DEPTH CONTROL DEVICE

DISPOSITIF DE CONTROLE DE L'IMMERSION

English Abstract

A depth control device (1) for a subsmersible body comprises a depth monitor
(5) and depth controller (6). A variable volume buoyant body operates under
the control of the depth controller and comprises three chambers, the first
(10) containing air and the secnod and third chambers (7, 8) containing
hydraulic fluid. The second and third chambers (7, 8) communicate reversibly
with one another by means of a tube (13) and bypass valve (12) in a pump (3),
so that a piston (9) at the entrance to the first chamber (10) is displaced in
response to movement of fluid between the second and third chambers. Thus, the
volume of the buoyant body is varied and hence the buoyancy of the
subsmersible body which it controls. To keep the submersible body at a
particular depth, a depth trigger value is set in the depth controller (6), so
that a reversible change in the volume of the buoyant body is initiated in
response to a change in depth sensed by the depth monitor (5) by pumping fluid
between the two chambers (7, 8). The depth monitor is generally a pressure
sensor and the pump is typically a swash plate pump.

French Abstract

L'invention concerne un dispositif de contrôle de l'immersion (1) destiné à équiper un corps submersible. Ce dispositif comprend un appareil de mesure de la profondeur (5) et un servo-régulateur de l'immersion (6). Un corps flottant de volume variable fonctionne sous le contrôle du servo-régulateur de l'immersion et comprend trois chambres, la première chambre (10) contenant de l'air et la deuxième et la troisième chambres (7, 8) contenant un fluide hydraulique. La deuxième et la troisième chambres (7, 8) communiquent l'une avec l'autre dans les deux sens au moyen d'un tube (13) et d'une soupape de dérivation (12) placés dans une pompe (3), de sorte qu'un piston (9) situé à l'entrée de la première chambre (10) se déplace en réponse au mouvement du fluide entre la deuxième et la troisième chambres. Ainsi, le volume du corps flottant est varié et, partant, la flottabilité du corps submersible qu'il contrôle. Pour maintenir le corps submersible à une profondeur donnée, une valeur de déclenchement d'immersion est fixée dans le servo-régulateur de la profondeur (6) de sorte qu'un changement réversible dans le volume du corps flottant soit déclenché en réponse à un changement de profondeur détecté par l'appareil de mesure de l'immersion (5), par pompage de fluide entre les deux chambres (7, 8). L'appareil de mesure de la profondeur est généralement une sonde de pression, et la pompe est généralement une pompe à plateau incliné fixe.

Claims

6
CLAIMS
1. A depth control device for a submersible body, the device comprising depth
monitoring means and depth control means; wherein the depth control means
comprises
a controller and a variable volume buoyant body; the buoyant body comprising a
first
chamber containing a first compressible fluid and second and third chambers in
communicating relationship with one another containing a second fluid, less
susceptible
to compression than the first; whereby a piston at the entrance to the first
chamber is
displaced in response to movement of the second fluid between the second and
third
chambers such that the volume of the buoyant body is varied; and wherein a
depth
trigger value is set in the depth control means such that the controller
initiates a
reversible change in the volume of the buoyant body in response to a change in
depth
sensed by the depth monitoring means, whereby the depth of the submersible
body is
maintained at substantially the depth trigger value.
2. A device according to claim 1, wherein the pump comprises a swash plate
pump.
3. A device according to claim 1 or claim 2, wherein the controller further
comprises means to receive control signals to set the depth trigger value,
such that the
depth of the submersible body may be changed.
4. A device according to any preceding claim, wherein the depth monitoring
means
comprises a pressure sensor.
5. A submersible body comprising one of a seismic streamer, a towed sonar
array, a
sonar buoy or a bathyscaph including at least one depth control device
according to any
preceding claim.

