CONTROLLING THE FLOW OF DRIVING FLUID TO A SYSTEM OF MULTIPLE HYDRAULIC MOTORS

Abstract: A device (20) for controlling hydraulic power supplied to a system of a plurality of hydraulic motors (10, 11) of a drill string for drilling a borehole into an earth formation, each hydraulic motor being arranged to drive a respective drill bit of the drill string, the device comprising means for dividing a stream of hydraulic fluid flowing through the drill string into a plurality of sub-streams (32, 33), each sub-stream driving a respective one of said motors, and control means for increasing the hydraulic power of a selected sub-stream and for decreasing the hydraulic power of at least one of the other sub-streams in response to an increase of fluid pressure drop across the motor driven by the selected sub-stream.

Description: CONTROLLING THE FLOW OF DRIVING FLUID TO A SYSTEM OF MULTIPLE HYDRAULIC MOTORS The present invention relates to drilling a borehole in an underground formation, wherein the drill bit is driven by means of a downhole hydraulic motor that is suspended in the borehole at the end of a drill string. The hydraulic motor is driven by the drilling fluid that circulates through the drill string. Such a hydraulic motor is also called a mud motor, and an example of the downhole hydraulic motor is a Moineau motor. In order to increase the power available to drive the drill bit, a multiple drilling motor is being proposed. Such a multiple drilling motor is a system of at least two peripheral hydraulic motors . It is an object of the present invention to provide a device for controlling the flow of driving fluid to a system of at least two hydraulic motors of a drill string. In accordance with the invention there is provided a device for controlling hydraulic power supplied to a system of a plurality of hydraulic motors of a drill string for drilling a borehole into an earth formation, each hydraulic motor being arranged to drive a respective drill bit of the drill string, the device comprising: means for dividing a stream of hydraulic fluid flowing through the drill string into a plurality of sub- streams, each sub-stream driving a respective one of said motors; and control means for increasing the hydraulic power of a selected sub-stream and for decreasing the hydraulic power of at least one of the other sub-streams in response to an increase of fluid pressure drop across the motor driven by the selected sub-stream. It is thereby achieved that if the pressure drop across one of the motors increases, for example due to increasing resistance encountered by the drill bit, the control system ensures that the motor receives more power to the expense of the power provided to at least one of the other motors. The total hydraulic power increase needed is thereby limited or remains constant. Suitably the control means is arranged to increase, for each motor, the hydraulic power of the sub-stream driving the motor and to decrease the hydraulic power of at least one of the sub-streams driving the other motors in response to an increase of fluid pressure drop across the motor. In a preferred embodiment each motor has a hydraulic fluid inlet provided with an inlet valve, and wherein the control means is arranged to control the inlet valve of the motor and at least one of the inlet valves of the other motors. Preferably the control system includes a mechanical system arranged to further close said at least one of the inlet valves in response to further opening of the inlet valve. Suitably the device comprises a block having a first surface and a second surface, which block is provided with a central cylinder and at least two peripheral cylinders arranged uniformly around the central cylinder, each cylinder having an inlet opening, a first port near the first surface and a second port near the second surface and being provided with a piston having a piston rod extending out of the first surface of the block, wherein the block is further provided with one supply channel for each cylinder that is connected to the inlet opening of the cylinder, and which device further comprises a free hanging control plate positioned near the first surface of the block, which control plate has a first surface facing the first surface of the block and a second surface, wherein the second port of each peripheral cylinder is in fluid communication with the inlet of a corresponding peripheral hydraulic motor, wherein the first port of the central cylinder is in fluid communication with the inlet of the central hydraulic motor, wherein the first port of each peripheral cylinder is in fluid communication with the discharge of a corresponding peripheral hydraulic motor, wherein the second port of the central cylinder is in fluid communication with the discharge of the central hydraulic motor, and wherein the piston rods pertaining to the pistons in the peripheral cylinders are in contact with the first surface of the control plate and the piston rod