Abstract:
The present invention generally provides a hydrogen gettering agent containing a fullerene compound, a protected optical fiber cable containing the hydrogen gettering agent and a method of making the same. According to some embodiments, the protected optical fiber cable is provided comprising a protective sheath, at least one optical fiber positioned within the protective sheath, and the hydrogen gettering agent surrounding the at least one optical fiber within the protective sheath.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to fiber optic cables, and more specifically relates to hydrogen gettering agents for use with fiber optic cables subjected to harsh environments, and for other similar applications.  
         BACKGROUND OF THE INVENTION  
         [0002]    There is an increasing need for low toxicity, low flammability liquid hydrogen gettering agent that can be used, for example, to help fiber optic cables survive the harsh environment encountered in down-hole fiber optic sensing applications. Such fiber optic cable is used, for example, to interconnect a down-hole fiber optic sensor with instrumentation located at the surface of a well bore.  
           [0003]    Down-hole environmental conditions typically include high temperatures and high partial pressures of hydrogen. Both molecular (H 2 ) and atomic (H) hydrogen from various sources, such as environmental hydrogen sulfide (H 2 S) and the corrosion of metallic components, may be present in the down-hole environment. It is well known that hydrogen permeation into optical fibers can significantly decrease the power of light transmitted through the fibers. Therefore, the fibers in these cables are typically protected from hydrogen by various combinations of hydrogen barriers (such as gold, aluminum or copper layers) and hydrogen gettering agents.  
           [0004]    Typically, these gettering agents are metallic (Y, Pd, Zr) or metal catalyzed unsaturated organic molecules that are dissolved or suspended in a petroleum or synthetic gel. These gettering gels are typically deployed within the inner diameter of an inner cable sheath, in intimate contact with the optical fibers. The function of these agents is to scavenge any hydrogen passing through the hydrogen barrier layer(s). Liquid gels are typically preferred to solids because they are easier to deploy within a cable. A shortcoming associated with existing gettering gels is their limited stability and H 2  scavenging density. A common gettering gel, sold under the trade name Sepigel (available from Seppic Corp. of Fairfield, N.J.), for example, will scavenge roughly three cubic centimeters (cc) of H 2  per gram at standard temperature and pressure, or 0.1348 grams-mole per kilogram (g-mole/kg). This is insufficient for high-density hydrogen exposure in some down-hole environments. Though the composition of Sepigel is a trade secret, typical unsaturated organic compounds found in gettering gels require a catalyst to promote appreciable hydrogenation, making it more complicated and more costly to manufacture a suitable agent, and making it hard to achieve high stability, low toxicity, and low flammability.  
           [0005]    Thus, there is a need for an improved hydrogen gettering gel for use with down-hole optical fiber cables and similar applications.  
         SUMMARY OF THE INVENTION  
         [0006]    One embodiment of the present invention provides a fiber optic cable comprising an protective sheath, at least one optical fiber positioned within the protective sheath, and a gettering agent surrounding the at least one optical fiber within the protective sheath, wherein the gettering agent contains a fullerene compound.  
           [0007]    Another embodiment of the present invention provides a gettering agent for use with a fiber optic cable, the gettering agent comprising a fullerene compound dissolved or suspended in a solvent. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    So that the manner in which the above recited embodiments of the invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0009]    [0009]FIG. 1 is a perspective view of an illustrative fiber optic cable that may be adapted to benefit from the present invention;  
         [0010]    [0010]FIG. 2 is a cross-sectional view of one embodiment of a fiber optic cable employing the hydrogen gettering agent of the present invention;  
         [0011]    [0011]FIG. 3 illustrates the hydrogenation of a carbon double bond that exemplifies the advantages of the present invention; and  
         [0012]    [0012]FIG. 4 is a flowchart detailing a method of manufacturing one embodiment of the present invention. 
     
    
       [0013]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.  
       DETAILED DESCRIPTION  
       [0014]    The present invention aims to provide a fiber optic cable that is more hydrogen resistant than those available in the prior art. Existing hydrogen gettering agents used in the manufacture of fiber optic cables are capable of scavenging limited densities of hydrogen and may prove less effective in many high-density hydrogen exposure environments. The present invention provides hydrogen gettering agents containing compounds that are non-toxic, high capacity, non-flammable, stable and active at appropriate temperatures without the need for a metallic catalyst. These gettering agents may be used to manufacture fiber optic cables that may be deployed at higher temperatures and higher H 2  partial pressures than those in the prior art.  
