U.S. patent application number 16/316848 was filed with the patent office on 2019-10-03 for method and apparatus for pre-loading a piezoelectric transducer for downhole acoustic communication.
This patent application is currently assigned to XACT Downhole Telemetry Inc.. The applicant listed for this patent is XACT Downhole Telemetry Inc.. Invention is credited to John Godfrey McRory, John-Peter Van Zelm, Dave Whalen, Xiaojun Xiao.
Application Number | 20190301280 16/316848 |
Document ID | / |
Family ID | 60951617 |
Filed Date | 2019-10-03 |
View All Diagrams
United States Patent
Application |
20190301280 |
Kind Code |
A1 |
Xiao; Xiaojun ; et
al. |
October 3, 2019 |
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.
Inventors: |
Xiao; Xiaojun; (Calgary,
Alberta, CA) ; Whalen; Dave; (St. Johns,
Newfoundland, CA) ; Van Zelm; John-Peter; (Calgary,
Alberta, CA) ; McRory; John Godfrey; (Calgary,
Alberta, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
XACT Downhole Telemetry Inc. |
Calgary |
|
CA |
|
|
Assignee: |
XACT Downhole Telemetry
Inc.
Calgary
AB
|
Family ID: |
60951617 |
Appl. No.: |
16/316848 |
Filed: |
July 7, 2017 |
PCT Filed: |
July 7, 2017 |
PCT NO: |
PCT/CA2017/050823 |
371 Date: |
January 10, 2019 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62360717 |
Jul 11, 2016 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
E21B 47/16 20130101;
E21B 47/26 20200501 |
International
Class: |
E21B 47/16 20060101
E21B047/16 |
Claims
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.
Description
FIELD
[0001] This disclosure relates generally to a downhole acoustic
transmitter having a pre-loaded piezoelectric transducer and a
method for pre-loading a piezoelectric transducer for use in
downhole communication such as downhole acoustic telemetry.
BACKGROUND
[0002] The evolution of modern oil and gas wells has led to
increases in both the depth of the wells and the complexity of the
procedures and equipment needed for drilling and completions
operations. Additionally, there is an ongoing need for improved
safety and efficiency in the drilling and completions process. The
combination of these factors has created a need for improved
visibility of the downhole conditions along the length of the drill
string and at the bottom hole assembly (BHA) during drilling and
completions operations. Downhole sensor measurements such as
downhole bore and annular pressure, drill string torque and
tension, and temperature can be transferred from a downhole
location to the surface through one of several known telemetry
methods.
[0003] One type of downhole communication method is wired drill
pipe telemetry, which offers very high bandwidths, but tends to be
expensive to deploy and prone to failure. Another known downhole
communication method is mud pulse telemetry which encodes sensor
data into pressure waves that are induced in the drilling fluid
flowing in the drill string. Drawbacks to mud pulse telemetry
include an inability to transmit when drilling fluid is not
flowing, and relatively low data rate transmissions which decrease
as the depth of the well increases. A third type of downhole
communication is electromagnetic (EM) telemetry, which transmits
digitally modulated electromagnetic waves through the formations
surrounding the drill string to a surface receiver. EM telemetry
does not require the flow drilling fluid and can provide a higher
data transmission rate than mud pulse telemetry, but can be
sensitive to the nature of the formations surrounding the well and
may not be well suited for deeper wells.
[0004] A fourth type of downhole communication is acoustic
telemetry, which has proven to be well suited for the modern
drilling environment. Acoustic telemetry is capable of transmitting
hundreds of bits per second, and since it uses the body of the
drill pipe as its transmission medium it is insensitive to the
surrounding formation or casing, and does not require any fluid
flow to enable the transmission of data.
[0005] There are currently three different implementations of
acoustic telemetry systems in downhole tools that use acoustic
telemetry: probe-based, clamp-on, and collar-based. These systems
typically comprise components including sensors, electronics,
batteries and an acoustic transmitter. The probe-based
implementation is mounted at least partially within the bore of the
drill pipe. The clamp-on implementation is mounted on the external
wall of the drill pipe. The collar-based implementation places the
components within an annular space in the downhole tool.
