Naval Safety Center




Acquisition Safety -- Confined Spaces

Introduction   |   Discussion   |   Recommendations   |   Conclusion   |   Resources


Confined spaces are compartments or enclosures that have limited openings for entry and exit, are not intended for continuous human occupancy, and are only suitable for temporary work such as inspections, maintenance, or repairs. These spaces may be oxygen deficient, contain fire, explosion or toxicity hazards, or hold liquid, sludge, or solids that create potential engulfment and/or drowning hazards. There are many such confined spaces aboard ship such as cargo holds, tanks, pump rooms, cofferdams, and duct keels. The Gas Free Engineering chapter of the Safety Afloat Manual (OPNAVINST 5100.19) provides perhaps the best overview of the scope of risk:  No routine hazard, the exception of ordnance, is as dangerous as the presence of lethal atmosphere in ship's spaces.  Entering these confined spaces without proper procedures and precautions could cause fires, explosions, bodily injury, illness, or death.

Proper planning in ship, facility, aircraft and other systems design to minimize or eliminate the need for confined space entry can save lives and life cycle costs.  This material provides an overview of risks and some measures for there mitigation that may be considered in system requirements, design and support.



Confined spaces create serious potential hazards that must be addressed during ship construction, use, repair and overhaul, and ship salvage and dismantlement. Working in confined spaces during each of these different ship life cycle phases presents challenges that require planning and Picture showing confined spaces under construction management of these spaces. Confined spaces are typically difficult to enter and exit, especially in an emergency. They may also contain hazardous atmospheres and other safety hazards as described below that could cause serious physical injury or death. General safety hazards in confined spaces include communication problems, entry and exit limitations, fall hazards, physical and mechanical hazards, etc. The U. S. Navy requires that its ships, aircraft, and shore facilities control their confined spaces by requiring special precautions, including training of personnel who work in and monitor these spaces and testing and approval by a certified gas free technician or marine chemist, before authorizing any person or persons to enter a confined space. Because of the special nature of confined spaces found aboard Navy ships, this section of the Acquisition Safety website will concentrate on the challenges of entering and working in shipboard confined spaces and the importance of planning in the ship design phase to minimize or eliminate the need for confined space entry.  Addressing risks of confined space work in the design and maintenance planning stage has the potential to reduce life cycle maintenance costs and risks to maintainers, especially shipyard workers.

Confined space hazards exists in large part because of the requirements of ship, aircraft and facility design to support effective operational performance, while minimizing space and weight and procurement costs.  These conflicting requirements may create potential difficulties in maintenance and require compromises in space configuration and planning for maintenance.

Hazardous Atmospheres 

The internal atmospheres of confined spaces may be oxygen deficient, flammable or explosive, toxic, or oxygen enriched, which may result in the risk of suffocation, fire and explosion or impaired physical capability for persons entering these spaces. A hazardous atmosphere is defined by its potential to disable and/or injure those exposed. It may be characterized by how much it differs from the normal air we breathe. Normal air is defined as approximately 21 percent oxygen, 73 percent nitrogen, a trace of carbon dioxide and a very small trace of other gases such as argon. If levels of these constituents change, whether up or down in concentration, then the atmosphere is considered hazardous. 

Asphyxiating Atmospheres: Oxygen deficiency may be caused by oxidation reactions such as fire or rusting. It also can take place during combustion of flammable substances, as in welding, heating, cutting, and brazing. Other causes for oxygen deficiency include the dilution of air with an inert gas (e.g., nitrogen or argon) and absorption by grains, chemicals, or soils. Normal air contains 20.9% oxygen; once the level drops below 19.5%, the air becomes hazardous to breathe.  As the level of oxygen is decreased in a space, the danger of asphyxiation for anyone entering that space increases. Oxygen deficiency provides minimal sensory warning. Symptoms may include ringing in the ears, dizziness and often-impaired cognitive functions. The victim may initially feel giddy and be otherwise impaired in his ability to sense the onset of problems. 

