Diving safety

The safety of underwater diving depends on four factors: the environment, the equipment, behaviour of the individual diver and performance of the dive team. The underwater environment can impose severe physical and psychological stress on a diver, and is mostly beyond the diver’s control. Equipment is used to operate underwater for anything beyond very short periods, and the reliable function of some of the equipment is critical to even short term survival. Other equipment allows the diver to operate in relative comfort and efficiency. The performance of the individual diver depends on learned skills, many of which are not intuitive, and the performance of the team depends on communication and common goals.

There is a large range of hazards to which the diver may be exposed. These each have associated consequences and risks, which should be taken into account during dive planning. Where risks are marginally acceptable it may be possible to mitigate the consequences by setting contingency and emergency plans in place, so that damage can be minimised where reasonably practicable. The acceptable level of risk varies depending on legislation, codes of practice and personal choice, with recreational divers having a greater freedom of choice.

Hazard control
The classic methods of hazard control are applied when reasonably practicable: The modes of diving can be considered levels of hazard control. An alternative mode of diving may include hazard elimination or substitution, engineering controls, administrative controls and personal protective equipment to reduce risk for a given activity, usually at considerable logistical cost, and often reducing operational flexibility.

Hazards to divers can be completely eliminated when a machine can do the job. There are a growing number of commercial, military and scientific applications where a remotely operated or autonomous underwater vehicle can produce satisfactory results. To a lesser extent this applies to atmospheric pressure diving, where the diver is not exposed to the environment as long as the suit integrity is maintained, but some of the hazards and risks remain. Saturation diving is a technique that allows divers to reduce the risk of decompression sickness (“the bends”) when they work at great depths for long periods of time.

Freediving, or breath-hold diving, is the original mode of diving, and was used for centuries in spite of limitations as it was the only option available. It is simple and inexpensive, but severely limited in the time available to do useful work at depth. The risk of drowning is relatively high, as the diver is limited to the oxygen supplied by a single breath, and the risk of hypoxic blackout underwater, followed by drowning, is significant.

Hypoxic blackout during freediving is a loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive, when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have caused it. It can be provoked by hyperventilating just before a dive, or as a consequence of the pressure reduction on ascent, or a combination of these. Victims are often established practitioners of breath-hold diving, are fit, strong swimmers and have not experienced problems before.

Divers and swimmers who blackout or grey out underwater during a dive will usually drown unless rescued and resuscitated within a short time. Freediving blackout has a high fatality rate but is generally avoidable. The risk cannot be quantified, but is clearly increased by any level of hyperventilation.

Freediving blackout can occur on any dive profile: at constant depth, on an ascent from depth, or at the surface following ascent from depth and may be described by a number of terms depending on the dive profile and depth at which consciousness is lost. Blackout during a shallow dive differs from blackout during ascent from a deep dive in that deep water blackout is precipitated by depressurisation on ascent from depth while shallow water blackout is a consequence of hypocapnia following hyperventilation.

Trained freedivers are well aware of this and competitions must be held under strict supervision and with competent first-aiders on standby. However this does not eliminate the risk of blackout. Freedivers are recommended to only dive with a ‘buddy’ who accompanies them, observing from in the water at the surface, and ready to dive to the rescue if the diver loses consciousness during the ascent.

Scuba diving
Diving using self-contained underwater breathing apparatus was developed after surface supplied diving, and was intended as a method of improving the mobility and horizontal range of the diver who is not restricted by a physical connection to a surface gas supply. The diver has a larger gas supply than the freediver, and this allows a greatly extended underwater endurance, and lower risk of drowning, but at the cost of higher risk from decompression sickness, lung over-pressure barotrauma, nitrogen narcosis, oxygen toxicity and hypothermia, all of which must be limited by procedural and engineering controls, and personal protective equipment.

For acceptable safety the diver must be able to survive any reasonably foreseeable single point of failure. For scuba equipment this implies that the failure of any single item of equipment should not put the diver out of reach of a breathing gas supply.

Open circuit
In the case of a single cylinder scuba set with a single first stage, and a single second stage, each of these items has a low but non-zero probability of failure. The components work in series – if any one of them fails, the system fails. It is equivalent to a single chain in which if any link fails, the chain breaks. When the dive is very shallow, the diver can safely escape to the surface, and when there is another diver right there with spare gas at the time of failure, they can share gas. At other times, a failure of a single item can kill the diver.

