Patent Description:
A drilling choke is a type of equipment that may be used in well drilling operations. In a well drilling operation known to those skilled in the art as managed pressure drilling, a drilling choke can be used to variably restrict flow of fluids from a well, in order to control fluid pressure in the well. It will, therefore, be readily appreciated that it is important to be able to accurately control operation of the drilling choke, so that as a result the fluid pressure in the well can be accurately controlled.

It is one of the objectives of the present disclosure to provide improvements to the art of controlling operation of a choke, such as a drilling choke. Such improvements can be useful in well operations other than managed pressure drilling (such as, well control, stimulation, water- or steam-flooding, etc.).

<CIT> describes a method of controlling a drilling choke in a controlled pressure drilling system. Pressure is monitored in the system, and a position of the drilling choke is monitored. An adjustment to the position of the drilling choke is determined based on the monitored pressure to control the drilling pressures. Rather than directly transferring motion from an actuator to the choke, the actuation of the actuator is transferred with a non-linear relationship relative to the position of the drilling choke. Generally, motion of the choke's internal trim (e.g., gate, ball, flapper, disc, etc.) is quicker when near a fully opened position and is slower when near a fully closed position.

The embodiment according to <FIG> and <FIG> is in line with the claimed invention. The remaining embodiments are useful for understanding the invention.

In managed pressure drilling (MPD) systems, drilling chokes or other forms of control valves are used to control wellbore pressure and a flow rate of fluids from a wellbore. A mechanism that opens or closes a choke typically consists of either a hydraulic piston that slides axially under control of hydraulic pressure (from a hydraulic power unit) or under control of a worm gear jack screw (driven by a hydraulic or electric motor).

For example, <FIG> illustrates a drilling choke arrangement <NUM>, such as used in managed pressure drilling. A hydraulic power unit (HPU) <NUM> feeds hydraulic pressure to a hydraulic motor <NUM>, which turns a jack screw to turn a worm gear <NUM>. An internal trim, such as a gate (not shown), within a drilling choke <NUM> is moved by the worm gear <NUM> to change an orifice for fluid flow in the choke <NUM> from an inlet <NUM> to an outlet <NUM>. A control unit (CU) <NUM> couples to the hydraulic pressure unit <NUM> and to a position sensor <NUM>. To choke the flow from the inlet <NUM> to the outlet <NUM>, the control unit <NUM> actuates the HPU <NUM>, motor <NUM>, worm gear <NUM>, etc., to adjust the choke's gate. Feedback from the position sensor <NUM> is used to monitor the position.

In this arrangement <NUM>, as well as an arrangement that uses a hydraulic piston, the input motion of the actuator <NUM> that causes the choke <NUM> to open and close correlates with a direct linear movement of the choke's internal trim (e.g., gate) by design. As a result of the design of the actuator <NUM>, a speed of the moving gate inside of the choke <NUM> will be the same throughout the time that the input motion is applied. This means that the closing speed of the choke <NUM> when there is a relatively high pressure drop across the choke <NUM> will be the same as the closing speed when the choke <NUM> is near a fully open position.

One obstacle to overcome when controlling pressure is trying to hold a specified wellbore pressure while operating with less than <NUM>% range of available motion of the choke <NUM> for all required pressure holds during a typical field run. For instance, <FIG> graphs a choke pressure drop versus position plot <NUM> for an example implementation in which particular choke positions (as a percentage open) relate to corresponding pressure drops (psi) produced by a choke. As the graphed line <NUM> indicates, most of the pressure drop (i.e., about <NUM>-psi to <NUM>-psi) that can be produced with the choke may fall within a range <NUM> of choke position (e.g., about <NUM>% to <NUM>%) that is essentially <NUM>% of the choke's total possible movement. Accordingly, in this instance, the choke will typically be operated within this <NUM>% of choke movement as the choke is used to manage pressure. As will be evident, precise control of the choke position is required to achieve the desired pressure drop. In fact, a control system can reach a point where each successive increment of <NUM>% of the choke gate motion to control the choke can cause a change of <NUM> psi pressure drop or more as the control system approaches the higher end of the pressure drop range.

When a control system is forced to move a choke at a constant rate along the full range of motion, a special emphasis must be placed on high precision pulses of control system input at the upper range of wellbore pressures. This requires the use of high precision machined components inside the choke or valve actuator as well as the HPU and further tweaking of the control system with periodic calibration to account for the loss of accuracy within the system that occurs over time. Several hundreds of man hours have been spent fine tuning, calibrating, and replacing parts to obtain the final increment of accuracy that is required to pass function testing of systems.

The emphasis of designing MPD control systems up to this point has been focused on precision control within a small range of motion of the choke or valve. A majority of man hours of factory acceptance testing have been spent fine-tuning these control systems and inspecting and replacing mechanical parts in the HPU and choke to achieve the high degree of precision that is required to precisely control the wellbore pressure. For the current designs, the total valid range of pressure control adds up to approximately <NUM>% of the total available gate motion. If the desired accuracy for managed pressure is +/-<NUM> psi in terms of pressure, then the accuracy of the gate motion can be limited to +/-<NUM>% of the full range which comes to less than +/-<NUM> inches (<NUM> inch = <NUM>).

This accuracy has been achieved with each new system, but it has come at the cost of hundreds of man-hours with each new factory acceptance test. Moreover, each MPD control system typically needs to be re-calibrated frequently to maintain the high level of control precision due to time-related deteriorating factors.

In one example, a method of controlling a drilling choke in a drilling system includes monitoring a parameter in the drilling system and monitoring a position of the drilling choke. An adjustment is determined to the position of the drilling choke based on the monitored parameter and the monitored position. An actuation of an actuator operably coupled to the drilling choke is produced to implement the adjustment. The actuation of the actuator is transferred with a transfer mechanism to motion of the drilling choke in a non-linear relationship relative to the position of the drilling choke. The monitored parameter is altered in response to the implemented adjustment of the drilling choke.

Note that the phrases "adjustment of the drilling choke," "motion of the drilling choke" and "position of the drilling choke" (and similar phrases) indicate an adjustment, motion or position of the drilling choke's internal trim (e.g., a gate or other flow restricting member).

Monitoring the parameter in a controlled pressure system can involve monitoring the parameter including flow-in, flow-out, density, and standpipe pressure, and monitoring the position of the drilling choke can involve obtaining an indication of the position from a position sensor operably connected to the drilling choke. The adjustment to the position of the drilling choke determined based on the monitored parameter and the monitored position can involve correlating a change from the monitored position to a new position of the drilling choke relative to a change from a current value for the monitored parameter to a new value for the monitored parameter. Altering the monitored parameter in response to the implemented adjustment of the drilling choke can include changing surface backpressure of the controlled pressure system.

The actuation of the actuator operably coupled to the drilling choke produced to implement the adjustment can include operating a hydraulic motor with a hydraulic power unit, operating an electric motor with a power supply, etc..

In transferring the actuation of the actuator with the transfer mechanism to the motion of the drilling choke in the non-linear relationship relative to the position of the drilling choke, the motion (e.g., speed) can be made quicker when near the position of being fully opened and can be made slower when near the position of being fully closed. In transferring the actuation of the actuator with the transfer mechanism to the motion of the drilling choke in the non-linear relationship relative to the position of the drilling choke, the motion (e.g., displacement) can be made greater when near the position of being fully opened and can be made smaller when near the position of being fully closed.

