GB2336000A

GB2336000A - Optic fibre cable with two overlapping C-section casings.

Info

Publication number: GB2336000A
Application number: GB9802700A
Authority: GB
Inventors: Simon Mark Rowland; Andrew James Mcdowell
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1998-02-09
Filing date: 1998-02-09
Publication date: 1999-10-06
Also published as: AU1738999A; WO2000049444A1; GB9802700D0

Abstract

An optical fibre cable comprises: (i) one or more optical fibres 8; (ii) a first C-section casing 16 surrounding the optical fibres; (iii) a second C section casing 18, positioned around the first C-section casing and overlaps at least part of the first C-section casing on either side of the opening 17 in the first C-section casing and (iv) a seal 20, formed between overlapping portions of the first and second casings on either side of the opening in the first C-section casing. Preferably both C-section casings are metallic and the seal is an epoxy or acrylic resin. Alternatively, the seal is formed by lap welding 34 and the C-section 28,30 may just overlap.

Description

2336000 Optical Fibre Cable This invention relates to optical fibre

communications cables. More particularly, the invention relates to optical fibre communications cables in which the fibres are contained in outer protective, usually metallic, loose casings, especially 5 tubes.

Optical fibres are now in common use in communications cables. They have a number of advantages compared to conventional copper cables. In particular, they have a large bandwidth, which means they can carry far more information than conventional copper cables. One disadvantage, however, is that mechanically they are very fragile. It is therefore necessary to protect the optical fibres against mechanical strain within the communications cables. One method of providing mechanical protection is to encase a number of optical fibres in a hollow, metal, loose tube.

A number of structures in which optical fibres are encased in outer metallic tubes are known. One example is in a so-called OPGW, or Optical Ground Wire. An OPGW is an electrical and mechanical analogue of a conventional high voltage earth wire, but incorporates a number of optical fibres for communications within it. A high voltage earth wire typically spans the tops of the steel supporting towers in an electrical power distribution network, to provide protection against lightning strikes and also to pass fault currents. A conventional earth wire, without optical fibres, may have a number of constructions, but typically comprises a plurality of galvanised steel wires, for mechanical strength, and a number of highly conductive aluminium wires, to provide electrical conductance. One common conventional earth wire is a so-called ACSR (Aluminium Conductor Steel Reinforced) wire, which comprises a central steel core, usually aluminium clad steel wires, and an outer ring of aluminium wires. The aluminium cladding on the steel wires gives increased conductivity, and also avoids corrosion problems between the wires. In the OPGW wire, one of the metallic strands, usually one of the steel strands, is replaced by a metal tube containing optical fibres. Typically 12, 24, 48 or even more wires are incorporated within the metal tubes.

Other examples where optical fibres are incorporated in outer metallic tubes, io usually steel, are:

(a) OPM (Optical Phase Wire) which is similar to OPGW but replaces one of the phase conductors in an electrical power distribution network, rather than the earth wire; (b) Submarine cables, where the metal outer tube provides protection against hydrostatic water pressure, stress corrosion, and hydrogen diffusion, Optical fibres used in communications cables are usually many kilometres in length, and it is not generally feasible to insert these long fibres longitudinally into the bore of a seamless metal tube. Therefore, instead, the usual procedure for forming metallic outer tubes around optical fibres is to start with a flat rolled strip of metal, to form this into a tube shape, with longitudinal edges abutting to form a seam, and then to laser weld the seam to form a butt weld. The tube wall is usually about 0. 2 mm thick, and the tube diameter is usually about 2.5 mm. The optical fibres, together with a filling compound, are typically incorporated into the tube during the forming and welding process, and special precautions are usually taken to protect the fibres from localised heating effects introduced by the laser welding operation. The tube forming process from a flat strip is described in EP-A-0299123 (Laser Armor Tech).

The known longitudinal butt weld seam achieved by the laser welding process is expensive and difficult to carry out. Special steps must be taken to avoid heat damage to the fragile fibres, beneath the weld, and if not carried out properly damage to the fibres may occur. Also when the finished steel seam welded tube is incorporated into a cable adjacent to aluminium conductors, the steel outer tube is in direct contact with a dissimilar metal, and electrochemical corrosion may occur.

