Underwater vehicle equipped with a hydraulic accumulator

Abstract

Underwater vehicle equipped with a hydraulic accumulator The underwater vehicle is equipped with a buoyancy adjustment system comprising a hydraulic accumulator (6) comprising a casing (8) inside which is delimited a fluid chamber (10) whose volume varies according to the pressure residing inside the fluid chamber (10), the fluid chamber (10) being in fluid communication with the surrounding water or in fluid communication with an external reservoir (28) containing a liquid (L), the external reservoir (28) being configured in such a way that the pressure of the liquid (L) contained in the external reservoir (28) varies according to the pressure of the surrounding water. Figure for abstract: Figure 1

Description

Title of the invention: Underwater vehicle equipped with a hydraulic accumulator

[0001] The present invention relates to the field of underwater vehicles, in particular underwater gliders.

[0002] The buoyancy of an underwater vehicle immersed in water is equal to the mass of the volume of water displaced by the underwater vehicle. The buoyancy of an underwater vehicle depends on the density of the water.

[0003] As a first approximation, the density of water is considered constant. However, the density of water increases with depth, particularly in a marine environment.

[0004] The buoyancy of the underwater vehicle therefore increases with depth and this increase in buoyancy increases the difficulty of the underwater vehicle in diving to great depths.

[0005] It is then necessary to compensate for the increase in buoyancy, for example using a propulsive force, generated for example by a propeller thruster, and / or using a ballasting system which makes it possible to modify the buoyancy of the underwater vehicle.

[0006] The energy consumed to generate the propulsive force or to operate the ballasting system negatively affects the autonomy of the underwater vehicle. This is particularly problematic in the case of an autonomous underwater vehicle such as an underwater glider designed to navigate autonomously for long periods.

[0007] One of the aims of the invention is to propose an underwater vehicle capable of diving to great depths while limiting energy consumption.

[0008] To this end, the invention proposes an underwater vehicle provided with a buoyancy adjustment system comprising a hydraulic accumulator comprising a casing inside which is delimited a fluid chamber whose volume varies as a function of the pressure residing inside the fluid chamber, the fluid chamber being in fluid communication with the surrounding water or in fluid communication with an external reservoir containing a liquid, the external reservoir being configured in such a way that the pressure of the liquid contained in the external reservoir varies as a function of the pressure of the surrounding water.

[0009] The hydraulic accumulator makes it possible to passively vary the buoyancy of the underwater vehicle as a function of the pressure of the surrounding water in which the underwater vehicle is immersed, and therefore in particular as a function of the depth at which the underwater vehicle is located.

[0010] Thus, the hydraulic accumulator makes it possible to compensate for variations in the density of the water in which the underwater vehicle is immersed.

[0011] According to other advantageous aspects, the underwater vehicle comprises one or more of the following optional characteristics, taken individually or in all technically possible combinations:

[0012] - the volume occupied by the external reservoir varies according to the volume of liquid contained in the external tank;

[0013] - the external tank has a dividing wall separating the liquid from the water in surrounding, the dividing wall being mobile and / or deformable under the effect of a pressure difference between the pressure of the liquid and the pressure of the surrounding water;

[0014] - the fluidic chamber is delimited by a deformable and / or mobile internal wall, the internal wall being subjected, on one side, to the pressure of the surrounding water or of the liquid present in the fluid chamber and, subjected on the other side, for example to the action of a pressurized gas present in a pneumatic chamber delimited by the internal wall and / or to the action of a spring;

[0015] - the hydraulic accumulator is calibrated so that when the volume of the fluidic chamber is minimal, the pressurized gas has a calibration pressure strictly greater than 1 bar and / or the spring has a non-zero prestress;

[0016] - the hydraulic accumulator is calibrated so that the volume of the chamber fluidic remains minimal as long as the surrounding water pressure is lower than a reference pressure, the volume of the fluidic chamber increasing with the surrounding water pressure when the surrounding water pressure is higher than the reference pressure;

