Turbine for driving a generator in a drill string

The invention is directed to a turbine with a housing mountable in a drill string and having an inlet opening and an outlet opening for a fluid conveyed through the drill string, and with an impeller arranged in the housing and adapted to be impinged upon by the fluid flowing into the housing, wherein the impeller is axially movably mounted and yieldably supported against the afflux direction in such a way that the supporting force of the yielding support increases as the axial movement of the impeller increases. To reduce the dependence of the turbine rotational frequency and turbine performance on the feed rate and to avoid overloading of the generator, the housing includes in the range of movement of the impeller a frontal first housing section of smaller inner diameter as seen in the afflux direction and a rear second housing section of larger inner diameter as seen in the afflux direction.

CROSS REFERENCE TO RELATED APPLICATIONS

FIELD OF THE INVENTION

This invention relates to a turbine for driving a generator, with a housing mountable in a drill string and having an inlet opening and an outlet opening for a fluid conveyed through the drill string, with at least one impeller arranged in the housing, which impeller is adapted to be impinged upon by the fluid flowing into the housing, is axially movably mounted and yieldably supported against the afflux direction in such a way that the supporting force of the yielding support increases as the axial movement of the impeller increases.

DESCRIPTION OF RELATED ART

In deep well drilling it is common practice to take measurements continuously while drilling by means of measurement systems installed in the drill string and to transmit the measurement results to the surface of the earth by means of telemetry devices. To generate the electric power required to operate the measurement and telemetry devices, use is generally made of a generator which is driven by a turbine arranged in the drill string. The turbine draws its drive energy from the drilling fluid stream which is conveyed through the drill string to the drill bit. The problem encountered with this approach however is that the feed rate of the drilling fluid fed through the drill string is dependent on the drilling conditions such as pump capacity, well depth and physical properties of the drilling fluid, to name but a few, and can be subject to severe fluctuations on a scale of 1 to 4. Such fluctuations are unsuitable for the drive of the generator connected to the turbine and would lead to hardly controllable fluctuations of rotational frequency and performance. It is necessary therefore to reduce the dependence of the turbine's performance and rotational frequency on the feed rate of the of the mud pump and to limit the drive performance of the generator to the desired degree.

In a turbine of the type initially referred to which is known from U.S. Pat. No. 6,864,593 B2, the impeller is coupled to the rotor of a generator and an axially adjustable throttling element of a throttling device in such a way that the variation of the pressure drop across the impeller, which results from a load change of the generator, produces an axial movement of the impeller and causes an adjustment of the throttling device. As a result of the adjustment of the throttling device, pressure pulses are produced in the drill string, which are intended for the transmission of signals to the earth's surface. No provision is made for a limitation of the rotational frequency and performance of the turbine.

From EP 0 069 530 A2 is known a bypass device for a turbine which is arranged in a drill string and has a valve which is arranged upstream from the turbine in the drill string in order to control the fluid stream bypassing the turbine. The valve is actuated by a piston arrangement which is acted upon in one direction by the pressure on the output side of the turbine and a compression spring, and in the opposite direction by the pressure on the input side of the turbine. The position of the valve varies in response to the pressure differential between input and output, thereby regulating the quantity of drilling fluid which gets to the turbine input and bypasses the turbine. By this means the output performance of the turbine should be maintained essentially constant in spite of changing operating conditions. Such a bypass device adds to the constructional expenditure, and its function may be impaired by particles in the drilling fluid.

In addition there is known from JP 04022766 A a speed controlling device for a turbine generator arranged in a drill string, wherein a valve is arranged at the turbine input and held in an open position by spring force. A bypass channel bypassing the turbine is provided parallel to the input of the valve. In this arrangement, the valve is increasingly closed as the feed rate of the supplied drilling fluid increases so that the bypass rate increases while the volume flow which reaches the turbine is kept essentially constant. This device has the disadvantage that a relatively large bypass cross section is always open so that in the presence of small feed rates the afflux to the turbine is too small. In addition there is the risk, particularly with the valve closed to greater degrees, of the valve passage becoming clogged with particles entrained in the drilling fluid.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a turbine of the type initially referred to for driving a generator in a drill string, whose maximum rotational frequency and drive performance are largely independent of the feed rate supplied. Desirably, the turbine enables a relatively high power output in the presence of a low feed rate and limits the rotational frequency and power output in the presence of a high feed rate in order to avoid overloading of the generator and the electric voltage stabilization.

