Method and apparatus for pre-loading a piezoelectric transducer for downhole acoustic communication

Abstract

A downhole acoustic transmitter has a pre-loaded piezoelectric transducer, an enclosure in which the piezoelectric transducer is housed, a preload spring that biases the transducer against a first end coupling of the enclosure, and an adjustable preload means mounted to the enclosure such that a selected compressive force is applied to the preload spring, which in turn urges the transducer against a face of the first end coupling such that a mechanical preload is applied to the transducer. The position of the adjustable preload means and the spring compliance are selected so that the level of mechanical preload applied to the transducer compensates for an expected amount of flexing of the acoustic telemetry transmitter due to varying tension and compression applied to the transmitter, thereby maintaining an effective preload on the transducer.

1 . A downhole acoustic transmitter for use in downhole communication, comprising:
 (a) an enclosure comprising a first end coupling, a second end coupling, a tubular outer housing having a first end coupled to the first end coupling and a second end coupled to the second end coupling, and an inner mandrel inside the outer housing and extending between the first and second end couplings such that an annular space is defined between the mandrel and the outer housing;   (b) a piezoelectric transducer in the annular space, and having a first end contacting an inner face of the first end coupling in an axial direction;   (c) a preload spring in the annular space and having a first end contacting a second end of the piezoelectric transducer in the axial direction;   (d) an adjustable preload means contacting the enclosure and a second end of the preload spring such that a compressive force in the axial direction is applied to the preload spring, which in turn compresses the piezoelectric transducer against the inner face of the first coupling.   
     
     
         2 . The downhole acoustic transmitter as claimed in  claim 1  wherein the adjustable preload means comprises one or more spacers contacting an inner face of the second end coupling. 
     
     
         3 . The downhole acoustic transmitter as claimed in  claim 1  wherein the adjustable preload means is a retaining ring attached to an inner surface of the outer housing. 
     
     
         4 . The downhole acoustic transmitter as claimed in  claim 1  wherein the adjustable preload means is a threaded nut attached to the mandrel. 
     
     
         5 . The downhole acoustic transmitter as claimed in  claim 1  wherein the piezoelectric transducer comprises an annular stack of annular piezoceramic discs with electrodes between each disc, wherein the annular stack is slidable over the mandrel. 
     
     
         6 . The downhole acoustic transmitter as claimed in  claim 1  wherein the preload spring is a metal tube slidable over the mandrel. 
     
     
         7 . The downhole acoustic transmitter as claimed in  claim 1  wherein the preload spring comprises one or more metal rods or tubes each extending in the axial direction in the annular space. 
     
     
         8 . The downhole acoustic transmitter as claimed in  claim 1  further comprising an acoustic tuning element in the annular space and attached to the second end of the piezoelectric transducer, the acoustic tuning element having a selected acoustic impedance that maximizes power transfer from the piezoelectric stack into the enclosure over a selected operating frequency bandwidth. 
     
     
         9 . The downhole acoustic transmitter as claimed in  claim 8  wherein the acoustic tuning element has a center frequency wherein the acoustic impedance of the acoustic tuning element matches the acoustic impedance of the piezoelectric stack and the selected operating frequency bandwidth is up to 15% of the center frequency. 
     
     
         10 . The downhole acoustic transmitter as claimed in  claim 9  wherein the acoustic tuning element comprises a metal cylinder having a first end attached to the second end of the piezoelectric transducer and a free second end. 
     
     
         11 . The downhole acoustic transmitter as claimed in  claim 9  wherein one or more of a mass density, mass distribution, length and cross sectional area of the acoustic tuning element is selected to provide the selected acoustic impedance. 
     
     
         12 . The downhole acoustic transmitter as claimed in  claim 1 , mounted in a telemetry tool or a repeater of a drill string, wherein the downhole acoustic transmitter has a configuration selected from a group consisting of: collar-based, clamp-on, and probe-based. 
     
     
         13 . A downhole acoustic telemetry node comprising:
 (a) one or more sensors for measuring a local borehole environment and one or more mechanical conditions of a drill string;   (b) a processor and memory communicative with the one or more sensors for storing measurements taken by the one or more sensors; and   (c) the downhole acoustic transmitter as claimed in  claim 1  communicative with the processor and memory for transmitting the measurement   
     
     
         14 . A method for acoustic transmission from a downhole location, comprising:
 (a) applying a compressive preload in an axial direction against a preload spring, which in turn compresses a piezoelectric transducer against an inner face of a first end coupling of an enclosure of a downhole acoustic transmitter, wherein the compressive preload is selected to place the piezoelectric transducer in compression over a range of expected operating conditions of the downhole acoustic transmitter; and   (b) applying a voltage to the piezoelectric transducer to generate an acoustic transmission.   
     
     
         15 . The method as claimed in claimed in  claim 14  further comprising tuning the acoustic impedance of the piezoelectric transducer by contacting an end of the piezoelectric transducer with an acoustic tuning element having a selected acoustic impedance such that when combined with an acoustic impedance of the preload spring, equals the acoustic impedance of the inner face of the first end coupling, wherein the end of the piezoelectric transducer contacting the acoustic tuning element also contacts the preload spring.