postulated that MASS would operate under one of four Degrees of Autonomy (DoA) as presented in Table 1[7] with a capability of dynamically switching between these. Such a switch may be triggered by changing environmental conditions, for instance [9]. As can be seen, one of the modes of operation is the above-mentioned remote control with or without seafarers' presence onboard. It is also expected that this type of MASS will be easier to implement than a fully autonomous one, for various reasons [10; 11]. Among them is the issue of safety [12], often named as the most important aspect of the operation of any socio-technical system. Most of the authors agree that the newly implemented autonomous ships shall be at least as safe as conventional ones [13; 14; 15], regardless of the DoA. Meanwhile, it has been raised that fully autonomous vessels will not necessarily meet this criteria [16], especially that they are expected to contribute to improve the maritime safety by eliminating, or at least greatly reducing, the occurrence of human errors [17]. However, no matter how autonomous the system is, some chances for the emergence of human factors remain [3; 18; 19]. Humans would need to be involved in the operations of the autonomous system at various points ranging from design [15] through maintenance [8] and trouble-shooting [20; 21; 22; 23; 24]. With respect to this issue, [PERSON] et al. applied an expert focus group method for identifying human factors present in the remote control of a ship [25]. [PERSON] et al. used probabilistic methods for modelling human-autonomy collaboration [26]. [PERSON] et al. investigated interrelations between tasks to be carried out by remote operators [27]. [PERSON] et al. also applied a non-participating observation of simulator runs, in order to study how seafarers handle remote supervision and control [28]. Similar ideas were investigated by [PERSON] et al. [29]. [PERSON] et al. investigated how certain risk factors can affect navigation of a remotely controlled ship on different legs of her voyage [30]. Despite all the above contributions, the question of what types of the human factors can potentially have the greatest influence on the occurrence of accidents involving remotely controlled vessels remains unanswered. The herein study aims to bridge this gap by collecting expert-elicited data to enhance the understanding of the development of potential accidents involving remotely-controlled merchant vessels. To this end, we employed a Human Factors Analysis and Classification System for Maritime Accidents (HFACS-MA, see Figure 1) [31] along with an expert study. The results suggest that factors falling under Condition of operator(s) and Failure to correct known problem categories are believed to affect the safety of remote vessels the most. This indicates the need of (1) proper safety management of the system (taking early and appropriate corrective actions) as well as (2) taking the condition of (remote) human operators into account even with the vessels themselves being unmanned. \begin{table}
\begin{tabular}{c c c} \hline
**DoA** & **Description** & **Definition** \\ \hline \multirow{4}{*}{1} & Ship with automated & Seafarers are on board to operate and control \\  & processes and & shipboard systems and functions. Some \\  & decision support & operations may be automated. \\ \multirow{2}{*}{2} & Remotely controlled ship with & The ship is controlled and operated from another \\  & seafarers on board & location, but seafarers are on board. \\  & Remotely controlled ship & Remotely controlled ship without seafarers on \\ \multirow{2}{*}{3} & Remotely controlled ship & board: The ship is controlled and operated from \\  & without seafarers on board & another location. There are no seafarers \\ \multirow{2}{*}{4} & \multirow{2}{*}{Fully autonomous ship} & The operating system of the ship is able to make \\  & & decisions and determine actions by itself \\ \hline \end{tabular}
\end{table} 
Table 1: Definitions of International Maritime Organization’s degrees of autonomy [7].