Abstract:
An improved oil and gas drilling control system which utilizes improved braking and feedback technology which, in turn, permits more precise weight-on-bit control and more smooth transitions of weight-on-bit than any existing technology. In addition, the system also permits more accurate feedback and control with respect to drilling depth, pipe transitions, and rate of penetration than prior systems.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is directed to methods and apparatus for use with subterranean drilling systems. More specifically, the present invention is directed to systems to maintain a constant and desired weight on a drilling stem to maximize penetration and drilling rates. 
   2. Description of the Prior Art 
   In earth drilling, particularly the drilling of oil and gas wells, the control of the drilling operation has usually been accomplished manually. Conventional drilling rigs utilize a draw works which is powered by an engine and operates most of the motor driven portions of the rig. The draw works includes a drum with a drill line wound on it which is fed off to lower drill pipe as the drilling is accomplished. The drill line is looped through a crown block in a double pulley relationship and the end of the line is connected to a fixed point end called the dead line. 
   As the pipe is lowered into the well during drilling, the weight of the pipe string on the drill bit is measured by the tension in the drill line. The tension in the drill line is commonly measured by a pressure sensor which converts tension to weight indication through a hydraulic line extending to a bit weight gauge on the drilling console. The rate of feed out of the drill line from the drum controls the bit weight and to a large extent the rate of drilling. The rate of feed out of the drill line from the drum is controlled by a hand brake operated by a conventional brake lever. In manually-operated drilling rigs, the driller has to monitor the operation of the equipment and operate the brake from time to time in response to the indications of the bit weight gauge to control the rate of feed out of the drill line and thus attempt to keep a fairly constant bit weight. 
   In recent years, there have been developed a number of automatic drilling systems. These systems are automatic in the sense that they provide some form of automatic control over the equipment. Many of these automatic drillers operate from the air supply of the drilling rig en route to the drillers control station. The components involved are mainly air clutches with various types of air dump valves to exhaust used air. 
   One such device is disclosed in U.S. Pat. No. 4,491,186 (“the &#39;186 patent”) as issued to Adler. The apparatus disclosed in the &#39;186 patent utilizes the rotation rate of a downhole mud motor as a parameter to determine the release of the drill string. While this invention has application to downhole mud motors, it requires the use of an in-hole tachometer which enhances operation cost. 
   Another automatic drilling system is disclosed in U.S. Pat. No. 5,474,142 (“the &#39;142 patent”) as issued to Bowden. The device illustrated in the &#39;142 patent operates off of bit weight and fluid pressure which act on a pair of Bourden tubes. While such a device is nominally effective as an automatic driller, the use of the Bourdon tubes creates a time lag and limits its sensitivity due to the necessity for pressurization of hydraulic fluid. 
   Yet another system is illustrated in U.S. Pat. No. 5,713,422 as issued to Dhirdsa. This system suffers from the use of multiple sensing assemblies to determine and calculate the rate of penetration, which assemblies are maintenance intensive and are thus problematic for long term operation. Those in the drilling industry desperately need to have an automatic driller that would significantly improve the constancy of WOB, and make necessary changes with more smoothness (known as “peeling the drum”) than is possible with existing systems and technology. The use of emerging bit technology such as PDC bits especially require a smooth action to prevent the breaking of expensive diamond cutters. 
   Also, the industry needs technology and systems which allow considerably faster drilling than is now possible. Existing automatic driller systems cannot be used in many circumstances, because the rate of penetration (ROP)is deemed to be too fast for conventional technology to keep up. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an improved drilling control system. 
   It is another object of the present invention to provide an improved drilling control system, which allows more precise control of weight on bit (“WOB”)than presently available systems. 
   It is another object of the present invention to provide an improved drilling control system, which allows more smooth transitions in WOB than presently available systems. 
   It is another object of the present invention to provide an improved drilling control system, which allows as rapid drilling progress as strata allows, rather than, as is the case with present technology, being limited by limitations of the driller system itself. 
   It is another object of the present invention to provide an improved drilling control system, which allows for greater degrees of control, accuracy and feedback information on the progress of drilling depths, etc. than allowed by presently available technologies in the drilling technology realm. 
   It is another object of the present invention to provide solutions to each of the problems or limitations addressed in the Background of the Invention section, regardless of whether such are enumerated individually as an object of the present invention. 
