Diving safety is the aspect of underwater diving operations and activities concerned with the safety of the participants. 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, or to remain healthy over the longer term. The performance of the individual diver depends on learned skills, many of which are not intuitive, and the performance of the team depends on competence, communication, attention 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, company policy, and personal choice, with recreational divers having a greater freedom of choice.
In professional diving there is a diving team to support the diving operation, and their primary function is to reduce and mitigate risk to the diver. The diving supervisor for the operation is legally responsible for the safety of the diving team. A diving contractor may have a diving superintendent or a diving safety officer tasked with ensuring the organisation has, and uses, a suitable operations manual to guide their practices. In recreational diving, the dive leader may be partly responsible for diver safety to the extent that the dive briefing is reasonably accurate and does not omit any known hazards that divers in the group can reasonably be expected to be unaware of, and not to lead the group into a known area of unacceptable risk. A certified recreational diver is generally responsible for their own safety, and to a lesser, variable, and poorly defined extent, for the safety of their dive buddy.
See main article: Safety.
Safety is not the absence of accidents. Safety is the presence of defences. Todd Concklin
Safety is the condition of being protected from harm, and also refers to the control of recognized hazards in order to achieve an acceptable level of risk. When one operates where it is not feasible to avoid or remove hazards completely, safety implies that defences have been set up to recover from foreseeable incidents and to mitigate their consequences to an acceptable level. The level of accepted risk may be imposed by a regulatory body, an organisation performing an activity to which risk is connected, or the individual exposed to the risk.
A distinction can be made between three types of safety:
See also: List of diving environments by type, List of diving hazards and precautions and Underwater environment. The underwater environment is alien to humans. When not actively hostile, it is unforgiving of errors, and some errors can escalate rapidly to a fatal conclusion. Many aspects of the underwater environment are static or predictable, others vary and may not be easily or reliably predictable, and must be managed as and when they occur. The reasonably predictable factors can be allowed for in the dive planning. Suitable equipment can be selected, personnel can be trained in its use and support provided to manage the foreseeable contingencies. When conditions are found to be other than predicted, plans may have to be changed. Sometimes conditions are better than expected, but other times they may be worse, and may deteriorate during the course of a dive to the extent that recovery becomes an emergency.
See also: diving equipment. Two basic classes of equipment are used by divers: Equipment necessary to do the planned dive, and equipment required to do the task for which the dive is necessary. Recreational divers may not require equipment for a task, but it is quite common for them to use a camera, and some will survey a dive site, or use a small lift bag to recover an anchor or diving shot. There are no particularly significant risks associated with tools commonly used by recreational divers. Commercial divers usually use tools of some kind while diving, and some of these tools can be very dangerous if used incorrectly, such as high-pressure water-jets, explosive bolts, oxy-arc cutting and welding and heavy lifting equipment and rigging.
Open circuit scuba is mechanically robust and reliable, but can malfunction when damaged, misused, poorly maintained, or occasionally due to unplanned circumstances. Provision of a completely independent emergency supply capable of providing sufficient breathing gas to allow the diver to surface safely from any point on the planned dive profile reduces the risk of a non-survivable out of gas incident to an extremely low level. This remains valid only as long as the emergency gas supply is within immediate reach of the diver, which is more reliably achieved by the diver carrying a bailout cylinder than by relying on a buddy or stand-by diver, who may not be where needed in an emergency.
Rebreathers have an intrinsically much higher risk[1] of mechanical and electrochemical sensor failure than open circuit scuba because of their structural and functional complexity, and some inherent characteristics of electro-galvanic oxygen sensors, but this can be mitigated by fault tolerant design which provides redundancy of critical items and by carrying sufficient alternative breathing gas supplies for bailout including any required decompression in case of failure. Designs that minimize risk of human-machine interface errors, and adequate training in procedures that deal with this area may help reduce the fatality rate. Two thirds of fatalities were associated with high risk behaviour or a high risk dive profile.