[0017] - the external reservoir comprises an elastically deformable bladder, a membrane elastically deformable and / or a piston subjected on one side to the action of the liquid and on the other side to the action of the surrounding water;

[0018] - the external tank and the casing are at least partly nested in one another the other;

[0019] - the underwater vehicle comprises a ballasting system, the ballasting system comprising a ballast tank fluidly connected to the external tank, and an actuator operable to force the transfer of liquid between the ballast tank and the external tank and / or the fluid chamber;

[0020] - the external reservoir and the fluidic chamber are fluidically connected in parallel to the actuator;

[0021] - the external reservoir and the fluidic chamber are fluidically connected in series to the actuator;

[0022] - the external reservoir is fluidly located between the actuator and the chamber fluidics;

[0023] - the ballast tank is received in a wet compartment of the underwater vehicle sailor;

[0024] - the external tank and / or the hydraulic accumulator are received in a com wet compartment of the underwater vehicle;

[0025] - the underwater vehicle is an underwater glider or an underwater profiler.

[0026] The invention and its advantages will appear on reading the description which follows, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0027] [Fig-1] [Fig.l] is a schematic view of an underwater vehicle equipped with a system buoyancy adjustment comprising a hydraulic accumulator according to an example, illustrating different configurations of the hydraulic accumulator;

[0028] [Fig.2] [Fig.2] is a schematic view of an underwater vehicle equipped with a system buoyancy adjustment comprising a hydraulic accumulator according to another example, illustrating different configurations of the hydraulic accumulator;

[0029] [Fig.3] [Fig.3] is a graph illustrating the buoyancy of an underwater vehicle without a hydraulic accumulator and the buoyancy of the same device equipped with a hydraulic accumulator;

[0030] [Fig.4] [Fig.4] is a schematic view of an underwater vehicle equipped with a system buoyancy adjustment system comprising a ballast system and a hydraulic accumulator, according to one example;

[0031] [Fig.5] [Fig.5] is a schematic view of an underwater vehicle equipped with a system buoyancy adjustment system comprising a ballast system and a hydraulic accumulator, according to another example;

[0032] [Fig.6] [Fig.6] is a schematic view of an underwater vehicle equipped with a system buoyancy adjustment system comprising a ballast system and a hydraulic accumulator, according to yet another example.

[0033] As illustrated in [Fig.l], the underwater vehicle 2 is configured to be immersed in water, in particular in a lake or in the sea.

[0034] The underwater vehicle 2 is for example equipped with a measuring system (not shown) for carrying out measurements in the water, for example temperature measurements, pressure measurements and / or salinity measurements.

[0035] The underwater vehicle 2 comprises a buoyancy adjustment system 4 configured to modify the buoyancy of the underwater vehicle 2.

[0036] The buoyancy adjustment system 4 is for example configured to modify the buoyancy of the underwater vehicle 2 as a function of the pressure of the surrounding water in a passive manner, i.e. without using energy to activate an actuator.

[0037] The buoyancy adjustment system 4 comprises a hydraulic accumulator 6 comprising a casing 8 inside which a chamber is delimited fluidic chamber 10 receiving a liquid, the volume of the fluidic chamber 10 varying according to the pressure of the liquid.

[0038] The hydraulic accumulator 6 is configured such that the volume of the fluid chamber 10 varies in conjunction with the pressure residing in the fluid chamber, i.e. such that the volume of the fluid chamber 10 increases when the pressure residing in the fluid chamber 10 increases and the volume of the fluid chamber 10 decreases when the pressure residing in the fluid chamber decreases.

[0039] The fluidic chamber 10 is for example delimited inside the envelope 8 by an internal wall 12 movable inside the envelope and / or deformable depending on the pressure residing in the fluidic chamber 10.

[0040] The internal wall 12 separates, for example, the fluidic chamber 10 from a closed pneumatic chamber 14 containing a gas G, the volume of the pneumatic chamber 14 varying inversely to the volume of the fluidic chamber 10.