   The driller system of present invention addresses each of the above objects and above-referenced problems, limitations and unmet desires in the drilling field. 
   In one preferred embodiment the present invention includes a system for controlling the release of a drill stem in a conventional drilling apparatus which includes a derrick with a crown block and a traveling block, a draw works and an engine where the draw works is powered by the engine and controlled by the clutch and brake. The draw works includes a drum on which is wound one end of a drill line which is wound up or released during the drilling operations. 
   The drill line extends through the crown block and traveling block and is connected at its opposite terminal end to a fixed point providing a deadline. The crown block and the traveling block form a pulley system for supporting a drill stem to raise or lower it during drilling operations. In this connection, when drill line is wound up on the drum, the traveling block is raised thereby raising the drill stem. 
   The system of the present invention provides means coupled to the deadline for obtaining a weight reading on the drill stem. This weight reading is usually in the form of an analog electrical signal. This analog electrical signal is supplied to a programmable logical controller, which transforms the analog electrical signal into a digital electrical signal. This electrical signal at a selected voltage or current is supplied to a gauging means in which has been programmed desired weight parameters. This gauging means then passes the signal to a control mechanism which uses an electric motor, coupled to a gearbox at the draw works. The motor&#39;s RPM rate depends on the voltage potential. 
   The present invention presents a number of advantages over prior art systems. 
   The electric motor mounted to a gearbox of the system of the present invention, under precise control of a computer unit, rotates at a substantially constant rate which is determined, by measurements of various parameters, to actuate the brake lever to a degree that a desired weight-on-bit is maintained by way of maintaining the associated rate of penetration (as indicated by the rate of movement of the drum on which the drill line is carried). 
   This represents a significant improvement over some old systems which utilized an air motor that would have to spin one shaft in the gearbox in one direction causing another perpendicular shaft to operate a pulley tied to a cable. Since the right angle design of gearboxes or old systems require motor rpm to cause a lifting of the brake handle, such a system is limited to how fast it can drill by the maximum rpm of the motor. 
   The use of this present system provides a number of advantages over presently available systems which may not be readily apparent, two of which are: (1) optimum weight-on-bit can be maintained quite precisely for any given drilling condition, thereby achieving maximum rate of penetration; and (2) precise and constant control of weight-on-bit avoids sudden, equipment-damaging torque changes. 
   In recent tests, the present driller system reduced rig “rotary torque” by 10 to 15 percent. This provides for improved ROP, decreased wear and tear on drill string joints and less unintended deviation in the drilled hole. 
   The addition of an encoder, described in more detail below, can provide an accurate feed back of the rig movements caused by the driller, and thereby provides several additional benefits: (1) it is an integral part of permitting the smooth drilling described above; (2) one is able to provide a display of the amount of footage drilled at any given point in time, how much footage since the last connection was made, and at what rate of penetration is being attained. 
   Users of the system of the present invention will be able to automate the process of “time drilling” which is used in virtually all directional and horizontal projects. Time drilling is a process used to start the deviated portion of a directional drilling operation. Through the use of mud motors and deviated sub connections deviation of the hole is started by drilling or moving the deviated bit forward for example, only one inch every five minutes. Currently this is performed by a human “driller” using a wrist watch and crude chalk markings on the “kelly” of the rig. 
   The present invention&#39;s system allows the directional drilling consultant to program the desired time and distance parameters and know that the work will be performed exactly to specifications resulting in a more accurate start to the directional portion of the project. This is a feature long requested among directional drilling personnel. 
   The information provided by the encoder and PLC also allows this driller to perform another function that is not available anywhere. “ROP Drilling” is exactly what it says—drilling only at a certain rate of penetration, even though the formation conditions could allow a faster rate. Some formations may be soft enough to be drilled at a fast rate, however other conditions such as gas pockets that need to be approached with caution to prevent a possible blow out, present the need for a driller that can control its ROP regardless of WOB. A variation of the before mentioned time drilling function allows this unit to perform this controlled “ROP Drilling”. 
   The overall sensitivity of the present system enables it to achieve more precise corrections for weight on bit. This is especially important when there exists the need to follow the contour of a producing formation. Such sensitivity is also helpful when using downhole mud motors to prevent damage and ensure smooth operation. This same simplicity also facilitates the retrofitting of existing rigs and drilling equipment. 
