, (
Dictionary of Technology), 1904 The bell is lowered into the water by cables from a
crane, gantry or A-frame attached to a floating platform or shore structure. The bell is
ballasted so as to remain upright in the water and to be negatively
buoyant, so that it will sink even when full of air. Hoses, supplied by
gas compressors or banks of
high pressure storage cylinders at the surface, provide
breathing gas to the bell, serving two functions: • Fresh gas is available for breathing by the occupants. • Volume reduction of the air in an open bell due to increasing hydrostatic pressure as the bell is lowered is compensated. Adding pressurized gas ensures that the gas space within the bell remains at constant volume as the bell descends in the water. Otherwise the bell would partially fill with water as the gas was compressed. The physics of the diving bell applies also to an
underwater habitat equipped with a
moon pool, which is like a diving bell enlarged to the size of a room or two, and with the water–air interface at the bottom confined to a section rather than forming the entire bottom of the structure.
Wet bell A wet bell, or open bell, is a platform for lowering and lifting divers to and from the underwater workplace, which has an air filled space, open at the bottom, where the divers can stand or sit with their heads out of the water. The air space is at ambient pressure at all times, so there are no great pressure differences, and the greatest structural loads are usually self weight and the buoyancy of the air space. A fairly heavy ballast is often required to counteract the buoyancy of the airspace, and this is usually set low at the bottom of the bell, which helps with stability. The base of the bell is usually a grating or deck which the divers can stand on, and folding seats may be fitted for the divers' comfort during ascent, as in-water decompression may be long. Other equipment that is carried on the bell includes cylinders with the emergency gas supply, and racks or boxes for tools and equipment to be used on the job. There may be a tackle for hoisting and supporting a disabled diver so that their head projects into the air space.
Type 1 wet bell The type 1 wet bell does not have an umbilical supplying the bell, because
diver's umbilicals supply the divers directly from the surface, similar to a
diving stage. Divers deploying from a type 1 bell will exit on the opposite side to where the umbilicals enter the bell so that the umbilicals pass through the bell and the divers can find their way back to the bell at all times by following the umbilical. Bailout from a type 1 bell is done by exiting the bell on the side that the umbilicals enter the bell so they no longer pass through the bell, leaving the divers free to surface.
Type 2 wet bell A gas panel inside the bell is supplied by the
bell umbilical and the emergency gas cylinders, and supplies the divers' umbilicals and sometimes
built-in breathing system (BIBS) sets. There will be racks to hang the divers' excursion umbilicals, which for this application must not be buoyant. Abandonment of a type 2 wet bell requires the divers to manage their own umbilicals as they ascend along a remaining connection to the surface.
Operation of a wet bell The bell with divers on board is deployed from the working platform (usually a vessel) by a
crane,
davit, or other mechanism with a
man-rated winch. The bell is lowered into the water and to the working depth at a rate recommended by the decompression schedule, and which allows the divers to
equalize comfortably. Wet bells with an air space will have the air space topped up as the bell descends and the air is compressed by increasing
hydrostatic pressure. The air will also be refreshed as required to keep the
carbon dioxide level acceptable to the occupants. The
oxygen content is also replenished, but this is not the limiting factor, as the oxygen
partial pressure will be higher than in surface air due to the depth. When the bell is raised, the pressure will drop and excess air due to expansion will automatically spill under the edges. If the divers are breathing from the bell airspace at the time, it may need to be vented with additional air to maintain a low carbon dioxide level. The decrease in pressure is proportional to the depth as the airspace is at ambient pressure, and the ascent must be conducted according to the
planned decompression schedule appropriate to the depth and duration of the diving operation. File:Campana húmeda.JPG|Wet bell exterior view File:Campan húmeda6.JPG|Wet bell interior showing bell gas panel File:Campana húmeda9.JPG|Wet bell hoisting winch File:Campana húmeda10.JPG|Wet bell supply gas panel (left) File:Campana húmeda11.JPG|Wet bell supply gas panel (right) File:Campana húmeda8.JPG|Wet bell umbilical deck storage File:Campana húmeda7.JPG|Wet bell bailout gas cylinders
Closed bell A closed, or dry, bell, also known as a personnel transfer capsule or submersible decompression chamber, is a