pertaining to the piston in the central cylinder is in contact with the second surface of the control plate. The invention will now be described in more detail with reference to the accompanying drawings, wherein Figure 1 shows schematically a multiple drilling motor provided with the device according to the present invention in a borehole; and Figure 2 shows schematically a cross-section of the device according to the present invention. Reference is now made to Figure 1, which shows schematically a multiple drilling motor 1 provided with the device according to the present invention (not shown) contained in a box 2 in a borehole 3 that is being drilled in an underground formation 4. The multiple drilling motor 1 comprises a system of two peripheral hydraulic motors 10 and 11 and one central hydraulic motor 12, the motors are arranged parallel to each other. Each of these hydraulic motors drives a drill bit, the drill bits are referred to with reference numeral 15. The peripheral hydraulic motors 10 and 11 are arranged uniformly around the central hydraulic motor 12. If the centres of the hydraulic motors are defined as the intersections of their central longitudinal axes and a horizontal plane, centres of the peripheral hydraulic motors 10 and 11 are the opposite end points of a line that goes through the centre of the central hydraulic motor 12, wherein the length of the line-segment from the centre of one peripheral hydraulic motor to the centre of the central hydraulic motor is equal to the length of the line-segment from the centre of the other peripheral hydraulic motor to the centre of the central hydraulic motor. The multiple drilling motor 1 is attached to the lower end of a drill string 18 that is suspended in the borehole 3. Through the drill string 18 drilling fluid is supplied via the device of the present invention contained in the box 2 to the drilling motor 1. Reference is now made to Figure 2 showing the device 20 according to the present invention for controlling the flow of driving fluid to a system of at least two peripheral hydraulic motors and one central hydraulic motor that is contained in the box 2 of Figure 1. The device 20 comprises a block 21, which block comprises three plates 21a, 21b and 21c that are secured onto each other. For the sake of clarity sealing means that are arranged between the plates have not been shown. The block 21 has a first surface 23 and a second surface 26. The block 21 is provided with a central cylinder 30 and two peripheral cylinders 32 and 33, wherein the peripheral cylinders 32 and 33 are arranged uniformly around the central cylinder 30. In the embodiment shown, the peripheral cylinders 32 and 33 are arranged at either side of the central cylinder 30 and parallel thereto. As will be explained now, each cylinder controls the flow of driving fluid to the corresponding hydraulic motor, thus the central cylinder 30 controls the flow of driving fluid to the central hydraulic motor 12 (see Figure 1) and the peripheral cylinders 32 and 33 control the flow of driving fluid to the peripheral hydraulic motors 10 and 11. To this end each cylinder 30, 32 and 33 has an inlet opening in the middle, a first port near the first surface 23 and a second port near the second surface 26. The inlet openings of the cylinders are referred to with reference numeral 35. The first ports of the peripheral cylinders 32 and 33 are referred to with reference numeral 38 and the second ports of the peripheral cylinders 32 and 33 are referred to with reference numeral 39, whereas the second port of the central cylinder 30 is referred to with reference numeral 42 and the first port with reference numeral 44. In each cylinder 30, 32, 33 a piston is located which piston is provided with a piston rod extending out of the first surface 23 of the block 21. The pistons in the peripheral cylinders 32 and 33 are referred with reference numeral 45 and the piston rods with 46, whereas the piston in the central cylinder 30 is referred to with reference numeral 47 and the piston rod with 48. The block 21 is provided with one supply channel 50 for each cylinder 30, 32 and 33, wherein each supply channel 50 is connected to the inlet opening 35 of the cylinders 30, 32 and 33. The supply channels are in fluid communication with the interior of the drill string 18 (see Figure 1) through which drilling fluid is supplied. Please note that in Figure 2 the other two supply channels have not been shown. Furthermore the device 20 comprises a free hanging control plate 55 positioned near the first surface 23 of the block 21, which control plate 55 has a first surface 56 facing the first surface 23 of the block 21 and a second surface 57. In order to supply the drilling fluid that drives the peripheral hydraulic motors 10 and 11 (see Figure 1), the second port 39 of the peripheral cylinder 32 is in fluid communication with the inlet of the peripheral hydraulic motor 10 and the second port 39 of the peripheral cylinder 