Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Line intended to be immersed in an aquatic environment
The invention relates to a line intended to be submerged in an
aquatic environment. A line of this kind can be employed to hold an abject at
a certain depth relative to the surface. An abject of this kind can for
example
be a passive or active sonar antenna.
Sonar antennas can be towed behind a surface vesse!. It can also
be useful to have an antenna which remains fixed in position. Ta that end, it
is known to use air-dropped sonar buoys. Once the buoy has reached the
surface of the water after the drop, if deploys a sonar antenna at a given
depth. The buoy is connected to the antenna by an electric/load-bearing
cable. The antenna receives sound information from the aquatic
environment. The antenna sends this information to the buoy via the
electric/load-bearing cable. In turn, the buoy sends, by radio, the
information
received from the sonar antenna, for example to the aircraft that dropped the
buoys.
This type of buoy has a drawback. The sonar antenna is
connected only to the buoy, and in the presence of marine currents the
assembly formed by the buoy and the antenna drifts at the mercy of the
current. Moreover, marine currents can be different at the surface and at the
depth at which the antenna is submerged. The electric/load-bearing cable is
then inclined, causing the antenna to be inclined as well. The inclination of
the antenna with respect to the vertical can compromise its mission. Indeed,
the inclination of the antenna causes the receiving sound lobes and, in the
case of an active antenna, the transmitting sound lobes to be inclined. The
inclination of the sound lobes impairs the performance of the sonar since
they might experience interference with the bottom or the surface.
One attempt to better keep the sonar antenna in position has been
to anchor the buoy and its antenna to the bottom. Anchoring makes the
antenna even more susceptible to the current.
The invention aims to remedy some or all of the abovementioned
problems by proposing a line that is submerged in the aquatic environment,
is anchored at one of its ends and has a buoy at the other end, with an object
being attached between the anchor point and the buoy. The line comprises
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means for keeping the abject vertical, even in the presence of a current in
the
aquatic environment.
More specifically, the invention relates to a line intended to be
subnnerged in an aquatic environment, characterized in that it comprises:
= a mooring configured to be placed on the bottom of the aquatic
environment and for immobilizing the line relative to the bottom,
= a buoy configured to float on the surface of the aquatic environment,
= an abject extending along a vertical axis, having a center of balance of
hydrodynamic forces when the abject is subjected to a horizontal water
current and called the hydrodynamic center, and having a center of
gravity vertically remote from the hydrodynamic center,
= a frame connected to the abject by a pivoting link with a substantially
horizontal axis passing through the hydrodynamic center,
= at least one fin extending vertically, whereby the abject can be oriented
relative to a horizontal water current,
= a first section of line connecting the mooring to the frame,
= a second section of line connecting the trame to the buoy.
Advantageously, the at least one fin is secured to the frame.
The abject comprises, for example, an upper part that is
configured to receive and/or transmit information to the aquatic environment
and a lower part that comprises utilities. Advantageously, the at least one
fin
faces the lower part without facing the upper part.
The pivoting link advantageously comprises two coaxial bearings
connecting the abject to the frame, the two bearings being arranged on either
side of the hydrodynamic center.
The line advantageously comprises a swivel arranged between the
mooring and the frame.
The swivel may be arranged between the first section of line and
the frame.
The abject may comprise an acoustic transmitter and an acoustic
receiver.
The second section of line advantageously comprises a cable that
is configured to send information tram the object to the buoy, and the buoy
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comprises a transmitter that is configured to send, through the air, the
information received from the object.
The invention will be better understood and further advantages will
become apparent upon reading the detailed description of one embodiment
provided by way of example, which description is illustrated by the attached
drawing, in which:
figure 1 shows an exemplary use of a submerged line according to
the invention;
figure 2 shows a submerged line according to the invention;
figure 3 shows, in greater detail, the Une from figure 2 at an object
attached to the line.
For the sake of clarity, the same elements will bear the same
references in the various figures.