Transferring with the transfer mechanism can involve transferring with the transfer mechanism operable between the actuator and the drilling choke. For example, the transfer mechanism can include a linkage operable between the actuator and the drilling choke, a crank and slider linkage operable between the actuator and the drilling choke, or a bell crank linkage operable between the actuator and the drilling choke. The transfer mechanism can be a gear arrangement, such as a planetary or elliptical arrangement, operable between the actuator and the drilling choke, can be a variable ratio chain and sprocket arrangement operable between the actuator and the drilling choke, or can be a variable transmission (such as, a continuously variable transmission) operable between the actuator and the drilling choke.

Transferring with the transfer mechanism can involve transferring with the transfer mechanism operable between a power source and the actuator. For example, the transfer mechanism can include a throttling valve operable between a hydraulic power source and a hydraulic motor of the actuator, a throttling valve operable between a hydraulic source of the power source and a hydraulic motor of the actuator and having a stem directly operated by movement of the choke, a drive operable between an electric power source and an electric motor of the actuator, or a control valve operable between a hydraulic power source and a variable speed hydraulic motor of the actuator. The transfer mechanism can include at least one control algorithm being operable to control the actuator according to the disclosed non-linear relationship.

In one example, a programmable storage device having program instructions stored thereon can cause a programmable control device to control a drilling choke in a drilling system. The teachings of the present disclosure can apply to a drilling choke of a drilling system, as discussed above, and can apply to controlling a choke in other implementations.

In one example, an apparatus for a pressure system includes a valve, a position sensor, an actuator, a controller, and a transfer mechanism. The valve is in operable communication with the pressure system. The position sensor is in operable communication with the valve and obtains a position of the valve trim, and the actuator is in operable communication with the valve. The controller is in operable communication with the position sensor and the actuator. The controller receives an adjustment to the position of the valve trim and produces an actuation of the actuator to implement the received adjustment. The transfer mechanism is operable between the actuator and the valve and transfers the actuation of the actuator to motion of the valve trim in a non-linear relationship relative to the position of the valve trim.

The valve can be a choke, a needle valve, a ball valve, a gate valve, a globe valve, a plug valve, a disc choke with plates, a butterfly valve, or other type of valve or choke. As noted, the valve can be a drilling choke in fluid communication with a borehole. In this arrangement, the controller is operable to monitor a parameter in the pressure system and to monitor the position of the drilling choke. The controller determines the adjustment to the position of the drilling choke based on the monitored parameter and the monitored position and produces the actuation of the actuator operably coupled to the drilling choke to implement the adjustment.

<FIG> shows an example of a closed-loop drilling system <NUM> adapted for controlled pressure drilling. As shown and discussed herein, this system <NUM> can be a Managed Pressure Drilling (MPD) system and, more particularly, a Constant Bottomhole Pressure (CBHP) form of MPD system. Although discussed in this context, the teachings of the present disclosure can apply equally to other types of controlled pressure drilling systems, such as other MPD systems (Pressurized Mud-Cap Drilling, Returns-Flow-Control Drilling, Dual Gradient Drilling, etc.) as well as to Underbalanced Drilling (UBD) systems, and to other types of drilling systems, as will be appreciated by one skilled in the art having the benefit of the present disclosure.

The drilling system <NUM> has a rotating control device (RCD) <NUM> from which a drill string <NUM>, a bottom hole assembly (BHA), and a drill bit <NUM> extend downhole in a wellbore <NUM> through a formation F. The rotating control device <NUM> can include any suitable pressure containment device that keeps the wellbore in a closed-loop at all times while the wellbore <NUM> is being drilled.

The system <NUM> also includes a standpipe (not shown), rig pumps <NUM>, mud tanks <NUM>, a mud gas separator <NUM>, and various flow lines, as well as other conventional components. In addition to these, the drilling system <NUM> includes an automated choke manifold <NUM> that is incorporated into the other components of the system <NUM>, such as a control system <NUM> having a control unit <NUM> and a power source <NUM>.

The control system <NUM> integrates hardware, software, and applications across the drilling system <NUM> and is used for monitoring, measuring, and controlling parameters in the drilling system <NUM>. In this contained environment of the closed-loop system <NUM>, for example, minute wellbore influxes or losses are detectable at the surface, and the control system <NUM> can further analyze pressure and flow data to detect kicks, losses, and other events. In turn, at least some operations of the drilling system <NUM> can be automatically handled by the control system <NUM>. Note that the scope of this disclosure is not limited to use of a valve or choke in a closed-loop drilling system.

To monitor operations, the control system <NUM> can use data from a number of sensors and devices in the system <NUM>. For example, one or more sensors can measure pressure in the standpipe. One or more sensors (i.e., stroke counters) can measure the speed of the rig pumps <NUM> for deriving the flow rate of drilling fluid into the drill string <NUM>. In this way, flow into the drill string <NUM> may be determined from strokes-per-minute and/or standpipe pressure. Alternatively, a flowmeter <NUM>, such as a Coriolis flowmeter downstream of the rig pumps <NUM>, can also be used to measure flow-in to the wellbore <NUM>.

One or more sensors can measure the volume of fluid in the mud tanks <NUM> and can measure the rate of flow into and out of mud tanks <NUM>. In turn, because a change in mud tank level can indicate a change in drilling fluid volume, flow-out of the wellbore <NUM> may be determined from the volume entering the mud tanks <NUM>.

The fluid data and other measurements noted herein can be transmitted to the control system <NUM>, which can in turn operate drilling functions. In particular, the control system <NUM> can use the control unit <NUM> and power source <NUM> to operate the automated choke manifold <NUM>, which manages pressure and flow during drilling and is incorporated into the drilling system <NUM> downstream from the rotating control device <NUM> and upstream of the gas separator <NUM>. Among other components, the manifold <NUM> has chokes <NUM>, a flowmeter <NUM>, pressure sensors (not shown), and other components. The controller or control unit <NUM> control operation of the manifold <NUM>, and the power source <NUM> (e.g., hydraulic power unit and/or electric motor) actuate the chokes <NUM>.

During operations, the system <NUM> uses the rotating control device <NUM> to keep the well closed to atmospheric conditions. Fluid leaving the wellbore <NUM> flows through the automated choke manifold <NUM>, which measures return flow (e.g., flow-out) and density using a flowmeter <NUM> installed in line with the chokes <NUM>. Software components of the control system <NUM> then compare the flow rate in and out of the wellbore <NUM>, the injection pressure (or standpipe pressure), the surface backpressure (measured upstream from the drilling chokes <NUM>), the position of the chokes <NUM>, and the mud density, among other possible variables. Comparing these variables, the control system <NUM> then identifies minute downhole influxes and losses on a real-time basis to manage the annulus pressure during drilling.

By identifying the downhole influxes and losses during drilling, for example, the control system <NUM> monitors circulation to maintain balanced flow for constant BHP under operating conditions and to detect kicks and lost circulation events that jeopardize that balance. The drilling fluid is continuously circulated through the system <NUM>, choke manifold <NUM>, and the Coriolis flowmeter <NUM>. As will be appreciated, the chokes <NUM> may fluctuate during normal operations due to noise, sensor errors, etc., so that the system <NUM> can be calibrated to accommodate such fluctuations. In any event, the system <NUM> measures the flow-in and flow-out of the well, detects variations, and operates the chokes <NUM> to account for the variations. In general, if the flow-out is higher than the flow-in, then fluid is being gained in the system <NUM>, indicating a kick. By contrast, if the flow-out is lower than the flow-in, then drilling fluid is being lost to the formation, indicating lost circulation.