   Still another advantage of the drilling system of the invention is its adaptability to monitor bit weight and/or bit torque and utilize one or both parameters as a determinix in the release of the drilling string. In such a fashion, selective control of downhole mud motors may be achieved. 
   Even more advantages of the invention will become obvious in light of the following detailed description of the preferred embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a partial schematic, diagrammatic view of one embodiment of the drawworks control system of the present invention. 
       FIG. 2  illustrates a diagrammatic view of the system of  FIG. 2 . 
       FIGS. 3A–B  illustrate various types of exemplary weight sensor assemblies for use with the control system of the invention. 
       FIG. 4  illustrates a schematic, partially diagrammatic view of one embodiment of a torque sensor which may be used in conjunction with the control system of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to the figures, shown therein and referred by the numeral  10  is a draw works control system, constructed in accordance with the present invention. 
   For purposes of description, the draw works control system  10  is shown in combination with a conventional rotary drilling rig  12 . The rotary drilling rig  12  consists of a draw works assembly  14  and a rotary drilling unit  16  which may be either a top drive or a table drive application. The draw works assembly  14  includes a traveling block  18  suspended from and applying tension to a cable  20 . The cable  20  has one end thereof wound on a drum  22 , the rotation of which is controlled by a power brake mechanism  24  and a prime mover, e.g. a diesel engine and/or a diesel-electric engine. The other end of the cable  20  is wound around an eccentrically mounted spool  26  and anchored to a storage drum  28 . The intermediate portion of the cable  20  is maintained in an elevated position via a crown block  30  in a conventional manner as illustrated. 
   As will be appreciated by those skilled in the art, a conventional brake mechanism  24  is comprised of a brake band  32  engageable with the drum  22  via a brake lever  34 , a brake lever biasing spring  36  connected between the brake lever  34  and a stationary rig or platform surface. It will be appreciated, however, that other braking systems may also be utilized in a manner consistent with the objectives of the invention. 
   The various elements comprising the draw works control apparatus illustrated in  FIGS. 1 and 2  are designed to be supplied with clean, dry, pressurized air from a suitable air supply source  50  which conventionally includes an off-on switch  52 . It is desirable in most applications to regulate the pressure of the air supplied to the various components comprising draw works control apparatus  10  by utilizing one or more regulators  54 . 
   One preferred embodiment of the draw works control system of the present invention is illustrated in  FIGS. 1 and 2 . By reference, however, to  FIGS. 3A and 3B , the system  10  includes a cable tension sensor assembly  41  which includes a sensor  44  and a transducer  65  to measure drill string weight. The sensor  44 , which may be any one of a number of commercially available sensors, is connected to cable  20  and senses the tension, and hence drill string weight, of cable  20 . 
   An exemplary sensor assembly  41  is illustrated in  FIG. 3A  in which is shown a sensor  44  coupled to a drilling line  20 . As illustrated, sensor  44  includes a deflection plug  61  which acts on a diaphragm  65  which is filled with hydraulic fluid. A second anchor type tension sensor assembly  70  is illustrated in  FIG. 3B  in which is illustrated a sensor  72  which includes a diaphragm  75 . In the case of each type of sensor assembly, an electrical output signal is created by the movement of the diaphragm which is acted on by the drill string  20 . It will be appreciated that still other sensor assemblies  41  may also be utilized with the control system  10  of the invention. 
   Referring principally to  FIGS. 2 and 3A , sensor  44  produces a 4–20 milliamps proportional output analog electrical signal, which is transmitted along electrical line  60  to a programmable logical controller (“PLC”)  70 , which preferably includes an analog to digital current converter  71 , such as a current converter made by Automation Direct. Converter  71  converts the 4–20 milliamps proportional output analog electrical signal to a scaled digital signal, e.g. a signal with a discrete value from 0 to 4095. A power supply  69  supplies electrical power to electrical components such as the PLC  70 . 
   The PLC  70  also receives an electrical signal representing a desired weight of bit (“WOB”) input from a touch-screen monitor  73 , on which the user may selectively enter or adjust the desired WOB or setpoint. The PLC, using program logic as will be explained below, then compares the current WOB (derived from the input from sensor assembly  41 ) to the desired WOB or set point. If the current weight on bit is less than the set point then the PLC will ramp up its digital output signal. This digital output signal will range from an output value of 0 to 4095. 