[0006] In a typical drilling or completions environment, a number
of acoustic transmitters can be spaced along the length of the
drill string. The most common type of acoustic transducer used
within downhole tools comprises a cylindrical piezoelectric stack
mounted in a collar-based implementation. Such a stack comprises a
number of thin piezoceramic discs layered with thin electrodes
between each disc which are connected electrically in parallel. As
is known in the art, such as disclosed in U.S. Pat. No. 6,791,470,
the entirety of which is incorporated by reference herein, an
advantage of the piezoelectric stack when compared to other
acoustic transducer types is that the acoustic impedance of the
stacked ring structure can be closely matched to the acoustic
impedance of the tool's structure thereby optimizing the transfer
of acoustic energy from the stack into the tool body, and
subsequently into the drill string. Any acoustic impedance mismatch
between the stack and the tool surrounding structure results in a
reduction in the acoustic output power of the tool.
[0007] The piezoelectric stack structure offers a large
displacement force combined with a high energy conversion
efficiency and high compressive strength, but offers little
resistance to tension, even that incurred when voltage is applied.
Due to its low tensile strength, it is common practice to place a
piezoelectric stack under a mechanical compressive preload along
the stack's axis of operation in order to maintain stack integrity
while being actuated. The magnitude of the preload can compensate
for dynamic forces, but also affects the mechanical energy output
from the stack. If there is no compressive preload or if the
compressive preload exceeds the blocking force of the piezoelectric
material, then there is no mechanical energy output from the stack.
An optimum preload level that will maximize the output mechanical
energy from the stack occurs when the stiffness of the preloaded
stack is equal to the stiffness of the mechanical load.
[0008] Referring to FIG. 1, a prior art collar-based piezoelectric
stack-type acoustic transmitter 301 comprises first and second
thermal expansion compensation rings 302a and 302b, a retaining
ring 303, end coupling 304, a steel outer housing 305, a mandrel
306, a pin 307, and a piezoelectric stack 308. The first and second
thermal expansion rings 302a and 302b are designed to compensate
for the difference between the thermal expansion of the steel
housing 305 and the piezoelectric stack 308. The mandrel 306 is
threaded into the end coupling 304, and the first thermal expansion
compensation ring 302a is slid down the mandrel 306 to an inner
face 309 of the end coupling 304. The piezoelectric stack 308 is
slid down the mandrel 306 to rest against the first thermal
compensation ring 302a. The second thermal compensation ring 302b
is slid down the mandrel 306 to rest against the end of the
piezoelectric stack 308, and the retaining ring 303 is placed on
the mandrel 306 against the second thermal compensation ring 302b.
The outer housing 305 is placed over the mandrel 306, first and
second thermal compensation rings 302a, 302b and the retaining ring
303 and threaded onto the end coupling 304. The pin 307 is threaded
into the housing 305 until the thread is shouldered, and the inner
face of the pin 310 is forced against the retaining ring 303 which
in turn forces the thermal compensation rings 302a, 302b and the
piezoelectric stack 308 against the immoveable inner face 309 of
the end coupling 304, thereby creating a compressive preload force
on the piezoelectric stack 308. The amount of compressive force on
the piezoelectric stack can be controlled by varying the length of
the retaining ring 303.
[0009] The prior art acoustic transmitter 301 will maintain a
positive compressive preload on the piezoelectric stack 308 over a
limited range of tension/compression on the downhole tool. However,
in deeper wells such as those drilled offshore, the
tension/compression applied to the downhole tool by external forces
can result in the tool flexing enough to either reduce the preload
to zero, or to compress the piezoelectric stack beyond its
compressive limits. Thus there is a need for a method of applying a
compressive preload to the piezoelectric stack in a downhole
acoustic transmitter that will maintain an effective preload over
the entire range of tension and compression applied to the downhole
tool by the drill string while operating in a downhole
environment.
