# A Review of Autonomous Tugboat Operations for Efficient and Safe Ship Berthing
## 3 Previous Research
In the work by [PERSON] [27], the vessel to be towed was modeled as a system with three active rudders, and tugboats were independently modeled. The modeling process considered both the dynamics of the tovline and the ship characteristics, such as the length of the towline and relative position of the tugboat. In this work, the leader-following control technique was applied to control the mothership. In the work by [PERSON] [28], the contact between the ship and each tugboat was considered as a point contact, and the speed constraint at the contact point was modeled. This work is notable because, for the first time, it considered the towing problem as a multibody dynamics problem that took into account several components, such as the ship, tugboat, towline, and environmental disturbances individually. A tracking control method was applied, which could manipulate the mothership to follow a straight path while maintaining the desired heading angle using tugboats. Lastly, [PERSON] [29] (2021) applied a hybrid system modeling technique in their study that considered ships, tugboats, and the continuous movement of the tugboats. Control was achieved using the PPO (Proximal Policy Optimization) algorithm, which is a \begin{table}
\begin{tabular}{c c} \hline \hline
**Reference** & **Main Features** \\ \hline
[18] & A tugboat is assumed (modeled) as a single thruster (2006) \\ \hline
[19] & Adaptive control techniques to compensate for the fluidity of hydrodynamic coefficients and unknown contact points (2007) \\ \hline
[2] & Uses RPI (redistributed pseudoinverse) algorithm, an improved thrust allocation algorithm for improved controllability (2011) \\ \hline
[24] & Modeling considering environmental disturbances such as wind, tide, and waves applying the SMC (Sliding Model Control) technique, one of the robust control techniques that is resistant to environmental disturbances such as wind, tide, and waves (2011) \\ \hline
[27] & Modeling considering the dynamics of the tug line leader-following control that follows the route by considering the tug line between the tug and the ship to be towed (2019) \\ \hline
[28] & Modeling considering multibody dynamics considering ships, tugboats, and environmental disturbances using simplified dynamic equations (2020) \\ \hline
[29] & A hybrid system modeling technique that considers ships and tugboats, and the continuous movement of tugboats is applied. Application of control method using PPO (Proximal Policy Optimization) algorithm, which is a reinforcement learning algorithm considering the actual port environment (including obstacles), simulated using PYGAME (2021) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Representative research for ship berthing using autonomous tugboats. reinforcement learning algorithm suitable for continuous action spaces. After applying the technique, it was tested in the Pygame simulation environment to confirm its usefulness. From the studies that applied control techniques from the early 2000s to recent years, where studies considered actual operation methods and applied control methods using artificial intelligence technology, the technology for autonomous berthing of ships using tugboats has significantly developed. Adaptive control techniques were introduced to compensate for uncertainties, while robust control techniques were considered to deal with environmental disturbances. However, most papers related to ship berthing using tugboats focused on modeling and control methods that assumed fully autonomous tugboats without addressing perception technology through sensors, such as cameras, AIS (Automatic Identification System), and radar, as well as optimal route creation, task allocation algorithms, and decision-making technology to maintain formation within the swarm. Therefore, to analyze previous research on the perception and decision-making aspects, we also searched for research work on single tugboats, USVs (Unmanned Surface Vehicle), or other robotics fields that could be applied to the ship berthing domain.