The essential aspect of surface-supplied diving is that breathing gas is supplied from the surface, either from a specialized diving compressor, high-pressure cylinders, or both. In commercial and military surface-supplied diving, a backup source of breathing gas should always be present in case the primary supply fails. The diver may also wear a cylinder called a "bail-out bottle," which can provide self-contained breathing gas in an emergency. Thus, the surface-supplied diver is much less likely to have an "out-of-air" emergency than a scuba diver as there are normally two alternative air sources available. Surface-supplied diving equipment usually includes communication capability with the surface, which improves the safety and efficiency of the working diver.
Surface-supplied equipment is required for diving in harsh contaminated environments under the US Navy operational guidance which was drawn up by the Navy Experimental Diving Unit, and by several other professional codes of practice.Surface-supplied diving equipment is required for a large proportion of the commercial diving operations conducted in many countries, either by direct legislation, or by authorised codes of practice, as in the case of IMCA operations.
See main article: Human factors in diving safety. Human factors are the physical or cognitive properties of individuals, or social behavior which is specific to humans, and influence functioning of technological systems as well as human-environment equilibria. The safety of underwater diving operations can be improved by reducing the frequency of human error and the consequences when it does occur. Human error can be defined as an individual's deviation from acceptable or desirable practice which culminates in undesirable or unexpected results.
Human error is inevitable and everyone makes mistakes at some time. The consequences of these errors are varied and depend on many factors. Most errors are minor and do not cause significant harm, but others can have catastrophic consequences. Examples of human error leading to accidents are available in vast numbers, as it is the direct cause of 60% to 80% of all accidents.In a high risk environment, as is the case in diving, human error is more likely to have catastrophic consequences. A study by William P. Morgan showed that over half of all divers in the survey had experienced panic underwater at some time during their diving career. These findings were independently corroborated by a survey that suggested 65% of recreational divers have panicked under water. Panic frequently leads to errors in a diver's judgment or performance, and may result in an accident. Human error and panic are considered to be the leading causes of dive accidents and fatalities.
Only 4.46% of the recreational diving fatalities in a 1997 study were attributable to a single contributory cause. The remaining fatalities probably arose as a result of a progressive sequence of events involving two or more procedural errors or equipment failures, and since procedural errors are generally avoidable by a well-trained, intelligent and alert diver, working in an organised structure, and not under excessive stress, it was concluded that the low accident rate in professional scuba diving is due to this factor. The study also concluded that it would be impossible to eliminate absolutely all minor contraindications of scuba diving, as this would result in overwhelming bureaucracy and would bring all diving to a halt.
Humans function underwater by virtue of technology, as our physiology is poorly adapted to the environment. Human factors are significant in diving because of this harsh and alien environment, and because diver life support systems and other equipment that may be required to perform specific tasks depend on technology that is designed, operated and maintained by humans, and because human factors are cited as significant contributors to diving accidents in most accident investigations
Professional diving is a means to accomplish a wide range of activities underwater in a normally inaccessible and potentially hazardous environment. While working underwater, divers are subjected to high levels of physical and psychological stress due to environmental conditions and the limitations of the life support systems, as well as the rigours of the task at hand.
Recreational, or sport divers, including technical divers, dive for entertainment, and are usually motivated by a desire to explore and witness, though there is no distinct division between the underwater activities of recreational and professional divers. The primary distinction is that legal obligations and protection are significantly different, and this is reflected in organisational structure and procedures.
Recreational diving has been rated more risky than snow skiing, but less risky than other adventure sports such as rock climbing, bungee jumping, motorcycle racing and sky diving. Improvements in training standards and equipment design and configuration, and increased awareness of the risks of diving, have not eliminated fatal incidents, which occur every year in what is generally a reasonably safe recreational activity. Both categories of diver are usually trained and certified, but recreational diving equipment is typically limited to freediving and scuba, whereas professional divers may be trained to use a greater variety of diving systems, from scuba to surface supplied mixed gas, saturation systems and atmospheric diving suits. A recreational diver may use some ancillary equipment to enhance the diving experience, but the professional will almost always use tools to perform a specific task.
Since the goal of recreational diving is personal enjoyment, a decision to abort a dive, for whatever reason, normally only affects the diver and his companions. A working diver faced with the same decision, must disappoint a client who needs and expects the diver's services, often with significant financial consequences. Therefore, the working diver often faces greater pressure to provide the service at the cost of reduced personal safety. An understanding of the human factors associated with diving may help the diving team to strike an appropriate balance between service delivery and safety.
