GB2300917A

GB2300917A - Underwater trailing antenna

Info

Publication number: GB2300917A
Application number: GB9608551A
Authority: GB
Inventors: Egidius Arens
Original assignee: STN Atlas Elektronik GmbH
Current assignee: Atlas Elektronik GmbH
Priority date: 1995-05-19
Filing date: 1996-04-25
Publication date: 1996-11-20
Anticipated expiration: 2016-04-25
Also published as: NO961972L; NO961972D0; GB9608551D0; FR2734443B1; GB2300917B; NO314820B1; DE19518461C1; FR2734443A1

Description

1 Underwater Trailing Antenna 2300917

The invention relates to an underwater trailing antenna of the type described by the specifications of Claim 1.

Such underwater trailing antennae are used as receivers for underwater sound. The trailing line that forms the acoustic portion, which is divided into sections of identical structure coupled to one another, may be of considerable length and is usually trailed through the water by a ship using a tow-rope and stowed on board coiled on a drum when not in use. Such trailing antennae wgight a considerable amount and when deployed, require a large towing force by the ship. When being reeled back on board, bulky and heavy cable drums are also needed. The volume and weight of the trailing line increase in proportion to the square of its diameter. The efficacy of a trailing antenna of this type is often reduced by stray noise which partially masks the useful signal. Suchperturbing signals may be produced within the electroacoustic transducers (hydrophones) and its associated electrical and electronic switching components. Other sources of perturbation arise due to vibration of the structural elements within the trailing line, the mechanical transmission of movements of the trailing line or the tow-rope, or due to the direct transfer of pressure waves caused by turbulence in the water surrounding the trailing line to the liquid filling in cases when the trailing line as is usual - is filled with a liquid such as oil to ensure sufficient buoyancy, or due to movement of the filling liquid through the internal components of the trailing line. Owing to the last-mentioned causes, the noise level is directly affected by the structure of the trailing line and the speed at which it is being towed, the noise level increasing as the speed becomes greater.

In a known underwater trailing antenna of the type described above (DE 29 41028 A1), it is attempted to ensure high signal sensitivity of the trailing antenna and a low interference level due to perturbations within the sheath, by incorporating at least one flexible tube, which is held concentrically by several flexible spacers 2 positioned some distance apart and which rest against the sheath. Within the said tube and axially displaced with respect to the said spacers there are flexible supports, each of which carries one hydrophone. This design of the trailing line leads to a two-stage mechanical decoupling between the hydrophones on the one hand and the sheath of the trailing line on the other hand, so that turbulence in the water and the movement of filling liquid inside the trailing line act in a less perturbing way upon the hydrophones.

However, a trailing line of this type has a relatively large diameter, which restricts its length if it is desired, on pulling it back on board, to stow the trailing line an a drum of acceptable volume. Moreover, the perturbing noise produced by the trailing line increases with increasing diameter, so that part of the effort to damp down the perturbations is frustrated by the increased thickness in question.

In another known underwater trailing antenna (WO 93/17356 AI), several longitudinally spaced hydrophones are arranged along the middle of a cylindrical housing closed at the ends, the said housing being filled completely with a gel that surrounds the hydrophones directly. The gel has damping properties and damps the noise reaching the hydrophone. The housing is located centrally within a flexible tubular outer sheath, surrounded by a polypropylene filler and held in position by a flexible wire. The wire is surrounded by a gel layer, which in turn is enclosed by a paper sheath. Tension strips made of KevIar are located A between the paper sheath and an inner sheath, the latter consisting of thermoplastic rubber and being enclosed by the outer sheath which is made of polyurethane. This trailing antenna structure again leads to a trailing line with large diameter, volume and weight.

The present invention proposes to provide an underwater trailing antenna of the type described initially, which while possessing good acoustic properties will also have a very small diameter and hence too, low volume and weight.

