GB2106264A

GB2106264A - Mineral insulated electric cable containing an optical fibre

Info

Publication number: GB2106264A
Application number: GB08225159A
Authority: GB
Inventors: William Mcneil; Edward Lewis Ross Gower
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1981-09-04
Filing date: 1982-09-03
Publication date: 1983-04-07
Also published as: GB2106264B

Abstract

In a mineral 4 insulated electric cable, the inner conductor 1, or one of the inner conductors, has a bore 2 extending throughout its length and has, loosely housed in the bore, an optical fibre 3 and/or an optical bundle. The cable is especially suitable for use in oil wells where the cable has to be capable of withstanding severe mechanical abuse and a communications link is required. 5 is an outer metal sheath. <IMAGE>

Description

SPECIFICATION An improved mineral insulated electric cable This invention relates to mineral insulated electric cables, that is to say cables of the kind comprising at least one conductor insulated from a surrounding metal sheath, and where there is more than one conductor from the other conductor or conductors, by compacted mineral insulating powder, usually magnesium oxide.

Mineral insulated electric cables have many important advantages arising from their unique construction. Perhaps the most important advantage is the performance of mineral insulated electric cables at elevated temperatures. With a metal sheath and a dielectric of compacted mineral insulating powder, the cable is Incombustible and can survive fires, very often to remain in working condition. Mineral insulated e,estr:c cables can withstand severe mechanical abuse, such as bending, twisting and even flattening to somme extent without unacceptable detsrioration of their electricai properties; they can there,tore be used under mechanically hazardous conditions without additional protection.Furthermore, with a choice of sheath materials from copper, chrome-nickel, stainless steel and inconel, mineral insulated electric cables can be made to resist a wide range of corrosive conditions.

It is an object of the present invention to provide an improved mineral insulated electric cable which, in addition to having all the advantages of mineral insulated electric cables hitherto proposed and used, provides at least one optical guide for the transmission of light.

According to the invention we provide an improved mineral insulated electric cable comprising at least one conductor insulated from a surrounding metal sheath, and where there is more than one conductor from the other conductor or conductors, by compacted mineral insulating powder, wherein the conductor or at least one of the conductors has a bore extending throughout the length of the conductor and has, loosely housed in the bore, at least one separate optical fibre and/or at least one optical bundle as hereinafter defined.

By virtue of being housed loosely in a bore extending lengthwise in the or a conductor, limited relative movement between the or each separate optical fibre and/or the or each optical bundle and the conductor can take place when the cable is bent or twisted.

By the expression "optical bundle" is meant a group of optical fibres or a group of fibres including one or more than one optical fibre and one or more than one non-optical reinforcing fibre or other reinforcing elongate element.

Light that can be transmitted along the or each optical fibre and/or optical bundle may be the ultra-violet, visible and infra-red regions of the electromagnetic spectrum but, preferably, the or each optical fibre and/or optical bundle is suitable for use in the communications field adapted for transmission of light having a wavelength within the range 0.8 to 2.1 micrometres.

The or each optical fibre and/or the or each optical bundle may have a continuous abrasionresistant coating of a hardened material, e.g.

enamel or an aluminium-based alloy, or may have a continuous coating of a buffer material, which material may be a hardened material or a soft material e.g. cellular plastics material. In some cases a continuous abrasion-resistant coating of a hardened material may overlie a continuous coating of a soft buffer material. It will be appreciated that the material selected for the abrasion-resistant coating or for the buffer coating will depend on the environment in which the mineral insulated electric cable is to be used.

In some circumstances, the bore of the or each conductor may be filled throughout the length of the conductor with a greasy filling medium which will permit relative sliding movement between the separate optical fibre or fibres and/or optical bundle or bundles and the conductor and which may be a water-impermeable medium. When a greasy filling medium is employed, it preferably consists of, or comprises as a major constituent, petroleum jelly.

The mineral insulated electric cable of the present invention may be manufactured- by any convenient method provided that appropriate precautions are taken to avoid damage to the or each separate optical fibre and/oropticalbundle.

