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RELATED CASE  
       [0001]     This application claims priority from U.S. Ser. No. 60/433,259 filed Dec. 13, 2002. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to a subsea coiled tubing injector and, more particularly, to a subsea injector with a pressure compensated drive system.  
       BACKGROUND OF THE INVENTION  
       [0003]     Coiled tubing has been used for decades in land-based hydrocarbon recovery operations to perform various well treatment, stimulation, injection, and recovery functions more efficiently than with threaded tubulars. In a conventional land-based operation, the coiled tubing injector may use a gear drive mechanism with conventional bearing assemblies to reliably and efficiently transmit power to the coiled tubing.  
         [0004]     While conventional coiled tubing injectors may work satisfactorily for land-based or shallow-water operations, they would not work in deeper water because the drive mechanism for the injector is not sufficiently protected from the subsea environment. Specifically, the hydrostatic pressure at such depths is sufficient to penetrate past the seals used on lubricated components such as the gear case and bearing assemblies of land-based equipment. A proposed solution to this problem is disclosed in U.S. Pat. No. 4,899,823, whereby the tubing injector is protected subsea by an enclosure surrounding substantially the entire tubing injector. Seals are provided between the enclosure and the coiled tubing above and below the injector. An obvious disadvantage of this solution is the size of the housing and complexity of enclosing the entire injector with the housing.  
         [0005]     An improved coiled tubing injector for subsea use is therefore desirable.  
       SUMMARY OF THE INVENTION  
       [0006]     A pressure-compensated tubing injector is disclosed for injecting coiled tubing into a subsea wellhead or flowline. The injector comprises a traction device including a plurality of opposing grippers carried on respective opposing chain loops for gripping engagement with the coiled tubing and longitudinally movable with the coiled tubing. A plurality of outboard bearing assemblies guide movement of the opposing chain loops. The bearing assemblies may comprise first and second pairs of bearing assemblies, each pair for guiding movement of a respective one of the opposing chain loops. A drive unit powers the opposing chain loops to move the chain loops and the grippers carried thereon. The drive unit includes a sealed gear case. A pressure compensator in communication with the sealed gear case is responsive to subsea pressure, such that pressure within the sealed gear case is functionally related to subsea pressure.  
         [0007]     The pressure compensator may be placed in communication with one or more of the outboard bearing assemblies, such that pressure within the one or more compensated outboard bearing assemblies is functionally related to subsea pressure.  
         [0008]     The pressure compensator may comprise a compensator housing structurally separate from the gear case and bearing assemblies and having a sealed internal cavity in communication with the sealed gear case. A movable element within the compensator housing is responsive to subsea pressure for varying a volume of the internal cavity. A biasing member may be included for biasing the movable element, preferably to increase pressure.  
         [0009]     Conduit may extend between the pressure compensator and the sealed gear case for placing the pressure compensator in fluid communication with the sealed gear case. Conduit may also extend between the pressure compensator and the one or more outboard bearing assemblies, for placing the pressure compensator in “direct” fluid communication with the bearing assemblies. Conduit may alternatively extend between the sealed gear case and the one or more outboard bearing assemblies, for placing the pressure compensator in “indirect” fluid communication with the bearing assemblies.  
         [0010]     The bearing assemblies may each comprise a self-contained a pressure compensator. A movable element is within a bore of a bearing shaft, and the bore is in fluid communication with a bearing cavity containing a lubricant within the bearing assemblies. The movable element is exposed on an inner surface to the lubricant and on an outer surface to subsea pressure.  
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a front view of a coiled tubing injector according to the present invention.  
         [0012]      FIG. 2  is a side view of the injector shown in  FIG. 1 .  
         [0013]      FIG. 3  is a pictorial view of a suitable pressure compensator shown in  FIG. 1 .  
         [0014]      FIG. 4  is an enlarged view of the traction system of the injector shown in  FIG. 1 , wherein the rollers are secured to the chain and ride along the support members.  
         [0015]      FIG. 5  is an enlarged view of an alternate embodiment of the traction system, wherein the rollers are secured to the support members, and the chain rides along the rollers.  