SUMMARY
[0010] According to one aspect, there is provided a downhole
acoustic transmitter for use in downhole communication, comprising
an enclosure, a piezoelectric transducer, a preload spring and an
adjustable preload means. The enclosure comprises 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. The piezoelectric transducer is in the
annular space, and has a first end contacting an inner face of the
first end coupling in an axial direction. The preload spring is in
the annular space and has a first end contacting a second end of
the piezoelectric transducer in the axial direction. The adjustable
preload means contacts 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.
[0011] The adjustable preload means can comprise one or more
spacers contacting an inner face of the second end coupling, or be
a retaining ring attached to an inner surface of the outer housing,
or be a threaded nut attached to the mandrel. The piezoelectric
transducer can comprise an annular stack of annular piezoceramic
discs with annular electrodes between each disc, wherein the
annular stack is slidable over the mandrel. The preload spring can
be a metal tube slidable over the mandrel, or can be one or more
metal rods or tubes each extending in the axial direction in the
annular space.
[0012] The downhole acoustic transmitter can further comprise an
acoustic tuning element in the annular space and attached to the
second end of the piezoelectric transducer. The acoustic tuning
element has a selected acoustic impedance that when combined with
the acoustic impedance of the preload spring, equals the acoustic
impedance of the inner face of the first end coupling. The acoustic
tuning element can comprise a metal cylinder having a first end
attached to the second end of the piezoelectric transducer and a
free second end. One or more of a mass density, mass distribution,
length and cross sectional area of the acoustic tuning element can
be selected to provide the selected acoustic impedance.
[0013] According to another aspect, there is provided a downhole
acoustic telemetry node which comprises one or more sensors for
measuring a local borehole environment and one or more mechanical
conditions of a drill string (e.g. pressure, temperature, tension,
compression and torque), a processor and memory communicative with
the one or more sensors for storing measurements taken by the one
or more sensors, and the downhole acoustic transmitter, which is
communicative with the processor and memory and is operable to
transmit the measurements.
[0014] According to another aspect, there is provided 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. The method can
further comprise 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.
BRIEF DESCRIPTION OF DRAWINGS
[0015] FIG. 1 is a schematic side sectioned view of a downhole
acoustic transmitter used in a downhole acoustic communication
system (PRIOR ART).
[0016] FIG. 2 is a schematic representation of a drill string
comprising a downhole acoustic communication system according to
embodiments of the invention.
[0017] FIG. 3 is a frequency response graph of a modulated acoustic
signal transmitted by the downhole acoustic communication system of
FIG. 2.
[0018] FIG. 4 is a schematic side sectioned view of a downhole
acoustic transmitter comprising a preload spring compressed by
adjustable spacers according to one embodiment of the
invention.
[0019] FIG. 5 is a schematic side sectioned view of a downhole
acoustic transmitter comprising a preload spring compressed by an
adjustable retaining ring according to another embodiment of the
invention.
[0020] FIG. 6(a) is a schematic side sectioned view of a downhole
acoustic transmitter comprising a preload spring compressed by an
adjustable retaining ring and an acoustic tuning element according
to another embodiment of the invention, and FIG. 6(b) is a detail
view of an interface of the acoustic tuning element and a
transducer stack of the downhole acoustic transmitter.
[0021] FIG. 7 is a schematic side sectioned view of a downhole
acoustic transmitter comprising a preload spring compressed by an
adjustable threaded nut according to another embodiment of the
invention.
[0022] FIG. 8 is a schematic side sectioned view of a downhole
acoustic transmitter comprising a preload spring compressed by an
adjustable threaded nut and an acoustic tuning element according to
another embodiment of the invention.
[0023] FIG. 9(a) is a graph showing a first resonance peak in an
example steel cylinder having a first constrained end and a second
free end, and FIG. 9(b) is a graph showing the magnitude of the
cylinder's acoustic impedance across a third acoustic passband of
the drill string as shown in FIG. 3.