Human factors are the influences on human behavior, and the resulting effects of human performance on a process or system.Safety can be improved by reducing the frequency of human error and the consequences when it does occur.
See main article: Fitness to dive. Fitness to dive is the medical and physical capacity of a diver to function safely in the underwater environment using underwater diving equipment and procedures. Psychological factors can also affect fitness to dive, particularly where they affect response to emergencies, or risk taking behavior. Some conditions affect fitness to function safely and effectively underwater in unpredictable ways, and may not be noticed until they show up under stress and precipitate an emergency. Other conditions do not necessarily prohibit the person from diving, but limit their ability to manage in difficult circumstances, and a realistic assessment of where the diver's personal limits lie can help prevent an emergency from developing. In professional diving, the diving medical examination is used to identify divers who are at risk in circumstances which are acceptable in a working environment. Recreational divers must take more personal responsibility for self-assessment before each dive. To some extent greater competence can mitigate a lower level of fitness, as the diver can correct mishaps more quickly and with less effort.
See main article: Situation awareness. Situational awareness is the perception of environmental elements and events with respect to time or space, the comprehension of their meaning, and the projection of their future status. It has been recognized as a critical, yet often elusive, foundation for successful decision-making across a broad range of situations. Lacking or inadequate situation awareness has been identified as one of the primary factors in accidents attributed to human error. The formal definition of situational awareness breaks it down into three components: perception of the elements in the environment, comprehension of the situation, and projection of future status. Situational awareness is recognised as necessary for diving safety, as a member of the dive team who is not aware of changing circumstances may fail to react appropriately in time to avoid serious difficulties.Situation awareness is limited by sensory input and available attention, by the diver's knowledge and experience, and by their ability to recognise, interpret and analyse the available information effectively. Attention is a limited resource, and may be reduced by distraction and task loading. Comprehension of the situation and projection of future status depend heavily on relevant knowledge, understanding, and experience in similar environments and situations. Team situation awareness may be less limited by these factors, as there can be a wider knowledge and experience base, but it is limited by the effectiveness of communication within the team.
Safety of underwater diving operations can be improved by reducing the frequency of human error and the consequences when it does occur.
See also: Diving team. A dive team can vary from a recreational buddy pair to a professional saturation diving team working 24 hours per day with dive and habitat support personnel on a dynamically positioned vessel. The primary purpose of a professional diving team is to improve safety for the working diver by providing backup and support, and to manage the surface equipment required for the operation. A buddy pair is a team intended to improve the safety of recreational divers, and in some circumstances succeeds in this aim, depending on the skills, situational awareness and compliance with procedures of both of the divers. Technical diving teams can vary between the recreational buddy pair at its worst to expedition teams with structure, competence and planning similar to professional teams. For buddy diving to be an effective means of improving safety, the buddy pairs should be set long enough before the dive to allow adequate familiarisation with each other's equipment, signals and procedures, where they differ, and discuss the dive plan, and kit-up should occur in close enough proximity for both divers to actually monitor the progress and details of pre-dive checks, and where necessary, to assist directly with fitting equipment.
For many applications, the minimum personnel requirement for a professional diving operation is a working diver, to do the job, a diver's tender to assist the diver and manage the umbilical or airline, a stand-by diver, competent and ready to go to the assistance of the working diver, and a supervisor, to co-ordinate the team, ensure that the plan is acceptably safe in terms of the organisational policies coded of practice and applicable legislation, ensure that the operation follows the plan as far as possible, and to manage any contingencies or emergencies that may come up during the operation. The primary responsibility of the supervisor of a professional diving team is the health and safety of the diving team. The minimum personnel requirement for a recreational dive in most places, is the diver. There are a few countries where further support is obligatory, generally a requirement to dive with a buddy, but in most places a recreational diver is responsible for their own safety and is legally free to dive without any support personnel. Recreational service providers may impose their own terms and conditions on customers, but this is generally a contractual option.