         [0016]      FIG. 6  shows a bearing assembly having a self-contained pressure compensator having a piston movable within a bore of a shaft.  
         [0017]      FIG. 7  shows a cutaway of the built-in pressure compensator of  FIG. 6 .  
         [0018]      FIG. 8  shows a cutaway of an alternate embodiment of the built-in pressure compensator using a diaphragm instead of a piston.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]      FIG. 1  shows a coiled tubing injector  10  for use in a subsea environment.  FIG. 2  is a side view of the injector  10  shown in  FIG. 1 . The injector  10  uses a traction assembly  12 , shown more closely in  FIG. 4 , to engage the coiled tubing  13  and drive the coiled tubing  13  into or out of a well (not shown). The traction assembly  12  comprises opposing chain loops  15  guided by bearing assemblies  52 . Gripping members  14  are secured to individual links  16  of the chain loops  15 , so as to grip the coiled tubing  13 . The gripping members  14  and the chain loops  15  thus move together longitudinally at the area of contact with the coiled tubing  13 , to move the coiled tubing  13  into or out of the well.  
         [0020]     A plurality of rollers  20 , as shown in  FIG. 1  and more closely in  FIG. 4 , are secured to the links  16  of the chain loops  15 , and roll along support members  19 . The support members  19  are moved laterally inwardly to urge the gripping members  14  into engagement with the coiled tubing  13  with sufficient force to grip the coiled tubing  13 . The rollers  20  allow for a large lateral load to be applied, preferably without inducing a significant longitudinal drag load.  FIG. 5  illustrates an alternate design, whereby the rollers  20  are instead secured to support members  17 , and the chain loops  15  instead ride along and move relative to the rollers  20 .  
         [0021]     The bearing assemblies  52  and an injector gear case  54  as shown in  FIG. 1  are both preferably sealed to retain lubricant and prevent intrusion of sea water. The bearing assemblies  52  are preferably outboard bearing assemblies, because the portion of the housing  55  adjacent the sealed gear case  54  may be open to seawater to accommodate the chain loops  15 . The chain loops  15  are typically routed over sprockets or gears (not shown) within the housing  55 , rotating about the axis of the bearings assemblies  52 , and the chain loops  15  are thus guided by the bearing assemblies  52 . A drive motor  11  drives the chain loops  15 , and is preferably hydraulically powered or possibly electrically powered. The gear case  54  may transmit energy from the drive motor  11  to the chain loops  15  using a plurality of gears within the gear case  54  and a drive shaft (not shown) sealably extending from the sealed gear case  54 .  
         [0022]     A commercially-available pressure compensator  60  is conceptually shown assembled with the injector  10  in  FIG. 1 , and illustrated more closely in  FIG. 3 . The pressure compensator  60  compensates pressure within the gear case  54 , and may also compensate pressure within each outboard bearing assembly  52  and other components of the injector  10  that are sealed and sensitive to pressure differentials, such as the rollers  20 . The pressure compensator  60  may include a compensator housing  64  structurally separate from and attached to a portion of the injector  10  such as the outer housing of the gear case  54 . Lubricant is contained within the housing  64 , which is sealed from seawater. Conventional tubing or other conduit  62  may be used to fluidly connect and pass lubricant between the pressure compensator  60  and the gear case  54 , the bearing assemblies  52 , the rollers  20 , and other sealed components. A piston or diaphragm indicated schematically by a movable element  66  is movable with respect to the housing  64 . According to basic physics, the pressure on a surface of the movable element  66  is substantially equal to the hydrostatic pressure. As the hydrostatic pressure surrounding the pressure compensator  60  increases, such as when the injector  10  is lowered into a subsea environment, the movable element  66  moves inwardly with respect to the housing  64 . This increases the internal pressure of the compensator  60  and of the sealed components plumbed therewith, such as the gear case  54 , the bearing assemblies  52 , and the rollers  20 . Accordingly, this reduces the pressure differential that would otherwise exist between the seawater environment and the interior of the sealed components. Ideally, air from the enclosed volumes of the sealed components is evacuated and replaced by the lubricant prior to deployment of the injector  10 , to ensure the reliable transfer of lubricant in response to movement of the movable element  66 .  
