Maritime Autonomous Surface Ships (MASS): Clarifying the IMO Definitions for Declarations
As digitalisation continues to shape the maritime sector, increasing attention is being paid to Maritime Autonomous Surface Ships (MASS) and the terminology associated with them. This is now becoming evident in documentation and declarations, where Members may be asked to state a vessel’s “degree of autonomy”.
Recent cases have shown that as declarations are updated to reflect MASS, the inclusion of “degree of autonomy” is creating uncertainty for conventionally operated vessels. This is not unexpected, as the terminology stems from ongoing regulatory work by the International Maritime Organization (IMO) and is intended to address emerging MASS operating models rather than routine declarations for conventional ships.
The IMO introduced the concept of “degrees of autonomy” as part of its Regulatory Scoping Exercise for MASS, the outcome of which is set out in MSC.1/Circ.1638. These definitions were designed to support a structured review of how existing conventions, such as SOLAS and COLREG, would apply to ships operating at varying levels of autonomy. They were not intended to serve as a classification tool for conventional ships currently in service, nor do they have any mandatory status.
Within this framework, the IMO refers to four degrees of autonomy. Degree 1 concerns ships with automated processes and decision support, where seafarers remain on board to operate and control shipboard systems and functions, with some operations potentially automated or unsupervised at times, but with seafarers ready to take control. Degree 2 refers to remotely controlled ships with seafarers on board, while Degree 3 applies to remotely controlled ships without seafarers on board. Degree 4 refers to fully autonomous ships capable of making decisions and taking actions independently.
At present, the regulatory framework for MASS is still under development. While a goal-based MASS Code is expected in due course, no mandatory instrument is in force. The only interim guidance issued by the IMO is MSC.1/Circ.1604, which applies only to trial operations, not to routine commercial trading.
It is important to draw a clear distinction between what constitutes a conventional vessel and what falls within the IMO’s concept of automation in the MASS context. A conventional vessel is navigated, operated and controlled by seafarers on board. This remains true even when the vessel is fitted with modern automation, including integrated bridge systems, engine room automation, monitoring systems, alarms and other decision-support tools. These systems are designed to assist the crew but do not replace the crew’s control of the vessel.
By contrast, an automated or autonomous vessel, as contemplated within the MASS framework, is one in which systems are intended to perform operational functions beyond normal crew assistance. This may involve controlling navigation, machinery, or other critical functions without direct onboard human input, or the ability to operate or control the vessel remotely from another location. The distinction, therefore, lies not in the presence of automation itself, but in whether that automation alters the vessel’s operating model by reducing, replacing, or relocating human control.
The source of much of the current misunderstanding lies in the wording of Degree 1. When read in isolation, it can appear to apply to many modern vessels. In reality, however, the automated processes and decision-support systems described are already standard across the global fleet. As such, applying Degree 1 to conventional ships on this basis misinterprets the IMO framework.
For Members, the practical position is straightforward. A conventionally operated vessel with a full complement of crew and no remote control or autonomous navigation capability should be treated as outside the MASS framework for declaration purposes. In such cases, the appropriate response remains “Not applicable”. A degree of autonomy should be assigned only where there is a clearly defined operational model involving remote control, shore-based intervention, or autonomous decision-making beyond the scope of normal shipboard systems.
Where vessels genuinely operate within an autonomous or semi-autonomous framework, the position becomes more nuanced. These cases typically involve trial programmes, dedicated system architectures, and specific approvals from Flag Administrations and Classification Societies. In such circumstances, supporting documentation is expected to demonstrate how the vessel is operated, how oversight is maintained, and how risks associated with reduced or remote human intervention are managed.
The current position reflects the transitional nature of the regulatory landscape. The IMO’s work on MASS is progressing, but until a formal code is adopted, the industry operates under a developing framework. In the meantime, careful interpretation of existing terminology is essential to avoid inconsistencies in declarations and potential misunderstandings about a vessel's capabilities.
Members are therefore encouraged to approach the autonomy classification with care and to ensure that declarations accurately reflect the vessel’s actual mode of operation. In practice, most conventionally operated ships should be declared as “Not applicable” for the vessel's degree of autonomy. Standard onboard automation does not make a vessel a MASS. Where there is any uncertainty, clarification should be sought at an early stage to ensure alignment and consistency.
