Patent Publication Number: US-8111960-B2

Title: Fiber optic cable systems and methods to prevent hydrogen ingress

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 12/139,973 filed Jun. 16, 2008, now U.S. Pat. No. 7,646,953 which is a continuation-in-part of U.S. patent application Ser. No. 11/397,791 filed Apr. 4, 2006, now U.S. Pat. No. 7,424,190 issued Sep. 9, 2008, which is a continuation of U.S. patent application Ser. No. 10/422,396 filed Apr. 24, 2003, now U.S. Pat. No. 7,024,081 issued Apr. 4, 2006. Each of the aforementioned related patent applications are herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention generally relate to fiber optic cable systems for use in harsh environments such as gas and oil wellbore applications. 
     2. Background of the Related Art 
     With advancements in the area of fiber optic sensors for use in harsh environments, there is an increasing need for fiber optic cables compatible with the harsh environmental conditions present in oil and gas wellbore applications. For example, fiber optic cables utilized in sensing applications within the wellbore must be able to operate reliably in conditions that may include temperatures in excess of 300 degrees Celsius, static pressures in excess of 138,000 kilopascal (kPa), vibration, corrosive chemistry and the presence of high partial pressures of hydrogen. The hydrogen tends to darken waveguides in the cable causing undesired attenuation. 
       FIG. 7  depicts one example of a conventional fiber optic cable  700  suitable for use in harsh environments such as oil and gas wellbore applications. The fiber optic cable  700 , shown in  FIG. 7 , includes a fiber in metal tube (FIMT) core  702  surrounded by an outer protective sleeve  704 . The FIMT core  702  includes an inner tube  706  surrounding one or more optical fibers  708 . Three optical fibers  708  are shown disposed within the inner tube  706  in the embodiment of  FIG. 7 . A filler material  710  is disposed in the inner tube  706  to fill the void spaces not occupied by the optical fibers  708 . The filler material  710  may also include a hydrogen absorbing/scavenging material to minimize the effects of hydrogen on the optical performance of the fiber  708 . The outer protective sleeve  704  includes a buffer material  712  and an outer tube  714 . The buffer material  712  provides a mechanical link between the inner tube  706  and the outer tube  714  to prevent the inner tube  706  from sliding within the outer tube  714 . Additionally, the buffer material  712  keeps the inner tube  706  generally centered within the outer tube  714  and protects the inner tube  706  and coatings formed thereon from damage due to vibrating against the outer tube  714 . 
     At least one of the inner or outer surfaces of the inner tube  706  is coated or plated with a low hydrogen permeability material  716  to minimize hydrogen diffusion into an area around the optical fibers  708 . As temperature increases, materials (e.g., the low hydrogen permeability material  716 ) of prior cables disposed around optical waveguides to provide hydrogen blocking become less effective since hydrogen diffuses faster through these materials. This susceptibility of the optical waveguides to attack by hydrogen in high temperature environments reduces service life of the cables. 
     Therefore, there exists a need for improved fiber optic cables and methods for use in harsh environments. 
     SUMMARY OF THE INVENTION 
     In one embodiment, a method of deploying an optic cable includes providing the cable which includes an inner tube having an optical fiber disposed inside the inner tube and an outer tube with an inner diameter sized for relative retention of the inner tube disposed inside the outer tube. In addition, the method includes positioning the cable at a location. Controlled flowing of a fluid between the inner and outer tubes removes hydrogen from within the cable positioned at the location. 
     For one embodiment, an optic cable system includes a cable and a source of fluid. The cable includes an inner tube having an optical fiber disposed inside the inner tube and an outer tube with an inner diameter sized for relative retention of the inner tube disposed inside the outer tube. A flow path disposed between the inner and outer tubes extends across a length of the cable and includes an inlet and an outlet. The source of fluid couples to the inlet of the flow path with the fluid being pressurized to achieve controlled fluid flow of the fluid through the flow path from the inlet toward the outlet that is defined. 
     According to one embodiment, a method of deploying an optic cable includes providing the cable, which includes an inner tube having an optical fiber disposed inside the inner tube and an outer tube with an inner diameter sized for relative retention of the inner tube disposed inside the outer tube. The method additionally includes lowering the cable into a wellbore. Removing hydrogen from within the cable while located in the wellbore occurs by circulating a fluid between the inner and outer tubes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof that 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. 
         FIG. 1  is a cross sectional view of one embodiment of a fiber optic cable suitable for use in oil and gas wellbore applications; 
         FIG. 2  is a partial sectional side view of the optic cable of  FIG. 1 ; 
         FIGS. 3A-E  are cross sectional views of alternative embodiments of a fiber optic cable suitable for use in oil and gas wellbore applications; 
         FIG. 4  is a cross sectional view of another embodiment of a fiber optic cable suitable for use in oil and gas wellbore applications; 
         FIG. 5  flow diagram of one embodiment of a method for fabricating a fiber optic cable suitable for use in oil and gas wellbore applications; 
         FIG. 6  is a simplified schematic of one embodiment of a fiber optic cable assembly line; and 
         FIG. 7  depicts one example of a conventional fiber optic cable suitable for use in oil and gas wellbore applications. 