Figure 1 shows, schematically, an aquatic environment 10 in
which one is trying to detect the presence of a submarine 11 that might pose
a threat to a surface vesse! 12. Ta that end, multiple sonar antennas 14 are
arranged in the area where the surface vesse! 12 is sailing, each one having
a fixed position with respect to the bottom. Each antenna 14 may be active
and comprises an acoustic transmitter and receiver, with the receiver
receiving an echo of a sound wave sent by the transmitter. The sought-after
echo is of course returned by the hull of the submarine 11. Alternatively, the
antenna may be passive. In that case, it comprises no sound transmitter and
merely detects sound waves in order to identify those coming from the
sought-after submarine 11, or more generally any threat approaching the
zone of interest containing the surface vesse! 12. More generally, the sonar
system comprising the various antennas 14 can operate in a bistatic mode in
which each antenna is configured to receive an echo of a sound wave that it
transmits. The sonar system may operate in a multi-static mode in which
each antenna is configured to receive an echo of a sound wave originating
from another antenna of the system.
Each sonar antenna 14 is arranged on a line that is submerged
between an anchoring point and a floating buoy. The buoy may receive
information from the antenna and send this information to a ground station
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15, for example via the intermediary of a satellite 16. The buoy may also
send information received from the antenna to other stations allowing
processing of the information, for example stations on board the surface
vessel 12 or of an airplane flying over the zone where the antennas 14 are
submerged. For transmission to the surface vessel 12 or an airplane, it is
possible to transmit directly without passing via the satellite 16, for
example
using VHF radio transmission.
In figure 1, the various antennas 14 are arranged in a circle
around the surface vessel 12. The invention is not limited to this
arrangement. It is for example possible to arrange the antennas in a straight
line. More generally, the invention may be implemented for a single antenna
14, and even for any object which one wishes to arrange submerged at a
given position and depth. Another mission of the object may be to measure
the velocity of a marine current at a predefined depth. The object is then
equipped with a water flow velocity sensor.
Figure 2 shows, in greater detail, the submerged line which in this
case bears the reference 20. The line 20 comprises a mooring 22 (also
known as a "sinker") that is configured to rest on the bottom 23 of the
aquatic
environment 10 so as to immobilize the line 20 with respect to the bottom 23.
A mooring is to be understood as any device that can remain immobile on the
bottom 23. The mooring 22 may be an inert mass, for example made of
concrete. This inert mass is heavy enough to immobilize the line 20. Any
other means that serves to immobilize the line 20 with respect to the bottom
23 may be implemented. The mooring 22 may be an anchor that can catch
on the bottom 23. If is possible to combine various immobilizing means, inert
mass and anchor. The mooring 22 is dimensioned such that the mooring 22
remains immobile on the bottom 23 when acted upon by a given current 25
that tends to exert a force on the line 20. At sea, if is possible to
experience
tidal currents which, in particular areas, can exceed 5 kn during spring
tides.
The mooring 22 is defined in dependence on the maximum drag force
experienced by the line 20 when acted upon by the maximum current existing
in the location where the line 20 is deployed.
The line 20 comprises a buoy 27, configured to float to the surface
28 of the aquatic environment 10, and a frame 30 that serves to maintain the
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vertical orientation of an abject which, in the example shown, is the sonar
antenna 14. In order to keep the buoy 27, the trame 30 and the mooring 22
secured ta one another, the line 20 comprises a line section 34 connecting
the mooring 22 ta the trame 30 and a line section 36 connecting the frame 30
5 ta the buoy 27. The two sections 34 and 36 may be cables or ropes.
The sonar antenna or more generally the abject 14 may be
autonomous. The abject 14 may be provided with a detector and a memory
which records, at predefined intervals, data collected by the detector.
Periodically, the line 20 may be raised ta recover the recorded data.
Alternatively, the abject 14 may share in real-time the data that it
collects, with a station external ta the line 20. Ta that end, the line
section 36
comprises a cable that is configured ta send information from the abject 14,
and for example the acoustic receiver thereof, ta the buoy 27. The line
section 36 may be electric and load-bearing and comprise for example
electrical conductors that form a core surrounded by load-bearing armor.
