A downhole power generator has a substantially tubular body. A cover surrounds at least a portion of the body. At least one piezoelectric element is disposed in a cavity in the body, the piezoelectric element acting cooperatively with the cover such that motion of the cover relative to the body causes the piezoelectric element to generate electric power. A method for generating power downhole comprises disposing a cover around at least a portion of a substantially tubular body; disposing at least one piezoelectric element in the body; and engaging the piezoelectric element with the cover such that motion of the cover relative to the body causes the piezoelectric element to generate electric power.

BACKGROUND OF THE INVENTION 
     The present disclosure relates generally to the field of power generation and more particularly to downhole power generation. 
     Electrical power for use in the downhole drilling environment may be supplied by batteries in the downhole equipment or by downhole fluid driven generators. Downhole fluid driven generators are prone to reliability issues. Downhole batteries may suffer reliability problems at high and low temperatures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present invention can be obtained when the following detailed description of example embodiments are considered in conjunction with the following drawings, in which: 
         FIG. 1  is a schematic of a drilling installation; 
         FIG. 2A  is a view of an example embodiment of a downhole generator; 
         FIG. 2B  is a cross-section of the downhole generator of  FIG. 2A ; 
         FIG. 2C  is another cross-section of the downhole generator of  FIG. 2A ; 
         FIG. 2D  is an enlarged view of bubble  2 D of  FIG. 2C ; 
         FIG. 3  shows examples of voltages generated by a piezoelectric generator; 
         FIG. 4  is a schematic showing one example of a circuit for converting power generated by piezoelectric elements; 
         FIG. 5A  is a view illustrating an example of an eccentric body for use in a downhole generator; 
         FIG. 5B  is a view illustrating an example of an eccentric sleeve for use in a downhole generator; 
         FIG. 5C  is a view illustrating an example of a sleeve having a single external blade for use in a downhole generator; 
         FIG. 6A  is an example of a downhole generator having a bearing mounted cover; 
         FIG. 6B  is a section of the downhole generator of  FIG. 6A  showing internal blades for activating the piezoelectric element assemblies; 
         FIG. 7  is an example of a downhole generator comprising radially moving blades interacting with piezoelectric elements; 
         FIG. 8  is an example of a downhole generator with blades on an outer surface of a cover; and 
         FIG. 9  shows a drill string having a plurality of spaced apart generators distributed therein. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereof are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Described below are several illustrative embodiments of the present invention. They are meant as examples and not as limitations on the claims that follow. 
     Referring to  FIG. 1 , a drilling installation is illustrated which includes a drilling derrick  10 , constructed at the surface  12  of the well, supporting a drill string  14 . The drill string  14  extends through a rotary table  16  and into a borehole  18  that is being drilled through earth formations  20 . The drill string  14  may include a kelly  22  at its upper end, drill pipe  24  coupled to the kelly  22 , and a bottom hole assembly  26  (BHA) coupled to the lower end of the drill pipe  24 . The BHA  26  may include drill collars  28 , an MWD tool  30 , and a drill bit  32  for penetrating through earth formations to create the borehole  18 . In operation, the kelly  22 , the drill pipe  24  and the BHA  26  may be rotated by the rotary table  16 . Alternatively, or in addition to the rotation of the drill pipe  24  by the rotary table  16 , the BHA  26  may also be rotated, as will be understood by one skilled in the art, by a downhole motor (not shown). The drill collars add weight to the drill bit  32  and stiffen the BHA  26 , thereby enabling the BHA  26  to transmit weight to the drill bit  32  without buckling. The weight applied through the drill collars to the bit  32  permits the drill bit to crush the underground formations. 
