METHOD AND DEVICE FOR POSITION CONTROL OF A WORKING ORGAN OF A DOWN-HOLE ACTUATOR

This invention relates to a method for position control of the working organ of a downhole actuator. More particularly, it relates to a method for position control of the working organ of a downhole actuator, the actuator being connected to a gripper and placed in a borehole . The working organ of the actuator is moved by means of a screw and nut connection driven by a motor. The invention includes also a device for practising the method.

A downhole actuator, often termed a "stroker", is used to a considerable extent for completion and maintenance tasks in a borehole. Typical tasks are the setting of plugs, replacement of valves and performance of cleaning operations.

The actuator is usually connected to a gripper, the gripper being arranged to be suspended against the borehole or pipe wall. A gripper and actuator assembly may be connected to, for example, a wire line or to coiled tubing but is often connected to a downhole tractor in order to be moved into ap- proximately horizontal borehole portions.

The working organ of the actuator, which often resembles a piston rod, is arranged to be moved, when the gripper is attached to the well or pipe wall, out or in by the actuator in the longitudinal direction of the borehole.

A considerable drawback of prior art actuators is that the relative position of the working organ relative to the actuator is not known. This causes uncertainty as to whether a performed operation was successful or not. It has turned out that the actuator will often have to be run down to the work site again, after it has been pulled to the surface, because an operation was unsuccessful. If the relative position of the working organ had been known, the operation could perhaps have been repeated while the actuator was at the work site.

The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art .

The object is achieved according to the invention through the features which are specified in the description below and in the claims that follow.

A method has been provided for the position control of the working organ of a downhole actuator, the actuator being connected to a gripper and positioned in a borehole, and the working organ of the actuator being moved by means of a screw and nut connection driven by a motor. The method is charac- terized by comprising:

- connecting a transmitter to the actuator, the transmitter being arranged to measure, directly or indirectly, the rotary angle of the screw;

- connecting the transmitter to a control unit, the control unit being arranged to receive a set value for how far the working organ is to be moved and, on the basis of the set value, to control the motor to rotate in the desired direction until the control unit calculates, on the basis of signals from the transmitter, that the set value has been reached;

- setting a set value; and

- seeing that the set value is reached.

The method provided can be carried out by the actuator being provided with a transmitter, the transmitter being arranged to measure, directly or indirectly, the rotary angle of the screw; and the transmitter being connected to a control unit, the control unit being arranged to receive a set value for how far the working organ is to be moved and, on the basis of the set value, to control the motor to rotate in a desired direction until, on the basis of signals from the transmitter, the control unit calculates that the set value has been reached.

It is appropriate for the transmitter to be connected to the motor and be driven by the shaft of the motor.

The display and operating means of the control unit is located at the surface. The other components belonging to the control unit, the control system itself being of a design known per se, may be placed in connection with the actuator or at the surface .

An actuator in accordance with the device described initially is well known to a person skilled in the art and is not de- scribed any further. Still, it may be worth noting that the motor of the actuator is normally supplied with energy from the surface via a cable and controlled by a downhole motor control receiving control signals from the surface. The motor control may form part of the control of the gripper or down- hole tractor.

When the working organ of the actuator is to be moved to a particular position relative to the actuator, the relative position is set as a set value by means of the operating means. The relevant relative position of the working organ is known by the control unit having received and stored signals from the transmitter, normally by the number of pulses re-

ceived after the working organ was at a reference point being known.

When a start signal is emitted, the motor rotates in the desired direction while the control unit checks whether the de- sired set value has been reached. When the set value has been reached, the control unit stops the motor. Control units of the kind relevant for use here will approximately continuously show the relative position of the working organ and can also be designed to output a signal when the set value has been reached.

A method and device according to the invention improve, compared with the prior art, the possibility of carrying out a successful downhole operation. This may lead to a considerable profit in terms of time and economy.

In what follows is described an example of a preferred method and embodiment which is visualized in the accompanying drawing, in which:

Figure 1 shows, schematically and partly in section, an actuator in accordance with the invention.

In the drawing the reference numeral 1 indicates an actuator which is connected to a gripper 2 and a downhole tractor 4, the actuator 1, gripper 2 and downhole tractor 4 being in a pipe 6 in a borehole 8.

The actuator 1 comprises an actuator housing 10 and an elon- gated working organ 12 which is arranged to be moved in and out of the free end portion of the actuator housing 10 in which the working organ 12 runs in a slide guide 14. The working organ 12 is prevented from rotating about the longitudinal axis 16 of the actuator 1. Furthest out the working organ 12 is formed with a threaded portion 18 for the connec-

tion of, for example, a tool not shown.

At its opposite end portion the working organ 12 is provided with a nut 20. The nut 20, most advantageously formed by a so-called spherical nut, extends on a complementarily match- ing screw 22.

The screw 22 is supported in and driven by an electric motor 24 which is placed in and secured to the actuator housing 10. The motor 24 is connected via a motor cable 26 to a motor control, not shown, of a design known per se.

A transmitter 28 on the motor 24 is connected to a control unit 30 via a transmitter cable 32. The transmitter 28 is arranged to emit signals dependent on the rotary angle of the shaft of the motor 24.

The control unit 30 communicates via a cable 34 with a con- trol panel 36 located at the surface. The cable 34 may be the same as the one supplying energy to the motor 24, gripper 2 and downhole tractor 4.

The control panel 36 comprises a display 38 and an operating means 40 including a start/stop switch 42.

When the actuator 1 is to be used, the actuator 1 is moved in the pipe 6 by means of the downhole tractor 4 to the position in the borehole 8 where an operation is to be carried out. Then the gripper 2 is activated, attaching to the pipe 6 to hold the actuator 1 in a particular position.

The position of the working organ 12 relative to the actuator housing 10 is shown on the display 38 and a new set value, possibly with position tolerances, for the relative position of the working organ 12 is entered into the control unit 30 by means of the operating means 40.

When the movement of the working organ 12 is to be started, a signal is given via the start/stop switch 42 to the control unit 30 which outputs a signal to the motor control, not shown, to start the motor 24 in the desired direction.

The motor 24 rotates while the transmitter 28 emits signals as the screw 22 is rotating and moving the working organ 12 in the direction towards the desired position.

When the control unit 30 calculates that the desired set value and, thereby, position has been reached, the motor 24 is stopped.