WO1986005543A1

WO1986005543A1 - Hydraulic power supply for use during well drilling operations

Info

Publication number: WO1986005543A1
Application number: PCT/GB1986/000150
Authority: WO
Inventors: James Harrison
Original assignee: Gearhart Geodata Services Ltd.
Priority date: 1985-03-15
Filing date: 1986-03-14
Publication date: 1986-09-25
Also published as: GB8506833D0; EP0215880A1

Abstract

A power supply arrangement (Fig. 2) for a well drilling installation enables hydraulic energy to be made available downhole for use in, for example, generating MWD pulses or operating hydraulic devices or for conversion to electrical energy. A piston (31) is exposed to the pressure difference between the interior of the drill string (1) and the return annulus (13) and compresses a resilient member (35) when circulation of drilling fluid commences. A one-way valve (33) locks the piston (31) against return movement when circulation of drilling fluid ceases. The lock can be released by a solenoid valve (37), allowing the stored energy of the resilient member (35) to be released as hydraulic energy in the circuit in which the valves (33, 35) are connected: the stored energy can be released at any time during the period of non-circulation of drilling fluid. An alternative power supply arrangement enables hydraulic energy to be made available while drilling fluid is being circulated and yet another arrangement enables hydraulic energy to be made available at the surface rather than downwhole.

Description

Hydraulic power supply for use during well drilling operations

The present invention relates to the provision of a supply of hydraulic power during a well drilling operation. The invention is concerned, in particular, with providing a downhole supply of power and also with providing a means of communicating with down- hole equipment during a well drilling operation.

The invention is applicable, more especially, to those drilling operations^' that employ so-called "measurement-while-drilling" (M D) techniques. M D techniques enable sub-surface measurements to be made while drilling is in progress and transmitted to the surface. An MWD system involves the use of one or more measuring instrument in the drill string in the vicinity of the drill bit for measuring physical para- meters of interest while drilling is in progress, together with appropriate telemetry for conveying the information obtained to the surface.

A known form of MWD system employs mud pulse telemetry, that is, mud pulses generated downhole are used to convey information to the surface.' The mud pulse generator is located in the drill string in the vicinity of the drill bit and the pulses are transmitted to the surface via the column of drilling fluid ("mud") in the borehole. Some form of downhole power in a borehole during a drilling operation is required to operate downhole devices, for example measuring and recording instru¬ ments and, in the case of an MWD system employing mud pulse telemetry, the mud pulse generator. Down- hole power is conventionally provided by batteries although turbines driven by the circulating mud have also been used as power sources. Compared with a turbine, a battery system is simpler and less costly , and offers greater flexibility but has the dis¬ advantage of a limited capacity. Operation of a battery system can also be affected by the elevated temperatures existing at the bottom of a borehole.

It is also desirable, during a drilling operation, to be able to transmit signals from the surface to downhole equipment. This can be done through the use of an electrical conductor within the drill string for carrying electrical signals from the sur¬ face to downhole equipment but the presence of such a conductor can give rise to problems. It is also common in the case of an MWD system using mud pulse telemetry, to control the transmission speed of the system by altering the rate of circulation- of drilling fluid at regular intervals: however, the use of such techniques is limited and causes inconvenience during drilling operations. It is an object of the invention to enable a downhole supply of power to be provided, which can be of comparatively-simple form but which can be re¬ newed. It is a further object of the invention to enable a communication system to be provided by which information can be passed from the surface to downhole equipment in a comparatively simple manner. The invention enables these objects to be achieved by providing a source of hydraulic power which, in one case, can provide a renewable source of. downhole power and, in another, can provide a means of communication between the surface and downhole equip¬ ment. •

According to the present invention there is pro- vided, for a well drilling installation, a power supply arrangement including means responsive to the pressure difference between the interior and the exterior of the drill string brought about by the circulation of drilling fluid to provide a source of hydraulic power for an auxiliary operation.

