GB2165961A

GB2165961A - Submarine optical fibre cable

Info

Publication number: GB2165961A
Application number: GB08525695A
Authority: GB
Inventors: Osamu Kawata; Nobuyuki Yoshizawa; Kiyoshi Funaki; Masami Inoue
Original assignee: OCC Corp; Nippon Telegraph and Telephone Corp
Current assignee: OCC Corp; Nippon Telegraph and Telephone Corp
Priority date: 1984-10-18
Filing date: 1985-10-18
Publication date: 1986-04-23
Also published as: GB8525695D0; GB2165961B; JPS6197607A

Abstract

The cable comprises an optical fibre unit which is formed by an optical fibre unit 2 including optical fibre cores 22. This unit is inside an external force resisting sheath 3 comprising a water pressure resisting inner metal tube 31, tension wires 32 and an outer metal tube 33. The sheath is in turn inside a water-proof jacket 4. The material and the radial thickness of the water pressure resisting inner metal tube 31 are selected so that only a part of the water pressure applied onto the cable is supported by the inner metal tube 31, while the remaining part of the water pressure is supported by the tension wires 32. Tension wires 32 are wound on the outer surface of the inner tube and arranged, in a cross section, side by side on a concentric circle so that adjacent wires 32 are in contact with one another. The reaction forces at the contact portions balance with the remaining part of the water pressure. <IMAGE>

Description

SPECIFICATION Submarine optical fibre cable The present invention relates to improvements in submarine optical fibre cables.

Attempts have been made to use optical fibres as transmission media in submarine cables. An already-proposed submarine optical fibre cable includes an optical fibre unit comprising at least one optical fibre core and a tension member which are covered with a buffer layer of an insulating material. In order to protect the optical fibre unit from external forces such as water pressure and tension, the optical fibre unit is enclosed in an external force resisting sheath, which is sometimes referred to as a pressure-resisting sheath. The sheath comprises an inner metal tube as a water-pressure resisting element, and at least one tension wire layer for withstanding tension applied to the cable. The tension wire layer is formed around the waterpressure resisting inner tube. An outer tube encloses the tension wire layer. The outer tube is inside a water-proof outer jacket.

According to a conventional design technique, the material and the radial thickness of the water pressure resisting inner metal tube are so selected that the tube can sufficiently resist water pressure at the maximum water depth of the sea or ocean where the cable is intended to be laid. This means that the material for the inner tube is limited to ones having a yield strength of a certain value or more, as described hereinafter. Furthermore, the radial thickness of the tube must be increased in case that the selected material has a comparatively low yield strength.

These result in the large weight and/or diameter of the cable and in difficulty of the cable production.

Our various examinations showed that only a part, or about 60%, of water pressure applied to the water-pressure resisting inner metal tube, the remaining part of the water pressure being supported by, mainly, the tension wire layer. In this connection, the tension wire layer should be formed so that a plurality of tension wires are arranged around the inner tube, in a cross section, with adjacent ones being in contact with one another. A part of the water pressure is balanced with the reaction forces acting on the contact portions, and the remaining portion of the water pressure is only applied to the water pressure resisting inner metal tube. This means that materials having a lower yield strength can be used as the inner metal tube and/or that the radial thickness of the tube can be reduced in comparison with the prior art.

The present invention is based on our abovedescribed novel knowledge.

According to the present invention, a submarine optical fibre cable is obtained which comprises an optical fibre unit inside a pressure resisting metal tube which is in turn inside an outer enclosure including at least one layer of a plurality of tension wires, which is characterized in that said a plurality of tension wires are wound on the outer surface of said pressure resisting tube and arranged, in the cross section, on a concentric circle so that adjacent ones are in contact with one another, whereby a part of water pressure applied to the cable is balanced with reaction forces at the contact portions between adjacent wires, said pressure-resisting tube being formed with a material and a radial thickness so that said pressure resisting tube can stand only the substantially remaining part of the water pressure.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which: Figures 1-3 are cross-sectional views of the different known submarine optical fibre cables; Figure 4 is a cross-sectional view of a pressure-resisting tube; Figure 5 is a partial sectional view of an external force resisting sheath for explaining a manner of tension wire layers standing a part of water pressure; Figure 6 is a cross-sectional view of a submarine optical fibre according to an embodiment of the present invention; and Figure 7 is a cross sectional view of a submarine optical fibre according to another embodiment.

According to Fig. 1, a known submarine optical fibre cable 1 comprises an optical fibre unit 2 inside an external force resisting sheath 3 as described hereinbefore. The sheath 3 is inside a water-proof and electric insulating jacket 4 composed of at least one layer (two layers 41 and 42 being shown) of, for example, a plastic resin material such as polyethylene.

