The Use Of Command Buoyancy Systems With Submersibles In The Recovery Or Manipulation Of Subsea Equipment
Authors: Baccaglini Russell , Irgon Joseph 
Published: October 2008

The most practical command buoyancy system is a compact unit with almost no submerged weight which releases an uncommonly large quantity of latent gas into an attached lift bag or flooded ballast tank when the buoyancy device is actuated either for chance emergencies or programmed purposes. Its design simplicity permits use of the device for object recovery and a variety of load support functions in almost any desired combination of lift capacity, operating depth and actuation sequence. It can be made an integral part of the load whenever desired, such as emergency buoyancy for the submersible per se. The command buoyancy technique can significantly improve the safety and economy of submersible operations involving manipulation or retrieval of submerged equipment.

Background

The most useful command buoyancy system is a latent buoyancy device comprised of a source of pressurized gas, an inflatable bag stored in the unit and an actuation mechanism. The components are best arranged in a compact package that is approximately neutrally buoyant.

The unit is actuated in discretionary ways. Lanyard, electrical signal, acoustic signal, elapsed time and hydrostatic pressure limit are typical actuation methods; payload orientation, cable break signal and velocity sensing element are less commonly needed. The actuation releases pressurized gas into the lift bag, which is simultaneously deployed either in a collapsed or flooded condition, as desired. The lift bag is optional when the unit is connected with a rigid ballast tank or pontoon. Since the latent buoyancy system has almost no submerged weight, the lift generated approximates closely theweight of seawater displaced by the gas.

The equivalent compressed gas cylinder would, moreover, have to tolerate a pressure of 50,000 psi or greater to yield the same standard cubic feet of gas A neutrally buoyant cylinder of this capability is clearly beyond the foreseeable state of art. On the other hand, stored compressed gas is instantly accessible inasmuch as a chemical reaction stage is not involved. Certain applications require the ready availability of store gas.

Three self-contained latent buoyancy devices powered either by a chemical gas generator (lithium hydride) or stored gas are shown in Figures 1 to 3. They differ essentially in their functional aspects.

The Model 1000/1000A unit (Figure 1) is a sophisticated chemically operated device that yields about 500 SCF of gas, which is equivalent to 1000 Ibs of lift generated at a depth of 1,000 feet. The design and operating characteristics have been reported previously (1-3). Preferably, the unit is attached to the payload before immersion. It may also be carried to the submerged object by diver or submersible. The passive unit displaces about one cubic foot and provides upwards of one-half ton of lift when actuated at 1,000 feet or at shallower depths. It is adaptable to the various means of actuation mentioned above and to multimodular service with simultaneous or sequential actuation of the individual units.

This particular model vents its buoyancy bag automatically as the load is picked up at the surface, or otherwise relieved, thus permitting a smooth transfer of the load to the pick-up crane.