Toxic Atmospheres: Toxic gases and vapors come from evaporation of fuel and solvents, or may be formed in the process of fermentation and during decomposition of both animal and vegetable material. Welding or brazing with metals such as mild steel, high strength and stainless steel produces toxic metal fumes and hazardous gaseous byproducts; recirculation of diesel exhaust emissions (used to suppress fuel tank atmospheres) will create a toxic atmosphere; and collection and holding tanks (CHTs) for sewage (blackwater) generate hydrogen sulfide, methane, and other hazardous byproducts in the tanks, piping, and valves.

Flammable and Explosive Atmospheres: This condition generally arises from vaporization of flammable liquids (fuels or solvents), byproducts of work such as spray painting and welding, chemical reactions, concentrations of combustible dusts, and desorption of chemicals from inner surfaces (bulkheads and decks) of the confined space. Welding or other hot work may liberate flammable vapors from combustible liquids previously stored in a compartment. Gas free procedures must evaluate not only the present material stored in an area, but must also consider previous cargoes and contents of adjacent spaces.

Other Common Shipboard Confined Space Challenges

Communication Problems: If a worker in a confined space should suddenly feel distressed and is not be able to summon help, an injury could become a fatality. Frequently, the body positions that are assumed in a confined space make it difficult for the standby person to detect an unconscious worker. Visual monitoring of the worker is often not possible because of the design of the confined space or location of the entry hatch. Effective process management and supporting OSHA regulations (29 CFR Part 1915 Subpart B -- Confined and Enclosed Spaces) require provision for communication between worker and monitor and the means for emergency rescue, which must be identified before confined space entry

Entry and Exit Limitation: The time it takes to enter and exit confined spaces may increase the hazards of exposure to the confined space atmosphere. 

Other Physical Hazards: While working in a confined space, workers can become fatigued or be exposed to extreme heat and cold, hazardous noise levels, vibration, or radiation. Drowning hazards include engulfment in sludge and other liquids. Additional physical hazards include inadvertent contact with electrical, rotating, or mechanical equipment, steam or other sources of burning heat, and moving parts. 

Monitoring and Display of Fluid Levels Inside Tanks: Some Navy ships have more than 150 tanks onboard that carry millions of gallons of diesel fuel, aviation fuel, potable water, feed water, oily waste, sewage (blackwater), etc. Other tanks are used for ballasting and list control. Traditionally, tanks installed onboard Navy ships used float type tank level indicators to monitor and display fluid levels in the tanks. These indicators consisted of a series of floats that were wired together inside of the tanks. The floats contained many internal electronic components to determine the tank fluid level. Whenever the floats needed maintenance, the tanks would have to be drained, opened, and freed of harmful gases before workers could enter them (see the Recommendations section for new Navy methodology).

Monitoring Tanks for Corrosion Inspection and Maintenance: Monitoring tanks aboard ships for corrosion maintenance is a Corrosion is a common problem in shipboard tanks very costly confined space problem. Tanks are difficult to inspect and evaluate. More than 4,000 shipboard tanks are opened, cleaned, and inspected each year at a cost of $32 million. Fifty percent of tank work is unplanned due to the inability to monitor tank conditions between inspections, which means that corrosion or other problems may create a repair need during the interval between cyclic inspections.

Fall Prevention for Entering and Working in Confined Spaces: Confined spaces also present the risk of slips, trips, and falls, especially where corrosion has caused rust, which damages ladders, walking surfaces, and anchor points for personal fall arrest systems. Many tanks have no proper fall protection equipment, anchors, or ladders inside. Shipboard tanks can be as deep as 60 feet requiring scaffolding to be set up to perform maintenance. Scaffolding can present another fall hazard. 



The first goal of new ship design should be to eliminate the need to enter confined spaces for maintenance, repairs, or other purposes. Where confined space entry is unavoidable, it is best to minimize the hazards involved in working in a confined space at the ship design stage and during the initial installation of ship equipment. 