According to another aspect of the present in¬ vention there is provided, for a well drilling installation a power supply arrangement including means responsive to the circulation of drilling fluid in the installation to provide a source of hydraulic power and means operable to release said hydraulic power selectively for an auxiliary operation. The means responsive to the circulation of drilling fluid may be responsive to the pressure difference between the interior and the exterior of the drill string.

The power supply arrangement may be located down- hole, in which case the auxiliary drilling operation may be the control of hydraulic devices downhole; the control of electrically-operated equipment downhole; the generation of mud pulses to provide a means of communication when there is no circulation of drill- ^• ing fluid, or actuation of a conventional mud pulse generator. Alternatively, the auxiliary drilling operation may be the transmission of information from the surface to downhole equipment, in which case the hydraulic arrangement is located at the surface. A power supply arrangement in accordance with either aspect of the invention may be incorporated as a unit in a drill collar for installation in a drill string or may form part of an MWD tool -for installation in a drill string. //The pressure differential responsive means may, for example be exposed to the pressure difference between the interior of the drill string and the return annulus. Alter¬ natively, the pressure differential responsive means may comprise a pair of pressure responsive members one of which members is exposed to the pressure in the drill string and the other of which is exposed to the pressure in the annulus. When the power supply arrangement is located at the surface, the pressure differential responsive means may be exposed to the pressure difference between the interior of the stand pipe of the rig and atmosphere.

In some embodiments of the invention, the hydraulic power is available while drilling fluid is being circulated (ie, when pressure in the stand- pipe and drill string is increased) . In other embodiments, the pressure responsive means is arranged to produce stored energy while drilling fluid is being circulated, the stored energy being available as hydraulic power when the circulation of drilling fluid ceases. The energy may ^'be stored in a resilient member.

For example, the pressure responsive means may be arranged to actuate a piston-and-cylinder assembly against a resilient bias while fluid is being circulated, to produce available hydraulic power in a hydraulic circuit to which the piston-and-cylinder assembly is connected. In one embodiment, the piston-and- cylinder assembly is locked against actuation and is selectively releasable to provide hydraulic power in the said hydraulic circuit while drilling fluid is being circulated. A one-way valve prmits the free return of the assembly under the resilient bias to its unactuated condition when the circulation of drilling fluid ceases. Alternatively, the piston-and- cylinder assembly is locked against return to its unactuated position and is selectively releasable to provide hydraulic power in the said hydraulic circuit when the circulation of drilling fluid ceases. The present invention further provides a method of generating hydraulic power in a well drilling installation, including the steps of senέing the difference in pressure between the interior and exterior ^• of the drill string; compressing a resilient device in dependence on that pressure difference and using the resilient device to generate power in a hydraulic circuit. According to another aspect of the invention, there is provided a method of generating hydraulic power in a well drilling installation including the steps of sensing the circulation of drilling fluid in the drill string; compressing/expanding a resilient device selectively in^' dependence on that circulation and using the resilient device to generate power in a hydraulic circuit.

By way of example, embodiments of the invention will now be described with reference to the accompany¬ ing drawings, in which:

Fig. 1 illustrates, schematically, a well drilling instalation;

Fig. 2 is a diagram of a downhole power supply arrangement for a well drilling in¬ stallation and illustrates one use of that arrangement; Fig. 3 is a diagram of an alternative form of downhole power supply arrangement;

Fig. 4 is a diagram of a signal generating arrangement for a well drilling in¬ stallation; Fig. 5 illustrates an MWD tool installed in a well drilling installation;

Figs. 6 and 7 illustrate alternative uses for the power supply arrangement of Fig. 2; Fig. 8 illustrates the installation of a power supply arrangement in a drill collar;

Fig. 9 illustrates an alternative power supply arrangement and its use, and Fig. 10 illustrates a power supply arrangement incorporating an alternative form of pressure-responsive device.