[0041] When the pressure in the fluid chamber 10 increases, the inner wall 12 moves and / or deforms in such a way that the volume of the fluid chamber 10 increases, the volume of the pneumatic chamber 14 decreases and the pressure of the gas G increases.

[0042] When the pressure in the fluid chamber 10 decreases, the inner wall 12 moves and / or deforms in such a way that the volume of the fluid chamber 10 decreases, the volume of the pneumatic chamber 14 increases, and the pressure of the gas G decreases.

[0043] The gas G contained in the pneumatic chamber 14 makes it possible to store elastic energy when the pressure of the liquid increases in the fluidic chamber 10 and when liquid enters the fluidic chamber 10, this elastic energy then making it possible to push the liquid outside the fluidic chamber 10 and to reduce the volume of the fluidic chamber 10 when the pressure of the liquid L decreases.

[0044] The hydraulic accumulator 6 advantageously comprises a calibration orifice 16 allowing the pneumatic chamber 14 to be filled with the gas G. The calibration orifice 16 allows the quantity of gas G in the pneumatic chamber 14 to be adjusted. The calibration orifice 16 is preferably provided with a valve.

[0045] The internal wall 12 is for example an elastically deformable bladder, an elastically deformable membrane, or a piston received sliding inside the casing 8.

[0046] In a variant in which the internal wall 12 is a piston received sliding inside the casing 8, the hydraulic accumulator 6 comprises for example, instead of the gas G or in addition to the gas G, a spring (not shown) arranged to act on the internal wall 12 in such a way that the spring opposes the movement of the wall internal 12 in the direction of an increase in the volume of the fluidic chamber 10.

[0047] When the pressure in the fluid chamber 10 increases, the inner wall 12 moves in such a way that the volume of the fluid chamber 10 against the spring increases. The spring stores energy.

[0048] When the pressure in the fluid chamber 10 decreases, the spring moves the inner wall 12 in such a way that the volume of the fluid chamber 10 decreases.

[0049] The casing 8 is preferably the external casing of the hydraulic accumulator 6.

[0050] The hydraulic accumulator 6 is for example arranged in such a way that the casing 8 is immersed in the surrounding water. In this case, the casing 8 is preferably configured to be non-deformable at pressures corresponding to the depth range of use of the underwater vehicle 2.

[0051] As illustrated in [Fig.l], the underwater vehicle 2 has a hull 20 delimiting a watertight compartment 22 into which the surrounding water does not penetrate and a wet compartment 24 into which the surrounding water penetrates. The casing 8 is located in the wet compartment 24.

[0052] As illustrated in [Fig.l], the fluidic chamber 10 is for example fluidically connected to the outside to allow the surrounding water to penetrate into the fluidic chamber 10. The fluidic chamber 10 contains surrounding water.

[0053] The casing 8 is for example provided with an orifice 26 passing through the casing 8 and opening into the fluid chamber 10 and to the outside.

[0054] As illustrated in [Fig.l] from left to right, if the pressure of the surrounding water increases (e.g., if the immersion depth of the underwater vehicle 2 increases), the volume of the fluid chamber 10 increases and surrounding water enters the fluid chamber 10. As a result, the buoyancy of the underwater vehicle decreases.

[0055] The surrounding water received in the fluid chamber 10 pushes the internal wall 12 against the gas G present in the pneumatic chamber 14 and / or the spring acting on the internal wall 12.

[0056] As illustrated in [Fig.l] from right to left, if the pressure of the surrounding water decreases (e.g. if the immersion depth of the underwater vehicle 2 decreases), the surrounding water present in the hydraulic chamber 10 is pushed out. As a result, the buoyancy of the underwater vehicle increases.

[0057] The surrounding water present in the hydraulic chamber 10 is for example pushed outwards by the internal wall 12 subjected to the action of the gas G present in the pneumatic chamber 14 and / or the spring.

[0058] In an exemplary embodiment, as illustrated in [Fig.2], the buoyancy adjustment system 4 comprises an external reservoir 28 containing a liquid L, the external reservoir 28 being fluidically connected to the fluidic chamber 10, for example via the orifice 26, in such a way that the liquid L circulates between the fluidic chamber 10 and the external reservoir 28.

