GB2154334A - Submarine optical fibre cable having welded metallic layer - Google Patents

Abstract

In an optical fibre submarine cable the ingress of hydrogen ions is reduced if not eliminated by at least one envelope which provides a barrier to hydrogen ions. In one embodiment a copper tape (L) is folded longitudinally around the cable core comprising strength member wires (G, H) split aluminium or copper tube (E) and optical fibre package (A). The abutting edges (L') are seam welded. In an alternative embodiment, or as a modification to the embodiment just described, the aluminium or copper tube (E) has its abutting edges (E') welded to a envelope (or a second such depth sufficient to provide the envelope) but insufficient to cause damage to the optical fibre package (A) through the heat from welding. <IMAGE>

Description

SPECIFICATION Submarine cable This invention relates to submarine optical fibre cables.

In 1 980 we, in collaboration with British Telecom, laid a first prototype experimental submarine optical fibre cable in Loch Fyne in Scotland and the performance of this cable has been monitored. The structure of the cable is shown in Fig. 1 of the drawing and, referring to that drawing comprises a central optical fibre package A which includes a package strength member A' of high tensile steel wire. Around the package strength member A' are eight single mode secondary coated optical fibres B which are held in the package with the strength member A' by means of a fibre package whipping C. We prefer a fibrous Kevlar ribbon which in the particular embodiment described has about 200 denier.

The fibre package is loosely housed in a tubular metallic core E which in this embodiment is aluminium. The tubular metallic core E has a single split E' whereby the optical fibre package is introduced into the core before the tubular member is closed.

The optical fibre package A and the annular gap between the fibre package and the internal bore D of the tubular member E, are impregnated and filled respectively with a water blocking compound. The main function of this water blocking compound is to substantially restrict longitudinal infiltration of water along the core should the cable become damaged.

Around the tubular core E is applied one layer of tensile strength elements for the cable, G. The layer G comprises twelve high tensile steel wires whose size is chosen in conjunction with the outer diameter of the core E, to be radially supported by the core and to bridge against one another so that in conjunction with the central tubular core E, there is formed a crush-resistant electrically conductive cable core having a tensile strength member which provides the tensile strength the cable is required to have, and also is crush-resistant so that the fibres loosely held in the central bore D of the tubular member are substantially free of compressive stresses in use of the cable.

Around the layer of strength member wires G is a copper tape with overlapping edges 'G' welded and which is compacted by drawing onto the strand.

Extruded around the outside of and directly onto the outer layer of strength member wires is a polyethylene sheath J which forms an electrical insulation between the outside and electric power or electric signal which is, in use of the cable, transmitted along the conductive central core E.

An armouring layer K is applied to the outside of the insulation J.

Since monitoring the performance of the cable we have become aware of a change taking place in the transmission loss of the optical fibres at certain wavelengths corresponding generally to impregnation of the fibre by hydrogen atoms. Although the change is slight it appears to be a gradual yet continuous change such that transmission at say 1.3 micron wavelength might eventually be impaired.

Our investigations lead us to believe that the likely source of the hydrogen is from outside the cable, probably the armour wires K reacting with the outside water, and from within the cable due to the use of mixed metals, i.e. copper, steel, aluminium, in the presence of an electrolyte. Whereas the polythene insulation J provides an effective water barrier it is relatively easily permeated by hydrogen atoms.

It is an object of the present invention to provide a cable construction which in realisation of this problem minimises permeation by hydrogen to the fibres while at the same time providing a cost effective manufacturing process.

According to one aspect of the present invention there is provided an optical fibre cable comprising at least one optical fibre housed within a core member of the cable, a tensile strength member for sustaining the tensile loads which will in use be borne by the cable, the core member comprising a split metal tube having a thickness in the range 20 thou to 70 thou and welded along the split to a depth in the range 1/3 to 2/3 the thickness of the tube wall whereby to provide at least one envelope around the fibre which is impermeable to hydrogen relative to the other component parts of the cable.