Fig. 1 illustrates a typical drilling installation in this case one employing an MWD system with mud • pulse telemetry, and shows the drill string 1 extending through the bore hole 3 from the rig 5 to the drill bit 7. When drilling is in operation, drilling fluid ( "mud" ) is circulated by the mud pump 9 through the stand pipe 11 to the interior of the drill string 1 and returns to the surface via the annulus 13 between the drill string and the wall of the bore hole 3. . The measuring instruments that form part of the MWD system are not shown in Fig. 1 but are located within the drill string 1 in the vicinity of the drill bit 7. These instruments may include, for example, a directional sensor (comprising a combined magnetometer and inclinometer) and gamma ray, resistivity and temperature sensors, details of which need not be described here. A mud pulse generator 15 is located within the drill string adjacent to the sensors and ^• is used to produce pulses in the fluid column within the bore hole 3: these pulses can be detected at the surface and are used to convey information from the downhole sensors.

Various forms of mud pulse generator- are known and need not be described in detail here. The generator 15 shown in Fig. 1 is a negative-pulse generator and includes a solenoid-actuated valve which, when operated, allows a short burst of high- pressure drilling fluid to

be by-passed directly, at 17, _frorn within the drill string to the return annulus. The effect of this is to produce, within the drilling fluid in the drill string, a negative pulse which is detected by an appropriate pressure transducer 19 connected to the stand pipe 11. The detected pulses are de-coded by an on-site computer 21 and information is recorded (23) and also presented on a visual display (25) for immediate evaluation. Pulses can be transmitted only when there is a sufficient pressure differential ^'between the drill string and the annulus, that is, when the mud pump_.9 is in operation. Data for transmission is recorded when the pump is shut^' down for a connection to be made to the drill string, and is then transmitted when the pump is re-started.

A downhole energy storage arrangement, suitable for use in the drilling installation of Fig. 1, is shown in Fig. 2. This arrangement can be used to .pro¬ vide power for the HvTD sensors and/or the mud pulse cenerator 15 of Fig. 1, or for any other appropriate urpose, as will be described later.

The energy storage arrangement shown in Fig. 2 is located within the drill string 1 adjacent the MvTD sensors and mud pulse generator 15 and comprises a double niston and cylinder assembly 27, the pistons 29, 31 of which are interconnected. The cylinder within which Di≤ton 31 is located contains drillinσ -fluid and is ooen, at A (on one side of the piston), to the interior of the drill collar 1 and, at B (on the other side of the piston) , to the return annulus 13. The second cylinder, within which piston 29 is located,^' forms part of a hydraulic circuit including a non-return valve 33 which interconnects cylinder ports C, D on opposite sides of the piston.

Tne piston 31 is loaded by a spring 35 into the position shown in Fig. 2 and occupies this position when the mud pump 9 (Fig. 1) is not operating or fluid pressure within the drill -collar 1 is not substantially higher than the pressure within the annulus 13.-

When the mud pump 9 s in operation, the pressure within the drill string increases and moves the piston 31 upwards (as seen in the drawing) against the spring 35 which is compressed. During this movement, drilling fluid enters the cylinder through port A and leaves, tlαrough the port B. The hydraulic piston 29 moves with the drilling fluid piston 31 and, in doing so, displaces hydraulic fluid through the non-return valve 33 from one side of the piston 29 to the other.

When the circulation of drilling fluid is stoooed, for example during the connection of a further length cf pipe to the drill string, the pressure differential across the piston 31 falls and, in the absence of the connection to piston 29, the spring 35 would return the piston 31 to its original position. However, valve 33 prevents the return flow of fluid in the hydraulic circuit so that the piston 29 is locked against downward movement (as seen in the drawing) and, with it, the piston 31. As a result, mechanical energy is stored in the spring 35 while the mud pump 9 is not running and this can be re¬ leased and utilized. Fig. 2 illustrates one way in which mechanical energy stored in the spring 35 can be released to provide mud pulse communication with the surface when there is no circulation of drilling fluid.. As described above, a conventional mud pulse generator operates only when drilling fluid is being circulated, and there is no way of providing con¬ ventional mud pulse communication with the surface when the circulation is stopped. If, however, a solenoid-operated valve 37 is connected across the piston 29 in parallel with the non-return valve

