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RELATED APPLICATION 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 11/779,376, filed Jul. 18, 2007, and entitled “OPTICAL TURNAROUND SYSTEM.” 
     
    
     BACKGROUND 
       [0002]    Optical fibers are used in a variety of sensing and other applications. For example, optical fiber has been used as a distributed temperature sensor in oilfield applications where the temperature profile can be applied in, for example, detection of water breakthrough, detection of leaks, and gas lift monitoring and optimization. The optical fiber is utilized by injecting light into the fiber, measuring the backscattered light, and then processing the results to determine temperature along the length of the fiber. 
         [0003]    Research has shown that distributed temperature sensor measurements are more accurate when performed in a double-ended configuration such that optical continuity exists between two optical fiber ends connected to a distributed temperature sensor control system. By preparing this double-ended configuration, light can be sent through the complete length of optical fiber from both directions and measurement correction is facilitated. However, the double-ended configuration requires that a continuous optical fiber extend down into a wellbore for the desired interval to be sensed, turnaround, and return to the surface. 
         [0004]    Attempts have been made to create turnarounds that route the optical fiber back to the surface. In one example, the turnaround has been formed with a metal tube doubled back on itself with both ends connected to additional tubing. The optical fiber is then routed through the tubing. This technique, however, requires many tubing connections that reduce system reliability while increasing deployment time. Many applications also are subject to space constraints which can create problems in properly controlling the bend of the optical fiber when routed through a turnaround. Exceeding the minimum bend radius of an optical fiber increases optical attenuation and can ultimately result in fiber breakage. 
       SUMMARY 
       [0005]    In general, the present invention provides a system and method for routing an optical fiber that can be used in a variety of sensing applications and environments. A continuous optical fiber is created such that optical continuity exists between two ends of the optical fiber. The optical continuity is created with the assistance of an optical turnaround constructed in a simple, dependable form able to control the bend of the optical fiber as it extends through the optical turnaround. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Certain embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
           [0007]      FIG. 1  is a front elevation view of a fiber optic system positioned in a wellbore with a corresponding well system, according to an embodiment of the present invention; 
           [0008]      FIG. 2  is a view of an optical fiber loop that may be adjusted in size within a protective housing, according to an embodiment of the present invention; 
           [0009]      FIG. 3  is a view of the optical fiber loop illustrated in  FIG. 2  combined with a seal assembly positioned to seal the protective housing to an optical fiber cable, according to an embodiment of the present invention; 
           [0010]      FIG. 4  is a cross-sectional view of an optical fiber turnaround, according to an alternate embodiment of the present invention; 
           [0011]      FIG. 5  is a cross-sectional view of the optical fiber turnaround illustrated in  FIG. 4  but rotated ninety degrees, according to an embodiment of the present invention; 
           [0012]      FIG. 6  is a cross-sectional view of the optical fiber turnaround taken generally a long line  6 - 6  in  FIG. 4 , according to an embodiment of the present invention; 
           [0013]      FIG. 7  is a front view of an embodiment of an insert that may be used in the turnaround illustrated in  FIG. 4 , according to an embodiment of the present invention; 
           [0014]      FIG. 8  is a top view of the insert illustrated in  FIG. 7 , according to an embodiment of the present invention; 
           [0015]      FIG. 9  is a front view of another embodiment of an insert that may be used in the turnaround illustrated in  FIG. 4 , according to an alternate embodiment of the present invention; and 
           [0016]      FIG. 10  is a top view of the insert illustrated in  FIG. 9 , according to an alternate embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those of ordinary skill in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
         [0018]    The present invention generally relates to a fiber optic system that utilizes an optical fiber in parameter sensing applications. The system utilizes optical fiber arranged in a double-ended configuration to provide optical continuity between two fiber ends that are connected to an appropriate control unit. The configuration facilitates sensing of the desired parameter by, for example, allowing light to be sent through the complete length of the optical fiber from both directions. The arrangement also facilitates measurement correction to provide for more accurate measurement of the desired parameter. In one embodiment, the fiber optic system is utilized in a well environment and the optical fiber is deployed in a wellbore drilled into a geological formation holding desired production fluids, such as hydrocarbon based fluids. 