33 with the inlet of the peripheral hydraulic motor 11. The amount of driving fluid delivered to a peripheral hydraulic motor 10 or 11 (see Figure 1) is controlled by the position of the piston 45 in the peripheral cylinder 32 or 33. This position is controlled by pressure drop over the peripheral hydraulic motor 10 or 11. Thereto, the discharge (not shown) of the peripheral hydraulic motor 10 is in fluid communication with the first port 38 of the peripheral cylinder 32 via a control conduit 58a, and the discharge (not shown) of the peripheral hydraulic motor 11 is in fluid communication with the first port 38 of the peripheral cylinder 33 via a control conduit 58b. For reasons that will become clear when the operation of the device 20 is described, the first port 44 of the central cylinder 30 is in fluid communication with the inlet of the central hydraulic motor 12 (see Figure 1) through the central delivery channel 60. The discharge (not shown) of the central hydraulic motor 12 (see Figure 1) is in fluid communication with the second port 42 of the central cylinder 30. The control plate 55 controls the movement of the two pistons 45 and of the piston 47. To this end, the piston rods 46 pertaining to the pistons 45 in the peripheral cylinders 32 and 33 are in contact with the first surface 56 of the control plate 55 and the piston rod 48 pertaining to the piston 47 in the central cylinder 30 is in contact with the second surface 57 of the control plate 55. The contacting means are the ball 64 and the two balls 65, respectively. During normal operation, drilling mud is supplied from the drill string 18 to the three hydraulic motors 10, 11 and 12 via the device 20 of the present invention. The device 20 is so designed that when the three hydraulic motors are loaded normally the two pistons 45 and the piston 47 are in their initial position, as shown in Figure 2. If one of the three drill bits 15 encounters a greater resistance, the pressure drop over the hydraulic motor driving that drill bit will increase. To be able to continue drilling, more drilling fluid has to be supplied to that hydraulic motor at the expense of the other two hydraulic motors. The device 20 of the present invention is capable of doing this. Assume that the central hydraulic motor 12 has to deliver more torque to the drill bit, and that consequently the pressure drop over the central hydraulic motor 12 increases. As a result the pressure at the discharge of the central hydraulic motor 12 decreases and thus the pressure under the piston 47 in the central cylinder 30. As the pressure above the piston 47 remains the same, there is an additional force on the piston 47 that forces the piston 47 downwards. By means of the ball 64 a downward force is exerted on the second surface 57 of the control plate 55. This force pushes the pistons 45 in the peripheral cylinders 32 and 33 downwards and they reduce the area of flow out of the inlet openings 35. Thus less driving fluid is supplied to the peripheral hydraulic motors 10 and 11, whereas more driving fluid is supplied to the central hydraulic motor 12 . If the peripheral hydraulic motor 10 has to deliver more torque to the drill bit, consequently the pressure drop over the peripheral hydraulic motor 10 increases. As a result the pressure at the discharge of the peripheral hydraulic motor 10 decreases and thus the pressure above the piston 45 in the peripheral cylinder 32. As the pressure under the piston 45 remains the same, there is an additional force on the piston 45 that forces the piston 45 upwards. By means of the ball 65 an upward force is exerted on the first surface 56 of the control plate 55. This force pushes the piston 45 in the peripheral cylinder 33 downwards and it reduces the area of flow out of the inlet opening 35. Thus less driving fluid is supplied to the peripheral hydraulic motor 11, whereas more driving fluid is supplied to the peripheral hydraulic motor 10. For the sake of simplicity it is assumed that the operating conditions of the central hydraulic motor 12 do not change. In this way the device according to the present invention provides a simple means for controlling the flow of driving fluid to a system of two peripheral hydraulic motors and one central hydraulic motor. In case there are three peripheral hydraulic motors, the centres of the three peripheral motors are located in the vertices of an equilateral triangle, of which the centre is formed by the centre of the central hydraulic motor. In that case the device has three peripheral cylinders of which the centres are located in the vertices of an equilateral triangle, of which the centre is formed by the centre of the central cylinder. The operation of the device for controlling the flow of driving fluid to a system of three peripheral hydraulic motors and one central hydraulic motor with three peripheral cylinders and one central cylinder is based on the same concepts as discussed with reference to Figures 1 and 2.

