Saturation diving is diving for periods long enough to bring all tissues into equilibrium with the partial pressures of the inert components of the breathing gas used. It is a diving mode that reduces the number of decompressions divers working at great depths must undergo by only decompressing divers once at the end of the diving operation, which may last days to weeks, having them remain under pressure for the whole period. A diver breathing pressurized gas accumulates dissolved inert gas used in the breathing mixture to dilute the oxygen to a non-toxic level in the tissues, which can cause potentially fatal decompression sickness ("the bends") if permitted to come out of solution within the body tissues; hence, returning to the surface safely requires lengthy decompression so that the inert gases can be eliminated via the lungs. Once the dissolved gases in a diver's tissues reach the saturation point, however, decompression time does not increase with further exposure, as no more inert gas is accumulated.^[1][2]

Saturation diving takes advantage of this by having divers remain in that saturated state. When not in the water, the divers live in a sealed environment which maintains their pressurised state; this can be an ambient pressure underwater habitat or a saturation system at the surface, with transfer to and from the pressurised living quarters to the equivalent depth underwater via a closed, pressurised diving bell. This may be maintained for up to several weeks, and divers are decompressed to surface pressure only once, at the end of their tour of duty. By limiting the number of decompressions in this way, and using a conservative decompression schedule the risk of decompression sickness is significantly reduced, and the total time spent decompressing is minimised. Saturation divers typically breathe a helium–oxygen mixture to prevent nitrogen narcosis, and limit work of breathing, but at shallow depths saturation diving has been done on nitrox mixtures.

Most of the physiological and medical aspects of diving to the same depths are much the same in saturation and bell-bounce ambient pressure diving, or are less of a problem, but there are medical and psychological effects of living under saturation for extended periods.

Saturation diving is a specialized form of diving; of the 3,300 commercial divers employed in the United States in 2015,^[3] 336 were saturation divers.^[4] Special training and certification is required, as the activity is inherently hazardous, and a set of standard operating procedures, emergency procedures, and a range of specialised equipment is used to control the risk, that require consistently correct performance by all the members of an extended diving team. The combination of relatively large skilled personnel requirements, complex engineering, and bulky, heavy equipment required to support a saturation diving project make it an expensive diving mode, but it allows direct human intervention at places that would not otherwise be practical, and where it is applied, it is generally more economically viable than other options, if such exist.

History

On December 22, 1938, Edgar End and Max Nohl made the first intentional saturation dive by spending 27 hours breathing air at 101 feet sea water (fsw) (30.8 msw) in the County Emergency Hospital recompression facility in Milwaukee, Wisconsin. Their decompression lasted five hours leaving Nohl with a mild case of decompression sickness that resolved with recompression.^[5]

Albert R. Behnke proposed the idea of exposing humans to increased ambient pressures long enough for the blood and tissues to become saturated with inert gases in 1942.^[6][7] In 1957, George F. Bond began the Genesis project at the Naval Submarine Medical Research Laboratory proving that humans could in fact withstand prolonged exposure to different breathing gases and increased environmental pressures.^[6][8] Once saturation is achieved, the amount of time needed for decompression depends on the depth and gases breathed. This was the beginning of saturation diving and the US Navy's Man-in-the-Sea Program.^[9] The first commercial saturation dives were performed in 1965 by Westinghouse to replace faulty trash racks at 200 feet (61 m) on the Smith Mountain Dam.^[5]

Peter B. Bennett is credited with the invention of trimix breathing gas as a method to eliminate high pressure nervous syndrome. In 1981, at the Duke University Medical Center, Bennett conducted an experiment called Atlantis III, which involved subjecting volunteers to a pressure of 2250 fsw (equivalent to a depth of 686 m in seawater), and slowly decompressing them to atmospheric pressure over a period of 31-plus days, setting an early world record for depth-equivalent in the process. A later experiment, Atlantis IV, encountered problems as one of the volunteers experienced euphoric hallucinations and hypomania.^[10]

