WO1995026514A1

WO1995026514A1 - Acoustic linear antenna and method for making same

Info

Publication number: WO1995026514A1
Application number: PCT/FR1995/000351
Authority: WO
Inventors: Dominique Grangeon; Jean-Claude Giannotta; Robert Fichaux; Alain Chalumeau; Bruno Tardy; Gilbert Roman
Original assignee: Thomson-Csf
Priority date: 1994-03-29
Filing date: 1995-03-22
Publication date: 1995-10-05
Also published as: AU2140995A; FR2718316B1; FR2718316A1

Abstract

Acoustic linear antennas and a method for making same in the form of a flexible flat tube consisting of an outer sheath (101) made of elastomeric material containing acoustic sensors (102) or electronic circuits attached to load take-up wires (104, 105) extending along the antenna. The inner space of the sheath is enclosed in a material (103) with similar acoustic properties to water. Long thin acoustic linear antennas that can be wound or unwound with a small diameter may thus be provided.

Description

ACOUSTIC LINEAR ANTENNA AND ITS MANUFACTURING METHOD

The present ^: invention relates to acoustic linear antennas for so ≤ marine use, both intended to be mounted on buildings or mobile vehicles that intended to be immersed in a fixed manner to constitute a detection network, for example for oil prospecting .

Linear acoustic antennas are known which are intended to be towed by a tractor-boat and which are essentially in the form of an elongated pipe of substantially circular section, often formed of several elements butted together. This pipe includes hydrophones as well as circuits for processing the signal from these hydrophones, for example preamplifiers, and it is filled with an acoustic coupling oil which provides a certain mechanical resistance to the antenna.

Also known, for example from French patent application No. 2,691,596 filed by the applicant on May 22, 1992, antennas which can be plated in a conforming manner against the hull of a building and which include piezoelectric polymer hydrophones such as PVDF.

In certain applications, it is desired to have linear antennas which can be submerged in a relatively large number to form a submerged surveillance network. The range of frequencies to receive is typically that of frequencies below 1,500 Hz. These antennas are likely to be brought to the surface and stored on board a carrier vessel to be redeployed at a location distant from the first.

This requires being able to store these antennas in a sufficiently small volume, which in practice and given the relatively large length (greater than 20 m for example) of each individual antenna, and their number, generally leads to wind them on drums of small diameter, for example less than 1 m, and to look for antennas of small section. It should be understood that when the antennas are unwound from the drums they do not have a curvature corresponding to a shape memory acquired during storage.

In addition, the antenna must both have sufficient flexibility to unwind or be unwound easily underwater and a sufficient stiffness to ensure the maintenance of correct straightness in operation.

The antenna must also be able to be deployed vertically, which requires a significant tensile strength, corresponding for example to forces greater than 3,000 Newtons. This tensile strength must extend to resistance to mechanical attack of all kinds, in particular from friction and shock applied during deployment of the antenna.

Of course, successive deployments and windings must not disturb the positioning in the antenna of its various elements, in particular hydrophones.

In addition, all of these properties must be able to be preserved under a wide range of environmental conditions, and in particular to withstand extreme storage temperatures, for example -30 ° C. + 65 ° C., operating temperatures comprised for example between - 2 ° C and 35 ° C, and an immersion pressure up to that corresponding to several thousand meters deep

In other applications, it is desired to have linear antennas which can be stretched over a hull or a building structure (for example a surface or submarine building), and also supporting numerous installation operations (deployment) and disassembly (winding), especially in the context of maintenance operations.

To fulfill all these conditions, the invention proposes a linear acoustic antenna of the type comprising a set of sensors and electronic conditioning circuits distributed in a flexible outer sheath itself filled with a coupling material, essentially characterized in that: the cross section of the sheath has a length and a width of substantially different dimensions giving the antenna a flat shape such as a ribbon,

- The coupling material does not significantly modify the stiffness of the antenna; and

- It further comprises a set of semi-rigid rods extending over the entire length and contributing to the mechanical strength of the antenna;

According to another characteristic, the antenna includes a set of supports distributed along the rods to support the sensors or the circuits and maintain them in position inside the sheath within the coupling material.

According to another characteristic, each support comprises a central part pierced with two holes and two end parts making it possible to receive and immobilize the rods.

According to another characteristic, the rods immobilized in the support ends make it possible to support the connection wires of the sensors.

