GB2178185A

GB2178185A - Optical fibre cable

Info

Publication number: GB2178185A
Application number: GB08518682A
Authority: GB
Inventors: Graham Kenneth Thornton
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-07-24
Filing date: 1985-07-24
Publication date: 1987-02-04
Anticipated expiration: 2005-07-24
Also published as: IT8621167A1; IT8621167A0; IT1197780B; GB8518682D0; FR2585851B1; GB2178185B; NO862947D0; FR2585851A1; NO862947L

Abstract

A well logging optical fibre cable comprises an optical fibre (F) housed in a longitudinally welded stainless tube (T) which is a loose fit around the fibre and in which there is a buoyancy fluid (O) which enables the fibre to more or less float in the tube and thus protects the fibre from damage, particularly when held vertically in lengths up to 5 or even 10Km in a well logging environment. Fig. 2 shows a method of temperature and pressure detection at the remote end of the fibre using moveable mirrors M, M' connected respectively to diaphragm D and temperature sensitive bar J. <IMAGE>

Description

SPECIFICATION Optical fibre cable This invention relates to optical fibre cable particularly, but not exclusively, for use in oil well logging instrumentation.

When drilling a new oil well it is necessary to sense pressures and temperatures at depths typically 5Km. At this depth temperatures can be in the region of 400"C and pressures can be in the region of 600 bar. Current techniques for measuring these temperatures and pressures use electronic circuits and semiconductor materials which can only be exposed to this type of environment for very short periods of time. It is therefore only possible to take a sample of pressure and temperature and then retract the equipment. It is not normally possible to leave the equipment in that environment for any length of time to take measurements over a period of time.

It is an object of the present invention to provide a data transmission system which can take remote measurements in environments of high temperatures and pressures, such as those discussed above, for a significant period of time.

According to one aspect of the present invention a data transmission system comprises an optical fibre housed within an hermetically sealed strain member tube and a buoyancy fluid within the tube for supporting the optical fibre, so that with the cable supported vertically the optical fibre will be protected against undue stress by its buoyancy within the surrounding buoyancy fluid.

Preferably a high pressure gland seals the fibre at least at the intended lower end.

Conveniently the system can have a pressure sensor which will modulate the light reflected back along the optical fibre or along a second return fibre, in proportion to the pressure being sensed. Instead of, or in addition, the system can have a temperature sensor which will modulate light reflected back along the fibre or along a separate return fibre in proportion to the temperature being sensed.

Preferably the or each sensor incorporates a reflector onto which light is focussed by a light focussing arrangement and the reflector is moved into or out of focus dependent upon the temperature or pressure being sensed.

In order that the invention can be clearly understood reference will now be made to the accompanying drawings in which: Figure 1 shows schematically a data transmission system for well logging according to an embodiment of the present invention and Figure 2 shows a sensor arrangement of Fig. 1 in greater detail.

Referring to Fig. 1 an optical fibre F (there could be more than one) is housed in a longitudinally-welded stainless steel tube T which is a loose fit around the fibre and is connected to a housing H which contains a ferrule G and a lens L to produce a complete, terminated cable assembly. The tube T has been laser-welded (or otherwise hermetically-sealed) and the tube is clamped using vacuum tight metal to metal (or other) seals S.

The whole system is liquid-filled, for example, with a silicone oil 0, of a prescribed density to match the density of the optical fibre and its primary coating. In this embodiment the preferred primary coating is silicone rubber.

The tube T is made from a material whose expansion co-efficient is closely matched to that of the optical fibre. This has the effect of minimising thermally-induced strain in the fibre when the whole assembly is placed in a environment where the temperature varies. One form of tube we prefer is a laser-welded stainless steel tube marketed by the company Armortech as their "K-tube". Preferably the tube T, the housing H and the ferrule G are made from "lnvar", which is an alloy of nickel. However there are other nickel alloys which can be selected to match a given glass fibre thermal expansion co-efficient, and which are in current use for making glass-to-metal seals.