33 (as shown in Fig. 2), then, by pulsed operation of the valve 37, the hydraulic lock on the piston 31 can be released in steps. The resulting stepped movement of the piston 31 will generate pulses in the annulus 13, via the port B, or in the drill string via port A and these pulses can be detected and used in the usual way to transmit information when drilling is not in progress which, in certain circum¬ stances, may be preferable to the conventional method of transmitting information only during drilling. As the energy is released, the pistons 29, 31 will move downwards under the action of the spring 35. When the circulation of drilling fluid recommences, the stored energy is replaced as described above.

The power required to operate the valve 37 in this arrangement can be provided by a downhole battery of any known type capable of meeting the re¬ quirements of a lower power solenoid actuator at the elevated temperatures that exist downhole. Such batteries are generally more able to withstand the elevated temperatures that exist downhole than batteries which have a higher power output.

Figs. 6 and 7 illustrate alternative ways of utilizing the energy that can be stored in the spring 35 of Fig. 2. In Fig. 6 a fluid-operated motor M and solenoid- operated valve 71 are shown connected across the piston 29 instead of the valve 37, with the output of the motor M being used to drive an alternator 72. By operating the valve 71 from, for example, a suitable downhole battery, the hydraulic lock on the piston 31 is released and fluid circulates through the motor M. The electrical power generated by the alternator 72 ^• can then be used for downhole electronic equipment, for example transducers or a recording device which is used to measure and record a downhole parameter whenever a connection is made in the drill string. Alternatively, or in addition, the power can be used for recharging or for augmenting that of the downhole battery.

In Fig. 7 a solenoid-operated valve 73 is shown connected across the piston 29 to control operation -of an hydraulic actuator 75. By operating the valve 73 from for example, a suitable downhole battery, the hydraulic lock on the piston 31 is released and fluid is applied via the valve 73 either to extend or to retract the actuator 75. Using such an arrangement, various downhole hydraulic devices can be operated either on command from the surface or automatically when the circulation of drilling fluid ceases. Devices that could be operated in this manner are calipers, seals, packers and extending probes. The power supply arrangement 27 to 33 of Fig.

2 is, advantageously, formed as a unit 75 as indicated in Fig. 8, which can be incorporated in a drill collar 77 of any appropriate size. During drilling, mud flows through the collar around the unit as indicated by the arrow 78 and hydraulic lines 79 enable connections to be made from the unit to, for example, a fluid motor unit and/or a hydraulic actuator unit contained within a drill collar 81 suspended below the power supply unit.

Alternatively, the pulse generating arrangement of Fig. 2 (ie including the solenoid valve 37) can be formed as a sub-unit 68 for use together with a conventional mud pulse generator sub-unit 64 in an MWD tool as illustrated in Fig. 5. These sub-units are joined to each other and to conventional electronics, sensor and battery units 66 by junction units 70 which carry the electrical connections ' through the assembly and also have radial fins which centralize the assembly within the drill collars 83. In such a tool, the sub-unit 64 operates in the con¬ ventional manner to generate "mud pulses" while drilling fluid is being circulated, and the unit 68 is used to generate pulses when drilling fluid is not being circulated.

If it is required that hydraulic energy should be delivered by the pis-ton 29 (Fig. 2 ) when the mud pump 9 is running rather than when the circulation of drilling fluid is stopped, then this can be achieved by reversing the non-return valve 33. In this case, the piston 31 will by hydraulically- locked against movement in an upwards direction (as seen in Fig. 2) when the mud pump is running: how- ever if the lock is released, for example by actuating a solenoid valve in the hydraulic circuit, then hydraulic energy becomes available and may be used as required, for example in any of the ways already described with reference to Figs. 2, 6 and 7.

