Combined Gas-Fluid Buoyancy System for Improved Attitude and Maneuverability Control for Application in Underwater Gliders

Since it was first published the conception of underwater glider by H. Stommel (1989), many designs have been proposed based on the variable buoyancy-gliding principle for application in underwater vehicles. Underwater gliders take advantage of the differential buoyancy of a body submerged in a fluid, the Archimedes principle, for converting part of this buoyancy force in surge velocity. For doing that, gliders need surfaces conveniently oriented for obtaining a resulting velocity in the desired surge direction. This hydrodynamic forward movement is obtained during the vehicle sinking and emerging paths, which is the most characteristic constraint of underwater gliders: the need of conducting a saw-tooth navigation pattern. The differential positive-negative buoyancy effect is based on a small increase of the vehicle volume. This increase is obtained by pumping some amount of liquid-fluid in external bladders, which makes the net buoyancy of the vehicle changing from negative (sinking) to positive (climb). However, bladders as buoyancy elements have limitations in the maximum allowable displacement volume and power requirements. Low value of differential buoyancy of ± (0.25 to ±0.45) provides low values of usable force limiting the dynamic capacity of the vehicle. As a result, speeds of around 0.50 m/s are claimed to be reachable by commercial gliders, although 0.15-0.30 m/s are the most frequent horizontal resultant speed obtained. This paper proposes a different concept on this buoyancy- based propulsion technique by considering the combination of compressed gas and liquid fluid. As an example of application, they are presented first results from the field conducted with the underwater glider Alba-14 HGL. The system proposed is based on a hybrid buoyancy gas/liquid device using oil and compressed air which is expected to be an interesting solution when increased speed and maneuverability may be required. Although the maximum range of the vehicle will be affected due to restrictions in the maximum capacity of compressed gas available and for the need of counting with an associated ballast- compensating system, this concept can provide additional booster ability for dealing with high environmental energetic areas in the ocean. Further works include additional field test of the Alba-14 HGL for testing extended long term operations as well as the validation of each of the individual subsystem proposed.