The invention relates specifically to a long distance submarine communications cable for use in a communications system requiring a high DC-voltage power supply, said cable comprising one or more optical fibres for transferring optical signals, and one or more electrical conductors for power feeding one or more intermediate units for regenerating said optical signals, and an insulation system as a covering for said one or more electrical conductors.

The invention furthermore relates to a method of manufacturing a long distance submarine communications cable for use in a communications system requiring a high DC-voltage power supply comprising the steps of providing a core element comprising one or more optical fibres for transferring optical signals, and arranging one or more electrical conductors for power feeding one or more intermediate units for regenerating said optical signals, and arranging an insulation system as a covering for said one or more electrical conductors.

The invention furthermore relates to the use of a long distance submarine communications cable according.

Description of Related Art: The following account of the prior art relates to one of the areas of application of the present invention, submarine cables for long distance optical communications.

The development of low loss optical fibres for long distance high speed communications has resulted in a decrease in the number of repeaters which are necessary in an optical communications system for the transmission of a certain amount of data or voice traffic over a certain distance. In other words, an increasing distance between the repeaters is possible. In a submarine communications cable, the power for electrically feeding the repeaters typically has to be supplied from one or more electrical conductors running parallel to the optical fibres in the cable. In a long distance submarine communications cable (such as used e. g. for a transatlantic connection) the electrical energy for the repeaters must sometimes be supplied over several thousands of kilometres, requiring DC-voltages in the kilovolt range. To isolate the high voltage electrical conductor (s) from the surrounding marine environment, such cables have traditionally been provided with an outer layer of a plastic material, typically of the Polyethylene (PE) type. The thickness of the PE layer is determined with a view to the voltage requirements of the system, the environment and the requirements to the mass density of the cable.

Norwegian laid out publication no. 167 778 discloses a submarine fibre optic telecommunications cable comprising centrally located armouring (e. g. steel threads) surrounded by a plastic material in which optical fibres are positioned in helical grooves. A watertight metal

sheath surrounds the central core of the cable, which again is surrounded by a plastic material such as polyethylene.

US-A-4,907,855 discloses a submarine fibre optic cable provided with a stranded layer of metal wires, which serve to supply energy for the regenerators. The layer of stranded wires is surrounded by an insulation layer, which in turn is protected by a stranded layer of sheathing wires. Both of the stranded layers of wires are embedded in a layer of a plastic, water-repellent material. The outer casing of the cable is formed by a metal composite layer sheath.

The increased demand for transmission bandwidth has resulted in the development of Gbit/s transmission systems using so-called Wave Division Multiplexing (WDM) and Dense Wave Division Multiplexing (DWDM) systems. In these systems several independent signals are transmitted at different optical wavelengths (hundreds of wavelengths in DWDM-systems) over one optical fibre, thus for example resulting in a transmission capacity of the order of 1000 Gbit/s (1 Tbit/s) of one optical fibre. This, on the other hand, increases the demand for electrical power for the intermediate regenerating units, e. g. comprising so- called fibre amplifiers (each typically comprising a section of Erbium-doped fibre, a laser pump, a section of dispersion compensating fibre, etc.). The distance between intermediate regenerating units may be in the range 50-100 km, i. e. for a cable of length 6-7000 km (such as a transatlantic cable) a number of the order of 100 units must be power fed from shore terminal stations via the cable. There is thus a need for increasing the power supply voltage of submarine fibre optical communications cables.

Summary : However, the increased need for electrical power for the intermediate regenerating units creates problems for a conventional cable construction in that an increased power supply voltage increases the necessary thickness of the insulation layer and thus the diameter of the cable and accordingly gives a relatively lower mass density.

The cable has to have a certain mass density to be able to position it safely on the seabed. A submarine cable further has to be armoured over certain sections of the cable in coastal areas where the probability of interference with fishing trawl and the like is large.