To then control pressure, the control system <NUM> introduces pressure and flow changes to the incompressible circuit of fluid at the surface to change the annular pressure profile in the wellbore <NUM>. To do this, the control system <NUM> uses the chokes <NUM> in the choke manifold <NUM> to apply surface backpressure within the closed loop, which can produce a reciprocal change in bottomhole pressure. In this way, the control system <NUM> uses real-time flow and pressure data and manipulates the annular backpressure to manage wellbore influxes and losses.

In the control process, the control system <NUM> uses internal algorithms to identify what event is occurring downhole, and reacts automatically. For example, the control system <NUM> monitors for any deviations in values during drilling operations, and alerts the operators of any problems that might be caused by a fluid influx into the wellbore <NUM> from the formation F or a loss of drilling mud into the formation F. In addition, the control system <NUM> can automatically detect, control, and circulate out such influxes and losses by operating the chokes <NUM> of the choke manifold <NUM> and performing other automated operations.

<FIG> illustrates an example of a process <NUM> for controlled pressure drilling. The control system (<NUM>) monitors various parameters of the drilling system (<NUM>), such as flow-in, flow-out, density, standpipe pressure, temperature, etc. (Block <NUM>). The control system (<NUM>) also monitors position of the drilling choke(s) (<NUM>) (Block <NUM>). The position information can be obtained from position sensor(s) operably connected to the drilling choke.

As drilling is performed, differences in the monitored parameters may indicate that operational changes are necessary, such as increasing or decreasing surface backpressure using the drilling choke(s) (<NUM>), so that decisions are continually made based on the monitored parameters (Decision <NUM>). If adjustment to the choke(s) (<NUM>) is necessary, the control system (<NUM>) determines what adjustment to the position of the drilling choke is needed based on the monitored parameters and the monitored position (Block <NUM>). As briefly described here, the control system (<NUM>) considers the current position of the choke(s), the current set point and correlates a change from the current position to a new position of the choke(s) relative to a desired change from a current set point of a monitored parameter to a new set point for the monitored parameter.

With the adjustment determined, the control system (<NUM>) produces actuation(s) of actuator(s) operably coupled to the drilling choke(s) (<NUM>) to implement the adjustment (Block <NUM>). Depending on the actuator(s) used and the power source, the actuation(s) can be electrical, hydraulic, etc..

In the implementation of the actuation(s), the transfer mechanism (<NUM>, see <FIG>) transfers the actuation(s) of the actuator(s) to motion of the drilling choke(s) (<NUM>) in a non-linear relationship relative to the position of the drilling chokes (<NUM>) (Block <NUM>). As generally depicted here (Item <NUM>), the non-linear relationship may tend to make the motion (i.e., speed) of the choke(s) (<NUM>) slower when near the position of being fully closed, whereas the motion (i.e., speed) of the choke(s) (<NUM>) may be quicker or faster by comparison when near the position of being fully opened. Likewise, the non-linear relationship may tend to make the motion (i.e., displacement) of the choke(s) (<NUM>) smaller when near the position of being fully closed, whereas the motion (i.e., displacement) of the choke(s) (<NUM>) may be larger by comparison when near the position of being fully opened. As noted, the additional control of the motion of the choke when near a more closed position is of particular interest with respect to controlling the flow, pressure, etc., through or in the choke.

In the end, the monitored parameter of the drilling system (<NUM>) can be properly altered in response to the implemented adjustment(s) of the drilling choke(s) (Block <NUM>). For example, increased surface backpressure can be applied to produce an increase in bottomhole pressure and counter a fluid influx from the formation.

<FIG> illustrates an embodiment of a choke <NUM>, such as a drilling choke used in managed pressure drilling, controlled by a control unit <NUM> and a power source <NUM>. An operational device <NUM> for operating the choke <NUM> includes a position sensor <NUM>, an actuator <NUM>, and a non-linear transfer mechanism <NUM> according to the present disclosure. An internal trim, such as a gate (not shown) within the choke <NUM>, is moved to change an orifice for fluid flow in the choke <NUM> from the inlet <NUM> to the outlet <NUM>.

The position sensor <NUM> is in operable communication with the control unit <NUM> to provide position information of the choke <NUM>, such as the position of the choke's internal gate or other internal trim mechanism (not shown). In general, the choke <NUM> can be a plug-style choke with trim that includes a gate and seat, such as used in a choke manifold of a drilling system. Other types of chokes could benefit from the teachings of the present disclosure, so that reference to the term "choke" as used herein can apply also to various types of valves capable of variably restricting fluid flow, such as needle valve, ball valve, gate valve, globe valve, plug valve, disc choke with plates and alignable orifices, butterfly valve, etc..

The actuator <NUM> is in operable communication with the power source <NUM>, which is operated by the control unit <NUM>. Based on system controls and determinations, the control unit <NUM> receives and/or determines an adjustment for the position of the choke <NUM> and produces an actuation of the actuator <NUM> to implement the adjustment. As noted, the purpose of the adjustment can be to control surface backpressure in a managed pressure drilling system. However, the scope of this disclosure is not limited to any particular purpose for adjusting a position of a choke or valve.

The non-linear transfer mechanism <NUM> is operable between the actuator <NUM> and the drilling choke <NUM>. Instead of allowing the actuation and adjustment to be applied directly to the choke <NUM>, the transfer mechanism <NUM> intermediates the transfer of the actuation from the actuator <NUM> to motion of the drilling choke <NUM> in a non-linear relationship relative to the position of the trim inside the drilling choke <NUM>. The input from the transfer mechanism <NUM> is thereby applied to the choke <NUM> to produce motion of a variable orifice in the choke <NUM>. (As noted, the internal trim in the choke <NUM> can be a gate, although other forms of trim can be used, such as ball, flapper, disc, etc.) In turn, the position sensor <NUM> detects the position information and feeds back to the control unit <NUM>.

In this and other configurations disclosed herein, the position sensor <NUM> is preferably coupled directly to the gate or other flow restrictor member <NUM> in the choke <NUM> to allow an accurate reading of the position for control purposes. In other examples, the position sensor <NUM> could detect a position of a member of the actuator <NUM> or the transfer mechanism <NUM>. The position sensor <NUM> can include a linear potentiometer, a Linear Variable Differential Transformer (LVDT), a proximity sensor, or any suitable device to detect position. Of course, other arrangements and different position sensors are possible.

The non-linear transfer mechanism <NUM> forces the nonlinear relationship between the actuation and the resulting valve motion such that the valve motion occurs more quickly when near a fully opened position and occurs more slowly when near a fully closed position. As a result of the changed correlation, a broader range of controlled motion can be effectively used to control the wellbore pressure, thereby decreasing the necessary positional accuracy of the actuator <NUM> (e.g., hydraulic or electric motor) coupled with the control unit <NUM> and power source <NUM>.

