Variable buoyancy pressure vessel

Last updated November 08, 2023

A variable buoyancy pressure vessel system is a type of rigid buoyancy control device for diving systems that retains a constant volume and varies its density by changing the weight (mass) of the contents, either by moving the ambient fluid into and out of a rigid pressure vessel, or by moving a stored liquid between internal and external variable volume containers. A pressure vessel is used to withstand the hydrostatic pressure of the underwater environment. A variable buoyancy pressure vessel can have an internal pressure greater or less than external ambient pressure, and the pressure difference can vary from positive to negative within the operational depth range, or remain either positive or negative throughout the pressure range, depending on design choices.

Several applications only need one cycle from positive to negative and back, to get down to depth and return to the surface between deployments, others may need tens to hundreds of cycles over several months during a single deployment, or may need near continuous but very small adjustments in both directions to maintain a constant depth, or neutral buoyancy at changing depths. Several mechanisms are available for this function, some are suitable for multiple cycles of variation between positive and negative buoyancy, others must be replenished between uses, and their suitability depends on the required characteristics for the specific application.

Uses of variable buoyancy diving systems

Variable buoyancy is a useful characteristic of any mobile underwater system that operates in mid-water without external support,^[1] and as such these systems are a major research topic in the field of underwater vehicles.^[2] Examples include submarines, submersibles, benthic landers, remotely operated and autonomous underwater vehicles,^[3] and ambient pressure and single atmosphere underwater divers.^[4]

A submarine can closely approach equilibrium when submerged, but have no inherent stability in depth. The sealed pressure hull structure is usually slightly more compressible than water and will consequently lose buoyancy with increased depth.^[5] For precise and quick control of buoyancy and trim at depth, submarines use depth control tanks (DCT) due to their function of controlling buoyancy and thereby depth)—also called hard tanks (due to their ability to withstand higher pressure), or trim tanks (due to their function of controlling trim). These are variable buoyancy pressure vessels. The amount of water in depth control tanks can be controlled to change the buoyancy of the vessel so that it moves up or down in the water column as a consequence of unbalanced buoyancy forces, or to maintain a constant depth as outside conditions (mainly water density) change, and water can be pumped between trim tanks to control longitudinal or transverse trim without affecting buoyancy.^[6]

The operating depth of underwater vehicles can be controlled by controlling the buoyancy, either by changing the overall weight or the displaced volume, or by vectored thrust. Buoyancy can be controlled by changing the overall weight of the vehicle at constant volume,^[7] or by changing the displaced volume at a constant vehicle weight. The resulting buoyancy is used to control heave velocity and hovering depth,^[7] and in underwater gliders a positive or negative net buoyancy is used to drive forward motion.

The Avelo scuba system uses a variable buoyancy pressure vessel, which is both the primary breathing gas cylinder and the scuba buoyancy compensator, with a rechargeable battery powered pump and dump valve unit which is demountable from the cylinder.^[4]^[8]

Variable buoyancy systems have been considered for depth control of tethered ocean current turbine electrical generation.^[9]

The type of variable buoyancy system best suited to an application depends on the precision of control required, the amount of change needed, and the number of cycles of buoyancy change necessary during a deployment.^[10]

Types of variable buoyancy systems

Several types of variable buoyancy system have been used, and are briefly described here. Some are based on a relatively incompressible pressure vessel, and are nearly stable with variation of hydrostatic pressure.

Ambient pressure buoyancy/ballast tanks (unstable with depth change), such as the main ballast tanks on a submarine, or an inflatable diver's buoyancy compensator ^[6]^[4] These are not pressure vessels, as the contents are at ambient pressure.
Weight discharge variable mass system. This is generally a system by which ballast of higher density than the surroundings is discharged, and once discharged the ballast is lost. The system is simple and appropriate for vehicles that only need to make a very limited number of buoyancy adjustments during a deployment.It is a common method of achieving positive buoyancy in an emergency as it is simple to arrange a fail-safe discharge mechanism. An analogous system for releasing fixed low density material is also possible.^[10]^: 21 These are also not pressure vessels as the weights or incompressible buoys are stored at ambient pressure.
One-way tank flood variable mass system. This is simply an empty tank that can withstand external working pressure, and can be partly or completely flooded by a control valve. The tank can be drained again at the surface for subsequent dives, but not while under pressure during a dive.^[10]^: 24
Pumped oil, constant mass, variable volume system. This method uses more power but is indefinitely repeatable while power lasts, as it does not discharge any consumables. A positive displacement pump transfers oil stored in a variable volume container inside a gas filled pressure vessel to an external variable volume container, incompressibly increasing the displaced volume of the vehicle. Return transfer may be by pressure difference controlled by a valve, or also pumped.^[10]^: 24
- Piston-driven oil, constant mass, variable volume system. This works very similarly to the pumped oil system, but the internal storage is in a cylinder with a piston which decreases or increases its volume using a mechanical drive, typically powered by an electric motor. In effect the piston acts as a pump.^[10]^: 25
Pumped water variable buoyancy system. Ambient water is moved into and out of the pressure vessel to change the overall density of the vessel, and thereby of the vehicle of which it is a component. In one direction this transfer may be possible by pressure difference, but in at least one direction it must be pumped. The process is repeatable while power lasts, as the ballast is drawn from the surroundings.^[10]^: 22