         FIG. 8  is a cross sectional diagrammatic view a wellbore with the fiber optic cable shown in  FIG. 3D  disposed in the wellbore and coupled to a pump for circulating fluid within the cable around an inner tube surrounding optical fibers of the cable. 
         FIG. 9  is an enlarged sectional view taken at  9  in  FIG. 8 . 
         FIG. 10  is a cross sectional view of a fiber optic cable as shown in  FIG. 3C  with addition of flow tubes between inner and outer tubes. 
         FIG. 11  is a cross sectional view of a fiber optic cable with annular flow paths. 
         FIG. 12  is a cross sectional view of the fiber optic cable shown in  FIG. 11  taken across line  12 - 12 . 
         FIG. 13  is a cross sectional view of a fiber optic cable having an outer tube surrounding a body having embedded therein one or more flow tubes and one or more tubes with fibers inside. 
         FIG. 14  is a cross sectional view of a fiber optic cable formed by an outer tube surrounding one or more flow tubes stranded with one or more tubes with fibers inside. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, wherever possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     Embodiments of the invention relate to reducing or preventing hydrogen darkening of an optical fiber located in a cable. While hydrogen may permeate through an outer surface of the cable, fluid circulating through the cable purges the hydrogen from within the cable. This circulation of the fluid occurs between an inner tube containing the fiber and an outer tube surrounding the inner tube. 
       FIG. 1  shows one embodiment of a fiber optic cable  100  suitable for use in oil and gas wellbore applications. The cable  100  comprises a fiber in metal tube (FIMT) core  102  disposed in a protective outer tube  104 . The FIMT  102  comprises an inner tube  106  surrounding one or more optical fibers  108 , three of which are shown in the embodiment depicted in  FIG. 1 . 
     The inner tube  106  is fabricated from a corrosion resistant material. Examples of suitable corrosion resistant metal alloys include, but are not limited to, 304 stainless steel, 316 stainless steel, INCONEL® 625 and INCOLOY® 825, among others. Examples of suitable plastics include, but are not limited to fluoropolymers, ethylene-chlorotrifluoroethylene, fluoroethylenepropylene, polyvinylidene fluoride, polyvinylchoride, HALAR®, TEFLON® and TEFZEL®, among others. The diameter of the inner tube  106  may be in the range of about 1.1 to about 2.6 millimeters (mm), and in an exemplary embodiment of the invention is about 2.4 mm. Although the inner tube  106  is described as being about 1.1 to about 2.6 mm in diameter, the diameter of the inner tube  106  may vary, depending upon the materials used and the number of optical fibers  108  to be placed in the inner tube  106 . 
     In one embodiment, the inner tube  106  has a wall thickness suitable for a seam welding process utilized to fabricate the tube from a coil of metal strip. For example, the wall thickness of the 304 stainless steel inner tube  106  may be about 0.2 mm to facilitate a continuous laser weld during a tube forming process. In another embodiment, the inner tube  106  has a wall thickness suitable for fabrication by plastic extrusion. 
     An optional plated barrier coating  110  may be disposed on at least one of the inner or outer surfaces of the inner tube wall. The barrier coating  110  may be coated, plated or otherwise adhered to the inner tube  106  and may be comprised of a low hydrogen permeability material, such as tin, gold, carbon, or other suitable material. The thickness of the barrier coating  110  is selected to slow the diffusion of hydrogen into the center of the inner tube  106  driven by a high partial pressure hydrogen environment present in some wells. Depending upon the barrier coating material, the coating thickness may be in the range of about 0.1 to about 30 microns or thicker. For example, a carbon barrier coating  110  may have a thickness of about 0.1 microns, while a tin barrier coating  110  may have a thickness of approximately 13 microns. In one embodiment, the barrier coating  110  includes a nickel seed layer disposed on the tube surface that provides an adhesion layer for an outer layer of low hydrogen permeability material. In applications where high partial pressures of hydrogen are not expected, the barrier coating  110  may be omitted. 
     In one embodiment, a protective outer coating  112  is disposed over the barrier coating  110 . The outer coating  112  is a protective layer of hard, scratch resistant material, such as nickel or a polymer such as polyamide, among others, that substantially prevents the barrier coating  110  from damage from contact with the outer tube  104 . The outer coating  112  may have a thickness in the range of about 0.5 to about 15 microns, depending on the selected material. 
     A filler material  114  is disposed in the inner tube  106  and substantially fills the void spaces within the inner tube  106  surrounding the optical fibers  108  to support and prevent the optical fibers  108  from moving excessively within the inner tube  106 . The filler material  114  has sufficient viscosity to resist the shear forces applied to it as a result of the weight of the optical fiber  108  when disposed in a vertical well installation at elevated temperatures, thereby supporting the optical fibers  108  without subjecting the fibers to the strain of their weight. The filler material  114  has an operating temperature range of about 10 to about 200 degrees Celsius. However, the cable  100  may be utilized over a wider temperature range. 