According to another aspect of the present invention there is provided an optical fibre cable comprising at least one optical fibre housed within a core member of the cable, a tensile strength member for sustaining the tensile loads which will in use be borne by the cable, and a metal tape surrounding the tensile strength member and having its abutting edges welded whereby to provide at least one envelope around the fibre which is impermeable to hydrogen relative to the other component parts of the cable.

According to a further aspect of the present invention there is provided an optical fibre cable comprising at least one optical fibre house within a metallic core member of the cable, a tensile strength member for sustaining the tensile loads which will in use be borne by the cable, the core member comprising a first split metal tube which has been closed around the optical fibres, and a second split metallic tube which has been closed around the first tube, the second tube having been welded whereby to provide at least one envelope around the fibre which is impermeable to hydrogen, relative to the other component parts of the cable.

According to yet another aspect of the present invention there is provided a method of making an optical fibre cabie, the cable comprising at least one optical fibre housed within a crush-resistant channel in a core member of the cable, and a tensile strength member for sustaining the tensile loads which will in use be borne by the cable, the method including the step of forming around the channel at least one envelope which is rendered impervious to hydrogen relative to the other component parts of the cable by a welding process.

In one embodiment of the invention the envelope comprises a metallic sheath of copper surrounding the core member and hermetically sealed. Conveniently this can be applied by folding a copper tape longitudinally about the core member and seam-welding the abutting edges of the copper tape to the extent that gaps or skips in the welding occupy less than 0.1 per cent of the length of the weld.

The copper tape is preferably about 1 5 thou thick which is a compromise between ease of handling during cable manufacture and relative impermeability to hydrogen.

In an alternative embodiment, where the core member comprises an extruded 'C' section which is closed around the optical fibre package and is in turn surrounded by the cable strength member element in the form of steel wires, then it is proposed to weld the split in the closed 'C' section to a depth sufficient to provide an effective barrier against hydrogen permeation but insufficient for the heat from welding to cause damage to the underlying optical fibre package. Preferably the depth of the weld is between one third and two thirds of the radial thickness of the split tube.Conveniently the weld is performed by an argon arc and the electrode is maintained in alignment with the split in the closed 'C' section by a thin guide blade located in the gap between the closing edges of the 'C' section, the blade controlling the orientation of the welding apparatus directly or by a servo-control arrangement. Alternatively due to heat transfer to the enclosed optical fibres, electron beam or high frequency welding may be preferred.

As an alternative embodiment it would be possible to combine the addition of the copper tape with welding the closed 'C' section tube to give a double barrier against hydrogen permeation. The 'C' section can be of aluminium or of copper.

In a further improvement where the strength member comprises steel wires in or around the core member, it is proposed to impregnate the interstices of the wires with a gas and water blocking filling material e.g.

silicon rubber.

In order that the invention can be clearly understood reference will now be made to the accompanying drawings in which: Figure 1 shows an optical fibre submarine cable similar to the experimental cable laid in Loch Fyne; Figure 2 shows the core of an optical fibre submarine cable according to one embodiment of the invention; Figure 3 shows the manufacture of the core of Fig. 2; Figure 4 shows in more detail part of Fig.

3; Figure 5 shows another part of the manufacturing process for welding the tube of Fig.

2; Figures 6, 7 and 8 show stages in the process of closing tube E of Fig. 5; Figure 9 shows part of the apparatus of Fig.

5; and Figure 10 shows an alternative embodiment in which the single tube E is replaced by a double tube E' and E".

Referring to Fig. 2 the same reference numerals as Fig. 1 are used to designate similar parts of the cable. Only the cable core is shown including the strength member wires. Prior to extruding the polythene dielectric over the core, a copper tape L is formed around the strength member and has abutting edges L' which are seam welded such that there are no gaps or skips or at least less than 0.1 per cent over the length of the core. Prior to forming the tape around the core, the interstices of the strength member are filled with a water and gas blocking compound e.g.

a silicone rubber which is relatively impermeable to hydrogen.