[0059] The liquid L present in the reservoir 28 and the fluidic chamber 10 is separated from the surrounding water.

[0060] The liquid L is for example a hydraulic oil compatible with the fluidic actuator 36

[0061] The external reservoir 28 is configured in such a way that the liquid L present in the external reservoir 28 is subjected to the pressure of the surrounding water.

[0062] The external reservoir 28 is at least partially submerged in the surrounding water.

[0063] The external reservoir 28 is configured in such a way that the volume occupied by the external tank 28 in the surrounding water varies depending on the quantity of liquid L present in the external tank 28.

[0064] The external reservoir 28 comprises for example a movable and / or deformable external wall 30 such that the liquid L is subjected to the pressure of the surrounding water through the external wall 30 and / or that the volume occupied by the external reservoir 28 in the surrounding water varies according to the quantity of liquid L present in the external reservoir 28.

[0065] In one example, as illustrated in [Fig.2], the outer wall 30 forms an elastically deformable bladder or membrane, subjected on one side to the action of the liquid and on the other side to the action of the surrounding water.

[0066] In a variant not shown, the external wall 30 forms a piston subjected on one side to the action of the liquid and on the other side to the action of the surrounding water.

[0067] As illustrated in [Fig.2] from left to right, if the pressure of the surrounding water increases (e.g. if the immersion depth of the underwater vehicle 2 increases), the pressure of the liquid L present in the external tank 28 increases together with the pressure of the surrounding water. Liquid L moves from the external tank 28 to the fluid chamber 10 via the orifice 26. The volume occupied in the surrounding water by the external tank 28 decreases. As a result, the buoyancy of the underwater vehicle 2 decreases.

[0068] The liquid L received in the fluidic chamber 10 pushes the internal wall 12 against the gas G present in the pneumatic chamber 14 and / or the spring acting on the internal wall 12.

[0069] As illustrated in [Fig.2] from right to left, in the event of a decrease in the pressure of the surrounding water (e.g. if the immersion depth of the underwater vehicle 2 decreases), the pressure of the liquid L present in the external tank 28 decreases together with the pressure of the surrounding water. Liquid L moves from the fluid chamber 10 to the external tank 28 via the orifice 26. The occupied volume into the surrounding water by the external tank 28 increases. As a result, the buoyancy of the underwater vehicle 2 increases.

[0070] The liquid L present in the fluidic chamber 10 is for example pushed outside by the internal wall 12 subjected to the action of the gas G present in the pneumatic chamber 14 and / or the spring.

[0071] The transfer of the liquid L between the external reservoir 28 and the hydraulic chamber 10 results solely from the action of the surrounding water, and in particular from the variation in the pressure of the surrounding water.

[0072] The buoyancy adjustment system 4 makes it possible to adjust the buoyancy passively, i.e. without using energy to operate an actuator, in particular in such a way that the buoyancy decreases when the pressure of the surrounding water increases and the buoyancy increases when the pressure of the surrounding water decreases.

[0073] This makes it possible to compensate for a variation in water density such as may occur between the water surface and great depths, for example depths greater than 500 m, in particular greater than 1,000 m.

[0074] The buoyancy adjustment system 4 is for example calibrated so that the variation in buoyancy due to the pressure as a function of the depth compensates for the variation in buoyancy due to the variation in density of the water as a function of the depth.

[0075] The fluid chamber 10 of the hydraulic accumulator 6 receiving the surrounding water ([Fig.l]) or the liquid L ([Fig.2]) has a minimum volume, corresponding to maximum buoyancy. The volume of the fluid chamber 10 is minimal when the pressure of the surrounding water is insufficient to cause the surrounding water or the liquid L to enter the fluid chamber.

[0076] The hydraulic accumulator 6 is for example calibrated so that the volume of the fluid chamber 10 remains minimal up to a reference pressure of the surrounding water. The reference pressure corresponding to a reference depth, which is for example a desired immersion depth for the underwater vehicle 2.