     The filler material  114  is also configured to allow the optical fibers  108  to relax and straighten with respect to the inner tube  106  due to differences in the coefficients of thermal expansion between the optical fiber  108  and the inner tube  106  and during spooling, deployment and use of the cable  100 . The filler material  114  also prevents chaffing of the coatings on the optical fibers  108  as a result of bending action during installation and vibration of the cable  100 . The filler material  114  also serves as a cushion for the optical fiber  108  against the surface of the inner tube  106  to avoid microbend losses across cable bends. Suitable compounds for the filler material  114  include conventional thixotropic gels or grease compounds commonly used in the fiber optic cable industry for water blocking, filling and lubrication of optical fiber cables. Optionally, the filler material  114  may be omitted. 
     To further reduce the effects of hydrogen on the optical fibers  108 , the filler material  114  may optionally include or be impregnated with a hydrogen absorbing/scavenging material  116 , such as palladium or tantalum, and the like. In one embodiment, the hydrogen absorbing/scavenging material  116  is a vanadium-titanium wire coated with palladium. Alternatively, the inner tube  106  may be coated with a hydrogen absorbing/scavenging material below the barrier coating  110  or on the interior surface  118  of the inner tube  106 , or such a hydrogen absorbing/scavenging material may be impregnated into the tube material, or any combination of the above. 
     The optical fibers  108  are selected to provide reliable transmission of optical signals through the cable  100  disposed in a gas or oil wellbore application. Suitable optical fibers  108  include low defect, pure silica core/depressed clad fiber. Alternatively, suitable optical fibers  108  include germanium doped single mode fiber or other optical fiber suitable for use in a high temperature environment. The optical fibers  108  disposed within the inner tube  106  may be comprised of the same type or of different types of materials. Although the invention is described herein as using three optical fibers  108  within the inner tube  106 , it contemplated that one or more fibers  108  may be used. The total number of fibers  108  and the diameter of the inner tube  106  are selected to provide sufficient space to prevent microbending of the optical fibers  106  during handing and deployment of the cable  100 . 
     As the fiber optic cable  100  has an operating temperature ranging at least between about 10 to about 200 degrees Celsius, a greater length of optical fibers  108  are disposed per unit length of inner tube  106  to account for the different coefficient of thermal expansion (CTE) represented by the optical fibers  108  and the inner tube  106 . The inner tube diameter is configured to accept an excess length of “serpentine over-stuff” of optical fiber  108  within the inner tube  106 . In one embodiment, the excess length of optical fiber  108  may be achieved by inserting the fiber  108  while the inner tube  106  is at an elevated temperature, for example, during laser welding of the inner tube  106 . The temperature of the inner tube  106  is controlled such that it approximates the anticipated maximum of normal operating temperature of the final installation. This process will lead to an excess length of fiber  108  of up to 2.0 percent or more within the inner tube  106  cooling of the inner tube. 
     The FIMT core  102  is surrounded by the outer tube  104  that is configured to provide a gap  120  therebetween. The gap  120  is filled with air or other non-structural material and provides sufficient isolation between the outer tube  104  and FIMT core  102  to prevent the various layers of the FIMT core  102  from excessively contacting the outer tube  104  and becoming damaged. As the FIMT core  102  and outer tube  104  are not retained in continuous contact with one another, the serpentine orientation of the FIMT core  102  within the outer tube  104  (shown in  FIG. 2 ) results in intermittent contact points  202  therebetween. The intermittent contact points  202  retain the inner tube  106  relative to the outer tube  104 , thus creating enough friction to prevent the inner tube  106  from moving within the outer tube  104  and damaging the coatings applied to the exterior of the inner tube  106 . 
     Returning to  FIG. 1 , the outer tube  104  is manufactured of a corrosion resistant material that easily diffuses hydrogen. The outer tube  104  may be manufactured of the same material of the inner tube  106  and may be fabricated with or without a coating of a low hydrogen permeability coating or hydrogen scavenging material. Examples of outer tube materials include suitable corrosion resistant metal alloys such as, but not limited to, 304 stainless steel, 316 stainless steel, INCONEL® 625 and INCOLOY® 825, among others. 
     In one embodiment, the outer tube  104  is seam welded over the FIMT core  106 . The weld seam  124  of the outer tube  104  may be fabricated using a TIG welding process, a laser welding process, or any other suitable process for joining the outer tube  104  over the FIMT core  102 . 
     After welding, the outer tube  104  is drawn down over the FIMT core  102  to minimize the gap  120 . The gap  120  ensures that the outer tube  104  is not mechanically fixed to the FIMT core  102 , thereby preventing thermally induced motion or strain during use at elevated temperatures and/or over temperature cycling, which may damage the barrier and/or outer coatings  110 ,  112  if the outer tube  104  were to slide over the inner tube  106 . 