The history of commercial saturation diving is closely linked to offshore oil and gas extraction. In the early 1960s exploration of the North Sea started on the premise that the Dutch gas fields might extend under the sea. This was borne out when the Gulf Tide rig hit the Ekofisk reservoir in 1969 and in 1971 Shell oil found the Brent oilfield between Norway and Shetland. From this time to the 1990s the industry developed the procedures and equipment for saturation diving from pioneering and experimental, with a somewhat dubious safety record, to a mature industry with greatly improved occupational health and safety.^[11]

When the North Sea drilling started, there was little diving support infrastructure in Europe, and the high wages attracted divers from the Gulf of Mexico oilfields, who introduced the fibre reinforced resin lightweight demand helmets from Kirby-Morgan, hot water suits from Diving Unlimited International, and the U.S. Navy Diving Manual, at the time the leading set of offshore diving procedures. Research and development money was available, and new technical developments were supported by the European Economic Community. A major challenge was developing saturation diving practices suitable to the common North Sea depth range of 100 to 180 m.^[11]

During the early drilling stages most of the diving work was for relatively short periods and was generally suitable for bell bounce diving, but the development of oilfield seabed infrastructure required much longer diver interventions, and saturation diving procedures were developed to suit. By 1982, a large amount of shallow maintenance work was becoming necessary, which brought in more air diving to service the rigs. By 2017 about 80% of North Sea diving was heliox saturation diving and the other 20% shallow air diving.^[11]

Excursion dives without decompression stops can be done both upward and downward from saturation storage pressure within limits, allowing the divers a range of working depths, and if work is required beyond excursion range, the divers can be compressed or decompressed in storage to suit the changed depth range. Further work was done by the United States Navy Experimental Diving Unit on excursion dives from February 1974 to June 1976, and the results published in the 1984 U.S. Navy Diving Manual.^[11] These tables used a partial pressure of oxygen of 0.35 to 0.4 bar during decompression, with quite slow decompression rates, which varied with the depth, getting slower as the depth decreased, with a 6-hour stop from midnight and a two-hour stop from 14:00 and a gas fraction limit of 22% for the last part of the ascent to reduce fire risk. The tables allowed decompression to start directly after return from a dive provided there had not been an upward excursion, as this was found to increase the risk of bubble development.^[11]

At the same time, the commercial diving contractor Compagnie maritime d'expertises (COMEX) had been developing slightly different decompression procedures, in which the oxygen partial pressures were higher, between 0.6 and 0.8 bar, and the ascent rates were faster to take advantage of the high P_O2. Continuous decompression without night stops was used, and excursions were allowed. Over time these were revised to use lower P_O2 and slower ascent rates, particularly at the shallower depths. Competing tables were thought to be used to gain competitive advantage, so in 1988 the Norwegian Petroleum Directorate organised a conference on saturation decompression safety under Val Hempleman,^[11] and in 1990 a conference to harmonise the saturation tables to be used in the North Sea in the Norwegian sector using input from five contractors. In 1999 the NORSOK U100 standard was published, which was a compromise using aspects of several of the tables, but which has proven in use to be sufficiently conservative and has a good safety record.^[11]

In the 1980s the Royal Navy were using an oxygen partial pressure of 0.42 bar for decompression from saturation, which is slightly higher than to 0.40 bar of the US Navy table. This reduced the time for decompression by a small percentage.^[12]

Saturation decompression in the Brazil oilfields took a slightly different route, and was originally based on company tables, until Brazil produced their own legislation in 1988, similar to that of the UK's Health and Safety Executive. In 2004 revised legislation was closer to the COMEX procedures.^[11]

By 2017 the system had settled into a chamber P_O2 of 0.5 bar while deeper than 15 msw, and limited to 22 to 23% at the end of decompression to limit fire risk.^[11]

Applications

Saturation diving has applications in scientific diving and commercial offshore diving.^[13]

Commercial offshore diving, sometimes shortened to just offshore diving, is a branch of commercial diving, with divers working in support of the exploration and production sector of the oil and gas industry in places such as the Gulf of Mexico in the United States, the North Sea in the United Kingdom and Norway, and along the coast of Brazil. The work in this area of the industry includes maintenance of oil platforms and the building of underwater structures. In this context "offshore" implies that the diving work is done outside of national boundaries.

