Load detection and indicator apparatus for well tubing or the like
FIELD OF THE INVENTION 
     The field of this invention is apparatus for detecting and indicating the amounts of loads or forces on objects such as well tubing supported by apparatus for moving the tubing into and/or out of a well. 
     BACKGROUND OF THE INVENTION 
     This invention is an improvement over the device disclosed in U.S. patent application Ser. No. 205,824, filed June 13, 1988. For a number of years, well tubing has been inserted into and removed from wells for performing various operations. Examples of apparatus for that purpose are shown in U.S. Pat. Nos. 3,182,877; 3,285,485; 3,313,346; 3,658,270; 3,675,719; 3,690,136; 3,690,381; 3,722,589; 3,722,594; and 3,791,447. U.S. Pat. Nos. 3,182,877 and 3,285,485 show an apparatus wherein the well tubing is lowered and raised through a vertically aligned two-gauge means for detecting and indicating loads or forces on the well tubing in both upward and downward directions. It has been common practice in more recent years to provide a pivoted mounting of such equipment in conjunction with a load cell for detecting and indicating the compressive loading or forces on the pipe in the downward direction only, an illustration of which is shown in FIG. 1 of the drawings, as will be more fully explained. 
     SUMMARY OF THE INVENTION 
     This invention relates to a new and improved apparatus for detecting and indicating loads on elongate objects such as well tubing or the like which moves relative to the apparatus. 
     More specifically, the apparatus is a load cell which is disposed concentrically below a well tubing injector apparatus with guide means to restrict angular deviation of the well tubing from the well axis and kinking of the well tubing in the event of a buckling load. The load cell has a separate compensating means for providing neutral buoyancy to the injector apparatus apart from the means for measuring either positive or negative loads on the injector apparatus relative to such neutral buoyancy position. The positive and negative measuring mechanisms are separated and arranged to ensure no lost motion during cross-over from positive to negative loads, or vice versa, and to also obtain independent operation of each of two gauges to assure accurate gauge readings for both positive and negative loads. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation which generally illustrates a typical prior art injector apparatus with a prior art load cell arrangement; 
     FIG. 2 is an elevation which generally illustrates the invention described in application Ser. No. 205,824, filed June 13, 1988, showing the load cell apparatus for injecting or withdrawing tubing from a well; 
     FIG. 3 is a view partly in section and partly in elevation, illustrating the load cell apparatus of application Ser. No. 205,824, filed June 13, 1988; 
     FIG. 4 is a view partly in section and partly in elevation, illustrating the load cell apparatus of the present invention when the tubing is not imparting a load or force in either direction on the apparatus; 
     FIG. 4A is a view similar to FIG. 4, but showing the load cell apparatus with a load applied to the load cell apparatus by the tubing as it is injected downwardly into the well; and 
     FIG. 4B is a view similar to FIG. 4, but showing the load cell apparatus when the tubing is exerting a load or force in the upward direction. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the drawings, the letter A designates generally the injector apparatus which is only schematically illustrated. A detailed example of an injector apparatus is disclosed in U.S. Pat. No. 3,285,485. Briefly, the injector apparatus A is adapted to feed or inject a tubing or pipe T between two endless drive chains 10 for the purpose of feeding or withdrawing the tubing T into or out of a well through wellhead equipment W at the upper end of the well. The typical wellhead equipment W illustrated schematically in the drawings includes blowout preventers and other controls which form no part of the present invention. The prior art apparatus shown in FIG. 1 is not shown in any patent so far as is known, but it is typical of the load cell apparatus used with well tubing in recent years. Considering the illustrating of such prior art apparatus in FIG. 1, as shown schematically, the injector apparatus A has a pair of endless chain drive assemblies 10 mounted on a base 12. The base 12 is pivotally mounted to the well platform P by a pivoted arrangement which is schematically shown as a downwardly extending pivot member 15 which is pivotally connected to an upstanding pivot member 16 mounted on the platform P, with a pivot pin 17 providing for the pivoting movement of the member 15 relative to the member 16. The member 15 is welded or is otherwise secured to the base 12 so that the central portion of the apparatus A can pivot or move about the pivot pin 17. 