     Alternatively, the outer tube  104  may be rolled or drawn down against the FIMT core  102 , where care is taken not to extrude or stretch the FIMT core  102  such that the excess length of the fibers  108  within the FIMT core  102  is not appreciably shortened. In embodiments where the outer tube  104  and the FIMT core  102  are in substantially continuous contact, the inner and outer tubes  106 ,  104  may be fabricated from the same material to minimize differences in thermal expansion, thereby protecting the coating applied to the exterior of the inner tube  106 . 
     An initial diameter of the outer tube  104  should be selected with sufficient space as not to damage the FIMT core  102  during welding. The outer tube  104  may be drawn down to a final diameter after welding. In one embodiment, the outer tube  104  has a final diameter of less than about 4.7 mm to less than about 6.3 mm and has a wall thickness in the range of about 0.7 to about 1.2 mm. Other outer tube diameters are contemplated and may be selected to provide intermittent mechanical contact between the inner tube  106  and the outer tube  104  to prevent relative movement therebetween. 
     To further protect the cable  100  during handling and installation, a protective jacket  122  of a high strength, protective material may be applied over the outer tube  104 . For example, a jacket  122  of ethylene-chlorotrifluoroethylene (ECTFE) may be applied over the outer tube  104  to aid in the handling and deployment of the cable  100 . In one embodiment, the jacket  122  may have a non-circular cross-section, for example, ellipsoid or irregular, or polygonal, such as rectangular. The protective jacket  122  may be comprised of other materials, such as fluoroethylenepropylene (FEP), polyvinylidene fluoride (PVDF), polyvinylchloride (PVC), HALAR®, TEFLON®, fluoropolymer, or other suitable material. 
     As the diameter of the outer tube  104  and optional protective jacket  122  result in a cable  100  that is much smaller than conventional designs, more cable  100  may be stored on a spool for transport. For example, a cable  100  having a diameter of about 3.2 mm may have a length of about 24 kilometers stored on a single spool, thereby allowing multiple sensing systems to be fabricated from a single length of cable without splicing. Furthermore, the reduced diameter of the cable  100  allows for more room within the wellhead and wellbore, thereby allowing more cables (or other equipment) to be disposed within the well. Moreover, as the cable  100  is lighter and has a tighter bending radius than conventional designs, the cable  100  is easier to handle and less expensive to ship, while additionally easier to deploy efficiently down the well. For example, conventional quarter 6.3 mm cables typically have a bending radius of about 101 mm, while an embodiment of the cable  100  having a 3.1 mm diameter has a bending radius of less than 76.2 mm, and in another embodiment, to about 50.8 mm. 
       FIG. 3A  illustrates a cross sectional view of another embodiment of a fiber optic cable  300  suitable for use in oil and gas wellbore applications. The cable  300  is substantially similar in construction to the cable  100  described above, having an FIMT core  306  disposed within a protective outer tube  104 . 
     The FIMT  306  comprises an inner metal tube  302  having a polymer shell  304  surrounding one or more optical fibers  108 . The inner tube  302  is fabricated similar to the metal embodiment of the inner tube  302  described above, while the polymer shell  304  may be applied to the exterior of the inner tube  302  by extruding, spraying, dipping or other coating method. The polymer shell  304  may be fabricated from, but is not limited to fluoropolymers, ethylene-chlorotrifluoroethylene, fluoroethylenepropylene, polyvinylidene fluoride, polyvinylchoride, HALAR®, TEFLON® and TEFZEL®, among others. Although the polymer shell  304  is illustrated as a circular ring disposed concentrically over the inner tube  302 , it is contemplated that the polymer shell  304  may take other geometric forms, such as polygonal, ellipsoid or irregular shapes. 
     An optional plated barrier coating (not shown) similar to the coating  110  described above, may be disposed on at least one of the inner or outer surfaces of at least one of the inner tube  302  or polymer shell  304 . In one embodiment, a protective outer coating (also not shown) similar to the outer coating  112  described above, is disposed over the barrier coating  110 . The outer coating  112  is a protective layer of hard, scratch resistant material, such as nickel or a polymer such as polyamide, among others, that substantially prevents the barrier coating  110  from damage from contact with the outer tube  104 . 
     The optical fibers  108  are selected to provide reliable transmission of optical signals through the cable  300  disposed in a gas or oil wellbore application. Although the invention is described herein as using three optical fibers  108  within the inner tube  302 , it is contemplated that one or more fibers  108  may be used. The optical fibers  108  may be disposed in filler material  114  that substantially fills the void spaces within the inner tube  302  surrounding the optical fibers  108 . The filler material  114  may optionally be impregnated with a hydrogen absorbing/scavenging material  116 , such as palladium or tantalum, and the like. 