[0024] FIG. 10 is a graph of the acoustic impedance of a
piezoelectric stack and the acoustic impedance of an acoustic
tuning element of an example downhole acoustic transmitter.
[0025] FIG. 11 is an acoustic amplitude vs. frequency graph
illustrating the acoustic output of a prior art acoustic
transmitter having a fixed preload, the acoustic transmitter with a
preload spring and an adjustable preload means as shown in FIG. 5,
and the acoustic transmitter with a preload spring, adjustable
preload means and an acoustic tuning element as shown in FIGS. 6(a)
and (b).
DETAILED DESCRIPTION
[0026] Directional terms such as "top", "bottom", "upwards",
"downwards", "vertically", and "laterally" are used in the
following description for the purpose of providing relative
reference only, and are not intended to suggest any limitations on
how any article is to be positioned during use, or to be mounted in
an assembly or relative to an environment.
[0027] Additionally, the term "couple" and variants of it such as
"coupled", "couples", and "coupling" as used in this description is
intended to include indirect and direct connections unless
otherwise indicated. For example, if a first device is coupled to a
second device, that coupling may be through a direct connection or
through an indirect connection via other devices and connections.
Similarly, if the first device is communicatively coupled to the
second device, communication may be through a direct connection or
through an indirect connection via other devices and
connections.
[0028] Furthermore, the singular forms "a", "an", and "the" as used
in this description are intended to include the plural forms as
well, unless the context clearly indicates otherwise.
[0029] The embodiments described herein relate generally to a
downhole acoustic transmitter having a pre-loaded piezoelectric
transducer and a method for pre-loading a piezoelectric transducer
for use in downhole acoustic communication such as downhole
telemetry. The transmitter comprises 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.
[0030] In some embodiments, the downhole acoustic transmitter
further comprises an acoustic tuning element positioned to contact
the piezoelectric transducer at the same end as the preload spring.
The acoustic tuning element is tuned such that the acoustic
impedance seen by the piezoelectric transducer at that end,
comprising the combination of the acoustic impedance of the tuning
element and the acoustic impedance of the preload spring at that
end, is equal to the acoustic impedance offered to the transducer
at the other end by the face of the first end coupling, thereby
maintaining the output power of the transducer while compensating
for any variations in the mechanical preload applied by the preload
spring.
[0031] Referring now to FIG. 2, one or more of the acoustic
telemetry transmitters can be installed in a drill string. Drill
string tubing 103 is suspended in a borehole 108 from a drilling
rig 102. The tubing 103 can extend for thousands of feet, and in a
typical deployment an acoustic transmitter is part of a telemetry
tool 105 in a bottom hole assembly (BHA) 104. Additional acoustic
transmitters can be included in repeaters 106 along the length of
the tubing 103, with the number of repeaters 106 and the spacing
between them determined by the along-string measurements required,
if any, at each of the additional locations, and the possible
necessity to repeat the acoustic signal if the distance to the
surface is too far to transmit successfully with a single acoustic
transmitter. The acoustic signal is received at the surface by a
receiver 107.
[0032] The acoustic transmitters in this embodiment have a
collar-based configuration, with the components of the acoustic
transmitter including the piezoelectric transducer, sensors,
electronics and batteries being mounted in a wall of a tubular
section of the repeater 106 or the telemetry tool 105. However, the
acoustic transmitters can have a probe-based or clamp-on
configuration according to other embodiments (not shown). As will
be described in more detail below, each acoustic transmitter
comprises a mandrel defining a through-bore which allows fluid to
pass through repeater 106 or telemetry tool 105. Each acoustic
transmitter is operable to transmit a modulated acoustic signal as
an extensional wave through the drill string components. The
connection of several lengths of tubing 103 of similar size and
dimensions is well known to form an acoustic frequency response
similar to a bandpass comb filter which comprises a number of
passbands alternating with stopbands as shown in FIG. 3. The
bandwidth of the modulated acoustic signal is limited by the
bandwidth of the acoustic passband used for the transmission,
although more than one passband can be used to transmit
simultaneously which increases the total bandwidth available for
the signal and hence the data rate. The telemetry signal travels to
the surface, either directly or through the repeaters 106, where it
is received and decoded by the receiver 107.