[0077] As long as the pressure of the surrounding water is lower than the reference pressure, the volume of surrounding water or liquid L present in the fluidic chamber 10 remains constant and minimal.

[0078] As long as the pressure of the surrounding water is lower than the reference pressure, the hydraulic accumulator 6 is inactive and does not modify the buoyancy of the underwater vehicle 2.

[0079] In the case of a hydraulic accumulator 6 in which a movable internal wall 12 delimiting the fluid chamber 10 is subjected to the action of a pressurized gas G and / or a spring, the pressure of the gas G and / or a prestress of the spring on the wall internal 12 when the volume of the fluid chamber 10 is minimal determine the reference pressure.

[0080] Advantageously, the pressure of the gas G when the volume of the fluidic chamber 10 is minimal is strictly greater than 1 bar and / or the preload of the spring is non-zero.

[0081] In particular, in the case of a hydraulic accumulator 6 using a pressurized gas G as illustrated in Figures 1 and 2, the gas G has a pressure (called "calibration pressure") when the volume of the fluid chamber 10 is minimal. The volume of the fluid chamber 10 begins to increase with an inlet of surrounding water or liquid L only when the pressure of the surrounding water or liquid L exceeds the calibration pressure of the gas G.

[0082] The calibration orifice 16 makes it possible to adjust the calibration pressure. Indeed, the adjustment of the calibration pressure is carried out by adjusting the quantity of gas G present in the pneumatic chamber 14 via the calibration orifice 16.

[0083] [Fig. 3] is a graph showing an abscissa axis on which the depth (expressed in meters) is plotted and a ordinate axis on which a weight (expressed in kg) is plotted. A first curve C1 illustrates the buoyancy (expressed in kg) of an underwater vehicle 2 displacing a volume of water V and without a hydraulic accumulator 6. A second curve C2 illustrates the buoyancy of the same underwater vehicle 2 equipped with a hydraulic accumulator 6 calibrated for a reference depth PR. A third curve C3 illustrates the quantity of surrounding water or liquid L having penetrated into the fluid chamber of the hydraulic accumulator 6 (expressed in kg) as a function of the depth.

[0084] As long as the underwater vehicle 2 equipped with the hydraulic accumulator 6 is located at an immersion depth less than the reference depth PR, surrounding water or liquid L does not penetrate into the hydraulic accumulator 6 and the buoyancy of the underwater vehicle 2 equipped with the hydraulic accumulator 6 (curve C2) is not modified compared to that of the underwater vehicle 2 without the hydraulic accumulator 6 (curve C1).

[0085] When the underwater vehicle 2 equipped with the hydraulic accumulator exceeds the reference depth PR, surrounding water or liquid L begins to penetrate into the fluid chamber 10 of the hydraulic accumulator 6. The buoyancy of the underwater vehicle 2 equipped with the hydraulic accumulator 6 is limited while that of the underwater vehicle without a hydraulic accumulator 6 increases linearly with depth.

[0086] The numerical values ​​of [Fig.3] correspond to an underwater vehicle 2 displacing a volume of water of 130 liters and a hydraulic accumulator 6 with pressurized gas having a calibration pressure of 70 bars.

[0087] Up to a reference depth PR of approximately 700 meters (corresponding to the calibration pressure of 70 bars), the buoyancy of the underwater vehicle 2 equipped with the hydraulic accumulator 6 (curve C2) is not or is little affected compared to that of the same underwater vehicle 2 without the hydraulic accumulator 6 (curve C1).

[0088] Beyond 700 meters depth, the buoyancy of the underwater vehicle 2 without a hydraulic accumulator 6 increases, reaching approximately 2 kg at a depth of 3500 m, whereas that of the underwater vehicle 2 equipped with a hydraulic accumulator 6 is limited, and varies here between - 400 g and + 400 g.

[0089] The hydraulic accumulator 6 facilitates the diving of the underwater vehicle 2 to great depths and makes it possible to increase its autonomy by avoiding the need to provide and / or activate a propulsion system to generate a propulsion force and / or activate a ballasting system.

