WO2003091782A1

WO2003091782A1 - Submarine communication cable with copper clad steel wires

Info

Publication number: WO2003091782A1
Application number: PCT/EP2003/050039
Authority: WO
Inventors: Gino Dugardeyn
Original assignee: N.V. Bekaert S.A.
Priority date: 2002-04-24
Filing date: 2003-03-05
Publication date: 2003-11-06
Also published as: AU2003219142A1

Abstract

A submarine optical fiber cable comprises pairs of optical fibers (32), a buffer tube (34) surrounding the pairs of optical fibers (32), one or more layers of steel wires (36, 38) surrounding the tube (34), and a dielectric coating (42) surrounding the layers of said steel wires (36, 38). At least one of the steel wires (36, 38) is a copper clad steel wire. A decreased electrical resistance can be obtained with only a limited increase in weight.

Description

SUBMARI NE COMMUN I CATION CABLE WITH COPPER CLAD STEEL WI RES

Field of the invention. The invention relates to a submarine optical fiber cable comprising pairs of optical fibers, a buffer tube surrounding the pairs of optical fibers, one or more layers of steel wires surrounding the tube, and a dielectric coating surrounding the layer or layers of steel wires.

Background of the invention.

Submarine optical fiber cables generally comprise pairs of optical fibers to transmit information, one or more layers of steel wires to protect the pairs of optical fibers and an electrical conductor material to provide electrical power to repeaters and branching units. These repeaters amplify the data signals. The electrical conductor material is present in the form of a strip or tube around the layers of steel wires.

Submarine optical fiber cables are known to transmit data signals from one continent to another. The distances between continents or between a continent and an intermediate landing point on an island can amount to several thousands of kilometers, for example 7000 km for a transatlantic network. The submarine fiber optical cables lie on the seabed, at depths of up to 8 km. At hazardous points on the trajectory, they can be buried under the seabed. These extreme circumstances put severe requirements on submarine optical fiber cables.

First of all, unlike aerial and land optical fiber cable systems there is no possibility to install intermediate power stations. Thus, due to the huge distances mentioned hereabove, the electrical resistance of the cable should be kept as low as possible.

Secondly, the steel wires fulfill an important mechanical role. One of the functions of the steel wires in a submarine cable is to protect the pairs of optical fibers against hydrostatic pressures at great depths. Another function of the steel wires is to prevent straining of the optical fibers, especially during installation of the cable at great depths. It is hereby understood that optical fibers break at elongations below 1 %.

Several alternatives could be considered to decrease the electrical resistance of the steel-copper construction in submarine optical fiber cables.

A first alternative could aim at replacing one or more steel wires by copper wires. Copper having a much lower tensile strength than steel, the tensile strength of the copper-steel construction, however, would decrease substantially. In order to fully compensate this loss of tensile strength, the remaining steel wires would have to be drawn beyond their deformation limit. Moreover, substitution of steel by the more deformable copper would endanger the locking mechanism of the wire construction around the buffer tube and the pairs of optical fibers.

A second alternative could aim at increasing the thickness of the copper strip or the copper tube. Here again, this would result in a substantial decrease in overall tensile strength of the copper-steel construction. And here again, in order to compensate fully this loss in tensile strength, the steel wires would have to be drawn beyond their deformation limit.

Summary of the invention. It is an object of the present invention to avoid the drawbacks of the prior art.

It is another object of the present invention to provide a submarine optical fiber cable with a decreased electrical resistance.

More particularly it is an object of the present invention to decrease the electrical resistance of a submarine optical fiber cable by 20 % to

50 % and more, with only a cable weight increase of 1 % to 5 %. According to the present invention there is provided a submarine optical fiber cable comprising pairs of optical fibers, a buffer tube surrounding the optical fibers, one or more layers of steel wires surrounding the buffer tube and a dielectric coating surrounding the layer or layers of steel wires. At least one of the steel wires and preferably all the steel wires are copper clad steel wires.

Within the context of the present invention the terms "copper clad steel wires" refer to steel wires around which a strip of copper forms a tube and is welded to the steel wire. A copper clad steel wire is to be distinguished from an electrolytically coated steel wire.

