# Digital Twin in the Maritime Domain: A Review and Emerging Trends
## 3 Adoption of Digital Twin Technology for the Marine Sector
### Automation of Marine Vehicles  
Automation can be considered the final step of striding towards a fully DT-based environment within the maritime sector. Autonomous ships are under extensive research all over the world and are being implemented due to the versatility of unmanned operations. As per the International Maritime Organization (IMO) regulation enforced in 2021, Maritime Autonomous Surface Ships (MASS) are to be regulated under four separate degrees, which are categorized as per the level of autonomy incorporated into the vessel [37]. A ship may also have the interchange between the four states of autonomy during its operation based on the type of vessel and designated task. This allows flexibility in asset management onboard with crew augmentation as per the dynamics and constraints of specific operations of the vessel. * Degree One: Ship with Automated Processes and Decision Support: Seafarers are present onboard; few functions may be operated onboard at times and operators are on standby and ready to take over control. * Degree Two: Remotely Controlled Ship with Seafarers Onboard: The entire ship will be operated remotely from the shore station, but seafarers are available onboard to take over control at any time. * Degree Three: Remotely Controlled Ship without Seafarers Onboard: The shore operator will control the functions of the entire ship remotely and have no humans onboard. * Degree Four: Fully Autonomous Ship: The intelligent control system of the vessel will take actions based on the decisions made by itself with full autonomy. As per [PERSON], Human Automation Interaction (HAI) in complex systems will be governed by the Level of Automation (LOA), which is highly essential to be considered during the design stage of any autonomous system [38]. * LOA-0: No Automation: Full-time human operation with the assistance of parameter indications and warnings. * LOA-1: Driver Assistance: Acceleration/deceleration/steering will be assisted by the operator by the inbuilt system but performed by the operator themself. * LOA-2: Partial Automation: Steering/acceleration/deceleration would be performed by the automated system and all remaining decisions will be performed by the operator themself. * LOA-3: Conditional Automation: Dynamic actions of the vessel will be performed by an automated system and the operator will intervene if required. Hence, a fully autonomous intelligent surface vessel would be designed to be operated in the LOA under the fourth degree of IMO regulations for MASS. However, careful assessment of the usage is to be analyzed since ships are high-value cost-sensitive assets voyaging for weeks at times without seeing a shore. Thus, autonomous systems must be designed at a high-fidelity robust standard to be capable enough to address all complex situations during the operation of the vessel. Autonomous Marine Vehicles (AMVs) are becoming popular both in commercial and military applications, as extensive research and development are underway all over the world [39]. It ranges from environmental surveys, cargo/passenger operations, hydrography, search and rescue, sea patrols, coastal protection, etc. They can be semi-submersible, conventional planing hulls, semi-planing hulls, and hydrofoils based on the sea surface, and these can be further augmented with the evitability of the limitations imposed by manning ergonomics [40; 41]. LOA can vary by the application and the different scenarios she undergoes during the operations. The applications of AMVs can be primarily divided into two parts: military and commercial. In military applications, DT can be incorporated into various missions in Anti-Submarine Warfare (ASW), Seabed Warfare (SBW), Mine countermeasures, Special operations forces support, Electronic Warfare (EW), Surveillance, Asymmetric Warfare, etc.