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 or "saturation spread", a hyperbaric environment on the surface, or an ambient pressure underwater habitat. This may be maintained for up to several weeks, 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.
- 1 History
- 2 Medical aspects
- 3 Operating procedures
- 4 Architecture of a surface saturation facility
- 4.1 Personnel transfer capsule
- 4.2 Handling system
- 4.3 Life support systems
- 4.4 Hot water system
- 4.5 Communication systems
- 4.6 Gas reclaim systems
- 4.7 Transfer chamber
- 4.8 Accommodation chambers
- 4.9 Recompression chamber
- 4.10 Mating flange for transportable chamber
- 4.11 Supply lock
- 4.12 Sanitation system
- 4.13 Main control console
- 4.14 Fire suppression system
- 4.15 Built in breathing systems
- 4.16 Gas supplies
- 4.17 Hyperbaric rescue and escape systems
- 4.18 Underwater habitats
- 5 Saturation diving depth records
- 6 Saturation diving in fiction
- 7 See also
- 8 References
- 9 Further reading
- 10 External links
On December 22, 1938, Edgar End and Max Nohl made the first intentional saturation dive by spending 27 hours breathing air at 101 feet 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.
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. 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. 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.
Peter B. Bennett is credited with the invention of trimix breathing gas as a method to eliminate High Pressure Nervous Syndrome. In 1981, at Duke University Medical Center, Bennett conducted an experiment called Atlantis III, which involved taking divers to a depth of 2,250 feet (685.8 metres), and slowly decompressing them to the surface over a period of 31-plus days, setting an early world record for depth in the process.
Decompression sickness (DCS) is a potentially fatal condition caused by bubbles of inert gas, which can occur in divers' bodies following the pressure reduction as they ascend. To prevent DCS, divers have to limit their rate of ascent, and pause at regular intervals to allow the pressure of gases in their body to approach equilibrium. This protocol, known as decompression, can last for many 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. This presents a problem for operations that require divers to work for extended periods at depth. However, after several hours under pressure, divers' bodies become saturated with inert gas, and no further uptake occurs. From that point onward, no increase in decompression time is necessary. The idea of saturation diving takes advantage of this by providing a means for divers to remain at depth for days. At the end of that period, divers need to carry out a single 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 relative comfort of the saturation habitat or decompression chamber, the risk of decompression sickness during the single exposure is further reduced.
High Pressure Nervous Syndrome
High Pressure Nervous Syndrome 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. HPNS is a limiting factor in future deep diving. HPNS can be reduced by using a small percentage of Nitrogen in the gas mixture.
Saturation diving (or more precisely, long term exposure to high pressure) can potentially cause 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 and osteonecrosis is not fully understood.
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 on-shore-hyperbaric-chamber-deep-scientific-test-dives. In 1981, during an extreme depth test dive to 686 metres they breathed the conventional mixture of oxygen and helium with difficulty and suffered trembling and memory lapses.
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) 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. On 20 November 1992, Comex diver Theo Mavrostomos was given the go-ahead to continue but spent only two hours at 701 msw (2300 ft). Comex had planned for the divers to spend four and a half days at this depth and carry out tasks.
Commonly, saturation diving allows professional divers to live and work at pressures greater than 50 msw (160 fsw) for days or weeks at a time. This type of diving allows for greater economy of work and enhanced safety for the divers. 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.
Increased use of underwater remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUV's) 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.
Compression to storage depth is generally at a limited rate to minimize the risk of HPNS and compression arthralgia. Norwegian standards specifies a maximum compression rate of 1 msw per minute, and a rest period at storage depth after compression and before diving.
Storage depth, also known as living depth, is the pressure in the accommodation sections of the saturation habitat — the ambient pressure under which the saturation divers live when not engaged in lock-out activity. Any change in storage depth involves a compression or a decompression, both of which are stressful to the occupants, and therefore dive planning should minimize the need for changes of living depth and excursion exposures, and storage depth should be as close as practicable to the working depth, taking into account all relevant safety considerations.
The hyperbaric atmosphere in the accommodation chambers and the bell are controlled to ensure that the risk of long term adverse effects on the divers is acceptably low. Most saturation diving is done on heliox mixtures, with partial pressure of oxygen in accommodation areas kept around 0.40 to 0.48 bar, which is near the upper limit for long term exposure. Carbon dioxide is removed from the chamber gas by recycling it through scrubber cartridges. The levels are generally limited to a maximum of 0.005 bar partial pressure, equivalent to 0.5% surface equivalent. Most of the balance is helium, with a small amount of nitrogen and trace residuals from the air in the system before compression.