Mechanism

A buoyancy tank that is within the pressure hull of the vehicle, as in a submarine, will be exposed to the internal pressure of the vehicle, so external pressure loads on the tank may be relatively low. In this case the ballast water transfer into the tank may not require pumping, though a positive displacement pump may still be useful to accurately control the volume of water admitted. Discharge of ballast water is against the external pressure, which will depend on depth, and will generally require significant work.^[6]

If the buoyancy tank is directly exposed to the ambient hydrostatic pressure, the external load due to depth can be high, but if the internal gas pressure is high enough, the pressure difference will be lower, and the pressure vessel is not subjected to high net external pressure loads which can cause buckling instability, which can allow a lower structural weight. In the extreme case the internal pressure is high enough to rapidly eject the water ballast at maximum operational depth, as in the case of the Avelo integrated diving cylinder and buoyancy control device. A pump is used to move ambient water into the pressure vessel against the internal pressure, compressing the gas further in proportion to volume decrease, so the entire internal volume is not available to hold ballast, as although the gas will decrease in volume, there will always be some gas volume remaining. The water and air in the pressure vessel may be separated by a membrane, diaphragm, free piston, or bladder to prevent pumping out air in some orientations, and to prevent the air from dissolving in the ballast water under high pressure.^[10]^[4]

Related Research Articles

A submarine is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely operated vehicles and robots, as well as medium-sized or smaller vessels, such as the midget submarine and the wet sub. Submarines are referred to as boats rather than ships irrespective of their size.

<span class="mw-page-title-main">Buoyancy</span> Upward force that opposes the weight of an object immersed in fluid

Buoyancy, or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. The pressure difference results in a net upward force on the object. The magnitude of the force is proportional to the pressure difference, and is equivalent to the weight of the fluid that would otherwise occupy the submerged volume of the object, i.e. the displaced fluid.

<span class="mw-page-title-main">Buoyancy compensator (diving)</span> Equipment for controlling the buoyancy of a diver

A buoyancy compensator (BC), also called a buoyancy control device (BCD), stabilizer, stabilisor, stab jacket, wing or adjustable buoyancy life jacket (ABLJ), depending on design, is a type of diving equipment which is worn by divers to establish neutral buoyancy underwater and positive buoyancy at the surface, when needed.

Diving physics, or the physics of underwater diving is the basic aspects of physics which describe the effects of the underwater environment on the underwater diver and their equipment, and the effects of blending, compressing, and storing breathing gas mixtures, and supplying them for use at ambient pressure. These effects are mostly consequences of immersion in water, the hydrostatic pressure of depth and the effects of pressure and temperature on breathing gases. An understanding of the physics is useful when considering the physiological effects of diving, breathing gas planning and management, diver buoyancy control and trim, and the hazards and risks of diving.

<span class="mw-page-title-main">Diving weighting system</span> Ballast carried by underwater divers and diving equipment to counteract excess buoyancy

A diving weighting system is ballast weight added to a diver or diving equipment to counteract excess buoyancy. They may be used by divers or on equipment such as diving bells, submersibles or camera housings.

A submersible is an underwater vehicle which needs to be transported and supported by a larger watercraft or platform. This distinguishes submersibles from submarines, which are self-supporting and capable of prolonged independent operation at sea.

A diving bell is a rigid chamber used to transport divers from the surface to depth and back in open water, usually for the purpose of performing underwater work. The most common types are the open-bottomed wet bell and the closed bell, which can maintain an internal pressure greater than the external ambient. Diving bells are usually suspended by a cable, and lifted and lowered by a winch from a surface support platform. Unlike a submersible, the diving bell is not designed to move under the control of its occupants, or to operate independently of its launch and recovery system.

An atmospheric diving suit (ADS) is a small one-person articulated submersible which resembles a suit of armour, with elaborate pressure joints to allow articulation while maintaining an internal pressure of one atmosphere. An ADS can enable diving at depths of up to 700 metres (2,300 ft) for many hours by eliminating the majority of significant physiological dangers associated with deep diving. The occupant of an ADS does not need to decompress, and there is no need for special breathing gas mixtures, so there is little danger of decompression sickness or nitrogen narcosis when the ADS is functioning properly. An ADS can permit less skilled swimmers to complete deep dives, albeit at the expense of dexterity.