Risk factors to minimize:  The forward to the ANSI Z117.19 Standard Safety Requirements for Confined Spaces acknowledges the critical role of design which influences the safety entry of confined spaces and identifies some factors in design that, when overlooked, may increase safety risks.  These include (1) means of entry which are too small, improperly located or designed in a way that complicates or inhibits escape (2) spaces which are unnecessarily obstructed, convoluted or otherwise hazardously configured (3) internal clearance that are too narrow for safe passage (4) spaces with distances which are excessively long and lack alternative means of escape (5) lack of provision for effective energy isolation (6) lack of features that enhance ventilation effectiveness (shipboard confined spaces often have structural partitions that complicate airflow movement) (7) structural weakness in walls, floors or pipes that may increase the hazards to entrants (8) absence of anchor points for retrieval devices and fall protection.

Provide Remote Monitoring and Inspection Systems

Remote monitor/inspection systems and automated cleaning systems eliminate or minimize the need for confined space entry. For example, the Navy is replacing float type tank level indicators with non-intrusive radar (radio detection and ranging) tank level indicators. The radar indicators are mounted externally to the tank. The radar indicator sends a pulse down the tank that is reflected off the fluid level; and the indicator measures the time it takes for the signal to return. The shorter the return time, the higher the fluid level is inside the tank. This signal is then converted into depth in inches to determine tank height. Existing sounding tables can be used to convert the reading from inches into gallons. 

Radar indicators are more accurate than the existing float type tank level indicators. Naval Research Lab's new Tank Monitoring System Sensor Radar indicators can measure fluid level to within one inch and are virtually maintenance free. If a unit fails, it can be replaced with a new one and reprogrammed in about one hour, because everything is external to the tank. It would take days to replace the old float type indicators. Tanks would have to be drained, opened, and freed of harmful gases before personnel could enter the confined space. Replacement of float type level indicators with non-intrusive radar indicators helps to eliminate the need for confined space entry inspection.

The Navy has also installed tank-monitoring systems that measure tank corrosion by monitoring changes in electric potential. Fluctuations in corrosion current and corrosion potential between the electrodes can be analyzed and correlated to rates of corrosion. These measurements provide information on rates of corrosion, stress corrosion cracking, and pitting. Tank Monitoring Systems allow tank inspections to become condition-based maintenance (CBM) instead of time-based as the tank's state of preservation is continuously monitored, making iExample of a robot inspection unit (courtesy of Inuktun Services Ltd)nspections current and avoiding unnecessary and costly confined space entry.

Other examples of remote monitoring/inspection systems and automated cleaning systems include:

•  Use of filters and external pumps to mix the water in tanks or an automated self-cleaning system to avoid the build-up of sludge in tanks.
•  Use of self-propelled video inspection units (some rated for use in hazardous locations), telescopic video inspection units, or telescopic valve stems to eliminate the need for confined space draining, purging, and entry during inspections.

Provide Access for Inspection (if remote sensing/monitoring is not available or entry is required)

Not all spaces and tanks will have remote sensing/monitoring, and for these spaces or tanks safe access must be provided. Condition based maintenance may require more frequent inspections. Designers should provide for ventilation, isolation of supply and drain lines, control of hazardous energy, ladders, anchorage, and walkways where possible. For example, the design of complex CHT tanks' aeration systems should anticipate the need for worker entry and occupancy. (see the McManus reference in the Resources section, which contains an excellent check list for confined space design concerns).

For merchant oil tankers and bulk carriers, there is a new International Maritime Organization (IMO) Safety of Life at Sea (SOLAS) rule requiring permanent means of access (PMA) or alternate means of access to the cargo and fuel storage areas. Since tank design is dependent on the arrangement of transverse and longitudinal bulkheads, designers need to anticipate these safety and health concerns early in the design stages.

Use Materials that Reduce the Need for Maintenance

Designing and selecting equipment should be done on the basis of reducing, even eliminating, worker exposure in confined spaces. Coating systems should be used that have longer service lives so workers do not need to enter confined spaces often. For example, the Navy has used improved commercial off-the-shelf mechanical seals for pump applications that last longer and are easier to install and maintain.

As part of the Rapid Cure Ship Tank Coatings Program, the Navy will utilize advanced coatings to extend the service life of:

•  Sea water tanks from 5 years to 20 years.
•  Collection, Holding and Transfer (CHT) tanks from 2 years to 8 years.
•  Fuel/comp fuel tanks from 5 years to 20 years.
•  Potable water tanks from 5 years to 20 years.