See main article: Efficiency–thoroughness trade-off principle. As both time and physical and financial resources are limited, there is a trade-off between safety and efficiency. If taken to extremes, concerns with safety could prevent all diving. This trade-off is acknowledged in occupational health and safety legislation, where precautions are required to be reasonably practicable, with reference to cost, benefit, available technology and other factors.
Recreational divers are often cautioned not to dive beyond the limits of their training and experience, as this increases the risk to the diver, and possibly also to other members of the dive team. Superficially this may seem logical, but it neglects to consider that formal training is focused on learning specific skills and assessment is against a minimum acceptable level of competence in those skills. These skills are intended to allow the diver to manage a more general range of conditions implied by the certification standard, and greater competence in the new skills is achieved by exercising those skills under a wider set of conditions than those encountered during training, which are of necessity, limited by circumstances. In reality, the range of diving that is considered within the limits of their training is generally defined in the training standard for each level of certification. The additional condition of being within the limits of their experience is not always specified in the same way. Some agencies will specify that the diver is certified to dive independently in conditions similar to those in which they were trained, but they do not specify how the similarity can be assessed.
There is also the matter of how and where the skills were originally developed and honed to become the standard practice taught to the learner in formal training programmes. These skills did not exist before the equipment was developed which makes them useful, so the pioneers of each mode of diving had to develop them from what was preciously available, sometimes while using unfamiliar equipment, often by trial and error. Exploration of a place where no-one has been before, or where no-one has reported on the conditions there, is explicitly beyond the limits of a divers experience, but it is done routinely by divers who explore new dive sites. Most of these divers do not experience problems on most of their exploratory dives, and at the end of the dive have extended their experience.
Those organisations which train divers at the higher levels of certification are more likely to caution divers to expand their experience gradually, making as few changes at a time as reasonably practicable, and ensuring that they remain competent at all relevant skills within the extended range as it is expanded.
There is a large variability between certification standards, and major philosophical differences regarding the minimum level of acceptable competence. Training in the minimum required skills specified for the certification, splitting certification into multiple courses for maximum diver convenience and agency profit, is common in mainstream recreational diver training, where the diver is responsible for their own safety and the choices they may make, and the agency is protected by legal waivers, and where entry-level training typically can be done over about three or four days, with four open-water dives worth of experience, and on-line self-study and automated assessment of the theoretical knowledge component. Fitness to dive is self assessed using a questionnaire. This has been shown to be an effective business strategy.
Ensuring during the entry level course that the diver is adequately skilled to deal with the range of conditions and equipment likely to be encountered at work is typical of professional diver training where training standards are set out by legislation for workplace health and safety, where entry-level training may be full time over a month, with several hours of confined water skills training and practice, a substantial classroom based theory and knowledge section, and about 30 open water dives, with practice and assessment of all critical skills and compulsory medical fitness assessment by a registered diving medical practitioner.
A few recreational/technical certification agencies provide entry-level training to intermediate levels.
See main article: List of diving hazards and precautions. Divers operate in an environment for which the human body is not well suited. They face special physical and health risks when they go underwater or use high pressure breathing gas. The consequences of diving incidents range from merely annoying to rapidly fatal, and the result often depends on the equipment, skill, response and fitness of the diver and diving team. The hazards include the aquatic environment, the use of breathing equipment in an underwater environment, exposure to a pressurised environment and pressure changes, particularly pressure changes during descent and ascent, and breathing gases at high ambient pressure. Diving equipment other than breathing apparatus is usually reliable, but has been known to fail, and loss of buoyancy control or thermal protection can be a major burden which may lead to more serious problems. There are also hazards of the specific diving environment, and hazards related to access to and egress from the water, which vary from place to place, and may also vary with time. Hazards inherent in the diver include pre-existing physiological and psychological conditions and the personal behaviour and competence of the individual. For those pursuing other activities while diving, there are additional hazards of task loading, of the dive task and of special equipment associated with the task.
Professional divers may be exposed to a wider range of hazards, some of which are inherent in the equipment used to reduce the risk of other hazards. Saturation diving is intended to reduce a relatively high risk of decompression sickness, but introduces other health and safety hazards of living at a high ambient pressure for extended periods, and transfer between pressurised spaces. Failure of a saturation system can be catastrophic and fatal to the occupants and bystanders. Such failures are seldom engineering failures, they are more often ergonomic design and operation failures, and usually systems are corrected after analysis of such failures.