[0033] According to a first embodiment and referring to FIG. 4, the
acoustic transmitter 401 used in the telemetry tool 105 and
repeater 106 generally comprises an enclosure, a transducer 405
housed within the enclosure, a preload spring 407 contacting one
end of the transducer 405, and one or more spacers 409 which
provide an adjustable means for applying a selected compressive
load (herein referred to as "preload") on the transducer 405 via
the preload spring 407. The enclosure comprises a first end
coupling 402, a tubular outer housing 403, a cylindrical inner
mandrel 404 and a second end coupling 410 (also referred to as a
"pin"). The first end coupling 402 has a body with threads on the
outer surface of the body ("external threads"), and a central bore
extending through the body. A first end of the inner mandrel 404 is
externally threaded and engages internal threads in the central
bore of the first end coupling 402 along a central axis. Both ends
of the outer housing 403 are internally threaded, with an
internally threaded first end engaging the external threads of the
first end coupling 402 and an internally threaded second end
engaging external threads of the second end coupling 410. The
second end coupling 410 has a body with a bore extending through
the body, and which engages a second end of the inner mandrel 404
by a threaded connection. When assembled, the enclosure defines a
through bore that extends through the central bores of the end
couplings 402, 410 and the bore of the mandrel 404, such that
drilling fluid can flow through the acoustic transmitter 401. The
assembled enclosure also defines a fluid-tight annular space 408
for housing the transducer 405, preload spring 407, and spacers
409.
[0034] The transducer 405 comprises a stack of thin annular
piezoceramic discs layered with thin annular electrodes between
each disc which are connected electrically in parallel (the
transducer is herein alternatively referred to as a "piezoelectric
stack" 405). As a result, the stack's electrical behavior is
primarily capacitive. Applying a high voltage charges the
piezoelectric stack 405 and causes it to increase and decrease in
length. It is this deflection that launches extensional waves into
the drill pipe (not shown). Data can be carried by the extensional
waves by modulating the voltage applied to the piezoelectric stack
405.
[0035] The piezoelectric stack 405 slides over the mandrel 404 and
has a first end that contacts an inner face of the first end
coupling 402. The preload spring 407 is shown in FIG. 4 as a coil
spring that slides over the mandrel 404 with a first end that
contacts a second end of the piezoelectric stack 405. However, the
preload spring 407 can alternatively have different forms,
including a metal cylinder (not shown) of selected length and
spring constant that slides over the mandrel 404, or one or more
metal rods or tubes (not shown) that extend axially in the annular
space between the mandrel 404 and the outer housing 403.
[0036] One or more spacers 409 slide over the mandrel 404 to
contact a second end of the preload spring 407. The pin 410 is
threaded onto the internally threaded second end of the outer
housing 403 such that an inner face of the pin 410 applies axial
pressure against the spacer(s) 409, which in turn applies an axial
compressive preload against the piezoelectric stack 405. Although
only one spacer 409 is shown in FIG. 4, additional spacers 409 can
be inserted depending on the desired preload to be applied to the
piezoelectric stack 405; that is, each spacer 409 has a certain
thickness, and the more spacers 409 inserted between the pin end
and the preload spring end, the higher the compressive preload will
be applied to the transducer 405. The properties of the preload
spring 407 are selected to provide a degree of compliance in the
preload applied against the transducer 405, i.e. to mitigate
against the varying external tensile and compressive forces imposed
on the acoustic transmitter 401 during drill string operation.