Saturation diving is standard practice for bottom work at many of the deeper offshore sites, and allows more effective use of the diver's time while reducing the risk of decompression sickness.^[2] Surface oriented air diving is more usual in shallower water.

Underwater habitats are underwater structures in which people can live for extended periods and carry out most of the basic human functions of a 24-hour day, such as working, resting, eating, attending to personal hygiene, and sleeping. In this context 'habitat' is generally used in a narrow sense to mean the interior and immediate exterior of the structure and its fixtures, but not its surrounding marine environment. Most early underwater habitats lacked regenerative systems for air, water, food, electricity, and other resources. However, recently some new underwater habitats allow for these resources to be delivered using pipes, or generated within the habitat, rather than manually delivered.^[14]

An underwater habitat has to meet the needs of human physiology and provide suitable environmental conditions, and the one which is most critical is breathing air of suitable quality. Others concern the physical environment (pressure, temperature, light, humidity), the chemical environment (drinking water, food, waste products, toxins) and the biological environment (hazardous sea creatures, microorganisms, marine fungi). Much of the science covering underwater habitats and their technology designed to meet human requirements is shared with diving, diving bells, submersible vehicles and submarines, and spacecraft.

Numerous underwater habitats have been designed, built and used around the world since the early 1960s, either by private individuals or by government agencies. They have been used almost exclusively for research and exploration, but in recent years at least one underwater habitat has been provided for recreation and tourism.^{[citation needed]} Research has been devoted particularly to the physiological processes and limits of breathing gases under pressure, for aquanaut and astronaut training, as well as for research on marine ecosystems. Access to and from the exterior is generally vertically through a hole in the bottom of the structure called a moon pool. The habitat may include a decompression chamber, or personnel transfer to the surface may be via a closed diving bell.

Employment

Saturation diving work in support of the offshore oil and gas industries is usually contract based.^[15]

Medical aspects

Decompression sickness

Decompression sickness (DCS) is a potentially fatal condition caused by bubbles of inert gas, which can occur in divers' bodies as a consequence of the pressure reduction as they ascend. To prevent decompression sickness, divers have to limit their rate of ascent, to reduce the concentration of dissolved gases in their body sufficiently to avoid bubble formation and growth. This protocol, known as decompression, can last for several hours for dives in excess of 50 metres (160 ft) when divers spend more than a few minutes at these depths. The longer divers remain at depth, the more inert gas is absorbed into their body tissues, and the time required for decompression increases rapidly.^[16] This presents a problem for operations that require divers to work for extended periods at depth, as the time spent decompressing can exceed the time spent doing useful work by a large margin. However, after somewhere around 72 hours under any given pressure, depending on the ingassing model used, divers' bodies become saturated with inert gas, and no further uptake occurs. From that point onward, no increase in decompression time is necessary. The practice of saturation diving takes advantage of this by providing a means for divers to remain at depth pressure for days or weeks. At the end of that period, divers need to carry out a single saturation decompression, which is much more efficient and a lower risk than making multiple short dives, each of which requires a lengthy decompression time. By making the single decompression slower and longer, in the controlled conditions and relative comfort of the saturation habitat or decompression chamber, the risk of decompression sickness during the single exposure is further reduced.^[2]

High-pressure nervous syndrome

High-pressure nervous syndrome (HPNS) is a neurological and physiological diving disorder that results when a diver descends below about 500 feet (150 m) while breathing a helium–oxygen mixture. The effects depend on the rate of descent and the depth.^[17] HPNS is a limiting factor in future deep diving.^[18] HPNS can be reduced by using a small percentage of nitrogen in the gas mixture.^[18]