         [0019]    Referring generally to  FIG. 1 , a fiber optic system  20  is illustrated according to one embodiment of the present invention. Fiber optic system  20  comprises an optical fiber sensor  22  that may comprise a distributed sensor  24  coupled to an appropriate control unit  26 . The control unit  26  may be selected from a variety of available optical control systems used to inject light along optical fiber. The control unit  26  is then used to measure the backscattered light and to process the results to determine the sensed parameter, e.g. temperature, along the length of the optical fiber. Temperature profiles can be used in well applications to evaluate a variety of formation and well equipment related characteristics, such as water breakthrough, leak detection, and gas lift monitoring and optimization. 
         [0020]    In the embodiment illustrated in  FIG. 1 , fiber optic system  20  is utilized in a well related application with the control unit  26  positioned generally at a surface location and optical fiber sensor  22  extending into a wellbore  28 . Wellbore  28  may be formed in a geological formation  30  holding desired production fluids, such as hydrocarbon based fluids that may be in the form of oil or gas. In many applications, wellbore  28  is lined with a wellbore casing  32  through which perforations  34  are formed to enable the flow of fluids between geological formation  30  and wellbore  28 . In this application, wellbore  28  extends downwardly from a wellhead  36  positioned at a surface  38 , e.g. the surface of the earth or a seabed floor. 
         [0021]    Optical fiber sensor  22  may be routed along wellbore equipment  40 , e.g. a tubing string, deployed in wellbore  28 . The wellbore equipment  40  may comprise a variety of wellbore completions, servicing tools or other equipment depending on the well related operation to be performed. Furthermore, the optical fiber sensor  22  can be routed within wellbore equipment  40 , along the exterior of wellbore equipment  40 , along the interior or exterior of wellbore casing  32 , or along a combination of regions. The optical fiber sensor  22  is designed to sense the desired parameter, e.g. temperature, along wellbore  28  or along a specific section of wellbore  28 . 
         [0022]    The optical fiber sensor  22  comprises a plurality of optical fibers  42 , as illustrated in  FIG. 2 . The optical fibers  42 , e.g. a pair of optical fibers  42 , have ends  44  which are connected by an optical fiber loop  46  to provide optical continuity extending between optical fibers  42 . The optical fiber loop  46  is functionally connected to ends  44  by fusion welding or other suitable connection processes that enable the establishment of optical continuity. In the example illustrated, optical fiber loop  46  is a high strength fiber segment that is packaged in a protective housing  48 . The fiber loop  46  is packaged in a manner that protects it during shipment and also from downhole environmental conditions once installed. The protective housing  48 , optical fiber loop  46  and related packaging comprise a fiber loop optical turnaround  50 . In this embodiment, the fiber loop optical turnaround  50  further comprises an adjustment device  52  provided to allow selective adjustment of optical fiber loop size. By way of example, adjustment device  52  may comprise a sleeve  54 , e.g. a tube, slidably mounted over optical fibers  42 . By sliding sleeve  54  toward or away from optical fiber loop  46 , the diameter of the loop can be adjusted to facilitate, for example, movement into protective housing  48  and subsequent expansion to the greatest diameter possible as limited by the inside diameter of protective housing  48 . 
         [0023]    Proper selection of a suitable optical fiber with which to form optical fiber loop  46  can be accomplished by a variety of methods. For example, lengths of optical fiber may be acquired for use in the optical turnaround  50  through a combination of testing and statistical flaw distribution. The optical fiber can be selected so as to have a statistical probability of sufficiently high strength to reliably form the optical fiber loop. In one example, lengths of optical fiber are taken from a batch of select fiber and subdivided into three sections or lengths. One length of fiber is set aside for use as optical fiber loop  46 , while the other lengths of optical fiber are tested to determine tensile strength/break load. If the strength of the tested lengths of fiber is sufficient to avoid experiencing damaging stress levels when creating a loop size needed for the optical turnaround, the set aside optical fiber is presumed suitable for use in forming loop  46 . Only a statistically insignificant probability exists that the set aside fiber has differing characteristics compared to those of the tested optical fibers. An alternative approach is to select the optical fiber for forming loop  46  from a fiber spool that is batch tested. A single test is performed on a sample of the fiber to verify strength. Additionally, through the use of appropriate optical fiber during manufacture of an optical fiber cable, it is feasible to create optical fiber loop  46  by exposing a sufficient length of optical fiber from the cable so that one of the fibers can be looped and fusion spliced to a second fiber in the cable. It should be noted that in some embodiments optical fiber loop  46  and optical fibers  42  can be formed from one continuous fiber. Use of these analysis and selection methods enables the determination of optical fiber having high strength for use in the optical turnaround. By locating suitable high strength fiber through selective/random testing and/or statistical/historical data, the optical fiber can be bent in a small diameter while still ensuring long-term reliability. The turnarounds discussed herein also can be formed with relatively short optical fiber loops, e.g. one meter or less, which further reduces the probability of defects in the optical fiber turnaround. 
