0001 The invention relates to a signal cable for transmission of optical signals in oil and gas wells, as appears from the preamble of patent claim 1.

BACKGROUND 0002 Signal cables are i.a. used in oil and gas wells for measurement of such parameters as pressure, temperature, tension, flow, and for fiber optic communication. The cables may have a length of 6 km or more for use in wells, and should tolerate temperatures of 200 C. and pressure over 1000 bar. In addition, the cables are vulnerable to corrosion because they are usually located in the liquid filled annulus between the casing and the tubing. 0003 NO patent-publication 175119 describes a cable for this application, where the fiber is inside a steel tube which protects it from the hydrostatic pressure which normally will be several hundred bar. The tube is stranded around a center element, and the fiber is able to move sidewise in the tube so that some elongation of the cable due to thermal expansion and/or mechanical tension, is possible without straining the fiber. 0004 For submarine cables, it is known that as a result of corrosion of, or at, the metal-containing cable, hydrogen is produced. The hydrogen diffuses into the optical fiber and results in attenuation of the light being transmitted. 0005 The attenuation can be permanent or reversible. Permanent losses are a result of a reaction between the hydrogen which diffuses into the optical fiber, and the silica glass, and the loss will generally increase with the hydrogen concentration, and with time. Reversible losses are the results of hydrogen molecules which diffuse into the optical fiber, but does not combine with the glass. The latter loss is proportional with the concentration of hydrogen. 0006 Representative partial pressures of hydrogen in submarine cables are about 1 bar. For submarine cables, one has tried to solve this problem by, for example, placing the optical fiber cable inside a copper tube in order to establish a barrier against diffusion of hydrogen into the optical fiber. 0007 In oil and gas wells it has surprisingly been found that considerable amounts of hydrogen are developed in the area outside and on the cable. Because of the high temperature prevailing in such wells, normally 100-200 C., the hydrogen will quickly penetrate into a steel tube which encapsulates the fiber, and diffuse into the optical fiber and cause losses. The partial pressure of hydrogen inside the pipe can be up to 100 bar. Even at much lower hydrogen pressures, the loss in the fiber will normally be so high that the cable is no longer usable for the intended purpose. 0008 The use of copper in such applications is, however, not practical, because of the high hydrostatic pressure to which the cable is exposed. Copper will further not be a particularly effective barrier against hydrogen at the high temperatures of interest.

OBJECT 0009 One object of the invention is to provide a signal cable for transmission of optical signals in oil and gas wells, said cable preventing or reducing the penetration of hydrogen into the fiber material. 0010 Another object of the invention is to provide such a device which also reduces the formation of hydrogen in the area around the fiber optical cable in oil/gas wells.

THE INVENTION 0011 The objectives are achieved with a cable according to the characterizing part of patent claim 1. Further advantageous features are evident from the dependent claims. 0012 The invention relates to a signal cable for transmission of optical signals in oil and gas wells, said cable comprising one or more optical fibers encapsulated in at the least one pressure and temperature resistant hollow and normally tube- or channel-shaped protection means, optionally placed by or integrated with a cable-construction, for example as described in NO patent publication 175119. 0013 According to the invention, the protection means is provided by a thin continuous surface coating of gold, or a gold-alloy which mainly comprises gold. 0014 According to the invention the surprising technical effect resulting from this new gold coating on fiber optic signal cables is firstly that the gold coating passivates the metal in the cover means resulting in a strong reduction in the formation of hydrogen on the surface of the cover means. Secondly, the solubility of hydrogen in gold is very low, and therefore the permeability rate for hydrogen in gold is very low. With a goldcoating on the cover means, for example a steel tube, with a few micrometers thickness, for example 1 -10 m, it is therefore possible to achieve a satisfactory and lasting signal transmission in the fiber optical cable for a period for several years, even with ambient hydrostatic pressure of several hundred bar, ambient temperature around 200 C., and in corrosive environments. 0015 Gold coating with such thickness can be established at the cover means with known methods, such as electroplating, on tube-lengths of several kilometers. 0016 The gold coating is preferably provided at the exterior surface of the cover means, both in order to passivate the cover material, and to establish a diffusion barrier against hydrogen. The gold coating may however, as an alternative or in addition, also be provided at the interior surface of the cover means, to establish a diffusion barrier against hydrogen. 0017 The signal cable according to the invention, may further comprise several cover means which encapsulate each other, for example concentric pipes or channels of steel, and one or more of the said surfaces of the concentric placed cover means, may be provided with gold coating. It is preferably the exterior surface of the cover means which is provided with a gold coating. This produces both a passivating effect and a barrier. A gold coating on the interior surface will only produce a barrier effect and besides be harder to apply than an exterior coating. 0018 The gold coated signal cable according to the invention, may be used in different cable constructions, for example, the one described in NO patent-publication 175119. In order to protect the gold coating on the signal cable according to the invention, during production, installation and operation, it would be advantageous to provide a thin protecting layer on the outside of the cover means, for example of a temperature resistant plastic, before the gold coated signal cable enters into the production process for the composite cable. Examples of applicable plastics are polyether keton (PEEK), perfluor alkoxy (PFA) and teflon. 0019 A plastic coating as described above, provided on the exterior surface of the cover means or at the exterior surface of the outermost cover means, may also have a passivating effect on the surface of the cover means and reduce local production of hydrogen. 0020 The thickness of the gold coating can be selected considering the range of use and signal cable construction. Multiple gold coatings demand less thickness per coating, and lower pressure/temperature in the surroundings also demands less thickness. Normally, the thickness of the coating should not be less than 2-3 micrometers in order to achieve a coating without pores. In the example below it is given an example of a calculation with a 3 micrometers thickness of the coating.

