Patent Application
20100294192
The present invention relates to the field of underwater devices, and more particularly, to a buoyancy system for controlling buoyancy of an underwater device.
An underwater glider is a type of underwater device that collects subsurface data in an observation region. The underwater glider is typically a torpedo shaped, winged device that moves through the water in a saw-tooth sampling pattern by changing its buoyancy. The underwater glider is neutrally buoyant, and typically includes a buoyancy system in its nose section.
The buoyancy system may be based on a displacement piston. To diver the displacement piston moves water into nose section of the underwater device. This makes the underwater glider's nose heavy. To ascend, water is pushed out of the nose section by the displacement piston. This makes the underwater glider's nose lighter.
Even in view of the advances made in buoyancy systems, there is still a need to improve such systems. For example, U.S. Pat. No. 6,131,531 discloses a selectively deformable buoyancy system. The buoyancy system includes a housing having walls defining an interior, sealable cavity. Changing the volume of the cavity controls buoyancy. The cavity has an original volume when the walls are maintained at or above a preselected temperature. The walls are deformed at temperatures below the preselected temperature to define a volume less than the original volume. The housing returns to the original volume when the temperature of the walls is raised above the preselected temperature.
Composite materials may be used as part of a buoyancy system, as disclosed in U.S. Pat. No. 4,482,590. In particular, implosion resistant macrospheres for use in buoyancy systems may be fabricated from synthetic foams, preferably from synthetic thermosetting polymeric resins. The implosion resistant macrospheres are primarily used in buoyancy devices at sea depths in excess of 4,500 feet.
In view of the foregoing background, it is therefore an object of the present invention to provide an improved buoyancy system for controlling buoyancy of an underwater device.
This and other objects, advantages and features in accordance with the present invention are provided by a buoyancy system comprising a chamber having a volume associated therewith, and a plurality of bladders within the volume of the chamber. Each bladder may contain a clathrate mixture in a liquid state. The chamber may include at least one opening to allow surrounding water to circulate within the volume. When the chamber is submerged in increasingly frigid surrounding water, the plurality of bladders may expand based on the clathrate mixture changing from the liquid state to a solid state. This thereby increases buoyancy by allowing less water to circulate within the volume of the chamber.
Similarly, when the chamber ascends in the increasingly warm surrounding water, the plurality of bladders may contract based on the clathrate mixture changing from the solid state to the liquid state. This thereby decreases buoyancy by allowing more water to circulate within the volume of the chamber.
Each bladder may comprise a water-tight enclosure so that the clathrate mixture therein does not directly contact the water. The clathrate mixture may comprise water and a clathrating agent. Each bladder may maintain a predetermined pressure on the clathrate mixture so that the clathrating agent does not vaporize when the clathrate mixture is in the liquid state. Vaporization of the clathrating agents would make an underwater device with such a buoyancy system permanently buoyant. The maintained minimum pressure thus depends on the clathrating agent, since each clathrating agent has a unique dissolution pressure.
Each bladder may comprise an elastic enclosure that expands as the clathrate mixture changes to the solid state. The elastic enclosure may comprise a thermally conductive material. The thermally conductive material advantageously allows the temperature of the surrounding water to be efficiently transferred to the clathrate mixture. As the water temperature cools, the clathrate mixture decreases density when it begins to freeze. The clathrate mixture expands as it freezes, similar to an ice cube that floats.
Once the clathrate mixture reaches a depth in the water where it can begin forming ice, each bladder expands as a result of the volume increase of the ice. This volume increase, multiplied for the total number of bladders, causes water to be forced out of the chamber. This decreases the overall mass while displacing the same volume of water. As a result, the buoyancy changes. The same concept applies in reverse as the ice melts. The bladders will shrink and the buoyancy system will weigh more as more water is allowed to enter the chamber, and its buoyancy will change again.
The buoyancy system may further comprise a respective spacer coupled between adjacent bladders so that the bladders are spaced apart from one another within the volume of the chamber. This advantageously helps with the transfer of heat from the water to the clathrate mixture since the water will surround each bladder, as compared to partially surrounding the bladders when they are bunched up against one another. Moreover, each bladder may be spherically shaped to provide a greater surface area for the water to contact, thereby improving heat transfer.
