## 2 Health Condition Monitoring
### Condition Monitoring
#### 2.3.1 Categories of Condition Monitoring  
There are many categories of condition monitoring such as online and offline, local and remote, data and video, equipment-level, and system-level. The condition monitoringof some special equipment, such as marine diesel engines, can be divided into conventional sensor monitoring and oil monitoring.  
1. Equipment-level condition monitoring  
The actual state monitoring methods often integrate a variety of monitoring methods. [PERSON] et al. proposed a method combining vibration data and performance data (cylinder pressures) to realize diesel engine condition monitoring [58]. [PERSON] et al. performed the condition monitoring by constructing the thermodynamic model of the main and auxiliary diesel engines [59]. To further enhance the oil monitoring technology into marine field application, [PERSON] et al. proposed a new online condition monitoring and remote FD system for marine diesel engines. The new system consisted of three parts, such as online signal acquisition, remote feature extraction, and fault diagnosis in the laboratory center [60]. To further improve the performance of monitoring and diagnostic, some researchers have made many contributions to the intelligent condition monitoring of MSAE. [PERSON] et al. presented a novel approach for clustering data containing measurements of physical parameters for a marine diesel engine cylinder. This model monitored the condition of the main engine by clustering parameter measurements under normal conditions [61]. [PERSON] et al. illustrated a fuzzy modeling approach utilizing IF-THEN rules and demonstrations of its usefulness [62];
2. System-level condition monitoring  
Different from single equipment condition monitoring, the system here can be large equipment on board, such as marine diesel engine and marine generator, or a combined system, such as the ship propulsion system, which can be further extended to the engine room monitoring system. At present, ship system-level condition monitoring mainly focuses on the diesel engine, propulsion system, cooling system, lubrication system, fuel system, and so forth. [PERSON] et al. presented a novel methodology for system-level engine performance monitoring, which utilizes non-report data with minimal data assumptions [20]. Condition monitoring was applied to the marine gas turbine propulsion system, and the real-time monitoring of system condition was realized [63]. Except for the proposed condition monitoring for the marine systems, ship AMS were implemented on modern vessels to support the efficient O&M [64]. A modular solution example of such a system is SeaPerformer which has been successfully implemented on a large number of merchant ships;
3. Visualization tools for condition monitoring  
There are many ways to express the results of condition monitoring, which can be roughly divided into curves or form (Figure 4a) or graphs (Figure 4b), and so forth. [PERSON] et al. judged the severity of engine fault according to the diesel engine images using the online ferrographic sensor [60]. [PERSON] et al. used self-organization mapping (SOM) topology graphs and tables to show the clustering of critical performance parameter data of engine cylinders to provide helpful data insight [61]. These curves or images can more simply and intuitively describe the health state of the diesel engine. However, it is usually only applicable to the case with a single parameter or few parameters. When monitoring the status of a multi-parameter system, the multi-parameter description method needs to be adopted. the radar chart method is usually used to describe the changes of system function [65], as shown in Figure 4c.