Assuming independence of failure events, each item that can cause failure of the combined system is a critical point of failure and increases the probability of system. For the system not to fail, all items must not fail according to the formula:

{\displaystyle {p}=1-\prod _{i=1}^{n}(1-p_{i})} {\displaystyle {p}=1-\prod _{i=1}^{n}(1-p_{i})}


{\displaystyle n} n – number of components
{\displaystyle p_{i}} p_{i} – probability of component i failing
{\displaystyle p} p – the probability of all components failing (system failure)
As a purely illustrative example, if there is a 1 in 100 probability of a regulator failure, and a 1 in 1000 probability of a scuba cylinder failure then

{\displaystyle p_{reg}=0.01} {\displaystyle p_{reg}=0.01}, and {\displaystyle p_{cyl}=0.001} {\displaystyle p_{cyl}=0.001}

{\displaystyle P_{fail}=1-(1-p_{reg})\times (1-p_{cyl})} {\displaystyle P_{fail}=1-(1-p_{reg})\times (1-p_{cyl})}
Substituting values:

{\displaystyle P_{fail}=1-(1-0.01)\times (1-0.001)} {\displaystyle P_{fail}=1-(1-0.01)\times (1-0.001)}
{\displaystyle =1-0.99\times 0.999} {\displaystyle =1-0.99\times 0.999}
{\displaystyle =1-0.98901} {\displaystyle =1-0.98901}
{\displaystyle =0.01099} {\displaystyle =0.01099} which is close to the sum of the two probabilities.
The example shows that each critical point of failure increases the probability of system failure by approximately that item’s probability of failure.

If there are two completely independent scuba sets at the diver’s disposal, either one of which is sufficient to allow the diver a safe return, then both sets must fail during the same dive to cause a fatal outcome. These items work in parallel – all must fail for the system to fail. The probability of this happening is extremely low for reliable equipment.

Assuming independence of failure events, each duplicate redundant item added to the system decreases the probability of system failure according to the formula:-

{\displaystyle {p}=\prod _{i=1}^{n}p_{i}} {p}= \prod_{i=1}^{n} p_{i}


{\displaystyle n} n – number of components
{\displaystyle p_{i}} p_{i} – probability of component i failing
{\displaystyle p} p – the probability of all components failing (system failure)
Taking two independent sets with the same probability of failure calculated in the example above:

{\displaystyle p_{left}=0.01099} {\displaystyle p_{left}=0.01099}, and {\displaystyle p_{right}=0.01099} {\displaystyle p_{right}=0.01099}

{\displaystyle P_{fail}=(p_{left})\times (p_{right})} {\displaystyle P_{fail}=(p_{left})\times (p_{right})}
Substituting values:

{\displaystyle P_{fail}=0.01099\times 0.01099} {\displaystyle P_{fail}=0.01099\times 0.01099}
{\displaystyle =0.00012078} {\displaystyle =0.00012078}
It is clear from the example that redundancy reduces the risk of system failure very rapidly, and conversely, that disregarding a failure of a redundant item increases the probability of system failure equally rapidly.

Closed circuit
See also: Electro-galvanic oxygen sensor § Managing cell failure in a life-support system
Open circuit scuba has a small number of fairly rugged and reliable components, each with a small number of failure modes and a low probability of failure. Most of these components remain present in closed circuit scuba, but there are also a number of additional items which could fail. Therefore, the rebreather architecture is inherently more likely to fail, and it is necessary to provide redundancy of critical components to provide reliability even approaching that of open circuit scuba. It is also more important to provide full redundancy of breathing gas supply as some rebreather failure modes do not allow safe ascent. Bailout to open circuit is the simplest and most robust option, but for dives where a long return under an overhead, or long decompression are necessary, open circuit can be impractically bulky. There is a point at which closed circuit bailout becomes a more manageable option, and the requirement for ability to return safely from any point on the planned dive profile makes it necessary for the breathing loop and gas supplies to be fully independent, though the ability to make use of the primary gas supply in the bailout rebreather can considerably extend the range for a small added complexity, using highly reliable components, but adding to the task loading of the diver.

A hazard specific to closed circuit rebreathers is failure of the oxygen partial pressure control system. The breathing gas mixture in a diving rebreather loop is usually measured using electro-galvanic oxygen sensors, and the output of the cells is used by either the diver or an electronic control system to control addition of oxygen to increase partial pressure when it is below the chosen lower set-point, or to flush with diluent gas when it is above the upper set-point. When the partial pressure is between the upper and lower set-points, it is suitable for breathing at that depth and is left until it changes as a result of consumption by the diver, or a change in ambient pressure as a result of a depth change.