Compression arthralgia

Compression arthralgia is a deep aching pain in the joints caused by exposure to high ambient pressure at a relatively high rate of compression, experienced by underwater divers. The pain may occur in the knees, shoulders, fingers, back, hips, neck or ribs, and may be sudden and intense in onset and may be accompanied by a feeling of roughness in the joints.^[19] Onset commonly occurs around 60 msw (meters of sea water), and symptoms are variable depending on depth, compression rate and personal susceptibility. Intensity increases with depth and may be aggravated by exercise. Compression arthralgia is generally a problem of deep diving, particularly deep saturation diving, where at sufficient depth even slow compression may produce symptoms. The use of trimix can reduce the symptoms.^[20] Spontaneous improvement may occur over time at depth, but this is unpredictable, and pain may persist into decompression. Compression arthralgia may be easily distinguished from decompression sickness as it starts during descent, is present before starting decompression, and resolves with decreasing pressure, the opposite of decompression sickness. The pain may be sufficiently severe to limit the diver's capacity for work, and may also limit the depth of downward excursions.^[19]

Dysbaric osteonecrosis

Saturation diving (or more precisely, long term exposure to high pressure) is associated with aseptic bone necrosis, although it is not yet known if all divers are affected or only especially sensitive ones. The joints are most vulnerable to osteonecrosis. The connection between high-pressure exposure, decompression procedure and osteonecrosis is not fully understood.^[21][22][23]

Extreme depth effects

A breathing gas mixture of oxygen, helium and hydrogen was developed for use at extreme depths to reduce the effects of high pressure on the central nervous system. Between 1978 and 1984, a team of divers from Duke University in North Carolina conducted the Atlantis series of onshore-hyperbaric-chamber-deep-scientific-test-dives.^[10] In 1981, during an extreme depth test dive to 686 metres (2251 ft) they breathed the conventional mixture of oxygen and helium with difficulty and suffered trembling and memory lapses.^[10][24]

A hydrogen–helium–oxygen (hydreliox) gas mixture was used during a similar on shore scientific test dive by three divers involved in an experiment for the French Comex S.A. industrial deep-sea diving company in 1992. On 18 November 1992, Comex decided to stop the experiment at an equivalent of 675 meters of sea water (msw) (2215 fsw) because the divers were suffering from insomnia and fatigue. All three divers wanted to push on but the company decided to decompress the chamber to 650 msw (2133 fsw). On 20 November 1992, Comex diver Theo Mavrostomos was given the go-ahead to continue but spent only two hours at 701 msw (2300 fsw). Comex had planned for the divers to spend four and a half days at this depth and carry out tasks.^[24]

Oxygen toxicity

Both acute and chronic oxygen toxicity are significant risks in saturation diving. The storage breathing gas exposes the divers to one continuous level of oxygen concentration for extended periods, on the order of a month at a time, which requires the gas in the habitat to be maintained at a long term tolerable partial pressure, generally around 0.4 bar, which is well tolerated, and allows for quite large accidental deviations without causing hypoxia. This may be increased during decompression, but as decompression may take over a week, the safely tolerable increase is limited, and at lower pressures oxygen partial pressure is also limited by fire hazard considerations.^[25][1]

Bell and excursion gas composition must suit the planned dive profile. A higher oxygen partial pressure may be tolerable over the working period, but it may be logistically preferable to use the same gas used for storage. Bailout gas may have a higher oxygen content. At one time the recommended bailout oxygen partial pressure was significantly higher than used in the main gas supply.^[26][27]

Thermal balance of the diver

Thermoregulation is the ability of an organism to keep its body temperature within specific bounds, even when the surrounding temperature is very different. The internal thermoregulation process is one aspect of homeostasis: a state of dynamic stability in an organism's internal conditions, maintained far from thermal equilibrium with its environment. If the body is unable to maintain a normal human body temperature and it increases significantly above normal, a condition known as hyperthermia occurs. The opposite condition, when body temperature decreases below normal levels, is known as hypothermia. It occurs when the body loses heat faster than producing it.

