\ldots\), X32 as the basic events, A, B, and C as intermediate events, and the ethane storage tank leakage risk T as the top event, we established a fault tree for ethane storage tank leakage risk, as shown in Figure 5.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Node** & **Basic Events** & **Node** & **Basic Events** & **Node** & **Basic Events** \\ \hline T & Ethane leak & X\({}_{2}\) & Material Defects & X\({}_{18}\) & Uneven soil quality \\ M\({}_{1}\) & Fracture & X\({}_{3}\) & Manufacturing defects & X\({}_{19}\) & Low geologic bearing capacity \\ M\({}_{2}\) & Destabilization & X\({}_{4}\) & High cyclical fluctuations & Sudden drop in tank \\ M\({}_{3}\) & Corrosion & X\({}_{5}\) & External variable loads & X\({}_{21}\) & Valve failure \\ N\({}_{1}\) & Defective storage tanks & X\({}_{6}\) & Concentrated stresses & X\({}_{22}\) & Insufficient liquid storage \\ N\({}_{2}\) & Excessive tank loading & X\({}_{7}\) & Residual stresses & X\({}_{23}\) & Strong wind \\ N\({}_{3}\) & Third-party sabotage & X\({}_{8}\) & Exceeding working pressure & X\({}_{24}\) & Poor corrosion resistance \\ N\({}_{4}\) & Natural disaster & X\({}_{9}\) & Operator error & X\({}_{25}\) & Presence of corrosive media \\ N\({}_{5}\) & Uneven geologic settlement & X\({}_{10}\) & Unauthorized work & X\({}_{26}\) & Thinning of internal coating \\ N\({}_{6}\) & Shrivel & X\({}_{11}\) & Inadequate management & X\({}_{27}\) & Peeling of internal coating \\ N\({}_{7}\) & Internal corrosion & X\({}_{12}\) & Internal debris accumulation & X\({}_{28}\) & Corrosion inhibitor failure \\ N\({}_{8}\) & External corrosion & X\({}_{13}\) & Mudslides & X\({}_{29}\) & Soil corrosion \\  & Failure of internal & & & \\ C1 & corrosion protection & & & \\ C2 & External corrosive environment & X\({}_{15}\) & Flood impact & X\({}_{31}\) & Failure of cathodic protection \\  & Failure of external & & & \\ C3 & corrosion protection & & & \\ X\({}_{1}\) & Construction defect & X\({}_{17}\) & Construction problems & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Symbols and corresponding events in the ethane storage tank leakage risk fault tree.