The bladders may form a three-dimensional array of bladders. The buoyancy system may further comprise a water permeable enclosure surrounding the plurality of bladders within the volume of the chamber. The water permeable enclosure advantageously prevents anyone of the bladders from escaping the chamber.
Another aspect of the present invention is directed to an underwater device comprising a housing, and a buoyancy system carried by the housing. The buoyancy system may be as defined above. The housing and the buoyancy system may be configured so that the underwater device is an underwater glider or a sonar buoy, for example.
Yet another aspect of the present invention is directed to a method for changing buoyancy of an underwater device comprising a buoyancy system as described above. The method may comprise placing the underwater device in the water, and submerging the underwater device based on the surrounding water entering the at least one opening within the chamber and contacting the plurality of bladders. The method may further comprise expanding the plurality of bladders based on the clathrate mixture changing from the liquid state to a solid state so that less water is circulated within the volume of the chamber, thereby changing the buoyancy of the underwater device. The method may further comprise contracting the plurality of bladders after having been expanded, with the contracting being based on the clathrate mixture changing from the solid state back to the liquid state so that more water is circulated within the volume of the chamber, thereby changing the buoyancy of the underwater device.
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.
Referring initially to
As will be discussed in greater detail below, the buoyancy system 20 changes buoyancy of the underwater device 10 based on the use of a plurality of bladders 30, where each bladder contains a clathrate mixture. The plurality of bladders 30 may also be referred to as a blister pack. The clathrate mixture comprises water and a clathrating agent, as will be discussed in greater detail below.
The bladders 30 are in contact with the water, and expand or contract based on the effects of the water's temperature on the clathrate mixture. The expansion or contraction of the bladders 30 affects the buoyancy of the underwater device 10.
The clathrate mixture changes from a liquid state to a sold state as the underwater device 10 submerges, and from the sold state back to the liquid state as the underwater device 10 rises. When the clathrate mixture is in the solid state, the mixture may also be referred to as a clathrate hydrate. Transition between the liquid and solid states is based on the temperature of the water surrounding the bladders 30. The sharpness of the saw-tooth-sampling pattern 22 depends on the rate of fusing the clathrate hydrate (bottom saw-tooth-sampling pattern) and on the rate of melting the clathrate hydrate (top saw-tooth-sampling pattern).
Referring now to
As readily appreciated by those skilled in the art, the clathrate mixture comprises water and a clathrating agent. The clathrating agent may include, but are not limited to, methane or propane, for example. The clathrate agent is not limited to a single type clathrating agent. The bladder 30 may include more than one type of clathrating agent. When the clathrate is in the solid state, the clathrate mixture is also referred to as a clathrate hydrate. Clathrate hydrates are crystalline compounds defined by the inclusion of a guest molecule within a hydrogen bonded water lattice. Gas hydrates are a subset of clathrate hydrates wherein the guest molecule is a gas at or near ambient temperatures and pressures. Such gasses include methane, propane, carbon dioxide, hydrogen, for example; although not all the gas hydrates are suitable for buoyancy modulation when their solidified state is denser than the liquid state (as is the case for carbon dioxide hydrate).
The clathrate mixture changes density when it freezes. As best illustrated in
Each bladder 30 comprises a water-tight enclosure so that the clathrate mixture therein does not directly contact the water. The water-tight enclosure could be a variety of plastics or rubber compounds that are elastic enough to accommodate the expanding clathrate as it freezes, but rigid enough to prevent any leaks between the ambient water and the clathrates.
Each bladder 30 maintains a predetermined pressure on the clathrate mixture so that the clathrating agent does not vaporize when the clathrate mixture is in the liquid state. For example, if the clathrating agent is propane, then the bladder maintains a pressure of at least 150 psi so that the propane does not vaporize when the underwater device 10 is at the surface of the water. Vaporization of the clathrating agent would make the underwater device 10 permanently buoyant. The maintained minimum pressure thus depends on the clathrating agent, since each clathrating agent has a unique dissolution pressure.