         [0024]    Referring to  FIG. 3 , one embodiment of fiber loop optical turnaround  50  is illustrated as combined with an optical fiber cable  56  to form optical fiber sensor  22 . In this embodiment, optical fiber loop  46  is established at the end of optical fiber cable  56  and isolated from direct contact with the well environment by housing  48 . Housing  48  also protects optical fiber loop  46  and optical fibers  42  from vibration and shock that may be incurred during installation of optical fiber sensor  22  and during service within wellbore  28 . Additionally, a potting compound  58  can be introduced into the interior of housing  48  to further protect the optical fibers and optical fiber loop. In one embodiment, housing  48  is partially filled with potting compound to secure optical fiber loop  46  within housing  48  after proper installation of the optical fiber loop. 
         [0025]    The housing  48  is sealed to optical fiber cable  56  to ensure fiber loop  46  and fibers  42  are isolated from direct contact with the surrounding well environment. In the embodiment illustrated, housing  48  and optical fiber cable  56  are sealed together by a suitable cable seal assembly  60 , such as an in-line splice. However, other methods of cable sealing can be used. One example of a suitable seal assembly, illustrated in  FIG. 3 , is an Intellitite Dry-Mate Connector (EDMC-R) available from Schlumberger Corporation. 
         [0026]    Construction of fiber loop optical turnaround  50  can be carried out according to several methodologies. One suitable approach involves initially preparing optical fiber cable  56  by stripping back the encapsulation material, straightening the cable, and holding the optical fiber cable via an appropriate assembly fixture or other mechanism. The cable seal assembly  60  is then slid onto optical fiber cable  56 , as illustrated in  FIG. 3 . Typically, the optical fiber cable  56  has a metal jacket which is removed for a length, and any filler material is stripped away to expose optical fibers  42 . Sleeve  54 , which may be in the form of a polyimide tube, is then slid over optical fibers  42  and inserted into optical fiber cable  56  until only a small length of the tube remains exposed. The optical fiber loop  46  is then functionally coupled to the ends  44  of optical fibers  42  by, for example, a fusion splice. Housing  48  can then be at least partially filled with potting compound  58  to secure the fiber loop  46 . Subsequently, sleeve  54  is slid toward optical fiber loop  46  until the loop is small enough to be inserted into housing  48 , and housing  48  is moved over the optical fiber loop  46  until the loop clears any restrictions. Sleeve  54  is then slid a short distance back into optical fiber cable  56  to allow fiber loop  46  to expand to, for example, the full inside diameter of housing  48 . A connection end  62  of housing  48  is moved over optical fiber cable  56  and into engagement with cable seal assembly  60  so that a seal can be formed between housing  48  and optical fiber cable  56 . 
         [0027]    Formation of the optical fiber turnaround in this manner avoids the addition of a variety of components into the overall tubing string. For example, no mandrels or shrouds are required to mount and protect the optical turnaround. Protection of optical turnaround  50  is achieved through the systems and procedures described above, resulting in significant cost savings. 
         [0028]    An alternate embodiment of optical turnaround  50  is illustrated in  FIGS. 4-6 . In this embodiment, a single tube  64  is used for routing optical fibers  42  and optical fiber loop  46 . It should be noted that optical fibers  42  and fiber loop  46  can all be part of a single optical fiber routed through the optical turnaround  50 . 
