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. 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, and as such these systems are a major research topic in the field of underwater vehicles. Examples include submarines, submersibles, benthic landers, remotely operated and autonomous underwater vehicles, and ambient pressure and single atmosphere underwater divers.
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. 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.
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, or by changing the displaced volume at a constant vehicle weight. The resulting buoyancy is used to control heave velocity and hovering depth, 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.
Variable buoyancy systems have been considered for depth control of tethered ocean current turbine electrical generation.
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.