As noted above, each bladder 30 comprises an elastic enclosure that expands as the clathrate mixture changes to the solid state. The elastic enclosure also comprises a thermally conductive material. The thermally conductive material advantageously allows the temperature of the water to be efficiently transferred to the clathrate mixture. As the water temperature cools, the clathrate mixture fuses into clathrate hydrate which decreases in density as the mass expands. This is similar to an ice cube that floats. As readily appreciated by those skilled in the art, some clathrate hydrates become more dense than their respective clathrate mixture states, and consequently, these clathrating agents are not appropriate for use with a buoyancy system 20. Once the clathrate mixture reaches a depth in the water where it can begin forming ice, each bladder 30 expands as a result of the volume increase of the ice.
When the clathrating agent is propane, for example, the propane fuses with water at about 6 degrees Celsius. This volume increase, multiplied for the total number of bladders 30, causes water to be forced out of the chamber 24. This decreases the overall mass while displacing the same change in volume of water. As a result, the buoyancy increases. The same concept applies in reverse as the clathrate hydrate melts. The bladders 30 will shrink and the buoyancy system 20 will weigh more as more water is allowed to enter the chamber 24, and its buoyancy will change again.
The size and number of bladders 30 within the chamber 24 will vary depending on the size or volume of the chamber, as well as the intended application of the underwater device 10. There needs to be enough bladders 30 to provide enough clathrate mixtures within the chamber 24 to effect a density change in the underwater device 10 to reverse its buoyancy. This would also depend on the volume and weight of the underwater device 10, and the desired climb or dive rates that may be required.
For illustrative purposes, the size of each bladder 30 may be within a range of about 1/16 to 2 inches, for example. The size of the chamber 24 is typically about 10 to 20% of the total volume of the underwater device 10. For an underwater device 10 that is about 25 cubic feet in volume, the chamber 24 would have a volume of about 2.5 to 5 cubic feet. There needs to be a sufficient number of bladders 20 to displace water from the chamber 24 so that there is an effect on buoyancy of the underwater device 10.
The number of bladders 30 within the chamber 24 may also compensate for failure of a certain number of bladders 30 that is expected over time. Consequently, additional bladders 30 may be included within the chamber 24 so that buoyancy can still be controlled even with the loss of a portion of the bladders 30.
As illustrated in
Referring now to
Another aspect of the invention is directed to a method for changing buoyancy of an underwater device 10 comprising a buoyancy system 20 as described above. Referring now to
The bladders 30 then expand at Block 76 as the clathrate mixture changes from a liquid state to a solid state so that less water is circulated within the volume of the chamber 24. This increases the buoyancy of the underwater device 10. As a result, the underwater device 10 floats toward the surface of the water at Block 78. The bladders 30 contact warmer water causing the clathrate mixture in the solid state to melt back into the liquid state.
The cycle of descending and ascending is continuously repeated in Blocks 74, 76 and 78. To end this cycle, the bladders 30 may rupture over time due to environmental causes at Block 80. Similarly, the bladders 30 may fail when their elastic properties become brittle over time at Block 82. Yet another option for ending this cycle is to scuttle the underwater device 10 by rupturing the bladders 30 on purpose at Block 84. The method ends at Block 86.
The blister pack approach has a significant lifecycle advantage over a piston cylinder device or a single large bladder device. Both these devices are disclosed in U.S. patent application Ser. No. 12/017,966, which is incorporated herein by reference in its entirety and is assigned to the current assignee of the present invention.
In the illustrated buoyancy system 20, the failure of any single bladder will have little effect on the overall performance of a buoyancy cycle. It would take a large number of bladder failures to terminate the buoyancy cycle. By eliminating single points of failure, an underwater device 10 including a plurality of bladders 30 will have a longer endurance. Its performance eventually will gradually diminish as individual bladder failures accumulate over time, as opposed to the catastrophic failure that would occur with a piston cylinder device or with a large bladder device. The blister pack bladder approach has a significant production advantage over the piston cylinder device or the large bladder device since the bladders 30 can be more easily mass produced.
Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.