     On the other side of the base 12 from the pivot, a conventional load cell L has been used in the past. Such load cell is schematically shown in FIG. 1 as having a piston 150 in a closed chamber 152, which has fluid below the piston 150. In the typical use of the apparatus A, the tubing T has been forced into a well through the well equipment W, where the well is under pressure so that it requires a pushing force downwardly on the tubing T to force it into the well. This is particularly true where the lower end of the tubing T has a check valve closing off the lower end of the tubing T, which is a typical way in which the tubing T is injected into a well. In the past, with the prior art arrangement of FIG. 1, the force or load on the tubing T has been measured with the load cell L by the chain drive assemblies 10 of the apparatus A tilting about the pivot pin 17 in a clockwise direction as viewed in FIG. 1. Such movement imparted a compressive force to the liquid in the chamber 152 below the piston 150 which was responsive to the amount of force or load on the tubing T. That force was indicated on a typical conventional gauge. 
     The invention of application ser. No. 205,824 is shown schematically in FIG. 2 and in detail in FIG. 3 of the drawings. The parts of the apparatus of this invention shown in FIG. 2 which are the same as in FIG. 1 are identified with the same letters and numerals as in FIG. 1. As can be seen in FIG. 2, the injector apparatus A is mounted on a load cell apparatus C which is disposed longitudinally directly below the apparatus A with the longitudinal passage between the endless chain assemblies 10 in alignment with the longitudinal opening through the well equipment W. The base 12 is mounted on the load cell apparatus C so that it does not pivot. Also, the load cell apparatus C is so constructed that readings of the amount of the load on the tubing T in either the downward direction or the upward direction can be obtained. 
     The load cell C illustrated in FIG. 2 is shown in detail in FIG. 3. In FIG. 3, the load cell apparatus C of this invention is shown positioned between and connected to the base 12 of the apparatus A and the platform P for the well. Apparatus A pushes tubing into the well via the central opening longitudinally through tubular guide 130. As upward or downward force is exerted by the tubing, Apparatus A moves slightly upwardly or downwardly along with lower piston 162 or upper piston 164, depending on the direction of motion. Load cell housing 140 remains stationary, causing an increase or decrease in pressure in fluid pressure sections 152, 154. This operation is similar to the functioning of a portion of the present invention, as will now be explained in more detail. 
     Referring now to FIG. 4, the present invention is illustrated. The base 12 is connected to the injector apparatus A. The load cell apparatus C of FIG. 4 is disposed between such base 12 and the platform P. The tubular guide is formed of two parts, an inner tubular guide member 32 and an outer tubular guide member 34. By means of a lower annular plate 36 and bolts 31, 33 the guide members 32 and 34 are connected to the base 12 and are movable longitudinally therewith. 
     The diameter of the inner surface 32a of the inner tubular guide member 32 is large enough for the tubing T to move freely through the member 32, but the annular spacing of such inner surface 32a from the tubing is close enough to serve as a guide for the tubing as it moves substantially vertically into and out of the well, and additionally to provide lateral support for the tubing T to prevent it from buckling if an obstruction is encountered by the tubing T when moving into the well. Further, the member 32 may be interchangeable with other guide members having smaller or larger diameters of the inner surface 32a to provide the foregoing features for tubing of different outside diameters. 
     The fluid pressure housing 40 is formed by a cylinder 42 and an upper annular plate 44. Bolts 41, 43 secure the housing sections 42 and 44 together and also to the platform 22. A fluid pressure chamber is provided between the guide cylinder 34 and the housing cylinder 42. Such chamber has an upper fluid pressure section 54 and a lower fluid pressure section 52. A central part of the cylinder 42 separates the sections 52 and 54. Cylinder 42 has suitable resilient seals 42a, 42b preferably made of rubber or other elastomer which seal against the external surfaces of upper piston 64 and lower piston 62, respectively. The upper piston 64 has a similar seal 64a and the lower piston 62 has a similar seal 62a for sealing against the inside annular surfaces of the housing cylinder 42. Also, a seal 44a is provided to seal the upper end of the fluid pressure chamber, and a seal 36a closes off the lower end of the fluid pressure chamber from compensating chamber 56. 
     The external surface of the guide member 34 has an annular lateral shoulder 34a which is in lateral alignment with the lower annular surface 44b of upper annular plate 44; these surfaces are engaged by the upper surface 64b of the piston 64 when the piston 64 is in the upper position shown in FIG. 4. The shoulder 34a engages the piston 64 to transmit downward movements of the guide 32, 34 to the piston 64, as will be explained. 
