GB2247325A - Optical fibre submarine cable armouring - Google Patents

Abstract

A submarine optical cable, where wire armouring is replaced by a corrugated metal tape 7, comprises a central multi-wire tensile strength member 1A, 4 containing optical films 1B, means 5 for preventing transmission loss due to hydrogen, an insulating layer 6, corrugated tube 7 tightly engaging the insulating layer and a sheath 8. Also shown are thermoplastic elastomeric, metallic pressure tube 2 and water-blocking compound 3. The tube 7 may be of steel or aluminium alloy. Means 5 may be a copper tube. <IMAGE>

Description

SUBMARINE CABLE ARMOURING This invention relates to optical fibre submarine cables, particularly such cables which are armoured.

Our British Patent 1550588 discloses a submarine cable in which the optical fibres are protected in a metallic pressure tube, in turn encased in a flexible wire strength member. This cable is intended more for deep water applications and, where it is required to power a repeater, the tube and strength member combine to provide a highly conductive path for electricity. The combined pressure tube and strength member provide a highly pressure resistant structure around the fibres to protect them, yet nevertheless remaining flexible so that creeling and storage on a drum, and lay over, e.g. a cable ship bow sheave, can be easily achieved.

Where the cable is laid in shallow water conventional wire armouring is applied over the insulation layer to protect the cable from ships' anchors and trawling, particularly beam trawl equipment. Because of the added tensile and torsional strength provided by the armouring and the fact that laying and retrieving the cable in shallow water induces less tensile strain to the cable, the central wire strength member can instead be of lower tensile strength e.g. mild steel wires in place ot the high tensile wires used in unarmoured deep water e.g.

over 500 metres depth.

Where the cable is to be used in unrepeatered links the conductivity of the tube and strength member and the degree of insulation are less important, making it possible to provide a smaller cable, which is also cheaper, but the armouring for shallow water remains a time consuming expensive operation.

It is an objective of the present invention to provide a submarine optical cable of the kind described having a cost-effective armouring.

According to the present invention there is provided a submarine optical cable comprising a central multi-wire tensile strength member containing a plurality of optical fibres, means for preventing the fibres suffering transmission loss through the effect of hydrogen, an insulating layer around the strength member, and a transversely-corrugated metal tube tightly engaging and adhering to the insulation layer, and a sheath surrounding and tightly engaging the steel tube, said tube acting as an armouring layer in place of the conventional wire armouring, said tensile strength member providing the main tensile strength of the cable.

In order that the invention can be clearly understood reference will now be made to the accompanying drawings in which: Fig 1 shows in cross section an optical fibre submarine cable according to an embodiment of the present invention Fig 2 shows in perspective the cable of Fig 1 with the various parts exposed, and Fig 3 shows a method of manufacturing the cable of Figs 1 and 2.

Referring to Figs 1 and 2 the cable comprises an optical fibre package 1 housed in a metallic pressure tube 2 which in this embodiment is made of mild steel.

The package 1 comprises a copper clad steel wire 1A, in this embodiment having nominal outer diameter of .7mm, embedded with a plurality of acrylic coated single mode fibres 1B, each with a nominal outer diameter of 0.25mm.

These fibres 1B and the copper clad steel wire 1A are laid straight and embedded in an extruded thermoplastic elastomer 1C.

Between the outer surface of the package 1 and the inner surface of the tube 2 is a water blocking compound 3.

The outer diameter of the mild steel tube 2 is in this embodiment 4.5mm.

Around the pressure tube 2 is a layer of high tensile steel wires each having a nominal diameter of 1.53mm in this embodiment. They are laid with a left-hand lay, the angle of lay being less than 100 and in this embodiment about 40, Around the strength members is formed a copper tube 5 which is partly swaged into the interstices of the strength member wires and this aspect is shown more clearly in Fig 2, the copper tube being shown schematlcally i Fic 1. The copper tube is hermetically sealed as by welding longitudinal edges of an applied copper tape, and this prevents hydrogen causing transmission loss in the fibres with the passage of time.

Around the copper tube, which has approximately 8mm outside diameter, is extruded a plastics layer 6 of high density polyethylene insulation having a nominal outer diameter of approximately llmm.

