Saturation diving

Saturation diving work in support of the offshore oil and gas industries is usually contract based.[14]

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.[15] 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 (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.[16] HPNS is a limiting factor in future deep diving.[17] HPNS can be reduced by using a small percentage of nitrogen in the gas mixture.[17]

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.[18] 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.[19] 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.[18]

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.[20][21][22]

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][23]

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.[23]

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, in 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.[24][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.[25][26]

There is some evidence of long term cumulative reduction in lung function in saturation divers.[27]

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.[28]

Dysbaric osteonecrosis is considered a consequence of decompression injury rather than living under saturation conditions.

A saturation diving team requires at the minimum the following personnel:[30]

• A diving supervisor (on duty during any diving operations)
• Two life-support supervisors (working shifts while there are divers under pressure)
• Two life-support technicians (also working shifts)
• Two divers in the bell (working diver and bellman – they may alternate during the dive)
• One surface stand-by diver (on duty when the bell is in the water)
• One tender for the surface stand-by diver

In some jurisdictions there will also be a diving medical practitioner on standby, but not necessarily on site, and some companies may require a diving medical technician on site. The actual personnel actively engaged in aspects of the operation are usually more than the minimum.[30]

Compression to storage depth is generally at a limited rate[31] 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.[31]

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.[31]

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.[1]

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,[32] 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 from the initial air content before compression, but tends to decrease over time as the system loses gas to lock operation, and is topped up with helium.[1]

Typical bell with stage and conventional clump weight system

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 complete 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.[1]

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.[33]

Section of an umbilical for a diving bell

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.[33]

A device called a bell cursor may be used to guide and control the motion of the bell through the air and the splash zone near the surface, where waves can move the bell significantly.[33]

Once the bell is at the correct depth, the final adjustments to pressure are made and after final checks, the supervisor instructs the divers 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.[29]^: 12

Breathing gas is supplied to the divers from the surface through the bell umbilical. If this system fails, the bell 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 cylinders of 50 litres capacity or larger, connected through pressure regulators to the gas panel.[29]^: 12

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.[30]^: 10–8 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 exhausted through a reclaim valve on the helmet, through the umbilicals and back to the surface, where the carbon dioxide is scrubbed and the gas boosted into storage cylinders for later use.

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.[34] 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.[34] Ascent rate during excursions is limited, to minimize the risk and amount of bubble formation.[32][35]

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 onward 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 and 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[35]

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 it 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.[35]

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.[34] The Norwegian standards do not allow decompression following directly on an excursion.[31]

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.[35] The divers' umbilicals are stored on racks inside the bell during transfer, and are tended by the bellman during the dive.[30]^: ch.13

The bell handling system lowers the diving bell of the US Navy's saturation fly-away diving system into the water

The bell is deployed from a gantry or A-frame, also known as a bell launch and recovery system (LARS),[30]^: ch.13 on the vessel or platform, using a winch. Deployment may be over the side or through a moon pool.[35]

• 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 (splash zone) 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 and above 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.

The transfer chamber is where the bell is mated to the surface saturation system for transfer under pressure (TUP). It is a wet surface 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.[38]

Accommodation chamber of a saturation spread

The accommodation chambers may be as small as 100 square feet.[40] This part is generally made of multiple compartments, including living, sanitation, and rest facilities, each a separate unit, joined by short lengths of cylindrical trunking. It is usually possible to isolate each compartment from the others using internal pressure doors.[38] Catering and laundry are provided from outside the system and locked on and out as required.

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.

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. The closed bell can be used for this purpose, but lighter and more easily portable chambers are also available. There will usually also be a mating flange for the hyperbaric rescue and escape system.

A small lock is used for transfer of supplies into and out of the pressurized system. This would normally include food, medical supplies, clothing, bedding etc.