For example, the actuator <NUM> may be a hydraulic motor or an electric motor providing revolutions to actuate the choke <NUM>. The non-linear transfer mechanism <NUM> can allow for five (<NUM>) revolutions of the motor <NUM> to control the applicable range of wellbore pressure instead of using just one (<NUM>) revolution. Consequently, the precision requirement for the actuator position can be relaxed by up to five (<NUM>) times.

Various types of non-linear transfer mechanisms <NUM> as disclosed herein can be used. In a first type, the transfer mechanism <NUM> can use an assembly of mechanical parts that force a nonlinear relationship between the input motion from the actuator <NUM> (e.g., rotational or linear motor or hydraulic cylinder) and the motion of the internal trim flow restrictor member in the choke <NUM>.

<FIG> illustrate a first non-linear transfer mechanism 120A of the present disclosure that uses a mechanical assembly. The mechanism 120A includes a crank and slider assembly having a rotating arm <NUM> with a hinged push rod <NUM> attached to a gate or other flow restrictor member <NUM> of a choke <NUM> (shown simplified). The rotating arm <NUM> is coupled at an input point <NUM> to rotational input R from the actuator <NUM>, which can be a hydraulic or electric motor, for example. Rotation of the rotating arm <NUM> about the input <NUM> transfers to the hinged push rod <NUM> via a moving pivot <NUM>. The rod <NUM> couples with another moving pivot <NUM> to the gate <NUM> so that the rod <NUM> changes the rotary motion of the arm <NUM> into reciprocating motion of the gate <NUM> of the choke <NUM>.

In this embodiment, the overall closing time of the choke <NUM> may remain the same as with a conventional design. However, with the choke <NUM> near an opened position (<FIG>), each increment of rotation of the arm <NUM> results in greater motion of the gate <NUM>. By contrast, with the choke <NUM> near a closed position (<FIG>), each additional increment of rotation of the arm <NUM> results in less motion of the gate <NUM>. Thus, the rotational input R of the actuator <NUM> is transferred by the mechanism 120A in a non-linear fashion to the motion of the gate <NUM>, which gives more control to the operation of the choke <NUM>.

<FIG> illustrate a second non-linear transfer mechanism 120B of the present disclosure that also uses a mechanical assembly. Here, the mechanism 120B includes a bell-crank linkage having a rotating arm <NUM> with a hinged push rod <NUM> attached to the gate <NUM> of the choke <NUM> (shown simplified). The rotating arm <NUM> is coupled at a pivot <NUM> to a linear input L from the actuator <NUM>, which can be a piston or linear motor, for example. Rotation of the rotating arm <NUM> about a fixed pivot <NUM> transfers to the hinged push rod <NUM> via a moving pivot <NUM>. The rod <NUM> couples with another moving pivot <NUM> to the gate <NUM>. The linear input L from the actuator <NUM> rotates the arm <NUM>, and the rod <NUM> changes the rotary motion of the arm <NUM> into reciprocating motion of the gate <NUM> of the choke <NUM>.

In this embodiment, the overall closing time of the choke <NUM> may remain the same as with a conventional design. However, with the choke <NUM> near an opened position (<FIG>), each increment of rotation of the arm <NUM> results in greater motion of the gate <NUM>. By contrast, with the choke <NUM> near a closed position (<FIG>), each additional increment of rotation of the arm <NUM> results in less motion of the gate <NUM>. Thus, the linear input L of the actuator <NUM> is transferred by the mechanism 120B in a non-linear fashion to the motion of the gate <NUM>, which gives more control to the operation of the choke <NUM>.

In the mechanical assemblies of <FIG>, the mechanisms 120A-B have a fixed transfer relationship due to the mechanics of the components, and the particulars of the fixed mechanics involved can be configured for a given implementation. In this way, a given mechanism 120A-B with one of a plurality of different mechanics can be used on a given choke <NUM> to suit the requirements of an implementation. Differences in the geometry and sizes of the arms <NUM>, <NUM> for example can achieve different control. Additionally, the pivot points <NUM>, <NUM> can be adjustable on the arms <NUM>, <NUM> to alter the mechanics, and/or arms <NUM>, <NUM> of different lengths and geometries may be used interchangeably to alter the mechanics.

For comparative purposes, <FIG> graphs a plot 90A of a choke's pressure drop (psi) versus control motion revolution (count) for an existing worm gear mechanism (e.g., <NUM>: <FIG>) alone and for an operational device (<NUM>) having a transfer mechanism (e.g., 120A-B: <FIG>) of the present disclosure. The plot 90A shows the change of slope for two pressure curves 92A, 94A as they relate to a number of motor revolutions. The first pressure curve 92A is shown for the existing worm gear mechanism (<NUM>) of the prior art, such as shown in <FIG> and as is currently used in a choke actuator. This first pressure curve 92A is compared to a second pressure curve 94A for an operational device having the crank-slider or bell crank mechanisms (120A-B) of the present disclosure. (This example of the mechanism (120A-B) uses a <NUM>:<NUM> reducer gearbox installed between the motor (<NUM>) and the mechanism (120A-B) to reduce <NUM> revolutions of the motor (<NUM>) into a ¼ of a revolution of the rotating arm (<NUM>). ) As the slope of the second curve 94A indicates, the mechanism 120A-B may provide approximately five (<NUM>) times more control resolution than the direct-drive worm gear mechanism (<NUM>) alone.

For comparative purposes, <FIG> graphs another plot 90B of choke closing speed (in/sec) versus time (sec) generally depicting the non-linear relationship. The plot 90B shows how the closing speed of the gate (<NUM>) does not vary with time for the existing worm screw mechanism (<NUM>: <FIG>) in curve 92B and shows how the closing speed of the gate (<NUM>) does vary with time for the disclosed crank-slider or bell crank mechanisms (120A-B) in curve 94B. The gate (<NUM>) closing speed (in/sec) is graphed relative to the choke closing position in time (sec) from opened to closed. As shown by curve 92B, the existing worm-gear mechanism (<NUM>) has a practically constant (linear) closing speed, which in this example is about <NUM>-in/sec, as the gate (<NUM>) goes from an opened position to a closed position over a time period of <NUM> to about <NUM>-sec. By contrast, the disclosed transfer mechanism (120A-B) shown by curve 94B has a varied closing speed that drops off over time, as the gate (<NUM>) goes from an open state to a closed state. The mechanism (120A-B) tend to make the motion (La, speed) of the choke (<NUM>) quicker or faster when near the position of being fully opened, whereas the motion (i.e., speed) of the choke (<NUM>) may be slower when near the position of being fully closed. This indicates that more refined closing can be achieved with the disclosed mechanisms (120A-B).

Looking at the relationship another way in a plot 90C of <FIG>, the non-linear relationship of the transfer mechanism (120A-B) can be viewed as actuation input from an actuator (i.e., input motion, degrees rotation of motor, linear displacement of piston, etc.) graphed relative to output of the choke (i.e., output motion, displacement of the choke's trim, etc.). For the worm gear arrangement, the transfer line 92C is constant in that each actuation input (degree rotation of the motor) results in output of the choke in a linear fashion as shown by the straight line. The correlation between the input and the output is linear, indicating that every increment (degree) of displacement of the actuator input produces a corresponding increment of displacement of the choke output.