Another technique for encasing optical fibres in a metal outer tube, which avoids the need to form a laser welded steel tube, is described in EP-A-0744638 and EP-A-0744639 (both to A.T & T). According to these references, which describe the formation of a submarine cable incorporating optical fibre, a steel closed C-section tube is first formed around a bundle of optical fibres, and then an outer copper tape is formed directly over the C-section tube in a close fitting manner, and then welded to itself alone, its abutting longitudinal edges. Welding of the copper tube is significantly easier than the laser welding technique described in EP-A-0299123, but the double layer construction of EP-A-0744638/9 still achieves a hermetic seal around the optical fibres.

Another double layer cable construction around optical fibres is described in EP-A-0693754 (Alcatel Kabel Beteiligungs-AG). According to this, a metal, e.g. stainless steel, tube is used to receive the optical fibres, and then an inherently closed thin metal layer is applied to surround the metal inner tube. The thin layer is typically about 10 to 20pim thick, and is typically applied electrolytically.

We have discovered another design of outer casing, for example a tube, especially a metal tube, for encapsulation of optical fibres, which avoids costly and difficult butt seam laser welding.

Accordingly, a first aspect of the invention provides an optical fibre cable comprising:

(i) one or more optical fibres; (ii) a first C-section casing surrounding the optical fibres; (iii) a second C-section casing, positioned around the first C-section casing such that the second casing covers the opening in the first C- section casing, and overlaps at least part of the first C-section casing; and (iv) a seal, formed between overlapping portions of the first and second casings, on either side of the opening in the first C-section casing.

The term "the opening in the C-section casing" is used herein to denote the longitudinal region between the tips of the "C" of each C-section casing. Where the C-section casing is substantially open in configuration, "the openinj between the tips may be several millimetres or even centimetres in distance. Where a C- section casing is substantially closed, i.e. the tips of the "C" are substantially touching, "the opening" may be substantially zero mm. in distance. However, it is defined herein as "the opening" for both an open and a closed C-section casing, since there is no direct seal, in either case, between the tips of the C-section casing.

In contrast to the dual layer covering described in EP-A-074463819, according to the present invention, the seal is provided BETWEEN the two layers. In the arrangements described in EP-A-0744638/9 the inner steel layer is not sealed, and the outer layer is sealed to itself by welding. The completely different approach of the present invention provides a number of advantages. One advantage is that the C section casings of the present invention may be, but need not be, substantially closed in cross-section. In EP-A-0744638/9 both concentric C-section tubes are described as substantially closed tubes. Indeed the outer tube must be closed in cross- section according to those patent applications, since abutting edges of the outer tube are sealed to each other to provide the hermetic seal around the optical fibres. A more open configuration, which is possible, but not necessary, according to the present invention, may be advantageous for certain applications. One advantage is cost of components, which is reduced for more open designs. Another advantage is improved access, e.g. for forming a termination, since there is a reduced thickness to be removed for access to the interior. In general the substantially closed configuration for at least the inner C-shaped casing is preferred in the present invention, since that provides a smooth interface adjacent the optical fibres, which minimises the possibility of damage.

Another advantage of the present invention is that since the seal is formed between overlapping layers of the C-section casings, the seal, if mad e by welding, is a lap bond weld, rather than a butt weld as used in EP-A-0299123 (Laser Armor Tech) and EP-A-0744638/9 (AT&T). Lap bond welds are easier to achieve, without damage to the underlying fibres, than butt welds. Thus, even if the C- section casings are made from steel, which is known to be difficult to weld, a relatively easy weld can be achieved, compared to the more difficult butt weld of the prior art.

Since the seal is between overlapping portions of the two C-section casings, it may be made by means other than welding. For example, a bonding material such as a resin may be positioned between overlapping portions of the C-section casings, on either side of any opening in the first C-section casing. Preferably the resin is positioned between substantially the entire overlap region between the two C-section casings. Properties that the resin should preferably have to provide an effective seal between the two C-section casings are that it should be capable of bonding a Imm to 1Omm strip of a metal or metallic alloy to a second strip of the same or a different metal or metallic alloy; that it should have an activation or curing process which matches the speed, tandem processing and health and safety requirements of a cable manufacturing process; that it will not react with petrochemical based thixotropic gels; that it provides a seal over a temperature range of -40'C to 17TC, that it will not degrade significantly in a typical external industrial or marine environment for a period of at least 20 years; and that it will not emit hydrogen. The bond formed should preferably have high mechanical strength when the tube is placed under tension, is pressurised internally, or subjected to longitudinal, transverse, or radial compression. As examples of suitable resins that may be used, there may be mentioned two component resins that set or cure when mixed, e.g. epoxies, heat activatable materials, such as hot melts, solders, especially low temperature solders, or structural acrylics.

