Enclosed spaces: what resolution MSC.581(110) changes on board

Enclosed spaces remain the leading cause of multiple fatal accidents on board merchant ships. The scenario never varies: a crew member climbs down into a ballast tank, a forepeak or a chain locker, loses consciousness within a few breaths, and two colleagues die trying to rescue him without breathing apparatus.

On 27 June 2025, at the 110^th session of its Maritime Safety Committee, the International Maritime Organization adopted resolution MSC.581(110), which revokes and replaces resolution A.1050(27), in force since 2011. This is not a tidying-up exercise: the text broadens the notion of dangerous space, sets numerical atmospheric thresholds, and introduces two documents that most ships do not yet carry on board.

This guide sets out what changes, what your safety management system must incorporate, and where to start.

What changes, at a glance

The broadened definitions: the real substantive change

This is the least visible and the most structuring contribution. Until now, the reasoning focused on the space being entered. The new text requires you to reason about what communicates with it.

• Enclosed space — a space with limited openings for entry and exit, with inadequate ventilation, or which is not designed for continuous human occupancy. These three criteria are alternative, not cumulative.
• Connected space — a space linked to a source of hazardous atmosphere by permanent or temporary means, a simple manual door for instance. It must be treated as dangerous until it has been tested.
• Adjacent space — a space sharing a common boundary with a dangerous space, with no opening between the two. The clarification is broad: outside the accommodation, many spaces fall into this category.
• Trapped hazardous atmosphere — an atmosphere that fills or empties at a rate different from that of the source space. In practical terms: ventilating the main tank does not guarantee that the adjacent pocket has cleared.

Two roles are also defined. The competent person has the operational level needed to form an informed judgement on the condition of the space. The responsible person is a management-level officer, authorised to permit entry. These two functions must not be confused, nor held by the same person at the same time.

The Enclosed Space Register

This is the most concrete new requirement, and the one that will be missing at the next audit if you do not anticipate it. Every ship must keep a register listing its enclosed spaces. For each one, it documents:

• the physical configuration and the entry and exit points;
• the hazards, both general and specific to that space;
• the connections with adjacent spaces;
• the means of ventilation and the methods for testing the atmosphere;
• the lighting available;
• the rescue equipment needed for an evacuation from that space;
• the securing and signage arrangements.

The register is not a fixed inventory: it must be reviewed periodically as part of the safety management system. A structural modification, a change of cargo or the addition of a CO₂ room will all make it evolve.

Its operational value goes well beyond compliance. A chief engineer joining a ship for the first time learns in a single reading which spaces call for what — exactly the kind of technical memory that crew rotation erases when nothing has fixed it in writing.

The dedicated emergency response plan

The company must establish an enclosed space emergency response plan, specific to each ship and integrated into the SMS. It targets the improvised-rescuer scenario head-on. It covers:

• raising the alarm and the crew muster points;
• consulting the Enclosed Space Register during the emergency, so as to know what the rescue team is exposing itself to;
• deploying a rescue team equipped with self-contained breathing apparatus;
• appointing an on-scene commander;
• evacuating the casualty and administering first aid;
• coordination with shore-based medical assistance.

SOLAS regulation III/19.3.6 also requires enclosed space entry and rescue drills. The useful question is not whether the drill appears in the drill record, but whether it has ever been run with the actual recovery harness, in the actual space, with a man of the build of the one who would have to be brought out. A drill held in a meeting room prepares you for nothing.

Atmospheric thresholds before entry

The text puts figures on the criteria that the atmosphere must meet before any entry.

CO₂ deserves a particular mention: the new text explicitly designates it as a powerful asphyxiant to be monitored during entry, and not only beforehand. Ships fitted with fixed CO₂ fire-extinguishing installations are directly concerned, as are those carrying certain solid bulk cargoes.

Gas detection equipment

SOLAS regulation XI-1/7 already requires portable atmosphere testing instruments to be carried. The new text sets out the outfit expected:

• at least two sets of gas detection equipment on board every ship;
• two additional sets for cargo ships whose operation requires regular entry into spaces liable to contain hazardous vapours;
• a personal gas detector for each person entering, measuring oxygen, CO₂, flammable gases and toxic gases, as well as any other gas identified by the risk assessment;
• equipment suited to the cargoes actually carried by the ship, current and previous.