<span class="mw-page-title-main">Underwater glider</span> Type of autonomous underwater vehicle

An underwater glider is a type of autonomous underwater vehicle (AUV) that employs variable-buoyancy propulsion instead of traditional propellers or thrusters. It employs variable buoyancy in a similar way to a profiling float, but unlike a float, which can move only up and down, an underwater glider is fitted with hydrofoils that allow it to glide forward while descending through the water. At a certain depth, the glider switches to positive buoyancy to climb back up and forward, and the cycle is then repeated.

A ballast tank is a compartment within a boat, ship or other floating structure that holds water, which is used as ballast to provide hydrostatic stability for a vessel, to reduce or control buoyancy, as in a submarine, to correct trim or list, to provide a more even load distribution along the hull to reduce structural hogging or sagging stresses, or to increase draft, as in a semi-submersible vessel or platform, or a SWATH, to improve seakeeping. Using water in a tank provides easier weight adjustment than the stone or iron ballast used in older vessels, and makes it easy for the crew to reduce a vessel's draft when it enters shallower water, by temporarily pumping out ballast. Airships use ballast tanks mainly to control buoyancy and correct trim.

A radio-controlled submarine is a scale model of a submarine that can be steered via radio control. The most common form are those operated by hobbyists. These can range from inexpensive toys to complex projects involving sophisticated electronics. Oceanographers and military units also operate radio-controlled submarines.

Ictíneo I was a pioneering submarine constructed in Barcelona, Spain in 1858–1859 by engineer Narcís Monturiol.

<span class="mw-page-title-main">Ascending and descending (diving)</span> Procedures for safe ascent and descent in underwater diving

In underwater diving, ascending and descending is done using strict protocols to avoid problems caused by the changes in ambient pressure and the hazards of obstacles near the surface such as collision with vessels. Diver certification and accreditation organisations place importance on these protocols early in their diver training programmes. Ascent and descent are historically the times when divers are injured most often when failing to follow appropriate procedure.

Ballast is dense material used as a weight to provide stability to a vehicle or structure. Ballast, other than cargo, may be placed in a vehicle, often a ship or the gondola of a balloon or airship, to provide stability. A compartment within a boat, ship, submarine, or other floating structure that holds water is called a ballast tank. Water should move in and out from the ballast tank to balance the ship. In a vessel that travels on the water, the ballast will remain below the water level, to counteract the effects of weight above the water level. The ballast may be redistributed in the vessel or disposed of altogether to change its effects on the movement of the vessel.

The trim of a diver is the orientation of the body in the water, determined by posture and the distribution of weight and volume along the body and equipment, as well as by any other forces acting on the diver. Both static trim and its stability affect the convenience and safety of the diver while under water and at the surface. Midwater trim is usually considered at approximately neutral buoyancy for a swimming scuba diver, and neutral buoyancy is necessary for efficient maneuvering at constant depth, but surface trim may be at significant positive buoyancy to keep the head above water.

The following outline is provided as an overview of and topical guide to underwater diving:

The following index is provided as an overview of and topical guide to underwater diving:

A buoyancy engine is a device that alters the buoyancy of a vehicle or object in order to either move it vertically, as in the case of underwater profiling floats and stealth buoys, or provide forward motion such as with underwater gliders and some autonomous aircraft.

Submarine rescue is the process of locating a sunk submarine with survivors on board, and bringing the survivors to safety. This may be done by recovering the vessel to the surface first, or by transferring the trapped personnel to a rescue bell or deep-submergence rescue vehicle to bring them to the surface. Submarine rescue may be done at pressures between ambient at depth, and sea level atmospheric pressure, depending on the condition of the distressed vessel and the equipment used for the rescue. Self-rescue of submarine personnel by buoyant free ascent at ambient pressure is considered submarine escape. Survivors may require recompression treatment for decompression illness.