Occupational hazard types can also be classified as biological, chemical, physical, and psychosocial hazards.
See also: List of diving hazards and precautions and Diving disorders. Diving related medical conditions, are conditions associated with underwater diving, and include both conditions unique to underwater diving, and those that also occur during other activities. This second group further divides into conditions caused by exposure to ambient pressures significantly different from surface atmospheric pressure, and a range of conditions caused by general environment and equipment associated with diving activities.
Disorders particularly associated with diving include those caused by variations in ambient pressure, such as barotraumas of descent and ascent, decompression sickness and those caused by exposure to elevated ambient pressure, such as some types of gas toxicity and excessive work of breathing. There are also non-dysbaric disorders associated with diving, which include the effects of the aquatic environment, such as drowning, which also are common to other water users, and disorders caused by the equipment or associated factors, such as carbon dioxide and carbon monoxide poisoning. General environmental conditions can lead to another group of disorders, which include hypothermia and motion sickness, injuries by marine and aquatic organisms, contaminated waters, man-made hazards, and ergonomic problems with equipment and tasks. Finally there are pre-existing medical and psychological conditions which increase the risk of being affected by a diving disorder, which may be aggravated by adverse side effects of medications and other drug use.
Treatment depends on the specific disorder, but often includes oxygen therapy, which is standard first aid for most diving accidents, and is hardly ever contra-indicated for a person medically fit to dive, and hyperbaric therapy is the definitive treatment for decompression sickness. Screening for medical fitness to dive can reduce some of the risk for some of the disorders.
The labels used to classify dives are not sufficiently precise for analysing risk. Terms like "recreational", "technical", "commercial", "military", "scientific" and "professional" are used but are not precisely defined, particularly for risk analysis as they do not identify specific contributors to diving risk. Categorisation by depth and obligation for decompression stops is also insufficient to classify risk.
The diving mode has a large influence on risk, and choice of diving mode is commonly based on the outcome of a risk assessment for the diving operation.
Risk management has three major aspects besides equipment and training: Risk assessment, emergency management and insurance cover.The risk assessment for a dive is primarily a planning activity, and may range in formality from a part of the pre-dive buddy check for recreational divers, to a safety file with professional risk assessment and detailed emergency plans for professional diving projects. Some form of pre-dive briefing is customary with organised recreational dives, and this generally includes a recitation by the divemaster of the known and predicted hazards, the risk associated with the significant ones, and the procedures to be followed in case of the reasonably foreseeable emergencies associated with them. Insurance cover for diving accidents may not be included in standard policies. There are a few organisations which focus specifically on diver safety and insurance cover, such as the international Divers Alert Network
Risk assessment is particularly important when planning exploration dives and when diving beyond one's current range of experience. Potential points of failure and methods for managing the risk may require research beyond existing personal knowledge. In all cases the aim is to set up circumstances in which if something fails, it will fail safely.
The tools of diving risk management include:
A JSA is a procedure which helps integrate accepted safety and health principles and practices into a particular task or job operation. In a JSA, each basic step of the analysis is to identify potential hazards and to recommend the safest way to do the job. In professional diving a JSA would be done for the planned task for a specific dive, and the result would be included in the dive briefing.
Administrative control procedures for specific hazards include:
Situation awareness helps the dive team members to be proactive in handling incidents
The Incident pit is a way of understanding the development of an incident from the pre-existing conditions, through the triggering incident, to the unrecoverable stage.
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 hazards and risks remain. Hazards can be substituted by using a different mode of diving when applicable. 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, at the cost of substituting other, lower risk, hazards, associated with living in a saturation environment.
See main article: freediving. 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 the competition rules require competitions to 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 is ready and able to dive to the rescue if the diver loses consciousness during the ascent. This recommendation has an obvious point of failure when the visibility does not allow the buddy to observe the diver throughout the dive.
See main article: 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.