Two methods of manufacture that may be employed are the method described in Patent Specification No. 937347 and the method described and claimed in the Complete Specification of Patent No. 1 554860 and, in each of these methods, the conductor or at least one of the conductors that is surrounded by a metal tube constituting the sheath tube will be of tubular form and will have at least one separate optical fibre and/or at least one optical bundle loosely housed in the bore of the conductor before the sheath tube is reduced in diameter to form a mineral insulated cable.

Preferably, in each of these methods, prior to introducing the or each separate optical fibre and/or optical bundle into the bore of the or a conductor, the internal surface of the tubular conductor may be maintained in a relatively smooth condition during initial processing of the tubular conductor by filling it with a meltable solid material, e.g. sodium hyposulphide.

Another method that may be employed is described and claimed in the Complete Specification of Patent No. 1 554859 and in this method at least one separate optical fibre and/or optical bundle is introduced into the bore of a tubular conductor as the conductor is being formed by causing at least one tape of metal or metal alloy to travel in the direction of its length; folding the advancing tape or tapes transversely or otherwise arranging the tape or tapes in such a way that adjacent longitudinal edges of the tape or tapes butt or overlap to form a tube; and feeding into the tube as it is being formed at least one separate optical fibre and/or at least one optical bundle, the relative cross-sectional sizes of the tube and the optical fibre or fibres and/or the optical bundle or bundles being such that the optical fibre or fibres and/or the optical bundle or bundles is or are loosely housed in the tube. The tube so formed may then be fed directly into the sheath tube during manufacture of the mineral insulated electric cable by the method of Patent No. 1 554859.

The invention is further illustrated by a description, by way of example, of a preferred mineral insulated electric cable with reference to the accompanying diagrammatic drawing which shows a transverse cross-sectional view of the cable, drawn on an enlarged scale.

Referring to the drawing, the mineral insulated electric cable comprises a central tubular electric conductor 1, an outer metal sheath 5 and, separating the conductor from and electrically insulating it with respect to the sheath, a body 4 of compacted magnesium oxide powder. The tubular conductor 1 has a bore 2 in which is loosely housed an optical fibre 3 which is of a length substantially greater than that of the bore of the conductor.

The mineral insulated electric cable of the present invention is especially suitable for use in oil wells where the cable has to be capable of withstanding severe mechanical abuse and a communications link is required.

Claims

1. A mineral insulated electric cable comprising at least one conductor insulated from a surrounding metal sheath, and where there is more than one conductor from the other conductor or conductors, by compacted insulating powder, wherein the conductor or at least one of the conductors has a bore extending throughout the length of the conductor and has, loosely housed in the bore, at least one separate optical fibre and/or at least one optical bundle as hereinbefore defined.

2. A mineral insulated electric cable as claimed in Claim 1, wherein the or each optical fibre and/or the or each optical bundle has a continuous abrasion-resistant coating of a hardened material.

3. A mineral insulated electric cable as claimed in Claim 1, wherein the or each optical fibre and/or the or each optical bundle has a continuous coating of a buffer material.

4. A mineral insulated electric cable as claimed in Claim 3, wherein the continuous coating of a buffer material on the or each optical fibre and/or the or each optical bundle is a soft material and a continuous abrasion-resistant coating of a hardened material overlies the continuous coating of a soft buffer material.

5. A mineral insulated electric cable as claimed in any one of the preceding Claims, wherein the bore of the or each conductor is filled throughout the length of the conductor with a greasy filling medium which will permit relative sliding movement between the separate optical fibre or fibres and/or optical bundle or bundles and the conductor.

6. A mineral insulated electric cable as claimed in Claim 5, wherein the greasy filling medium is a water-impermeable medium.

7. A mineral insulated electric cable as claimed in any one of the preceding Claims, wherein the or each separate optical fibre and/or the or each optical bundle is of a length substantially greater than that of the bore of the conductor in which it is loosely housed.

8. A mineral insulated electric cable substantially as hereinbefore described with reference to and as shown in the accompanying drawing.