By contrast, the transfer mechanism (120A-B) provides a different correlation between the input and output. As shown, the transfer line 94C initially has a steeper slope near the open state (<NUM>), which indicates that a given displacement of the input produces a greater displacement of the output. Yet, near the closed state (<NUM>), the slope of the transfer line 94C is less than the <NUM>:<NUM> ratio of the standard transfer line 92C. This lower slope indicates that a given displacement of the input results in a smaller displacement of the output. Accordingly, the non-linear relationship may tend to make the motion (i.e., displacement) of the choke(s) (<NUM>) smaller when near the position of being fully closed (<NUM>), whereas the motion (i.e., displacement) of the choke (<NUM>) may be larger by comparison when near the position of being fully opened (<NUM>).

Another example of the operational device <NUM> depicted in <FIG> combines the mechanical mechanism 120B shown in <FIG> with a hydraulic cylinder <NUM> as the actuator. A piston of the cylinder <NUM> attaches to the back side of the rotating arm <NUM>. <FIG> show the bell crank mechanism 120B in conjunction with the cylinder <NUM> during stages of operation. The orientation and arrangement of the hydraulic cylinder <NUM> with respect to the rotating mechanism 120B is such that the initial driving motion from the sliding piston yields a relatively larger rotating motion than the final driving motion. The resulting variable rotating speed combined with the mechanical advantage of the rotating mechanism 120B provides additional resolution to the control as the gate (<NUM>) approaches the closed position.

<FIG> illustrates a choke <NUM> controlled by a control unit <NUM> and a power source <NUM> and operated by another operational device <NUM> according to the present disclosure. As shown, the operational device <NUM> for operating the choke <NUM> includes a position sensor <NUM>, an actuator <NUM>, and a non-linear transfer mechanism 120C. Here, the actuator <NUM> is a hydraulic motor powered by hydraulics from a hydraulic power unit (HPU) also referred to as the power source <NUM> in this example. The actuator <NUM> couples to the choke <NUM> with a worm gear arrangement <NUM> or or other suitable mechanism, which transfers the rotation of the motor <NUM> into reciprocal movement of the internal trim or flow restrictor member (not shown) in the choke <NUM>.

The non-linear transfer mechanism 120C includes a hydraulic throttling valve coupled to the hydraulics between the hydraulic motor <NUM> and the hydraulic power unit <NUM>. The control unit <NUM> uses one or more control algorithms <NUM> that send control signals to the hydraulic throttling valve 120C coupled with the hydraulic motor <NUM>. (If more than one control algorithm <NUM> is available, selection of a particular control algorithm <NUM> can be performed by the control unit <NUM> based on calibration, operating parameters, etc.) The controlled throttling from the signals causes the hydraulic motor <NUM> to speed up and slow down the motor revolutions according to feedback coming from the position sensor <NUM>. Accordingly, these signals for controlling the speed of motor revolutions are used to achieve the purposes disclosed herein-namely slowing the closing speed of the internal trim (e.g., gate <NUM> or other flow restrictor member) in the choke <NUM> when near a fully closed position.

<FIG> illustrates a related configuration where a non-linear transfer mechanism 120C includes a hydraulic throttling valve coupled between the hydraulic motor <NUM> and the hydraulic power unit <NUM>. Here, an operation stem of throttling valve 120C is physically moved by (directly operated by, attached to, or connected to) the stem on the choke <NUM>, alongside the position sensor (not shown). The throttling valve 120C thereby moves with the movement of the choke <NUM> and correspondingly throttles the hydraulics to the motor <NUM>.

<FIG> shows the hydraulic throttling valve 120C in more detail in two operating conditions (open-<FIG> and closed-<FIG>). The operation stem <NUM> of the valve 120C is physically moved by (or attached to) the choke stem (not shown) so the operation stem <NUM> can reciprocate inside a throttle housing <NUM>. A throttling head <NUM> of the stem <NUM> can move relative to a flow restriction or orifice <NUM> to throttle flow from an inlet <NUM> (coupled to the power unit <NUM>) to an outlet <NUM> (coupled to the motor <NUM>).

In <FIG>, the throttling valve 120C is open with the throttling head <NUM> moved away from the flow restriction <NUM> for free flow of the hydraulics from the inlet <NUM> to the outlet <NUM>. As the choke (<NUM>) moves closer to closing, however, the throttling valve's head <NUM> also moves closer to closing in the flow restriction <NUM>, which slows down the flow of hydraulic fluid from the HPU <NUM> at the inlet <NUM> to the motor <NUM> at the outlet <NUM>. This in turn slows down the closing speed of the gate (<NUM>) of the choke (<NUM>).

<FIG> illustrates a related configuration where a non-linear transfer mechanism 120D includes a drive controller coupled between an electric motor <NUM> as the actuator and a power supply <NUM> as the power source. Here, the control unit <NUM> uses one or more control algorithms <NUM> that send control signals to the drive controller 120D coupled with the electric motor <NUM>. (If more than one control algorithm <NUM> is available, selection of a particular control algorithm <NUM> can be performed by the control unit <NUM> based on calibration, operating parameters, etc.). The controlled drive from the signals causes the electric motor <NUM> to speed up and slow down the motor revolutions according to feedback coming from the position sensor <NUM>. Accordingly, these signals for controlling the speed of motor revolutions can be used to slow the closing speed of the internal trim (e.g., gate <NUM>) in the choke <NUM> when near a fully closed position.

In the embodiments of <FIG>, the overall closing time of the choke's gate (<NUM>) can remain constant according to its particular design. However, the speed of the hydraulic or electric motor <NUM> driving the choke's gate (<NUM>) can be changed over time to allow a more linearized pressure over time profile near the closed position. As such, the closing speed can be adjusted to mimic what is graphed in <FIG> to produce better control of the resulting pressure drop similar to what is graphed in <FIG>.

Another non-linear transfer mechanism 120E according to the present disclosure shown in <FIG> can use a gear arrangement coupled between the actuator <NUM> and the choke <NUM>. For example, the actuator <NUM> can be a hydraulic or electric motor that couples by a worm gear arrangement <NUM> to the choke <NUM> so that rotation of the motor <NUM> transfers to reciprocal movement of the internal trim or flow restrictor member (not shown) in the choke <NUM>. The gear arrangement 120E alters the transfer of the rotation in a non-linear fashion. For example, the gear arrangement 120E can use a pairing of variable radii and/or variable pitch gears.

As shown in <FIG>, an example of a pair <NUM> of variable radii gears can include elliptical gears <NUM>, <NUM>. Using the pair <NUM> of the elliptical gears <NUM>, <NUM> can change the rotational speed of the output gear <NUM> relative to the input gear <NUM>, which would add more resolution to the control unit <NUM> as the internal trim in the choke <NUM> approaches the closed position.

As will be appreciated with the benefit of the present disclosure, other gear mechanisms can be used for the transfer mechanisms of the present disclosure. An arrangement having a chain between elliptical sprockets could be used in a manner similar to the planetary gears. Similarly, a continuously variable transmission can be used to make the transfer in the non-linear relationship.

Another operational device <NUM> depicted in <FIG> for operating a choke <NUM> includes a position sensor <NUM>, an actuator <NUM>, and a non-linear transfer mechanism 120F according to the present disclosure. The actuator <NUM> is a hydraulic motor <NUM> with built-in variable speed. The variable speed motor <NUM> has a pilot port <NUM> accessed via a control valve <NUM> to actuate an internal mechanism in the motor <NUM> and change the required displacement of hydraulic fluid per revolution.