Around the electrical insulation 6 is formed an adhesive coated corrugated mild steel tape 7 having overlapped longitudinal edges 7A and 7B. In this embodiment the adhesive is a hot melt adhesive pre-coated on both sides of the mild steel tape. In this embodiment the tape has a thickness of 155 microns and the coating a thickness of about 60 microns on either side.

As the tape is formed around the insulation 6, corrugated with the overlapped edges in place, an outer plastics sheath 8 of high density polyethylene is extruded over the corrugated tape 7 and the heat of extrusion causes the coatings 7C and 7D on either side of the tape 7 to bond to the plastics insulation 6 and to bond also to the sheath 8. Fig 1A shows in detail an axial section of the part of the cable to illustrate how the insulation 6 has surface indentations which are formed during the extrusion of the sheath 8.

The advantage of the proposed form of armouring in place of conventional steel wire armouring, is that it is substantially cheaper and provides increased torsional stiffness to the cable whilst being torsionally balanced and therefore not generating torque under load as is the case for traditionally armoured cables. This is important when the cable is being laid e.g. on the sea bed since any residual torsion set up in the cable by, for example traditional strength member wires, can cause the cable to throw loops on the sea bed which is very undesirable or during laying in storage tanks. The proposed armouring will help prevent this by providing increased torsional stiffness whilst being torsionally balanced.

The cable described has a nominal outer diameter of just under 16mm with a specific gravity of 2.2 grams per cubic centimetre. It. has an ultimate tensile strength of 50 kN, and insulation resistance better than 5 x 101l ohms per kilometre and a composite conductor resistance of less than 1.5 ohms per kilometre.

The armouring 7 can have a thickness in the range 100 m crons to 0.5mn. The hot melt adhesive layers in this embodiment are, medium density polyethylene coating. The corrugations have a peak-to-peak spacing in the range 0.8 to 1.5mm.

As can be seen in Fig lA, any air entrapment between the underside of the tape 7 and the insulation 6 is minimal if not eliminated by the combination of adhesive layer 7C and the deformation of the surface of the insulation 6 which effectively "fill" the troughs in the corrugated tape.

As an alternative or in addition to adhesive layer 7C described, a hot melted adhesive could be applied onto the tape by an applicator as it is wrapped around the insulation layer 6 to glue it to the layer 6.

Furthermore it would be possible to rely solely on penetration during extrusion of sheath 8 into the interstices of the corrugations to provide the necessary adhesion between sheath 8 and tape 7 without the need for adhesive layer 7D.

It would be possible, but more expensive to use a metal other than mild steel for the armour tape 7, for example different grade of steel, or aluminium alloy.

Instead of the copper tape 5, fibres could be individually protected against the affects of hydrogen by individual hermetic coatings. However, for telemetry and/or electroding purposes the resistance of the combination of items 5, 4 and 2 need to be less than say 5 ohms per kilometre.

Claims (8)

1. A submarine optical cable comprising a central multi-wire tensile strength member containing a plurality of optical fibres, means for preventing the fibres suffering transmission loss through the effect of hydrogen, an insulating layer around the strength member, and a transversely-corrugated metal tube tightly engaging the insulation layer, and a sheath surround and tightly engaging the steel tube, said tube acting as an armouring layer in place of the conventional wire armouring, said tensile strength member providing the main tensile strength of the cable.

2. A cable as claimed in claim 1, wherein the steel tube is formed by a longitudinal tape with overlapping longitudinal edges.

3. A cable as claimed in claim 1 or 2, wherein the steel tube is made of mild steel, stainless steel or other high strength ferrous or non-ferrous alloys.

4. A cable as claimed in any preceding claim, wherein the cable outside diameter does not exceed 25mm.

5. A cable as claimed in any preceding claim, wherein the thickness of the steel tube lies in the region 150-500um.

6. A cable as claimed in any preceding claim wherein the steel tube is bonded to the insulation and to the sheath by an adhesive.

7. A cable as claimed in claim 6, wherein the adhesive substantially fills any voids between the steel tube and the insulation.

8. A cable substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.