3 According to the present invention there is provided an underwater trailing antenna with a trailing line made up from several sections, each of which has an elastic, tubular sheath and contains a number of hydrophones held centrally and at fixed distances apart from one another, characterized in that the sheath is made from polyethylene with a specific density lower than 1 g/cm3 and preferably equal to 0.96 g/CM3, and is completely filled with a soft gel which encloses the said hydrophones directly, the specific density of the said gel being lower than 1 g/cm3 and preferably equal 3 to 0.97 g/cm The underwater trailing antenna according to the invention has the advantage that the diameter of the trailing line itself is determined exclusively by the diameter of the hydrophones used, with no additional radial space required for spacers, foam tubes or other damping elements. The tensile forces during towing are borne by the polyethylene (PE) sheath, so that the usual tension strips inside the trailing line are also no longer needed. The hydrophones, if necessary with associated preamplifiers, are held in position both axially and in the radial direction, by a gel which sets after being filled into the sheath. Only during the manufacturing process is it necessary to adopt means of centring and spacing out the hydrophones within the sheath of the trailing line section until the gel has been filled in and has set. Compared with conventional trailing antennae, the diameter of the trailing line in this case can therefore be made much smaller and may, for example, amount to no more than 2.5 cm. This small trailing line diameter makes it possible on the one hand to have a trailing antenna of great length, which has an advantageous effect on its directional sensitivity, and on the other hand allows convenient and rapid winding out of the trailing antenna from the towing vessel and, when it is brought back on board again, permits it to be stowed in a relatively small space. With its trailing line of small diameter, the trailing antenna is light in weight and easy to manipulate, which increases the readiness of the operating personnel to let it out and bring it in more frequently and permits it to be used on smaller ships, such as corvettes or submarines. The specific density of the PE sheath and gel established according to the invention makes it unnecessary to fill the trailing line with oil in order to achieve weightcompensating buoyancy, and this makes the trailing antenna more environment 4 friendly, since no oil can leak out if the trailing line is damaged. The PE sheath and the gel can eventually be disposed of in an environmentally acceptable way, and they do not create any special waste disposal problems.

in accordance with one preferred version of the invention, the sheath has an elasticity modulus of approximately 100 N/mm2 and the gel has an elasticity modulus of 3 to 5 Nlmm2. The gel is water-repelling and non-viscous in its initial form, but after being filled into the sheath it sets to a gel- like vulcanisate. For preference, the gel used is that marketed by Wacker Co. under the product name SilGel 612, a 2-component silicone rubber characterized by high fluidity, extremely low vulcanized hardness, and marked self-cohesion. Appropriate mixing of the two components in the correct proportions ensures that once set, the vulcanisate will be of the desired softness.

For production technological reasons the trailing line can only be produced in part-sections of a given length, which is much shorter than the desired length of the entire trailing line. The latter is therefore formed by joining together a number of so-termed trailing line sections. For this, according to an advantageous design version of the invention, the ends of the D-olyethylene sheaths of successive trailing line sections are pushed one into another and welded together. At the joins, the current and signal conductors of copper or L ght- conducting fibres are connected together, for example by splicing.

J During the process of producing the trailing line sections, to ensure that the hydrophones are very uniformly distributed and centred inside the sheath, they are held in place by means of a loose nylon netting with a few elastic longtudinal strands. The elementary skeleton so formed is permeated by the still fluid gel. This allows the sheath to be rotated while the gel is setting, so that the elementary skeleton will remain at the centre of the tube. The elementary skeleton can if necessary also be centred by means of soft, thin bristles attached to it, which support it relative to the inner wall of the sheath.

in a preferred version of the invention, the sheath of the last trailing line section in the sequence has an end section with extremely large surface roughness. This surface roughness is preferably achieved by longitudinal folds, preferably 1 1 distributed uniformly around the circumference of the sheath, which at the end of the sheath project outwards and are produced, for example, by creasing together two circumferentially adjacent sheath sections in such a way that their internal surfaces come into contact in the radial direction along a certain distance. If the last section of the trailing line is shaped in this way, then when the underwater trailing antenna is being towed it will be kept taut and any undulations or flapping either side of its longitudinal axis will. be reduced to a minimum. In this way the flow noise produced by the antenna itself is considerably reduced, as also is the drag force to be overcome by the towing vessel.

The measures according to the invention result in an underwater trailing antenna which, at a towing speed of 20 kn in a number-3 swell, and with a pick-up frequency of about 10 Hz, will still give sufficiently good reception results.

One embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:- Fig. 1:

Fig. 2:

Fig. 3:

Fig. 4:

Fig. 5:

A side view of a surface vessel at sea, towing an underwater trailing antenna according to the invention with a towing cable and trailing line. A longitudinal cutout section of the trailing line of the trailing antenna in Fig. 1 A segment of the trailing line in the area of two adjacent trailing line sections, shown in longitudinal section. A cutout side view of the last trailing line section of the trailing line. A section along the line V-V in FigA, enlarged.

The underwater trailing antenna shown in side view schematicc-My in Fig. 1 consists of a trailing line 10 and a tow-rope or towing cable 11, as well as a sotermed VIM (Vibration Isolation Module) 12 located between the towing cable 11 and the trailing line 10. The VIM 12 joins the trailing line 10 to the towing cable 11, and the latter is attached to a towing vessel 13, here shown as a surface vessel but which could also be a submarine. On board of the vessel the towing 6 cable 11 is attached to a coiling drum 14 onto which the towing cable 11 and the trailing line 10 are wound when the trailing antenna is drawn in. The trailing line 10, which is formed of numerous line segments or sections 101, constitutes the acoustic part of the trailing antenna and contains - as is shown in particular hydrophones 15. All the sections 101 of the trailing line 10 are constructed in the same way except for the last in the series, section 1011, which is modified at its end.