Claims: C L A I M S 1. A device for controlling hydraulic power supplied to a system of a plurality of hydraulic motors of a drill string for drilling a borehole into an earth formation, each hydraulic motor being arranged to drive a respective drill bit of the drill string, the device comprising: means for dividing a stream of hydraulic fluid flowing through the drill string into a plurality of sub- streams, each sub-stream driving a respective one of said motors; and - control means for increasing the hydraulic power of a selected sub-stream and for decreasing the hydraulic power of at least one of the other sub-streams in response to an increase of fluid pressure drop across the motor driven by the selected sub-stream. 2. The device of claim 1, wherein the control means is arranged to increase, for each motor, the hydraulic power of the sub-stream driving the motor and to decrease the hydraulic power of at least one of the sub-streams driving the other motors in response to an increase of fluid pressure drop across the motor. 3. The device of claim 1 or 2, wherein each motor has a hydraulic fluid inlet provided with an inlet valve, and wherein the control means is arranged to control the inlet valve of the motor and at least one of the inlet valves of the other motors. 4. The device of claim 3, wherein the control system includes a mechanical system arranged to further close said at least one of the inlet valves in response to further opening of the inlet valve. 5. The device of any one of claims 1-4, wherein the device comprises a block having a first surface and a second surface, which block is provided with a central cylinder and at least two peripheral cylinders arranged uniformly around the central cylinder, each cylinder having an inlet opening, a first port near the first surface and a second port near the second surface and being provided with a piston having a piston rod extending out of the first surface of the block, wherein the block is further provided with one supply channel for each cylinder that is connected to the inlet opening of the cylinder, and which device further comprises a free hanging control plate positioned near the first surface of the block, which control plate has a first surface facing the first surface of the block and a second surface, wherein the second port of each peripheral cylinder is in fluid communication with the inlet of a corresponding peripheral hydraulic motor, wherein the first port of the central cylinder is in fluid communication with the inlet of the central hydraulic motor, wherein the first port of each peripheral cylinder is in fluid communication with the discharge of a corresponding peripheral hydraulic motor, wherein the second port of the central cylinder is in fluid communication with the discharge of the central hydraulic motor, and wherein the piston rods pertaining to the pistons in the peripheral cylinders are in contact with the first surface of the control plate and the piston rod pertaining to the piston in the central cylinder is in contact with the second surface of the control plate. 6. Device according to claim 5 for controlling the flow of driving fluid to a system of two peripheral hydraulic motors and one central hydraulic motor, which device comprises two peripheral cylinders. 7. Device according to claim 5 for controlling the flow of driving fluid to a system of three peripheral hydraulic motors and one central hydraulic motor, which device comprises three peripheral cylinders. 8. The device substantially as described hereinbefore with reference to the drawings.

Inventors: BRAITHWAITE STEPHEN RICHARD, HEIJNEN WILHELMUS HUBERTUS PAU, WORRALL ROBERT NICHOLAS, BRAITHWAITE, STEPHEN, RICHARD, HEIJNEN, WILHELMUS, HUBERTUS, PAULUS, MARIA, WORRALL, ROBERT, NICHOLAS
Applicants: BRAITHWAITE STEPHEN RICHARD, HEIJNEN WILHELMUS HUBERTUS PAU, SHELL INT RESEARCH, WORRALL ROBERT NICHOLAS, BRAITHWAITE, STEPHEN, RICHARD, HEIJNEN, WILHELMUS, HUBERTUS, PAULUS, MARIA, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V., WORRALL, ROBERT, NICHOLAS
Application year: 2000
Main classification: E21B21
IPC codes: E21B4, E21B21, E21B44