[0037] That is, the physical environment imposed on the acoustic
transmitter 401 can be particularly challenging, with the telemetry
tool 106 in particular being subjected to extreme ranges of
pressure, temperature, and tension/compression, all of which vary
as a function of the tool's placement in the drill string, depth,
and the rig's operational state. The orientation of the borehole
108 containing the tubing 103 can be vertical with an inclination
of 0 degrees, or may have one or more deviations in orientation
along its length resulting in changes of inclination as high as 90
degrees. Due to the length of the tubing 103 and the deviations in
its orientation, the tensile and compressive forces that the
telemetry tool 106 are subjected to during rig operations can be
very high. For example, the telemetry tool 106 may be subject to
pressures up to 30 kpsi, tensions over 1,000,000 pounds, and
temperatures up to 175.degree. C. Of particular concern to the
piezoelectric stack 405 is the flexing of the tool structure under
various load conditions. These varying load conditions can affect
the mechanical energy output by the piezoelectric stack 405 as the
compressive load on the piezoelectric stack 405 varies. In the
extreme, the piezoelectric stack 405 can be depolarized due to
excessive compression caused by compression on the tool 106, or be
damaged when the stack compression falls below safe operating
levels during periods of high tension on the tool 106.
[0038] Because a selected compressive preload is applied to the
piezoelectric stack 405 by the spacers 409 via the preload spring
407, the piezoelectric stack 405 can be subjected to relatively
large variations in compressive load as the tool 106 is subjected
to changes in the drill string tension and compression during the
rig's operations. The amount of compressive preload applied to the
piezoelectric stack 405 by the preload spring 407 and spacers 409
can be selected by selecting the spring constant of the preload
spring 407 and selecting the number of spacers 409 between the
preload spring 407 and the pin 410. An appropriate compressive
preload maintains a positive compressive preload on the stack 405
over the entire range of tension and compression expected to be
applied to the telemetry tool 105 by the drill string during a
drilling operation. Determining the appropriate preload will be
evident to one skilled in art based on certain properties of the
drill string, borehole, reservoir, and drilling operation. Once the
appropriate preload is determined, a spring 407 with a suitable
spring constant and a suitable number of spacers 409 can be
selected to provide the appropriate preload.
[0039] Referring to FIG. 5 and according to another embodiment of
the acoustic transmitter 501, a retaining ring 509 is used instead
of spacers 409 to apply a compressive preload to a transducer 505
via a suitable preload spring 507. Like the first embodiment, this
alternative embodiment also comprises an enclosure having first and
second end couplings 502, 510, and an outer housing 503 and a
mandrel 504 that connect to the end couplings 502, 510 to form a
fluid-tight annular space 508 in which the transducer 505, preload
spring 507 and retaining ring 509 are housed. The retaining ring
509 is fixedly mounted to the inner surface of the outer housing
503 in a location that provides the desired compressive preload to
the transducer 505.
[0040] Referring to FIGS. 6(a) and (b) and according to another
embodiment, an acoustic transmitter 601 has the same elements as
the acoustic transmitter 501 shown in FIG. 5, and is further
provided with an acoustic tuning element 606 that serves to match
the acoustic impedance of the piezoelectric stack 605 with the
first end coupling 602, thereby maintaining optimal power output by
the acoustic transmitter 601. Like the embodiment shown in FIG. 5,
the acoustic transmitter 601 generally comprises an enclosure, a
transducer 605 comprising the piezoelectric stack, a preload spring
607, and a retaining ring 609 for applying an axial compressive
preload on the transducer 605 via the preload spring 607. The
enclosure comprises a first end coupling 602 with an inner face
611, a tubular outer housing 603, a cylindrical inner mandrel 604
and a second end coupling 610 ("pin"). The acoustic tuning element
606 has a metal tubular body with a first end for contacting the
piezoelectric stack 605 and an open second end 612.