A first advantage can be explained as follows. The copper cladding of the steel wires is done at an intermediate step during the manufacturing of the copper clad steel wires. After the copper cladding, the steel wires are further subjected to a mechanical deformation, e.g. by drawing. Copper is a softer component than steel and functions as a type of lubricant during the drawing and makes higher deformations of the steel wire core easier to obtain. So the obtainable degree of deformation with a copper clad steel wire is much higher than with a non copper clad or uncoated steel wire. So with copper clad steel wires a higher tensile strength of the wire steel core can be obtained. This means that with copper clad steel wires less steel cross-section is required for a same reinforcing and protection function. This results in the possibility of using more copper in the cable and in thus in a reduction of the electrical resistance.

A second advantage is as follows. The steel wires form one or more layers around the buffer tube. This layered structure is often slightly deformed in order to obtain a contiguous annular unity around the buffer tube in order to protect the buffer tube and the optical fibres against the high pressures exercised by the huge amount of water above the submarine optical fiber cable. In case of the invention copper clad steel wires are used in this layered structure. Being a softer material than steel, copper will 'flow' easily between all interstices so that the contiguous annular unity contains less porosities than when no copper clad steel wires are used.

It has been established that by using copper clad steel wire the electrical resistance of the submarine fiber optic cable can be reduced by 20 % to 50 % and more, with only a weight increase of 1 % to 5 %.

The reduction of the electrical resistance in submarine cables. results in the use of lower voltages with a same number of pairs of optical fibers or the possibility for more pairs of optical fibers with a same level of voltage or a combination hereof.

A lower voltage leads to an increased system reliability and uptime, since in case of an electrical fault the single end feeding becomes easier. Surges in case of an electrical defect will be lower. Another advantage of a lower voltage is that the cable electrical insulation layer can be thinner, which means a lower insulation cost and a smaller cable diameter. The smaller cable diameter means less cable joints and more cable length on one ship.

Yet another advantage of a lower voltage is that distances can now be achieved which are currently impossible without a landing point, because of power source limits (typically max. 22 kV) .

The reduction of the electrical resistance in submarine cables also results in the amplification of more pairs of optical fibers or channels per pair of optical fibers. This means a substantial increased data transfer capacity of the submarine optical fiber cable.

The invention involves a change in design or structure of an existing submarine optical fiber cable. This change in design, however, can be obtained without any changes in the cable manufacturing process. The only modification involved is a change of type of steel wire (copper clad steel wire versus a non-coated steel wire) in the strander. The steel wires preferably have a carbon content of 0.80 % or more in order to obtain the desired final tensile strengths. Examples of carbon contents are 0.82 %, 0.86 %, 0.92 % and even 1 .10 %.

At least one of said steel wires has less than 0.80 % of an element chosen amongst the group consisting of chromium, vanadium, nickel, copper, boron, molybdenum, cobalt, zirconium, titanium and wolfram, or less than 0.80% of a combination of the elements of the group. Such micro-alloying elements are preferably added to the steel composition in order to decrease the level of deformation required to obtain a predetermined final tensile strength. A suitable example of an element is chromium present in an amount of about 0.20 %.

These steel compositions belong to the prior art as such at least since

1988 and are disclosed e.g. in JP-A-63-192846. The existent applications of these steel compositions are in the field of high strength springs and of high-tensile steel cord. Up to now no attempts have been made to clad these high-carbon steels with copper. Although no reason can be found in the literature for the lack of attempts in this direction, one reason might be the fear for formation of martensite spots during the welding of the copper strip and the resulting fractures during the subsequent drawing steps. The inventors have discovered that martensite spots can be avoided by avoiding heat treatments during the cladding operation.

Preferably the copper clad steel wires have a tensile strength of more than 1800 MPa, e.g. greater than 2000 MPa, e.g. ; greater than 2200 MPa, calculated over the whole cross-section, the copper cross- section included.

The steel cores in the copper clad steel wires preferably have a round cross-section.

Preferably two or more layers of steel wire surround the buffer tube. Most preferably the copper clad steel wires have a tensile strength of at least 1800 MPa, calculated over the whole cross-section, copper cross-section being included. The thickness of the copper in these copper clad steel wires is preferably at least 3% of the copper clad steel wire diameter, yielding a composite conductivity of 21 % I ACS, in order to provide for the necessary electrical conductivity. For a composite conductivity of 30% IACS, the copper thickness is

6.5% of the steel wire diameter.