Provide Adequate Ventilation

Ventilation is one of the most effective means of controlling hazardous atmospheres in confined spaces. Providing adequate ventilation in confined spaces avoids build-up of contaminants or combustible atmospheres. Consider designs that will facilitate ventilation of the space. Ventilation modeling, including finite element analysis, may support designs and configurations that reduce purging time, minimize "dead spots" and facilitate more rapid availability. Factors to consider:

•  Clean air replaces contaminated air by natural or forced (mechanical) ventilation.
•  Supply fan intake has to be located away from flammable or toxic air.
•  Exhaust fan outlets should be positioned to avoid re-circulation of contaminants.

See the Ventilation section of this Acquisition Safety website for more information.

Prevent Invasion of Contaminants to Confined Spaces

•  Diesel exhaust emissions should be prevented from re-circulating into confined spaces.
•  Every fuel oil separator should be of efficient design and substantial construction. Provisions should be made to prevent overpressure in any fuel oil separator part and to prevent the discharge of oil vapor into confined spaces.

Design Adequate Means of Entry and Exit

Design adequate and convenient means of entry and exit for persons who may be required to wear personal protective equipment, a breathing apparatus, and protective Temporary ventilation tube is routed through a butter-worth hole minimizing interference with access and egress clothing. A good example is to have a "butterworth opening" or separate entry hole for all support equipment so personnel are not required to enter through this same access point. Designers should recognize the need for two hatches for spaces into which workers must enter along with the "butterworth hole" for ventilation. Ventilation ducts or hoses should not impede personnel access or exit through hatches. To avoid costly retrofits, include "butterworth" hole and additional hatch designs before loading is calculated and the overall design structure is frozen. The same is true for design of lightening holes in tank baffles and girders. These should be positioned so workers can move from section to section of the tank without undue climbing.

Provide for Fall Hazard Protection

To prevent fall hazards in confined spaces, provide fixed ladders, platforms, guardrails, and anchor points for personal fall arrest systems. Puget Sound Naval Shipyard developed a device that can be inserted into D-holes - cut-outs in transverse bulkheads (metal separations inside shipboard tanks) - designed to accommodate a safety boot for climbing. These devices provide a certified anchor point for attachment of scaffolding or anchorages for personal fall arrest systems.  When tank configurations are being upgraded or redesigned, there should be consideration for placing of D ring cutouts or other anchor points in locations where they can provide effective anchorage points for supporting scaffolding and fall protection anchorages to facilitate subsequent entry.  Seemingly minor space configuration changes can greatly reduce the cost and complexity of installing required scaffolding and thus reduce life cycle costs and hazards to maintenance personnel.

Bulkheads inside some tanks have passages/penetrations that permit movement of fluids, while reducing free surface area and the rapid movement of liquids within tanks ("sloshing") that would occur without such partitions.  These penetrations are also used for passage of personnel during maintenance operations.  Design measures to consider, if structural considerations permit, include placing some of these passages at the lower level of tanks to facilitate passage of personnel during maintenance and making these spaces as large as feasible.  Concurrently, consideration should also be given to placing passage holes and associated D ring holes near the top of tanks to facilitate installation of scaffolding near the top of the tank.

Anchor Point Assembly in training mock-up

Plan for Emergency Rescue

The greatest number of confined space fatalities within construction and general industry are actually would-be rescuers.  (see CDC/NIOSH CONFINED SPACES Topical Overview and Rekus-Complete Confined Space Handbook in Resources Section.  Therefore, preplanning, education and stringent program oversight are critical.

Confined spaces that will be entered by ship's crew and shipyard workers should be configured for the removal of injured or unconscious personnel. This means that hatches and trunks should be able to handle various stretchers (Stokes baskets, Oregon spine splint, Reeves sleeve or stretcher) and be configured to accommodate high angle rescue. Pad eyes and anchor points should be available for high angle rescue and hoisting stretchers. Adequate hoisting points should also be provided for movement of materials and equipment.