         [0029]    As illustrated in  FIG. 4 , the tube  64  is deformed to create an optical fiber turnaround region  66  having a span  68  greater than the undeformed internal diameter  70  of tube  64 . The deformation of tube  64  to create turnaround region  66  also enables the use of an optical fiber loop  46  having a diameter greater than the internal diameter  70  of tube  64 . In the embodiment illustrated, tube  64  comprises a deformable, metal tube. The tube  64  is pinched or otherwise deformed from its exterior to create span  68  in one direction and a narrower structure in the perpendicular direction, as further illustrated in  FIGS. 5 and 6 . 
         [0030]    The optical fiber turnaround  50  illustrated in  FIGS. 4-6  also can be combined with an insert  72  that is inserted into tube  64  so that it resides in optical fiber turnaround region  66 . The insert  72  is sized to receive optical fiber and to hold optical fiber loop  46 . The insert  72  also is sized to support tube  64  in optical fiber turnaround region  66  (see  FIG. 6 ) to prevent the tube from further collapsing under external pressure. 
         [0031]    In creating this embodiment of optical fiber turnaround  50 , insert  72  is initially placed within tube  64  at the desired optical fiber turnaround region  66 . An appropriate pressing tool is then used to pinch tube  64  from the outside to reduce its dimension in the direction of pinching and to increase its dimension in the opposite or perpendicular direction. The wall of tube  64  is pinched until it touches insert  72  which allows the insert  72  to support tube  64  against further collapse. The extremities of tube  64  on one or both sides of optical fiber turnaround region  66  can remain undeformed to enable connection to other tubes or to enable termination using suitable pipe termination fittings. 
         [0032]    As further illustrated in  FIGS. 7 and 8 , insert  72  may comprise a passage  74  through which optical fiber loop  46  extends. The passage  74  may have a curved portion  76  or other suitably shaped portion to enable the optical fiber loop to be placed inside tube  64  in a desired shape and curvature. Furthermore, curved portion  76  may be formed with a trough  78 , when viewed in cross-section, to hold and protect the optical fiber loop  46 . By way of example, insert  72  also may comprise a generally flat midsection  80  positioned between a pair of a larger ends  82 , as illustrated best in  FIG. 8 . 
         [0033]    The optical fiber loop  46  can be formed by allowing a single optical fiber  42  to be routed through passage  74  and around at least a portion of insert  72  until it is allowed to turn around and extend back up through tube  64 . The diameter of the optical fiber loop is larger than the inside diameter  70  of tube  64  and may, for example, be in the range of more than 1 and less than 1.57 times the inside diameter  70 . In many applications, a suitable diameter for optical fiber loop  46  is in the range of 1.1 to 1.4 times larger than the inside diameter  70  of tube  64 . 
         [0034]    In some embodiments, optical fiber loop  46  and the optical fiber sections  42  extending from loop  46  can be further protected by an auxiliary sheath or tube  80  disposed around the optical fiber. The auxiliary tube  80  can be made from appropriate, flexible materials including polytetrafluoroethylene (PTFE) or thin-walled metal. 
         [0035]    Another embodiment of insert  72  is illustrated in  FIGS. 9 and 10 . This latter embodiment is similar to the embodiment described with respect to  FIGS. 7 and 8 , but it includes a lengthened section  84 , as best illustrated in  FIG. 9 . A distal end  86  of section  84  is collapsed on itself (see  FIG. 10 ) to enable the end of the insert and/or the end of tube  64  to be closed. Distal end  86  can be sealed by a linear weld  88  or other suitable sealing mechanism. 
         [0036]    The embodiments of optical turnaround  50  can be formed from a variety of materials and components. Additionally, the optical turnarounds  50  can be used to facilitate construction of a variety of optical fiber sensor systems that are used in many environments and applications. The optical fiber sensor system and turnaround are suited to well related applications, but the system and methodology also can be applied in other applications. 
         [0037]    Accordingly, although only a few embodiments of the present invention have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this invention. Such modifications are intended to be included within the scope of this invention as defined in the claims.

Summary:
A technique is provided for utilizing an optical fiber in a variety of sensing applications and environments by beneficially routing the optical fiber. A continuous optical fiber is created to provide optical continuity between two ends of the optical fiber. The optical continuity is created with the assistance of an optical turnaround constructed in a simple, dependable form able to control the bend of the optical fiber as it extends through the optical turnaround.