     As shown in  FIG. 1 , BHA  26  may include an MWD tool  30 , which may be part of the drill collar section  28 . As the drill bit  32  operates, drilling fluid (commonly referred to as “drilling mud”) may be pumped from a mud pit  34  at the surface by pump  15  through standpipe  11  and kelly hose  37 , through drill string  14 , indicated by arrow  5 , to the drill bit  32 . The drilling mud is discharged from the drill bit  32  and functions to cool and lubricate the drill bit, and to carry away earth cuttings made by the bit. After flowing through the drill bit  32 , the drilling fluid flows back to the surface, indicated by arrow  6 , through the annular area between the drill string  14  and the borehole wall  19 , or casing wall  29 . At the surface, it is collected and returned to the mud pit  34  for filtering. In one example, the circulating column of drilling mud flowing through the drill string may also function as a medium for transmitting pressure signals  21  carrying information from the MWD tool  30  to the surface. In one embodiment, a downhole data signaling unit  35  is provided as part of MWD tool  30 . Data signaling unit  35  may include a pressure signal transmitter  100  for generating the pressure signals transmitted to the surface. 
     MWD tool  30  may include sensors  39  and  41 , which may be coupled to appropriate data encoding circuitry, such as an encoder  38 , which sequentially produces encoded digital data electrical signals representative of the measurements obtained by sensors  39  and  41 . While two sensors are shown, one skilled in the art will understand that a smaller or larger number of sensors may be used without departing from the principles of the present invention. The sensors  39  and  41  may be selected to measure downhole parameters including, but not limited to, environmental parameters, directional drilling parameters, and formation evaluation parameters. Such parameters may comprise downhole pressure, downhole temperature, the resistivity or conductivity of the drilling mud and earth formations, the density and porosity of the earth formations, as well as the orientation of the wellbore. 
     The MWD tool  30  may be located proximate to the bit  32 . Data representing sensor measurements of the parameters discussed may be generated and stored in the MWD tool  30 . Some or all of the data may be transmitted by data signaling unit  35 , through the drilling fluid in drill string  14 . A pressure signal travelling in the column of drilling fluid may be detected at the surface by a signal detector unit  36  employing a pressure detector  80  in fluid communication with the drilling fluid. The detected signal may be decoded in information handling system  33 . For purposes of this disclosure, an information handling system may comprise any instrumentality or aggregate of instrumentalities operable to compute, classify, process, transmit, receive, retrieve, originate, switch, store, display, manifest, detect, record, reproduce, handle, or utilize any form of information, intelligence, or data for scientific, control, or other purposes. The pressure signals may comprise encoded binary representations of measurement data indicative of the downhole drilling parameters and formation characteristics measured by sensors  39  and  41 . Information handling system  33  may be located proximate the rig floor. Alternatively, information handling system  33  may be located away from the rig floor. In one embodiment, information handling system  33  may be incorporated as part of a logging unit. Alternatively, other types of telemetry signals may be used for transmitting data from downhole to the surface. These include, but are not limited to, electromagnetic waves through the earth and acoustic signals using the drill string as a transmission medium. In yet another alternative, drill string may comprise wired pipe enabling electric and/or optical signals to be transmitted between downhole and the surface. 
     In one example, a generator  102  provides electrical power and may be located in BHA  26  to provide at least a portion of the electrical power required by the various downhole electronics devices and/or sensors. 
     Also referring to  FIGS. 2A-2D , in one example, generator  102  comprises a tubular body  202  that may be coupled into drill string  14 . Flow passage  201  provides a passage for the flow of drilling fluid through body  202 . In this example, the axis  203  of flow passage  201  is approximately coincident with the axis of rotation of the drill string proximate body  202 . A plurality of longitudinal cavities  230  may be formed around the outer circumference of tubular member  202 . In the example shown, six cavities  230  are formed around tubular member  202 . Alternatively, a greater or fewer number of cavities may be formed around tubular member  202 . A piezoelectric assembly  212  may be disposed in each cavity  230 . For example, piezoelectric assemblies  212   a - f  may be disposed in cavities  230   a - 230   f , respectively. 
     In one embodiment, each piezoelectric assembly  212  may comprise a stack of piezoelectric elements  211  encased in flexible potting material  210 . In one embodiment, each piezoelectric element  211  is separated by an adjacent piezoelectric element  211  by a distance L. The intermediate space between each adjacent element may be filled with flexible potting material  210 . In one example, approximately the same thickness of potting material  210  separates the bottom piezoelectric element from the bottom of cavity  230 . 