Alternatively, with the valve 33 in the reversed position, the hydraulic energy delivered by piston 29 can be used to provide the actuating power for the conventional mud pulse generator 15 of Fig. 1. The solenoid-actuated mud valve of the generator is conventionally powered by a battery and, because a high-power solenoid is required, such an arrangement suffers from the disadvantage that it consumes a sub¬ stantial amount of the available downhole battery energy. Fig. 9 illustrates an arrangement which en¬ ables the life of the downhole battery to be -in¬ creased substantially. The arrangement is- generally similar to that shown in Fig. 7 but, as already in¬ dicated, the non-return valve 33 is reversed and, in addition, the actuator 75 is shown coupled to the conventional mud valve 85 which, when operated, allows a small volume of mud to pass from the interior of the drill string to the return annulus thereby generating a negative pulse in the drill string which can be detected at the surface. When the mud pump 9 (Fig. 1) is running, the piston 31 is hydraulically-locked against movement in an up- wards .direction (as seen in Fig. 9) but, by actuating the valve 73, the lock is released and the piston 29 moves upwards to deliver fluid to the actuator 75 and thereby operate the mud valve 85. The valve 73 is actuated by power supplied from a downhole battery but requires substantially less power than the high-power solenoid conventionally employed to operate the mud valve directly. The valve 73 can be actuated on command from the surface when drilling is in progress or can be actuated automatically when drilling commences. When drill¬ ing is halted and the circulation of mud ceases, ^• the piston 31 returns to its original position under the action of spring 35, accompanied by return movement of the piston_.29 and flow of fluid through the rion-return valve 33.

An alternative arrangement for providing hydraulic energy downhole is illustrated in Fig.

3. In this arrangement, two fluid-filled pressure bags 40, 42 are used to sense the pipe/annulus pressure differential instead of the drilling fluid piston 31 of Fig. 2. The bags are both connected in the hydraulic circuit, with one bag 40 being located within the drill string and the other 42 within the annulus. The bag 40 is connected, via a non-return valve 46 to a hydraulic cylinder 44 on one side of the piston 48 and the bag 42 is connected directly to the cylinder on the other side of the piston. As shown in the drawing, the piston 48 is resiliently-biased by a spring 50. When drilling fluid is being circulated and the pressure in the drill string is substantially higher than the pressure in the annulus, hydraulic fluid is displaced from the bag 40 via the non-return valve 46 to the cylinder 44 and moves the piston 48 against the bias of the spring 50: this, in turn, displaces hydraulic fluid from the cylinder to the bag 42 which expands as the bag 40 collapses.

When the circulation of drilling fluid is stopped, the pressure in the drill string falls but the spring 50 is unable to return the cylinder 44 and re-expand the bag 40 due to the presence of the non-return valve 46. Energy is, accordingly, now stored in the system and can be utilized as described. above with reference to Figs. 2, 6 and 7 by connecting approp¬ riate hydraulic circuitry across the valve 46 at T, P.

A further arrangement which utilizes hydraulic power- generated from the pressure differential between the interior and the exterior of the drill string is shown in Fig. 4. In this case, the arrangement is not located downhole but at the surface and is used

to provide a means of communicating, from the surface, with downhole equipment during a drilling operation.

As in Fig. 2 , the arrangement comprises a double piston-and-cylinder assembly 52, the pistons 54, 56 of which are interconnected. The cylinder 54A, within which piston 54 is located, is connected at one_. end (at A') to the stand pipe ^'11 (Fig. 1)^" and is^" vented at the other end (at B^•) . The second cylinder 56A, within which piston 56 is located, forms part of a hydraulic ^"circuit-including a non-return valve 58 and a solenoid valve 60 connected in parallel with each other between the cylinder ports C, D¹ on opposite sides of the piston 56.

