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CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a national phase of International Application PCT/IB2011/001059 which claims priority to U.S. Provisional Application No. 61/310,099 filed on Mar. 3, 2010, the disclosures of which are incorporated herein by reference for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The current invention relates to a coiled tubing injector and, more particularly, to a mounting arrangement for a coiled tubing injector of the type used for inserting and withdrawing coiled tubing into and out of a well bore. 
     BACKGROUND OF THE INVENTION 
     The use of coiled tubing injectors for drilling oil and gas well has risen dramatically in recent years. More particularly, the use of coiled tubing injectors in the use of directional drilling has gained widespread acceptance. 
     In the drilling of vertical, directional, or horizontal wells, there is a need for accurately controlling the weight on the drill bit (WOB). Accurate control of WOB is particularly critical when either directional or horizontal wells are being drilled. In directional or horizontal wells, the weight on the drill bit affects the angular deviation of the drill hole away from the vertical. By obtaining an accurate time measurement of the duration of travel of the rotary bit within the well bore, together with providing a way of accurately limiting the loads that are placed on the drill bit, it is possible to execute delicate and sophisticated drilling operations while minimizing downhole tool failures and maximizing the life of the drill bits. 
     U.S. Patent Publication 2008/0296013 (&#39;013 Publication), incorporated herein by reference for all purposes, discloses a top mounted injector for coiled tubing injection comprising an injector supported from a mounting component in a support system e.g. a mast, the mounting component including a carrier which is engageable with the mast for transferring to the mast the forces exerted on the mounting component from the injector component during the injection and withdrawal of tubing by the injector component. The &#39;013 Publication discloses a strain gauge deployed between the injector and the mounting component for providing continuing indication of the forces developed in injecting or withdrawing the tubing from the borehole and consequently the force transferred between the injector to the mast through the mounting component. However, in the arrangement shown in the &#39;013 Publication, vis-à-vis determining accurate WOB, the arrangement in the &#39;013 Publication suffers from the fact that the injector is suspended via one strain gauge and a hinge. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, there is provided a coiled tubing injector assembly mounted in a mast or other support which permits more accurate determination of WOB. 
     In another aspect of the present invention, there is provided a coiled tubing injector assembly, including a coiled tubing injector, an injector mount and load cells, particularly in the form of load cell pins, interconnecting the mount and the injector. 
     The coiled tubing injector system of the present invention can comprise a coiled tubing injector having a guide arch and a mount, the injector being interconnected to the mount by at least two load cells. The load cells are positioned between the injector and the mount, such that any forces exerted on the injector by flexing or twisting of the guide arch and/or coiled tubing guided by the arch are detected by the load cells and subsequently accounted for so that an accurate WOB measurement is achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevational view of one embodiment of the present invention. 
         FIG. 2  is an elevational side view of the embodiment shown in  FIG. 1 . 
         FIG. 3  is an elevational view of another embodiment of the present invention; and 
         FIG. 4  is a schematic showing the use of a summing computer with the coiled tubing assembly of the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In prior art coiled tubing injector systems, load cells are commonly used to measure WOB. However, with these prior art systems, WOB is rarely accurate because the goose neck or guide arch on the injector exerts forces on the injector, which affects the reading of the load cells. In particular, as tubing is spooled from the storage reel into the wellbore, it exerts a force which tries to pull the guide arch toward the injector. However, the guide arch effectively acts as a lever exerting a counter-upward pull. Accordingly, as coiled tubing is unspooled from the reel, the load fluctuates and it is not uncommon for the load cell readings to go from positive to negative quickly, thus rendering the measurement of WOB inaccurate if not meaningless. The above problem is solved by the present invention. 
     Referring first to  FIGS. 1 and 2 , there is shown one embodiment of the present invention wherein the coiled tubing injector is bottom mounted. The coiled tubing injector, shown generally as  10 , includes a pair of continuous linked drive chains,  12  and  14 , having opposed flights on opposite sides of the passage of the coiled tubing  16  therebetween. As is well known to those skilled in the art, the drive chains  12 ,  14  carry a series of gripping blocks  18 ,  20 , respectively, to grip the coiled tubing, as it is injected into or pulled from the well. The chains  12 ,  14  are driven by a pair of upper, drive sprockets  20  and  22 , respectively. Chains  12 ,  14  are also rotably mounted on lower, idler sprockets  24  and  26 , respectively. 