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 failure. For the system not to fail, all items must not fail according to the formula:
{p}=1-
| n | |
| \prod | |
| i=1 |
(1-pi)
where:
n
pi
p
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
preg=0.01
pcyl=0.001
Pfail=1-(1-preg) x (1-pcyl)
Pfail=1-(1-0.01) x (1-0.001)
=1-0.99 x 0.999
=1-0.98901
=0.01099
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:-
{p}=
| n | |
| \prod | |
| i=1 |
pi
where:
n
pi
p
Taking two independent sets with the same probability of failure calculated in the example above:
pleft=0.01099
pright=0.01099
Pfail=(pleft) x (pright)
Pfail=0.01099 x 0.01099
=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.
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.
See also: 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. Surface oriented diving means that the dive starts and ends at surface pressure, which may involve staged decompression stops, gas switches, or surface decompression.
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 also 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. Surface supplied systems routinely include voice communication between diver and supervisor, making it relatively easy to monitor the condition of the diver from the surface.
See main article: 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 state where the diver's tissues have absorbed the maximum concentration of inert gas possible for that depth due to the diver being exposed to breathing gas at that pressure for periods in the order of 24 hours. This is significant because once the tissues become saturated, the time required 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 or pressurised 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 or contract. By limiting the number of decompressions in this way, the risk of decompression sickness and the total time required for decompression of each diver 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.
See also: Atmospheric diving suit. 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 approximately one atmosphere. The ADS can be used for very deep dives of up to 2300feet 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 greatly 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, or as personal protective equipment.
See main article: Remotely operated underwater vehicle. 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.
See main article: Administrative controls. Administrative controls include medical screening, planning and preparation for diving and training in essential skills. These requirements may be specified by regulation, code of practice, operations manual or terms or conditions of a contract.
See also: List of legislation regulating underwater diving, Occupational safety and health, Code of practice and Operations manual. Professional diving is generally regulated by occupational safety and health legislation, which can vary between jurisdictions, but tends to have some common features.
In most jurisdictions, recreational diving is unregulated, and an entirely untrained person is not legally hindered from scuba or freediving at their own risk in public access bodies of water. There is to some extent industry self-regulation, and most service providers will require customers to show evidence of competence appropriate to the service requested. This is usually a matter of presenting a recognised C-card, and is intended mainly to limit liability. The other instrument commonly used to limit liability is a waiver signed by the customer as part of the conditions of service.
In special cases exemptions may be granted for specific classes of diving operations, or for specific diving projects.
USA – Depending on state legislation, public safety diving in the USA may fall under state or federal occupational safety and health legislation. Federal legislation applies where there is no relevant state legislation and the divers are employees diving as part of their occupation.If they fall under federal legislation they are exempt (excluded) from specific requirements of 29 CFR Part 1910, Subpart T, Commercial Diving Operations, only during diving activities incidental to police and public-safety functions the purpose of which is to provide search, rescue, or public-safety diving services. The exemption was written to include the ability to deviate from safe diving practices under limited conditions where compliance would be impracticable due to time constraints or the possible consequences of failing to perform the task overwhelm the risks taken using available facilities. This exclusion does not apply during training, recovery operations, searches where there is no reasonable probability of rescue of a living person or there is no real and immediate public safety hazard. The specific federal legislation does not apply to volunteers where there is no employer/employee relationship. Scientific diving in the US is also exempt from 29 CFR Part 1910, Subpart T, Commercial Diving Operations provided that such diving is under the auspices of the American Academy of Underwater Sciences, and complies with all requirements for the exemption, which are tied to the AAUS Standards for Scientific Diving Certification and Operation of Scientific Diving Programs.
See also: Fitness to dive. 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, and has to be done by a doctor who is competent in diving medicine, and in these cases fitness can not always be established by prescriptive rules.
Psychological factors can affect fitness to dive, particularly where they affect response to emergencies, or risk taking behaviour. Overconfidence and trait anxiety are both undesirable characteristics in a diver. 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.
Fitness to dive can be modified to some extent by training. Physical fitness can be improved to give the diver better capacity to deal with physical challenges, and properly conducted stress exposure training can improve situational awareness and the ability to focus on relevant responses under stress.
See also: Dive 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 an overview of diving activities, a schedule of diving operations, specific information on the dive plan, contingency plans, emergency plans, and a budget.
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, but diverging to an existing contingency plan is not normally considered deviation from the plan. 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.