The apparatus for carrying out these operations is illustrated in Fig. 3. Referring to Fig.

3 the cable core as shown in Fig. 2 but without the copper tape and designated generally by the reference numeral 1 is fed either directly from a stranding machine or as illustrated from a cable drum 2, on which it has been stored following the strength member stranding operation. The core is drawn from the drum through an impregnating station 4 where a gas and water blocking compound 5 such as silicone rubber is forced by a positive displacement pump into an extruding head and die block assembly 6. The interstices of the wires G and H become impregnated and coated with the compound, the temperature of the head 1 3 and the pressure of extrusion are adjusted so that complete impregnation is achieved.

The coated and impregnated core 1 proceeds to the next station 7 where it enters a folding copper tape L which is drawn from a storage drum 8. The station 7 comprises a series of closure rollers which act to fold the tape around the core.

Just beyond station 7 is a welding station 10 and this is shown schematically in Fig. 4.

Referring to Fig. 4 an argon arc welding station comprises an electrode 11 of an arc welding torch 1 2 which is held stationary in the welding position close to the final rollers or die of station 7. As the core 1 and the copper tape L of the cable are drawn through the welding station, the abutting edges L' are seam welded together. Following the welding operation the copper tube formed from the tape is drawn down through a sizing die 1 3 so that the tube is a tight fit around the strength member wires H.

Following the die 1 3 the weld is tested by eddy current or ultrasonic equipment to check that less than 0.1 % skips or gaps are present.

The copper tape L provide an effective barrier to permeation of hydrogen atoms from outside the cable core. In addition or instead of these measures, it is also proposed to seal the aluminium or copper tube E by welding the abutting edges E'. This is illustrated in Fig. 5.

Referring to Fig. 5 the 'C' section E is drawn from a drum 21 (it could instead be folded up from flat tape) and the optical fibre package A is simultaneously drawn from a drum 22. The thickness of the tube E is at least 20 thou, preferably 45 thou, but could be thicker. In the preferred embodiment the external diameter of the tube E is 0.24 inches. The width of the longitudinally-extending gap between edges of the 'C' section E is just large enough to allow the optical fibre package A to enter the tube and the arrangement at the location X in Fig. 5 is shown in Fig. 6 in cross section. The tube E then enters a closing die or set of rollers at station 23 which almost closes the tube as shown in Fig.

7. One of the top rollers 24 is shown in Fig. 9 and this roller has a horizontal axis and a central fin 25 which fits in the closing gap between the edges of the tube E so as to maintain the gap or seam to be welded in line with the welding apparatus 26 immediately following the station 23. At location Y the arrangement is as shown in Fig. 7, but at location Z the seam W is welded and sideways pressure is averted by rollers such as 27 so as to upset the weld W and ensure the now abutting edges are pressed together during welding. Fig. 8 shows the arrangement at the location Z and beyond.

Various types of welding apparatus 26 may be used such as TIG welding using a nonconsumable electrode (gas tungsten arc welding), electron beam welding, high frequency welding, laser welding, plasma arc welding.

Due to heat transfer to the enclosed optical fibres electron beam welding or H.F. welding is preferred.

Instead of using the finned roller such as is shown in Fig. 9, to maintain the tube seam in a fixed alignment, a separate fin fixed to the apparatus at the station 23 could be used for the same purpose.

It wold also be possible to use a seam following device indicated by reference numeral 26A in Fig. 5 which senses the seam and moves the welding apparatus or beam sideways to compensate for any sideays drift of the seam caused by the tube twisting slightly during the closing process. This could be used as well as, or in place of, the fixed seam locating technique described above. An advantage of H.F. welding is its comparative insensitivity to seam wander.

In the embodiments described the thickness of the copper tape L is 45 thou. It could, however, lie anywhere in the range 20 to 70 thou.