     FIG. 4 illustrates the modified load cell C in the neutral position when the tubing T is not exerting a force in either direction on the base 12. Preferably, the compensating chamber 56 is connected to a liquid pump P through a pressure regulator R, by means of a connector pipe 70 extending from opening 36b in movable member 36, or alternatively to an accumulator (not shown) whose function is to provide more or less constant pressure to the compensating chamber 56 while not restricting downward movement of members 32, 34, 36 and also providing pressurized make-up fluid to the compensating chamber 56 during upward movement. In the preferred embodiment, the liquid in the compensating chamber 56 is pressurized by the pump a sufficient amount to apply a lifting force to the base 12 and the portion of the apparatus A which is supported thereon so as to offset the weight of such apparatus, thus making it weightless as if floating on the liquid in the compensating chamber 56. Because of such floating arrangement, the variations which appear on the gauges 82, 84 are indications of only the amount and direction of the force exerted by the tubing T which is supported by the apparatus A. 
     Referring to FIG. 4A, when a downward force is exerted by the tubing T on the apparatus A, the piston 64 is moved downwardly from the position of FIG. 4 to the position of FIG. 4A due to the downward movement of the guide cylinders 32, 34. Such downward movement of the piston 64 applies a compressive force on the liquid in the upper fluid pressure chamber section 54. Such increase in pressure is transmitted through the opening 42d to gauge 84. It is to be noted that the lower piston 62 does not move downwardly when the piston 64 moves downwardly since they are operable and movable independently of each other. Any tendency of the piston 62 to move downwardly would tend to create a vacuum in chamber section 52 which would hold it in its position as illustrated in FIG. 4A. Also, it will be understood that the amount of movement of the piston 64 illustrated in FIG. 4A is exaggerated for purposes of illustration. 
     Referring now to FIG. 4B, the apparatus is shown in a position wherein the tubing T is exerting an upward longitudinal force on the base 12. Such upward force causes the lower piston 62 to be moved upwardly by the upward movement of the plate 36 and the guide members 32, 34, upon engagement of the upper surface 36c with the lower surface 62b of piston 62. The fluid in the lower fluid pressure chamber section 52 is pressurized in response to the amount of force in the upward direction from the tubing T and the increased pressure is indicated through a port 42c connected to the lower gauge 82. 
     An intrinsic feature of this invention is that during the initial filling of the fluid chamber sections 52, 54, both pistons 62, 64 are compelled to move away from each other and they come to rest against the shouldered parts of the movable guide cylinder 34 and plate 36 (FIG. 4), thereby ensuring intimate contact therebetween, thereby assuring that there is no slack in the movements of the pistons 62, 64 during use, regardless of the manufacturing tolerances of those parts. Such removal of slack or gaps in the movements of the pistons 62, 64 is valuable since working travel of the movable pistons 62, 64 is measured in very small increments. Thus, the present invention provides for a high degree of accuracy in the force indications on the gauges 82, 84. 
     Further, because the two chamber sections 52, 54 are divided by an impermeable barrier with seals 42a, 42b (FIG. 4), inaccuracies due to communication between the two chambers sections 52, 54 are prevented. Therefore, separate independent fluid or pressure signals are reflected on the gauges 82 and 84. Each of the pistons 62, 64 is free to move independently of the other piston and neither is attached to the movable guide cylinder 34. Of prime importance is the fact that, only in the neutral position, i.e., when no load is placed on the load cell C in either direction by the tubing T, does the movable guide cylinder 34 contact both pistons 62, 64 simultaneously (FIG. 4). The moment that the movable guide cylinder 34 moves, it loses contact with one piston but remains in contact with the opposite piston. It is for this reason that the chamber sections 52 and 54 are never subjected to a vacuum-causing load; the error producing section drag, which would be present in a single piston structure, never exists in the present invention. 
     Although the invention has been disclosed for use with well tubing which is disposed in a vertical or substantially vertical position, it should be understood that the load cell disclosed herein may be used in directions other than vertical and with equipment other than tubing strings. 
     The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction may be made without departing from the spirit of the invention.