The invention further provides a method of manufacturing such a linear acoustic antenna, essentially characterized in that it comprises the steps of:

- Manufacturing of a sheath having an oblong section identical to that of the final section of the antenna;

- preparation of a strand of rushes on which are fixed supports which support the antenna sensors or the circuits themselves;

- Wiring of these sensors or of these circuits by wires which are stranded on at least one of these rods;

- Deposition inside this sheath of an adhesion primer; - introduction of the rosary in the sheath; and

- filling the interior of this sheath with a material of low hardness acoustically suitable for water in the liquid state and solidifying at the end of the operation.

According to another characteristic, after having deposited the primer inside the sheath, let it dry, then place this sheath in a mold formed by two assembled shells delimiting an interior space whose section is that of the antenna to be obtained, this sheath is pressed on the interior walls of the mold by vacuum, this mold is slightly inclined relative to the horizontal and injects the filling material from the lower end of the mold.

Other features and advantages of the invention will appear clearly in the following description, presented by way of nonlimiting example - with reference to the appended figures which represent:

- Figure 1, a cross-sectional view of an antenna according to the invention;

- Figure 2, a partial view of a support for the internal sensors of this antenna;

- Figure 3, a partial top view of the internal chain comprising the sensors fixed on the supports of Figure 2; and - Figure 4, a cross-sectional view of a mold intended to maintain the sheath of the antenna during its filling with the filling material.

The number, size and positioning of the sensors depend on the performance expected from the antenna and the frequency band considered; moreover, the sensors must not disturb the ability of the antenna for deployments and windings. In a preferred embodiment using flat sensors, made for example of piezoelectric polymer, 25 pairs of such sensors will be placed in an antenna 40 m long. This length and the type of sensors are given for information only and can be adjusted as required.

The material constituting the outer sheath is an elastomer resistant to external aggressions (mechanical such as friction, tearing ..., chemical, due to the marine environment ...) and compatible from the point of view of adhesion with the filling material. The filling material can be a material

"thermosetting" or thermofusible, for example a polyurethane. The word "thermosetting" here means that the material is liquid when it is introduced cold into the sheath, and that it then solidifies, in principle under the effect of heat. However, it remains "soft" so that the antenna is flexible. The semi-rigid rods are used to support the tensile forces while increasing in a controlled way the stiffness in bending of the antenna. In a preferred embodiment, cylindrical rods of resin-fiberglass composite material will be used. Each sensor is placed inside the antenna as shown in the section of FIG. 1. The sensor 102 is immobilized substantially in the center of the sheath 101. After filling, it is held in place by the filling material 103.

On both sides of the sensor are semi-rigid rods 104 and 105. These rods make it possible to obtain the necessary rigidity and to support the tensile forces. In this exemplary embodiment, two rods of relatively small diameter, for example 2 mm, 104, are used located closer to the sensor and two other rods of larger diameter, for example 5 mm, are located closer to the outer sides of the sheath. These rods of larger diameter make it possible to maintain the various electrical cables which themselves allow the signals from the sensors and the preamplifiers to be propagated as well as the electrical supply voltages of these same preamplifiers. These cables 106 are stranded around the rods 105. At each end of the antenna, two flat flanges have been placed which are glued to the sheath and on which the rods are blocked in order to allow the recovery of force. These flanges are provided with an opening which makes it possible to connect a connector to the cables 106 and thus to connect the antenna to the external operating members, placed for example on a submerged beacon and provided with transmission means in the case of a fixed detection network.

The manufacture of such an antenna poses various problems of positioning the internal organs and filling resolved by the method according to the invention. We start by preparing a chain of sensors and electronic circuits attached to the rods and joined to the various power and transmission cables.

For this, centering supports 201 are used such as that shown partially in FIG. 2 seen from the side and completely in FIG. 3 representing a part of the chain seen from above. In a preferred embodiment, such supports 201 will be made of plastic. They include a central part 202 pierced with holes 203 making it possible to introduce the rods 104, and ends 204 where the large rods 105 are housed. This system of supports and rods makes it possible to immobilize the sensors and the electrical circuits one compared to others in all directions.

In the next step, the large rods 105, previously covered with the connection cables, are fixed to the ends 204 of the supports 201. The immobilization of these rods relative to the supports can be ensured by bonding.