[0090] As illustrated in [Fig.4], the buoyancy adjustment system 4 optionally includes a ballasting system 32 configured to actively modify the buoyancy of the underwater vehicle 2.

[0091] The ballast system 32 comprises a ballast tank 34 fluidly connected to the external tank 28 via a fluidic actuator 36 configured to force the transfer of liquid L between the ballast tank 34 and the external tank 28. The fluidic actuator 36 is for example a pump.

[0092] The ballast system 32 is configured such that the transfer of liquid between the ballast tank 34 and the external tank 28 is controlled solely by the activation of the fluidic actuator 36.

[0093] The liquid L contained in the ballast tank 34 is not subjected to the pressure of the surrounding water.

[0094] To do this, the ballast tank 34 is installed inside the hull 20 of the underwater vehicle 2, so as not to be subjected to the pressure of the surrounding water, in particular by being arranged in a watertight compartment 22 of the hull 20.

[0095] As illustrated in [Fig.3], the external reservoir 28 and the fluid chamber 10 of the pressure accumulator 6 are fluidically connected in parallel to the fluid actuator 36.

[0096] Alternatively, as illustrated in [Fig.5], the external reservoir 28 and the fluid chamber 10 of the hydraulic accumulator 6 are fluidically connected in series to the fluid actuator 36. The external reservoir 28 is for example fluidically located between the ballast tank 34 and the pneumatic chamber 14 of the hydraulic accumulator 6.

[0097] Advantageously, as illustrated in [Fig.6], the external reservoir 28 and the hydraulic accumulator 6 are at least partially nested within each other. This allows a compact arrangement of the buoyancy adjustment system, in particular when the hull 20 of the underwater vehicle has a hydrodynamic profile.

[0098] As illustrated in [Fig.5], the hydraulic accumulator 6 is for example at least partially nested in the external tank 28. In particular, the external tank 28 at least partially surrounds the hydraulic accumulator 6. The external tank 28 delimits a space in which the hydraulic accumulator 6 is at least partially received.

[0099] In a variant not shown, the configuration is reversed. The external reservoir 28 is for example at least partially fitted into the hydraulic accumulator 6. In particular, the hydraulic accumulator 6 at least partially surrounds the external reservoir 28. The hydraulic accumulator 6 delimits a space in which the external reservoir 28 is received at least partially.

[0100] In operation, to reduce the immersion depth of the underwater vehicle 2, the fluidic actuator 36 is activated to force the transfer of liquid L from the ballast tank 34 to the external tank 28. This causes an increase in the volume occupied in the surrounding water by the external tank 28 and an increase in the buoyancy of the underwater vehicle.

[0101] To increase the immersion depth of the underwater vehicle 2, the fluidic actuator 36 is activated to force the transfer of liquid L from the external tank 28 to the ballast tank 34. This causes a reduction in the volume occupied in the surrounding water by the external tank 28 and a reduction in the buoyancy of the underwater vehicle.

[0102] The transfer of liquid L between the external tank 28 and the ballast tank 34 makes it possible to actively modify the buoyancy of the underwater vehicle 2.

[0103] The fluid connection between the external reservoir 28 and the fluid chamber 10 of the hydraulic accumulator 6 makes it possible to passively modify the buoyancy of the underwater vehicle 2, for example to compensate for a variation in buoyancy due to a variation in the density of the water.

[0104] It is therefore not necessary to use the ballasting system 32 to compensate for a variation in buoyancy due to a variation in density of the water.

[0105] As a result, it is possible to control the buoyancy of the underwater vehicle 2 by limiting the energy expenditure. It is therefore possible to provide an underwater vehicle 2 with satisfactory energy autonomy.

[0106] The underwater vehicle 2 is for example an underwater glider. An underwater glider is configured to navigate underwater autonomously by alternating ascent phases and descent phases during which the underwater glider advances due to a propulsive force generated by the vertical movement of the underwater glider, for example due to the shape of the hull of the underwater vehicle 2 or the presence hydrodynamic appendages, such as fins. The vertical movement of the underwater glider is, for example, generated using the ballast system 32 with low energy consumption.

