Abstract:
A mobile service rig for servicing an oil well includes a variable speed engine and a multi-speed transmission that selectively powers a drive wheel for transport, a hoist for lifting and lowering well-related components, and a hydraulic circuit for a tong used in tightening and loosening sucker rods or tubing. A speed adjuster operatively coupled to the engine limits the speed of the engine when the tong is operating, while a flow restriction limits the rate of hydraulic fluid flowing through the tong. Such an arrangement reduces power consumption, reduces heat, and avoids over tightening a sucker rod connection.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention generally pertains to mobile service rigs for wells and more specifically to a mobile service rig that includes an engine powering a hoist and a tong. 
     2. Description of Related Art 
     Oil wells and wells for other fluids typically include a well casing, tubing, sucker rods and a reciprocating drive unit. A well casing is what lines the well bore and usually comprises a long string of relatively large diameter pipe interconnected by threaded couplings known as collars. Casings generally define the overall diameter and depth of a well bore. Well tubing typically comprises a long string of pipe sections whose threaded ends are also interconnected by threaded couplings. The tubing extends down through the casing and provides a conduit for conveying oil or some other fluid to the surface of the well. A submerged reciprocating pump attached to the lower end of the tubing draws the fluid from the annulus between the inside diameter of the casing and the outside diameter of the tubing, and forces the fluid up through the tubing to the surface. To operate the pump, a string of sucker rods extends through the tubing to serve as a long reciprocating connecting rod that couples the submerged pump to a reciprocating drive unit at ground level. A string of sucker rods typically includes numerous sucker rods whose ends are interconnected by a threaded rod coupling. 
     Servicing oil wells and other types of wells can involve a variety of tasks that include, but are not limited to, installing or removing sections of casing, sucker rods, tubing and pumps. The various tasks each have their own particular needs. 
     When working with sucker rods, a rod tong is often used for making-up and/or disassembling a string of rods. A typical rod tong is a hydraulically powered wrench that turns one sucker rod relative to an adjacent one so that one or the other screws into or unscrews from the rods&#39; adjoining coupling. Since sucker rods are continuously subjected to a pulsating or reciprocating load, fatigue may cause a rod coupling to separate if the coupling had been over or under tightened when it was first installed. Thus, sucker rods should be tightened in a precise manner. 
     The assembly of tubing is less critical, as tubing is generally stationary in a well bore. To assembly or disassemble tubing, a tubing tong is often used, which also is a hydraulically powered wrench. Tubing tongs have serrated teeth that grip the outer wall of two adjacent tube sections, and then tighten the two sections into their mating coupling. The operation typically involves substantially more power than what is required when working with sucker rods, as the diameter of tubing is significantly larger than that of rods. 
     Removing or replacing sections of casing often involves heavy lifting by way of a hoist operating at fill capacity. Full-power lifting may be required when the casing is stuck and difficult to remove from the well bore, or may be required simply due to the casing being relatively heavy. The hoist is also needed, but at a much lower lifting capacity, when installing or removing sucker rods. For tubing or for setting a pump, the hoist is generally operated at some intermediate capacity between that used for casings and sucker rods. 
     Since there are numerous tasks involved in servicing a particular well, and various wells can be hundreds of miles apart, it would be advantageous to equip a single vehicle with the all equipment needed to perform the various tasks. It would be further advantageous to provide such a vehicle with a single engine or prime mover to power the various equipment. However, that can be difficult to do, as the power requirements vary broadly among the various operations. 
     For example, to power or propel such a vehicle down the highway or to operate its hoist at full capacity may require a 400 hp diesel engine, while tightening or loosening sucker rods may only require 10 hp. Tightening or loosening tubing may require 30 hp. Thus if a single hydraulic pump is used to power both tubing tongs and rod tongs, such a pump should be able to provide 30 hp for tubing even though only 10 hp would be needed for tongs. Likewise, a single diesel engine should be able to provide 400 hp for vehicular transport and heavy hoisting even though only 30 hp is needed to power the hydraulic pump. The resulting power imbalances of such a system create some serious problems, particularly when installing or removing sucker rods. 