0021 In the enclosed drawings there are shown schematic embodiments of the invention, where 0022 FIG. 1 shows a schematic cross section of a first embodiment of a signal cable according to the invention, 0023 FIG. 2 shows a cross section corresponding to FIG. 1, of a second embodiment, and 0024 FIG. 3 shows a cross section corresponding to FIG. 1, of a third embodiment. 0025 FIG. 1 shows an optical fiber 1 in a tube-shaped cover means 2. The exterior surface of the cover means 2, er provided with a thin coating 3 of gold. 0026 FIG. 2 shows another embodiment where the optical fiber 1 is in a first tube-shaped cover means 2a, which again is encapsulated in a generally concentric second tube-shaped cover means 2b. In this embodiment both the exterior surfaces of the cover means 2a and 2b are provided with a gold coating 3a and 3b. 0027 FIG. 3 shows a third embodiment where the optical fiber 1 is in a tube-shaped cover means 2. The exterior surface of the cover means 2 is in this embodiment covered with a gold coating 3 and a plastic coating 4 in order to give the gold coating a mechanical protection.

EXAMPLE 0028 This calculation example is put forward in order to illustrate the barrier effect of gold, with a coating thickness of 3 micrometers as a function of temperature and time, in comparison with an unprotected steel tube. 0029 The table which is given below, presents the calculation results for an unprotected steel tube (Inconel 625) and a steel tube provided with a 3 micrometers thick gold coating after 1 and 5 years, at a partial pressure of hydrogen at 1 and 100 atmospheres at 100 and 200 C. It is assumed that the steel tube has an outer diameter of 1.8 mm, and an inner diameter of 1.4 mm. These are realistic dimensions for most applications. The hydrogen permeability for gold is about 1104 exp(15840/T)mol/cm sec.{square root}atm. The hydrogen permeability for Inconel 625 is about 8.16107 exp(7232/T)mol/cm sec.{square root}atm. T is the absolute temperature in K. 1TABLE 1Calculated pressure of hydrogen in a steel tube with and without a3 m thick gold coating.Pressure inside tubePressure inside tube withOuterwithout coatingAu-coatingTemppressure1 year5 year1 year5 year170 C.1114 1032 102atmatmatmatmatm170 C.1001001004 1022 101atmatmatmatmatm200 C.1114 1020.18atmatmatmatmatm200 C.1001001000.432.0atmatmatmatmatm

0030 As can be seen from the table, a gold-coating of 3 m produces a substantial reduction of the hydrogen pressure surrounding the optical fiber inside the steel-tube, even after 5 years. Without the gold coating the pressure inside the tube, will approach the outer pressure after only a few days. 0031 The invention thus provides an improved signal cable which makes it possible to transmit optical signals during a longer time period without substantial loss of signals, in oil and gas wells under high pressure and temperature and in surroundings with high partial pressure of hydrogen.