AU665196B2 - Towed array jacket - Google Patents

Description

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66 5 19

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION S F Ref: 212047 FOR A STANDARD PATENT

ORIGINAL

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Name and Address of Applicant: Actual Inventor(s): Address for Service: :u.

Invention Title: ASSOCIATED PROVISION Application No( PK8061 z* 4 Australia Sonar Systems Pty Ltd Innovation House First Avenue Technology Park South Australia 5095

AUSTRALIA

Allan Lloyd Carpenter Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia Towed Array Jacket AL APPLICATION DETAILS s) [33] Country E321 Application Date AU 28 August 1991

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;4 The following statement is a full description of this invention, including the best method of performing it known to me/us:- 5815/3 2- The present invention relates to towed arrays and, In particular, to the jackets for such towed arrays.

It has long been known that the properties of the material comprising the jacket of liquid-filled towed acoustic arrays act as a "mechanical filter". This filter removes unwanted noise, such as that resulting from boundary layer turbulence as the array is towed through the water, whilst allowing the transmission of wanted acoustic signals with little attenuation. This "filtering" function is most influenced by two material properties of the jacket, namely Its elastic strength and damping. In the terminology of materials science, these properties are Tensile Modulus (Youngs Modulus) and the Loss Tangent (that is the ratio of the real and complex parts of the Tensile Modulus determined under dynamic conditions).

Theoretical studies of the towed array's filtering function indicate that an array jacket (termed a hosewall) will provide the most effective filtering when both the Tensile Modulus and the Loss Tangent have a high value. However, unfortunately the simultaneous presence of high values for both properties in a single materialsis virtually unknown. If fact, for most known engineering materials, a high value of one property tends to preclude a high value o the other. Thus, materials with a high Tensile Modulus, such as glass fibres, aramid (KEVLAR) fibres, and carbon fibres exhibit very low (substantially zero) damping, whilst the materials used In damping applications such as pitch and plastic putties (mastics) exhibit a very low (substantially zero) Tensile Modulus.

Therefore, in general, the choice of the material or materials from which the jacket has been made, has been a compromise to obtain hitherto acceptable, but not high, values of both the Tensile Modulus and the Loss Tangent.

A further practical consideration which has hitherto been taken into account Is that prior art liquid filled towed arrays used methods of assembly which require the jacket or hosewall to be expanded (non-collapsed) by the application of an interior pressure for insertion of the interior components within the hose wall. This requirement placed a practical limit on the Tensile Modulus and, In particular, the tangential Tensile Modulus, of the jacket as a whole.

The present invention seeks to overcome this fundamental problem in material science by the provision of a jacket having both a Tensile SModulus and Loss Tangent which are as high as possible. The present 0703E 71 0 D.

-3invention finds particular application in the production of modular towed arrays of the type disclosed in Australian Patent No. 662132 in the name of Australia Sonar Systems Pty Ltd, since the assembly of such a modular array does not require the application of interior pressure to the jacket or hosewall in order to enable the interior components to be inserted within each module of the array.

According to one aspect of the present invention there is disclosed a jacket for a liquid filled towed acoustic array, said jacket being hollow and cylindrical and comprising an inner layer of plastics material, one or more fibres of high Tensile Modulus extending generally circumferentially over said inner layer, an intermediate layer of plastics material of high damping characteristics bonded to said inner layer and having said fibre(s) embedded therein, and an outer layer provided over said intermediate layer and having longitudinal strength members embedded therein, wherein said fibre(s) is/are glass fibre, aramid fibre, carbon fibre or high tensile steel fibre, or a mixture of two or more thereof.

15 The invention further discloses a method of manufacturing a jacket for a liquid filled towed acoustic array, the method comprising the steps of: providing an inner layer of plastics material; winding one or more fibres of high Tensile Modulus generally circumferentially over said inner layer, said fibre(s) being glass fibre, aramid fibre, 20 carbon fibre or high tensile steel fibre, or a mixture of two or more thereof; extruding an intermediate layer of plastics material of high damping characteristics over said inner layer so that said fibre(s) is embedded in said I intermediate layer; and extruding an outer layer over said intermediate layer, said outer layer having longitudinal strength member(s embedded therein.