         [0015]    A particularly attractive new hydrogen getter is the class of buckminsterfullerenes, popularly known as buckyballs. Buckminsterfullerenes are highly unsaturated, and thus provide a large number of available sites for hydrogen absorption. Further, resonance stability keeps the buckyballs from polymerizing, a problem with several other potential hydrogen gettering materials. Still further, buckyballs do not need a catalyst to enable them to getter hydrogen at relatively low temperatures, so long as they are dissolved in a solvent. While buckyballs typically exhibit a relatively low solubility in solvents (e.g., 3% by weight in benzene or Sepigel) when compared to conventional getters, the high gettering capacity still allows for high hydrogen gettering capacities relative to the materials that are typically stable in down-hole environments.  
         [0016]    For some embodiments of the present invention, buckminsterfullerene may be suspended in a liquid solvent, such as benzene. However, to facilitate deployment for applications like protecting optical fiber, for other embodiments, the buckminsterfullerene may be suspended in a gel solvent. It will be appreciated by those skilled in the art that several gel solvents, including Sepigel, are non-toxic and non-flammable (unlike benzene, which is toxic, carcinogenic, flammable and highly regulated) and thus may be advantageously adapted to benefit from the present invention.  
         [0017]    [0017]FIG. 1 is a perspective view of an illustrative fiber optic cable  10  that may be adapted to benefit from the present invention. The cable  10  may be used in a well bore  27  of an oil or gas well; however, the present invention is not limited in utility to solely down-hole applications. Optical fibers ( 16 ,  17  in FIG. 2) are selected to provide reliable transmission of optical signals between a first end  25  and a second end  26  of the cable  10 . For example, the signals may be transmitted between a fiber optic sensor  28  positioned with the well bore  27  (e.g., proximate the first end  25 ) and optical processing equipment  30  located outside the well bore  27  and above ground (e.g., proximate the second end  26 ). It is the molecular and atomic hydrogen present within the well bore  27  that decreases optical power and therefore compromises the reliability of the optical signals transmitted between the first and second ends  25 ,  26  of the cable  10 .  
         [0018]    [0018]FIG. 2 is a cross-sectional view of one embodiment of a fiber optic cable  10  employing the hydrogen gettering agent of the present invention. The cable  10  includes a protective sheath  13  surrounding one or more optical fibers  16 ,  17 . Although FIG. 2 depicts a cable  10  having two optical fibers  16 ,  17 , it is to be appreciated by those skilled in the art that any number of optical fibers may be used; however, the number is limited by the diameter of the protective sheath  13  such that sufficient space must be provided to prevent microbending of the optical fibers during handling and deployment of the cable  10 . The protective sheath  13  is surrounded by one or more outer protective layers  33  that may optionally include a fiber in metal tube (FIMT) core  11 , buffer material  35  and/or an outer tube  38 .  
         [0019]    The protective sheath  13  may be a laser welded tube, e.g., a length-wise laser welded tube, manufactured from any suitable material, including a corrosion resistant material, such as a corrosion resistant metal alloy or a high-strength polymer. The protective sheath  13  diameter may be in the range of 1.1 to 2.6 mm, and in an exemplary embodiment of the invention is 2.4 mm. Although the protective sheath  13  is described as being 1.1 to 2.6 mm in diameter, the diameter of the protective sheath may vary over a wide range, depending upon the materials used and the number of optical fibers to be placed in the protective sheath  13 . The protective sheath wall thickness is selected to be sufficient for the laser welding process.  
         [0020]    Surrounding the protective sheath  13  is a barrier layer  19  of low hydrogen permeability material, such as tin, aluminum, copper, gold, carbon, or other suitable low hydrogen permeability material. Alternatively, the protective sheath  13  may be coated or plated with the barrier layer  19 . The thickness of the barrier layer  19  is selected to provide a barrier to a high partial pressure hydrogen environment. Depending upon the selection of material, the barrier layer thickness may be in the range of 0.1 to 15 microns. For example, a carbon layer may have a thickness as thin as 0.1 microns, while a tin layer may be approximately 1.3 microns in thickness. The barrier layer  19  may be over coating  21  with a protective layer of hard, scratch resistant material, such as nickel or a polymer such as polyamide. The over coating  21  may have a thickness in the range of 2 to 15 microns, depending on the material. A method of manufacturing such a fiber optic cable is illustrated by the flowchart in FIG. 4.  
