AU661892B2 - A towed acoustic array - Google Patents

Description

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AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIRCATION FOR A STANDARD PATENT S F Ref: 234375

ORIGINAL

Name and Address of Applicant: Actual Inventor(s): Address for Service: Invention Title: Australia Sonar Systems Pty Ltd Innovation House West First Avenue Technology Park South Australia 5095

AUSTRALIA

Raymond Frederick Farmer and Allan Lloyd Carpenter Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia A Towed Acoustic Array cc''

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Caa at ASSOCIATED PROVISIONAL APPLICATION DETAILS [31] Application No(s) [33] Country PL.1309 AU [32] Application Date 12 March 1992 The following statement i a full description of this invention, including the best method of performing it known to me/us:- CC C* C C. C o..

o.o e °oe' 5815/3 L p-~ 1 -2- BACKGROUND OF THE INVENTION This invention relates to tow cables for towed acoustic arrays.

Conventional towed arrays, or streamers, have consisted of towing cable, which must be negatively buoyant to achieve submersion to the required depth, a forward Vibration Isolation Modules (VIM) to minimise the transmission of mechanical vibration from the tow cable to the acoustic array, an array of acoustic (and non-acoustic) sensors, and an aft VIM to minimise transmission of vibration from the 'flapping tail' forward to the acoustic array.

This invention relates to the construction and form of the tow cable which links the towing vessel to the head of the acoustic streamer and, through its density, determines the depth at which the streamer is towed.

Conventional tow cables have typically comprised a multicore inner portion, surrounded by several layers of helically wound steel armouring, which provides the required density, tensile strength for towing and extreme robustness to resist the rigours of handling during deployment and retrieval operations. Regretfully, this construction has also brought disadvantages, including the limitations of limited flexibility (large bend radius), a relatively large diameter (to achieve the target Specific Gravity of 3 or more) and, with the normal circular section, a low resistance to excitation into strumming by periodic vortex shedding a problem usually treated by the addition of difficult-to-handle 'flags' along the cable length. The dominance of the cable mechanical characteristic by the steel armouring results in very little internal damping to attenuate any periodic cable strumming that is excited. rc,' Vt 3 3 tt4C 3 BRIEF SUMMARY OF THE INVENTION It is an object of the present invention to try to provide measures to address at least three of the above problem areas: a. cable size, b. the lack of damping inherent in the cable structure, and c. cable form, and its sensitivity to strumming.

Accordingly, in one aspect; the invention provides a tow cable for acoustic arrays, the tow cable comprising: a. an inner core of relatively high density, high damping material, b. a covering of high tensile strength surrounding the core, and ;.ihk-630E c -3c. at least one layer of resiliently deformable material of high internal damping surrounding the high tensile covering, wherein the exterior surface of the tow cable is provided with at least one longitudinally extending inflection.

In a preferred embodiment of the invention, the exterior surface of the tow cable comprises at least two longitudinally extending surfaces which intersect each other longitudinally. The longitudinally extending surfaces can be either curved or flat. The inflection can be either a ridge or a groove. In a preferred embodiment, there are three longitud-ially extending curved surfaces which intersect to form three longitudinal ridges.

Preferably, the longitudinally extending inflection(s) is/are arranged along a helical or twisted path(s) along the cable.

In a preferred embodiment, strength members and data transmission members are provided surrounded by one or more of the layers of resiliently deformable material of high internal damping.

In a further aspect of the present invention, there is provided a tow cable suitable for incorporating into a towed array streamer, comprising; a. a central core of relatively high density, high damping material, b. a helically wrapped or braided layer of fibres of high tensile strength, c. a layer of deformable resilient material, d. a layer of helically wrapped or braided cables for power and data transmission, e. a layer of helically wrapped or braided fibres to provide a protective armouring for the power/data cable layer, and f. an outer extrusion of thermoplastic material in an 30 approximately triangular or trochoidal form.

Preferably, the 'triangular' form is subjected to a regular twist to present a changing form along its length to the cross flow, thereby further minimising the chance of any local areas of instability being established. However, it will be appreciated that forms other than triangular or trochoidal could be utilised.

Preferably, the power/data cable layer is enclosed in an Sextrusion of thermoplastic material. In one preferred embodiment the °2 central core is made of lead.

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.e0 4-Jk D6 30 E -1 I 4 -4- In the above embodiment the three identified performance aspects have been addressed size is reduced by the use of high density materials, damping is increased by the use of materials with high internal damping qualities, and the form chosen is insensitive to vortex shedding, thereby minimising the initiation of the forcing function for cable strumming.

BRIEF DESCRIPTION OF THE DRAWINGS Various embodiments of the invention will now be described more fully, by way of example, with reference to the drawings, of which; Figure 1 schematically illustrates one possible cross-section of implementation; Figure 2 schematically illustrates the proposed longitudinal twist of the trochoidal cross-section; and Figure 3 schematically illustrates a number of other possible profiles of the exterior surface of the cable.

DETAILED DESCRIPTION OF THE EMBODIMENTS As shown in Figure 1, one aspect of a preferred embodiment involves the provision of a high density, high damping core of a material such as lead surrounded by a layer of helically wrapped or braided strength fibres of a material such as steel (or alternatively, of aramid fibres). Over this is provided a layer of a resilient, deformable thermoplastic material such as polyurethane to provide for movement of the subsequent data-bearer layer under strain, so that no strain is applied to the data-bearers themselves.

