# A Review of Autonomous Tugboat Operations for Efficient and Safe Ship Berthing
## 3 Previous Research  
In recent years, there has been a growing interest in the development of autonomous tugboats owing to their potential benefits in terms of cost-effectiveness and safety. To gain a comprehensive understanding of the progress and trends in this field, we present Table 2, which offers an overview of related work conducted from the early 2000s to recent times. The survey table categorizes the related works based on their distinct characteristics, such as perception, decision-making, modeling, and control techniques. Such classification enables researchers to gain a better understanding of the various approaches and methods utilized in the development of autonomous tugboats, thus identifying gaps and opportunities for future research and development in this promising field. Perception:  
* P1--Localization technology;
* P2 --Sensor fusion technology. Decision Making:  
* D1--Path Planning:  
* A. Path planning in the open sea;
* B. Path planning in a port environment;
* C. Reactive path planning. * D2--Determining the swarm formation;
* D3--Algorithm for task allocation. Control:  
* C1--Adaptive control;
* C2--Robust control;
* C3--Reinforcement-learning-based control;
* C4--Tracking control. Modeling:  
* M1--Modeling of tugboat as a thruster;
* M2--Modeling considering environmental disturbance;
* M3--Modeling considering multibody dynamics;
* M4--Hybrid system modeling;
* M5--Modeling using simplified dynamic equations. Infrastructure and Operation:  
* I1--Tugboat planning;
* I2--Allocation problem: \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reference** & **Project** & **Content** &
\begin{tabular}{c} **Participating** \\ **Company** \\ \end{tabular} & **Autonomous Level** \\ \hline \multirow{6}{*}{[16]} & \multirow{6}{*}{[PERSON] (2015)} & \multicolumn{2}{c}{Presenting a conceptual} & \multirow{6}{*}{[PERSON] (2015)} \\  & & & model of an & \\  & & autonomous tugboat & \\  & & considering the twoline & \\  & & between the & \\  & & mothership and the & \\  & & tugboat & \\ \hline \multirow{6}{*}{[17]} & \multirow{6}{*}{[PERSON]/ADP (2020)} & \multicolumn{2}{c}{Aiming to develop the} & \multirow{2}{*}{[PERSON], Abu} & \multirow{2}{*}{Aim for Level 3} \\  & & world’s first remotely & \\ \cline{1-1}  & & operated unmanned & \\ \cline{1-1}  & & commercial tugboat & \\ \cline{1-1}  & & capable of operating in & \\ \cline{1-1}  & & a wide range & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._* A. Berth Allocation Problem (BAP);
* B. Quay Crane Assignment Problem (QCAP). * I3--Mooring System;
* I4--Energy efficiency and eco-friendliness. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Perception** & **Decision Making** & **Control** & **Modeling** & \begin{tabular}{c} **Infrastructure and** \\ **Operation** \\ \end{tabular}  \\ \hline
[18] & - & D1A & C2,C4 & M1 & - \\ \hline
[19] & - & D1A & C1,C4 & M1 & - \\ \hline  
Figure 9 shows the timeline of notable studies, and Table 3 contains brief descriptions of these studies. In an early study in the early 2000s, tugboats were modeled as azimuth propellers installed in motherships, and it was assumed that the tugboats only exerted pushing forces [18]. Following this work, adaptive control techniques were applied in subsequent studies [19; 21; 23] to compensate for uncertainty in the contact point between the tugboat and the vessel, as well as the hydrodynamic parameter values. [PERSON] [2] minimized the power consumption of the tugboat and improved controllability by utilizing the redistributed pseudoinverse (RPI) algorithm to determine the thrust and direction of each tugboat. In a later study [24], the sliding mode control technique, a robust control approach, was applied to environments with disturbances, such as wind, current, and waves. It is important to note that the studies discussed in this paragraph modeled the tugboats as thrusters. Therefore, it cannot be considered swarm control. Moreover, these studies did not consider the towing operation of tugboats using towlines.