An important feature of the system is the silicone oil filling medium. This has a number of purposes: 1. To render the optical fibre neutrally buoyant, through a correct choice of oil and thickness of primary coating on the fibre, when the cable assembly is hanging vertically, for example.

2. To transmit stress evenly throughout the length of the fibre, over and above any hydrostatic pressure-induced stresses.

3. The tube T can support a crushing pressure defferential in the region of 600 bar when a suitable diameter and wall thickness is chosen. The liquid ensures that this differential pressure is not transmitted to the fibre which experiences only the hydrostatic pressure due to the column of silicone oil.

4. During winching and handling of the cable compressive stresses are relieved by fluid movement.

All these characteristics contrasted with the situation where the fibre might be surrounded by a polymer medium. In such a.case stresses due to mechanical loading or thermal stressing of the outer tube T could only be relieved by "creep" of the polymer relative to the glass fibre. This could lead to excessive crushing stress on the fibre or shear stressed caused by movement axially of the polymer.

In manufacturing the system shown in Fig. 1 it is proposed to fill the tube T by using a pressurised feed of silicone oil into one end of the tube and fibre assembly, with a bleed vent at the other end. It is possible to fill a 10Km length of cable in less than a few hours.

When the cable is hung vertically, in an environment where the external temperature and pressure are elevated above normal ambient conditions, such as in oil well logging, the result of the encapsulation technique is: a. differential thermal expansion and the consequent mechanical strain is minimised by choosing a tube material with a thermal expansion co-efficient which matches that of the glass fibre; b. the silicone oil is selected to have a specific gravity which matches that of the glass fibre plus its primary coating. The fibre thus "floats" in the filling medium and the fibre need not support its own weight when hung vertically; c. to provide a means of stress relief when the cable is winched under load. Any compressive loading of the tube T is transmitted into the fluid which can only sustain the local hydrostatic pressure.

Reference will now be made to Fig. 2 of the drawings. Referring to Fig. 2 there is shown an extension of the housing H of Fig. 1. In this extension H' there is located a diaphragm D and a mirror M attached to the diaphragm D. The diaphragm is secured around its periphery between flanges J and K. The internal cavity of housing extension H' is filled with gas at normal ambient pressure and when there is a pressure differential across the diaphragm D then the diaphragm will flex and move the mirror M from the focus of the light beam B from the lens L. Thus light reflected back into the lens will be less as the mirror becomes more and more defocussed as the pressure externally of the housing increases.

With suitable callibration is it thus possible to get an absolute pressure measurement.

In addition to the pressure sensor there is a temperature sensor comprising a temperature sensitive bar N which has a significant linear temperature co-efficient of expansion and carries at its end a mirror M' arranged to receive a light beam B' focussed on the mirror M' by a second lens L', itself coupled to a second optical fibre F' which is encapsulated in a metal tube in exactly the same way as the fibre previously described. Thus there would be two or more such fibres each individually encapsulated in a hermetic metal tube.

It is proposed that the individual encapsulated fibres are incorporated into strain cables or electro-optic cables.

In the embodiment described the diameter of the tube T would be of the order of imam.

Typical primary coated fibres have DO of 250-350 microns. 250 microns is normal for 50/125 micron multimide fibre.

The encapsulating arrangement described reduces induced strain from external pressure, external changes in temperature, (especially elevated temperatures up to about 400"C), tensile loads when the optical fibre is hung vertically, in lengths up to and exceeding 5Km, bending during winching of a cabled length of the optical fibre, and compression during winching of a cabled length of the optical fibre.

As described the high temperature capability of this cabled fibre can be exploited by using passive optical sensors which do not rely on electronic circuits and semiconductor materials for their operation.

Claims

1. A data transmission system comprises an optical fibre housed within an hermetically sealed strain member tube and a buoyancy fluid within the tube for supporting the optical fibre, so that with the cable supported vertically the optical fibre will be protected against undue stress by its buoyancy within the surrounding buoyancy fluid.