The optical fibre unit 2 includes a central tension member 21, one or more optical fibre cores 22 (six cores being shown) and a buffer layer 23. The tension member 21 is made of, for example, a steel wire. Each optical fibre core 22 is an optical fibre element coated with a thin plastic resin layer and is stranded along the tension member 21. The buffer layer 23 holds the optical fibre cores 22 and the tension member 21 together and covers them. The buffer layer 23 is formed by winding, for example, polyprophelene yarns, or by moulding a plastic resin, for example, silicone rubber.

The external force resisting sheath 3 is composed of an inner metal tube 31, tension wires 32 such as steel wires (two layers of wire being shown) wound on the inner tube 31, and an outer metal tube 33 surrounding the tension wires 32.

Referring to Fig. 2, another known submarine optical fibre cable shown therein has an arrangement similar to that of Fig. 1, except that the tension member (21 in Fig. 1) is omitted and that the jacket is composed of a single layer. The similar parts are represented by the same reference numerals as Fig. 1, and the detailed description is omitted.

Referring to Fig. 3, a still another known submarine optical fibre cable is also similar to that of Fig. 1, except that a single tension wire layer 32 is used and that the inner tube 31 is formed of an assembly of three special segments each having a fan-shape cross section.

In any one of the above-described known cables, the inner tube 31 is designed to resist the entirety of water pressure at the maximum water depth where the cable is intended to be laid, while the tension wires 32 is for resisting tensile force applied to the cable. The outer metal tube 33 is a cover of the tension wire layer and is not intended to stand any external force.

A conventional manner for designing the inner tube 31 will be described referring to Fig. 4.

Providing that water pressure is represented by Pe at a water depth where the cable is intended to be laid, the entire water pressure Pe applies to the inner tube 31, as shown in Fig.

4.

The inner diameter 2 r of inner tube 31 is determined by the outer diameter of the given optical fibre unit 1. When a material having a yield strength of Ao is selected for the inner tube 31, the radial thickness t of the inner tube is determined by the following equation (1) for resisting the water pressure Pe, as is known in the prior art:

where K is a safety factor, and usually 1.5.

In an example where the cable is laid at a water depth of 8000 m while the outer diameter of the optical fibre unit 2 being 2.8 mm, Pie=8 kg/mm2 and the inner diameter 2-r=3.0 mm taking into consideration a clearance of 0.1 mm between the inner surface of the inner tube and the outer surface of the optical fibre unit. Under the condition, various values of the radial thickness t are given in the following Table 1 in response to variation of A of the used material.

Table 1

A0(kg/mm2) 10 15 20 25 30 5 40 45 50 t(mm) - - - 6.0 1.85 1.18 0.87 0.70 0.58 When 'is is 24 (kg/mm2) or less, the denominator (L,,-2-KPe) in the radical sign (ç) in equation (1) becomes zero (0) or negative. Therefore, material having the yield strength i.0 of 24 (kg/mm2) or less can not be used for the inner tube 21.

As the yield strength, a tensile strength of material is usually used in the prior art.

A series of aluminum alloys of &num;5000 order in JIS (Japanese Industrial Standards) H4000 have a tensile strength of 25-30 kg/mm2. Therefore, those aluminum alloys can be used for the inner tube. However, the radial thickness is 1.85-6 mm and is rather thick as seen in Table 1, so that the inner tube can be hardly made from a piece of flat plate or band of such an aluminum alloy by a tube forming machine. Therefore, the inner tube 31 must be formed as an assembly of a plurality of special-shaped aluminum alloy segments as shown in Fig. 3. This results in difficulty of production and an increased cost.

When a material having a higher tensile strength of, for example, 35-45 kg/mm2 is selected, the radial thickness is reduced to be 0.70-1.18 mm as shown in Table 1. However, the material having the high tensile strength is hard copper or iron. Since either hard copper or iron is large in density in comparison with aluminum alloy, a cable modulus (a ratio of a cable breaking strength to a cable weight in water) can not be made large, so that the optical fibre core failure probability increases.

In order to solve the problems in the known submarine optical fibre cables, we tried various examinations and know that a part of the water pressure applied to the cable was supported, borne or stood by the tension wire layer. Therefore, the inner tube is not necessary to resist the entire water pressure. That is, if each tension wire layer is formed so that a plurality of tension wires are arranged, in a cross section, side by side around the inner tube and are in contact with one another, reaction forces acting on the wires at the contact portions balance with a part of the water pressure.

Referring to Fig. 5, since water pressure Pe applies on the entire surface of the cable uniformly, all wires 32 receive radial inward forces. Accordingly, each wire 32 pushes adjacent opposite wires 32 at contact portions P and therefore, receives reaction forces F, from the opposite wires. The reaction forces F, are directed to the center 0 of the wire and the resultant force F2 of the two reaction forces F, directs in a radial outward direction of the cable. Thus, a part of the water pressure Pe is balanced with the resultant force F2 of reaction forces F,.