pressure vessel for human occupancy which is lowered into the sea to the workplace, equalised in pressure to the environment, and opened to allow the divers in and out. These functional requirements dictate the structure and arrangement. The internal pressure requires a strong structure, and a sphere or spherically ended cylinder is most efficient for this purpose. When the bell is underwater, it must be possible for the occupants to get in or out without flooding the interior. This requires a pressure hatch at the bottom. The requirement that the bell reliably retain its internal pressure when the external pressure is lowered dictates that the hatch open inward, so that internal pressure will hold it closed. The bell is lowered through the water to working depth, so must be negatively buoyant. This may require additional ballast, which may be attached by a system that can be released from inside the bell in an emergency, without losing pressure, to allow the bell to float back to the surface. Locking onto a deck decompression chamber or saturation system at the surface is possible either from the bottom or the side. Using the bell bottom hatch for this purpose has the advantage of only needing one hatch, and the disadvantage of having to lift the bell up and place it over a vertical entry to the chamber. A bell used in this way may be called a personnel transfer capsule. If decompression is done inside the bell, it may be referred to as a submersible decompression chamber. The bell bottom hatch must be wide enough for a large diver fully kitted with appropriate
bailout cylinders, to get in and out without undue difficulty, and it can not be closed while the diver is outside as the umbilical is tended through the hatch by the
bellman. It must also be possible for the bellman to lift the working diver in through the hatch if he is unconscious, and close the hatch after him, so that the bell can be sealed and pressurised for the ascent. A lifting tackle is usually fitted inside the bell for this purpose, and the bell may be partially flooded to assist the procedure. The internal space must be large enough for a fully kitted diver and bellman (the
stand-by diver responsible for manning the bell while the working diver is locked out) to sit, and for their umbilicals to be stowed neatly on racks, and the hatch to be opened inwards while they are inside. Anything bigger will make the bell heavier than it really needs to be, so all equipment that does not need to be inside is mounted outside. This includes a framework to support the ancillary equipment and protect the bell from impact and snagging on obstacles, and the emergency gas and power supplies, which are usually racked around the framework. The emergency gas supply (EGS) is connected to the internal gas panel. The part of the framework that keeps the lower hatch off the bottom is called the
bell stage. It may be removable, which can facilitate connection to a vertical access chamber lock. The bell umbilical is connected to the bell via through hull fittings (hull penetrations), which must withstand all operating pressures without leaking. The internal gas panel connects to the hull penetrations and the diver's umbilicals. The umbilicals will carry main breathing gas supply, a communications cable, a
pneumofathometer hose, hot water supply for suit heating, power for helmet mounted lights, and possibly gas reclaim hose and video cable. The bell umbilical will usually also carry a power cable for internal and external bell lighting. Hydraulic power lines for tools do not have to pass into the interior of the bell as they will never be used there, and tools can also be stored outside. There may be an emergency through-water communications system with a battery power supply, and a location transponder working on the international standard 37.5 kHz. The bell may also have viewports and a medical lock. A closed bell may be fitted with an umbilical cutter, a mechanism which allows the occupants to sever the bell umbilical from inside the sealed and pressurised bell in the event of an umbilical snag that prevents bell recovery. The device is typically hydraulically operated using a hand pump inside the bell, and can shear the umbilical at or just above the point where it is fastened to the top of the bell. Once cut, the bell can be raised and if the umbilical can then be recovered, it can be reconnected with only a short length lost. An external connection known as a
hot stab unit which allows an emergency umbilical to be connected by a ROV or diver to maintain life support in the bell during a rescue operation may be fitted. Hot water, breathing gas, electrical power and communications connections are likely to be provided. The divers in the bell can be monitored from the diving control point by
closed circuit video, and the bell atmosphere can be monitored for volatile hydrocarbon contamination by a hyperbaric hydrocarbon analyser which can be linked to a topside repeater and set to give an alarm if the hydrocarbon levels exceed 10% of the anaesthetic level. The bell may be fitted with an external emergency battery power pack, carbon dioxide scrubber for the internal atmosphere, and air conditioner for temperature control. Power supply is typically 12 or 24V DC. The bell may be provided with ballast to give it negative buoyancy so that it will sink when lowered by the winch. It may be possible to release the ballast from inside the sealed bell for an emergency buoyant ascent in the event of a lifting winch failure. One such system allows the ballast to be lowered enough to allow the bell to rise two metres, giving more clearance to access the bottom hatch. After entering and sealing the bell, the occupants can complete the release, making the bell buoyant so it can float to the surface. A bell will be provided with equipment to rescue and treat an injured diver. This will normally include a small tackle to lift the disabled diver into the bell through the bottom hatch and secure them in an upright position if needed. A bell flooding valve, also known as a may be available to partially flood the interior to aid in lifting a disabled diver into the bell. Once inside and secure, the bell is cleared of water using the blow-down valve to fill the interior with breathing gas at ambient pressure and displace the water out through the hatch. A first aid kit will be carried.
