## 3 Fault Diagnosis
### Epilog  
This section reviews the FD methods on-board. The advantages and disadvantages of different diagnosis methods are analyzed, and their applications in the shipping field are summarized. Although the FD methods have achieved good application onboard, they still face many problems.  
* Under normal conditions, some MSAEs are not allowed to run to failure as an unexpected failure could result in a breakdown of the ship, maritime traffic accidents, casualties, and environmental pollution. Consequently, failure data in various modes are difficult to be obtained in marine fields.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Methodologies** & **References** & **Marine Objects** \\ \hline \multirow{8}{*}{Physics model-based method} & & ([PERSON] et al., [PERSON] et al., & Feedwater system \\  & Bond graph & [PERSON] and [PERSON], & Condenser \\  & [PERSON] et al., [PERSON] et al., [PERSON] et al.) & Power system \\  & & [73; 74; 121; 122; 123] & Diesel engine \\ \cline{2-3}  & Parity space & ([PERSON] et al.) [124] & Dynamic \\  & & & positioning system \\ \cline{2-3}  & Parameter estimation & ([PERSON] et al., [PERSON] et al., & Raisers \\  & [PERSON] et al. [PERSON] and [PERSON], & Current turbine \\  & [PERSON], et al.) [76; 125; 126; 127; 128] & Polusion system \\ \cline{2-3}  & & ([PERSON] et al., [PERSON] et al., & Underwater thruster \\  & Observers & [PERSON] et al., [PERSON] and [PERSON], & Propulsion system \\  & [PERSON] and [PERSON], [PERSON] et al.) & Unmanned \\  & & [78; 129; 130; 131; 132; 133] & surface vehicle \\ \hline \multirow{8}{*}{Knowledge-based methods} & \multirow{2}{*}{Rule-based reasoning} & ([PERSON] et al., [PERSON] et al., [PERSON], & \\  & [PERSON] and [PERSON].) & Power system \\  & & [81; 85; 134; 135] & Diesel engine \\ \cline{2-3}  & Fuzzy logic-based reasoning & ([PERSON] and [PERSON], [PERSON] & \\  & and [PERSON], [PERSON] and [PERSON], [PERSON] & \\  & and [PERSON],) [88; 89; 90; 136] & Boiler Proulsion system Bearing \\ \cline{2-3}  & Neural network-based reasoning & ([PERSON] et al.) [91] & Diesel engine \\ \hline \multirow{8}{*}{Knowledge-based methods} & \multirow{2}{*}{Signal processing} & ([PERSON] et al., [PERSON] et al., & \\  & [PERSON] et al., [PERSON] et al., & \\  & [PERSON] et al., [PERSON] et al., & \\  & [PERSON] and [PERSON].) [42; 137; 138; 139; 140; 141; 142] & Object engine \\ \cline{2-3}  & \multirow{2}{*}{Multivariate statistical analysis} & [PERSON] et al., [PERSON] et al., & \\  & [PERSON] et al.) [41; 143; 144; 145; 146] & Turbine generators \\ \cline{2-3}  & & ([PERSON] et al., [PERSON] and & \\  & & [PERSON], [PERSON] et al., [PERSON] et al., & \\  & & [PERSON] et al., 2020; & Turbocharger \\  & & [PERSON] and [PERSON].) & Ship systems \\  & & [50; 61; 147; 148; 149] & \\ \hline \multirow{8}{*}{Hybrid approaches} & \multirow{2}{*}{Method fusion} & ([PERSON] et al., 2021; [PERSON] et al., & \\  & [PERSON] et al., [PERSON] et al., & \\ \cline{1-1}  & & [PERSON] et al., & \\ \cline{1-1}  & [PERSON] et al., [PERSON] et al.) & & \\ \cline{1-1}  & & [40; 48; 50; 113; 114; 150; 151] & \\ \cline{1-1} \cline{2-3}  & Data and information fusion & ([PERSON] et al., [PERSON] et al.) [37; 114] & Current turbine \\ \hline \hline \end{tabular}
\end{table}
Table 3: Summary of application to MSAE fault diagnosis.  
* Marine machinery, such as main engines, propulsion systems, always works under rough sea conditions. Some external uncertainties from the outside environment (wind, wave, and current) and internal environment (vibration, noise, and electromagnetic) are mixed into the measurement parameters, thus increasing the difficulty of fault pattern recognition.
* Working modes identification is important. The MSAEs work alternately in a variety of working modes. When the modes are different, the optimal value (baseline value) and limits (thresholds) of the same variable will be significantly different, which will increase the difficulty of fault identification.