Accuracy and reliability of measurement is important in this application for two basic reasons. Firstly, if the oxygen content is too low, the diver will lose consciousness due to hypoxia and probably die, or if the oxygen content is too high, the risk of central nervous system oxygen toxicity causing convulsions and loss of consciousness, with a high risk of drowning becomes unacceptable. Secondly, decompression obligations cannot be accurately or reliably calculated if the breathing gas composition is not known. Pre-dive calibration of the cells can only check response to partial pressures up to 100% at atmospheric pressure, or 1 bar. As the set points are commonly in the range of 1.2 to 1.6 bar, special hyperbaric calibration equipment would be required to reliably test the response at the set-points. This equipment is available, but is expensive and not in common use, and requires the cells to be removed from the rebreather and installed in the test unit. To compensate for the possibility of a cell failure during a dive, three cells are generally fitted, on the principle that failure of one cell at a time is most likely, and that if two cells indicate the same PO2, they are more likely to be correct than the single cell with a different reading. Voting logic allows the control system to control the circuit for the rest of the dive according to the two cells assumed to be correct. This is not entirely reliable, as it is possible for two cells to fail on the same dive.

Surface oriented surface supplied diving
Surface-supplied diving is diving using equipment supplied with breathing gas using a diver’s umbilical from the surface, either from the shore or from a diving support vessel, sometimes indirectly via a diving bell.

The copper helmeted free-flow standard diving dress is the version which made commercial diving a viable occupation, and although still used in some regions, this heavy equipment has been superseded by lighter free-flow helmets, and to a large extent, lightweight demand helmets, band masks and full-face diving masks. Breathing gases used include air, heliox, nitrox, oxygen and trimix. Gases with raised oxygen fraction are used to reduce decompression obligation and accelerate decompression, and gases containing helium are used to reduce nitrogen narcosis. Both applications reduce the risk to the diver when applicable.

The primary advantages of conventional surface supplied diving over scuba are lower risk of drowning and considerably larger breathing gas supply than scuba, allowing longer working periods and safer decompression.

Surface supplied diving systems improve safety by virtually eliminating the risk of a lost diver, as the diver is physically connected to the surface control point by the breathing gas supply hose, and other components of the umbilical cable system. They also significantly reduce the risk of running out of breathing gas during the dive, and allow multiple redundancy of gas supply, with main and secondary surface supply, and a scuba bailout emergency gas system. Use of helmets and full-face masks help protect the diver’s airway in case of loss of consciousness. These can be considered engineering controls of the hazards.

Saturation diving
Decompression sickness occurs when a diver with a large amount of inert gas dissolved in the body tissues is decompressed to a pressure where the gas forms bubbles which may block blood vessels or physically damage surrounding cells. This is a risk on every decompression, and limiting the number of decompressions can reduce the risk.

“Saturation” refers to the fact that the diver’s tissues have absorbed the maximum partial pressure of gas possible for that depth due to the diver being exposed to breathing gas at that pressure for prolonged periods. This is significant because once the tissues become saturated, the time to ascend from depth, to decompress safely, will not increase with further exposure.

In saturation diving, the divers live in a pressurized environment, which can be a saturation system – a hyperbaric environment on the surface – or an ambient pressure underwater habitat. This may continue for up to several weeks, usually with the divers living at the same or very similar ambient pressure to the work site, and they are decompressed to surface pressure only once, at the end of their tour of duty. By limiting the number of decompressions in this way, the risk of decompression sickness is significantly reduced at the cost of exposing the diver to other hazards associated with living under high pressure for prolonged periods. Saturation diving is an example of substitution of a hazard expected to present a lower risk than surface oriented diving for the same set of operations.

Atmospheric pressure diving
Atmospheric pressure diving isolates the diver from the ambient pressure of the environment by using an atmospheric diving suit (ADS), which is a small one-person articulated submersible of anthropomorphic form which resembles a suit of armour, with elaborate pressure joints to allow articulation while maintaining an internal pressure of one atmosphere. The ADS can be used for very deep dives of up to 2,300 feet (700 m) for many hours, and eliminates the majority of physiological dangers associated with deep diving; the occupant need not decompress, there is no need for special gas mixtures, and there is no danger of decompression sickness or nitrogen narcosis, and a drastically reduced risk of oxygen toxicity. Hard suit divers do not even need to be skilled swimmers, as swimming is not yet possible in atmospheric suits. The current generation of atmospheric suits are more ergonomically flexible than earlier versions, but are still very limited in personal mobility and dexterity compared to an ambient pressure diver. Use of an atmospheric suit may be considered as substituting a relatively low risk of crushing for a higher risk of decompression sickness and barotrauma, by using the suit as an engineered barrier between the diver and the hazards.