The armouring process gets increasingly complicated the larger the cable diameter and the cable becomes increasingly difficult to handle: The cable becomes heavier and more voluminous per unit length, resulting in increasing costs pr. unit length (large amounts of materials, complicated production, etc.), smaller lengths of the cable can be handled by a given cable ship, i. e. the ship must make more shifts to lay out a given length of cable (requiring more time, and more sea joints, thus being more expensive), etc.

The object of the present invention is to provide a long distance submarine fibre optical communications cable comprising electrical conductors for power feeding intermediate units and a method of manufacture of said cable, which allows a compact construction at high supply DC-voltages.

This is, as disclosed in claim 1, achieved according to the invention in that it comprises an electrically conducting, watertight sheath on the outer surface of said insulation system.

It should be emphasized that the term "comprises/comprising"when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other stated features, integers, steps, components or groups thereof.

In the present context, the terms inner'and outer'in relation to features of a submarine cable are taken to mean that seen relative to the central core of the cable, an inner'surface or layer is located closer to the core than an outer'surface or layer. In other words an outer surface'is located closer to the marine environment than an inner surface'.

In the present invention, the outer sheath of metal enclosing the insulation system is provided to be able to increase the voltage per unit length of the cable. By keeping the insulation system dry by means of the outer metal sheath, a higher supply voltage may be used for a given thickness of the insulation layer. A further reduction of the insulation thickness is achieved by supplying the insulation with electrical field limiting semiconducting layers on its inside and outside surfaces.

By utilizing the merits of the invention a more compact cable may be constructed, i. e. a cable with dimensions that allow an appropriate armouring over the relevant sections of the cable where fishing industry is present.

It has the further advantage that it may be designed to have an appropriate mass density over the sections of the cable where no armouring is necessary (typically the deep sea'part of cable) even at relatively high power supply DC-voltages (in the lOs of kilovolt range) where a conventional cable construction would have a problem due to the (necessary) increased insulation layer thickness

that would increase the cable diameter but decrease the mass density.

A cable construction according to the invention has the advantage over a conventional cable of being more compact for a given power supply voltage, resulting in that less material per unit length is needed, an easier handling as regards armouring and longer lengths per shift to lay out, leading to a reduced cost per bit'.

The problem of the prior art cables having an insulating outer sheath in direct contact with the marine environment is that a substantial electrical potential difference may be established in the insulating sheath during a short circuit situation leading to considerable damage of the cable.

For a submarine communications cable, the power is ON during the layout of the cable. This, in combination with the increasing power supply voltage, requires a careful design of the combined system comprising the power supply, the cable, the regenerating units, the cable ship, and the sea to ensure that the personnel is safe in handling the cable in all possible situations.

When, as disclosed in claim 2, said electrically conducting outer sheath comprises at least one electrically semiconducting layer on its outer surface, it is ensured that the outer metal sheath has the same potential as the surrounding water (or contact parts with the ship during layout). Thus a short-circuit situation will lead the resulting current to the water (or to the ground connection of contact parts with the ship during layout) thus avoiding a damage of the cable over longer lengths. In addition to a relatively thick second layer of a semiconducting material (e. g. a several mm thick

layer of a polymeric with an appropriate amount of carbon black) possibly applied by extrusion or some other convenient technique, a relatively thin (sub mm) first layer of semiconducting material may be applied directly to the outer surface of the electrically conducting outer sheath by an appropriate coating technique (e. g. in the form of a hotmelt'adhesive layer). The semiconducting first or second layers may be present alone or in combination.

In the present context, the term an electrically semiconducting layer'is taken to mean that the electrical conductivity of the layer in question has a value between that of a corresponding metal layer and an insulating layer. In other words that the semiconducting material has a specific resistance between that of Copper (1,7-10-6 Qcm) and a good electrical insulator (~ 1017 Qcm).

The semiconducting outer layer in contact with the marine environment ensures that the metal sheath has the same electrical potential as the surrounding water. An advantage of the invention is that the damage of a short circuit between the electrical conductors in the cable core and the outer metal sheath, which e. g. may occur during a cable rupture, will be limited since current will be able to flow to the water thus avoiding the damage to extend over a longer length of the cable. The minimisation of damages is especially important for a long distance submarine cable, which is often located at great sea depths for which a repair may be extremely costly.