British mini-bell system A variant of this system used in the North Sea oilfields between early 1986 and the early 90s was the Oceantech Minibell system, which was used for
bell-bounce dives, and was operated as an open bell for the descent, and as a closed bell for the ascent. The divers would climb into the bell after stowing their umbilicals on outside racks, remove their helmets for outside storage, seal the bell, and return to the surface, venting to the depth of the first decompression stop. The bell would then be locked onto a deck decompression chamber, the divers transferred under pressure to complete decompression in the chamber, and the bell would be available for use for another dive.
Breathing gas distribution Breathing gas supplies for the bell comprise a primary gas supply, a reserve gas supply and an emergency gas supply carried on the bell. The divers will also carry bailout gas in scuba cylinders, or as a
semi-closed circuit rebreather, sufficient to get them back to the bell in the event of an umbilical supply failure. Primary gas, or main gas supply may be compressed air, which is usually supplied by a low pressure breathing air compressor, or mixed gas, which is usually provided in manifolded clusters of high-pressure storage cylinders, commonly referred to as "
quads". Primary gas is connected to the main gas panel throughout the diving operation except when it fails or a problem is being corrected, during which time the divers are switched over to reserve gas. Reserve gas, or secondary gas, which is connected to the main gas panel and available for immediate use by opening the supply valve, may also be supplied by low pressure compressor, or from high pressure storage. It has the same composition as the main gas supply.
Decompression gas, when used, is also supplied via the main gas panel. It may be the same gas as the primary gas, or an oxygen enriched mixture, or pure oxygen. Gas switching for in-water decompression in a wet bell is not the preferred procedure for commercial diving, as the entire breathing gas delivery system must be oxygen clean, and as a decompression chamber is required on site when a specified limit of obligatory decompression is planned, it is more convenient to do
surface decompression on oxygen (SurDO2) in the chamber. The relative safety of surface decompression and in-water decompression is uncertain. Both procedures are accepted by health and safety regulatory bodies. is carried on the bell, usually in a small number of 50 litre high-pressure cylinders connected to the bell gas panel. This should be the same gas as the primary gas. On closed bells there is an additional supply of pure oxygen if the bell has a carbon dioxide scrubber for the bell atmosphere. On a type 2 wet bell or a closed bell this emergency gas can be distributed to the divers from the bell gas panel operated by the bellman, through the excursion umbilicals. Each diver carries an emergency gas supply (bailout gas) sufficient to get back to the bell under any reasonably foreseeable circumstances of umbilical supply failure of primary, reserve, and bell emergency gas supplies. The main gas distribution panel is located at the control point for the diving operation, and operated by the
gas man, who may also be a diver, or if the gas is air, it may be directly operated by the
diving supervisor.
Bell gas panel The bell gas panel is a manifold of valves, pressure regulators, pipes, hoses and gauges mounted inside a closed bell, and under the canopy of a type 2 wet bell, and is operated by the
bellman. When a helium reclaim system is in use, the return hose for the reclaimed gas passes through the bell gas panel and a
back-pressure regulator on its way to the surface. The bell gas panel is supplied with primary and secondary gas supplies from the main gas panel through the bell umbilical, and with on-board emergency gas from the cylinders carried on the bell. Pressure of each gas supply is shown by a gauge on the panel before and after regulation.
Deployment of a modern diving bell Diving bells are deployed over the side of the vessel or platform, or through a moonpool, using a gantry or A-frame from which the clump weight and the bell are suspended. On
dive support vessels with in-built saturation systems the bell may be deployed through a
moon pool. The bell handling system is also known as the launch and recovery system (LARS).