     In one embodiment, piezoelectric element  211  comprises a piezoelectric film material. Examples include, but are not limited to, polyvinylidene fluoride (PVDF) and copolymers, such as a copolymer of PVDF and trifluoroethylene, and a copolymer of PVDF and tetrafluoroethylene. Alternatively, piezoelectric element  211  may comprise a piezoelectric ceramic material such as lead zirconium titanate (PZT) and barium titanate (BATiO 3 ), or a piezoelectric crystalline material, for example, quartz, or any other material that exhibits piezoelectric properties. In yet another embodiment, piezoelectric element  211  may comprise a piezoelectric fiber-composite material. 
     In one example, cover  204  is a substantially cylindrical member that fits around the section of tubular body  202  housing the piezoelectric assemblies  212 . Cover  204  extends in each axial direction, beyond cavity  230  and has an internal spline  206  formed on at least a portion of inner surface  217  thereof. Internal spline  217  engages a mating external spline  208  formed on an outer surface  219  of body  202 . As shown in  FIGS. 2B-2D , spline  206  is sized such that there is a gap, G, between the inner surface  217  of spline  206  and the outer surface  219  of spline  208 . Gap, G, allows cover  204  to move radially due to interaction of cover  204  with the borehole wall  19 . In one example, flexible potting material  210  extends outward to contact spline surface  215  of cover  204 . Flexible potting material  210  may be adhered to the bottom of spline surface  215  by a suitable adhesive material  213 . Alternatively, potting material  210  may not be adhered to spline surface  215 . 
     In another embodiment, see  FIG. 8 , at least one blade  280  is attached to the outside of cover  204  to enhance contact with the borehole wall. While shown with three blades  280 , any number of blades may be used. Attachment may be by any suitable mechanical process, including, but not limited to, mechanical fasteners, welding, and brazing. Alternatively, at least one blade may be formed integrally to the outside of cover  240  using any suitable forming process. For example, the cover and the at least one blade may be machined from a single bar. 
     In one example during drilling operations, drill string  14  and/or drill collar section  28  rotates. During rotation, cover  204  may be forced radially into contact with borehole wall  19 . This contact will cause cover  204  to move radially with respect to body  202  causing compression of piezoelectric element assembly  212  and generating a voltage increase  302 , see  FIG. 3 , across the piezoelectric elements  211 . As cover  204  moves away from the wall, cover  204  may move back to a neutral position with the voltage of the piezoelectric assembly returning to its base level  300 . If the potting material in each cavity  230  is adhered to spline surface  215  in each cavity  230 , the compression on one side of cover  204  results in cover  204  stretching the piezoelectric assembly on the opposite side of body  202 , resulting in a negative voltage  304 . Similarly, as cover moves away from the wall, cover  204  may move back to a neutral position with the voltage  304  of the piezoelectric assembly returning to its base level  300 . In the case where the potting compound is not adhered to spline surface  217 , only compression is applied to piezoelectric elements  211  such that only the positive voltage  302  is generated. 
     In another drilling example, body  202  may experience cyclical bending stresses such that body  202  deflects with respect to cover  204 . Such cyclic motion produces simultaneous cyclical compression and tension on piezoelectric element assemblies  212  on opposite sides of body  202 , if piezoelectric element assemblies  212  are adhesively coupled to cover  204 . The cyclical loading will produce cyclical positive and negative voltages that may be fed into suitable circuitry for use downhole. 
     In one example, also referring to  FIG. 4 , each piezoelectric element  211  comprises piezoelectric material  240  described previously. Piezoelectric material  240  has a conductive material  241  disposed on the upper and lower surfaces thereof. As loads are applied to piezoelectric assembly  212 , the voltage/charge generated is fed in parallel from each piezoelectric element  211  to a rectifier  260 , through a smoothing/filter capacitor, and to load  262 . Load  262  may comprise additional electronic circuits  218 , housed in electronics cavity  216 . Electronics cavity  216  may be a longitudinal cavity similar to cavity  230 . Alternatively, electronics cavity  216  may encompass the circumferential volume around body  202 . Circuits  216  may comprise voltage converters, a processor, and a memory in data communication with the processor for storing programmed instructions to control the energy storage and/or distribution to other downhole devices and/or tools in drill string  14 . In one example, power from piezoelectric element assemblies  212  may be used to charge capacitors and/or rechargeable batteries. 