Description

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DEPTA CONTROL DEVICE
This invention relates to a depth contro) device, in particular for use with
submersible bodies.
Certain submersible bodies, e.g. towed arrays, such as towed sonar arrays and
seismic streamers, or sonar buoys, are required to maintain a predetermined
depth.
Other submersible bodies are deposited on the sea bed and must then be
retrieved. Still
others, such are autonomous underwater vehicles (AUV's) require an effective
depth
control mechanism as they move through the water. Conventionally, the towed
arrays
are towed behind a ship and are required to maintain neutral buoyancy at a
particular
depth. A sinker is applied to the end of the array nearest to the towing ship
to position
that end of the array correctly and the remainder of the array is made
neutrally buoyant
at the desired depth, either by adding and subtracting ballast from the array
or pumping
fluid, such as kerosene, into its housing until the correct level of buoyancy
is reached.
The disadvantage of these methods is that they are expensive in ship time and
manpower
and if the array is moved to an area where the salinity of the sea is
different, the whole
procedure must be repeated.
Another system which has been proposed is to attach moveable hydrofoils along
the towed array and control the angle at which the hydrofoils are set, so that
as the array
is towed through the water its depth and stability can be maintained.
US5619474
describes a system of this type. However, these hydrofoils create noise and
can impair
the performance of seismic sensors on a seismic streamer.
US 3909774 suggests incorporating pressure sensors in sections of a seismic
streamer towed by a ship to control inflow and outflow of a buoyancy control
iiquid held
in a pipeline alongside the streamer and serving multiple sections. The
problem with this
system is that it is bulky and relies on a connection to a pump and supply on
the ship for
the additional fluid pipeline required, so it is unsuitable for small bodies
or independent
vessels, such as the AUV.
Retrieval of bodies, which do not have their own form of propulsion, from the
sea bed is typically done by attaching an air bag and providing a tank of
compressed air
which inflates the air bag to give the body sufficient buoyancy to return to
the surface.
Another way is to ballast the body e.g. with lead shot, then to disengage the
ballast, so

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that the natural buoyancy of the body raises it to the surface. Both of these
methods are
only suitable for single use and disposal of ballast is potentially damaging
to the
environment.
A device for deploying instrumentation has been proposed which is dependent on
the external water pressure acting on a piston until the piston comes up
against a spring ,
and then the piston continues to compress the spring until, at a predetermined
depth, a
spike beneath the piston impales a gas canister. The gas released forces the
piston back
up and provides sufficient buoyancy to raise the device back to the surface of
the water.
The instrument is retrieved and the readings recorded during the device's
passage can
then be read. This system suffers from the similar problems to the ballast or
air bag
systems, that the devices must be set up for a specific depth in a particular
area of water
and can only be used for one depth setting without adjustment and
replenishment of the
gas canister. Use of this system in towed arrays would involve significant
time and
expense.
In accordance with a first aspect of the present invention, a depth control
device
for a submersible body, comprises depth monitoring means and depth control
means;
wherein the depth control means comprises a controller and a variable volume
buoyant
body; the buoyant body comprising a first chamber containing a first
compressible fluid
and second and third chambers in communicating relationship with one another
containing a second fluid, less susceptible to compression than the first;
whereby a piston
at the entrance to the first chamber is displaced in response to movement of
the second
fluid between the second and third chambers such that the volume of the
buoyant body is
varied; and wherein a depth trigger value is set in the depth control means
such that the
controller initiates a reversible change in the volume of the buoyant body in
response to
a change in depth sensed by the depth monitoring means, whereby the depth of
the
submersible body is maintained at substantially the depth trigger value.
The present invention uses a buoyant body, the volume of which may be
reversibly changed, to either increase or decrease the buoyancy of the body in
response
to a change in depth sensed by the depth monitoring means, and thereby
maintain the
depth at a set value. The invention uses a closed system which transfers the
second fluid
between two chambers, so that pressure is exerted indirectly on the more
compressible
first liquid allowing a self contained, compact device to be produced.