Bell handling system A closed bell handling system is used to move the bell from the position where it is locked on to the chamber system into the water, lower it to the working depth and hold it in position without excessive movement, and recover it to the chamber system. The system used to transfer the bell on deck may be a deck trolley system, an overhead gantry or a swinging A-frame. The system must constrain movement of the supported bell sufficiently to allow accurate location on the chamber trunking even in bad weather. A bell cursor may be used to control movement through and above the splash zone, and heave compensation gear may be used to limit vertical movement when in the water and clear of the cursor, particularly at working depth when the diver may be locked out and the bell is open to ambient pressure.
Bell umbilical The bell umbilical supplies gas to the bell gas panel, and is separate from the divers' excursion umbilicals, which are connected to the gas panel 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, as this reduces the risk of the umbilical snagging on underwater obstructions. Wet bell handling differs from closed bell handling in that there is no requirement to transfer the bell to and from the chamber system to make a pressure-tight connection, and that a wet bell will be required to maintain a finely controlled speed of descent and ascent and remain at a fixed depth within fairly close tolerances for the occupants to decompress at a specific ambient pressure, whereas a closed bell can be removed from the water without delay and the speed of ascent and descent is not critical. A bell diving team will usually include two divers in the bell, designated as the working diver and bellman, though they may alternate these roles during the dive. The bellman is a
stand-by diver and umbilical tender from the bell to the working diver, the operator of the on-board gas distribution panel, and has an umbilical about 2 m longer than the working diver to ensure that the working diver can be reached in an emergency. This can be adjusted by tying off the umbilicals inside the bell to limit deployment length, which must often be done in any case, to prevent the divers from approaching known hazards in the water. Depending on circumstances, there may also be a surface stand-by diver, with attendant, in case there is an emergency where a surface oriented diver could assist. The team will be under the direct control of the
diving supervisor, will include a winch operator, and may include a dedicated surface gas panel operator.
Clump weight Deployment of a diving bell usually starts by lowering the clump weight, which is a large ballast weight suspended in the bight of a cable which runs from a winch, over a sheave on one side of the gantry, down to the weight, round a pair of sheaves on the sides of the weight, and back up to the other side of 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 clump weight cable, and has a fairlead on each side which slides along the cable as it is lowered or lifted. Deployment of the bell is by a separate cable attached to the top, which runs over a sheave in the middle of the gantry. As the bell is lowered, the fairleads prevent it from rotating on the deployment cable, which would put twist into the umbilical and risk loops or snagging. The clump weight cables therefore act as guidelines or rails along which the bell is lowered to the workplace, and raised back to the platform. If the lifting winch or cable fails, and the bell ballast is released, a positively buoyant bell can float up and the cables will guide it to the surface to a position where it can be recovered relatively easily. The clump weight cable can also be used as an emergency recovery system, in which case both bell and weight are lifted together. An alternative system for preventing rotation on the lifting cable is the use of a
cross-haul system, which may also be used as a means of adjusting the lateral position of the bell at working depth, and as an emergency recovery system.
Bell stage A bell stage is an open framework below the bell which prevents the bell lower lock from getting too close to the clump weight or seabed, ensuring that there is space for the divers to safely exit and enter the bell. This can be deployed either as part of the bell, or as part of the clump weight. The bell stage may be fitted with baskets for carrying tools and equipment.
Bell cursor A bell cursor is a device 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, to and from a depth where the water motion is less energetic. It can either be a passive system which relies on additional ballast weight or an active system which uses a controlled drive system to provide vertical motion. The cursor has a cradle which locks onto the bell and which moves vertically on rails to constrain lateral movement. The bell is released and locked onto the cursor in the relatively still water below the splash zone.
Moon pool aeration system On some diving support vessels the moon pool is provided with an aeration system which blows large volumes of low pressure air into the water of the moon pool through submerged nozzles. This converts the water to two-phase foam of lower density than the water, which exerts less force on the bell when it passes between the air above and the unaerated water below the interface. The density change is relatively gradual, and water motion of waves and vortices in the aerated zone is attenuated, so shock loads are significantly reduced.
Heave compensation Heave compensation equipment is used to stabilise the depth of the bell by counteracting vertical movement of the handling system caused by movements of the platform, and usually also maintains correct tension on the guide wires. It is not usually essential, depending on the stability of the platform.
Cross-hauling Bell cross-hauling is the use of a cable connected to the bell to move the bell laterally when this is useful, such as when the workplace is not close to directly below the LARS. Cross-hauling systems are cables from an independent lifting device, and may also be used to limit rotation and as an emergency bell recovery system. ==Use with hyperbaric chambers==