Drilling choke with matched actuator

A method of controlling pressure in a wellbore comprising calibrating a choke system comprising including a choke having an inlet and an outlet, a central flow passage which extends between the inlet and outlet, and a plug which is movable to vary the degree to which fluid flow along the central flow passage is restricted, the choke system further comprising an actuator which is operable to move the plug to vary the degree to which fluid flow along the central flow passage is restricted, the plug being movable between a closed position in which the plug substantially prevents flow of fluid along the central flow passage and an open position in which flow of fluid along the central flow passage is restricted by the plug to a minimum extent, and a positioner which is configured to receive from an electronic controller a signal representing the desired position of the plug output, the positioner being connected to the actuator and configured to control operation of the actuator so that the plug is moved in accordance with the control signal, the calibration of the choke system comprising connecting to the positioner an electronic controller which is programmed to output a control signal representing the desired position of the plug, connecting to the electronic controller a flowmeter which is configured to provide a flow signal representing the rate of flow of fluid from the pump to the inlet and a pressure sensor which is configured to provide a pressure signal representing the fluid pressure at the inlet, connecting the inlet to a reservoir of fluid via a pump which is operable to pump fluid from the reservoir into the inlet at varying flow rates, and using gain scheduling to determine optimum values of proportional gain and integral gain required for control of the choke using a proportional differential and integral controller at a plurality of different rates of flow of fluid along the central flow passage, wherein the method further comprises installing the calibrated choke system in a return line of the drilling system so that the central flow passage forms part of the return line and fluid returning from the wellbore enters the choke system at the inlet, connecting the positioner to an electronic controller which has a memory and which is programmed with a PID algorithm and to send to the positioner an electrical signal representing the desired position of the plug, connecting the controller to a flow meter and a pressure sensor which are configured to send to the controller electrical signals representing the fluid flow rate and fluid pressure at the inlet, wherein the optimum values of the proportional gain and integral gain determined during calibration of the choke system are stored in the memory of the controller, and the controller is programmed to use these, in combination with the signals received from the flow meter and pressure sensor, to select the appropriate proportional gain and integral gain to be used in the PID algorithm in determining the control signal to be sent to the positioner whenever there is a deviation of the measured pressure from the desired pressure at the inlet.

A drilling system comprising a drill string which extends into a wellbore to form an annular space in the wellbore around the drill string, a return fluid flow conduit which is connected to the annular space, and a choke system comprising a choke having an inlet and an outlet, a central flow passage which extends between the inlet and outlet, and a plug which is rotatable about an axis of rotation to vary the degree to which fluid flow along the central flow passage is restricted, the choke system further comprising an actuator which is operable to rotate the plug to vary the degree to which fluid flow along the central flow passage is restricted, the plug being movable between a closed position in which the plug substantially prevents flow of fluid along the central flow passage and an open position in which flow of fluid along the central flow passage is restricted by the plug to a minimum extent, the choke being mounted in the return fluid flow conduit so that the central flow passage forms part of the return fluid flow conduit and fluid returning from the wellbore enters the choke system at the inlet, the plug being provided with a plug passage which has a first end and a second end and which is arranged so that when the plug is in its open position, fluid can pass along the central flow passage of the choke from the inlet to the outlet of the choke via the plug passage from the first end of the second end of the plug passage, wherein the plug is provided with a removable plug insert which surrounds the second end of the plug passage.