GB2188954A

GB2188954A - Moulded nodules on strainwire

Info

Publication number: GB2188954A
Application number: GB08608948A
Authority: GB
Inventors: Frank Richard Attenborough
Original assignee: Britoil Ltd; STC PLC
Current assignee: Britoil Ltd; STC PLC
Priority date: 1986-04-12
Filing date: 1986-04-12
Publication date: 1987-10-14

Abstract

A plastics sheathed strainwire (1), e.g. of non-metallic material, includes at regular intervals nodules (3) of a material formed, e.g. by injection moulding, on the strainwire sheathing (2). In use, the strainwire may tow hydrophones located at the nodules for accurate spacing. The nodules are moulded onto the strainwire in a mould assembled from two of the parts 10, Fig. 2. <IMAGE>

Description

SPECIFICATION Moulded nodules on strainwire This invention relates to strainwires or ropes, such as are employed in towed seismic arrays, to which are attached at regular intervals devices such as hydrophones, electronic circuit assemblies or simple mechanical spacer elements.

The use of modern synthetic materials such as aramid fibres have enabled the development of high tensile strength rope structures having low extension. The term 'strainwire' is used herein to include non-metallic strain resistant ropes such as those made from aramid fibres or equivalent materials. Conveniently such strainwires are sheathed in an extruded plastics material such as polythene or a thermoplastic elastomer which is a co-polyester.

One problem encountered on using such sheathed strainwires is preventing the attached devices from slipping, either during assembly or subsequently in use. Typically the towed array or streamer comprises a series of hydrophone sections separated by nodes.

Such an array is shown for example in UK specification No. 2 149 19 A. The hydrophone sections are generally 100 m in length and each carry hydrophones spaced at regular intervals. For correct functioning of the array it is essential that the hydrophones are positioned relative to one another with a high degree of accuracy. In a conventional technique the hydrophone fixing positions are defined by spring clips which are clamped to or driven into the strainwire and to which the hydrophones are subsequently attached. It is difficult to perform this operation with a high degree of accuracy, particularly as the strainwire must be maintained under tension during the operation.

According to the present invention there is provided a plastics sheathed strainwire including at regular intervals nodules of moulded material formed on the strainwire sheathing.

The invention also provides an apparatus for moulding plastics material nodules at regular intervals along a plastics sheathed strainwire comprising a two-part mould having mating faces each having formed therein one half of a channel conforming closely to the profile of a sheathed strainwire which will pass through the closed mould and each face including at least one enlarged recess is the respective half channel such that the two recesses together define a nodule mould cavity through which the sheathed strainwire passes.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 depicts a portion of a sheathed strainwire, and Figure 2 illustrates one half of an injection mould.

Referring to Fig. 1, a strainwire 1 of, for example, aramid fibres is sheathed with an extruded plastics sheath 2. At regular intervals along the sheathed strainwire there is formed a nodule 3 formed e.g. by injection moulding.

Typically the strainwire may comprise bundles of aramid fibres laid up to form a rope Smm in diameter with an extruded sheath of polyethylene, the overall diameter being 8mm. Nodules of nylon are formed on the sheath at intervals typically of 150mm or 223.5mm, each nodule having an external diameter of lOmm and a length of 20mm. Injection moulding of nylon on polyethylene is preferred as this technique ensures a good adhesion due to the shrinkage on cooling of the moulded nylon. The invention is not restricted to the use of nylon moulded onto polyethyelene, other materials can be used, e.g. the nodules can be moulded in polyethylene on a strainwire sheathed in a co-polyester thermoplastic elastomer.

To form the nodules on the strainwire a split injection mould can be used, one half of which is shown in Fig. 2. Each half of mould 10 has formed in its mating surface 10a a half channel 11 passing right across the mould from one side to the other, the two halves when placed together providing a bore which is a close fit around a sheathed strainwire passing through the closed mould. Each half mould has one or more enlarged recesses or cavities 12 which together define nodule mould cavities when the mould is closed. In use the sheathed strainwire is laid in the channel 11 in one half of the mould, the two halves of the mould are closed up, the fibre tensioned and the nylon or other material is injected into the cavity 12 via a tab gate feed 13 and inlet port 14. The tab gates provide centreing of the strainwire in the mould by the application of opposed injection forces.The tags (not shown) that are formed on the nodule corresponding to the positions of the tab gates are subsequently trimmed off to leave a smooth profile. When the injection process is completed the two halves of the mould are separated, the nodule ejected and the strainwire is moved longitudinally the required amount and then the mould is closed again on the strainwire and tensioned to perform the next injection moulding. It is clear that the moulds can have two or more channels side by side so that several lengths of strainwire can be processed simultaneously.

In use hydrophones or other devices are mounted on the nodules of the strainwire, the nodules providing the necessary longitudinal reference for accurate assembly. Two similar untreated strainwires are provided each with a plurality of bushes said bushes being a sliding fit on the strainwires. The busses are positioned by sliding along the respective strainwires so that, when all these strainwires are placed together to form the array, each nodule abuts a pair of bushes whereby the strainwires are spaced in an array of equilateral triangular cross-section. The assembly is then protected by a large diameter plastics hose.

The assembly forms part of a towed array, there being a plurality of similar hydrophone arrays separated by nodes coupled therebetween.

Claims

1. A plastics sheathed strainwire including at regular intervals nodules of a moulded material formed on the strainwire sheathing.

2. A strainwire as claimed in claim 1, wherein said nodules are formed by injection moulding.

3. A strainwire according to claim 1 or 2, wherein the nodules are moulded in nylon on a polyethylene sheathed strainwire comprised of aramid fibres.

4. A strainwire including nodules substantially as described with reference to Fig. 1 of the accompanying drawings.

5. A hydrophone assembly for a towed seismic array, the assembly including first, second and third parallel plastics sheathed strainwires so disposed as to form in crosssection the verteces of an equilateral triangle, and hydrophones supported at regular intervals on the first strainwire, wherein the first strainwire is provided with plastics nodules modulated thereto and on which the hydrophones are mounted, and wherein said second and third strainwires are provided each with slidable bushes so positioned as to abut corresponding nodules on the first strainwire to define the spatial relationship of the strainwires.

6. A towed seismic array incorporating a strainwire as claimed in any one of claims 1 to 4 on a hydrophone assembly as claimed in claim 5.

7. An apparatus for moulding plastics material nodules at regular intervals along a plastics sheathed strainwire comprising a two-part mould having mating faces each having formed therein one half of a channel conforming closely to the profile of a sheathed strainwire which will pass through the closed mould and each face including at least one enlarged recess in the respective half channel such that the two recesses together define a nodule mould cavity through which the sheathed strainwire passes.

8. An apparatus for moulding nodules on a strainwire substantially as described with reference to Fig. 2 of the accompanying drawings.