# Ship's Digital Twin--A Review of Modelling Challenges and Applications
## 4 Applications
### Maintenance and Condition Monitoring  
Condition-Based Maintenance (CBM) is of great importance to the maritime industry as it can prevent unexpected failures due to the wear-off and/or stress fatigue that occur over time, and which can cause significant expenses or even disasters. The DT concept for the purpose of CBM is further justified as the blackout incident of the Viking Sky cruise ship could have been avoided, as stated in [51], by simulating and detecting possible fault scenarios using the DT of a ship. The incident occurred due to the loss of lubrication oil pressure of generator diesel engines, caused by heavy sea and low lubricating oil levels in the tank. Such a failure forced engines into a shutdown procedure, which caused a blackout and, consequently, loss of propulsion and near grounding, thus forcing the crew and passengers to abandon the ship. The effect of propeller and hull fouling on ship speed was estimated in [8]. The fouling process is relatively fast and its impact on the overall efficiency of the ship can be estimated to within a few months, which opens opportunities for maintenance optimisation as a typical sailing interval takes about four to five years. The research results presented in [8] showed positive correlations between the hull and propeller condition and the speed loss. Moreover, inadequate hull and propeller performance is estimated to reduce the efficiency of the entire world fleet by 9-12%, as stated in [8]. As the data acquired can be used to make predictions and track the actual condition, it is possible to schedule maintenance according to these calculations, i.e., DT's predictions. Furthermore, the recorded data may be used to determine maintenance efficiency, based on the recorded condition before and after maintenance activity. DTs can be used to simulate, analyse, and predict environmental effects such as wind, waves, and ocean currents on the ship structure. In [16], analysis was performed to estimate the fatigue damage and calculate the cumulative damage based on the collected data on a real cruise ship during 970 days of operation. The resulting fatigue damage estimates were further used to conclude the structural damage and remaining fatigue life of a cruise ship balcony. In [20], analysis was performed for the purpose of condition monitoring and maintenance of a ship drivetrain, and vibration analysis. The results indicated a lower simulation accuracy with the increase in model fidelity, thus dictating further research as low-fidelity models were found inappropriate for digital twin applications. In [11], research was conducted for the purpose of condition motoring of ship structures, thus identifying the ship structures to be monitored and adequate strain sensor location. The paper [25] proposed a DT of a ship electric power system for the purpose of the offline simulation of maintenance and subsystem upgrades. Research has been conducted to investigate requirements for the MBD methodology, as well as the HIL (Hardware-in-the-Loop) capabilities of proposed methodology. In [13], implementation possibilities of the DT concept were researched by conducting full-scale measurements on a ship, thus installing sensors and analysing acquired data. The results of the research imply further actions such as the implementation of real-time data acquisition, processing, and transmission infrastructure ensuring benefits for the stakeholders of a ship by means of optimising maintenance intervals, remote assistance, structural stress analysis, autonomous operation, and improving the safety of navigation. In [26], a DT of a ship-synchronous generator was proposed for the purpose of condition monitoring and mechanical defects identification. Transient analysis was conducted, making it possible to identify abnormalities in the frequency spectrum of the forces, electrical current, and voltage, thus identifying mechanical malfunctions. The proposed approach still needs to be validated.