# Ship's Digital Twin--A Review of Modelling Challenges and Applications
## 7 Conclusions  
Over 100 published papers on the topic of a digital twin (DT) are considered in order to investigate the use of DTs in maritime industry, and 51 of those serve as references for writing this paper, while 19 papers are found dedicated to the digital twins of a ship or ship subsystems. The analysis presented in this paper suggests a continuous positive trend in the number of SDT-related publications in the last four years that justifies the research of an SDT concept.  
One of the goals of the work presented in this paper is to investigate the steps of the SDT formulation process and to make a review of the methods used in the process, communication protocols, as well as to describe the applications in maritime industry. An iterative four-step procedure for creating an SDT is proposed, with data acquisition, data processing, modelling, and validation of the model making those steps. The steps proposed in this paper for the SDT formulation are justified as those steps can be related to the steps found in most (52%) of the reviewed papers, and 3 out of 4 steps can be found used in 95% of the reviewed publications.  
Methods used to form DT models are various, ranging from traditional physical-based mathematical modelling to modern data-based modelling approaches such as artificial neural networks. As an SDT concept relies on data, it would be expected for data-based modelling, and especially artificial intelligence, to take over the prime lead in this area.  
Figure 7: Percentage of the application purposes as presented in reviewed papers.  
However, classical physical-based modelling using mathematical modelling and FEM methods were used in 44% of the cases analysed, while data-based modelling was used in 30% of the cases. The results suggest the variability of the methods used and case dependency; hence, the modelling method is chosen based on the application purpose, possibilities, and case-based constraints.  
Based on the review presented in the paper, it can be concluded that the ship's digital twin can be usefully and efficiently exploited in terms of maintenance scheduling, predictive maintenance, fault prediction, real-time fault detection, optimisation of the processes onboard the ship, e.g., fuel consumption reduction, pollution impact reduction, ship's hull condition monitoring, and overall cost reduction, where the most common SDT applications focus on condition monitoring and condition-based maintenance, as found in 42% of the cases reviewed. The analysis showed that the propulsion system (18%), ships structure (18%), and electrical systems (23%) are the most common case-studies.  
Communication standards and protocols used in the SDT concept are well-known and verified by use in various industries through the past twenty years. A commonly used reference model for communication standards comparison is the ISO OSI reference model according to which some of the standards, such as the CIP protocol family, EtherCAT, and MTConnect, cover multiple layers of the reference model. Considering the communication standards available, it can be concluded that one of the major challenges regarding the use of SDTs is the real-time communication with the real object.  
Conceptualization, N.A., P.M. and M.K.; methodology, N.A. and P.M.; formal analysis, P.M. and M.K.; investigation, N.A.; resources, N.A. and P.M.; writing--original draft preparation, N.A.; writing--review and editing, P.M., M.K. and N.A.; visualization, N.A.; supervision, P.M. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding. APC was funded by the Faculty of Maritime Studies, University of Split, Croatia.  
Not applicable.  
The authors declare no conflict of interest.