1 Fig. 2 shows a cutout longitudinal section of a segment 101 of the trailing line 10. A number of hydrophones 15 are located inside an elastic, tubular sheath 16 made of polyethylene (PE). The hydrophones 15 are positioned along the centre line inside the sheath, and are longitudinally evenly spaced. The hydrophones 15, which may also be directly associated with preamplifiers and analog/digital convertors, are interconnected by electric conductors 17 made of copper or by lightconductor fibres for the purpose of current supply and signal transmission. The hydrophones 15 are held in position by a gel 18, which directly encloses the hydrophones 15 and completely fills the inside of the sheath 16. In Fig. 2 this filling is indicated by dots. The gel used is a 2-component silicone rubber which is non-viscous and vulcanizes at room temperature, marketed by W'a=ker Co. under the trade name SilGel 612. Both components of this silicone rubber are non-viscous with good flow properties, and when combined, they vulcanize at room temperature to form a gel-like vulcanisate whose softness or hardness can be controlled by varying the mixing ratio of the two components. The gel 18 is adjusted to give a relatively soft n-dx, for example by choosing a mixing ratio of the two components equal to 1:1 or 0.8:1. Once set, the gel retains its final consistency, with a specific density of 0.97 g/cm3, and it has a marked vibration damping effect. The polyethylene sheath 16 has a similar specific density of less than 1 91cm 3, preferably being chosen at 0.96 g/cm3, so that together with the gel, a buoyancy is obtained sufficient to compensate for the weight of the hydrophones 15 and the electrical leads 17.

To manufacture section 101 of the trailing line 10, the hydrophones 15 associated with section 101 are positioned in accurate longitudinal positions with respect to one another, held in place by a very loose nylon net not shown in the figure, with a few relatively elastic longitudinal strands. The so-termed 7 elementary skeleton so produced is permeated inside the sheath 16 by the stillfluid gel 18, and measures are adopted to ensure that during the setting of the gel 18, the hydrophones 15 will remain centred within the sheath 16. Such measures could for example involve rotating the sheath 16 during the setting of the gel 18, so that the skeleton will remain at the middle, or soft and thin bristles could be attached to the skeleton, which hold the hydrophones 15 centrally by resting against the inner walls of the sheath.

As shown by the longitudinal sketch of Fig. 3, the trailing line is assembled from its individual sections 101 by pushing the adjacent ends of the sheaths 16 of neighbouring sections 101 into one another and welding them together. Before this is done, the electric leads 17 of the hydrophones 15 in the two sections 101 must be connected at the junctions, for example by splicing. The joints formed between the electric leads 17 are designated with the number 19 in Fig. 3.

Referring now to Figs 4 and 5, to keep the trailing line 10 largely straight and to suppress undulating movements while it is being towed, the sheath 16 of the final section 101' in the series has an end segment with very high surface roughness, which creates eddles in the water flowing over the said end segment. To achieve this, longitudinal folds 20 uniformly distributed around the circumference of the said end segment are formed by creasing together two circumferentially adjacent sheath sections in such a way that the inner surfaces of those sheath sections 161, 162 and 162, 163 and 163, 164 and 164,161 come together in the radial direction over a certain distance. This causes the end segment of the last section 101' of the trailing line 10 to taper towards its free end, where the said folds 20 are most pronounced, while gradually merging into the sheath section with a smooth surface in the opposite direction.

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Claims

CLAIMS Underwater trailing antenna with a trailing line (10) made up from several sections (101), each of which has an elastic, tubular sheath (16) and contains a number of hydrophones (15) held centrally and at fixed distances apart from one another, characterized in that the sheath (16) is made from polyethylene with a specific density lower than 1 g/cm3 and preferably equal to 0. 96 g/cm3, and is completely filled with a soft gel (18) which encloses the said hydrophones (15) directly, the specific density of the said gel being lower than 1 g/cm3 and preferably equal to 0.97 g/cm3.

1

2.

Trailing antenna according to Claim 1, characterized in that the sheath (16) has an elasticity modulus of approximately 100 N/mm2 and the gel (18) 2 has an elasticity modulus of

3 to 5 N/mm 3. Trailing antenna according to Claim 1 or Claim 2, characte =Jed in that the gel (18) is water-repellent.

