GB2253717A - Manufacture of optical fibre cable using welded metal strip - Google Patents

Abstract

A metal strip 4 is folded at 5 around optical fibres 1 as they are fed longitudinally, and the edges of the strip are welded at 7 to form a moisture barrier tube 6, there being sufficient clearance between the fibres and the heat-affected zone of the tube to prevent damage to the fibres, and the diameter of the formed tube being subsequently reduced by passage through reducing rollers 11-13. The metal of the tube is such as to provide adequate protection of the fibres against tensile forces and external pressure. and is preferably one in which the tensile strength is increased by the reducing process. Central member 3 and water-blocking compound insertion 8, 9 are also shown. <IMAGE>

Description

THE MANUFACTURE OF OPTICAL FIBRE CABLE This invention relates to the manufacture of optical fibre cables and especially cables for submarine application.

An important consideration in the design of an optical fibre cable for submarine use, is to ensure that the individual optical fibres are not the subject of mechanical strain during manufacture, transportation, installation, service or recovery.

Further, since it is known that the action of moisture and/or hydrogen is to damage the optical fibres, it is necessary to provide an environment wherein the fibres are shielded from these elements.

The requirements to take account of the foregoing is well known to designers of submarine cables and is usually accomplished by the incorporation of a layer, or layers, of high tensile material (typically steel wire) applied over the optical fibre package, either overlaying or underlying a tubular member which provides a moisture and hydrogen barrier for the underlying optical fibres.

It is also known from Japanese Patent Disclosure No. 51-99032 that, by the selection of suitable material, the combined requirements of protection of the optical fibre elements from tensile forces, external pressure, moisture and hydrogen can be met in design by the tubular member alone. Such a design makes a most economic use of materials. However the Disclosure fails to describe a method whereby such a cable could be efficiently produced.

The encapsulation of an optical fibre or plurality of optical fibres within a continuous high tensile tube of such dimension as to provide protection for the optical fibres, must be accomplished without damaging or stressing said fibres. Patent Application No. GB 2029047A recognises the practical difficulty of forming a continuous metal tube without causing damage to underlying optical fibres, and proposes to form the tube from a strip folded around the fibres, with the edges spaced apart, and closing the resultant gap by an overlying pressure resisting layer, the need for welding the edges and the likelihood of damage to the fibres due to overheating being thereby avoided. However the presence of a gap can in some instances result in a region of weakness.

Accordingly an object of the present invention is to provide a method of manufacturing an optical fibre cable having an alternative form of continuous protective metal tube.

According, therefore, to the invention, in a method of manufacturing an optical fibre cable, the optical fibres are enclosed as they are fed in a longitudinal direction by a continuously fed metal strip folded around the fibres and the edges welded together to form a tubular moisture barrier, the diameter of which, at the welding stage, provides sufficient clearance between the fibres and the heat affected zone of the welded tube to prevent damage to the optical fibres due to overheating, the diameter of the formed tube being subsequently reduced by passage through one or more sets of reducing rollers, and the metal forming the resultant tube being such as to provide adequate protection of the fibres against tensile forces and external pressure.

Preferably the metal forming the tube is one in which the tensile strength is increased by the reducing process. The use of such a metal has the advantage that the material of the initial metal strip has a lower tensile strength than that of the finally formed and reduced tube, thus facilitating the folding of the strip around the optical fibres.

It will, of course, be appreciated that the initial strip must, in any case, be sufficiently ductile for it to be formed readily into a tube, and formed of a metal which is easily weldable.

Furthermore the reduction in the diameter of the tube should produce hardening of the zone adjacent the weld, which may have been softened by the welding operation, as well as the main part of the tube material.

A suitable metal is EN 25 Steel, although other metals could alternatively be used.

Where the optical fibres and strip are fed in a horizontal direction the strip is preferably folded upwards around the fibres so that welding of the edges of the strip takes place above the fibres.

However welding can take place with the fibres and strip fed in directions other than horizontal, in some cases vertically.

Means may be provided for introducing a water blocking material of any convenient kind into the tube, so as to surround the optical fibres, and the formed tube is preferably enclosed in a dielectric sleeve, for example of polythene extruded around the tube subsequent to the reduction process, in order to prevent corrosion of the tube when immersed in water.

In some cases means may be provided for cooling the optical fibres in the region of the weld, for example by feeding a suitable cooled gas around the fibres or between the fibres and the edges of the folded strip at the weld position. Such a gas is preferably an inert gas, such as argon, which will also serve to protect the strip material in the vicinity of the weld against oxidation.

One method of manufacturing a submarine optical fibre cable in accordance with the invention will now be described by way of example with reference to Figures 1 and 2 of the accompanying schematic drawing, in which Figure 1 illustrates diagrammatically and not to scale apparatus for carrying out said method, and Figure 2 represents a cross sectional view of the formed cable.

