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.

Variable buoyancy is a useful characteristic of any mobile underwater system that operates in mid-water without external support.[1] Examples include submarines, submersibles, benthic landers, remotely operated and autonomous underwater vehicles, and underwater divers.

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

Pumped oil constant mass, variable volume system.
Top: Low volume, low buoyancy.
Bottom: High volume, high buoyancy state.
PV = Pressure vessel
Bi = Internal bellows
Bo = External bellows
P = pump
CV = Check valve
V = Valve
Piston operated oil constant mass, variable volume system.
Top: Low volume, low buoyancy.
Bottom: High volume, high buoyancy state.
PV = Pressure vessel
Bo = External bellows
P = Piston
C = Cylinder
R = Piston rod
M = Motor
Pumped water constant volume, variable mass system.
Left: Low internal pressure, pumped both ways.
Right: High internal pressure, ballast ejected by internal gas pressure.
PV = Pressure vessel
LP = Low internal pressure
HP = High internal pressure
P = pump
CV = Check valve
IV = Inlet valve
OV = Outlet valve

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.


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]

See also


  1. ^ a b Jensen, Harold Franklin III (June 2009). "2.2. Variable Buoyancy Benefits". Variable Buoyancy System Metric (Thesis). Massachusetts Institute of Technology. pp. 19–20.
  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.
  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.
  4. ^ a b c d "Technology: The Avelo Solution". diveavelo.com. Avelo Labs. Retrieved 24 November 2021.
  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.
  6. ^ a b c "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.
  7. ^ a b 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.
  8. ^ "New Tank Lets Scuba Divers Ditch Their BCD". Scuba Diving. PADI Media. 22 November 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.
  10. ^ a b c d e f g Jensen, Harold Franklin III (June 2009). "3. Current VB Systems". Variable Buoyancy System Metric (Thesis). Massachusetts Institute of Technology. pp. 21–26.