Around this layer is provided a layer of helically wrapped (or braided) power and data bearers comprising bearers including coaxial cables, twisted pairs, and fibre optic bearers. To ensure t water-blocking, this layer may be enclosed in an extrusion of thermoplastic material such as polyurethane. Around this is t 30 wrapped or braided a layer of protective fibres of material such as steel or aramid fibre, and over this, in a roughly trochoidal cross-section, is extruded an outer layer of thermoplastic material such as polyurethane.

1 Figure 2 shows the extruded cable, illustrating the trochoidal 5 cross section and the twist of this section along the length (9) of the cable. This twisting of the trochoidal section causes the i ridges to effectively rotate as the cable moves through water and therefore minimises vortex shedding and cable strumming.

,i *Ijk1O630E Figure 3 to shows a number of other cross-sectional shapes which could be used for the exterior of the cable. Figure 3(a) shows a cable having a substantially circular cross-sectional shape but having a single inflection or ridge In Figure the cable is again of substantially circular shape, but having a groove (11) in place of the ridge (10) of Figure Figure 3(c) shows a cable whose exterior surface is formed by two curved surfaces (12) and (13) intersecting at two ridges (14) and In Figure the exterior shape of the cable is that of a square having four sides (17) intersecting to form four ridges Figure 3(e) shows a cable having an exterior surface composed of a number of concavely curved surfaces which intersect to provide ridges Finally, in Figure 3(f) there is shown a cable whose exterior surface is formed by a concavely curving surface (21) and two convexly curving surfaces (22) and (23) which intersect to provide three ridges It will be i I appreciated that various other shapes of exterior surface could be provided without :1 departing from the scope of the invention. It will also be appreciated that in each case, 15 the shape is preferably twisted along the length of the cable.

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

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Claims (12)

1. A tow cable for acoustic arrays, the tow cable comprising: a. an inner core of relatively high density, high damping material, b. a covering of high tensile strength surrounding the core, and c. at least one layer of resiliently deformable material of high internal damping surrounding the high tensile covering, wherein the exterior surface of the tow cable is provided with at least one longitudinally extending inflection.

2. A tow cable according to claim I, wherein the exterior surface of the tow cable comprises at least two longitudinally extending surfaces which intersect each other longitudinally.

3. extending

4. extending least one

6. least one A tow cable according to claim 2, wherein the longitudinally surfaces are curved. A tow cable according to claim 2, wherein the longitudinally surfaces are flat. A tow cable according to any preceding claim, wherein at inflection is a ridge. A tow cable according to any preceding claim, wherein at inflection is a groove. t csi a CL c0 g ~re E r, .r c

7. A tow cable according to claim 1, wherein the exterior surface has three longitudinally extending curved surfaces which intersect to form three longitudinal ridges.

8. A tow cable according to any preceding claim, wherein the longitudinally extending inflection(s) is/are arranged along a helical or twisted path(s) along the cable.

9. A tow cable according to any preceding claim, further comprising strength members and data transmission members surrounded by Sone or more of the layers of resiliently deformable material of high 30 internal damping. *5t1 0. A tow cable suitable for incorporating into a towed array streamer, the cable comprising; a. a central core of relatively high density, high damping S material, b. a helically wvapped or braided layer of fibres of high tensile strength, c. a layer of deformable resilient material, a -4 *1'MIiR10630E QQ -7- d. a layer of helically wrapped or braided cables for power and data transmission, e. a layer of helically wrapped or braided fibres to provide a I'rotective armouring for the power/data cable layer, and f. an outer extrusion of thermoplastic material having an approximately triangular or trochoidal outer contour.

11. A tow cable according to claim 10, wherein the triangular contour is subjected to a regular twist to present a changing form along its length to the cross flow.

12. A tow cable according to claim either 10 or claim 11, wherein the power/data cable layer is enclosed in an extrusion of thermoplastic material.

13. A tow cable according to any one of claims 10, 11 or 12, wherein the central core is made of lead.

14. A tow cable substantially as hereinbefore described with reference to the drawings. DATED this FOURTH day of MARCH 1993 Australia Sonar Systems Pty Ltd Patent Attorneys for the Applicant SPRUSON FERGUSON i i rhk/0630E A Towed Acoustic Array ABSTRACT A tow cable comprises a high density, high damping core of a material such as lead surrounded by a layer of helically wrapped or braided strength fibres of a material such as steel or alternatively, of aramid fibres. Over this is provided a layer of a resilient, deformable thermoplastic material such as polyurethane to provide for movement of the subsequent data-bearer layer under strain, so that no strain is applied to the data-bearers themselves. Around this layer is provided a layer of helically wrapped (or braided) power and data bearers comprising bearers including coaxial cables, twisted pairs, and fibre optic bearers. To ensure water-blocking, this layer may be enclosed in an extrusion of thermoplastic material slich as polyurethane. Around this is wrapped or braided a layer of protective fibres of material such as steel or aramid fibre, and over this, in a roughly trochoidal cross-section, is extruded an outer layer of thermoplastic material such as polyurethane. The trochoidal cross section is twisted along the length of the cable. This twisting of the trochoidal section causes the ridges to effectively rotate as the cable moves through water and therefore minimises vortex shedding and cable strumming. Figure 1. It -Ij .7 r 34I I rhk/0630E