# Performability Evaluation of Autonomous Underwater Vehicles Using Phased Fault Tree Analysis
## 3 Performability Evaluation Method Based on Phased Fault Trees
### 4.2. Phased FT for AUVs
#### 4.2.1 FT for Launch  
During the launch phase, the USV conducts separation control, and communication is used to exchange status updates. As a result, consideration is given to the propulsion system, the navigation system for position and attitude control, the communication system for sharing mission and status information, and the controller for separation control. The following Figure 2(a) displays the FTA results. Comprehensive information regarding each specific component can be found in Tables 1 and 2. The sub-FT RA is the FT of the reliability of the driving unit, which considers the state of the motor and power supply and leakage of the driving unit. The sub-FT PA, which includes the performance of the motor, equipment inspection module, power module, and other equipment, is the FT linked to the driving part. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Event** & **Meaning** & **Event** & **Meaning** & **Event** & **Meaning** & **Event** & **Meaning** \\ \hline R1 & Propeller breakage & R12 & Motor overcurrent & R23 & Controller PCM & R34 & Noise \\ \hline R2 & Yaw actuator failure & R13 & Motor aging & R24 & \begin{tabular}{c} Communication unit \\ signal bus failure \\ \end{tabular} & R35 & \begin{tabular}{c} Integrated antenna \\ assembly failure \\ \end{tabular} \\ \hline R3 & Pitch actuator failure & R14 & Motor driver failure & R25 & \begin{tabular}{c} Satellite \\ communication \\ failure \\ \end{tabular} & R36 & \begin{tabular}{c} Forward-looking \\ MBS failure \\ \end{tabular} \\ \hline R4 & Buoyancy pump & R15 & \begin{tabular}{c} Leak in power \\ supply for motor \\ \end{tabular} & R26 & \begin{tabular}{c} WIFI communication \\ failure \\ \end{tabular} & R37 & \begin{tabular}{c} Undersea terrain \\ exploration \\ MBS failure \\ \end{tabular} \\ \hline R5 & Air bladder leak & R16 & \begin{tabular}{c} Motor power \\ transformation \\ module (PTM) \\ \end{tabular} & R27 & \begin{tabular}{c} RF communication \\ failure \\ \end{tabular} & R38 & \begin{tabular}{c} Underwater \\ camera failure \\ \end{tabular} \\ \hline R6 & Short & R17 & \begin{tabular}{c} Motor power \\ charging module (PCM) \\ \end{tabular} & R28 & \begin{tabular}{c} LTE-maritime \\ communication \\ failure \\ \end{tabular} & R39 & \begin{tabular}{c} Side scan \\ sonar failure \\ \end{tabular} \\ \hline R7 & Shaft misalignment & R18 & \
\begin{tabular}{c} Complex navigation \\ computer failure \\ \end{tabular} & R29 &
\begin{tabular}{c} Communication unit \\ serial bus failure \\ \end{tabular} & R40 & Depth gauge failure \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Basic reliability event description of the fault tree. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Event** & **Meaning** & **Event** & **Meaning** & **Event** & **Meaning** & **Event** & **Meaning** \\ \hline R8 & Leak detection failure & R19 & Autonomous control computer failure & R30 & Underwater ultrasonic communication transducer failure & R41 & Pressure sensor failure \\ \hline R9 & Case breakage & R20 & Emergency buoyancy control device failure & R31 & Underwater wireless optical transmitter failure & R42 & CTD failure failure \\ \hline R10 & O-ring breakage & R21 &
\begin{tabular}{c} Leak in power supply for control unit \\ \end{tabular} & R32 & USBL failure & R43 & ADCP failure \\ \hline R11 & O-ring corrosion & R22 & Controller PTM & R33 & GPS failure & R44 & DVL failure \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._  
Figure 3: The design of fault trees for each mission: (**a**) launch, (**b**) mission execution, (**c**) recovery, and (**d**) charging. When changes in the performance factors had a substantial impact on the overall operating state of the system, an OR gate was employed to connect them. The performance factors are connected via an AND gate in situations where a degradation in one component's performance can be compensated for by other components or where the impact on the system as a whole is anticipated to be minimal.