A towed acoustic array

Abstract: A tow cable comprises a high density, high damping core of a material such as lead (1), surrounded by a layer of helically wrapped or braided strength fibres of a material such as steel (2), or alternatively, of aramid fibres. Over this is provided a layer of a resilient, deformable thermoplastic material such as polyurethane (3), 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 (3) is provided a layer of helically wrapped (or braided) power and data bearers (4), 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 (5), such as polyurethane. Around this is wrapped or braided a layer of protective fibres (6), 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 (7), 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.

Description: BACKGROUND OF THE INVENTION This invention relates to tow cables for towed acoustic arrays. Conventional towed arrays, or streamers, have consisted of towing required depth, 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. The 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 as disclosed in US Patent 4,402,069 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. Some tow cables comprise, as disclosed in EP Applications 0 416 992 and 0 478 422 some inflections which extend longitudinally along the exterior surface of the cable but straightforwardly and with no twist, thus increasing the rigidity of the cable. 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 cale for acoustic arrays, according to claim 1. In a further aspect of the present invention, there is provided a tow cable according to claim 9. 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 (1), surrounded by a layer of helically wrapped or braided strength fibres of a material such as steel (2), (or alternatively, of aramid fibres). Over this is provided a layer of a resilient, deformable thermoplastic material such as polyurethane (3), 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 (3) is provided a layer of helically wrapped (or braided) power and data bearers (4), 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 (5), such as polyurethane. Around this is wrapped or braided a layer of protective fibres (6), 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 (7), such as polyurethane. Figure 2 shows the extruded cable, illustrating the trochoidal cross section (8), and the twist of this section along the length (9) 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 3 (a) to (f) 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 (10). In Figure 3(b), the cable is again of substantially circular shape, but having a groove (12) in place of the ridge (11) of Figure 3(a). Figure 3(c) shows a cable whose exterior surface is formed by two-curved surfaces (13) and (14) intersecting at two ridges (15) and (16). In Figure 3(d), the exterior shape of the cable is that of a square having four sides (17) intersecting to form four ridges (18). Figure 3(e) shows a cable having an exterior surface composed of a number of concavely curved surfaces (19), which intersect to provide ridges (20). 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 (24). It will be appreciated that various other shapes of exterior surface could be provided without departing from the scope of the invention. It will also be appreciated that in each case, the shape is preferably twisted along the length of the cable. The foregoing describes only some aspects of the present invention, and modifications obvious to those skilled in the art can be made without departing from the scope of the present invention.

Claims: 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 density damping surrounded the high tensile covering, characterized in that the exterior surface of the tow cable is provided with at least one longitudinally extending inflection which is arranged along a helical or twisted path along the cable. A tow cable according to claim 1, wherein the exterior surface of the tow cable comprises at least two longitudinally extending surfaces which intersect each other longitudinally. A tow cable according to claim 2, wherein the longitudinally extending surfaces are curved. A tow cable according to claim 2, wherein the longitudinally extending surfaces are flat. A tow cable according to any preceding claim, wherein at least one inflection is a ridge. A tow cable according to any preceding claim, wherein at least one inflection is a groove. A tow cable according to claim 1, wherein the exterior surface has three longitudinally extending curved surfaces which intersect to form three longitudinal ridges. A tow cable according to any preceding claim, further comprising strength members and data transmission members surrounded by one or more of the layers of resiliently deformable material of high internal damping. A tow cable suitable for incorporating into a towed array streamer, the cable 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 braised cables for power and data transmission, e. a layer of helically wrapped or braised fibres to provide a protective armouring for the power/data cable layer, characterized in that it further comprises an outer extrusion of thermoplastic material having an approximately triangular or trochoidal outer contour, which is arranged along a helical or twisted path along the cable. A tow cable according to claim 9, wherein the triangular contour is subjected to a regular twist to present a changing form along its length to the cross flow. A tow cable according to claim either 9 or 10, wherein the power/data cable layer is enclosed in an extrusion of thermoplastic material. A tow cable according to any one of claims 9, 10 or 11, wherein the central core is made of lead.

Inventors: CARPENTER ALLAN LLOYD, FARMER RAYMOND FREDERICK, CARPENTER, ALLAN LLOYD, FARMER, RAYMOND FREDERICK
Applicants: AUSTRALIA SONAR SYST, AUSTRALIA SONAR SYSTEMS PTY LIMITED
Application year: 1993
Main classification: G01V1
IPC codes: G01V1