The pressurised compartments of the system are connected through access trunking: relatively short and small diameter spools bolted between the external flanges of the larger compartments, with pressure seals, forming passageways between the chambers, which can be isolated by pressure doors.[38]

The life support system provides breathing gas and other services to support life for the personnel under pressure. It includes the following components:[38]

• Breathing gas supply, distribution and recycling equipment: scrubbers, filters, boosters, compressors, mixing, monitoring, and storage facilities
• Chamber climate control system – control of temperature and humidity, and filtration of gas
• Instrumentation, control, monitoring and communications equipment
• Fire suppression systems
• Sanitation systems

The 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. Primary gas supply, power and communications to the bell are through a bell umbilical, made up from a number of hoses and electrical cables twisted together and deployed as a unit.[35] This is extended to the divers through the diver umbilicals.[38]

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 sanitation systems and equipment function are monitored and controlled.[35]

Divers working in cold water, particularly when breathing helium based gases, which increase the rate of heat transfer, may rapidly lose body heat and suffer from hypothermia, which is unhealthy, can be life-threatening, and reduces diver effectiveness. This can be ameliorated with a 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 can be used to heat the breathing gas before it is inhaled, the divers breathing gas is mainly heated on dive's below 150 metres and region specific will dictate what temperature the water is heated too that will then flow through the diver's hot water suit to keep the diver warm.[35][38]

Helium and high pressure both cause hyperbaric distortion of speech. The process of talking underwater is influenced by the internal geometry of the life support equipment and constraints on the communications systems as well as the physical and physiological influences of the environment on the processes of speaking and vocal sound production.[41]^: 6, 16 The use of breathing gases under pressure or containing helium causes problems in intelligibility of diver speech due to distortion caused by the different speed of sound in the gas and the different density of the gas compared to air at surface pressure. These parameters induce changes in the vocal tract formants, which affect the timbre, and a slight change of pitch. Several studies indicate that the loss in intelligibility is mainly due to the change in the formants.[42]

The difference in density of the breathing gas causes a non-linear shift of low-pitch vocal resonance, due to resonance shifts in the vocal cavities, giving a nasal effect, and a linear shift of vocal resonances which is a function of the velocity of sound in the gas, known as the Donald Duck effect. Another effect of higher density is the relative increase in intensity of voiced sounds relative to unvoiced sounds. The contrast between closed and open voiced sounds and the contrast between voiced consonants and adjacent vowels decrease with increased pressure.[43] Change of the speed of sound is relatively large in relation to depth increase at shallower depths, but this effect reduces as the pressure increases, and at greater depths a change in depth makes a smaller difference.[42] Helium speech unscramblers are a partial technical solution. They improve intelligibility of transmitted speech to surface personnel.[43]

The communications system may have four component systems.[35]

• The hardwired 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 hardwired 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

Helium Quad: breathing gas storage asset

Gas storage and blending equipment are provided to pressurize and flush the system, and treatment gases should be available appropriate to the planned storage depths. Bulk stock of premixed gas is usually provided to suit the planned depth of the operation, and separate bulk stock of helium and oxygen to make up additional requirements, adjust chamber gas composition as the oxygen is used up, and mix decompression gas.[38]

Bulk gas is usually stored in manifolded groups of storage cylinders known as "quads", which usually carry about 16 high pressure cylinders, each of about 50 litres internal volume mounted on a frame for ease of transport, or larger frames carrying larger capacity high pressure "tubes". These tube frames are usually designed to be handled by intermodal container handling equipment, so are usually made in one of the standard sizes for intermodal containers.