The control unit <NUM> uses one or more control algorithms <NUM> that send control signals to the control valve <NUM> coupled between the hydraulic power unit <NUM> and the pilot port <NUM> of the variable speed motor <NUM>. (If more than one control algorithm <NUM> is available, selection of a particular control algorithm <NUM> can be performed by the control unit <NUM> based on calibration, operating parameters, etc.). The control valve <NUM> can be a solenoid operated three-way valve, as shown. The controlled pilot feed from the control valve <NUM> causes the variable speed motor <NUM> to speed up and slow down according to feedback coming from the position sensor <NUM>. Accordingly, this control for controlling the speed of motor revolutions can be used to slow the closing speed of the internal trim or flow restrictor member in the choke <NUM> when near a fully closed position. This configuration allows for finer control of the choke <NUM> at select positions based on feedback from the position sensor <NUM>. The variable speed hydraulic motor <NUM> can also be implemented with any combination of the aforementioned embodiments.

A combination of the previously disclosed embodiments could be used together to enhance the control capabilities of a choke <NUM> at critical pressures. The gear arrangement 120E of <FIG> can be used in conjunction with the mechanisms 120A-B of <FIG> or can be coupled to the motor <NUM> of <FIG>.

As one particular example, <FIG> illustrates a choke <NUM> controlled by a control unit <NUM> and a hydraulic power unit <NUM> as the power source. The actuator <NUM> is a hydraulic motor in communication with the hydraulic power unit <NUM>. A combination of the non-linear transfer mechanisms 120A-E of <FIG> can used to transfer/control the actuation. In particular, a bell crank mechanism 120A as in <FIG> is coupled between the motor <NUM> and the choke <NUM> for adjusting motion of the choke's gate (<NUM>), and a gear box 120E having gears as in <FIG> is coupled between the bell crank mechanism 120A and the motor <NUM>. The gear box 120E coupled between the motor <NUM> and the rotating arm of the mechanism 120A can alter the number of motor revolutions to move the gate (<NUM>) between fully-open to fully-closed in order to refine control as desired.

Finally, a hydraulic throttling valve 120C as in <FIG> can be operable between the power unit <NUM> and the hydraulic motor <NUM>. One or more control algorithms <NUM> may be used by the control unit <NUM> to control the throttling valve 120C. The bell crank mechanism 120B and gear box 120E have a fixed transfer relationship built into the mechanics of the mechanisms. By contrast, the throttling valve 120C can have a selective transfer relationship based on the control algorithm used. As will be appreciated, these and other combinations of the various mechanisms <NUM> disclosed herein can be used for controlling the choke <NUM>.

The additional precision for the control system comes from a built-in mechanical component, or software algorithm or combination of the two that causes the choke's internal trim, gate or flow restrictor member (<NUM>) to move more slowly as it nears the closed position. The use of an entirely mechanical device <NUM> to provide the additional range of control in some examples removes the need to have extra electrical components and software algorithms added to the design.

Although disclosed herein as applying to automated designs using powered actuation for a choke from a power source such as a hydraulic power unit or electric power supply, a manually operated choke can also benefit from the non-linear transfer mechanisms of the present disclosure. This may be especially evident for the mechanical assemblies disclosed herein where the rotational input R (see <FIG>) or the linear input L (see <FIG> & <FIG>) is provided by a manual device operated by personnel.

Representatively illustrated in <FIG> is a portion of another example of the drilling system <NUM> which can embody principles of this disclosure. The choke <NUM> is depicted as being connected between the wellbore <NUM> and the mud gas separator <NUM>, mud tanks <NUM> and rig pumps <NUM>, as in the example of <FIG>. However, the scope of this disclosure is not limited to use of a choke connected between a wellbore and a mud gas separator, mud tanks and rig pumps. Instead, any of the chokes <NUM> described herein may be connected between a variety of other types of equipment.

The operational device <NUM> used to operate the choke <NUM> in the <FIG> example includes a non-linear transfer mechanism 120F which transfers motion from the actuator <NUM> to the worm gear arrangement <NUM>. The actuator <NUM> in this example is a motor (such as, an electric or hydraulic motor) which is operatively connected to the power source <NUM> and produces a rotational input R to the transfer mechanism 120F. Other types of actuators may be used in keeping with the scope of this disclosure.

The worm gear arrangement <NUM> could be replaced, for example, by a bell crank mechanism (such as, the mechanism 120A) or another type of mechanism, or combination of mechanisms. In other examples, a separate mechanism may not be used between the transfer mechanism 120F and the choke <NUM>.

In the <FIG> example, the transfer mechanism 120F comprises a continuously variable transmission (CVT). The CVT transfers motion from the actuator <NUM> to the choke <NUM> (via the worm gear arrangement <NUM> in this example) in a non-linear manner. Preferably, a rate of actuation of the choke <NUM> (such as, a speed of displacement of the internal trim or flow restrictor member <NUM> of the choke, see <FIG>) is slower when the choke is proximate its closed or most flow restrictive position or configuration, than when the choke is proximate its open or least flow restrictive position or configuration.

Operation of the CVT is controlled by the control unit <NUM> which, as described above, can include one or more control algorithms <NUM> (see <FIG>, <FIG>, <FIG> & <FIG>). In this example, the control algorithm <NUM> is adapted to send control signals to the transfer mechanism 120F. (If more than one control algorithm <NUM> is available, selection of a particular control algorithm <NUM> can be performed by the control unit <NUM> based on calibration, operating parameters, etc.). The signals cause the transfer mechanism 120F to speed up and slow down the rotational speed transferred to the worm gear arrangement <NUM> by changing an effective gear ratio of the CVT according to feedback coming from the position sensor <NUM> (see <FIG>, <FIG>, <FIG>, <FIG> & <FIG>). Accordingly, these signals for controlling the effective gear ratio of the CVT can be used to slow the closing speed of the internal trim, gate or other flow restrictor member <NUM> in the choke <NUM> when near the fully closed position.

The overall closing time of the choke's gate or other flow restrictor member <NUM> can remain constant according to its particular design. However, the speed of the rotational input R from the transfer mechanism 120F to the worm gear arrangement <NUM> driving the choke's flow restrictor member <NUM> can be changed over time in response to the control signals, in order to allow a more linearized pressure over time profile near the closed position. For example, the closing speed can be adjusted to mimic what is graphed in <FIG> to produce better control of the resulting pressure drop, similar to what is graphed in <FIG>.

Referring additionally now to <FIG>, an example of internal components of the non-linear transfer mechanism 120F is representatively illustrated. In <FIG>, an effective gear ratio of the transfer mechanism 120F is relatively high (in this example, a rotational output speed is greater than a rotational input speed). The transfer mechanism 120F may be in this relatively high gear ratio configuration when the flow restrictor member <NUM> of the choke <NUM> is near its fully open or least flow restrictive position.

As depicted in <FIG>, an input shaft <NUM> receives a rotational input from the actuator <NUM>. A radially enlarged generally conical contact surface <NUM> is formed on (or secured to) an end of the input shaft <NUM>. The contact surface <NUM> is in frictional contact with a radially enlarged generally planar contact surface <NUM> formed on (or secured to) an end of an output shaft <NUM>. The output shaft <NUM> is connected to and provides a rotational output to the worm gear arrangement <NUM>.