         [0023]     The external pressure compensator  60  may be plumbed to the gear case  54  via conduit  62  to place the pressure compensator  60  in communication with the gear case  54 . The bearing assemblies  52  may then be placed either in “direct” communication with the pressure compensator  60  by plumbing directly between the pressure compensator  60  and bearing assemblies  52 , or “indirect” communication by plumbing from the gear case  54  to the bearing assemblies  52 . Alternatively, multiple external compensators (not shown) may be used to plumb to selected components. For example, one compensator  60  may be plumbed to the gear case  54 , and directly or indirectly to the two upper bearing assemblies  52  closer to the gear case  54 , and another compensator (not shown) may be positioned more closely and plumbed to the lower bearing assemblies  52  further from the gear case  54 .  
         [0024]     Instead of plumbing an external compensator to the bearing assemblies  52 , the bearing assemblies  52  may include a self-contained pressure compensator  70  within a bore  72  of a shaft  74 , as shown conceptually in  FIG. 6  and in closer detail in a cutaway view of  FIG. 7 . A piston  78  is sealed with the shaft bore  72  by a sealing member, which may be an o-ring  75 . The bore  72  is in fluid communication with a lubricant-containing bearing cavity  73  via flow passageway  69 . An optional spring  71  is secured adjacent an outer side  79  of the piston exposed to the subsea environment, and is secured at one end to the shaft  74  with a plate  76  or other securing member. The spring  71  selectively biases the piston  78  inwardly or outwardly. Preferably, the spring  71  biases the piston  78  inwardly to compress the volume of the bore  72  and cavity  73 , which results in an overbalancing pressure on the lubricant in the bearing cavity  73 . The pressure overbalancing further protects against intrusion of seawater into the bearing cavity  73  and bearings  80 , by offsetting the oppositely-directed subsea pressure attempting to infiltrate into the sealed cavity  73 .  
         [0025]     The cutaway view of  FIG. 8  shows a less preferred embodiment of the pressure compensator  70  of  FIGS. 6 and 7 . A flexible diaphragm  81  is used instead of the piston  78  within the bore  72  of the shaft  74 . The optional spring  71  biases the diaphragm  81  as it did the piston  78 .  
         [0026]     The coiled tubing injector of this invention is not limited to downhole recovery operations. For example, the tubing injector may also be used to perform pipeline maintenance operations. The pipeline version of the coiled tubing injector may be landed on the seabed and attached to an access valve in the pipeline using a lightweight connector. The pressure control system may consist of a gate valve a shear ram, and a set of strippers. Tools and/or fluid may then be conveyed in and out of the pipeline using the coiled tubing. Because the coiled tubing may be used to pull the tools back from where they were launched, there is no need for a pigging loop. The use of coiled tubing also allows various fluids to be pumped into the pipeline, which would be especially beneficial for removing sand or paraffin.  
         [0027]     Although specific embodiments of the invention have been described herein in some detail, it is to be understood that this has been done solely for the purposes of describing the various aspects of the invention, and is not intended to limit the scope of the invention as defined in the claims which follow. Those skilled in the art will understand that the embodiment shown and described is exemplary, and various other substitutions, alterations, and modifications, including but not limited to those design alternatives specifically discussed herein, may be made in the practice of the invention without departing from the spirit and scope of the invention.

Summary:
A pressure compensated tubing injector for injecting coiled tubing into a subsea wellhead or flowline. A traction device including grippers carried on respective opposing chain loops provides gripping engagement with the coiled tubing to longitudinally move the coiled tubing. A plurality of sealed outboard bearing assemblies guide movement of the chain loops. A drive unit powers the chain loops and includes a sealed gear case. One or more pressure compensators are placed in communication with the sealed gear case and one or more of the bearing assemblies, to compensate for subsea pressure and prevent intrusion of seawater. The bearing assemblies may include self-contained pressure compensators.