Schematic diagram of a heliox breathing gas reclaim system

• BGP: bell gas panel
• S1: first water separator
• BP1: bell back-pressure regulator
• U: bell umbilical
• F1: first gas filter
• BP2: topside back-pressure regulator
• R1, R2: serial gas receivers
• F2: second gas filter
• B: booster pump
• Sc1, Sc2: parallel scrubbers
• C: gas cooler
• S2: last water separator
• VT: volume tank
• PR: pressure regulator
• MGP: main gas panel

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.[36] The recovered gas is passed through a scrubber system to remove carbon dioxide, filtered to remove odours and other impurities, and pressurised into storage containers, where it may be mixed with oxygen to the required composition.[44] Alternatively the recycled gas can be more directly recirculated to the divers.[45]

During extended diving operation very large amounts of breathing gas are used. Helium is an expensive gas and can be difficult to source and supply to offshore vessels in some parts of the world. A closed circuit gas reclaim system can save around 80% of gas costs by recovering about 90% of 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.[44]

A reclaim system will typically consist of the following components:[44][45]

Topside components:

• A reclaim 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, with low-pressure carbon dioxide scrubber towers, filters' receivers and back-pressure regulator which will remove carbon dioxide and excess moisture in a condensation water trap. Other gases and odours can be removed by activated carbon filters.
• A gas booster, to boost the pressure of the reclaimed gas to the storage pressure.
• A gas volume tank
• 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.
• Dive control panel
• A bell gas supply panel, to control the supply of gas to the bell.

Underwater components:

• 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 excursion umbilicals, with supply and exhaust hoses between the bell and the divers
• Reclaim helmets which supply gas to the divers on demand, with reclaim back-pressure regulators which exhaust the exhaled gas to the return line.
• Bell back-pressure regulator with water trap

In operation the gas supply from the reclaim system is connected to the topside gas panel, with a backup supply at a slightly lower pressure from mixed gas storage which will automatically cut in if the reclaim supply pressure drops. The bellman will set onboard gas supply to a slightly lower pressure than surface supply pressure to the bell gas panel, so that it will automatically cut in if surface supply is lost. After locking out of the bell the diver will close the diverter valve and open the return valve on the helmet, to start the gas reclaim process. Once this is running, the reclaim control panel will be adjusted to make up the metabolic oxygen usage of the diver into the returned gas. This system will automatically shut down oxygen addition if the flow of exhaled gas from the diver fails, to avoid an excessive oxygen fraction in the recycled gas. There is an indicator light to show whether the return gas is flowing.[45]

The gas supplied to the diver's helmet passes through the same hoses and demand valve as for the open circuit system, but the exhaled gas passes out into the reclaim valve at slightly above ambient pressure, which is considerably above atmospheric pressure, so the flow must be controlled to prevent dropping the helmet internal pressure and causing the demand valve to free-flow. This is achieved by using back-pressure regulators to control the pressure drop in stages. The reclaim valve itself is a demand triggered back-pressure regulator, and there is another back pressure regulator at the bell gas panel, and one at the surface before the receiver tanks. Each of these back-pressure regulators is set to allow about a 1 bar pressure drop.[45]

Exhaust gas returns to the bell through the diver's umbilical exhaust hose, where it passes through a water separator and trap then through a back-pressure regulator which controls the pressure in the exhaust hose and which can be monitored on a pressure gauge in the bell and adjusted by the bellman to suit the excursion depth of the diver. The gas then passes through the bell umbilical exhaust hose to the surface via a non-return valve and another water trap. When the gas enters the surface unit it goes through a coalescing water separator and micron particle filter, and a float valve, which protects the reclaim system from large volumes of water in the event of a leak at depth. Another back-pressure regulator at the surface controls the pressure in the bell umbilical. The gas then passes into the receiver tanks, where oxygen is added at a flow rate calculated to compensate for metabolic use by the diver.[38]

Before entering the boosters, the gas passes through a 0.1 micron filter. The gas is then boosted to storage pressure. Redundant boosters are provided to keep the system running while a booster is serviced. The boosters are automatically controlled to match the diver's gas consumption, and the boosted gas passes through a scrubber where the carbon dioxide is removed by a material like sodalime. Like the boosters, there are at least two scrubbers in parallel, so that they can be isolated, vented and repacked alternately while the system remains in operation. The gas then passes through a cooling heat exchanger to condense out any remaining moisture, which is removed by another 1 micon coalescing filter before it reaches the volume storage tank, where it remains until returned to the gas panel to be used by the divers. While in the volume tank, the gas can be analysed to ensure that it is suitable for re-use, and that the oxygen fraction is correct and carbon dioxide has been removed to specification before it is delivered to the divers.[38] If necessary any lost gas can be compensated by topping up the volume tank from the high pressure storage. Gas from the volume tank is fed to the topside gas panel to be routed back to the bell and diver.[45]