When, as disclosed in claim 3, said electrically conducting outer sheath is provided with at least one electrically insulating layer on its outer surface for

mechanical protection and to protect the electrically conducting outer sheath from the marine environment, it is ensured that the metal-water surface of the cable is protected against corrosion. In addition to a relatively thick second layer of an insulating material (e. g. a several mm thick layer of Polyethylene) possibly applied by extrusion or some other convenient technique, a relatively thin (sub mm) first layer of insulating material may be applied directly to the outer surface of the electrically conducting outer sheath by an appropriate coating technique (e. g. in the form of a hotmelt'adhesive layer). The insulating first or second layers may be present alone or in combination. Further the relatively thick insulating second layer may be present in combination with a relatively thin (sub mm) first layer of semiconducting material, where the thin coating of semiconducting material is applied directly to the outer metal surface and the relatively thick layer of insulating material is applied thereon e. g. by extrusion.

When, as disclosed in claim 4, said at least one electrically insulating layer is provided with an electrically semiconducting layer on its outer surface, it is ensured that it is possible to test the insulation sheath (located between the outer metal sheath and the semiconducting layer) by a voltage test.

In a preferred embodiment, as disclosed in claim 5, an electrically semiconducting layer is arranged adjacent to the inner surface of said electrically conducting outer sheath. The semiconducting layer can e. g. be a relatively thin (sub mm) layer applied directly to the inner surface of the electrically conducting outer sheath by an appropriate coating technique (e. g. in the form of a hotmelt'adhesive layer applied to the sheet metal in advance of the cable construction). It may alternatively

be constituted by the outmost layer of the insulation system.

In a preferred embodiment, as disclosed in claim 6, said insulation system comprises an insulation layer provided with a semiconducting layer on its inner and outer surfaces. The combined use of an electrically conducting outer sheath and an insulation system comprising an insulation layer provided with semiconducting inner and outer layers allows a more compact cable design compared to conventional cable designs even allowing at the same time decreasing the cable diameter and increasing the maximum supply voltage for feeding the repeaters of the communications system. Such a design of the insulation system leads to a well defined electrical field distribution in the insulation and no electrical discharges on the insulation surfaces. The electrical field is limited to the insulation layer since the semiconducting layer on the outer surface of the insulation layer is short-circuited to the outer metal sheath and the semiconducting layer on the inner surface of the insulation layer is short-circuited to the one or more electrical conductors for power feeding the regenerating units.

An especially compact cable construction is, as outlined in claims 7 and 8, achieved if the inner and/or outer semiconducting layer (s) of the insulation system is (are) constituted by relatively thin layer (s) applied to the respective electrical conductor (s) and/or to the outer surface of the insulation layer by a coating technique.

This construction is additionally simple in construction, requiring fewer process steps.

In a preferred embodiment, as disclosed in claim 7, said inner semiconducting layer of said insulation system is

applied to the outer surface of said one or more electrical conductors by a coating technique.

In a preferred embodiment, as disclosed in claim 8, said outer semiconducting layer of said insulation system is applied to the outer surface of said insulation layer by a coating technique.

A method of manufacturing a long distance submarine communications cable for use in a communications system requiring a high DC-voltage power supply comprising the steps of providing a core element comprising one or more optical fibres for transferring optical signals, and arranging one or more electrical conductors for power feeding one or more intermediate units for regenerating said optical signals, and arranging an insulation system as a covering for said one or more electrical conductors is furthermore provided by the present invention. When, as disclosed in claim 9, it comprises the step of applying an electrically conducting, watertight outer sheath to the outer surface of said insulation system, the same advantages as are achieved for the corresponding product claim are provided.

