Introduction
An enclosed space is any space which may contain a hazardous atmosphere or oxygen deficiency and which has limited access, inadequate ventilation or is not designed for continuous occupancy and includes, but is not limited to, cargo holds, ballast and fuel tanks, pumprooms, cofferdams, bilge spaces, chain lockers, void spaces and engine crankcases. The activity of entering enclosed spaces on ships presents a significant blend of familiarity and danger. Despite decades of established guidelines, training and lessons learned from past incidents, enclosed space entry continues to result in serious accidents and loss of life across all sectors of the maritime industry.
Enclosed space entry should be treated as a critical shipboard operation within the Safety Management System (SMS), requiring effective planning, risk assessment and strict adherence to established procedures, supported by coordination between ship and shore personnel. Although regulations, guidance and training are widely available, incidents continue to occur across the industry.
Experience from incident investigations indicates that such occurrences are rarely the result of a lack of procedures, but are more commonly associated with poor judgement, incorrect assumptions and insufficient hazard recognition. Personnel may enter a space under the assumption that it is safe and become exposed to oxygen-deficient or toxic atmospheres, with subsequent rescue attempts sometimes leading to additional casualties. These incidents may involve experienced personnel and are typically linked to a combination of factors including inadequate planning, procedural deviations, ineffective communication and a failure to fully recognise the risks associated with enclosed spaces.
The IMO Resolution MSC.581(110) establishes clearer expectations and a more structured approach to enclosed space entry, with increased emphasis on hazard identification, risk assessment, atmospheric testing and emergency preparedness to support consistent, ship-specific practices. This Risk Alert outlines the practical implications of these requirements and highlights areas where breakdowns in control may occur.
Why Enclosed Spaces Remain So Dangerous
Enclosed spaces can be misleading. Many do not appear hazardous, are entered frequently and may have been accessed safely on numerous occasions. This familiarity can reduce caution and repeated safe entry should not be taken as an indication of continued safety, as conditions may change without warning. A fundamental characteristic of an enclosed space is its potential for rapid atmospheric change, often without any visible indication. A space confirmed as safe at one point in time may quickly become unsafe due to oxygen depletion, the accumulation of toxic gases or the build-up of flammable vapours, which can make the space lethal within minutes.
Carbon dioxide (CO₂) has been identified as a contributing factor in a number of recent fatalities, particularly in spaces associated with organic cargoes, bulk cargoes, cargo residues and fumigation operations. Unlike many hazards, CO₂ provides no sensory warning and a space may appear normal immediately prior to incapacitation.
Incident reports have also shown that secondary casualties are common, as the instinct to assist a collapsed colleague can lead to unplanned rescue attempts and multiple fatalities. The revised IMO guidance places increased emphasis on preventing such occurrences through improved hazard identification, control measures and preparedness.
What latest IMO Guidelines Change in Practice
The revised guidance reinforces that entry into enclosed spaces is a high-risk activity requiring deliberate control, rather than a routine task. A fundamental principle is that enclosed spaces should not be entered until they have been assessed, tested and confirmed safe. This approach must be applied consistently, regardless of prior experience with the space or perceived operational urgency, recognising that previous safe entry is not an indication of current safety. The revised guidance also strengthens expectations around planning and supervision. Entry shall not be undertaken as an isolated activity and all entries must be properly planned, risk assessed and controlled, with a trained attendant present at all times and empowered to stop the operation if conditions deteriorate.
In addition, the guidance introduces and clarifies key definitions to support more effective hazard identification, risk assessment and incident analysis. Hazards may also arise from connected or adjacent spaces and should be considered as part of the assessment. These include definitions of enclosed spaces, connected and adjacent spaces, trapped hazardous atmospheres and the roles of competent persons, responsible persons and attendants, together with the requirement for a ship specific enclosed space register. Collectively, these updates are intended to improve industry consistency in identifying risks and managing enclosed space operations in practice.
Atmospheric Testing / Raising the Bar
Gas detection equipment shall be maintained, calibrated and bump-tested in accordance with the manufacturer’s instructions and company procedures. Testing should be carried out by a competent person. Personnel should be trained not only in the operation of the equipment, but also in understanding its limitations, including sensor response, alarm settings and potential cross-sensitivities, as well as the need to test at representative locations within the space. Prior to entry and throughout occupancy, atmospheric testing shall confirm that conditions are within safe limits. As a minimum, the atmosphere should be verified as follows –
- oxygen content at or above 20.9% by volume;
• carbon dioxide below 0.5% (5,000 ppm);
• flammable gases or vapours at less than 1% of the Lower Flammable Limit (LFL), which represents the minimum concentration at which a gas or vapour can ignite; and
• toxic gases or vapours below 50% of the relevant Occupational Exposure Limit (OEL), which defines the maximum concentration to which personnel can be exposed without adverse health effects.
These limits provide a safety margin to account for instrument accuracy, potential stratification and the possibility of rapid changes in atmospheric conditions. Atmospheric conditions shall not be assumed to remain stable. Testing must remain valid for the duration of the work and repeated as necessary.
Vessels should ensure that gas detection equipment is suitable for the ship type, cargo history and associated hazards. Standard four-gas detectors may not be sufficient where carbon dioxide monitoring is required.
Where personnel are required to enter or remain within an enclosed space, the use of personal gas detectors capable of continuous monitoring should be considered, particularly in situations where atmospheric conditions may change during the work.