Remotely operated underwater vehicles
A remotely operated underwater vehicle (ROV) is an unoccupied, highly maneuverable, tethered mobile underwater device operated by a crew aboard a base platform. They are linked to the base platform by a neutrally buoyant tether or, often when working in rough conditions or in deeper water, a load-carrying umbilical cable is used along with a tether management system (TMS). The purpose of the TMS is to lengthen and shorten the tether so the effect of cable drag where there are underwater currents is minimized. The umbilical cable is an armored cable that contains a group of electrical conductors and fiber optics that carry electric power, video, and data signals between the operator and the TMS. Where used, the TMS then relays the signals and power for the ROV down the tether cable. Most ROVs are equipped with at least a video camera and lights. Additional equipment is commonly added to expand the vehicle’s capabilities. These may include sonars, magnetometers, a still camera, a manipulator or cutting arm, water samplers, and instruments that measure water clarity, water temperature, water density, sound velocity, light penetration, and temperature. ROVs are commonly used in deep water industries such as offshore hydrocarbon extraction, where they can carry out many tasks previously requiring diver intervention. ROVs may be used together with divers, or without a diver in the water, in which case the risk to the diver associated with the dive is eliminated altogether.

Administrative controls
Administrative controls include medical screening, planning and preparation for diving and training in essential skills.

Legislation, codes of practice and organisational procedures
Exemptions from regulations for emergency public safety diving – applicable in some jurisdictions only where there is a possibility of rescuing a survivor.

Medical screening
Fitness to dive, (also medical fitness to dive), is the medical and physical suitability of a diver to function safely in the underwater environment using underwater diving equipment and procedures. Depending on the circumstances it may be established by a signed statement by the diver that he or she does not suffer from any of the listed disqualifying conditions and is able to manage the ordinary physical requirements of diving, to a detailed medical examination by a physician registered as a medical examiner of divers following a procedural checklist, and a legal document of fitness to dive issued by the medical examiner.

The most important medical is the one before starting diving, as the diver can be screened to prevent exposure when a dangerous condition exists. The other important medicals are after some significant illness, where medical intervention is needed there and has to be done by a doctor who is competent in diving medicine, and can not be done by prescriptive rules.

Psychological factors can affect fitness to dive, particularly where they affect response to emergencies, or risk taking behaviour. The use of medical and recreational drugs, can also influence fitness to dive, both for physiological and behavioural reasons. In some cases prescription drug use may have a net positive effect, when effectively treating an underlying condition, but frequently the side effects of effective medication may have undesirable influences on the fitness of diver, and most cases of recreational drug use result in an impaired fitness to dive, and a significantly increased risk of sub-optimal or inappropriate response to emergencies.

Pre-dive preparation and planning
Dive planning is the process of planning an underwater diving operation. The purpose of dive planning is to increase the probability that a dive will be completed safely and the goals achieved. Some form of planning is done for most underwater dives, but the complexity and detail considered may vary enormously.

Professional diving operations are usually formally planned and the plan documented as a legal record that due diligence has been done for health and safety purposes. Recreational dive planning may be less formal, but for complex technical dives, can be as formal, detailed and extensive as most professional dive plans. A professional diving contractor will be constrained by the code of practice, standing orders or regulatory legislation covering a project or specific operations within a project, and is responsible for ensuring that the scope of work to be done is within the scope of the rules relevant to that work. A recreational (including technical) diver or dive group is generally less constrained, but nevertheless is almost always restricted by some legislation, and often also the rules of the organisations to which the divers are affiliated.

The planning of a diving operation may be simple or complex. In some cases the processes may have to be repeated several times before a satisfactory plan is achieved, and even then the plan may have to be modified on site to suit changed circumstances. The final product of the planning process may be formally documented or, in the case of recreational divers, an agreement on how the dive will be conducted. A diving project may consist of a number of related diving operations.

A hazard identification and risk assessment procedure is the basis of a large part of dive planning. The hazards to which the divers will be exposed are identified, and the level of risk associated with each is evaluated. If the risk is deemed to be excessive, control methods will be applied to reduce the risk to an acceptable level, and where appropriate, further controls will be set in place to mitigate the effects if an incident does occur.