Bell operations and lockouts may also be done at between 0.4 and 0.6 bar oxygen partial pressure, but often use a higher partial pressure of oxygen, between 0.6 and 0.9 bar, which lessens the effect of pressure variation due to excursions away from holding pressure, thereby reducing the amount and probability of bubble formation due to these pressure changes. In emergencies a partial pressure of 0.6 bar of oxygen can be tolerated for over 24 hours, but this is avoided where possible. Carbon dioxide can also be tolerated at higher levels for limited periods. US Navy limit is 0.02 bar for up to 4 hours. Nitrogen partial pressure starts at 0.79 bar, but tends to decrease over time as the system loses gas to lock operation, and is topped up with helium.
Deployment of divers
Deployment of divers from a surface saturation complex requires the diver to be transferred under pressure from the accommodation area to the underwater workplace. This is generally done by using a closed diving bell, also known as a Personnel Transfer Capsule, which is clamped to the lock flange of the accommodation transfer chamber and the pressure equalized with the accommodation transfer chamber for transfer to the bell. The lock doors can then be opened for the divers to enter the bell. The divers will suit up before entering the bell and completing the pre-dive checks. The pressure in the bell will be adjusted to suit the depth at which the divers will lock out while the bell is being lowered, so that the pressure change can be slow without unduly delaying operations.
The bell is deployed over the side of the vessel or platform using a gantry or A-frame or through a moon pool. Deployment usually starts by lowering the clump weight, which is a large ballast weight suspended from a cable which runs down one side from the gantry, through a set of sheaves on the weight, and up the other side back to the gantry, where it is fastened. The weight hangs freely between the two parts of the cable, and due to its weight, hangs horizontally and keeps the cable under tension. The bell hangs between the parts of the cable, and has a fairlead on each side which slides along the cable as it is lowered or lifted. The bell hangs from a cable attached to the top. As the bell is lowered, the fairleads guide it down the clump weight cables to the workplace.
The bell umbilical is separate from the divers' umbilicals, which are connected on the inside of the bell. The bell umbilical is deployed from a large drum or umbilical basket and care is taken to keep the tension in the umbilical low but sufficient to remain near vertical in use and to roll up neatly during recovery.
Once the bell is at the correct depth, the final adjustments to pressure are made and after final checks, the supervisor instructs the working diver(s) to lock out of the bell. The hatch is at the bottom of the bell and can only be opened if the pressure inside is balanced with the ambient water pressure. The bellman tends the working diver's umbilical through the hatch during the dive. If the diver experiences a problem and needs assistance, the bellman will exit the bell and follow the diver's umbilical to the diver and render whatever help is necessary and possible. Each diver carries back mounted bailout gas, which should be sufficient to allow a safe return to the bell in the event of an umbilical gas supply failure.
Breathing gas is supplied to the divers from the surface through the bell umbilical. If this system fails, the ball carries an on-board gas supply which is plumbed into the bell gas panel and can be switched by operating the relevant valves. On-board gas is generally carried externally in several storage cylinder of 50 litres capacity or larger, connected through pressure regulators to the gas panel.
Helium is a very effective heat transfer material, and divers may lose heat rapidly if the surrounding water is cold. To prevent hypothermia, hot-water suits are commonly used for saturation diving, and the breathing gas supply may be heated. Heated water is produced at the surface and piped to the bell through a hot-water line in the bell umbilical, then is transferred to the divers through their excursion umbilicals.
The umbilicals also have cables for electrical power to the bell and helmet lights, and for voice communications and closed circuit video cameras. In some cases the breathing gas is recovered to save the expensive helium. This is done through a reclaim hose in the umbilicals, which ducts exhaled gas through a reclaim valve, through the umbilicals and back to the surface, where the carbon dioxide is scrubbed and the gas boosted into storage cylinders for later use.