     With sucker rods, the rod tong typically operates at something less than 30 hp, while the hoist operates at a relatively low capacity (e.g., low weight, fast speed) to quickly move the sucker rods into position. The rod tong can preferably tighten or loosen a sucker rod coupling within the time it takes the hoist to get another rod into position. Thus, the hoist and the rod tong work in concert in removing or installing a string of sucker rods. To keep such an operation moving smoothly, an operator preferably does not divide his attention between the operations of the hoist and speed of the diesel engine (which powers the hoist and the pump that powers the rod tong). Thus, the operator typically just runs the engine at full speed, with the hoist transmission in low gear to keep the hoist operating at a reasonable speed. This wastes fuel, may tend to shorten the life of the engine, and generates a tremendous amount of waste heat in the hydraulic system that drives the rod tong. 
     SUMMARY OF THE INVENTION 
     To conserve fuel and reduce heat generated by a rod tong hydraulic circuit, it is an object of the invention to limit the engine speed of a mobile service rig when installing or removing rod tongs. 
     Another object of the invention is to provide a mobile service rig for servicing wells that includes a common engine for powering a drive wheel, a hoist and a hydraulic circuit for a tong, such that the speed of the engine is reduced in response to feedback from the hydraulic circuit. 
     Another object is to provide a mobile service rig for servicing wells that includes a common engine for powering a drive wheel, a hoist and a hydraulic circuit for a tong, such that the hydraulic circuit includes a flow restriction whose flow coefficient increases with a decrease in a pressure differential applied across the restriction, whereby the flow rate of fluid through the hydraulic circuit does not vary proportionally with changes in the pressure differential. 
     Yet another object of the invention is to provide a mobile service rig with a hydraulic system that includes a common hydraulic pump to selectively drive a rod tong and a tubing tong, and provide such a system with an appropriate flow restriction. 
     A further object is to provide a mobile service rig with a single engine driving a single transmission, which in turn selectively powers both a hoist and a drive wheel, and provide such a rig with an appropriate speed adjuster for the engine. 
     These and other objects of the invention are provided by a mobile service rig that includes an engine and a transmission that selectively powers a drive wheel, a hoist and a hydraulic circuit for a tong. A speed adjuster operatively coupled to the engine unattendantly limits the speed of the engine when the tong is operating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic view of a mobile service rig according to at least one embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A service rig  10 , of FIG. 1, includes a truck frame  12 ; an operator cab  14 ; at least one drive wheel  16 ; two front wheels  18 ; a diesel engine  20  (capable of about 400 hp); a hoist  22 ; and a transmission  24 , such as a General Motors or Allison transmission that includes one input shaft  26 , a first output shaft  28  and a second output shaft  30 . Input shaft  26  is coupled to engine  20 , second output shaft  30  is coupled to drive wheel  16  for propelling service rig  10  along the road, and first output shaft  28  is coupled to hoist  26  through a drive train  30  (e.g., gears, sprockets, chains, etc.). A clutch  32  selectively engages and disengages drive train  30  and a cable take-up reel  34  of hoist  22 . The rotation of reel  34  determines the drawing in and paying out of a cable  36  for respectively raising and lowering of a hook  38 . Service rig  10  also includes a hydraulic circuit  40  when connected to a tong  42 , wherein the term, “tong” refers to a tool adapted to torque two sucker rods  44  and  46  that are connected by a threaded coupling  48 . 
     Circuit  40  includes a hydraulic pump  50  (capable of about 30 hp), a flow restriction  52 , a main pressure relief valve  54 , a directional valve  56 , a secondary pressure relief valve  58  and a hydraulic motor  60  of tong  42 . An air actuated clutch  62  or dog clutch may couple hydraulic pump  50  to a flywheel or drive gear of transmission  24 , or may couple pump  50  more directly to engine  20  (e.g., via the engine&#39;s harmonic balancer). A discharge line  64  from pump  50  delivers pressurized hydraulic fluid through flow restriction  52  and onto an inlet port  66  of valve  56 . The hydraulic fluid returns to a suction port  68  of pump  50  by way of a return line  70 , which is connected to an exhaust port  72  of valve  56 . Valve  56  could comprise one or more valves in various configurations; however, in this example, valve  56  is a four-way, three-position spool valve that is manually actuated with a spring-return to a central neutral position. 