The sanitation system includes hot and cold water supply for washbasins and showers, drainage, and marine toilets with holding tank and discharge system.[35]

It is common for the control room to be installed in an ISO intermode container for convenience of transport. There are three main control panels, for life support, dive control and gas management.[46]

The gas management panel includes pressure regulation of gases from high pressure storage, and distribution to the consumers. Gases will include air, oxygen and heliox mixes[46]

The chamber control panel will typically include depth gauges for each compartment, including trunking, blowdown and exhaust valves, oxygen monitoring and other gas analysis equipment, make-up system for oxygen replenishment, valves for supplying therapeutic breathing mixture, closed circuit television monitoring displays, and monitoring systems with alarms for temperature and pressure in the system chambers.[46]

The dive control panel will include depth gauges for bell internal and external pressure, diver and bellman depth, and trunking pressure for transfer to the accommodation chambers. There will also be breathing gas pressure gauges and control valves for each diver, and blowdown and exhaust valves for the bell interior, diver communications systems with speech unscramblers, a through-water emergency communications system to the bell, controls, monitors and recording equipment for helmet and bell mounted video cameras, oxygen analysers for diver breathing gas, oxygen and carbon dioxide analysers for bell and reclaim gas, alarms for reclaim gas flow, dynamic positioning and hot water supply.[46]

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.[35]

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.[35]

Hyperbaric escape module

Hyperbaric escape module launch control room

Hyperbaric rescue chamber recovery drill

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:[47]

• 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 cannot be dealt with on site.
• A "lost" bell (a bell which has been broken free of lifting cables and umbilical; the actual position of the bell is usually still known with considerable accuracy).

A hyperbaric lifeboat or rescue chamber may be provided for emergency evacuation of saturation divers from a saturation system.[36] 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 by a surface pressure crew while the chamber occupants are under pressure. It must be self-sufficient for several days at sea, in case of a delay in rescue due to sea conditions. It is possible to start decompression after launching if the occupants are medically stable, but seasickness and dehydration may delay the decompression until the module has been recovered.[48]^{: Ch. 2}

The rescue chamber or hyperbaric lifeboat will generally be recovered for completion of decompression due to the limited onboard life support and facilities. The recovery plan will include a standby vessel to perform the recovery.[49]

IMCA recognises that though the number of hyperbaric evacuations which have been successfully carried out is small, and the likelihood of an incident needing hyperbaric evacuation is extremely low, the risk is sufficient to justify requiring the equipment to be available. The original meaning for the term hyperbaric evacuation system covered the system that actually transported the divers away from the working hyperbaric system such as a hyperbaric rescue chamber, a self-propelled hyperbaric lifeboat, or hyperbaric rescue vessel, all of which float and carry short term life-support systems of varied endurance, but it has more recently come to include all of the equipment that would support a hyperbaric evacuation, such as a life support package that can be connected to a recovered hyperbaric rescue unit, to provide interim life support until decompression facilities are available, and the hyperbaric reception facility where divers can be decompressed and treated in relative comfort. The four main classes of problem that must be managed during a hyperbaric evacuation are thermal balance, motion sickness, dealing with metabolic waste products, and severely cramped and confined conditions.[48]^{: Ch. 2}

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 transfer between bells at ambient pressure.[47] 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.[47]

Evacuation of a single diver who is medically stable, or a single diver with an attendant, may be possible using a hyperbaric stretcher or a small portable chamber if the duration of the trip is short, the pressure is suitable and the locking flanges are compatible.