Also depicted in <FIG> are outer peripheries 172A & 174A of the respective contact surfaces <NUM>, <NUM>. At a Contact Point, rotational displacement of the input and output shafts <NUM>, <NUM> are the same. In the <FIG> configuration, the Contact Point is at or near the outer peripheries 172A & 174A of the contact surfaces <NUM>, <NUM>. However, since a radius of the contact surface <NUM> is less than a radius of the contact surface <NUM>, the rotational speed of the output shaft <NUM> will be greater than a rotational speed of the input shaft <NUM>.

In the <FIG> example, the position of the Contact Point can be varied by means of a rack and pinion arrangement <NUM>. Operation of the rack and pinion arrangement <NUM> can be controlled by control signals from the control system <NUM>, for example, in response to the output from the position sensor <NUM>. Thus, the control signals can cause the rack and pinion arrangement <NUM> to place the transfer mechanism 120F in the <FIG> configuration (such as, by operating a motor or other type of actuator (not shown) connected to a pinion of the arrangement <NUM>) when the position sensor <NUM> indicates that the flow restrictor member <NUM> is near its fully open or least flow restrictive position.

Referring additionally now to <FIG>, the internal components of the non-linear transfer mechanism 120F are representatively illustrated in a relatively reduced effective gear ratio configuration. In the <FIG> configuration, the Contact Point is at or near the outer periphery 174A of the contact surface <NUM>, and near a center of the contact surface <NUM>. Thus, the rotational speed of the output shaft <NUM> will be less than a rotational speed of the input shaft <NUM>.

The position of the Contact Point has been varied (compared to the <FIG> configuration) by operation of the rack and pinion arrangement <NUM> in response to the control signals from the control system <NUM>, which are produced in response to the output from the position sensor <NUM>. Thus, the control signals cause the rack and pinion arrangement <NUM> to place the transfer mechanism 120F in the <FIG> configuration (such as, by operating the motor or other type of actuator (not shown) connected to the pinion of the arrangement <NUM>) when the position sensor <NUM> indicates that the flow restrictor member <NUM> is near its fully closed or most flow restrictive position.

In this manner, the position of the flow restrictor member <NUM> in the choke <NUM> can be more precisely adjusted near the fully closed or most flow restrictive position of the flow restrictor member. Stated differently, more rotational output from the actuator <NUM> is required to produce a given displacement of the flow restrictor member <NUM> when the flow restrictor member is nearer its fully closed or most flow restrictive position. An actuation rate (e.g., displacement speed of the flow restrictor member <NUM>, or change in position per given actuator output, or rate of change of the choke Cv) is relatively slow, therefore, when the flow restrictor member is nearer its fully closed or most flow restrictive position.

In addition, the control unit <NUM> can adjust the rack and pinion arrangement <NUM>, so that less rotational output from the actuator <NUM> is required to produce a given displacement of the flow restrictor member <NUM> when the flow restrictor member is nearer its fully open or least flow restrictive position. An actuation rate (e.g., displacement speed of the flow restrictor member <NUM>, or change in position per given actuator output, or rate of change of the choke Cv) is relatively fast, therefore, when the flow restrictor member is nearer its fully open or least flow restrictive position.

Between the closed/most restrictive and open/least restrictive positions, the effective gear ratio of the transfer mechanism 120F can be continuously varied by the control unit <NUM> via the rack and pinion arrangement <NUM>. Note that use of the rack and pinion arrangement <NUM> is not necessary in keeping with the scope of this disclosure, since other types of actuation mechanisms may be used instead to vary the relative positions of the contact surfaces <NUM>, <NUM>.

The position of the flow restrictor member <NUM> in the choke <NUM> influences the flow coefficient Cv of the choke. The flow coefficient Cv is given by the well-known equation: <MAT> where Q is the fluid flow rate, SG is the specific gravity of the fluid, and dP is the differential pressure across the choke.

At the fully open or least flow restrictive position of the flow restrictor member <NUM>, the Cv of the choke <NUM> will generally be at or near a maximum value. Conversely, at the fully closed or most flow restrictive position of the flow restrictor member <NUM>, the Cv of the choke <NUM> will generally be at or near a minimum value.

In one example of a managed pressure drilling application (such as, depicted in <FIG>), a CVT 120F is connected between the actuator <NUM> and the choke <NUM> (such as, via the worm gear arrangement <NUM>). A feedback loop is created with the control system <NUM> using the output of the position sensor <NUM> to gradually change to the relatively low gear ratio configuration of <FIG> while the choke flow restrictor member <NUM> approaches a low Cv configuration of the choke (e.g., with the flow restrictor member proximate its closed or most flow restrictive position). This provides an increase in sensitivity while holding higher pressure in the wellbore <NUM> at low flow rates. The CVT 120F can also be used to address other issues encountered while drilling that can generally create unwanted spikes in downhole pressure (such as, when cycling out debris from the choke <NUM>).

In a variety of different examples, the gear ratio of the CVT 120F can be changed either manually or with an automated feedback loop as described above. The output speed of the CVT 120F can be reduced as the choke <NUM> nears the closed or minimal Cv position. The CVT 120F, thus, provides a narrower pressure control window when the choke <NUM> is in its optimal control range.

Referring additionally now to <FIG>, an example of a graph <NUM> of actuation rate versus flow coefficient Cv is representatively illustrated. Two points <NUM>, <NUM> are indicated on the <FIG> graph <NUM>. The point <NUM> corresponds to a position of the flow restrictor member <NUM> at or near its fully closed or most flow restrictive position. The point <NUM> corresponds to a position of the flow restrictor member <NUM> at or near its fully open or least flow restrictive position.

At the point <NUM>, the Cv of the choke <NUM> is relatively low (since the flow rate Q through the choke <NUM> is reduced and the differential pressure dP across the choke is increased), and the actuation rate is relatively low, as compared to the point <NUM>. The rate of change of the Cv is relatively low at the point <NUM>, corresponding to the relatively low actuation rate. At the point <NUM>, the Cv of the choke <NUM> is relatively high (since the flow rate Q through the choke <NUM> is increased and the differential pressure dP across the choke is decreased), and the actuation rate is relatively high, as compared to the point <NUM>. The rate of change of the Cv is relatively high at the point <NUM>, corresponding to the relatively high actuation rate.

Between the points <NUM>, <NUM>, the effective gear ratio of the transfer mechanism 120F can be continuously varied by the control unit <NUM>, so that there is a linear relationship between the actuation rate and the choke Cv, as indicated by the dashed line <NUM> in the <FIG> graph <NUM>. Alternatively, the control unit <NUM> can vary the gear ratio of the transfer mechanism 120F, so that there is a non-linear relationship between the actuation rate and the choke Cv between the points <NUM>, <NUM>, as indicated by the dashed curves <NUM>, <NUM> in the graph <NUM>.

The transfer mechanism 120F may be used in place of, or in combination with, any of the other transfer mechanisms 120A-E described above. Accordingly, any of the operational devices <NUM> described above can be controlled (e.g., using the control unit <NUM> and control algorithm <NUM>), so that the actuation rate of the choke <NUM> is reduced as the flow restrictor member <NUM> displaces toward its fully closed, most flow restrictive or minimum Cv position. The actuation rate (and, thus, the rate of change of the Cv) of the choke <NUM> can be increased as the flow restrictor member <NUM> displaces toward its fully open, least flow restrictive or maximum Cv position. The actuation rate (and, thus, the rate of change of the Cv) can be reduced as the choke Cv decreases, and the actuation rate can be increased as the choke Cv increases.