[0041] The acoustic tuning element 606 is slid over the mandrel 604
such that the first end attaches to the piezoelectric stack 605 by
a threaded connection, while leaving an annular space 608 between
the outer surface of the mandrel 604 and the inner face of the
acoustic tuning element 606. The preload spring 607 is slid over
the mandrel 604 into the annular space 608 between the mandrel 604
and the acoustic tuning element 606 to contact the end of the
piezoelectric stack 605. The outer housing 603 is slid over the
assembly and threaded onto the external threads of the first end
coupling 602, and the retaining ring 609 is slid over the mandrel
604 and comprises external threads which engage with internal
threads of the outer housing 603 such that a compressive preload is
applied to the piezoelectric stack 605 via the preload spring 607;
consequently the piezoelectric stack 605 is compressed between the
preload spring 607 and the inner face 611 of the first end coupling
602. The retaining ring 609 does not contact the second end 612 of
the tuning element 606; therefore, the second end 612 of the tuning
element remains "open". The pin 610 is threaded into the outer
housing 603 and mandrel 604 to close and seal the annular space 608
but does not contribute to the preload on the piezoelectric stack
605.
[0042] The acoustic tuning element 606 comprises a resonant
structure that is tuned such that when it is attached to the end of
the piezoelectric stack 605 its acoustic impedance reduces the
piezoelectric stack 605 compliance at the frequencies being
transmitted, and restores the acoustic match between the
piezoelectric stack 605 and the first end coupling 603 without
affecting the preload applied to the piezoelectric stack 605 by the
preload spring 607.
[0043] For optimal acoustic output power, the piezoelectric stack
605 should be matched at either end with acoustic impedances equal
to that of the piezoelectric stack 605; however the additional
compliance of the preload spring 607 reduces the acoustic impedance
seen by the piezoelectric stack 605 at the end at which the preload
force is applied. The acoustic impedance of a segment of a cylinder
of length l can be determined using the four-pole matrix solution
to the wave equation. The four-pole solution can be written as:
[ F ( x + l ) V ( x + l ) ] = [ cos ( kl ) iz sin ( kl ) i sin ( kl
) z cos ( kl ) ] [ F ( x ) V ( x ) ] ##EQU00001##
in which
k = ( 2 .pi. f c ) , ##EQU00002##
where c is the wave speed which is defined as
c = E .rho. ##EQU00003##
where E is the Young's modulus of the cylinder material and .rho.
is the mass density of the material. The force at one end of the
cylinder at x+l can be written as
F(x+l)=F(x)cos(kl)+izV(x)sin(kl) Equation 1
in which z is the wave impedance of the cylinder which is defined
as z=.rho.ca, and a is the cross sectional area of the cylinder. In
the case of a cylinder with an open end F(x+l)=0, resulting in an
acoustic impedance at the opposing end of the cylinder of:
Z a ( x ) = F ( x ) V ( x ) = - iz sin ( kl ) cos ( kl ) = - iz tan
( kl ) Equation 2 ##EQU00004##
[0044] wherein i indicates the imaginary part of a complex number
and is defined as the sqrt(-1).
[0045] For example, a steel cylinder 3.2 m long and 0.1 m in
diameter and a 3800 mm.sup.2 cross sectional area can be used to
represent the combined acoustic impedance of a preload spring and
an acoustic tuning element; the acoustic impedance at a first end
of the cylinder given a free end at the second end of the cylinder
can be calculated using Equation 2. The resulting acoustic
impedance contains resonant peaks and nulls which occur at
frequencies corresponding to integer multiples of quarter
wavelengths of the first resonant frequency. FIG. 9(a) shows the
first resonance occurring at a cylinder length of l=.lamda./4. The
resonant impedance peak shown in FIG. 9(a) is too high to be of any
use, however the acoustic impedance level on the higher frequency
side of the resonance peak is low enough to be useful. FIG. 9(b)
shows the magnitude of the cylinder's acoustic impedance across the
third acoustic passband of the drill string as shown in FIG. 3. The
properties of the tuning element disclosed here is only one
possible example; the impedance behavior of the tuning element can
be controlled through choice of materials, the length of the tuning
element, the mass of the tuning element and the distribution of the
mass along the length of the tuning element.