A documented dive plan may contain elements from the following list:

Overview of Diving Activities
Schedule of Diving Operations
Specific Dive Plan Information

Following the plan
A basic strategy of risk management is to plan an operation and then conduct it, as far as reasonably practicable, according to the plan. If this is done, the risks will have been assessed and the equipment chosen will be suitable. Deviation from the plan brings in unassessed factors. In professional diving where a diving operation plan must be drawn up, variation from the plan generally requires reassessment of risk and recording of the deviation and any measures that were found necessary to manage the changed circumstances. In recreational diving, the diver is free to plan or not, and to change the plan on whim, but technical diving certification agencies generally encourage divers to “plan the dive and dive the plan”, as this is considered good practice for safety, and is the same strategy used by professionals.

Standard operating procedures and codes of practice are used to reduce the amount of detail required in dive planning. These documents provide much of the necessary detail of how frequently encountered tasks should be performed, using methods which have been tested and found to be effective, efficient and acceptably safe. When standard procedures are used, it is not necessary to detail those procedures in the dive plan, as the team members should be familiar with them already.

Standard operating procedures are the procedures identified by the diving contractor as the recommended or required way of performing a range of routine activities and codified in a document. Following SOPs is generally a condition of employment for the diving team, and the provision of SOPs may be a requirement of health and safety regulations. The document is often called the operations manual, diving manual or something similar. For example, the US Navy Diving Manual, NOAA Diving Manual,

Codes of Practice are procedures identified by a larger population as preferred methods for a similar range of activities. They may be a set of industry best practice recommendations, such as the IMCA Code of Practice for offshore diving, a government regulated set of recommendations, or a regulated set of requirements which must be followed.

Training, practice and experience
To make effective use of standard procedures, the diving team must be competent in the procedures, particularly the diving and emergency skills. These skill sets are the basis of the standard operating procedures, and have themselves been standardised to a degree where they are largely internationally accepted, and are portable between organisations without requiring much re-learning. A large part of the variation is connected to different equipment and equipment configurations, and operators need to become familiar with new equipment under controlled conditions before operating in the field. This is the realm of formal training for diving certification, which is normally done by registered diving schools and instructors, and equipment rating and familiarisation, which may be done by the employer or by diver training schools, depending on the risks and complexity of the training, and how much unfamiliar equipment is involved. For example, basic operation of an unfamiliar mode of life support equipment like surface supplied diving or a rebreather is likely to be learned at a school, while the details of operating a different model of non-diving equipment, like a hydraulic bolt-tensioner is likely to be learned from a skilled operator of that equipment, or at a manufacturer’s familiarisation workshop. It is common practice to record such training and the associated assessment in the diver’s logbook, as well as any certificate which may be issued.

Appropriate response to minor life-support equipment malfunctions which can be corrected by the diver is very important for diving safety. The diver is expected to deal with a number of small problems promptly and correctly before the situation escalates. Dealing with such problems as a dislodged or flooded mask, or free-flowing regulator, or correctable buoyancy fault should be done before the situation deteriorates to an emergency. A basic understanding of the physics and physiology of diving should give the diver the ability to predict the consequences of possible responses to unfamiliar contingencies. A diver with inadequate understanding may respond inappropriately to an emergency outside of their training and experience, which though unlikely remains possible. Repeated practice beyond initial competence of standard responses to the more likely contingencies develops a “muscle memory” response, which helps the diver perform the correct response under stress, and when more than one problem occurs simultaneously. It is possible to never experience one of these problems, and some divers may never need the skills in practice, but divers who do not practice the skills are more likely to be overtaken by circumstances if something does go wrong. The practice of stress-training in benign conditions, where the diver is task loaded with an increasing level of simulated problems and must deal with them, is thought to develop the diver’s confidence in their ability to manage an emergency effectively, which may give them the ability to avoid panic and continue to respond usefully to the situation, giving a better chance of survival.

Continued occasional practice of emergency procedures after initial training ensures that the skills are not lost due to lack of use. Divers who have not practiced their skills for several months or years are at higher risk of accidents when first returning to the water, and refresher courses and checkout dives in benign conditions are available to get the skills back to standard and thereby reduce risk of an accident.

Personal protective equipment
A large part of personal diving equipment can be classified as personal protective equipment.

Breathing apparatus
Exposure suits – Wetsuits, Dry suits, and hot-water suits provide thermal protection to the diver. Where thermal protection is not necessary, divers may wear overalls as protection against stings, cuts and abrasions which could be caused by contact with the environment.
Diving helmets provide thermal protection and impact protection for the diver’s head. Neoprene hoods provide protection against high volume sound, often produced by the breathing apparatus, but also from other sources.
Gloves and boots serve similar functions underwater to those they provide at the surface.

Source from Wikipedia