For a composite conductivity of 40% IACS, the copper thickness is 10% of the steel wire diameter.

In a preferable embodiment of the present invention, the copper clad steel wires in the submarine cable are in a cold deformed state so that the copper clad steel wires have a tensile strength of more than

2700 - C x 27 MPa preferably more than 3000 - C x 30 MPa and most preferably more than

3150 - C x 31 .5 MPa as measured over its whole cross-section, the copper section included. C is the electrical conductivity expressed in percentage of IACS (=

International Annealed Copper Standard) . The above formulae are valid in a range of 15% IACS to 70% IACS, e.g. in a range of 20% IACS to 70% IACS.

Examples of copper clad steel wires in this preferable embodiment of a submarine optical fiber cable are : tensile strength R_m greater than 2400 MPa and an electrical conductivity of 21 % IACS ; tensile strength R_m greater than 2100 MPa and an electrical conductivity of 30% IACS ; tensile strength R_m greater than 1800 MPa and an electrical conductivity 40% IACS. Brief description of the drawings.

The invention will now be described into more detail with reference to the accompanying drawings wherein

FIGURE 1 shows a cross-section of a prior art submarine cable ;

FIGURE 2 shows a cross-section of an invention submarine cable.

Description of the preferred embodiments of the invention.

FIGURE 1 shows a cross-section of a prior art submarine cable 10. Pairs of optical fibers 12 are located in the center. The pairs of optical fibers 12 are surrounded by a buffer tube 14, which can be made of steel. Two layers of steel wires 16, 18 surround the buffer tube 14. A first layer of steel wires 16 with an equal diameter and a second layer of steel wires 18 where a smaller diameter alternates with a greater diameter. A copper strip 20 is deformed around the second layer of steel wires 18 and forms together with the steel wires 16, 18 an annular structure. A dielectric layer 22 for electric insulation surrounds the annular structure. Other layers of steel (armoring) wires may be provided around the dielectric layer 22 (not shown).

FIGURE 2 shows a cross-section of an invention submarine cable 30. The pairs of optical fibers 32 are located in the center and are surrounded by a buffer tube 34 out of steel. Two layers of copper clad steel wires 36, 38 are stranded around the buffer tube 34. An additional copper strip 40 is deformed around the two layers of copper clad steel wires 36, 38. Due to the plastic deformation the copper of the strip 40 has flown together with the copper on the copper clad steel wires, filling the voids between the wires. In this way a contiguous annular steel copper structure is formed around the buffer tube 32.

Not all steel wires of the first and second layer need to be copper clad. Depending upon the desired combination of electrical and mechanical properties one may decide to copper clad only part of the steel wires. As mentioned hereabove, apart from the choice of the type of steel wires, nothing has changed in the cable production process.

Another way to tune the final properties of the submarine optical fiber cable is to vary the thickness of the copper on the steel wires.

A copper clad steel wire as used in the invention submarine cable can be manufactured as follows.

Starting product is a wire rod with a diameter of about 1 1 .0 mm. The wire rod composition comprises a carbon content ranging from

0.80 % to 1 .10 %, a silicon content ranging from 0.10 % to 2.0 %, a manganese content ranging from 0.10 to 1 .60 %, and a chromium content ranging from 0.05 % to 0.80 %, the remainder is iron and unavoidable impurities. For example, the carbon content may be 0.92 %, the silicon content 0.20 %, the manganese content 0.30 %, the chromium content 0.20 %, a nickel content of 0.02 %, an aluminum content of 0.001 % and nitrogen quantities ranging from 24 PPM to 30 PPM. Sulfur and phosphorous are present in amounts not exceeding 0.03 %. The wire rod is subjected to a patenting treatment and to a cold drawing deformation until a diameter of about 9.0 mm. The cold drawn wire is cleaned, however without using a heat treatment therefor, in order to avoid martensite spots. Thereafter, a copper strip is cold clad around the drawn steel wire until a diameter of about 9.50 mm. This is done by cleaning the copper strip, by cutting it to its dimensions and by forming a tube around the steel wire. The copper strip is then welded to the steel wire. The steel wire with the copper strip is then cooled and rinsed, dried and the copper strip is subsequently closed around the steel wire.