The piston 54 is loaded by a spring 62 into the position shov in Fig. 4 and occupies this position when the mud pump 9 (Fig. 1) is not operating. When the pump is running, the pressure of the drilling fluid in the stand pipe 11 acts on the piston 54 via the port A- against the action of the spring 62 but the piston is unable to move because it is hydraulically locked by the non-return valve 58 which prevents the flow of fluid from Dort C' to port D- and hence prevents movement of the hydraulic piston 56. However, if the solenoid valve 60 is operated under these circumstances, the hydraulic lock on the piston 54 will be released and the piston will move against the spring 62: this results in a ^• - IS - negative pressure pulse in the stand pipe which will be transmitted downhole via the drilling fluid. If the solenoid valve 60 is operated in a series of steps, then a series of negative pulses will be transmitted downhole: These pulses can be measured at or near the drill bit and can be used as a means of communication, for example to carry commands to downhole equipment or to select data stored downhole.

When the mud pump 9 is stopped, the piston 54 returns to its original position (shown in Fig. 4) under the action of the spring 62 and, with it, the piston 56. During this movement, the non-return valve 58 allows the free passage of fluid from port D¹ to port C of the hydraulic-cylinder 56A. Instead of using a single drilling fluid cylinder 54A in which the piston 54 is moved in steps to produce a series of pulses, a plurality of such cylinders could be used each of which produces a res- oective one of the series of pulses by movement of its piston through a complete stroke: this enables more accurate and repeatable pulses to be produced. Alter¬ natively, by using two sets of cylinders, one larger than the other, series of larger and smaller pulses can be generated. If it is required that the mud pulses should be transmitted downhole when the mud pump 9 is not running, then this can be achieved by reversing the non-return valve 58. In this case, the piston 54 will be able to move under the pressure of the drilling fluid and against the action of the spring 62 when the pump is running but will be hydraulically locked against return movement when pumping ceases. If the solenoid valve 60 is operated under these circumstances the piston 54 will move in the return direction under the action of the spring 62 and produce a positive pulse in the stand pipe which will be transmitted downhole.

It will be appreciated that the spring-biased drilling fluid piston 54 of Fig. 4 could be replaced by a pressure-bag arrangement as used in Fig. 3. In each of the arrangements described above, a spring (35, 50, 62) is employed to resiliently- bias the piston that responds to the pressure difference between the interior and the exterior of the drill string. This is advantageous in that the piston then has a comparatively long stroke which facilitates the controlled use of the hydraulic power c=.nerated by movement of the piston: however, alternative resilient devices could be used.

In each of the arrangements described above, changes in pressure caused by starting or stopping the mud pump 9 are utilized. However, changes in pressure brought about in other ways could be utilized when appropriate. For example, changes in the rate of flow of drilling fluid will cause changes in the pressure difference between the interior and the exterior of the drill string and so also will the natural pulsations in the drilling fluid caused by the strokes of the mud pump 9 (Fig. 1).

It is also possible, in each of the arrangements illustrated in Figs. 2 to 4, 6, 7 and 9, to replace the piston 31, 54 or the fluid-filled bags 40, 42 (both of which arrangements respond to the pressure difference between the inta±or and the exteLor of the drill string) by a device which functions by forming a restriction within the drill string and responds, to the pressure differential generated across that restriction by the flow of drilling fluid. Such a device is shown in Fig. 10 and comprises a tubular body formed in two parts 90, 91 one of which (90) telescopes over the other (91) which is fixed in the drill collar 93. A shoulder 92 on the tubular body forms a restriction in the flow path within the drill collar 93 so that, when drilling fluid is circulating, a pressure differential is created which forces the upper part 90 of the tubular down over the lower part 91. The part 90 is coupled to a piston 94 (corres- ponding to the piston 29 of Fig. 2) which also move downwards forcing fluid through the non-return valve 95. When the circu¬ lation of drilling fluid ceases, the non-return valve 95 prevents return movement of the piston 94 under the action of spring 96 but the hydraulic lock can be released and the hydraulic power which is thereby made available can be utilized in any of the ways described above with reference to Figs. 2, 6 and 7.