         [0021]    The protective sheath  13  is filled with a hydrogen gettering agent  22 , which fills the void spaces within the protective sheath  13  and surrounds the optical fibers  16 ,  17 . Alternate designs may be envisioned in which the gettering agent  22  surrounds the optical fibers  16 ,  17 ; for example, the gettering agent  22  may be used as a coating on one or more components of the cable  10  (e.g., the protective sheath,  13 , the optical fibers  16 ,  17 ), or it may be incorporated into the material of the sheath  13  or an outer layer (e.g., in a silicone resin). Additional layers of gettering agent  22  may even be provided between the sheath  13  and outer cable layers. As illustrated in FIG. 2, the gettering agent  22  is in intimate contact with the optical fibers  16 ,  17 . The function of the gettering agent  22  is to scavenge any hydrogen that passes through the hydrogen barrier layer  19 . The gettering agent  22  of the present invention consists of fullerene molecules  25  dissolved or suspended in an organic gel such as Sepigel, petroleum jelly, or a synthetic silicone gel. In the embodiment illustrated in FIG. 2, buckminsterfullerene (C 60 ) is incorporated in the gettering agent  22 . Because fullerene compounds are characterized by double carbon bonds (C=C), C 60  features a high degree of unsaturation that provides sites for hydrogenation; that is, it is particularly adept at scavenging environmental H 2 , because the hydrogen will add across (i.e., react with) the double bonds present in the gettering agent  22  (via the C 60  component) before it can permeate the optical fibers  16 ,  17 . FIG. 3 illustrates the saturation of a double carbon bond (such as those present in fullerene compounds) by a hydrogen molecule.  
         [0022]    Furthermore, typical unsaturated organic compounds found in commercially available gettering agents require a catalyst to promote any appreciable hydrogenation; however, the use of C 60  in a gettering agent has been shown to reduce the need for a catalyst. For example, it has been shown that thermodynamically favorable reactions of up to thirty-six hydrogen molecules per one C 60  molecule can be achieved without the use of a catalyst. Adding a catalyst may achieve an even higher ratio of hydrogenation. In addition, fullerene compounds in general exhibit high degrees of solubility in organic solvents, making them particularly well suited for incorporation into commercial gettering gels.  
         [0023]    Thus a gettering agent containing C 60  in a gel agent or coating can be applied to or incorporated in a fiber optic cable to improve hydrogen gettering efficiency. The efficiency of this resultant gettering agent will be proportional to the concentration of C 60 . It is estimated that a one-percent addition to Sepigel will increase gettering by an order of magnitude. Based on the solubility of the particular fullerene in a chosen solvent (gel), concentrations from 0.01% to 50% can be deployed. However, a fullerene concentration between 0.1% and 3% may result in optimal stability.  
         [0024]    Further, the improved gettering capacity resulting from the use of a fullerene compound in the hydrogen gettering agent will reduce, and in some cases eliminate, the need for a hydrogen barrier layer  19 , making the production of the fiber optic cable  10  less complex and less costly.  
         [0025]    While the preceding description has focused primarily on downhole applications, embodiments of the present invention may also be used in applications. For example, in applications with less demanding temperatures and H 2  partial pressures, such as in undersea telecommunications cables, the invention could be implemented as a fullerene impregnated thermoplastic sheath over an optical fiber or fibers.  
         [0026]    Thus the present invention represents a significant advancement in the field of hydrogen gettering agents for fiber optic cable design. The gettering agent incorporates a fullerene compound that improves hydrogen gettering efficiency so dramatically that the need for both a hydrogenation catalyst and a hydrogen barrier layer on portions of the cable is either reduced or eliminated. Therefore, cable designs may be achieved that are more hydrogen resistant and may be deployed at higher temperatures and higher H 2  partial pressures. Furthermore, the optical power of the fiber optic cable is maintained, while the overall complexity and cost of the cable design is reduced.  
         [0027]    While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.