Consider Other Design and Support Equipment Modifications

•  Provide suitable illumination, which will be sufficient for safe entry, conducting work, and exiting.  Design of maintenance support operations to provide for safe and low-voltage LED lighting should be considered to reduce both risk and cost for support activities.  LED lighting may be justified on the basis of energy savings.
•  Provide sufficient room for persons to work in other than stooped or cramped positions.
•  Use catalytic converters and ventilation to prevent buildup of carbon monoxide levels in confined spaces.
•  Provide a voice or alarm-activated explosion proof type of communication system if visual monitoring of the worker is not available because of the design of the confined space or location of the entry hatch.

Optimize Types of Personal Protective Equipment

In situations where Navy or contractor personnel find it necessary to enter a confined space, proper personal protective equipment must be used. In the past few years, Navy ships have been equipped with excellent gas-detection equipment, such as the Ultra 4 gas analyzer. They also have excellent breathing devices, such as the supplied-air respirator, with a backup self-contained breathing apparatus (SCBA). For ventilating confined spaces, Navy ships typically use a fire main, pressure-driven exhaust fan, with elephant trunks. Protective clothing, such as chemically resistant coveralls and rubber gloves, can protect personnel entering confined spaces from developing occupational dermatitis or from absorbing toxic hazards (such as fuels) through their skin.



Design accommodations made for safe crew and worker confined space entry, inspection, and repair are relatively inexpensive compared to the costs of retrofits, destruction of equipment and property from fires or explosions, and tragic loss of personnel. 

The best solution for avoiding confined space entry hazards is to eliminate the need to enter the confined space. Remote monitor/inspection and cleaning systems can be utilized to avoid entering or minimize the need for confined spaces. Designing and selecting material or equipment with a long service life will also help to minimize the need to enter confined spaces. 

When confined space entry is unavoidable, designing for safe entry and for the safety of workers once inside is the key. Considerations for confined spaces that should be emphasized during the design stage are prevention of contaminant incursion, adequate ventilation, fall protection, adequate lighting, means of emergency rescue, communication, and ergonomics. Reducing hazard levels saves lives and property, increases worker productivity, saves time, and reduces life cycle costs. 


Resources/Best Practices

•  DoD/Navy Instructions and Regulatory Requirements
•  Confined Space Design Guidance Documents
•  Confined Space Regulations
•  Confined Space Standards
•  General Confined Space References

Supporting management process:  Risk control confined space entry is part of an overall management process which includes policy instructions for implementation of required regulations and good practice education of senior and working level personnel, monitoring of spaces, design and pre-planning for potential rescue, planning for routine and emergency communications and program review and oversight.

DoD/Navy Instructions, and Regulatory Requirements and Guidance:

ASTM F-1166-07  
(2013) Standard Practice for Human Engineering Design for Marine Systems, Equipment and Facilities - Establishes general human engineering design criteria for marine vessels and systems, subsystems, and equipment contained therein. The Standard provides a useful tool for the designer to incorporate human capabilities into a design and may be invoked in design criteria.

ASTM F-1337-91  
Standard Practice for Human Engineering Program Requirements for Marine Systems, Equipment and Facilities - Establishes and describes the requirements for applying human engineering to development and acquisition of ships and marine systems, equipment and facilities.

DoD Instruction 5000.02  
"Operation of the Defense Acquisition System," 11/25/2013 - Establishes a simplified and flexible management framework for translating mission needs and technology opportunities, based on approved mission needs and requirements, into stable, affordable, and well-managed acquisition programs that include weapon systems and automated information systems.

MIL-STD-46855A  - 24 May 2011 superseding MIL-HDBK-46885A 17 May 1999
Human Engineering Program Process and Procedures - This handbook provides human engineering (HE) (a) program tasks, (b) procedures and preferred practices, and (c) methods for application to system acquisition.  MIL-STD-46855 is the primary tasking document used by the services to specify human engineering efforts during system acquisition.  It supports the human factors engineering discipline independently or as a part of Human System Integration Initiatives.  MIL-STD-46855 is also written to accommodate a wide range of products, including samll equipment items as well as major systems.

Mil Std 1472G  
(01/11/2012) DoD Design Criteria Standard Human Engineering - This standard establishes general human engineering criteria for design and development of military systems, equipment, and facilities. See section 5.7 Workspace Design and Section 5.9 Design for Maintainer.