[0046] FIG. 6(b) shows a detailed view of the internal components
of the acoustic transmitter 601. In particular, a first mechanical
interface 613 is shown between the first end coupling 602 and the
piezoelectric stack 605, and a second mechanical interface 615 is
shown between the piezoelectric stack 605 and both the cylindrical
acoustic tuning element 606 and the preload spring 607. At the
first mechanical interface 613, in an acoustically matched system
the acoustic impedance Z.sub.1 of the first end coupling 602 would
be the same as the acoustic impedance of the piezoelectric stack
605. This condition is also true for the acoustic impedance Z.sub.2
at the second mechanical interface 615. However if only the preload
spring 607 is applied then the compliance of the preload spring 607
is too high to offer the required acoustic impedance and the output
power of the piezoelectric stack 695 is reduced. The addition of
the acoustic tuning element 606 reduces the compliance of the
preload, restoring the acoustic impedance to the required value.
Ideally, the acoustic tuning element 606 has a selected impedance
that when combined with the acoustic impedance of the preload
spring 607, equals the acoustic impedance at the first mechanical
interface 613, i.e. the acoustic impedance of the first end
coupling 602.
[0047] To demonstrate, given a common piezoelectric material with a
density of 7.5 Mg/m.sup.3, and a Young's modulus of 9.9*1010
N/m.sup.2, then a piezoelectric stack with a length of 0.142 m and
a cross sectional area of 4200 mm.sup.2 will have a wave impedance
of 114 Kg/s. FIG. 10 shows that the combined acoustic impedance of
the tuning element and the preload spring (labeled "cylinder" in
FIG. 10) is equal to that of the piezoelectric stack at 640 Hz
("center frequency"), with a useable operating frequency bandwidth
across the 600 Hz to 700 Hz bandwidth of the third passband of the
drill string. In other words, the usable range of acoustic
impedance of the tuning element 606 in this example is between 70
kg/s and 160 kg/s for a selected operating frequency bandwidth of
600-700 Hz. While the usable operating frequency bandwidth of the
tuning element in this case is about 15% of the center frequency,
the usable operating frequency bandwidth and resulting usable
acoustic impedance range of the tuning element can vary based on
the physical properties of the piezoelectric stack and enclosure,
as well as on the operating conditions. Generally speaking, the
acoustic impedance of the tuning element can be within a selected
range that maximizes acoustic power transfer from the piezoelectric
stack into the enclosure over a selected usable operating frequency
bandwidth.
[0048] Instead of spacers 409 or a retaining ring 509, other types
of adjustable preload means can be used to provide a compressive
preload to the transducer via the preload spring. For example,
referring to FIG. 7 and according to another embodiment, an
acoustic transmitter 701 comprises a threaded nut 709 that is
mounted to a mandrel 704 to apply a selected compressive preload to
a transducer 705 via a preload spring 707. Like the previous
embodiments, an enclosure comprising first and second end couplings
702, 710, an outer housing 703 and the mandrel 704 provides a fluid
tight space 708 to house the transducer 705, preload spring 707,
and threaded nut 709. Optionally and as shown in FIG. 8, an
acoustic tuning element 706 similar to the previous embodiments can
be installed to match the acoustic impedance of the transducer 705
with the first end coupling 702, thereby maintaining optimal power
output by the acoustic transmitter 701.
[0049] While the illustrative embodiments of the present invention
are described in detail, it is not the intention of the applicant
to restrict or in any way limit the scope of the appended claims to
such detail. Additional advantages and modifications within the
scope of the appended claims will readily be apparent to those
skilled in the art. The invention in its broader aspects is
therefore not limited to the specific details, representative
apparatus and methods, and illustrative examples shown and
described. Accordingly, departures may be made from such details
without departing from the spirit or scope of the general
concept.
* * * * *