Excursions from storage depth
It is quite common for saturation divers to need to work over a range of depths while the saturation system can only maintain one or two storage depths at any given time. A change of depth from storage depth is known as an excursion, and divers can make excursions within limits without incurring a decompression obligation, just as there are no-decompression limits for surface oriented diving. Excursions may be upward or downward from the storage depth, and the allowed depth change may be the same in both directions, or sometimes slightly less upward than downward. Excursion limits are generally based on a 6 to 8 hour time limit, as this is the standard time limit for a diving shift. These excursion limits imply a significant change in gas load in all tissues for a depth change of around 15m for 6 to 8 hours, and experimental work has shown that both venous blood and brain tissue are likely to develop small asymptomatic bubbles after a full shift at both the upward and downward excursion limits. These bubbles remain small due to the relatively small pressure ratio between storage and excursion pressure, and are generally resolved by the time the diver is back on shift, and residual bubbles do not accumulate over sequential shifts. However, any residual bubbles pose a risk of growth if decompression is started before they are fully eliminated. Ascent rate during excursions is limited, to minimize the risk and amount of bubble formation.
Decompression from saturation
Once all the tissue compartments have reached saturation for a given pressure and breathing mixture, continued exposure will not increase the gas loading of the tissues. From this point onwards the required decompression remains the same. If divers work and live at pressure for a long period, and are decompressed only at the end of the period, the risks associated with decompression are limited to this single exposure. This principle has led to the practice of saturation diving, and as there is only one decompression, and it is done in the relative safety and comfort of a saturation habitat, the decompression is done on a very conservative profile, minimising the risk of bubble formation, growth and the consequent injury to tissues. A consequence of these procedures is that saturation divers are more likely to suffer decompression sickness symptoms in the slowest tissues, whereas bounce divers are more likely to develop bubbles in faster tissues.
Decompression from a saturation dive is a slow process. The rate of decompression typically ranges between 3 and 6 fsw (0.9 ad 1.8 msw) per hour. The US Navy Heliox saturation decompression rates require a partial pressure of oxygen to be maintained at between 0.44 and 0.48 atm when possible, but not to exceed 23% by volume, to restrict the risk of fire
|1600 to 200 fsw||6 fsw per hour|
|200 to 100 fsw||5 fsw per hour|
|100 to 50 fsw||4 fsw per hour|
|50 to 0 fsw||3 fsw per hour|
For practicality the decompression is done in increments of 1 fsw at a rate not exceeding 1 fsw per minute, followed by a stop, with the average complying with the table ascent rate. Decompression is done for 16 hours in 24, with the remaining 8 hours split into two rest periods. A further adaptation generally made to the schedule is to stop at 4 fsw for the time that is would theoretically take to complete the decompression at the specified rate, i.e. 80 minutes, and then complete the decompression to surface at 1 fsw per minute. This is done to avoid the possibility of losing the door seal at a low pressure differential and losing the last hour or so of slow decompression.
Decompression following a recent excursion
Neither the excursions nor the decompression procedures currently in use have been found to cause decompression problems in isolation. However there appears to be significantly higher risk when excursions are followed by decompression before non-symptomatic bubbles resulting from excursions have totally resolved. Starting decompression while bubbles are present appears to be the significant factor in many cases of otherwise unexpected decompression sickness during routine saturation decompression. The Norwegian standards do not allow decompression following directly on an excursion.
Architecture of a surface saturation facility
The "Saturation System" typically comprises either an underwater habitat or a surface complex made up of a living chamber, transfer chamber and submersible decompression chamber, which is commonly referred to in commercial diving and military diving as the diving bell, PTC (Personnel Transfer Capsule) or SDC (Submersible Decompression Chamber). The system can be permanently placed on a ship or ocean platform, but is more commonly capable of being moved from one vessel to another by crane. The entire system is managed from a control room (van), where depth, chamber atmosphere and other system parameters are monitored and controlled. The diving bell is the elevator or lift that transfers divers from the system to the work site. Typically, it is mated to the system utilizing a removable clamp and is separated from the system tankage bulkhead by a trunking space, a kind of tunnel, through which the divers transfer to and from the bell. At the completion of work or a mission, the saturation diving team is decompressed gradually back to atmospheric pressure by the slow venting of system pressure, at an average of 15 metres (49 ft) to 30 metres (98 ft) per day (schedules vary). Thus the process involves only one ascent, thereby mitigating the time-consuming and comparatively risky process of in-water, staged decompression normally associated with non-saturation ("mixed gas diving or sur-D O2") operations.
The divers use surface supplied umbilical diving equipment, utilizing deep diving breathing gas, such as helium and oxygen mixtures, stored in large capacity, high pressure cylinders. The gas supplies are plumbed to the control room, where they are routed to supply the system components. The bell is fed via a large, multi-part umbilical that supplies breathing gas, electricity, communications and hot water. The bell also is fitted with exterior mounted breathing gas cylinders for emergency use.