The thus clad steel is then subjected to a series of drawing steps until a final diameter is reached. Depending on the amount of deformation, intermediate patenting steps may be applied to facilitate the final drawing steps and to reach to proper wire properties. In order to obtain 21 % IACS a copper layer thickness of nominal 3% of the wire diameter is required, for 30% IACS a copper layer thickness of nominal 6.5% is required and for 10% IACS a copper layer thickness of nominal 10% is required.

As a matter of a first example only, in comparison with a prior art cable electrical resistance of 1 .0 Ohm/km, it has been possible to decrease the electrical resistance with 20% with only a weight increase of 1 % for 21 % IACS, to decrease the electrical resistance with 28% with only a weight increase of 2.1 % for 30% IACS and to decrease the electrical resistance with 37% with only a weight increase of 3.2% for 40% IACS.

As a matter of a second example only, in comparison with a prior art cable resistance of 1 .6 Ohm/km, it has been possible to decrease the resistance to 1 .07 Ohm/km with only a weight increase of 1 .3% for 21 % IACS, to decrease the resistance to 0.89 Ohm/km with only a weight increase of 2.7% for 30% IACS and to decrease the resistance to 0.73 Ohm/km with only a weight increase of 3.4% for 40% IACS.

Generally, it has been possible to decrease the electrical resistance of a submarine optical fiber cable by 20% to 50% or even more, e.g. by 30% to 50%, with only a weight increase of 1 % to 5%, e.g. of 3% to 4%.

Claims

1. A submarine optical fiber cable comprising pairs of optical fibers, a buffer tube surrounding said pairs of optical fibers, one or more layers of steel wires surrounding said tube, a dielectric coating surrounding said one or more layers of steel wires, characterized in that at least one of said steel wires is a copper clad steel wire.

2. A submarine optical fiber cable according to claim 1 , wherein all said steel wires are copper clad steel wires.

3. A submarine optical fiber cable according to any one of the preceding claims, wherein at least one of said steel wires has a carbon content of more than 0.80 %.

4. A submarine optical fiber cable according to any one of the preceding claims, wherein at least one of said steel wires has less than 0.80 % of an element chosen amongst the group consisting of chromium, vanadium, nickel, copper, boron, molybdenum, cobalt, zirconium, titanium and wolfram, or a combination of the elements of the group.

5. A submarine optical fiber cable according to any one of the preceding claims, wherein said copper clad steel wires have a tensile strength of more than 1800 MPa, calculated over the whole copper clad steel wire cross-section, copper cross-section being included.

6. A submarine optical fiber cable according to any one of the preceding claims, wherein said copper clad steel wires have a copper thickness of 3 % of the wire diameter, yielding a composite wire conductivity of 21 % IACS.

7. A submarine optical fiber cable according to any one of the preceding claims, wherein said steel wires have a round cross-section.

8. A submarine optical fiber cable according to any one of the preceding claims, wherein two layers of steel wire surround said buffer tube.

9. A submarine optical fiber cable according to any one of the preceding claims, wherein said copper clad steel wires have a tensile strength of at least 1800 MPa, calculated over the whole cross-section, copper cross-section being included, and wherein said copper clad steel wires have a copper thickness of 3 % of the wire radius, yielding a composite wire conductivity of 21 % IACS.

10. A submarine optical fiber cable according to claim 1 , wherein said copper clad steel wires are in a cold deformed state so that they have a tensile strength of more than 2700 - C x 27 MPa as measured over the whole cross-section of the steel wire, and where C is the electrical conductivity expressed in percentage of IACS.

1 1 . A submarine optical fiber cable according to claim 10, wherein said copper clad steel wires have a tensile strength of more than 3000 - C x 30 MPa as measured over the whole cross-section of the steel wire.

12. A submarine optical fiber cable according to claim 11 , wherein said copper clad steel wires have a tensile strength of more than 3150 - C x 31 .5 MPa as measured over the whole cross-section of the steel wire.