[0107] The underwater vehicle 2 is for example a profiler. A profiler is configured to dive and ascend, without necessarily moving forward, to take measurements as a function of depth, for example in a water column.

[0108] The underwater vehicle 2 is for example equipped with a measuring system for carrying out measurements during its movement in the water, for example temperature, pressure and / or salinity measurements.

Claims

Claims

1. An underwater vehicle provided with a buoyancy adjustment system comprising a hydraulic accumulator (6) comprising a casing (8) inside which is delimited a fluid chamber (10) whose volume varies as a function of the pressure residing inside the fluid chamber (10), the fluid chamber (10) being in fluid communication with the surrounding water or in fluid communication with an external reservoir (28) containing a liquid (L), the external reservoir (28) being configured in such a way that the pressure of the liquid (L) contained in the external reservoir (28) varies as a function of the pressure of the surrounding water.

2. An underwater vehicle according to claim 1, wherein the volume occupied by the external tank (28) varies as a function of the volume of liquid contained in the external tank (28).

3. An underwater vehicle according to claim 1 or 2, wherein the external tank (28) has a separating wall (30) separating the liquid (L) from the surrounding water, the separating wall (30) being movable and / or deformable under the effect of a pressure difference between the pressure of the liquid (L) and the pressure of the surrounding water.

4. Underwater vehicle according to claim 3, in which the fluid chamber (10) is delimited by a deformable and / or movable internal wall (12), the internal wall (12) being subjected, on one side, to the pressure of the surrounding water or of the liquid (L) present in the fluid chamber (10) and, subjected on the other side, for example to the action of a pressurized gas (G) present in a pneumatic chamber (14) delimited by the internal wall (12) and / or to the action of a spring.

5. Underwater vehicle according to claim 4, in which the hydraulic accumulator (6) is calibrated such that when the volume of the fluid chamber (10) is minimal, the pressurized gas (G) has a calibration pressure strictly greater than 1 bar and / or the spring has a non-zero prestress.

6. An underwater vehicle according to any one of the preceding claims, wherein the hydraulic accumulator (6) is calibrated so that the volume of the fluid chamber (10) remains minimal as long as the pressure of the surrounding water is lower than a reference pressure (PR), the volume of the fluid chamber (10) increasing with the pressure of the surrounding water when the pressure of the surrounding water rumbling is higher than the reference pressure (RP).

7. An underwater vehicle according to any preceding claim, wherein the external tank (28) comprises an elastically deformable bladder, an elastically deformable membrane and / or a piston subjected on one side to the action of the liquid and on the other side to the action of the surrounding water.

8. An underwater vehicle according to any preceding claim, wherein the external tank (28) and the casing (8) are at least partly nested within each other.

9. An underwater vehicle according to any one of the preceding claims, comprising a ballast system (32), the ballast system (32) comprising a ballast tank (34) fluidly connected to the external tank, and an actuator (36) activatable to force the transfer of the liquid (L) between the ballast tank (34) and the external tank and / or the fluid chamber (10).

10. An underwater vehicle according to claim 9, wherein the external tank (28) and the fluid chamber (10) are fluidically connected in parallel to the actuator (36).

11. An underwater vehicle according to claim 9, wherein the external tank (28) and the fluid chamber (10) are fluidically connected in series to the actuator.

12. An underwater vehicle according to claim 11, wherein the external reservoir (28) is fluidly located between the actuator (36) and the fluid chamber (10).

13. An underwater vehicle according to any one of claims 9 to 11, wherein the ballast tank (32) is received in a wet compartment (24) of the underwater vehicle.

14. An underwater vehicle according to any preceding claim, wherein the external tank (28) and / or the hydraulic accumulator (6) are received in a wet compartment (24) of the underwater vehicle.

15. An underwater vehicle according to any preceding claim, wherein the underwater vehicle is an underwater glider or an underwater profiler.

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