4. Trailing antenna according to any of Claims 1 to 3, characterized in that the gel (18) in its original form is non-viscous and sets to the desired final consistency after being filled into the sheath (16).

5. Trailing antenna according to any of Claims 1 to 4, characterized in that conductors (17) for signal and current transmission pass along the inside of the sheath (16) filled with the gel (18).

6. Trailing antenna according to any of Claims 1 to 5, characterized in that the sheath ends of successive sections (101) of the trailing line (10) are pushed one into another and welded together.

1 9 Trailing antenna according to Claim 5, characterized in that the conductors (17) in successive sections (101) of the trailing line (10) are connected together by splicing.

8. Trailing antenna according to any of Claims 1 to 7, characterized in that during the manufacture of the trailing line sections (101), the hydrophones (15) are held in place by a loose Nylon net with a few elastic longitudinal strands, with which they form a skeleton, the said skeleton being permeated inside the sheath (16) by the gel (18) when the latter is introduced in its still non-viscous state.

9. Trailing antenna according to any of Claims 1 to 8, characterized in that the sheath (16) of the last section (101') in the series forming the trailing line (10) has an end segment with an extremely large surface roughness.

10. Trailing antenna according to Claim 9, characterized in that the surface roughness is formed by longitudinal folds (20), preferably distributed uniformly around the circumference of the sheath(A), which project radiaUy outwards at the end of the sheath (16).

11. Trailing antenna according to Claim 10, characterized in that the longitudinal folds (20) are formed by creasing together pairs of circumferentially adjacent sheath sections (161 to 164), such that their internal surfaces come together in the radial direction over some distance.

12.

Underwater trailing antenna substantially as hereinbefore described with reference to the accompanying drawings.

CLAIMS Underwater trailing antenna with a trailing line (10) made up from several sections (101), each of which has an elastic, tubular sheath (16) and contains a number of hydrophones (15) held centrally and at fixed distances apart from one another, characterized in that the sheath (16', is made from polyethylene with a specific density lower than 1 g/cm3 and is completely filled with a sof t gel (18) which encloses the said hydrophones (15) directly, the specific density 0.1 3 the said gel being lower than 1 g/cm 2. Trailing antenna according to claim 1, characterized in that the specific density of the polyethylene for the sheath (16) is equal to or substantially equal to 0.96g/cm3.

Trailing antenna according to claim 1 or 2, characterized in that the specific density of the soft gel (18) is equal to or substantially equal to 3 0.97g/cm 4.

Trailing antenna according to claim 1, 2 or 3, characterized in that the sheath (16) has an elasticity modulus of approximately 100 N/mm2 and the 2 gel (18) has an elasticity modulus of 3 to 5 N/mm Trailing antenna according to claim 1, 2, 3 or 4, characterized in that the gel (18) is waterrepellent.

6 - Trailing antenna according to any of Claims 1, to 5, characteraJed in that the gel (18) in its original form is non-viscous and sets to the desired final consistency after being filled into the sheath (16).

11 Trailing antenna according to any of Claims 1 to 6, characterized in that conductors (17) for signal and current transmission pass along the inside of the sheath (16) filled with the gel (18).

Trailing antenna according to any of Claims 1 to 7, characterized in that the sheath ends of successive sections (101) of the trailing line (10) are pushed one into another and welded together.

9. Trailing antenna according to claim 7 or claim 8 as appendant on claim 7, characterized in that the conductors (17) in successive sections (101) of the trailing line (10) are connected together by splicing.

10. Trailing antenna according to any of Claims 1 to 9, characterized in that during the manufacture of the trailing line sections (101), the hydrophones (15) are held in place by a loose Nylon net with a few elastic longitudL-ial strands, with which they form a skeleton, the said sk,-.",eton be,-ng permeated inside the sheath (16) by the gel (18) when the latter is introduced in its still non-viscous state.

TraLILrig antenna according to any of Claims 1 to 10 r-haracter--zed in that the sheath (16) of the last section (101') in the series forming the =ahng line (10) has an end seament. with an extremely large surface rc,-, z.-.--.--=s.

12. Trailing antenna according to claim 11, characterized in that the surface roughness is formed by longitudinal folds (20) which project radially outwards at the end of the sheath (16).

13. Trailing antenna according to claim 12, characterized in that the longitudinal folds are distributed uniformly or substantially uniformly around the circumference of the sheath (16).

0.

11, Trailing antenna according to claim 12 or 13, characterized in that the longitudinal folds (20) are formed by creasing together pairs of circumferentially adjacent sheath sections (161 to 164), such that their internal surfaces come together in the radial direction over some distance.

15. Underwater trailing antenna substantially as hereinbefore described with reference to the accompanying drawings.

4