Body heat is lost by respiratory heat loss, by heating and humidifying (latent heat) inspired gas, and by body surface heat loss, by radiation, conduction, and convection, to the atmosphere, water, and other substances in the immediate surroundings. Surface heat loss may be reduced by insulation of the body surface. Heat is produced internally by metabolic processes and may be supplied from external sources by active heating of the body surface or the breathing gas.^[28]

Heat transfer to and via gases at higher pressure than atmospheric is increased due to the higher density of the gas at higher pressure which increases its heat capacity. This effect is also modified by changes in breathing gas composition necessary for reducing narcosis and work of breathing, to limit oxygen toxicity and to accelerate decompression. Heat loss through conduction is faster for higher fractions of helium. Divers in a helium based saturation habitat will lose or gain heat fast if the gas temperature is too low or too high, both via the skin and breathing, and therefore the tolerable temperature range is smaller than for the same gas at normal atmospheric pressure.^[28]

The heat loss situation is very different in the saturation living areas, which are temperature and humidity controlled, in the dry bell, and in the water.^[29]

The alveoli of the lungs are very effective at heat and humidity transfer. Inspired gas that reaches them is heated to core body temperature and humidified to saturation in the time needed for gas exchange, regardless of the initial temperature and humidity. This heat and humidity are lost to the environment in open circuit breathing systems. Breathing gas that only gets as far as the physiological dead space is not heated so effectively. When heat loss exceeds heat generation, body temperature will fall.^[28]

Exertion increases heat production by metabolic processes, but when breathing gas is cold and dense, heat loss due to the increased volume of gas breathed to support these metabolic processes can result in a net loss of heat, even if the heat loss through the skin is minimised.

This section needs expansion with: more detail on heat loss and effects of changes to the gas mixture (Neves and Thomas)[28]. You can help by adding to it. (February 2024)

Health effects of living under saturation conditions

There is some evidence of long term cumulative reduction in lung function in saturation divers.^[30]

Saturation divers are frequently troubled by superficial infections such as skin rashes, otitis externa and athlete's foot, which occur during and after saturation exposures. This is thought to be a consequence of raised partial pressure of oxygen, and relatively high temperatures and humidity in the accommodation.^[31][12]

Dysbaric osteonecrosis is considered a consequence of decompression injury rather than living under saturation conditions.^{[citation needed]}

Long term cumulative exposure to high oxygen partial pressures is associated with accelerated development of cataracts.^[32]

Duration of exposure and surface intervals

The Diving Medical Advisory Council recommends that under normal circumstances the duration of a saturation dive should not exceed 28 days, and the interval between saturation exposures should generally equal the duration of the previous exposure, with a cumulative exposure of not more than 182 days in any 12 month period.^[33]

Operating procedures

Saturation diving allows professional divers to live and work at pressures greater than 50 msw (160 fsw) for days or weeks at a time, though lower pressures have been used for scientific work from underwater habitats. This type of diving allows for greater economy of work and enhanced safety for the divers.^[1] After working in the water, they rest and live in a dry pressurized habitat on, or connected to, a diving support vessel, oil platform or other floating work station, at approximately the same pressure as the work depth. The diving team is compressed to the working pressure only once, at the beginning of the work period, and decompressed to surface pressure once, after the entire work period of days or weeks. There are accepted safe upward and downward excursion limits based on the storage depth. Excursions to greater depths require decompression when returning to storage depth, and excursions to shallower depths are also limited by decompression obligations to avoid decompression sickness during the excursion.^[1] Most of the diving skills required for saturation diving are the same as for surface-oriented surface-supplied diving.

Increased use of underwater remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) for routine or planned tasks means that saturation dives are becoming less common, though complicated underwater tasks requiring complex manual actions remain the preserve of the deep-sea saturation diver.^{[citation needed]}

A person who operates a saturation diving system is called a life support technician (LST).^[34]: 23

Personnel requirements

A saturation diving team requires at the minimum the following personnel:^[35]

• A diving supervisor (on duty during any diving operations)
• Two life-support supervisors (working shifts while there are divers under pressure)
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