Cargo history should be taken into account when assessing potential hazards. Particular caution is required following the carriage of organic cargoes, coal, wood products, grain, fishmeal, steel products subject to oxidation, fumigated cargoes, petroleum products and chemicals, as well as in inerted spaces and spaces adjacent to cargo or ballast tanks. The absence of smell shall not be taken as an indication that the atmosphere is safe. Relevant cargo information, including Safety Data Sheets where applicable, should be considered.
The Enclosed Space Register – From Generic to Ship-Specific
One of the key improvements introduced by the latest IMO guidance is the requirement for each vessel to maintain a ship-specific Enclosed Space Register as part of its SMS. The Enclosed Space Register shall be maintained on board and ashore and kept up to date to reflect changes in cargo, ballast, contents or operational conditions that may affect the risks associated with a space. The register should be supported by a risk assessment for each space and used to identify hazards, assess risks and determine the control measures required prior to entry.
The Enclosed Space Register serves a purpose beyond simply listing cargo holds, ballast tanks and other enclosed spaces. It is intended to document the characteristics, hazards and associated control measures for each enclosed space, including connected and adjacent spaces, and how their atmospheres may be influenced by operational or cargo-related factors.
As a minimum, the Register should contain sufficient information to support effective hazard identification, risk assessment and safe entry planning for each space. Accordingly, the Register should include –
- all enclosed spaces on board, together with connected and adjacent spaces;
• the physical layout of each space, including access and egress points and any limitations affecting entry, exit or movement;
• physical hazards within the space, including vertical ladders, unguarded openings, poor lighting, wet or slippery conditions, excessive heat or other configuration-related hazards;
• hazards associated with each space, including how the atmosphere may change depending on cargo, ballast, contents or operational conditions;
• specific atmospheric hazards, including oxygen depletion or enrichment, toxic gases, flammable vapours and the potential for trapped hazardous atmospheres;
• ventilation arrangements, including fixed or portable systems, equipment location and the estimated time required to achieve safe atmospheric conditions;
• arrangements for atmosphere testing, including the means of testing and representative sampling locations;
• where applicable, information related to any additional technology used to determine the condition of the enclosed space;
• lighting arrangements, including temporary or intrinsically safe lighting where appropriate;
• locking arrangements and appropriate “safe to enter” / “unsafe to enter” signage;
• cargo- and contents-related considerations, including previous and current cargoes, fumigation, residues, fuel oils, slops, ballast operations and any oxygen-depleting conditions;
• equipment necessary to facilitate emergency rescue from the space; and
• any additional information necessary to support risk assessment, safe entry planning and emergency response arrangements.
When properly utilised, the Enclosed Space Register supports risk assessment, permit-to-work processes and emergency planning. If it is treated only as a documentation exercise, its effectiveness is reduced. The Register should be used to support decision-making based on the conditions at the time, rather than being relied upon as a static record.
The permit-to-work system should be time-limited and should not be treated as a standing approval. If ventilation stops, the work team leaves the space, conditions change, cargo or ballast operations commence, or there is any doubt regarding the atmosphere, the permit should be suspended and the space re-tested before re-entry.
Emergency Response and Planning
The updated IMO guidance places significant emphasis on the need for planned and ship-specific emergency response arrangements for enclosed spaces. Experience indicates that one of the most common causes of additional casualties is the attempt to rescue after an individual has collapsed in an enclosed space without adequate planning or control.
Effective emergency plans should be practical, clearly structured and regularly rehearsed. They should define roles and responsibilities, identify the equipment required, establish communication arrangements and ensure that decision-making authority is clearly understood.
Risk assessment should not be limited solely to the space intended for entry. Connected and adjacent spaces may contain trapped hazardous atmospheres or allow gases to migrate through pipelines, ducts, valves, structural openings or residues. These spaces should therefore be considered potentially hazardous until they have been appropriately assessed, isolated where necessary, ventilated and tested.
Prior to entry, all relevant systems should be effectively isolated and secured. This includes, where applicable, valves, pipelines, pumps, agitators, hydraulic systems, electrical equipment, cargo systems and inert gas lines.
Lock-out/tag-out procedures should be applied as appropriate, and all isolations should be verified before entry is authorised.
Drills conducted in accordance with SOLAS requirements should assess not only the readiness of equipment but also the ability of personnel to respond appropriately. This includes the ability to recognise when it is unsafe to enter a space, which is often a critical factor in preventing further casualties.
Enclosed space entry drills should, where appropriate, reflect realistic emergency scenarios, including casualty recovery and should verify communication arrangements, coordination between personnel and the use of rescue and safety equipment. They should also allow for the assessment of response time and competence and reinforce the need to avoid unplanned or impulsive rescue attempts.
Conclusion
Enclosed space fatalities stem not from obscure hazards or intricate technical failures, but from well-known risks, recurring issues and preventable decisions. The revised IMO recommendations offer a clearer and more structured framework. However, their effectiveness depends on their effective implementation on board vessels.
Care should be taken where shore personnel, surveyors, contractors, stevedores or terminal representatives are involved. Such personnel may not be familiar with the vessel’s configuration, previous cargoes, ventilation arrangements or the presence of connected or adjacent spaces. Entry should not be permitted unless the vessel’s enclosed space procedures have been fully followed. This includes atmosphere testing by a competent person, the issue of a valid entry permit, the establishment of effective communication arrangements and the availability of appropriate rescue equipment and procedures.
Effective control of enclosed space entry is not achieved through procedures alone, but through their consistent application and sound judgement in their use. Decisions should be based on current conditions and not on assumptions derived from previous experience.
This Risk Alert is intended to assist Members in their loss prevention efforts and should not be considered a substitute for flag State requirements or the procedures contained within company Safety Management Systems.