     The outer tube  104  is configured to intermittently contact the FIMT core  306  while substantially maintain a gap  120  as described above. The intermittent contact between the outer tube  104  and FIMT core  306  prevents the FIMT core  306  from moving within the outer tube  104  while advantageously minimizing the outer diameter of the cable  300  as compared to conventional designs. 
       FIG. 3B  depicts a cross sectional view of another embodiment of a fiber optic cable suitable for use in oil and gas wellbore applications. The cable is substantially similar in construction to the cable  300  described above, having an FIMT core  336  disposed within a protective outer tube  104 , except that the FIMT core  336  includes a plurality of fins  332 . 
     In one embodiment, the FIMT core  336  includes an inner metal tube  302  having a polymer shell  334  disposed thereover. The fins  332  extend outwardly from the polymer shell  334 . The fins  332  are typically unitarily formed with the shell  334  during an extrusion process, but may alternatively be coupled to the shell  334  through other fabrication processes. Ends  338  of the fins  332  generally extend from the shell  334  a distance configured to allow a gap  340  to be defined between the ends  338  and the wall of the outer tube  104 . The gap  340  allows the FIMT core  336  to be disposed within the outer tube  104  in a serpentine orientation (similar to as depicted in  FIG. 2 ), thereby allowing intermittent contact between the FIMT core  336  and the outer tube  104  that substantially secures the core  336  and outer tube  104  relative to one another. 
     In an alternative fiber optic cable  330 , as depicted in  FIG. 3C , the outer tube  104  may be sized or drawn down to contact the fins  332  of the FIMT core  336 , thus mechanically coupling the FIMT core  336  to the outer tube  104 . In this embodiment, a gap  120  remains defined between the shell  334  and outer tube  104  to substantially protect the FIMT core  336  and any coatings disposed thereon, while the mechanical engagement of the outer tube  104  and fins  332  prevent movement of the core  336  within the outer tube  104 . Moreover, the space defined between the fins  332  provides spacing between the FIMT core  336  and the outer tube  104  to prevent damage of the FIMT core  336  during welding. Additionally, the fins  332  may be slightly compressed during the reduction in diameter of the outer tube  104  so that the FIMT core  336  is not stretched or extruded in a manner that substantially removes the excess length of fiber within the FIMT core  336 . 
       FIG. 3D  depicts a cross sectional view of another embodiment of a fiber optic cable  350  suitable for use in oil and gas wellbore applications. The cable  350  is substantially similar in construction to the cable  330  described above, having a fiber in tube (FIT) core  356  disposed within a protective outer tube  104 , except that the FIT core  356  includes a plurality of fins  352  extending from a polymer inner tube  354  that surrounds at least one optical fiber  108  without an intervening metal tube. 
     The fins  352  are unitarily formed with the polymer inner tube  354  during an extrusion process, but may alternatively be coupled to the inner tube  354  through other fabrication processes. During fabrication, the optical fiber  108  is disposed in the polymer inner tube  354  while the tube  354  is in an expanded state, for example, immediately after the polymer inner tube  354  is extruded or after heating the tube. As the polymer tube  354  cools and shrinks, the length of optical fiber  108  per unit length of polymer tube  354  increases, thereby allowing enough optical fiber  108  to be disposed within the polymer tube  354  to ensure minimal stress upon the optical fiber  108  after the polymer tube  354  has expanded when subjected to the hot environments within the well. 
     Ends  358  of the fins  352  generally extend from the polymer inner tube  354  a distance configured to allow a gap to be defined between the ends  358  and the wall of the outer tube  104  or to contact the outer wall  104  as shown. In either embodiment, a gap  120  remains defined between the polymer inner tube  354  and outer tube  104  to substantially protect the FIT core  356  and any coatings disposed thereon. 
       FIG. 3E  depicts a cross sectional view of another embodiment of a fiber optic cable  360  suitable for use in oil and gas wellbore applications. The cable  360  is substantially similar in construction to the cable  350  described above, having a FIT core  366  disposed within a protective outer tube  104 , except that the FIT core  366  defines a polymer without fins and without an intervening metal tube. 
     The FIT core  366  has a polygonal form, such as a triangle or polygon (a square is shown in the embodiment depicted in  FIG. 3E ). However, it is contemplated that the FIT core  366  may take other geometric forms, such as polygonal, ellipsoid, circular or irregular shapes, where the FIT core  366  has a different geometric shape than the inner diameter of the outer tube  104 . 
     In the embodiment depicted in  FIG. 3E , the FIT core  366  includes corners  368  that generally extend from the FIT core  366  a distance configured to allow a gap to be defined between the corners  368  and the wall of the outer tube  104  or to contact the outer wall  104  as shown. In either embodiment, a gap  120  remains defined between the FIT core  366  and outer tube  104  to substantially protect the FIT core  366  and any coatings disposed thereon. 