This last point is the one that gets forgotten. A standard four-gas detector does not cover the specific vapours of certain cargoes. The question to ask is a simple one: does our equipment measure what this ship carries?

Three rules that admit no exception

1. Entering alone is prohibited. No circumstance justifies it, no duration excuses it. An attendant stays at the entrance, in communication.
2. The entry permit is valid for eight hours at most. Beyond that it has to be renewed, which means testing again. An atmosphere that is safe at eight in the morning is not necessarily safe at six in the evening.
3. Every enclosed space is presumed dangerous until it has been declared safe by a competent person, with measurements to back it up. The presumption always runs in that direction — including for a space entered without incident the day before.

Bringing your SMS into compliance: where to start

The approach comes down to six moves, in this order.

1. Survey the ship's enclosed spaces, taking in the connected and adjacent spaces that the old definition left aside. This is the longest piece of work, and it is done on board, not from ashore.
2. Build the register from that survey, with the expected headings for each space.
3. Reassess the risks in the light of the new definitions, particularly for trapped atmospheres and CO₂.
4. Check the detection outfit: number of sets, personal detectors, suitability for the cargoes, calibrations up to date.
5. Write the emergency response plan specific to each ship and integrate it into the SMS.
6. Train and drill, under realistic conditions, and record those drills.

Approach your flag State and your classification society to find out the arrangements and the timetable for application that they adopt: an IMO resolution of this kind is then carried through into flag instructions.

Keeping the register alive rather than filing it away

An Enclosed Space Register sitting in a binder in the master's office ticks the box at audit and is of no use to anyone on the day a ballast tank has to be opened. Three requirements of the new text translate directly into functions of a technical system.

The register must be available for consultation at the moment of entry, including in an emergency, and therefore from the deck and without a satellite connection. Attaching each space to the ship's asset tree, like any other item of equipment, makes it available where it is actually used.

It must be reviewed periodically. A periodic review with no scheduled due date never takes place. A maintenance plan carries a document review just as well as an oil change.

Permits, measurements and drills must leave a trace, dated and attributed. That is what distinguishes, at audit, a system that is applied from a system that is merely written — the same principle as the one described in our article on work permits and lock-out on board.

To these are added detector calibration and breathing apparatus checks, which are maintenance tasks in their own right, with their own due dates and their own evidence.

Frequently asked questions

Is MSC.581(110) mandatory?

It is a resolution setting out recommendations, and it revokes A.1050(27). Its practical force comes from the way it interlocks with mandatory requirements — the ISM Code, SOLAS regulations III/19.3.6 and XI-1/7 — and from its adoption by flag States and classification societies. At audit and at inspection, this is the text that will be referred to from now on.

Which spaces must appear in the register?

All those that meet the definition of an enclosed space, broadened to connected and adjacent spaces: ballast tanks, forepeaks, chain lockers, tanks, cofferdams, sumps, pipe ducts, CO₂ rooms, hull void spaces. The list is specific to each ship.

Is a four-gas detector enough?

Not necessarily. The equipment must cover the gases identified by your risk assessment, including those of the cargoes carried. CO₂ receives particular attention in the new text.

What becomes of an entry permit beyond eight hours?

It ceases to be valid. Extending the work calls for a new permit, preceded by a new atmosphere test.

Does the register replace the entry permit?

No, the two are complementary. The register describes the space on a permanent basis; the permit authorises a given entry, at a given time, once the conditions have been verified.

In summary

MSC.581(110) does not overturn the principles — test, ventilate, never enter alone — but it broadens the scope of the danger and calls for two documents that many ships do not yet have: an Enclosed Space Register and a dedicated emergency response plan. Producing them takes field work that cannot be improvised the day before an audit.

At Smart Sailors, we design a marine CMMS built so that this kind of requirement lives on board rather than in a binder: documentation attached to the ship's asset tree, scheduled review due dates, permits and drills recorded, and all of it available offline. Request a demonstration on your own procedures.

This article presents the resolution for information only and does not replace the official IMO text or the instructions of your flag State. The arrangements for application depend on the flag, the type of ship and its operation. Refer to your classification society and your maritime administration.