Advantageous embodiments of the turbine are disclosed in claims2to11.

According to the present invention, the turbine comprises a housing mountable in a drill string and having an inlet opening and an outlet opening for a fluid conveyed through the drill string, and it has at least one impeller arranged in the housing and adapted to be impinged upon by the fluid flowing into the housing, wherein the impeller is axially movably mounted and yieldably supported against the afflux direction in such a way that the supporting force of the yielding support increases as the axial movement of the impeller increases, and wherein the housing surrounds the range of movement of the impeller with a frontal first housing section of smaller inner diameter as seen in the afflux direction and a rear second housing section of larger inner diameter as seen in the afflux direction.

In the turbine of the invention, the turbine wheel is displaced in the housing by the dynamic pressure of the fluid impinging on the turbine wheel, which pressure is dependent on the feed rate, and in addition by the pressure differential when loaded in the afflux direction, with the impeller entering more and more into the housing section with the larger inner diameter with the displacement proceeding. The turbine of the invention thus compensates in a self-regulating way for variations in the feed rate of the supplied fluid by a displacement of the turbine wheel into a relief region of the turbine housing. The volumetric efficiency of the turbine decreases as a result, and the rise in turbine rotational frequency and turbine performance is dampened or restricted. In this manner, the dependence of the turbine rotational frequency and turbine performance on the feed rate is reduced, and overloading of the generator and the connected electrical devices is avoided. Correspondingly, the labor involved in providing the electric voltage stabilization can be kept smaller, which is a great advantage given the conditions prevailing in the well, such as high temperatures and restricted space. The embodiment of the invention affords the added advantage of dispensing with the need for bypass devices of the aforementioned type, thereby avoiding attendant disadvantages. The outer diameter of the turbine housing may be dimensioned such as to approach the inner diameter of a drill string closely, so that the turbine wheel may have a relatively large outer diameter with short blade lengths. This enables a sufficient turbine power to be achieved for driving the generator with only a low feed rate.

According to another proposal of the invention, for achieving a certain control characteristic the first and the second housing section may be separated from one another by a radial shoulder, or a transition region with continually changing inner diameter may be provided between the two housing sections. Furthermore it has proved advantageous for the diameter difference between the first and the second housing section to amount to at least 15%, particularly at least 20%.

An advantageous construction of the turbine and a favorable mounting of the impeller are achievable according to a further proposal of the invention by arranging for the impeller to be secured to an axially movably and rotatably mounted shaft, with the shaft being preferably a hollow shaft which is mounted on a stationary axle extending through the turbine. For driving the generator, the shaft of the impeller may be coupled to the generator shaft either through a coupling which compensates for axial movements or rigidly, with the latter case requiring the rotor of the generator to be equally mounted axially movably as the shaft of the impeller.

According to another proposal of the invention, the yielding support of the impeller or the shaft may be provided by a compression spring fixedly supported on the housing. Moreover, the yielding support may include mutually repelling permanent magnets which transmit the supporting force of the compression spring to the shaft without friction. Alternatively, it is also possible to provide a friction bearing or an anti-friction bearing to transmit the supporting force between the compression spring and the impeller or the impeller shaft. Preferably, the compression spring is mounted on the axle extending through the turbine and takes support on the axle in axial direction.