Claims

1. A well drilling installation in which there is provided a power supply arrangement including means responsive to the pressure difference between the interior and the exterior of the drill string brought about by the circulation of drilling fluid to provide a source of hydraulic power for an auxiliary operation.

2. A well drilling installation in which there is provided a power supply arrangement including means responsive to the circulation of drilling fluid in the installation to provide a source of hydraulic power and means operable to release said hydraulic power selectively for an auxiliary operation.

3. An installation as claimed in claim 2, in which the means responsive to the circulation of drilling fluid is responsive to the pressure difference between the interior and the exterior of the drill string.

4. An installation as claimed in any one of claims 1 to 3, in which the power supply arrangement is located downhole.

5. An installation as claimed in claim 1 or claim 3, in which the pressure differential responsive means is exposed to the pressure difference between the interior of the drill string and. the return annulus. 6. An installation as claimed in claim 1 or claim 3, in which the pressure differential responsive means comprises a pair of pressure responsive members one of which members is exposed to the pressure in the drill string and the other of which is exposed to the pressure in the return annulus.

7. An installation as claimed in any one of claims 1 to 3, in which the power supply arrangement is located at the surface. " 8- An installation as claimed in claim 1 or claim 3, in which the pressure-differential responsive means is exposed to the pressure difference between the interior of the stand pipe of the rig ' and atmosphere. 9. An installation as claimed in any one of the preceding claims, in which the hydraulic power is available while drilling fluid is being circulated.

10. An installation as claimed in any one of. claims 1 to 8, in which the power supply arrangement is arranged to produce stored energy while drilling fluid is being circulated, the stored energy providing a source of hydraulic power which is available when the circulation of drilling fluid changes.

11. An installation as claimed in claim 10, in which the power supply arrangement includes a resilient member in which energy is stored.

12. An installation as claimed in claim 10 or claim 11, in which the hydraulic power is available when the circulation of drilling fluid has ceased. 13. An installation as claimed in claim 1 or claim 3, in which the pressure-differential responsive means is arranged to actuate a piston-and-cylinder assembly against a resilient bias while drilling fluid is being circulated, to provide a source of hydraulic power which is available, while drilling fluid is being circulated, in a hydraulic circuit in which the piston-and-cylinder assembly is connected. 14. An installation as claimed in claim 13 when appendant to claim 3, in which the piston-and-cylinder assembly is locked against actuation and is selectively releasable to provide hydraulic power in the said hydraulic circuit. 15. An installation as claimed in claim 14, including valve means connected to permit the free return of the assembly under the resilient bias to its unactuated condition when the circulation of drilling fluid ceases. 16. An installation as claimed in claim 1 or claim 3, in which the pressure-differential responsive means is arranged to actuate a piston-and-cylinder assembly against a resilient bias while drilling fluid is being circulated and in which the return of the assembly under the resilient bias to its unactuated position when the circulation of drilling fluid ceases produces available hydraulic energy in a hydraulic circuit in which the assembly is connected. 17. An installation as claimed in claim 16 when appendant to claim 3, in which the piston-and-cylinder assembly is locked against return to its unactuated position and is selectively releasable to provide hydraulic power in the said hydraulic circuit.

18. An installation as claimed in claim 4, in which the auxiliary operation is the control and/or operation of downhole equipment.

19. An installation as claimed in claim 4, in •"which the auxiliary operation is the generation of pressure pulses in the drilling fluid for trans¬ mission to the surface.

20. An installation ^'as claimed in claim 7, in which the auxiliary operation is the generation of pressure pulses in the drilling fluid for transmission downhole.

21. An installation as claimed in claim 1 or claim 3, in which the pressure-differential responsive means is responsive- to pressure changes caused by starting and/or stopping the circulation of drilling fluid.

