WO2015192887A1

WO2015192887A1 - Optical submarine cable

Info

Publication number: WO2015192887A1
Application number: PCT/EP2014/062669
Authority: WO
Inventors: Josep Maria MARTIN REGALADO; Carles FANDOS FERRES; Ruben BARRIO FARGAS
Original assignee: Prysmian S.P.A.
Priority date: 2014-06-17
Filing date: 2014-06-17
Publication date: 2015-12-23

Abstract

Optical submarine cable (1) comprising at least one optical fibre (101) housed inside a hollow central metallic element (102), and at least one monitoring electrical conductor (103) in an outer external position with respect to the central metallic element (102), wherein the monitoring electrical conductor (103) is arranged in direct mechanical and electrical contact with the hollow central metallic element (102).

Description

OPTICAL SUBMARINE CABLE

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DESCRIPTION

Background of the invention

Field of the invention

The present invention relates to the field of optical fibre cables (optical cables), particularly of optical submarine cables. More specifically, the present invention relates to an optical submarine cable with electric conductors for monitoring the cable status.

Overview of the related art

An optical submarine cable typically comprises a plurality of optical fibres contained in a central element (typically a steel tube) surrounded by a polymer layer. Depending on the specific application, one or more armour layers, typically comprising steel wires, are provided in radial external position with respect to the polymer layer.

An optical submarine cable can be subjected to significant stresses both during its deployment and in operation, and may thus incur damages.

Therefore, the possibility of monitoring an optical submarine cable after deployment thereof and in operation, for checking its integrity and, in the event, locating (i.e., determining the position along the cable of) a fault in the cable, for repair purposes, is welcome.

Alternated Current (AC) tone tracking is a known reliable and accurate technique for monitoring an optical submarine cable and locating the location of faults, as from, for example, INNOVATUM Systems Manual Revision 3 12 December 2001, Section 6.

In order to implement AC tone tracking or an equivalent monitoring system for optical cable, an electric conductor path has to be provided in the cable structure, for the propagation of a monitoring tone or signal.

As reported, for example, by EP 704734, a longitudinally welded tube or a helically wound tape of electrically conductive material can be used as monitoring conductor in optical submarine cables. The welded tube or the helically wound tape, is arranged around the central element of the cable housing the optical fibres.

Alternatively, monitoring conductors in form of wires can be provided around the central element.

EP 704734 discloses a sea cable that has at least one central element (cladding tube), at least one optical fibre disposed therein and at least two measuring conductors insulated from each other within a common insulating sleeve or insulating layer. In particular, the measuring conductors are spaced from the central element such that between the central element and the measuring conductors, an insulating layer is arranged. Such disposition has the advantage that interfering electrical potentials between the central element and the measuring conductors are avoided.

Summary of the present invention

The Applicant observes that the known optical submarine cable construction having a longitudinally welded tube or a helically wound tape of electrically conductive material as monitoring conductor has the disadvantage that the starting component, i.e. the metallic tape to be longitudinally welded or helically wound is available in limited lengths and welds are needed (also in the case of the tape) and these can constitute weak points, besides increasing the manufacturing complexity.

The Applicant observed that monitoring conductors made of separately insulated wires and the central element housing the optical fibres, as in EP 704734, may be locally at electric potential different for each other, and electric discharges can arise in case of discontinuities in the insulation layer(s).

Such discontinuities are not unlikely to happen because the manufacturing of different insulating layers (the one around the central element and those surrounding the monitoring conductors) is carried out by different extrusion steps that, besides making the manufacturing process longer/more expensive, can result is an uneven adhesion among the extruded layer, thus causing an uneven distribution of the insulation around the monitoring conductors and central element.

The Applicant has found that by providing monitoring conductors in form of wires in electrical contact with the metallic central element housing the optical fibres of the optical submarine cable, it is ensured that the monitoring conductors and the metallic central element are at the same electric potential, and any risk of discharge is avoided.

Also, an insulating layer can be provided to collectively surround monitoring conductors and metallic central in a single-step process, thus decreasing the manufacturing costs and times.

According to one aspect, the present invention provides an optical submarine cable comprising at least one optical fibre housed in a hollow central metallic element, and at least one monitoring electrical conductor in an outer external position with respect to the central metallic element, wherein the monitoring electrical conductor is arranged in direct mechanical and electrical contact with the hollow central metallic element.

Advantageously, a plurality of optical fibre housed in a hollow central metal element of the cable of the invention.

Advantageously, the cable of the invention comprises at least two monitoring electrical conductors, preferably at least three monitoring electrical conductors. The monitoring electrical conductor of the invention can be in the form of a wire or of joined wires.

The monitoring electrical conductor can be arranged either parallel to the longitudinal axis of the hollow central element or wound around the hollow central element.

In the cable of the invention an electrically insulating layer can collectively surround and enclose the monitoring electrical conductor and the hollow central element. The insulating layer is made of an extrudable material, for example a polymeric material, preferably polyethylene or a polyethylene based compound.

The electrically insulating layer of the cable of the invention is preferably a single layer of extruded material and can be applied in a manufacturing single step, therefore substantially avoiding the probability of discontinuity occurrence throughout its thickness.

The cable of the invention can comprise one or more armour layers, preferably made of armour wires. Advantageously, a separator layer can be interposed between two adjacent armour layers.

An outer sheath, preferably in polymeric material, can be the outermost layer of the optical submarine cable of the invention.

Thanks to the fact that the monitoring electric conductor(s) is(are) in direct mechanical and electrical contact with the optical cable central element, it is ensured that the monitoring electric conductor(s) and the central element are at the same electric potential, thereby any risk of discharge among the monitoring electric conductor(s) and the central element is avoided.