       FIG. 4  depicts another embodiment of a cross sectional view of another embodiment of a fiber optic cable  400  suitable for use in oil and gas wellbore applications. The cable  400  is substantially similar in construction to the cables described above, except that the cable  400  includes an expanded polymer spacer  402  that applies a force against an outer tube  104  and an FIMT core  102  that bound the spacer  402 . 
     The polymer spacer  402  may be a foamed polymer, such as urethane or polypropylene. In one embodiment, the polymer spacer  402  may be injected and foamed between the outer tube  104  and the FIMT core  102  after the outer tube  104  has been welded. In another embodiment, the polymer spacer  402  may be disposed over the FIMT core  102  and compressed during a diameter reducing step applied to the outer tube  104  after the welding. In yet another embodiment, the polymer spacer  402  may be applied to the exterior of the FIMT core  102 , and activated to expand between the outer tube  104  and the FIMT core  102  after welding. For example, the polymer spacer  402  may be heated by passing the cable  400  through an induction coil, where the heat generated by the induction coil causes the polymer spacer  402  to expand and fill the interstitial space between the outer tube  104  and the FIMT core  102 . As the polymer spacer  402  is biased against both the outer tube  104  and the FIMT core  102 , any well fluids that may breach the outer tube  104  are prevented from traveling along the length of the cable  400  between the outer tube  104  and the FIMT core  102 . 
       FIGS. 5-6  are a flow diagram and simplified schematic of one embodiment of a method  500  for fabricating the optic cable  330 . The reader is encouraged to refer to  FIGS. 5 and 6  simultaneously. 
     The method  500  begins at step  502  by extruding a polymer tube  602  through a die  620  around at least one or more optical fibers  604 . The optical fibers  604  may optionally be sheathed in a seam welded metal tube as described with reference to  FIG. 1 , and as described in U.S. Pat. No. 6,404,961, incorporated by reference. As the polymer tube  602  is formed, the one or more optical fibers  604  are deployed from a first conduit or needle  612  extending through the die  620  into the tube  602  to a point downstream from the extruder  606  where the polymer comprising the tube  602  has sufficiently cooled to prevent sticking of the fibers  604  to the tube wall at step  504 . The one or more optical fibers  604  are disposed in the tube  602  at a rate slightly greater than the rate of tube formation to ensure a greater length of optical fiber  604  per unit length of polymer tube  602 . 
     At an optional step  506 , a filler material  608  may be injected into the interior of the polymer tube  602  to fill the void spaces surrounding the optical fibers  604 . The filler material  608  is injected from a second conduit or needle  610  extending through the die  620  of the polymer tube  602  to a suitable distance beyond the extruder to minimize any reaction between the cooling polymer tube  602  and the filler material  608 . The filler material  608  may optionally be intermixed with a hydrogen absorbing/scavenging material. 
     At an optional step  508 , the polymer tube  602  may be coated with a barrier material  614 . The barrier material may be applied by plating, passing the tube  602  through a bath, spraying and the like. In one embodiment, the barrier material  614  is plated on the polymer tube  602  by passing the tube through one or more plating baths  618 . 
     At an optional step  510 , a protective outer sleeve  624  is formed around the polymer tube  602 . The outer sleeve  624  may include seam welding a metal strip  626  to form the sleeve  624  around the polymer tube  602 . The protective outer sleeve  624  may also include a polymer jacket  628  applied over the sleeve  624 . The polymer jacket  628  may be formed by spraying or immersing the sleeve  624  in a polymer bath after welding. If a protective outer sleeve  624  is disposed over the polymer tube  602 , the metal sleeve  624  may be drawn down into continuous contact with the polymer tube  602  at step  512 . 
     Thus, a fiber optic cable suitable for use in harsh environments such as oil and gas wellbore applications has been provided. The novel optic cable has unique construction that advantageously minimizes fabrication costs. Moreover, as the novel optic cable has a reduced diameter that allows greater spooled lengths of cable facilitates more efficient utilization as compared to conventional cable designs, thereby minimizing the cost of optical sensing systems that utilize optic cables in oil field applications. 
       FIG. 8  shows a cross sectional diagrammatic view of a wellbore  800  with the fiber optic cable  350  shown in  FIG. 3D  disposed in the wellbore  800 . The cable  350  couples to a flow control device, such as a pump  802  and/or valve, for circulating fluid within the cable  350 . The pump  802  receives the fluid from a source  804  for inputting the fluid into the cable  350  as depicted by an arrow indicating flow  806 . For some embodiments, the fluid in the source  804  may be pressurized such that no additional pumping of the fluid is required. The source  804  holds or otherwise provides the fluid, which may be a liquid or gas selected such that circulation of the fluid through the cable  350  flushes away hydrogen that may be diffusing into the cable  350 . 