   The digital output signal is sent along a first signal path  77  to a variable frequency drive (“VFD”)  75  which will, in turn, send a variable amount of alternating electrical current at a variable frequency along a second signal path  79  to an electric motor  82 . In this way, the amount of current sent to the electric motor  82  (and, accordingly, its RPM) will depend on the value of the output signal from the PLC  70 . 
   The electric motor  82  drives a conventional draw works gearbox  89  with a clutched cable reel  92  rotatably carried on an output shaft  91 . Cable reel  92  carries cable  90  which, in turn, is attached to brake handle  34 , in the conventional manner. Quite contrary to convention, however, electric motor  82  drives gearbox  89  continuously, at a nearly constant RPM. This is in stark contract to conventional systems which dramatically ramp up and ramp down the speed of the gearbox for attempting to stay within rate of penetration settings. Such lack of precision in conventional systems is the product of a lack of precision feedback and control of the present system, and of the use of conventional air motor drives for draw works gearboxes, which, of course, cannot be controlled with any precision. 
   The RPM of electric motor  82  is, as mentioned above, the product of the signal output of VFD  75  and, for reasons described hereafter, will be that substantially constant rate which optimally maintain the ROP which will, in turn, assure the desired WOB. 
   PLC  70  continuously compares the desired WOB to the extrapolated WOB and adjusts the RPM of motor  82  in such a way that, when balanced against the mechanical effect of movement of drum  22  via a conventional drum unit, flexible shaft and overriding clutch mechanism (not shown separately in the drawings), cable  90 , and with it, brake handle  34  are drawn to a degree that the desired WOB, via precise management of the ROP is maintained. In other words, by substantially, constantly measuring the WOB, and adjusting the RPM of motor  82 , PLC  70  ensures that a substantial state of equilibrium exists between the tension on cable  90  and brake handle  34  and the opposite tending forces of the mechanical feedback from movement of drum  22  such that the desired ROP and WOB are constantly assured. 
   The precise management of RPM of electric motor  82 , and with it, all the desired parameters described above, is achieved by certain functionalities which are products of the software or firmware by which PLC  70  operates. PLC  70  has, as mentioned above, an output range of 0–4095. When the WOB setpoint and actual WOB match, the output is 0. However, as WOB decreases (as earth is drilled away from under the drill bit) PLC output increases. 
   The principle operation of the PLC  70 &#39;s software or firmware is summarized as follows: The program works on X range of weight variance from the setpoint representing the maximum PLC output. For example let us say that at one point in time, the PLC has the range set to 10 which represents 10,000 lbs of variance below the setpoint. If 30,000 lbs is the desired WOB, then  4095  output would be attained at 20,000 lbs WOB. One should never actually reach 4095 in output during normal drilling, because the system would correct for such a variance before reaching that point (no more than 500 lbs. WOB variance from either side of the setpoint). 
   Now, let us say that drilling is occurring at a 35 foot per hour rate, and the output of the PLC is averaging in the range of 800 output. This results in the hertz range out to the motor averaging in the range of 18 hz. So, for this ROP (35 fph) a hertz output in the range of 18 will keep the rig within 500 lbs of the setpoint. 
   If drilling rates were to never change, nothing else would be necessary. However, such is not the case. ROP changes constantly, and so the driller too needs to constantly change to keep the smooth drilling pace both at faster and slower rates. 
   The PLC is constantly monitoring the relationship between WOB and WOB setpoint. PLC  70  can be set to make adjustments to the range up or down according to that relationship every 0.3 seconds. Returning to our example: suppose the drill bit encounters slightly softer formation and the earth drills away faster causing a loss of WOB. Now, in order to maintain the desired WOB, one needs to drill faster (increase the ROP). 