Accordingly, only the remaining part of the water pressure Pe is applied to the inner tube 31.

This was confirmed by the finite element method (FEM).

The remaining part is about 60% of the water pressure Pe in the case that two tension wire layers are used. For use of a single tension wire layer, it increases and is about 63% of water pressure Pe.

The present invention is based on the novel knowledge.

Referring to Fig. 6, an optical fibre cable 1 according to an embodiment has a general arrangement similar to Fig. 1. The similar parts are represented by the same references as Fig. 1 and description of them is omitted for the purpose of simplification of the description.

The embodiment shown in Fig. 6 is different from the prior art in Fig. 1 in two points. That is, it is necessary that each tension wire layer is formed so that each one tension wire 32 is in contact with other wires adjacent thereto as shown in Fig. 5. Furthermore, the pressure resistance of the inner tube 31 can be reduced in comparison with the prior art of Fig. 1.

As described above referring to Fig. 5, a part of water pressure Pe applied to the cable is supported by the tension wire layer.

In an example where the water depth is 8000 m and r= 1.5 mm, similar to the abovedescribed example, the pressure applied to the inner tube is about 8 kg/mm2X0.6=4.8 kg/mm2.

Thus, providing that Pe in equation (1) is 5 kg/mm2, various values of the radial thickness t of the inner tube were calculated for various values of A(,. The calculated data are shown in Table 2.

Table 2

A0(kg/mm2) | 10 |15 15 20 25 30 35 40 t(mm) - - 1.5 0.87 0.62 0.48 0.40 In this case, materials having A" of 15 (kg/mm2) or less can not be used for the inner tube because the denominator (AO-2 K-Pe) in the radical sign (ç) in equation (1) becomes zero or negative. However, materials having i." more than 15 (kg/mm7) can be used for the inner tube, and the radial thickness is smaller than the prior art. For example, aluminium alloy having i."=20-25 (kg/mm2) could not be used in the prior art, but it can be used according to the present invention.Furthermore, the radial thickness is 0.87-1.5 mm which is considerably small in comparison with the prior art wherein aluminum alloy of i.l,=25-30 (kg/mm7) is used.

Therefore, the inner tube can be made in a form of a pipe from a thin aluminum alloy tape by the use of a tube forming machine. Accordingly, the cable can be readily produced with a reduced production cost. Furthermore, since the aluminum alloy can be used, the cable modulus is small.

Use of copper of i."=35-40 (kg/mm2) can be also accepted that the radial thickness is considerably reduced in comparison with the prior art.

Referring to Fig. 7, another embodiment shown therein has a general arrangement similar to Fig. 6 except that the single tension wire layer and the single layer of the water-proof jacket are used. The similar parts are represented by the same reference numerals in Fig. 6.

The use of the single tension wire layer is supported by the use of aluminum alloy in place of copper for the outer tube 33 which results in the reduced weight. The radial thickness of the inner tube is designed to be larger than the embodiment of Fig. 6 under the same condition for the other factor because the part of water pressure supported by the single tension wire layer is slightly smaller than that supported by the dual layers as described above.

Claims

1. A submarine optical fibre cable having a given design water pressure, comprising an optical fibre unit inside a pressure-resisting metal tube which is in turn inside an outer enclosure including at least one layer of tension wires, the tension wires being wound on the outer surface of the pressure resisting tube and arranged, in cross section, on a concentric circle so that adjacent wires are in contact with one another, whereby a part of the water pressure applied to the cable is balanced with reaction forces at the contact points between adjacent tension wires, the pressure-resisting tube being formed of a material and with a radial thickness such that this tube can withstand only part of the design water pressure, whereas the tube and tension wires together can withstand the design water pressure.

2. A submarine optical fibre cable as claimed in claim 1, wherein the part of the design water pressure withstood by the tension wires is about 60% of the water pressure.

3. A submarine optical fibre cable as claimed in claim 1 or 2, wherein the pressure-resisting tube is formed of aluminium alloy metal.

4. A submarine optical fibre cable as claimed in claim 1 or 2, wherein the pressure-resisting tube is formed of copper metal.

5. A submarine optical fibre cable as claimed in any of claims 1-4, wherein the optical fibre unit comprises a tension wire, at least one optical fibre core and a buffer of an insulating material covering both the tension member and the optical fibre core.

6. A submarine optical fibre cable as claimed in any of claims 1 to 5, wherein the outer enclosure comprises the tension wire layer formed by the tension wires wound around the pressure-resisting tube, an outer metal tube around the tension wires, and a water-proof outer jacket around the outer metal tube.

7. A submarine optical fibre cable as claimed in claim 6, wherein the outer enclosure comprises an additional tension wire layer formed by tension wires wound around the first said tension wire layer.

8. A submarine optical fibre cable as claimed in any of claims 1 to 7, wherein the pressureresisting tube is a seam welded pipe formed from a metal tape by a tube former.