While in the water the divers will often use a hot water suit to protect against the cold. The hot water comes from boilers on the surface and is pumped down to the diver via the bell's umbilical and then through the diver's umbilical.
Personnel transfer capsule
A closed diving bell, also known as Personnel Transfer Capsule or Submersible Decompression Chamber, is used to transport divers between the workplace and the accommodations chambers. The bell is a cylindrical or spherical pressure vessel with a hatch at the bottom, and may mate with the surface transfer chamber at the bottom hatch or at a side door. Bells are usually designed to carry two or three divers, one of whom, the bellman, stays inside the bell at the bottom and is stand-by diver to the working divers. Each diver is supplied by an umbilical from inside the bell. The bell has a set of high pressure gas storage cylinders mounted on the outside containing on-board reserve breathing gas. The on-board gas and main gas supply are distributed from the bell gas panel, which is controlled by the bellman. The bell may have viewports and external lights. The divers' umbilicals are stored on racks inside the bell during transfer, and are tended by the bellman during the dive.
- The handling system must be able to support the dynamic loads imposed by operating in a range of weather conditions.
- It must be able to move the bell through the air/water interface in a controlled way, fast enough to avoid excessive movement caused by wave action. A bell cursor may be used to limit lateral motion through the splash zone.
- It must keep the bell clear of the vessel or platform to prevent impact damage or injury.
- It must have sufficient power for fast retrieval of the bell in an emergency, and fine control to facilitate mating of the bell and transfer flange, and to accurately place the bell at the bottom.
- It must include a system to move the bell between the mating flange of the transfer chamber and the launch/retrieval position.
Life support systems
A life support system for the bell provides and monitors the main supply of breathing gas, and the control station monitors the deployment and communications with the divers. Gas supply to the bell is through a bell umbilical, made up from a number of hoses and electrical cables twisted together and deployed as a unit.
The accommodation life support system maintains the chamber environment within the acceptable range for health and comfort of the occupants. Temperature, humidity, breathing gas quality and equipment function are monitored and controlled.
Hot water system
A diver hot water system heats filtered seawater and pumps it to the divers through the bell and diver umbilicals. This water is used to heat the breathing gas before it is inhaled, and flows into the diver's exposure suit to keep the diver warm.
The communications system may have 4 component systems.
- The ubiquitous hardwire intercom system, an amplified voice system with speech unscrambler to reduce the pitch of the speech of the occupants of the pressurized system. This system will provide communications between the main control console and the bell and accommodation chambers. This two-way system is the primary communications mode.
- Wireless through-water communications between bell and main control console is a backup system in case of failure of the hardwire system with the bell.
- Closed circuit video from cameras on the bell and diver helmets allow visual monitoring of the dive and the divers by the supervisor.
- A sound powered phone system may be provided as a backup voice communication system between bell and control console
Gas reclaim systems
A helium reclaim system (or push-pull system) may be used to recover helium based breathing gas after use by the divers as this is more economical than losing it to the environment in open circuit systems. The recovered gas is passed through a scrubber system to remove carbon dioxide, filtered to remove odours, and pressurised into storage containers, where it may be mixed with oxygen to the required composition.
During extended diving operation very large amounts of breathing gas are used. A closed circuit gas reclaim system can save around 80% of gas costs by recovering the helium based breathing mixture. Reclaim also reduces the amount of gas storage required on board, which can be important where storage capacity is limited. Reclaim systems are also used to recover gas discharged from the saturation system during decompression.
A reclaim system will typically consist of the following components:
- A control console, which controls and monitors the booster pump, oxygen addition, diver supply pressure, exhaust hose pressure and make-up gas addition.
- A gas reprocessing unit, which will remove carbon dioxide in a scrubber and excess moisture in a condensation water trap.
- A gas booster, to boost the pressure of the reclaimed gas to the storage pressure.
- A storage system of pressure vessels to hold the boosted and reconstituted gas mixture until it is used. This functions as a buffer to allow for the variations of gas volume in the rest of the system due to pressure changes.
- A bell gas supply panel, to control the supply of gas to the bell.
- The bell umbilical, with the supply and exhaust hoses between the topside system and the bell.
- Internal bell gas panel to supply the gas to the divers, and bell reclaim equipment, which controls the exhaust hose back-pressure, and can shut off the reclaim hose if the diver's gas supply is interrupted. A scrubber for the bell atmosphere and water trap would be included.