     The injector mount comprises a base  28  having an upwardly extending frame  32  attached thereto, base  28  being supported on a mast or the like, a portion of which is shown as  30 . 
     As can be seen, mount  28  comprises part of a generally rectangular frame, shown generally as  32 . Frame  32  has a top portion  34 , to which is connected a guide arch  36 , well known to those skilled in the art. 
     As best seen in  FIG. 2 , injector  10  is connected to spaced beams  40  and  42  in a manner described hereafter. Extending upwardly from base  28  are eye brackets  44 ,  46 ,  48  and  50 . Eye brackets  44 - 50  have eyes or holes therethrough, which are in register. In like fashion, beams  40  and  42  have spaced apertures which are in register with the eyes in brackets  44 - 50 . As can be seen in  FIG. 1 , beam  40  has one such aperture  52  and a second such aperture  54 . It will be appreciated that beam  42  is of like construction. When beams  40  and  42  are properly positioned, the eyes in brackets  40 - 50  are in register with the apertures in the beams  40 ,  42 . With reference to  FIG. 1 , it can be seen that beam  40  has a first aperture  52  and a second aperture  54 . Received in the eyes of brackets  44  and  46  and the aperture  54  in beam  40  is a load cell in the form of a load pin  60 . In like fashion, a second load pin  62  is received in the eyes of brackets  48  and  50 , and the registering aperture in beam  42 . It will also be appreciated, as can be seen from  FIGS. 1 and 2 , that beams  40  and  42  are interconnected to eye brackets on both ends in the manner shown in  FIG. 2 . In other words, there are four load pins interconnecting injector  10  via the beams  40  and  42  to mount or base  28  by virtue of eight eye brackets, four of which are shown as  44 - 50  by means of four load pins, two of which are shown as  60  and  62 . Accordingly, any weight or force on injector  10 , including string weight downhole is transmitted to the load pins. 
     Referring now to  FIG. 3 , there is shown another embodiment of the present invention wherein the injector is top mounted as opposed to the embodiment shown in  FIGS. 1 and 2 , wherein the injector  10  is bottom mounted. In other words, in the embodiment shown in  FIGS. 1 and 2 , the injector  10  rests on the load pins, whereas in the embodiment shown in  FIG. 3 , the injector is suspended from the load pins. 
     Referring now to  FIG. 3 , the injector shown generally as  70  comprises first and second endless chains  72  and  74  carrying gripping blocks  76  and  78 , respectively. Chain  72  is mounted on drive sprocket  80  and caller sprocket  82 . In like manner, chain  70  is mounted on drive sprocket  84  and caller sprocket  86 . Guide arch  36  is connected to the upper surface  88  of the mount  90 , mount  90  being connected or supported by a mast, a portion of which is shown as  92 . As in the case of the embodiment shown in  FIGS. 1 and 2 , in the embodiment shown in  FIG. 3 , eight eye brackets, two of which are shown as  94  and  96 , are connected to mount  90 . First and second beams, only one of which is shown as  98 , are interconnected to mount  90  via four load pins, two of which are shown as  100  and  102  in a manner similar to that described in connection with the embodiment shown in  FIGS. 1 and 2 . In essence, while in the embodiment shown in  FIGS. 1 and 2 , injector  10  rests upon four load pins, in the embodiment shown in  FIG. 3 , injector  10  is suspended by four load pins. 
     As is well known to those skilled in the art, in general load cells utilize strain gauge technology. In the most basic form, load cells convert force into an electrical signal, which can then be converted to measure weight or force in a number of different applications. Thus, load cells can be used to measure compression, tension, bending or shear. Although the present invention has been described with particular reference to use of load measuring pins, commonly known as load pins, other types of load cells could be employed if desired albeit that mounting complexity might be increased. Thus, for example, compression load cells, tension load cells, tension and compression load cells, beam load cells, load measuring shackle, load monitoring links are all types of load cells that could be used in connection with the present invention. 