Alternatively, the line section 36 may consist of a load-bearing cable around
which is coiled an electric cable that sends information between the abject 14
and the buoy 27. The buoy 27 comprises a transmitter 38 that is configured
ta send, through the air, the information received tram the abject 14. The
transmitter may be of any kind, for example a radio or optical transmitter.
The length L of the line 20 between the mooring 22 and the buoy
27 is defined in dependence on the depth P of the aquatic environment 10
where the line is intended ta be deployed. The depth P is the distance
between the surface 28 and the bottom 23. The aquatic environment 10 may
experience tides, and the length L of the line 20 must take into account the
tidal range. Advantageously, the buoy 27 always floats at the surface 28 in
order ta continuously send its information by means of its transmitter 38. The
length L of the line 20 is then greater than the depth P at the highest tide.
Alternatively, it is possible ta reduce the length L such that the buoy 27 can
float only intermittently, for example at low tide. This reduced length may be
useful in the case of an abject 14 provided with a recorder. Recovery of the
data then takes place when the buoy 27 is floating. The length of each of the
two line sections 34 and 36 may also be adjusted ta account for the
bathycelerity profiles of the zone in which the line 20 is submerged. When the
length L of the line 20 is greater than the depth P, the abject 14 and the
buoy
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27 will move around the mooring 22 at the mercy of the current 25. This
creates, in particular, an oscillation around the position of the mooring 22
when the line 20 is submerged in an environment where the tides produce
alternating currents. Thus, the line sections 34 and 36 are inclined with
respect to the vertical. This inclination presents difficulties for
maintaining the
orientation of the object 14 with respect to the vertical direction. Holding
the
object 14 is useful, as stated above, for a sonar listening mission. This
holding is also useful for a mission for measuring marine currents. These
difficulties are solved by the invention.
The object 14 has external shapes that serve to define a center of
balance of hydrodynamic forces when the object 14 is subjected to a
horizontal water current 25. This center is more simply referred to as the
hydrodynamic center 40. In a first approach, the position of hydrodynamic
center 40 does not depend on the intensity of the current, but solely on the
forms of the object 14. When the object 14 is held by ifs hydrodynamic center
40, the hydrodynamic forces exerted by a horizontal current above the
hydrodynamic center 40 balance out the same forces exerted below the
hydrodynamic center 40. For example, when the object 14 is a vertically
oriented cylinder, the hydrodynamic center 40 is located at half the height of
the object 14. The position of the hydrodynamic center 40 may depend on
the surface state of the object 14. lt is possible to determine ifs position
by
trials in a reference aquatic environment with a current of predetermined
intensity.
The object 14 also has a center of gravity 42. The object 14 is
configured such that ifs center of gravity 42 is at a vertical distance from
its
hydrodynamic center 40.
At least one pivoting link 43 of essentially horizontal axis 45
passing through the hydrodynamic center 40 connects the frame 30 and the
object 14. The distance between the axis of the pivoting link 43, passing
through the hydrodynamic center 40, and the center of gravity 42 naturally
encourages a stable position of the object 14, which holds itself vertically
whether in the absence or presence of a current 25, the center of gravity 42
being located below the hydrodynamic center 40.