According to the invention, an increase in turbine performance while the housing's outer diameter is maintained unchanged is accomplishable in simple manner by mounting in the housing several impellers at an axial relative distance on an axially movably and rotatably mounted shaft, with the housing surrounding each impeller with a housing section that covers the axial range of movement of the respective impeller and includes a first housing section of smaller inner diameter and behind it in the afflux direction a second housing section of larger inner diameter. Moreover, the housing may include an annular baffle section between the housing sections encompassing the respective impellers, in which several flow-regulating baffles extending axially and radially inwardly are arranged at a substantially uniform distance from each other. By arranging one or several further impellers, energy is once again withdrawn from the fluid after its exit from the impeller first impinged upon by the fluid, as a result of which the drive performance available for driving the generator is increased significantly.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The turbine1illustrated inFIG. 1and the generator2illustrated inFIG. 2form in combination a constructional unit mountable in a drill string for deep well drilling in order to generate electric power intended to supply measurement instruments and to drive pulsating telemetry devices. The turbine1is shown in the drawing in such a way that the afflux end points upwards. The lower end of the turbine1is then adjoined by the upper end of the generator2.

The turbine1has a housing3which is composed of three housing sections screwed together. The housing3has a cylindrical middle section4of an outer diameter which is smaller than the inner diameter of the drill string pipes into which the turbine1can be installed according to the intended purpose, with allowance only for the amount of play that is necessary for installation. Adjoining the two ends of the middle section4towards the housing ends are conically tapering housing sections5,6which have a plurality of longitudinally extending slots for the formation of inlet openings7or outlet openings8for the fluid which is conveyed from a pump arranged above ground through the drill string in the direction of the drill bit. Above and hence upstream from the inlet openings7, the housing3is closed by a wall9. As becomes apparent fromFIG. 2, the lower end of the housing3is bolted on a housing10of the generator2.

Extending through the housing3in the longitudinal direction is an axle12which is composed of one central pipe13running throughout the length and several sleeves arranged on the pipe13in a manner preventing relative rotation and axial displacement. The pipe13has one end secured and sealed in the wall9. The opposite end of the pipe13is fixedly connected with a wall14which closes the lower end of the housing10of the generator2. The bore of the pipe13forms a channel for routing cables through which devices arranged upstream from the turbine1, for example, measuring instruments, may be connected to devices arranged downstream from the generator2.

A hollow shaft16is rotatably and axially displaceably mounted on the axle12. The hollow shaft16is composed of three elements161,162,163screwed together. The elements161,162are located in the turbine1. The element163is arranged in the generator2and forms the generator shaft17. To carry the hollow shaft16on the axle12, a first friction bearing18is arranged in the turbine1and a second friction bearing19is arranged in the generator2. Both friction bearings18,19include an inner bushing20formed fast with the axle and a movable outer bushing21. The bushings20,21are made of a wear-resistant material with suitable sliding properties. The length of the inner bushings20is greater than that of the outer bushings21and dimensioned sufficiently long to enable the hollow shaft16to be displaced in axial direction for a predetermined length without the sliding surfaces of the bushings20,21becoming disengaged.

Arranged at the upper end of the hollow shaft16concentrically with the axle12is an annular permanent magnet23. Two more annular permanent magnets24,25are located on either side of and spaced from the permanent magnet23. The permanent magnet24is inserted in a recess in the wall9. The permanent magnet25is axially slidably mounted on the pipe13in the interior of the hollow shaft16and secured against relative rotation on the pipe13. On the side facing away from the permanent magnet23, the permanent magnet25takes support upon a stop sleeve26of the axle12by means of a compression spring27. The poles of the permanent magnets23,24,25are oriented in such a way that the permanent magnets repel each other. The permanent magnets23,24,25and the compression spring27guide the hollow shaft16in axial direction, with the permanent magnets23,24,25effecting a friction-free transmission of the guide forces. In the absence of fluid impinging on the turbine1, the hollow shaft16is held in the axial position shown inFIG. 1.

An annular bearing element29is axially movably arranged in a bore28of the generator housing10at the opposite lower end of the hollow shaft16. The bearing element29forms in combination with the end surface of the adjacent bushing21an axial bearing30which may also fulfill a sealing and filter function. On the side of the bearing element29facing away from the axial bearing30, the bore28accommodates a compression spring31which bears against the wall14closing the bore28and urges the bearing element29against the bushing21.