     Referring now to  FIG. 4 , there is shown an embodiment of the present invention wherein the load cells, be it the embodiment of  FIGS. 1 and 2  or the embodiment of  FIG. 3 , have their outputs connected to a computer, preferably a summing computer shown generally as  120 . As noted above, load cells produce an electrical signal which is ultimately converted to force or weight. To accomplish this, and in the case of the present invention in the embodiment where four load cells are employed, typically the signal from each load cell would be sent to computer  120 . The summing computer, or for that matter, a PLC, can determine what force or weight is being exerted on each load cell, in the case of the present invention, usually weight, which can then be summed to determine the WOB. As noted, when an injector is in use, uneven loading on the injector can occur, meaning that the load on one load cell is not the same as load on another load cell. In the case of the present invention, it would not be uncommon for the injector to be slightly canted such that the weight on the load cells on one end of the beams would be greater than the weight on the load cells on the opposite end of the beams. 
     In prior art coiled tubing injectors, it was common to use a single load cell in an attempt to measure WOB. However, because there are so many other forces, primarily from the guide arch and/or the coiled being guided thereby, a single load cell will not provide an accurate WOB. In the present invention, there are at least two load cells, and they are positioned between the injector mount and the injector, such that any force exerted by the guide arch and/or the coil tubing is detected and accounted for by the summing computer. For example, assume, as is shown in the preferred embodiments, there are four load cells in a generally rectangular pattern as per the embodiments described above. If it is now assumed that there is 1000 lbs. acting directly in the middle of the rectangle defined by the four load cells, each of the load cells will see 250 lbs. If the injector is now pulled 45° in the direction of the guide arch, the top left load cell; e.g., load cell  60  in  FIG. 2 , would show nothing, while the bottom right load cell would show 500 lbs. But the summing computer, gathering data from all the load cells, will still see 1000 lbs. Assuming that 1000 lbs. is the accurate WOB, then any force exerted by the guide arch has been taken into account, meaning the WOB measurement is correct. In effect, summing computer  120  takes an average of the readings of the four load cells in the preferred embodiment described above regardless. It will be understood that at times the coil is being pushed into the wellbore and at other times it is being pulled out. Accordingly, the WOB can be negative. At a minimum, there must be two load cells and one of them must be positioned proximal the first chain drive; e.g., chain  12  while the other load cell must be positioned proximal the other chain drive; i.e., chain  14 , but not necessarily the same distance. Also, the two load cells cannot be positioned on the same side of the center line of the injector as determined by the path of the coiled tubing through the injector. It will further be understood that where only two load cells are used in the manner just described, WOB readings might not be as accurate because of the various ways the guide arch can flex, twist or swivel on the frame. However, the present invention clearly contemplates the use of two load cells positioned so as to provide a WOB measurement with any forces exerted by the guide arch being accounted for. In a more general sense, the more load cells that are employed, the more accurate the measurement. While in the preferred embodiment described above, there are four load cells in a generally rectangular pattern, it will be understood that four load cells in a diamond pattern (as viewed in plan view) or for that matter in a circular pattern (as viewed in plan view) would also work effectively. Indeed, any pattern and any number of load cells can be employed as long as the pattern is such that any forces which are not a result of WOB are detected by the load cells. 
     Although specific embodiments of the invention have been described herein in some detail, this has been done solely for the purposes of explaining 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 are 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 its scope.

Summary:
A coiled tubing injector assembly having an arch for guiding coiled tubing into the injector and, including an injector selectively engageable with coiled tubing for forcing the coiled tubing through the injector in an upward or downward direction. There is also an injector mount and the injector is interconnected to the mount via a plurality of load cells, such that any forces exerted on the injector by flexing or twisting of the arch and/or coiled tubing guided by the arch are detected by the load cells.