The C-section casings are preferably shaped so that in cross-section they provide part of a circle. However, C-section casings which in section form part of an ellipse, or other similar oval shape, are also envisaged. Preferably, the C-section casings, in section, have an axis of symmetry. However, this is not necessary. All that is required are two generally C-section casings that co-operate together, so that an outer casing overlaps at least part of the inner casing, and covers the opening in the first C-section casing.

Preferably the second (outer) C-section casing overlaps at least 10% of the outer surface of the first C-section casing. In other embodiments, the second Csection casing overlaps at least 30%, or at least 50%, or at least 70%, or at least 90%, or at least 95% of the outer surface of the first C-section casing. In an especially preferred embodiment the second (outer) casing overlaps substantially the entire outer surface of the firs C-section casing.

Preferably the minimum peripheral (e.g. circumferential) displacement (as hereinafter defined) between the openings in the first and second Csection casings is at least 20', more preferably at least 35', even more preferably at least 45', especially preferably at least 60', at least 90', at least 120', or at least 150', and most preferably about 180'.

The angle of peripheral displacement can be measured in the following manner for C-section casings forming, in section, part of a circle, regular oval, or ellipse.

First draw a line from one tip (the selected tip) of the C of the first Csection casing, to the centre of the circle or ellipse of the cross-section of the casing. Secondly, draw a line from the tip of the C of the second C-section casing which is nearest to the selected tip of the first casing to the centre of the circle or ellipse of the cross section of the second casing. Then measure the angle between the two lines drawn and record it. Repeat the process for the initially non-selected tips of the two C- section casings, and record the angle between the two new lines drawn. The smaller of the two angles recorded is the minimum peripheral displacement between the openings in the two C- section casings.

For C-section casings which in section do not form part of a circle or regular oval, or ellipse, a similar process may be carried out using the centre of gravity of a section of the tube as the "centre" to which the lines are drawn for measuring minimum peripheral displacement.

In a preferred embodiment, both first and second C-section casings are substantially closed, both form part of a circle, regular oval, or ellipse in cross-section, and the peripheral displacements between the openings in the C-section casings (i.e. where the tips of each C abut) is 180', i.e. the abutting edges of the first closed Csection casino, is diametrically opposed to the abutting edges of the second C-section CI casing.

As mentioned above, either or both the C-section casings may be substantially closed, or have a more open configuration. Where one or both casings are not closed C> the angle between lines drawn from the tips of the "C" to the centre (as hereinbefore defined) of the section at most 120', more preferably at most 90', especially at most 0.

Where the seal between the two casings is formed by a lap weld between overlapping portions of the casings, the area of overlap of the layers may be as little as 5%, more preferably 10%, 15% or 20% of the total periphery of the casings. For certain applications, especially for strength reasons, even larger areas of overlap are preferred, especially substantially 100% overlap. Where the seal between overlapping portions of the casings is formed by a resin between the layers, the outer face of the inner casing and the inner face of the outer casing preferably have substantially 100% overlap, so substantially 100% of their area is available for adhesion, by the resin, to each other.

Preferably the second C-section casing is a close fit around the first Csection casing. Preferably the space between overlapping portions of the C- section casings measured laterally (e.g. radially for C-section casings which form part of a circle in section) is at most I mm, more preferably at most 0.75min.

In certain embodiments according to the invention, the outer periphery of the outer casing may have a profiled shape, for fitting adjacent other similar shaped articles, e.g. other casings. For example, the outer periphery of the outer casing may have a hexagonal shape, or the shape of a sector of a circle or ellipse. The inner periphery of the outer casing is preferably shaped to co-operate with the outer surface of the inner casing. This will most usually be curved, e.g. circular or elliptical.