     A termination  808  of the cable  350  includes either a cross-over area for flowing the fluid back through other flow paths of the cable  350  as described further herein or a vent, which may include a one-way valve assembly to let the fluid escape into the wellbore  800  without permitting ingress into the cable  350 . Returning the fluid back to surface for discharge avoids potential complications of introducing the fluid in the wellbore, such as subsequent separation and removal of the fluid from other wellbore fluids or possible fluid locks created by the termination  808  being in a trapped volume, and facilitates ensuring no breach of the cable  350  in the wellbore  800  as may occur with problems from venting in the wellbore  800 . One way flow of the fluid through the cable  350  thus may offer a more desirable approach in other applications where the cable  350  is not deployed in the wellbore  800 . 
       FIG. 9  illustrates an enlarged sectional view taken at  9  in  FIG. 8 . The flow  806  of the fluid traverses along a length of the cable  350  through a first path  119  defined by one of the gaps  120  between the inner and outer tubes  354 ,  104  of the cable  350 . The first path  119  retains the flow  806  of the fluid in at least part of an annular area around the inner tube  354  surrounding the optical fibers  108  of the cable  350 . Referring to  FIG. 3D , the fins  352  may isolate the first path  119  formed by one of the gaps  120  from one or more other circumferential spaced ones of the gaps  120 . Another one of the gaps  120 , for example, may form a second path  121  separate from the first path  119 . In operation, the fluid may flow through the second path  121  in an opposite direction from the flow  806  of the fluid in the first path  119  to enable return of the fluid upon reaching the termination  808  (shown in  FIG. 8 ). As with other embodiments described herein, the termination  808  may include apertures, such as through the fins  352 , or spacing between components at ends where, for example, at least the fins  352  terminate prior to reaching an enclosing end face of the cable  350 . Such exemplary arrangements at the termination  808  enable flow to cross-over from the first path  119  to the second path  120 . The flow  806  of the fluid may occur through any or all of the gaps  120 . For some embodiments such as those where venting occurs at the termination  808 , the flow  806  of the fluid passes in one direction around the inner tube  354  making isolation between the first and second paths  119 ,  121  not required as shown by example in  FIG. 3B . 
     Regardless of configuration, the fluid may circulate through the cable  350  between first and second terminal ends of the cable  350 . The circulation occurs for a period of time as required for removal of hydrogen during life of the cable  350 , such as longer than one of a day, a week and a month. To provide the flow  806  across a desired length of the cable  350 , the fluid enters and exits the cable  350  via an inlet and outlet at selected locations, such as proximate one of the ends of the cable  350  or respectively at each end of the cable  350  if no return of the fluid is desired. The inlet and outlet being defined makes controlled fluid flow through the first and second paths  119 ,  121  possible. 
     Operations to remove hydrogen utilize controlled circulation of the fluid through the cable  350 . For some embodiments, sizing of the first and second paths  119 ,  121 , pressure of the fluid in the source  804 , and/or operation of the flow control device such as the pump  802  or a valve at where the flow  806  exits the cable  350  controls the flow  806  of the fluid into the cable  350 . For some embodiments, flow rate of the fluid through the cable  350  may provide an exchange rate of all the fluid within the cable  350  of about once per day. This exchange rate may correspond to the flow rate being between 0.05 cubic meters and 1.5 cubic meters per day, as the cable  350  may contain less than about 0.03 cubic meters of fluid per 3000 meters of the cable  350 . In some embodiments, the flow  806  of the fluid may occur in intermittent pulses instead of a continuous flow to conserve the fluid. 
     In some embodiments, the source  804  includes the fluid that may be a mixture and that may be non-hydrogen containing. As size of the paths  119 ,  121  in the cable  350  through which the fluid is flowing decreases, liquids become more difficult to circulate making gases more desirable in some applications. Exemplary gases for the fluid in the source  804  include air, nitrogen, helium, fluorine, argon, oxygen, neon, krypton, xenon, radon, carbon monoxide, carbon dioxide and mixtures thereof. Further, the fluid from the source  804  may contain hydrogen scavenging compounds such as fullerenes including buckminsterfullerenes, carbon tetrachloride, perfluorohexane, potassium iodate and mixtures thereof. 
       FIG. 10  shows a cross sectional view of the fiber optic cable  330  as shown in  FIG. 3C  with addition of first and second flow tubes  900 ,  901  between the inner and outer tubes  302 ,  104 . While two of the flow tubes  900 ,  901  are shown, the cable  330  may include any number of the flow tubes  900 ,  901 . As described with reference to  FIG. 8 , flowing of fluid through one or more of the flow tubes  900 ,  901  and returning the fluid through the gap  120  flushes hydrogen from within the cable  330  prior to the hydrogen reaching the optical fibers  108 . The flow tubes  900 ,  901  can withstand pressures necessary to establish circulation of the fluid. Passing the fluid inside the flow tubes  900 ,  901  facilitates in ensuring that a distinct and stable flow path extends across all of the cable  330  prior to the fluid being returned back through the cable  330 . 