The 'C' section tube can be of aluminium instead of copper. As an alternative form of construction shown in Fig. 10 it is proposed to use a thin section of aluminium or copper (e.g. between 1 5 to 55 thou thick) which, after closure to form a first tube E', is surrounded by a similar metal tube E" which is longitudinally seam welded W' along the abutting edges and compacted around the inner closed section. In the preferred embodiment the inner tube thickness would be 31.5 thou and the outer thickness 1 5 thou. To provide an effective hydrogen barrier the welded seam must have low porosity and a skip length summation less than 0.1 % of the total length. This applies to welding the tube E, the outer tube E" (Fig. 10) and the tape weld L'.

Claims (14)

1. An optical fibre cable comprising at least one optical fibre housed within a core member of the cable, a tensile strength member for sustaining the tensile loads which will in use be borne by the cable. the core member comprising a split metal tube having a thickness in the range 20 thou to 70 thou and welded along the split to a depth in the range 1/3 to 2/3 the thickness of the tube wall whereby to provide at least one envelope around the fibre which is impermeable to hydrogen relative to the other component parts of the cable.

2. An optical fibre cable comprising at least one optical fibre housed within a core member of the cable, a tensile strength member for sustaining the tensile loads which will in use be borne by the cable, and a metal tape surrounding the tensile strength member and having its abutting edges welded whereby to provide at least one envelope around the fibre which is impermeable to hydrogen relative to the other component parts of the cable.

3. An optical fibre cable comprising at least one optical fibre house within a metallic core member of the cable, a tensile strength member for sustaining the tensile loads which will in use be borne by the cable, the core member comprising a first split metal tube which has been closed around the optical fibres, and a second split metallic tube which has been closed around the first tube, the second tube having been welded whereby to provide at least one envelope around the fibre which is impermeable to hydrogen, relative to the other component parts of the cable.

4. A cable as claimed in claim 1, wherein the thickness of the split tube is about 45 thou and the external diameter of the tube is about 0.24 inches.

5. A cable as claimed in claim 1, 3 or 4, comprising a metal tape surrounding the tensile strength member and having its abutting edges welded whereby to provide a second envelope around the fibre which is relatively impermeable to hydrogen.

6. A cable as claimed in any preceding claim, wherein interstices within the cable are filled with a gas and water blocking filling material.

7. A cable as claimed in any preceding claim, wherein the metal tape has a thickness in the range 10 to 20 thou.

8. A cable as claimed in any preceding claim, comprising a tubular dielectric body extruded over the core.

9. A cable as claimed in claim 8, wherein the dielectric body is extruded directly onto said metal tape.

10. A cable substantially as hereinbefore described with reference to Figs. 1, 2, 3, and 4, modified or not with reference to Figs. 5 to 9, or to Fig. 1 and Fig. 5 to 9, of the accompanying drawings.

11. A method of making an optical fibre cable the cable comprising at least one optical fibre housed within a crush-resistant channel in a core member of the cable, and a tensile strength member for sustaining the tensile loads which will in use be borne by the cable, the method including the step of forming around the channel at least one envelope which is rendered impervious to hydrogen relative to the other component parts of the cable by a welding process.

1 2. A method as claimed in claim 10, wherein the envelope is formed by folding a longitudinally-applied copper tape over the core member and seam welding the abutting edges of the tape.

13. A method as claimed in claim 10 or claim 11, wherein said one envelope, or a second envelope, is formed by providing an open metal extrusion or tape, closing the extrusion or tape around said optical fibre, and welding the adjacent longitudinal edges to a depth sufficient to provide the envelope but insufficient to cause damage to the fibre.

14. A method of making an optical fibre cable substantially as hereinbefore described with reference to Figs. 1, 2, 3 and 4, modified or not with reference to Figs. 5 to 9, or to Figs. a and Figs. 5 to 9, of the accompanying drawings.