A wide range of materials may be used for each of the C-section casings. As examples may be mentioned metals, for example steels (various grades including stainless), aluminium, copper, Inconel (trademark), and silver, and polymeric materials such as polyesters (e.g. polybutylene terephthalate (PBT) and polyethylene terephthalate (PET)), polyvinylidenefluoride (PVDF), polyolefins, and polyamides. The C-section casings may be made from the same materials or from different materials. One embodiment of the invention provides a first (inner) C-section casing made from stainless steel, and a second (outer) C-section casing made from aluminium. Both C-section casings are preferably substantially closed. This embodiment is particularly desirable for use adjacent other aluminium strands, e.g. in an OPGW, for example in an OPGW analogue of an ACSR as described earlier in the specification. The stainless steel inner casing provides optimum mechanical protection and strength, and the outer aluminium casing provides extra conductivity, and also avoids any problem of corrosion between the optical wire and the adjacent aluminium strands since the metals in contact are the same.

Up to now, the invention has been described with reference to two overlapping casings. It is also envisaged that three or more overlapping casings could be used, each with its opening peripherally displaced relative to the opening of its neighbours. Increasing the number of overlapping casings may advantageously enhance the seal, but may also increase the weight, which could be a disadvantage for some applications.

A second aspect of the present invention provides a method of sealing one or more optical fibres within a tubular outer casing, comprising:

(i) forming a first C-section casing around the optical fibres; (ii) forming a second C-section casing around the first C-section casing such that the second casing covers the opening in the first C-section casing, and overlaps at least part of the first C-section casing; and (iii) forming a seal between overlapping portions of the first and second casings on either side of the opening in the first C-section casing.

is Preferred features described above, referring to the optical fibre cable according to the first aspect of the invention, also apply to the method according to the invention.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein:

Figure 1 is a cross-section through a prior an OPGW; Figures 2 and 3 are cross-sections through two optical fibre cables according to the present invention; Figure 4 is a schematic sectional view showing the casings of Figure 3 in a different relative orientation; Figure 5 is a schematic view showing another embodiment in which the C- section casings are not closed.

Referring now to the drawings, Figure 1 shows an OPGW according to the prior art. It comprises a central core of steel wires 2, surrounded by an outer ring of aluminium wires 4. Each stainless steel wire 2 is clad with an aluminium layer 6. The steel provides mechanical strength, and the aluminium provided conductance to the ground wire. In the arrangement one of the steel wires has been replaced by a bundle of 24 optical fibres 8 contained in an outer stainless steel loose tube 10. The fibres 8 are surrounded by gel 12. The stainless steel tube has been made by forming a flat rolled sheet into a tubular configuration and laser welding the longitudinal seam 14 according to EP- A-0299123 (Laser Armor Tech).

The present invention is concerned with other arrangements which may intr alia be used to replace the longitudinally laser welded stainless steel tube 10 of the prior art.

Figure 2 shows an optical fibre cable according to the present invention. It comprises a first, inner C-section casing 16 made from stainless steel, having a diameter of from 0.9mm to 1Omm, and a wall thickness of 0.15 to 0.2mm. The Csection casing 16 is substantially closed, that is the tips of the C substantially abut, and the opening between the tips of the C is substantially zero mm in distance. The tips of the C-section casing 16 are not sealed to each other directly in any way. Surrounding the first Csection casing 16 is a second C-section casing 18. It is made from aluminium, has a diameter of 1.2 to 10Amm, and a wall thickness of 0.15 to 0.2mm. The thickness of the aluminium casing is greater than would be achieved by vapour deposition on an inner layer. The thicker layer provides enhanced mechanical strength. The C-section casing 18 is also substantially closed in cross-section, as described above, but the tips of the C-section casing 18 are not sealed directly to each other in any way. The openings in the casings 16 and 18, as indicated by reference numerals 17 and 19 respectively, are substantially diametrically opposed, i.e. the openings in the C-section casings are circumferentially displaced by 180'.

Both casings 16 and 18, in section form a circle. Between the casings 16 and 18 is positioned a resin 20 made of epoxy. This fills substantially the entire space between the casings 16 and 18. The radial spacing between the outer wall of casing 16 and the inner wall of casing 18 is about 0.75mm. According to this embodiment, although there is no direct seal between the tips of either of the casings 16 and 18, i.e. there is no complete tube formed, there is nonetheless a seal, provided by the resin, preventing external contaminants entering the interior of casing 16. Within casing 16 are positioned optical fibres 8 and a gel 12.