It may now be fully appreciated that the above disclosure provides significant advancements to the art of controlling operation of a choke, such as a drilling choke or a valve capable of variably restricting fluid flow. In examples described herein, an actuation rate or rate of change of a flow coefficient Cv of the choke <NUM> can be varied based at least in part on a position of a flow restrictor member <NUM> of the choke. As the flow restrictor member <NUM> displaces toward a fully closed or most flow restrictive position (with a corresponding decreased Cv), the rate of change of the flow coefficient Cv can be decreased to provide for enhanced precision of adjustment of the flow coefficient Cv.

An apparatus for use with a subterranean well is provided to the art by the above disclosure. In one example, the apparatus can include: a choke <NUM> comprising a flow restrictor member <NUM> having at least first and second positions (e.g., corresponding to points <NUM>, <NUM>), a flow coefficient Cv of the choke <NUM> with the flow restrictor member <NUM> in the first position being less than the flow coefficient Cv of the choke <NUM> with the flow restrictor <NUM> in the second position; and an operational device <NUM> configured to displace the flow restrictor member <NUM> between the first and second positions at a variable actuation rate. The actuation rate with the flow restrictor member <NUM> in the first position being less than the actuation rate with the flow restrictor member <NUM> in the second position.

The operational device <NUM> may comprise a non-linear transfer mechanism <NUM> connected between the choke <NUM> and an actuator <NUM>. The non-linear transfer mechanism <NUM> may comprise a continuously variable transmission 120F.

A control unit <NUM> may be operatively connected to the operational device <NUM>. The control unit <NUM> may comprise a control algorithm <NUM> adapted to vary an effective gear ratio of the continuously variable transmission 120F as the flow restrictor member <NUM> is displaced between the first and second positions. The control algorithm <NUM> may be adapted to vary a rate of change of the flow coefficient Cv as the flow restrictor member <NUM> is displaced between the first and second positions. The control algorithm <NUM> may be adapted to reduce the rate of change of the flow coefficient Cv as the flow restrictor member <NUM> is displaced toward the first position.

A restriction to flow through the choke <NUM> with the flow restrictor member <NUM> in the first position may be greater than a restriction to flow through the choke <NUM> with the flow restrictor member <NUM> in the second position.

A method is also provided to the art by the above disclosure. In one example, the method may comprise: displacing a flow restrictor member <NUM> of a choke <NUM>, thereby decreasing a flow coefficient Cv of the choke <NUM>; and decreasing a rate of change of the flow coefficient Cv in response to the step of decreasing the flow coefficient Cv of the choke <NUM>.

The step of displacing the flow restrictor member <NUM> of the choke <NUM> may comprise operating an actuator <NUM> and a non-linear transfer mechanism <NUM> connected to the choke <NUM>. The step of operating the non-linear transfer mechanism <NUM> may comprise varying a gear ratio of the non-linear transfer mechanism <NUM>.

The non-linear transfer mechanism <NUM> may comprise a continuously variable transmission 120F. The step of operating the non-linear transfer mechanism <NUM> may comprise displacing a first contact surface <NUM> of the continuously variable transmission 120F relative to a second contact surface <NUM> of the continuously variable transmission 120F.

The method may include displacing the flow restrictor member <NUM>, thereby increasing the flow coefficient Cv of the choke <NUM>; and increasing the rate of change of the flow coefficient Cv in response to the step of increasing the flow coefficient Cv of the choke <NUM>.

The step of decreasing the rate of change of the flow coefficient Cv may include varying a gear ratio of a continuously variable transmission 120F connected to the choke <NUM>.

A drilling system <NUM> for use with a subterranean wellbore <NUM> is also described above. In one example, the drilling system <NUM> can include: a choke <NUM> configured to variably restrict fluid flow, the choke <NUM> comprising a flow restrictor member <NUM>; an actuator <NUM>; and a continuously variable transmission 120F connected between the actuator <NUM> and the choke <NUM>. The continuously variable transmission 120F is configured to cause an actuation rate to vary based on a position of the flow restrictor member <NUM>.

The drilling system <NUM> may include a control unit <NUM> configured to control operation of the continuously variable transmission 120F so that the actuation rate decreases as a restriction of the fluid flow increases.

The drilling system <NUM> may include a control unit <NUM> configured to control operation of the continuously variable transmission 120F so that the actuation rate decreases as a flow coefficient Cv of the choke <NUM> decreases.

The drilling system <NUM> may include a position sensor <NUM> which senses the position of the flow restrictor member <NUM>. An effective gear ratio of the continuously variable transmission 120F may be variable based on an output of the position sensor <NUM>.

An inlet <NUM> of the choke <NUM> may be configured to receive the fluid flow from the wellbore <NUM>. An outlet <NUM> of the choke <NUM> may be connected upstream of at least one rig pump <NUM>.

The continuously variable transmission 120F may be configured to decrease a rate of change of a flow coefficient Cv of the choke <NUM> as the flow coefficient Cv decreases.

Although various examples have been described above, with each example having certain features, it should be understood that it is not necessary for a particular feature of one example to be used exclusively with that example. Instead, any of the features described above and/or depicted in the drawings can be combined with any of the examples, in addition to or in substitution for any of the other features of those examples. One example's features are not mutually exclusive to another example's features. Instead, the scope of this disclosure encompasses any combination of any of the features.

Although each example described above includes a certain combination of features, it should be understood that it is not necessary for all features of an example to be used. Instead, any of the features described above can be used, without any other particular feature or features also being used.

It should be understood that the various embodiments described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of this disclosure. The embodiments are described merely as examples of useful applications of the principles of the disclosure, which is not limited to any specific details of these embodiments.

The terms "including," "includes," "comprising," "comprises," and similar terms are used in a non-limiting sense in this specification. For example, if a system, method, apparatus, device, etc., is described as "including" a certain feature or element, the system, method, apparatus, device, etc., can include that feature or element, and can also include other features or elements. Similarly, the term "comprises" is considered to mean "comprises, but is not limited to.

Claim 1:
An apparatus for use with a subterranean well, the apparatus comprising:
a choke (<NUM>) comprising a flow restrictor member (<NUM>) having at least first and second positions (<NUM>, <NUM>), a flow coefficient Cv of the choke (<NUM>) with the flow restrictor member (<NUM>) in the first position (<NUM>) being less than the flow coefficient Cv of the choke (<NUM>) with the flow restrictor member (<NUM>) in the second position (<NUM>); and
an operational device (<NUM>) configured to displace the flow restrictor member (<NUM>) between the first and second positions (<NUM>, <NUM>) at a variable actuation rate, the actuation rate with the flow restrictor member (<NUM>) in the first position (<NUM>) being less than the actuation rate with the flow restrictor member (<NUM>) in the second position (<NUM>), in which the operational device (<NUM>) comprises a rack and pinion arrangement (<NUM>) which varies a contact point between a generally conical contact surface (<NUM>) and a generally planar contact surface (<NUM>).