OPNAVINST 5100.19 Series 
Navy Safety and Occupational Health (SOH) Program Manual for Forces Afloat - Establishes policy, procedures, and actions for implementing the Navy’s safety and health program. See Chapter B8, Gas Free Engineering.

OSHA General Industry Standard for Confined Space Entry (29 CFR 1910.146)


Confined Space Design Guidance Documents:

Appendix A to 1910-146 - Permit-Required Confined Space Decision Flow Chart - Should be consulted both in the design phase and prior to any entry in order to support risk management, including preparation for any emergency.

Guidance Notes for the Application of Ergonomics to Marine Systems (2013) Ergonomics (August) 
ABS developed the Guidance Notes to be used in concert with human-system interface design processes.  Although compliance is not required, ABS encourges designers, owners and operators to consider adopting this guidance where feasible.  American Bureau of Shipping, ABS Plaza, 16855 Northchase Drive, Houston, TX 77060 USA .


Confined Space Regulations:

29 CFR Part 1915 Subpart B 
Confined and Enclosed Spaces and Other Dangerous Atmospheres in Shipyard Employment.


Confined Space Standards:

ANSI Z117.1 (Confined Spaces) May 19, 2010 -  Safety Requirements for Confined Spaces - This revised standard provides minimum safety requirements to be followed while entering, exiting, and working in confined spaces at normal atmospheric pressure. Available from the American Society of Safety Engineers .


General Confined Space References:

Complete Confined Spaces Handbook, John F. Rekus, copyright 1994, Lewis Publishers - This book provides plant managers, supervisors, safety professionals, and industrial hygienists with recommended procedures and guidance for safe entry into confined spaces.

CDD/NIOSH Confined Spaces Topical Overview - or

Enclosed Spaces and Other Dangerous Atmospheres 
OSHA Ship Repair website e-tool

OSHA Safety and Health Topics - Confined Spaces

Confined Space Entry, An AIHA Protocol Guide. 2nd Ed., AIHA Press 2001
SOLAS Chapter II-1, Part A-1 Structure of Ships Regulation 3-6
Access to and within spaces in the cargo area of oil tankers and bulk carriers (2002 rev.)

Confined Space Safe Practice Rec. No 72 
International Association of Classification Societies (IACS).

Human Factors Conference, London, 27 – 29 September 2000, Royal Institution of Naval Architects - Human factors in ship design and operation provides an opportunity to reduce cost, improve safety, increase effectiveness and improve conditions on board ships. Details of papers presented at this conference are available at 

International Maritime Organization (IMO) 
Safety of Life at Sea (SOLAS) rule for merchant oil tankers and bulk carriers requiring permanent means of access (PMA) or alternate means of access to the cargo and fuel storage areas.

Navy Shipboard Corrosion Maintenance  
NAVSEA, Corrosion Control Division, Beau Brinckerhoff ‘s presentation at the 2002 DoD Maintenance Symposium, describes Navy innovative, cost savings projects.

Navy Shipboard Corrosion Maintenance  
NAVSEA, Corrosion Control Division, E. Dail Thomas II‘s presentation describes Navy innovative cost savings projects.

Naval Safety Center Website Presentation
Describes the dangers, procedures, and equipment used for confined space entry of Coast Guard boarding team.

NFPA 301 Code for Safety to Life from Fire on Merchant Vessels 2001 Edition

NFPA 306 Control of Gas Hazards on Vessels 2001 Edition - guidelines for Marine Chemists and hazards of Confined and Enclosed Spaces aboard ships.

Ergonomic Interventions in the Building, Repair, and Dismantling of Ships, 2000, Stephen D. Hudock, Ph.D., CSP

Remote Tank Monitoring and Inspection Methods  
Naval Sea System Command (NAVSEA), Carderock Division - use of tank level indicators on Navy ships.

Safety and Health in Confined Spaces, Neil McManus, copyright 1999 Lewis Publishers - comprehensive review of all aspects of confined space entry including hazard assessment, Standards Guidelines, and protective measures.

American Industrial Hygiene Association - Confined Spaces Committee

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How to Contribute

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Last Revision:  September 03, 2014