     Wires (not shown) may be run in passages  232  and  234  to power other devices in body  202  and/or in other downhole systems external to body  202  via suitable connectors. Electronics cover  214  fits over electronics cavity  216  and seals electronics cavity  216  from the external environment via seals  220 . In one example electronics cover  214  is threaded onto body  202  by threads  222  and  223  formed on electronic cover  214  and body  202 , respectively. In one embodiment, a plurality of generators  102  may be connected to a common electrical bus for combining power from the generators  102 , when higher power is required. 
     In one embodiment, also referring to  FIG. 5A , body  502  is formed such that the center  504  of body  502  is displaced from the center  506  of rotation  506  of drill string  14 . This forms an eccentric body that is substantially always in contact with the borehole wall  19  thereby generating electric power. In this example, flow passage  501  is approximately concentric with the axis of rotation of drill string  14  proximate body  502 . 
     In another embodiment, see  FIG. 5B , an eccentric section  513  is formed on sleeve  514  using techniques known in the art. Eccentric section  513  extends outward from sleeve  514  and contacts borehole wall  19  as drill string  14  rotates thereby generating electric power. Alternatively, see  FIG. 5C , a single blade  515  may be attached to sleeve  204  to effect an eccentric geometry such that rotation of drill string  14  causes blade  515  into contact with borehole wall  19  thereby generating electric power. 
     In another embodiment, referring to  FIGS. 6A and 6B , cover  604  is mounted on bearings  620  such that cover  604  and body  602  are rotatable relative to each other. A plurality of stabilizer blades  605  may be attached or integrally formed on cover  604 . Blades  605  may be straight blades, as shown in  FIG. 6B , spiral blades known in the art, or any other suitable blade geometry. In one example, at least one of blades  605  may contact borehole wall  19  such that cover  604  and blades  605  are substantially stationary with respect to borehole wall  19 . As shown in  FIG. 6B , at least one internal blade  606  may be positioned in an internal cavity  609  in cover  604 . A spring  608  forces internal blade  606  into contact with piezoelectric element assembly  212  during rotation of body  602  by drill string  14 . The contact of internal blade  606  causes compression of piezoelectric element assembly  212  causing generation of a voltage/charge that may be collected as described previously. As shown, multiple internal blades  606  may be positioned around cover  604  to increase the frequency of contact of internal blades  606  with piezoelectric element assemblies  212 . Spring  608  may be an elastomer spring or a metallic spring, for example a leaf spring. 
     In yet another embodiment, referring to  FIG. 7 , body  702  has at least one longitudinal cavity  730  formed therein that accepts a piezoelectric element assembly  212 , previously described. A blade  710  may be disposed in contact with a potting material  210 , previously described, such that radial motion of blade  710 , for example, due to interaction of at least one blade  710  with borehole wall  19  causes compression of piezoelectric element assembly  212  thereby generating electric power. Three blades  710  are shown in  FIG. 710 . Any suitable number of blades, including a single blade, may be used. 
     While generator  102  is described herein as located in BHA  26 , it will be appreciated that a plurality of generators  102  may be spaced out within drill string  14 , see  FIG. 9 . Each generator  102  may contain sensors and a telemetry transmitter and/or receiver. 
     One skilled in the art will appreciate that the amount of power generated is related to the number of piezoelectric element assemblies in a particular body. In addition, as described previously, any number of generator bodies may be electrically connected to a common power bus to provide additional power. For example, the embodiments described above may be configured to generate on the order of 20-100 milliwatts for use, for example, in a repeater configuration, and up to about 20 watts for powering, for example, devices in a BHA. 
     One skilled in the art will appreciate that, the stacking of the piezoelectric elements may be accomplished using different orientations, for example, a longitudinal stacking. In one embodiment, both longitudinal and radial stacking may be used to enhance the generation of electrical power from multiple vibration modes and sources. In one embodiment, transient torsional motion, for example stick-slip motion, may interact with and deform the potting material to impart compression and/or tension loads on the piezoelectric elements, in any of the configurations described above, to generate electrical power. 
     Numerous variations and modifications will become apparent to those skilled in the art. It is intended that the following claims be interpreted to embrace all such variations and modifications.