Secured to the element162of the hollow shaft16are two impellers34,35arranged in spaced relationship to each other. The impellers34,35are located in the cylindrical middle section4of the housing3between the inlet openings7and the outlet openings8. The impellers34,35define with the edges of their blades a circular-ring-shaped range of movement which, due to the axial displaceability of the hollow shaft16, extends for some length from the illustrated initial position of the impellers34,35in the direction of the generator. The housing3surrounds the axially extending range of movement of each impeller34,35with two different housing sections36,37and38,39, respectively. The frontal housing sections36,38as seen in the direction of flow are cylindrical and have an inner diameter which is only slightly greater than the outer diameter of the range of movement of the impeller34and35, respectively. As seen in the direction of flow, the housing sections37,39lie downstream from the housing sections36,38and have a cylindrical inner wall of an inner diameter which, depending on the required control range, is around 15 to 25% greater than the inner diameter of the housing sections36,38. Radially outwardly extending shoulders40,41form the transitions between the different housing sections36,37and38,39, respectively. The shoulders40,41have a steep, frusto-conical annular surface. Depending on the desired control characteristic, the shoulders40,41may also have a flatter shape. Alternatively, a transition region with varying slope may be provided.

Also arranged in the housing3upstream from the impeller35is a baffle ring43which has an inner diameter substantially equal to the inner diameter of the housing section38and includes several circumferentially spaced flow-regulating baffles44which extend radially inwardly in close proximity to the hollow shaft16and axially. The function of the flow-regulating baffles44is to retard the eddies formed in operation on the outlet side of the impeller34and to direct the fluid into the appropriate direction onto the impeller35.

The generator2has a rotor45with a plurality of magnet segments47which produce a bipolar magnetic field. The magnet segments47are arranged in longitudinal grooves46of the generator shaft17and are covered by a sleeve48made of a thin-walled, nonmagnetic material which embraces the grooved area of the generator shaft17and effects a seal to the outside. The rotor45is surrounded by generator windings49and iron cores50which are arranged in the generator housing10behind a stator sleeve51made of a thin-walled, nonmagnetic material of low electric conductivity to avoid eddy currents. Between the sleeve48and the stator sleeve51is a narrow gap which is in communication with the outlet openings8and through which the fluid is able to reach the friction bearing19and also enter the bore28. The generator windings49are arranged in a closed chamber52of the housing10, which chamber is filled with an insulating liquid. By means of pressure compensating pistons53slidably arranged in bores54, the pressure in the chamber52is adjusted to the respective pressure in the area of the outlet openings8.

When the fluid flow passes through the inlet openings7to impinge on the turbine1, a pressure differential is created at the impellers34,35as a result of their flow resistance, which generates a pressure force acting upon the hollow shaft16in the direction of the generator2. When the amount of fluid supplied is still relatively small, the force of the compression springs27,31and the repelling force between the permanent magnets23,25offers sufficient resistance for the hollow shaft16and the impellers34,35mounted thereon to be held in the position shown inFIG. 1. When the feed rate increases, the pressure differential on the impellers34,35increases likewise and with it also the axial loading of the hollow shaft16. As a result of the higher loading, the compression springs27,31can be somewhat compressed, causing the hollow shaft16to be displaced in the direction of the generator2. When the displacement of the hollow shaft16remains so small that the impellers34,35do not leave the housing sections36,38, the volumetric efficiency of the impellers34,35changes only insignificantly, the maximum rotational frequency and performance of the turbine1increase in accordance with the increased feed rate. With the feed rate continuing to increase, the correspondingly increasing loading of the hollow shaft16displaces the impellers34,35initially in part beyond the control edges of the shoulders40′,41into the region of the housing sections37,39. As a result, the volumetric efficiency of the impellers34,35deteriorates so that the further increase in the maximum rotational frequency and performance becomes weaker. With a still further increase in the flow rate, the impellers34,35are ultimately moved entirely into the region of the housing sections37,39. This reduces the volumetric efficiency of the impellers34,35still further, enabling a significant part of the flow rate to flow around the impellers34,35. In this way, the maximum rotational frequency and performance of the turbine1is effectively limited, and overloading of the generator2and the connected systems is avoided.

Aside from the flow rate delivered to the turbine1, also the torque load of the impellers34,35has an impact on the pressure differential on the impellers34,35. Measured by the impact of the flow rate, this impact is however significantly smaller and does not impair the performance for driving the generator.