- Diver umbilicals, with supply and exhaust hoses between the bell and the divers
- Reclaim helmets, which supply gas to the divers on demand, and exhaust the exhaled gas to the return line.
The transfer chamber is where the bell is mated to the surface saturation system. It is a wet chamber where divers prepare for a dive and strip off and clean their gear after return.Connection to the bell may be overhead, through the bottom hatch of the bell, or lateral, through a side door.
Multiple compartments. Living, sanitation, rest facilities,
A recompression chamber may be included in the system so that divers can be given treatment for decompression sickness without inconveniencing the rest of the occupants. The recompression chamber may also be used as an entry lock, and to decompress occupants who may need to leave before scheduled.
Mating flange for transportable chamber
One or more of the external doors may be provided with a mating flange or collar to suit a portable or transportable chamber, which can be used to evacuate a diver under pressure.
A small lock used for transfer of supplies into and out of the pressurized system. This would normally include food, medical supplies, clothing, bedding etc.
This includes hot and cold water supply for washbasins and showers, drainage, and marine toilets with holding tank and discharge system.
Main control console
Fire suppression system
Firefighting systems include hand held fire extinguishers to automatic deluge systems. Special fire extinguishers which do not use toxic materials must be used. In the event of a fire, toxic gases may be released by burning materials, and the occupants will have to use the built-in breathing systems (BIBS) until the chamber gas has been flushed sufficiently. When a system with oxygen partial pressure 0.48 bar is pressurized below about 70 msw (231fsw), the oxygen fraction is too low to support combustion (less than 6%), and the fire risk is low. During the early stages of compression and towards the end of decompression the oxygen levels will support combustion, and greater care must be taken.
Built in breathing systems
Built in breathing systems are installed for emergency use and for treatment of decompression sickness. They supply breathing gas appropriate to the current function, which is supplied from outside the pressurized system and also vented to the exterior, so the exhaled gases do not contaminate the chamber atmosphere.
Gas storage and blending equipment to pressurize and flush the system, and treatment gases appropriate to the planned storage depths.
Hyperbaric rescue and escape systems
A saturated diver who needs to be evacuated should preferably be transported without a significant change in ambient pressure. Hyperbaric evacuation requires pressurised transportation equipment, and could be required in a range of situations:
- The support vessel at risk of capsize or sinking.
- Unacceptable fire or explosion hazard.
- Failure of the hyperbaric life support system.
- A medical problem which can not be dealt with on site.
- A "lost" bell.
A hyperbaric lifeboat or rescue chamber may be provided for emergency evacuation of saturation divers from a saturation system. This would be used if the platform is at immediate risk due to fire or sinking, and allows the divers under saturation to get clear of the immediate danger. A hyperbaric lifeboat is self-contained and can be operated from the inside by the occupants while under pressure. It must be self-sufficient for several days at sea, in case of a delay in rescue due to sea conditions. The occupants would normally start decompression immediately after launching.
Bell to bell transfer may be used to rescue divers from a lost or entrapped bell. This will generally occur at or near the bottom, and the divers can transfer between bells at ambient pressure.
It is possible in some circumstances to use a bell as a rescue chamber to transport divers from one saturation system to another. This may require temporary modifications to the bell, and is only possible if the mating flanges of the systems are compatible.
Scientific saturation diving is usually conducted by researchers and technicians known as aquanauts living in an underwater habitat, a structure designed for people to live in for extended periods, where they can carry out almost all basic human functions: working, resting, eating, attending to personal hygiene, and sleeping, all while remaining under pressure beneath the surface.
Saturation diving depth records
The diving depth record for off shore diving was achieved in 1988 by a team of professional divers of the Comex S.A. industrial deep-sea diving company performing pipe line connection exercises at a depth of 534 meters of sea water (msw) (1752 fsw)in the Mediterranean Sea during a record scientific dive.
In 1992 Greek diver Theodoros Mavrostomos  achieved a record of 701 msw (2300 fsw) in an on shore hyperbaric chamber. He took 43 days to complete the scientific record dive, where a hydrogen–helium–oxygen gas mixture was used as breathing gas.
The complexity, medical problems and accompanying high costs of professional diving to such extreme depths and the development of deep water atmospheric diving suits and ROVs in offshore oilfield drilling and production have effectively prevented non-atmospheric manned intervention in the ocean at extreme depths.
Saturation diving in fiction
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