Referring first to Figure 1, a plurality of optical fibres 1 protected by plastics sheaths 2 and wound in a helical fashion around a central member 3, for example of Kevlar, which serves to protect the fibres against tensional forces during the manufacturing process, are fed, together with a strip 4 of EN 25 steel, 63mm wide and approximately 1.5mm thick, in a horizontal direction, as indicated by the arrow, to a tube-forming stage in which the strip is folded by means of a series of rollers, as at 5, into the form of a tube 6, the edges of the strip being welded together by means of an arc welding head 7.

A water-blocking compound, such as petroleum jelly or other suitable gelatinous material, is simultaneously introduced into the tube so as to cover the fibre elements, this conveniently being achieved under pressure through a pipe 8 having its outlet 9 downstream of the welding position. The optical fibre package, which has an external diameter of about 12mm, is sufficiently spaced from the weld to protect the fibres against overheating, although a cooling gas, for example argon at a suitably low temperature, may be fed into the tube between the fibres and the weld as added protection. Argon or other suitably inert gas used for such a purpose will also serve to protect the strip against oxidation in the vicinity of the weld.

From the weld area the formed tube is fed to a series of reduction stages (two only of which are shown at 11 and 12) where rollers 13 reduce the external diameter of the tube by stages to approximately 15mm, the tube then closely surrounding the optical fibre package as at 16.

It will be appreciated that the optical fibre package, which may include a heat resisting layer applied as a binder over the layer of optical fibres, must be fed into the forming tube at a suitably greater rate than the fed strip, in order to prevent the fibres being subjected to undue tension due to the extension of the tube during the tube-reduction stage.

The formed tube 16 is finally covered by a dielectric sheath, for example of polythene, as shown at 14 in Figure 2.

It has been found that the working of the steel of the tube during the reduction process has the effect of increasing its tensile strength.

The tube 16 serves, not only to provide protection of the optical fibre elements against radial compression forces, but also provides the main protection of the fibres against tensional forces.

However it will be appreciated that the dimensions of the strip, the work-hardening characteristics of the metal, and the ultimate dimensions of the formed tube can readily be selected to suit any partciular conditions of use.

It will also be appreciated that the metal must also have a sufficiently high tensile strength and Youngs Modulus so that the tube itself, without additional strain bearing members, can support the cable during installation, repair and other handling in use, without the tube being so rigid as to prevent the cable being coiled for storage or in the hold of a cable laying vessel.

Each of the sheaths 2 may contain a single fibre 1, as shown, or a plurality of fibres.

If the cable is designed for use in relatively shallow waters the sheath is conveniently surrounded by armouring in the form of steel wires, 17, 18 in the form of two layers wound helically around the sheath with opposite lays to provide an anti-torsional arrangement. Alternative forms of armouring may be used to suit the particular conditions in which the cable is installed. For cables in deep water the armour is generally not required.

Claims (13)

1. A method of manufacturing an optical fibre cable wherein the optical fibres are enclosed as they are fed in a longitudinal direction by a continuously fed metal strip folded around the fibres and the edges welded together to form a tubular moisture barrier, the diameter of which, at the welding stage, provides sufficient clearance between the fibres and the heat affected zone of the welded tube to prevent damage to the optical fibres due to overheating, the diameter of the formed tube being subsequently reduced by passage through one or more sets of reducing rollers, and the metal forming the resultant tube being such as to provide adequate protection of the fibres against tensile forces and external pressure.

2. A method according to Claim 1 in which the metal used for forming the tube is one in which the tensile strength is increased during the reducing process.

3. A method according to Claim 2 wherein the metal used for forming the tube is EN25 steel.

4. A method according to any preceding claim in which the fibres and strip are fed in a horizontal direction, and the strip is folded upwards around the fibres so that welding of the edges of the strip takes place above the fibres.

5. A method according to any preceding claims including feeding a water blocking material into the tube so as to surround the optical fibres.

6. A method according to any preceding claims wherein a sleeve of plastics material is applied around the tube subsequent to the reduction process.

7. A method according to Claim 6 wherein the plastics material comprises a sleeve of polythene which is extruded around the tube.

8. A method according to any precedtng claim including the step of cooling the optical fibres in the region of the weld.

9. A method according to Claim 8 wherein cooling of the optical fibres is effected by feeding a cooled gas around the fibres or between the fibres and the edges of the folded strip at the weld position.

10. A method according to Claim 9 wherein the gas consists of argon.

ll. A method according to any preceding claim wherein a heat resisting layer is applied over the fibres upstream of the welding position.

12. A method of manufacturing an optical fibre cable carried out substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawing.

13. An optical fibre cable formed by a method according to any preceding claim.