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3
A pump with an integral return valve is most suitable for the device of the
invention and preferably, the pump comprises a swash plate pump.
The present invention uses a swash plate pump which increases or decreases the
buoyancy of the device by transfernng fluid between two reservoirs to alter
the volume
of fluid in a first chamber which provides the device with its buoyancy. There
is no limit
on how often this can be done, unlike systems which rely on compressed air,
which have
a limited usable life without refilling.
Preferably, the controller further comprises means to receive control signals
to
set the depth trigger value, such that the depth of the submersible body may
be changed.
The invention can be operated to maintain a pre-set depth or to change the
depth
at which the body is held, by adjusting the buoyancy for a new depth trigger
value.
Preferably, the depth monitoring means comprises a pressure sensor.
Other ways of measuring depth include active acoustic means.
In accordance with a second aspect of the present invention a submersible body
1 S comprising one of a seismic streamer, a towed sonar array, a sonar buoy or
a bathyscaph
including at least one depth control device according to the first aspect.
Another way of deploying sonar is in a sonar buoy, instead of a towed array,
which is usually tethered at a fixed depth and recovered by detaching itself
from the
tether. A depth control device of the invention would allow the buoy to be
moved to
different depths to obtain data.
An example of a depth control device according to the present invention will
now
be described with reference to the accompanying drawings in which:-
Figure 1 illustrates a device according to the invention.
Fig. 1 shows an example of a depth control device 1. The device 1 comprises a
housing 2, a swash plate pump 3, a motor 4, a pressure transducer 5,
electronic control
circuits 6, first and second reservoirs 7, 8 and an air chamber 10. The first
reservoir 7 is
inside the housing 2 and a piston 9 is provided between the reservoir 7 and
the air
chamber 10. This piston 9 moves when hydraulic fluid, typically oil, is pumped
between
the first reservoir 7 and the second reservoir 8 by means of a tube 13 and
bypass valve
12. The second reservoir is bounded by a second piston 11, separating the oil
from
water outside the housing, which moves to accommodate the distribution of the
volume

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4
of oil between the two reservoirs 7, 8. The volume of the air chamber 10
varies to
increase the buoyancy of the device 1 in response to a sensed increase in
pressure or vice
versa. The housing is preferably made of metal to be robust.
The control circuits 6 include a processor which may be programmed with a
S pressure corresponding to a desired depth at which the device 1 is to be
maintained and
when the pressure exceeds that programmed value, the buoyancy of the device 1
is
increased by operation of the swash plate pump 3 to pump hydraulic fluid from
the first
reservoir 7 to the second reservoir 8, allowing the air to expand into the
space vacated
so increasing the volume of the air chamber 10 and the resultant buoyancy. If
the
pressure is below the programmed value, the buoyancy is reduced by reversing
operation
of the motor 4 and opening a bypass valve 12 which allows hydrostatic pressure
on the
piston 11 to push oil back from the second reservoir into the first reservoir,
so
compressing the piston 9 and reducing the volume of the air chamber 10. The
fluid in
the air chamber 10 is not restricted to air, other gases could be substituted.
The device may be connected to a body for which it is to control the depth via
a
watertight connector 13. The control circuitry may also include a receiver to
receive
control signals to change the programmed depth. The control signals could be
delivered
via internal cabling which is already in place in a towed array and connects
directly to the
ship or for a sonar buoy or other non-towed body, control signals may be
transmitted
from a remote transmitter on shore or on a ship or an aircraft. The active
nature of the
device of the present invention allows submersible bodies to be maintained at
a
predetermined depth or for the depth to be changed according to the
circumstances. By
making setting up pre-programmed consecutive variations to the programmed
depth a
body can operate at one or more depths for a period, surface and then move to
a new
depth. Alternatively, if a particular characteristic within a water column is
of interest,
additional sensors may be used to follow that characteristic and to control
the
programmed depth, so that a buoy passes through only the vertical section of
the water
column in which that characteristic is apparent.
The device can also be used to position and hold objects at a required depth
e.g.
for positioning equipment underwater by lowering the object under control to a
pre-set
depth and holding it at that depth whilst it is connected to other equipment.
This

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reduces the cost of deployment as compared with an air bag system as currently
used for
this purpose which requires divers, cranage or ballasting.

Representative Drawing