     In the neutral position, valve  56  connects discharge line  64  to return line  70  and closes off two valve ports  74  and  76 . One hydraulic line  78  connects valve port  74  to a motor port  80  of motor  60 , and second hydraulic line  82  connects valve port  76  to a second motor port  84 . Manually actuating valve  56  in one direction connects discharge line  64  and return line  70  to lines  78  and  82  respectively, which drives motor  60  in a direction that tightens or “makes” a sucker rod connection. Actuating valve  56  in the opposite direction connects discharge line  64  and return line  70  to lines  82  and  78  respectively, which reverses the rotation of motor  60  for unscrewing or “breaking” a sucker rod connection. 
     When making a connection, secondary pressure relief valve  58  limits the pressure that can be applied across motor  60 , thus helping to limit the extent to which a connection can be tightened. Relief  58  is preferably adjustable to suit sucker rods of various diameter. The main pressure relief valve  54  serves to limit the overall pressure that can be applied to hydraulic circuit  40 . Typical pressure relief settings of relief valves  58  and  54  might be 800 psig and 2,000 psig, respectively. 
     In some instances, hydraulic fluid at an appropriate pressure, but at an excessively high volume or flow rate, may allow tong  42  to accelerate to an exceptionally high speed before a sucker rod connection reaches what is known as its shoulder point. The shoulder point is where an axial face of a rod comes into metal-to-metal contact with a mating axial face of a coupling. In other words, the shoulder point is where the connection just begins tightening into a strained preloaded condition. If tong  42  is running excessively fast upon reaching the shoulder point, the rotational momentum of tong  42  plus the rotational momentum of a rotating sucker rod may provide enough kinetic energy to over tighten the connection, regardless of what pressure relief valve  58  opens. This is especially likely to occur if engine  20  is driving pump  50  at full speed; however, the problem may also occur at lower speeds. 
     Thus, flow restriction  52  is used to limit the volume or flow rate of hydraulic fluid passing through discharge line  64 . Ideally, restriction  52  would provide a constant flow rate (e.g. 14 gpm), regardless of how fast engine  20  is driving pump  50 . However, one economical solution to the problem is achieved by selecting a flow restriction whose flow coefficient increases with a decrease in a pressure differential applied across the restriction. An example of such a flow restriction is a model NS1600 COLORFLOW needle valve, by Parker Hannifin Corporation, of Elyrie, Ohio. The term, “flow coefficient” is defined as a ratio of the fluid flow to the pressure differential (e.g., gpm divided by psig). For example, when engine  20  is operating at 2,500 rpm, flow restriction  52  might convey 14 gpm, and when engine  20  slows down to 1,250 rpm (half of its original speed), the flow of hydraulic fluid might only drop 2 gpm to convey 12 gpm. Thus, the flow through restriction  52  might only change slightly with drastic changes in engine speed. 
     This allows engine  20  to run at full speed without delivering an excessive rate of flow to tong  42 , and also allows the speed of engine  20  to be reduced to a speed that more closely matches the relatively low power requirements of tong  42 . Reducing the speed of engine  20  lowers the pressure in discharge line  64  to a level below the pressure at which main relief valve  54  opens. In contrast, if relief valve  54  were to open to relieve pressure exceeding its set limit, a significant amount of heat could be generated at relief valve  54 . For example, if pressure relief valve  54  had to open to limit the pressure in discharge line 64 to 2,000 psig, and doing so allowed valve  54  to convey 10 gpm from discharge line  64  at 2,000 psig to return line  70  at zero psig, then about 30,000 Btu/hr (comparable to 11.6 hp) of waste heat is generated at valve  54 . Thus, it may be beneficial to reduce the speed of engine  20  so that pump  50  has a discharge pressure that is less than the pressure at which main relief valve  54  opens. 
     This can be accomplished by providing service rig  10  with a speed adjuster  86 , i.e., a device that selectively determines whether engine  20  operates at a lower speed mode or a higher speed mode. A lower speed mode can be a first range of speeds and the higher speed mode can be a second range of speeds, with the average of the first range being lower than that of the second range. Some overlap of the two ranges is possible. 