The use of monitoring electric conductors in the form of metal wires (instead of a metal tape) is also advantageous because metal wire lengths as long as 50 Km can be supplied, long enough to cover the length of the cable to be manufactured, thereby avoiding the need of welds to join different spans of monitoring electric conductors.

The above and other features and advantages of the present invention will appear clearly by reading the following detailed description of exemplary and non- limitative embodiments of the present invention, making reference, for a better intelligence, to the annexed drawing which is a schematic cross-sectional view of an optical submarine cable according to an embodiment of the present invention.

For the purpose of the present description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term "about". Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein. Brief description of the drawings

Figure 1 shows a schematic cross section of an optical submarine cable according to an embodiment of the present invention.

Detailed description of exemplary embodiments of the present invention

An optical submarine cable according to an example of the invention, globally denoted 1, comprises a plurality of optical fibres 101 (e.g., 48, and up to 96 optical fibres) accommodated within an elongated tubular central element 102 (hollow central element) made of metal, e.g. of stainless steel. For the purposes of the present description, by "optical fibre" it is meant an optical core transmitting light surrounded by one or more protective layers.

The central element 102 may also contain, in addition to the optical fibres 101, element/s capable of absorbing hydrogen and/or water, e.g. a gel or one or more filament optionally supporting a water swellable powder.

The central element 102 can be manufactured by longitudinally welding, preferably laser welding, a metal foil around the plurality of optical fibres 101.

Monitoring electric conductors 103 are arranged externally to and in mechanical and electrical contact with the central element 102.

In an exemplary embodiment of the present invention, each monitoring electric conductor 103 is a metal wire, e.g. of copper or aluminium or composite thereof.

The monitoring electric conductors 103 can for example be arranged parallel to the longitudinal axis of the central element 102, or they can be wound around the central element 102, in any case being in mechanical and electrical contact with the central element 102.

In the example shown, three monitoring electric conductors 103 are provided, being arranged approximately angularly regularly spaced (120°) from each other. However, the number of monitoring electric conductors 103 to be provided is not limitative, and can depend on the required overall cross-sectional area of the monitoring electric conductors 103 (in order to create a suitable current path for the monitoring signals), on the individual cross-sectional area of the monitoring electric conductors 103, on the overall outer diameter of the central element 102. In general, at least one monitoring electric conductor 103 is provided, possibly with one or more backup monitoring electric conductors 103 for backup purposes.

Considering the shown example of three monitoring electric conductors 103, assuming a cable 1 outer diameter of 12 mm, each monitoring electric conductor 103 can have a diameter of 1.6 mm. When more than one monitoring electric conductor 103 is provided, it is preferable, although not strictly necessary, to arrange them symmetrically around the longitudinal axis of the central element 102. The central element 102 and the monitoring electric conductors 103 are covered by an electrically insulating layer 104, e.g. in polymeric material such as polyethylene. The electrically insulating layer 104 collectively surrounds and encloses the central element 102 and the monitoring electric conductors 103.

In the present embodiment, the electrically insulating layer 104 is surrounded by two armour layers 105 and 107, for example formed by a plurality of metallic armour wireslOSa, 107a helicoidally wound.

All of the armour wires 105a, 107a can be made of steel, e.g. of galvanized steel. Alternatively, some of the armour wires 105a, 107a can be made of steel and some of a polymeric material, e.g. polyethylene.

Preferably, a separator layer 106 is provided between the reinforcing layers 105 and 107; for example, the separator layer 106 is in polymeric material, e.g. in polyethylene.

A protective sheath 108 is preferably formed over the reinforcing layer 107. For example, the protective sheath 108 is formed by extrusion of a polymeric material, for example polyethylene, polypropylene or copolymers thereof.

Thanks to the fact that the monitoring electric conductor(s) is(are) in direct mechanical and electrical contact with the optical cable central element, it is ensured that the monitoring electric conductor(s) and the central element are at the same electric potential, thereby any risk of electrostatic discharge is avoided.

The use of monitoring electric conductors in the form of metal wires (instead of a metal tape) is also advantageous because metal wire lengths as long as 50 Km can be supplied, thereby avoiding the need of welds to join different spans of monitoring electric conductors.

The electrical conductor/s of the cable of the invention is/are suitable for monitoring the status of the optical submarine cable comprising them and for locating a possible fault thereof. According to a monitoring technique, such as AC tone tracking, all of the electrical conductors present in the cable of the invention are forward conductors, the seawater surrounding the optical cable performing as return conductor.

Claims

1. Optical submarine cable (1) comprising at least one optical fibre (101) housed inside a hollow central metallic element (102), and at least one monitoring electrical conductor (103) in an outer external position with respect to the central metallic element (102), wherein the monitoring electrical conductor (103) is arranged in direct mechanical and electrical contact with the hollow central metallic element (102).

2. The optical submarine cable of claim 1 comprising at least two monitoring electrical conductors (103).

3. The optical submarine cable of claim 1, wherein the electrical conductor (103) is in the form of a wire or of joined wires.

4. The optical submarine cable of claim 1, wherein the electrical conductor (103) is arranged parallel to a longitudinal axis of the hollow central element (102).

5. The optical submarine cable of claim 1, wherein the electrical conductor (103) is wound around the hollow central element (102).

6. The optical submarine cable of claim 1 comprising an electrically insulating layer (104) collectively surrounding and enclosing the electrical conductor (103) and the hollow central element (102).

7. The optical submarine cable of claim 5, wherein the electrically insulating layer (104) is made of an extrudable material.

8. The optical submarine cable of claim 1, comprising one or more armour layers (105,107).