22. An installation as claimed in claim 1 or claim 3, in which the pressure-differential responsive means is responsive to pressure changes caused by varying the rate of flow of drilling fluid.

23. An installation as claimed in claim 1 or claim 3, in which the pressure-differential responsive means is responsive to pressure pulses caused by operation of a pump that circulates the drilling fluid.

24 . A well drilling installation comprising a drill string, a drill at the end of the drill string and a pump for circulating drilling fluid under pressure through the drill string to the drill, the drilling fluid returning around the outside of the drill string to remove debris from the drilling site; a device exposed to the pressure inside the drill string and the pressure outside the drill string to respond to the difference between those pressures due to the circulation of drilling fluid, and a hydraulic circuit to which the device is connected to provide power in said circuit for an auxiliary operation..

25. A well drill installation comprising a drill string, a drill at the end of the drill string and a pump for circulating drilling fluid under pressure through the drill string to the drill, the drilling fluid returning around the outside of the drill string to remove debris from the drilling site; a device responsive to the circulation of drilling fluid; a hydraulic circuit to which.the device is connected to provide power in said circuit for an auxiliary operation, and means operable to release said power selectively.

26 . An installation as claimed in claim 24 or claim 25, in which che said device is a resilient device arranged to be compressed in response to a change in the circulation of drilling fluid, and means for allowing a regulated expansion/contraction of the device to release power selectively in the hydraulic circuit.

27. A method of generating hydraulic power in a well drilling installation, including the steps of sensing the difference in pressure between the interior and exterior of the drill string; com- •;pressing a resilient device in dependance on that pressure difference and using the resilient device to generate power in a hydraulic circuit.

28^'. A method of generating hydraulic power in a well drilling installation including the steps of sensing the circulation of drilling fluid in the drill string; compressing/expanding a resilient device selectively in dependance on that circulation and using the resilient device to generate power in a hydraulic circuit. 29. A drill collar for use in the drill string of a well drilling installation, the drill collar incorporating a power supply arrangement including means responsive to the pressure difference between the interior and the exterior of the drill string brought about by the circulation of drilling fluid to provide a source of hydraulic power for an auxiliary operation.

30. A drill collar for use in the drill string of a well drilling installation, the drill collar incorporating a power supply arrangement including means responsive to the circulation.- of drilling fluid^' in the installation to provide a source of hydraulic power and means operable to release said hydraulic power selectively for an auxiliary operation.

31. An MWD tool for use in a well drilling installation, the tool incorporating a. power supply arrangement including means responsive to the pressure difference between the interior and the exterior of the drill string brought about by the circulation of drilling fluid to provide a source of hydraulic power for an auxiliary operation.

32. An MWD tool for use in a well drilling installation, the tool incorporating a power supply arrangement including means responsive to the circulation of drilling fluid in the installation to provide a source of hydraulic power and means operable to release said hydraulic power selectively for an auxiliary operation.

33. A tool as claimed in claim 32/ in which the auxiliary operation is^" the generation of pressure pulses in the drilling fluid for transmission to the surface when drilling fluid is not being circulated.

34. A tool as claimed in claim 33, which also includes a pulse generating unit operable to generate pressure pulses for transmission to the surface when drilling fluid is being circulated.

35. A well drilling installation including a power supply arrangement substantially as des- cribed herein with reference to, and as shown in, any one of Figs. 2 to 4, 6, 7, 9 and 10 of the accompanying drawings.

36. A^'method as claimed in^' claim 27 or claim ^■ 28, 'substantially as described herein with reference to the accompanying drawings.

37. A drill collar incorporating a power supply arrangement, substantially as described herein with reference to Fig. 8 and any one of Figs. 2, 3, 6,

7, 9 and 10 of the accompanying drawings. 38. An MWD tool incorporating a power supply arrangement, substantially as described herein with reference to Figs. 2 and 5 of the accompanying drawings.