Enclosed space entry on ships: the procedure under IMO MSC.581(110)

What counts as an enclosed space on a ship?

MSC.581(110) defines an enclosed space as one that may contain a hazardous atmosphere or lack of oxygen and has any of these characteristics:

• limited openings for entry and exit;
• inadequate ventilation; or
• not designed for continuous human occupancy.

The definition names cargo holds, bilge spaces, ballast and other tanks, pump-rooms, chain lockers and engine crankcases, but is not limited to them. The revised recommendations also warn that unsafe atmospheres can build up in places crews visit every day, such as forecastle stores, compressor rooms and inert gas rooms, when they connect to a hazardous space.

Two new definitions matter in practice:

What changed from A.1050(27) to MSC.581(110)?

MSC.581(110) keeps the structure of the 2011 recommendations but adds a company-level safety strategy and several firm expectations. The resolution invited the IMO Assembly to revoke A.1050(27). The main changes for operators:

What is an Enclosed Space Register?

An Enclosed Space Register is the ship-specific list of every enclosed space on board, with the information needed to assess entry. MSC.581(110) says it should be kept up to date, particularly after loading cargoes that may affect a space, and should form the basis of the enclosed space emergency response plan. The register should record:

• the layout of the space, access and exit points, and connected spaces;
• physical hazards such as vertical ladders, unguarded openings, poor lighting or heat;
• connections to adjacent spaces;
• specific hazards, such as the effect of ballast water treatment on the atmosphere in ballast tanks;
• fixed and portable ventilation, and the time needed to achieve safe air changes;
• lighting, including intrinsically safe lighting where needed;
• the means of atmosphere testing;
• locking and "safe to enter" or "unsafe to enter" signage;
• the rescue equipment needed for that space.

The register should be reassessed when a space's contents change, for example when treated sewage or grey water is stored temporarily in a ballast tank.

What is the enclosed space entry procedure on ships, step by step?

The procedure below follows sections 4 to 8 of MSC.581(110). Your SMS may add steps for your ship types and cargoes.

1. Assess the risk. A competent person assesses the space using the register, cargo information and Safety Data Sheets, and decides whether there is minimal risk, a risk that could arise during the work, or an identified risk to health or life. The space is treated as hazardous until proved safe.
2. Authorise entry. No one opens or enters the space without authority from the Master or the responsible person nominated by the Master. Where any risk to health or life is identified, an entry permit is issued, valid for no more than eight hours. Hot work, isolation or working at height permits may also be needed.
3. Isolate and ventilate. Hazards are isolated as far as possible and the space is thoroughly ventilated.
4. Test the atmosphere. With ventilation stopped and conditions stable, a trained person tests at several levels using calibrated instruments.
5. Prepare communication and rescue. Communication and evacuation signals are agreed and tested, an attendant is nominated and briefed, and rescue and resuscitation equipment is ready at the entrance with an agreed rescue plan.
6. Equip the entry team. Personnel wear calibrated personal gas detectors and the right PPE. At least two people enter; single person entry is not permitted.
7. Monitor during the work. The atmosphere is retested frequently, ventilation is kept running, and anyone leaves at once if conditions deteriorate or ventilation fails. After a break, the atmosphere is retested and recorded before re-entry.
8. Close out. When work ends, the space is secured against entry and its signage updated.

As far as practicable, entry should take place in daylight or normal working hours, when reserve personnel are available.

What atmosphere readings make a space safe to enter?

MSC.581(110) asks for steady readings of all of the following, after a risk assessment targeted at the space:

If these conditions cannot be met, the space is ventilated further and retested. If the atmosphere remains or is suspected to be unsafe, section 9 applies: positive-pressure breathing apparatus, personal gas detectors, rescue harnesses and, unless impractical, lifelines. Emergency escape breathing devices are not suitable for entry. The resolution notes that national requirements may set different ranges, and that testing for oxygen or flammability does not measure toxicity.

What do SOLAS regulations require for enclosed spaces?

MSC.581(110) is a set of recommendations. Two SOLAS regulations make parts of it mandatory:

• SOLAS III/19: enclosed space entry and rescue drills. In force since 1 January 2015 (resolution MSC.350(92)). Under paragraph 3.3, crew members with enclosed space entry or rescue responsibilities take part in a drill at least once every two months. Under paragraph 3.6, each drill includes checking and using PPE, communication equipment and procedures, atmosphere testing instruments, and rescue equipment and procedures, plus instruction in first aid and resuscitation.
• SOLAS XI-1/7: atmosphere testing instrument. In force since 1 July 2016 for SOLAS cargo ships of 500 GT and above. Ships carry an appropriate portable instrument able to measure at least oxygen, flammable gases or vapours, hydrogen sulphide and carbon monoxide. A personal detector worn inside the space does not meet this requirement.

MSC.581(110) also links entry procedures to the ISM Code: they should be among the key shipboard operations under section 7, and crew training should follow sections 6.3 and 6.5. It recommends reviewing the emergency response plan after each drill. See our ISM Code and SMS guide.

What do vetting inspectors check on enclosed space entry?

Vetting inspectors check that the procedure exists, that the records show it was followed, and that the crew can explain and demonstrate it.

For dry bulk and general cargo ships, RightShip's data puts enclosed space entry at the top of its high-risk findings. In its November 2025 webinar, RightShip reported 529 high-risk findings against RISQ question 4.5 (enclosed space entry procedure, evidence it was followed, and crew training) across 4,985 inspections from January to 12 November 2025. That was the largest number for any question. See our RightShip RISQ 3.2 guide.

For tankers and gas carriers, SIRE 2.0 inspectors test crew familiarity through human response questions, so officers may be asked to talk through a recent entry permit, a gas test or the rescue plan. See our SIRE 2.0 guide.

Common gaps to check on your own ships:

• no Enclosed Space Register, or one copied from a sister ship;
• permits with no stated validity, or left open past the shift;
• gas readings recorded as "OK" instead of values, or without the instrument's calibration date;
• drills recorded every two months with no detail of what was practised;
• attendants who are unsure whether they may enter to help (they should not, until help arrives);
• rescue equipment that has never been tried at the access openings of the ship's own spaces;
• shore workers entering holds without a joint risk assessment.

An example from our audits. A pattern we meet on board: the permit, the gas readings and the rescue plan are all in order on paper, but when the crew walk through a rescue from the space named on the permit, the plan does not fit the space. The stretcher will not pass the access opening, or the tripod and lifeline are stowed at the other end of the ship. Only a drill in the actual space finds this, which is why drills should move between different spaces and the rescue plan should be reviewed after each one.

How should a company prepare its ships for MSC.581(110)?

Start with the documents, then test them on board:

1. Revise the SMS procedure against MSC.581(110) and any flag circular, including single person entry, permit validity and the new definitions.
2. Build an Enclosed Space Register for each ship from a walk-round, not a template.
3. Check gas detection equipment: carriage, calibration, bump testing and CO2 capability.
4. Name competent and responsible persons and record their training, including who is competent to calibrate and use the instruments.
5. Run realistic drills in different spaces, and review the rescue plan after each one.
6. Brief shore personnel with a schematic plan at the gangway.
7. Audit it. Include enclosed space entry in internal audits and the management review.

To check your own procedure against the resolution, use our compliance checklist.

Electronic permits help keep the permit, risk assessment and gas readings together. PTW X7 is the permit to work module of Marine X7. For how permits work for hot work and enclosed spaces, see our article Permit to work system for ships: hot work and enclosed spaces.

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