The next step consists in wiring in a known manner the sensors or circuits by joining them to the wires supported by the rods 105. The rosary thus obtained is then ready to be placed in the sheath. To increase the adhesion between the external sheath and the filling material, a bonding primer can be used by deposition on the internal face of the sheath.

The sheath being thus prepared, it is then placed in the molding tool shown in FIG. 4. This tool comprises a lower part 401 joined to an upper part 402 by means of bolts 403. These two parts have the form of two gutters, the meeting of which delimits an interior space having substantially the dimensions of the completed antenna. The sheath is placed in part 401 and the assembly is closed with part 402. Vacuum sockets, represented in the figure by the pipe 404, are distributed along the mold so as to allow the air to be sucked in. content between the mold and the sheath to properly press the latter against the walls of the mold.

The string formed by the rods, the sensor supports, the sensors or preamplifiers, and the wiring, which has been made up as described above, are then introduced. For this one can for example pass through the interior of the sheath two cords using a needle and come fix one side these cords to the two outer rods. By pulling on the other side of the cords and carefully guiding the rosary, it will easily penetrate the sheath and come into place. So as to obtain a very homogeneous assembly, in particular at the ends, the invention then proposes to fix from this moment the end flanges of the antenna by coming to glue them to the ends of the sheath, as well as to the rods which will pass through holes in these flanges. The skein of connection wires will pass through openings in the center of the flanges so that the connection connectors can be fixed in a later step. We can then at the end of this step level off at the level of these flanges the part of the rods that comes out.

To facilitate the injection of the filling material, the invention proposes to cover the ends of the mold with parts having the form of boxes provided with an injection head. The filling material is introduced at the lower end of the tool, while the air outlet and, at the end of the operation, the excess material takes place at the upper ends.

The internal shape of these boxes will be provided so as to obtain an overmolding of the end flanges which are themselves glued to the sheath.

The filling speed of the sheath depends on the viscosity of the filling product, the wettability of the sheath by this product, the density of objects placed inside the sheath and the curing time of the product. The flow rate of the material injection machine must take these different parameters into account.

The excess filling material having been discharged through the injection head located in the upper part of the mold, the pressure exerted throughout the system and the volume of excess material must compensate for the shrinkage associated with hardening of the material.

The material is hardened, we can then dismantle the mold and we have a practically finished antenna. The finishing operations consist in eliminating the excess material linked to possible end leaks, then in connecting and fixing the antenna connection devices. These devices may possibly be overmolded to further improve the seal.

Claims

1. Acoustic linear antenna of the type comprising a set of sensors (102) and electronic conditioning circuits distributed in a flexible outer sheath (101) itself filled with a coupling material (103), characterized in that:

- The cross-section of the sheath has a length and a width of substantially different dimensions giving the antenna a flat shape such as a ribbon, - the coupling material does not substantially modify the stiffness of the antenna; and

2. Antenna according to claim 1, characterized in that it comprises a set of supports (201) distributed along the rods (104, 105) for supporting the sensors (102) or the circuits and keeping them in position at the inside the sheath within the coupling material.

3. Antenna according to claim 2, characterized in that each support comprises a central part (202) pierced with two holes

(203) and two end parts (205) for receiving and immobilizing the rods (105).

4. Antenna according to claim 3, characterized in that the rods (105) immobilized in the support ends (204) make it possible to support the connection wires (106) of the sensors (102).

5. Method for manufacturing a linear acoustic antenna according to any one of claims 1 to 4, characterized in that it comprises the steps of:

- Manufacturing a sheath (101) having an oblong section identical to that of the final section of the antenna;

- Preparation of a string formed of rods (104, 105) on which are fixed supports (201) which themselves support the sensors (102) of the antenna or the circuits;

- Wiring of these sensors or of these circuits by wires (106) which are stranded on at least one of these rods (105) .;

- Deposition inside this sheath of an adhesion primer; - introduction of the rosary in the sheath (101); and

6. Method according to claim 5, characterized in that after having deposited the primer inside the sheath, the latter is left to dry, this sheath is then placed in a mold formed by two assembled shells delimiting an interior space whose section is that of the antenna to be obtained, this sheath is placed on the interior walls of the mold by vacuum, this mold is slightly inclined relative to the horizontal and the filling material is injected from the lowest end of the mold.