   By repeatedly comparing setpoint WOB to actual WOB, the PCL  70  will detect this change of circumstance. Let us use a 150 lbs. as a detected variance from setpoint after the softer strata is encountered. PLC  70  will then subtract 300 lbs (as an example, depending on programming) from the above mentioned range of 0–10,000 lb variance range. Now 4095 of output would theoretically happen at 9,700 lbs away from the setpoint, rather than the earlier 10,000 lbs. With the reduction of the overall range, the output at approximately 500 lbs. away may now average 850 in PLC output, resulting in average hz. output of the VFD being 20 hz. This results in more gearbox speed and therefore more ROP. The PLC will continue to decrease the overall range as long as the WOB remains below the setpoint. Then, when the WOB is over the setpoint, the opposite process begins, causing a increase in range and a reduction in hertz output per lb. away from the setpoint. In this format the WOB will float slightly above and below the setpoint maintaining that constant drilling or “peel” but at the same time keep the variance from set point with 500 lbs. to either side. 
   Programming to achieve the above results are well within the skills of a competent programmer upon reference to this disclosure, and actual code examples or routines are not required for present purposes. 
   One additional aspect of the PLC logic deserves mention: If one considers the above basic premises, one would suspect that when WOB is at zero variance from the WOB setpoint the PLC output would be zero, and the electric motor  82  and the attached gearbox  89  would, therefore, be stopped and then spin up as the WOB fell below the setpoint. That is the way drillers of the past would operate, and the result was very much less than optimal smoothness of operations. 
   Because there should be a certain amount of gearbox rotation to move the brake handle to any degree, one should maintain some degree of output or “lead” in the range so that the right gearbox rpm can be attained with minimal variance from the setpoint. In other words, if PLC  70  has determined from the example above that a 9,700 lbs variance range is required to provide the needed hertz and rpm for the given conditions, PLC  70  may shift 2000 lb. of that range above the setpoint and leave the remaining 5700 lb. below the setpoint. That way, one attains the desired 850 PLC output and 20 htz. output to achieve the necessary rpm basically at the setpoint. This way, as stated above, the WOB will float slightly above and below the setpoint, thereby maintaining the desired constant “peel”—gearbox  89 , and therefore, the action of the brake handle  34  never stops. 
   A system including the above-described features and components provides a number of benefits not previously available in the art. As mentioned above in more detail, these benefits, when compared to existing driller control system technology, include: (1) more precise and consistent control of weight-on-bit; (2) smoother transitions between weight-on-bit settings; (3) more precise information feedback for monitoring depth of drilling, time for component change-out, etc.; and (4) elimination of driller control system limitations on rate of penetration. 
   By reference to  FIG. 4 , in a rotary table system utilizing a non-electric power source, e.g. a diesel engine (not shown), a hydraulic signal is taken from an idler wheel tension sensor  100  which in turn is coupled to a transducer, e.g. a transducer as manufactured by M.D.—Totco. Sensor  100  mounts against the drive chain  102  such that idler wheel  103  is disposed in contacting relation to said chain  102 , as illustrated. Thus, as drive chain  102  rotates, pressure is applied against wheel  103  which in turn applies pressure to hydraulic piston  107 , thereby increasing the fluid pressure within the hydraulic line  109 . Hydraulic line  109  in turn is coupled to a transducer  110 . 
   Transducer  110  sends an electric signal to a PLC, with an appropriate input, the specifics of which would be readily apparent to anyone reasonably skilled in the field upon reference to this disclosure. An increase in hydraulic signal as reported to the PLC will be interpreted as an increase in hook load and therefore a decrease in WOB. Therefore, the electrical signal would, in that condition, then be increased to create gearbox movement to increase the WOB. 
   As described above with respect to bit weight, the PLC with its touch-screen input, allows the operator to set desired parameters for tool torque. If the measurement of this parameter below the set value, the PLC ramps up the output signal and conversely if the WOB is greater than the set point the PLC will ramp down the output signal, all resulting in the change of WOB, and, therefore, the torque in the manner described elsewhere herein. 
   Incorporating the features and components of the present invention&#39;s system into conventional drilling apparatuses and equipment are well within the skill of those in the art, upon reference to this description. In addition, selection of specific components to meet the descriptions and functionalities referenced herein are also within the reasonable skills of those in the art, once provided with this description. 
   Although particular detailed embodiments of the apparatus and method have been described herein, it should be understood that the invention is not restricted to the details of the preferred embodiment. Many changes in design, composition, configuration and dimensions are possible without departing from the spirit and scope of the instant invention.