     Perhaps the simplest form of a speed adjuster is a switch  88 , which is schematically illustrated to encompass a variety of switches including, but not limited to, mechanical mechanisms (e.g. a governor  90  driven by engine  20 ), pneumatic mechanisms (e.g., diaphragms, vacuum lines, pneumatic valves, etc.), electrical mechanisms, electromechanical mechanisms (e.g., an engine driven alternator  92  that serves as one example of a tachometer by providing an output voltage or frequency that varies with engine speed), manually actuated electrical switches, electromechanically actuated switches (e.g., solenoid actuated relay), solid state switches (e.g., transistor, triac, diac, computer, programmable logic controller, etc.), transducers, sensor actuated switches (pressure sensor, flow sensor, temperature sensor, etc.), vehicle cruise control mechanisms, and “soft switches,” such as those of a touch screen monitor. Switch  88 , in some embodiments, simply acts directly or indirectly upon a fuel injector  94  to regulate or simply restrict incoming fuel  96  to supply a desired limited rate of supply fuel  98  to engine  20 . For example, closing switch  88  could limit incoming fuel  96  to provide an average engine speed of 1,250 rpm. Opening switch  88 , as shown in FIG. 1, could simply disable itself to allow engine  20  to be controlled in the usual manner of a conventional accelerator pedal  100 , or could allow a full rate of supply fuel  98  to provide an average engine speed of 2,500 rpm. Switch  88  preferably has maintained open and closed positions to allow engine  20  to operate at either of its higher or lower speed modes without ongoing operator attention. In other words, switch  88  is preferably adapted to unattendantly maintain engine  20  at its lower or higher speed modes. 
     In some embodiments, switch  88  provides an input signal  102  to a control  104  (e.g., a computer), which in response thereto provides an output  106  that determines the speed mode of engine  20  Examples of input signal  102  includes, but is not limited to, feedback  108  from governor  90 , feedback  110  from alternator  92 , and feedback  112  from a sensor  114 . Sensor  114  is schematically illustrated to encompass various sensors including, but not limited to a fluid pressure sensor that senses the pressure in discharge line  64 , a temperature sensor that senses some predetermined temperature associated with hydraulic circuit  40 , and a fluid flow sensor that senses the flow rate of hydraulic fluid passing through hydraulic circuit  40 . 
     In some embodiments, sensor  114  is a flow sensor, and feedback signal  112  represents the rate of hydraulic fluid flowing through discharge line  64 . Control  104  then adjusts output  106  so that engine  20  drives pump  68  at a speed that produces a predetermined flow rate of hydraulic fluid, such as 14 gpm. In other embodiments, sensor  114  is a pressure sensor, and feedback signal  112  represents the pressure in discharge line  64 . Control  104  then adjusts output  106  so that engine  20  drives pump  50  at a speed that produces a predetermined pressure in discharge line  64 , such as 1,950 psig or some other predetermined pressure just below the pressure at which main relief valve  54  is set to open, thereby ensuring valve  54  normally remains closed. 
     Transmission  24  has multiple speed positions to selectively provide at least a low-gear operation and a high-gear operation, wherein the ratio of speed of first output shaft  28  to input shaft  26  is higher in the high-gear operation than in the low-gear operation for operating hoist  22  at various speeds. High-gear operation can be used for light, rapid hoisting, and low-gear can be used for heavy lifting. Likewise, the ratio of speed of second output shaft  30  to input shaft  26  is higher in the high-gear operation than in the low-gear operation when mobile service rig  10  is traveling down a road. Thus, operating transmission  24  in high-gear and running engine  20  in its lower speed mode renders rig  10  operable in a reduced power mode that is suitable for normal tong operations and rapid light hoisting of sucker rods. Operating engine  20  in its higher speed mode renders rig  10  operable in a higher power mode that is suitable for heavy lifting; however, the higher power mode is also suitable for tong operations if desired. Shifting transmission  24  among its various speed positions can be carried out by conventional linkage and clutch arrangements that are well know to those skilled in the art. 
     Although the invention is described with reference to a preferred embodiment, it should be appreciated by those skilled in the art that various modifications are well within the scope of the invention. Therefore, the scope of the invention is to be determined by reference to the claims that follow.