# Ship's Digital Twin--A Review of Modelling Challenges and Applications
## 2 SDT Concept Development and Methods Used
### Modelling and Modelling Methods
Furthermore, MBD represents a modelling method entirely based on simulation models, and its use was proposed in [25] to form a DT of a ship electric power system. FEM and the 3D panel method were used in [16] to perform an analysis with reference to the Classification Society ClassNK guidelines for direct load analysis and strength assessment. A DT of a cruise ship structure was created from a 3D model. Combined weather data and data from the AIS (Automatic Identification System) were used as the basis for the analysis using Rayleigh distributions in 3D NAPA software. Ship speed and wave direction were used as input variables for the 3D panel method, where wave scatter plots were created from the time series of wave conditions at specific positions of a ship. In [20], the use of FEM was proposed to analyse the ship drivetrain based on data acquired by implementing sensors on a laboratory drivetrain test rig model. Similarly, ref. [11] proposed an FEM and ANN approach to form two models of a ship structure, while ref. [26] proposed an FEM approach to build the DT of a ship-synchronous generator and conducted transient analysis using wavelet transformations and Fourier analysis. Mathematical modelling of the ship behaviour for the DT development was been proposed in [22], as well as in [18], where the power system of a ship was modelled using mathematical equations along with blackout dynamics and generator load distribution. Few blackout prevention methods have been proposed based on power consumption monitoring, such as fast load reduction. A DT model of a ship's hull, by fusing several models based on FEM, CAD, and machine learning, was proposed in [12]. Ship Power and Propulsion Systems were modelled in [17], in which the diesel engine was modelled by a transfer function with the fuel index as the input variable and the generated torque as the output variable. An alternator was modelled in the dq-reference frame, as a three-phase synchronous machine with salient poles, where the swing equation was used to describe the rotor dynamics. The fuel supply system was modelled as a PI controller that controls the fuel index as a function of the difference between measured and reference speed, i.e., speed error. An automatic voltage regulator (AVR) controlling the excitation voltage was modelled as a regulator with an error as an input, calculated as a difference between measured and reference voltage. Switchboards were modelled as a set of breakers described by resistance. Three-phase induction propulsion motors, powered by AC/AC inverters, were modelled in the dq-reference frame, where direct torque control with space vector modulation was used as the control strategy. Fixed-pitch propeller dynamics were modelled with varying torque as a function of propulsion power and shaft speed, while auxiliary and hotel loads were modelled as three-phase loads with adjustable reactive and active power for different types of load simulation. Batteries were modelled as lithium-ion type with voltage as a function of current and state of charge. The systems were modelled in Matlab/Simulink and wrapped as Functional Mock-up Units (FMUs) as four sub-simulators using the Open Simulator Platform (OSP) architecture based on the FMI (Functional Mock-up Interface) standard as described in [39]. Modelling of a three-phase electrical motor in a dq-reference frame can be found explained in [40]. The BEM was proposed and used in [24] to develop a DT of a scaled ship model used for remote control. The scaled model of a ship was tracked using a stereoscopic system with five reflective targets, while the DP (Dynamic Positioning) system transmitted the rotational speed of the propellers and the azimuth angle to the operator's screen.