References

1 2 Jensen, Harold Franklin III (June 2009). "2.2. Variable Buoyancy Benefits". Variable Buoyancy System Metric (Thesis). Massachusetts Institute of Technology. pp. 19–20.
↑ Ranganathan, Thiyagarajan; Thondiyath, Asokan. Design and Analysis of Cascaded Variable Buoyancy Systems for Selective Underwater Deployment (PDF). Proceedings of the 13th International Conference on Informatics in Control, Automation and Robotics (ICINCO 2016). Vol. 2. SCITEPRESS – Science and Technology Publications, Lda. pp. 319–326. doi:10.5220/0005979903190326. ISBN 978-989-758-198-4.
↑ Worall, Mark; Jamieson, A.J.; Holford, A.; Neilson, R.D.; Player, Michael; Bagley, Phil (July 2007). A variable buoyancy system for deep ocean vehicles. OCEANS 2007 - Europe. doi:10.1109/OCEANSE.2007.4302317 – via Researchgate.
1 2 3 4 "Technology: The Avelo Solution". diveavelo.com. Avelo Labs. Retrieved 24 November 2021.
↑ Moore, C.S. (August 1974). "Intact Stability: Submerged equilibrium: Stability in depth". In Comstock, John P. (ed.). Principles of Naval Architecture (Revised ed.). New York, NY: Society of Naval Architects and Marine Engineers. pp. 111–112.
1 2 3 "The Fleet Type Submarine Online: Submarine Trim and Drain Systems. Navpers 16166". maritime.org. Retrieved 1 January 2022– via San Francisco Maritime National Park Association.
1 2 Tiwari, Brij Kishor; Sharma, Rajiv (8 April 2020). "Design and Analysis of a Variable Buoyancy System for Efficient Hovering Control of Underwater Vehicles with State Feedback Controller". Journal of Marine Science and Engineering. MDPI. 8 (4): 263. doi: 10.3390/jmse8040263 .
↑ "New Tank Lets Scuba Divers Ditch Their BCD". Scuba Diving. PADI Media. 22 November 2021.
↑ Hasankhani, Arezoo; VanZwieten, James; Tang, Yufei; Dunlap, Broc; De Luera, Alexandra; Sultan, Cornel; Xiros, Nikolaos (July 2021). "Modeling and Numerical Simulation of a Buoyancy Controlled Ocean Current Turbine". International Marine Energy Journal. 4 (2): 47–58. doi: 10.36688/imej.4.47-58 . S2CID 238999433.
1 2 3 4 5 6 7 Jensen, Harold Franklin III (June 2009). "3. Current VB Systems". Variable Buoyancy System Metric (Thesis). Massachusetts Institute of Technology. pp. 21–26.

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[Jensen_2009a-1] 1 2 Jensen, Harold Franklin III (June 2009). "2.2. Variable Buoyancy Benefits". Variable Buoyancy System Metric (Thesis). Massachusetts Institute of Technology. pp. 19–20.

[Ranganathan_and_Thondiyath_2016-2] Ranganathan, Thiyagarajan; Thondiyath, Asokan. Design and Analysis of Cascaded Variable Buoyancy Systems for Selective Underwater Deployment (PDF). Proceedings of the 13th International Conference on Informatics in Control, Automation and Robotics (ICINCO 2016). Vol. 2. SCITEPRESS – Science and Technology Publications, Lda. pp. 319–326. doi:10.5220/0005979903190326. ISBN 978-989-758-198-4.

[Worall_et_al_2007-3] Worall, Mark; Jamieson, A.J.; Holford, A.; Neilson, R.D.; Player, Michael; Bagley, Phil (July 2007). A variable buoyancy system for deep ocean vehicles. OCEANS 2007 - Europe. doi:10.1109/OCEANSE.2007.4302317 – via Researchgate.

[Avelo_2021-4] 1 2 3 4 "Technology: The Avelo Solution". diveavelo.com. Avelo Labs. Retrieved 24 November 2021.

[Comstock_1974-5] Moore, C.S. (August 1974). "Intact Stability: Submerged equilibrium: Stability in depth". In Comstock, John P. (ed.). Principles of Naval Architecture (Revised ed.). New York, NY: Society of Naval Architects and Marine Engineers. pp. 111–112.

[Navpers_16166-6] 1 2 3 "The Fleet Type Submarine Online: Submarine Trim and Drain Systems. Navpers 16166". maritime.org. Retrieved 1 January 2022– via San Francisco Maritime National Park Association.

[Tiwari_and_Sharma_2020-7] 1 2 Tiwari, Brij Kishor; Sharma, Rajiv (8 April 2020). "Design and Analysis of a Variable Buoyancy System for Efficient Hovering Control of Underwater Vehicles with State Feedback Controller". Journal of Marine Science and Engineering. MDPI. 8 (4): 263. doi: 10.3390/jmse8040263 .

[Scuba_Diving_2021-8] "New Tank Lets Scuba Divers Ditch Their BCD". Scuba Diving. PADI Media. 22 November 2021.

[Hasankhani_et_al_2021-9] Hasankhani, Arezoo; VanZwieten, James; Tang, Yufei; Dunlap, Broc; De Luera, Alexandra; Sultan, Cornel; Xiros, Nikolaos (July 2021). "Modeling and Numerical Simulation of a Buoyancy Controlled Ocean Current Turbine". International Marine Energy Journal. 4 (2): 47–58. doi: 10.36688/imej.4.47-58 . S2CID 238999433.

[Jensen_2009b-10] 1 2 3 4 5 6 7 Jensen, Harold Franklin III (June 2009). "3. Current VB Systems". Variable Buoyancy System Metric (Thesis). Massachusetts Institute of Technology. pp. 21–26.

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