# Digital Twin in the Maritime Domain: A Review and Emerging Trends
## 1 Introduction  
The technological development of the world is ever incrementing, and humankind is searching for innovative pathways to perform the tasks demanded by industries or services with improved efficiency and effectiveness. The digitalization of the world boomed with the evolution of Cloud computing, Internet of Things (IoT) [1, 2], Big Data analytics, Virtual Reality (VR), Augmented Reality (AR), Artificial Intelligence (AI) [3] with Machine Learning, Deep Learning and Neural Networks, etc. The same has led the way to the development of the Cyber-Physical System (CPS), which is hailed as the epitome of the manufacturing and consumer service sector [4]. It is an automated system with a connection of physical reality featured with computing structures with smart networking tools. The technologies of 5G and Tactile Internet [5], with the provision of ultra-reliable ultra-low-delay, have enhanced communication and feedback between entities across oceans. To create the fusion between cyberspace and physical space, Digital Twin (DT) was introduced as a rational solution in associating the two extents. The concept of DT has become a real application within a limited time, and some disciplines are surpassing the expectations and predictions made during the early conceptual inception of DT.  
In the present world, simulation has become an integral part of system development for every engineering discipline. Starting from solving design problems using numerical algorithms in the 1960s, simulation has taken us to the digital era, where simulation is integrated into the life cycle of the particular product including design, testing, manufacturing, commissioning, operating, and decommissioning [6]. This process of evolution has opened the way to DT, which is more versatile and dynamic than the physical twin concept. Ever since, the concept of DT has become a reality, expanding its outreach to various disciplines around the world including the maritime domain.  
This survey is an endeavor to review and understand the DT technology and the meticulous footprint it has made within the maritime domain to comprehend the future compatibility to meet the upcoming high-tech requirements. Past technological milestones in DT evolution in the marine sector are highlighted in this article, which includes an evaluation of ongoing research and applications around the globe. Further, scientometric analysis has also been performed to identify the key research points such as author contributions, author impact research collaborations, demographic research impact, trending keywords, etc. The main contributions of this publication can be condensed as follows.  
* A comprehensive literature review on DT from the initial notion up to the year 2022 with a focus on the maritime domain.
* Proposed a generic four-layer DT framework for a marine vessel.
* Scientometric analysis of the published research related to marine DT within the duration of 20 years including demographic collaboration and trending keywords.
* The progress of development and implementation of DT technology in the maritime sector, thus providing insight for researchers with key challenges and gaps within the marine DT domain.  
This paper is structured in the following manner. Followed by the Introduction in Section 1, Section 2 describes the origination of the DT concept with nomenclature clarification. In the Section 3, an overview of the implication pathway of DT technology in the marine industry is discussed. In Section 4, various DT-based applications in different sectors in the maritime domain are discussed, while Section 5 describes the prerequisites for a DT-based system along with the overall DT framework for the marine sector. In Section 6, a comprehensive bibliometric study is carried out using the scientometric analysis approach. Section 7 describes the key challenges and future trends. Finally, the conclusion of the entire effort is provided in Section 8.