       FIG. 11  illustrates a cross sectional view of a fiber optic cable with a filler body  910  disposed between inner and outer tubes  102 ,  104  in a spaced relationship with both the inner and outer tubes  102 ,  104 . In one embodiment, aluminum that may be extruded over or wrapped and welded around the inner tube  102  forms the filler body  910 . In some embodiments, the inner tube  102  may be concentric with the outer tube  104  and may be made of metal. Separation between the filler body  910  and the outer tube  104  creates an outer annular flow path  911  concentric with an inner annular flow path  912  provided by spacing between the inner tube  102  and the filler body  910 . The filler body  910  isolates the annular flow paths  911 ,  912  from one another until cross-over is desired. In some embodiments, an outer diameter of the filler body  910  being about 0.1 to 0.3 mm smaller than an inner diameter of the outer tube  104  and an outer diameter of the inner tube  102  being about 0.1 to 0.3 mm smaller than an inner diameter of the filler body  910  establish the annular flow paths  911 ,  912 . 
       FIG. 12  shows a cross sectional view of the fiber optic cable shown in  FIG. 11  taken across line  12 - 12 . Referring again to  FIG. 8 , fluid flow through the annular flow paths  911 ,  912  removes any hydrogen that permeates through the outer tube  104  to reduce or eliminate impact of hydrogen on optical fibers  108  contained in the inner tube  102 . Pressurization and return of the fluid can occur respectively through either of the annular flow paths  911 ,  912 . For example, input flow  921  may pass through the outer annular flow path  911  for a cable length prior to being returned through the inner annular flow path  912  as depicted by return flow  922 . The input and return flows  921 ,  922  as depicted, respectively enter and exit the outer and inner annular flow paths  911 ,  912 , thereby illustrating, for example, the inlet and the outlet. 
       FIG. 13  illustrates a cross sectional view of a fiber optic cable having an outer tube  104  surrounding a filler body  930 , first and second flow tubes  932 ,  933  embedded in the filler body  930 , and an inner tube  102  also embedded in the filler body  930  with fibers  108  inside the inner tube  102 . For some embodiments, extrusion of aluminum over the inner tube  102  and the flow tubes  932 ,  933  forms the filler body  930 . Centralization of the inner tube  102  within the filler body  930  prevents length mismatch between the inner and outer tubes  102 ,  104  as a result of coiling or uncoiling. Pressurization of the flow tubes  932 ,  933  with fluid at a first end of the flow tubes  932 ,  933  passes the fluid to a second end of the flow tubes  932 ,  933  where the fluid exiting the second end of the flow tubes  932 ,  933  is directed back along an annulus  931  between the filler body  930  and the outer tube  104 . A gap between an outer diameter of the filler body and an inner diameter of the outer tube  104  creates the annulus  931 , which is purged of hydrogen by the fluid that is circulated therein. 
       FIG. 14  shows a cross sectional view of a fiber optic cable formed by an outer tube  104  surrounding first and second flow tubes  941 ,  942  stranded or twisted together with an inner tube  940  that has optical fibers  108  inside. The flow tubes  941 ,  942  and inner tube  940  for some embodiments include a coating to hold the flow tubes  941 ,  942  and inner tube  940  together. Gaps  120  of open space exist external to the flow tubes  941 ,  942  and inner tube  940  and inside of the outer tube  104 . Like other embodiments described herein, flowing fluid through the flow tubes  941 ,  942  in a first direction along a cable length and then flowing the fluid back through the gaps  120  along the cable length in a second direction that is opposite the first direction flushes away any hydrogen gas that has migrated through the outer tube  104  before the hydrogen gas can build a partial pressure enough to penetrate the inner tube  102  containing the optical fibers  108 . 
     Compared to designs that introduce a gas barrier in direct contact with optical fibers, embodiments of the invention provide several advantages. For example, the gas barrier in direct contact with the optical fibers requires leaving the fibers in an unsupported condition and subject to free movement. Further, purging hydrogen at the optical fiber itself means that any hydrogen in flow for the gas barrier can interact with the optical fiber as there is nothing blocking the hydrogen from access to the fibers. Redundant hydrogen barriers or hydrogen scavenging techniques on the other hand can benefit from reduced hydrogen concentrations when hydrogen is purged ahead of such redundant hydrogen barriers. Moreover, integration of purging fluid inflow and outflow in a single cable facilitates deployment of the cable. Another exemplary advantage provided by embodiments of the invention includes ability to incorporate various cable design aspects in which optical fibers are disposed without being limited to running of bare optical fibers within the gas barrier. 
     Based on the foregoing, various arrangements exist for providing one or more flow paths in a cable between an inner tube containing optical fibers and an outer tube surrounding the inner tube. The flow paths enable flowing of fluid to prevent hydrogen ingress to the optical fibers as exemplarily illustrated for various embodiments herein even though other configurations, such as shown in  FIG. 1 , may provide suitable flow paths. Although the invention has been described and illustrated with respect to exemplary embodiments thereof, the foregoing and various other additions and omissions may be made therein and thereto without departing from the spirit and scope of the present invention.