Two embodiments of the present invention will now be described with reference to the drawings in which: i Fig. 1 is a fragmentary partially cross-sectioned perspective view of a jacket in accordance with a first embodiment of the present invention, and Fig. 2 is a view similar to Fig. 1 but of a jacket of the second embodiment of the present invention.

Theoretical studies of the filtering function of the hosewall of the array have indicated that ideally a high circumferential Tensile Modulus is required to limit "bulge wave" propagation but that a low axial or longitudinal Tensile Modulus is preferable to limit the transmision of axial vibrations. In addition, a low sheer modulus is ideally required to limit th transmission, by viscous sheer, of vibrational energy through the hosewall or jacket. Skih vibrational energy is input to the jacket via its outer boundary ©from the boundary layer trbulence caused by the array being towed thtough the water. i A first embodimeniof a composite material that can provide the I lN!\bkI00305!BPD ,b.r;t r i4 required combination of high Tensile Modulus and high Loss Tangent will now be described with reference to Fig. 1. The composite material is formed from short lengths (typically 2-4mm) of reinforcing fibre 1 which are mixed with a thermoplastic material 2 to form the jacket 3. The reinforcing fibre 1 is preferably either glass fibre, aramid fibre, or carbon fibre or a mixture thereof and provides the desired high Tensile Modulus. The thermoplastic material 2 provides a relatively high level of viscoelastic damping (that Is a high, or at least medium, Loss Tangent). The fibres 1 and thermoplastic material 2 are mixed, or compounded, by the material supplier and the material for the jacket 3 provided in granular form to the extrusion contractor. The manufacture of the jacket 3 is undertaken by normal extrusion processing.

As indicated in Fg. 1, the "chopped" fibres 1 adopt a substantially random orientation in the resulting extrudate thereby resulting in the jacket 3 having relatively homogeneous mechanical properties. As a consequence, substantially uniform values of the Tensile Modulus and Sheer Modulus in each of the three orthogonal axes f aie obtained. The fibres 1 provide the increased value of the Tensile Modulus desired of the jacket 3, and the thermoplastic material 2 provides the damping. Suitable thermoplastic materials Inclhde (EVA).

The overall, or averaged, values of the Tensile Modulus, sheer modulus and Loss Tangent of the jacket 3 will depend upon the same r r Iproperties of the fibres 1 and material 2, and also upon the length of the fibres 1, the ratio of the fibres 1 to thermoplastic material 2, and the adhesion between the fibres 1 and thermoplastic material 2, The above described arrangement results in an improvement over the single materials currently available, however, two relatively minor drawbacks still remain. These are that as the jacket 3 provides the functional integrity of the array, it must be impermeable to the, 30 typically paraffln-based, fill fluid. Thus the thermoplastic material 2 must represent a compromise between damping, tensile strength, and the A necessary impermeability.

I tr t Secondly, the random orientation of the fibres 1 provides some load transfer in the axial direction, which Is clearly not preferred, and can also increase the sheer modulus in all axes, contrary to the desired i minimlsing of this property,.

These two drawbacks with the first embodiment lead to the second embodiment of the present invention which will now be described with CSH/0703E i reference to Fig. 2. As seen therein, the overall jacket 13 is formed from an inner jacket 14 of conventional hosewall material such as polyvinyl chloride (PVC) or polyurethane thermoplastic, which material has a low permeability to the paraffin-based fill fluid and is also relatively strong. The inner jacket 14 is overwound with a spiral of continuous reinforcing fibre 11. As before the fibre 11 can be glass, aramid or carbon fibres, however, metals such as high tensile steel are also suitable. A mixture of various types of fibre could also be used.

The diameter and pitch of the fibre 11 are able to be selected to suit.

Over this fibre 11 is extruded an intermediate layer 15 of very high damping material, such as a hot-melt EVA. This intermediate layer bonds strongly to both the fibres 11 and the inner jacket 14.