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In the example shown, the frame 30 surrounds the object 14. Line
section 34 is attached to the frame 30 at an attachment point 44 and line
section 36 is attached to the frame 30 at an attachment point 46. In the
absence of a current, when the two line sections 34 and 36 are aligned
vertically, the hydrodynamic center 40, the center of gravity 42 and the two
attachment points 44 and 46 are also aligned along a vertical axis 47 of the
object 14. Thus, the various vertical forces applied to the frame 30, that is
to
say the forces tram the two line sections 34 and 36 and the force due to the
weight of the object 14, are ail aligned. This alignment serves to keep the
axis of the pivoting link horizontal. Advantageously, and as in the example
shown, the pivoting link 43 is established by means of two coaxial bearings
48 and 50 which allow the object 14 to rotate, about the axis 45, with respect
to the frame 30. The two bearings 48 and 50 are positioned on either side of
the hydrodynamic center 40. The presence of these two bearings 48 and 50
avoids the object 14 being supported in a cantilever manner with respect to
the frame 30. This arrangement of the hydrodynamic center 40, the center of
gravity 42 and the pivoting link 43 serves to keep the object 14 vertical. In
other words, the axis 47 passing through the hydrodynamic center 40 and the
center of gravity 42 remains vertical. It is still possible for the object 14
to
rotate on its axis 47. The line sections 34 and 36 may be very long, and
twisting of these sections is possible. It is possible to know this rotation
by
fitting the object 14 with a compass. However, the axis 45 of the pivoting
link
43 may align itself with the current 25. With this orientation, if the frame
30 is
inclined with respect to the vertical, the axis 45 of the pivoting link 43 is
no
longer horizontal and the axis 47 of the object 14 passing through its
hydrodynamic center 40 and its center of gravity 42 no longer remains
vertical. In order to stabilize the object 14 in rotation about its axis 47,
it is
possible to fit the object 14 with at least one vertical fin. In the presence
of a
current 25, this fin points in the direction of the current 25 and serves to
keep
the axis 45 of the pivoting link 43 perpendicular to the current.
However, the presence of one or more fins attached to the object
14 has a drawback by changing the shape of the object 14. Fins of this kind
can for example hamper the propagation of acoustic waves and it is
advantageous for the object 14 to remain rotationally symmetric about its
vertical axis 47. The presence of fins attached to the object 14 has another
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drawback linked ta the fact that these fins change the hydrodynamic behavior
of the abject 14. The fins attached ta the abject 14 present a risk of
instability
in the position of the hydrodynamic center 40. In the event of turbulence in
the current, the presence of fins on the abject 14 could weaken the effect of
keeping it in the vertical position. In order ta ensure the stability of the
orientation of the abject 14, without adding any extra physical features, at
least one fin extending vertically is attached ta the frame 30. In the
presence
of a current 25, the frame 30 aligns itself with the axis of the current 25
and
the abject 14 follows the orientation of the frame 30. In the example shown,
the frame 30 is equipped with two fins 52 and 54, each located close ta one
of the bearings 48 and 50.
Figure 3 shows the abject 14 and the frame 30. The abject 14
comprises an upper part 60, referred ta as the active part, comprising sound
transmitters and/or receivers, or other types of sensor, and a lower part 62
comprising utilities such as a battery and electronic modules which in
particular format information received from the sensors or receivers, or in
the
direction of the transmitters. The presence of a battery in the lower part 62
tends ta lower the position of the center of gravity 42. The active part 60
receives and/or transmits information ta the aquatic environment 10, such as
sound waves for example, while the lower part 62 communicates only
internally with the abject 14 or ta the electric/load-bearing cable of section
36.
Advantageously, the fins 52 and 54 face the lower part 62 without facing the
upper part 60. Thus, the fins do not hamper the propagation of information in
the aquatic environment 10. The fins 52 and 54 are located in the lower part
of the frame 30, below the bearings 48 and 50.
The orientation of the abject 14 depending on the current 25 by
virtue of the fins 52 and 54 may be braked by line section 34. lndeed, this
orientation requires twisting of line section 34. Small twists are often
possible.
However, reversai of the current 25 is possible, such that it is necessary ta
be able ta twist through 180 , or even several full turns. In order ta
facilitate
the orientation of the abject, the line 20 comprises a swivel 64 arranged
between the mooring 22 and the frame 30. The swivel 64 provides the frame
30 with the freedom ta rotate with respect to the mooring 22, about a
longitudinal axis of line section 34. The shackle 64 is advantageously
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arranged between line section 34 and the frame 30, and constitutes
attachment point 44 so as to avoid any twisting of line section 34. lt is also
possible to arrange another swivel located between the frame 30 and the
buoy 27, for example to allow the buoy 27 to turn freely about line section
36.
In practice, this other swivel may be installed in the absence of an
electric/load-bearing cable connecting the buoy 27 to the object 14. To that
end, it is possible to arrange a recorder in the object 14 so as to permit
subsequent retrieval of information processed in the object 14, such as
information received from a sonar antenna belonging to the object.