When, as disclosed in claim 10, it further comprises the step of applying an electrically semiconducting layer on the outer surface of said electrically conducting, watertight sheath, the same advantages as are achieved for the corresponding product claim are provided.

When, as disclosed in claim 11, the step of arranging an insulation system as a covering for said one or more electrical conductors comprises the substeps of arranging an inner semiconducting layer around said one or more electrical conductors, and arranging an insulating layer around said inner semiconducting layer, and arranging an

outer semiconducting layer around said insulating layer, the same advantages as are achieved for the corresponding product claim are provided. The semiconducting layers may e. g. be applied by extrusion. In a preferred embodiment the semiconducting layers are applied to the outer surface of the one or more electrical conductors for feeding the regenerating units and/or to the inner surface of the outer metal sheath by a coating technique, e. g. in that the sheets used for the electrical conductors and/or the outer sheath and/or the outer surface of the insulation layer of the insulation system are coated with a semiconducting material. This has the advantages of yielding an easy addition of the semiconducting layers (possibly present from the start on the metallic foils) and an especially compact cable construction.

The use of a long distance submarine communications cable according to any one of claims 1-8 is moreover claimed by the present invention. The same advantages as are achieved for the corresponding product claims are thereby provided.

Brief Description of the Drawings: The invention will be explained more fully below in connection with a preferred embodiment and with reference to the drawings in which: fig. 1 shows a conventional construction of a communications cable, fig. 2 shows a conventional construction of a communications cable adapted for a high DC-power supply,

fig. 3 shows a communications cable according to the invention, and fig. 4 shows a communications cable according to the invention with an outer semiconducting layer.

The figures are schematic but of equal scale and simplified for clarity, and they just show details which are essential to the understanding of the invention, while other details are left out. Throughout, the same reference numerals are used for identical or corresponding parts.

Detailed Description of Embodiments: Fig. 1 shows a conventional construction of a communications cable.

Fig. 1 shows a known conventional communications cable for a power supply of repeaters with a DC voltage of some few kilovolts. The cable includes a fibre optic element 1, a tensile armouring 2 and an electrical conductor 3 for power feeding. Then follow the insulation layer 4 and a polymeric sheath 5.

Fig. 2 shows a conventional construction of a communications cable adapted for a high DC-power supply.

Fig. 2 shows a conventional communications cable with the components 1, 2 and 3 as for fig. 1 but upgraded for a high power supply DC voltage and therefore with an increased thickness of the insulation layer 4. The cable is protected by the polymeric sheath 5.

Fig. 3 shows a communications cable according to the invention. The cable includes a fibre optic element 1, a tensile armouring 2 and an electrical conductor 3 for power feeding of repeaters. The insulation layer 4 forms together with the semiconducting layers 6 and 7 an insulation system for high DC voltage supply of repeaters. The cable is supplied with a metallic layer 8, which at any time secures a dry insulation system.

The outer metal sheath 8 may comprise metals such as steel, Aluminium, Copper etc. The sheath may be made of sheets of metal that are folded and/or welded to provide a watertight protection of the cable. The sheets of metal may be provided with coatings of insulating or semiconducting material (not shown) on one of its sides (typically the outer) or on both.

The polymeric sheath surrounding the outer metallic layer provides a mechanical and corrosion protection of the cable. The sheath material can be insulating or semiconducting or a combination with an insulating sheath, which is covered with a thin semiconducting layer for sheath voltage testing as shown in fig. 4. In the embodiment shown in fig. 3, the sheath material is semiconducting (50) (which e. g. may be achieved by the addition of appropriate amounts of carbon black to the insulation), but it might alternatively be insulating.

Fig. 4 shows a communications cable according to the invention with an outer semiconducting layer. The same elements as depicted in fig. 3 and as described above are present in fig. 4, only the polymeric sheath 51 surrounding the outer metallic layer is insulating.

Additionally the insulating sheath 51 is supplied with a thin semiconducting layer 9 for sheath voltage testing.

Some preferred embodiments have been shown in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject-matter defined in the following claims.