Preferably, in order to complete the jacket 13, the intermediate layer 15 is covered by an extruded tough outer jacket 16, preferably of a polyurethane thermoplastic. If desired, within the outer jacket 16 can be embedded longitudinally extending strength members 17 preferably of aramid fibre. The strength members 17 are preferably co-extruded with the outer jacket 16.

It will be apparent that the fibre 11 provides continuous, unbroken reinforcing along the tangential axis (or circumferential direction) but leaves the jacket strength in the axial direction substantially unchanged. The intermediate layer 15 of high damping material provides low sheer strength across the jacket 13 and the required high damping in all three orthoganal axes. The preferred co-extrusion of the strength member 17 with the outer jacket 16 provides the required load transfer from the strength members 17 through the jacket 13 to any winch or array handling system (not illustrated), but these strength members 17 (which can carry axial vibrations) are isolated from the array interior by the intermediate damping layer 15. i It will be apparent to those skilled in the art that the above i described arrangements provide, to a large extent, the nonhomogeneous mix j of material properties sought for increasing the acoustic performance of S the towed array. It will be apparent that the first embodiment is relatively inexpensive to implement, but more limited in its effectiveness whilst the second embodiment is of increased effectiveness, but is more expensive to implement. As a consequence, which one of the two above described embodiments of the present invention will be implemented in a particular array, will depend upon a compromise of CSH/0703E Ti~yg

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-6economic and performance factors. However, irrespective of which embodiment is selected, the result Is an improved performance over the prior art.

The foregoing describes only two embodiments of the present invention and modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the present invention.

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Claims (7)

1. A jacket for a liquid filled towed acoustic array, said jacket being hollow and cylindrical and comprising an inner layer of plastics material, one or more fibres of high Tensile Modulus extending generally circumferentially over said inner layer, an intermediate layer of plastics material of high damping characteristics bonded to said inner layer and having said fibre(s) embedded therein, and an outer layer provided over said intermediate layer and having longitudinal strength members embedded therein, wherein said fibre(s) is/are glass fibre, aramid fibre, carbon fibre or high tensile steel fibre, or a mixture of two or more thereof.

2. A jacket according to claim 1, wherein said inner layer is of polyvinyl chloride or polyurethane thermoplastic material.

3. A jacket according to claim 1, wherein said strength members are of aramid fibre.

4. A method of manufacturing a jacket for a liquid filled towed acoustic array, the method comprising the steps of: providing an inner layer of plastics material; winding one or more fibres of high Tensile Modulus generally circumferentially over said inner layer, said fibre(s) being glass fibre, aramid fibre, carbon fibre or' high tensile steel fibre, or a mixture of two or more thereof; 20 extruding an intermediate layer of plastics material of high damping characteristics over said inner layer so that said fibre(s) is embedded in said intermediate layer; and extruding an outer layer over said intermediate layer, said outer layer having longitudinal strength members embedded therein. 25

5. A method according to claim 4, wherein said inner layer is of polyvinyl chloride or polyurethane thermoplastic material.

6. A method according to claim 4, wherein said strength members are of i aramid fibre. i

7. A jacket for a liquid filled towed acoustic array substantially as herein described and as shown in Fig. 2 of the accompanying drawings. DATED this Twenty Third Day of October 1995 Australia Sonar Systems Pty Ltd Patent Attorneys for the Applicant I SPRUSON FERGUSON I C C I I ccl c C 4p I- [N\IlbklOO305oBFD r _I ABSTRACT A jacket for a liquid filled towed array is formed from a composite material formed from short lengths (typically 2-4mm) of reinforcing fibre which are mixed with a thermoplastic material to form the jacket The reinforcing fibre is preferably either glass fibre, aramid fibre, or carbon fibre or a mixture thereof and provides the desired high Tensile Modulus. The thermoplastic material provides a relatively high level of viscoelastic damping (that is a high, or at least medium, Loss Tangent). The fibres and thermoplastic material are mixed, or compounded, by the material supplier and the material for the jacket provided in granular form to the extrusion contractor. The manufacture of the jacket is undertaken by normal extrusion processing. Figure 1 t IC i I i f CC CSH/0703E I I