Figure 3 shows another embodiment similar to Figure 2, except that the inner casing 16' and the outer casing 18' are both formed from steel, and the resin 20 of the Figure 2 embodiment is replaced by lap welds 22 between overlapping portions of the casings on either side of the opening 24 in the first casing 16'. The welds 22 are shown in the Figure near to the tips of the C-section tubing 18', but they could be anywhere between overlapping portions, provided a weld is provided on either side of the opening 24. The relative positions, and the sizes of the casings 16 and 18 in Figure 3 is the same as that for the embodiment shown in Figure 2.

Figure 4 is a schematic sectional view showing the arrangement of Figure 3, but with the opening 24 in casing 16' oriented so that it is circumferentially displaced by an angle A' (a right angle in the drawing) relative to the opening 26 in the outer casing 18'. Preferred minimum values for the angle A' are given earlier in the specification. The optical fibres are not shown in Figure 4 for clarity.

Figure 5 is a schematic sectional view showing a first C-section casing 28 and an outer C-section casing 30. Neither of the casings is closed, but outer casing 30 covers the opening 32 in the inner casing 28, and overlaps a portion of the casing 28 on either side of the opening 32. The minimum angular circumferential displacement of the two casings 28 and 30 is defined earlier in the specification as the smaller of the angles marked AI and A2 in the Figure. Both casings 28 and 30 are formed of the same metal, and are sealed to each other by lap welds 34: one on either side of the opening 32 in the inner casing 28.

Any of the embodiments shown in Figures 2 to 5 can be used in place of the longitudinally laser welded stainless steel tube of Figure 1. The embodiments in which the outer casing is aluminium are most preferred in this arrangement, to avoid corrosion problems with adjacent aluminium strands.

CLAWS 1. An optical fibre cable comprising:

(i) one or more optical fibres; (ii) a first C-section easing surrounding the optical fibres; (iii) a second C-section casing, positioned around the first C-section casing such that the second casing covers the opening in the first C- section casing, and overlaps at least part of the first C-section casing, on either side of the opening in the first C-section casing; and (1v) a seal, formed between overlapping portions of the first and second casings, on either side of the opening in the first C-section casing.

0 2. An optical fibre cable according to claim 1, wherein the first Csection casing, andlor the second C-section casing is in the form of a substantially closed tube.

3. An optical fibre cable according to claim 1 or 2, wherein the seal between overlapping portions of the casings on either side of the opening in the first Csection casing is provided by lap welding.

4. An optical fibre cable according to any of claims 1 to 3, wherein the seal between overlapping portions of the casings on either side of the opening in the first C-section casing is provided by resin.

An optical fibre cable according to claim 4, wherein the resin is an epoxy or an acrylic resin.

6. An optical fibre cable according to any preceding claim, wherein each C-section C> casing is shaped so that, in cross-section, it forms part of a circle, or ellipse.

7. An optical fibre cable according to any preceding claim, wherein the second casing overlaps at least 10% of the outer surface of the first C-section casing.

8. An optical fibre cable according to any preceding claim, wherein the minimum peripheral displacement between the opening in the first and second casings is at least 20 degrees.

An optical fibre cable according to claim 8, wherein the said peripheral displacement is about 180 degrees.

10. An optical fibre cable according to any preceding claim, wherein one or both of the C-section casings are made from steel, aluminium, copper, Inconel or silver.

11. An optical fibre cable according to any preceding claim wherein the first and second casings are made of the same material.

12. An optical fibre cable according to any of claims 1 to 10, wherein the first and second casings are made of different materials.

13. An optical fibre cable according to any preceding claim wherein the space between overlapping portions of the C-section casings is at most 0. 75mm.

14. A method of scaling one or more optical fibres within a tubular outer casing, comprising:

(i) forming a first C-section casing around the optical fibres; ' (ii) forming a second C-section casing around the first C-section casing such that the second casing covers the opening in the first C-section casing, and overlaps at least part of the first C-section casing; and (iii) forming a seal between overlapping portions of the first and second casings on either side of the opening in the first C-section casing.