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CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit under 35 USC §119 of the filing date of international application PCT/US02/27861 filed Sep. 3, 2002, the disclosure of which is incorporated herein by this reference. 
   TECHNICAL FIELD 
   The present invention relates generally to subterranean well apparatus and, in an embodiment described herein, more particularly provides an improved acoustic transmission system for use in subterranean well applications. 
   BACKGROUND 
   In subterranean well completions, of both surface and subsea types, a metal tubing structure such as production tubing is typically supported from an appropriate metal hanger structure and extends downwardly therefrom through the wellbore portion of the completion which is normally lined with a metal casing structure. It is often desirable to monitor the state of various downhole well parameters such as, for example but not by way of limitation, the temperatures and pressures within the tubing and external to the tubing in an annular space defined between the tubing and the casing. Many times the desired sensing locations for these well parameters are thousands of feet downhole. Thus, signals indicative of the sensed well parameters must correspondingly be tubing wall-transmitted upwardly through great distances via the wellbore (and a lengthy undersea riser in a subsea application) to a predetermined signal receiving location. 
   Various techniques have previously been proposed for generating and transmitting these well parameter signals. One such technique has been to transmit acoustic signals upwardly through the downhole metal wall portion of the tubing structure and then to the signal receiving location, via the wall portion of the remainder of the tubing structure, for conversion to, for example, digital or analog electrical signals. 
   A substantial impediment to successfully utilizing this acoustic-based signal transmission technique has been the necessary presence of a metal hanger structure from which the metal tubing structure is supported. In a subsea application, this metal hanger structure is typically a fluted hanger assembly, and in a surface application it is typically a slip structure. In either case, due to the metal-to-metal contact between the hanger structure and the tubing the hanger structure substantially dissipates an acoustic signal reaching it via a downhole portion of the tubing wall. 
   Accordingly, the acoustic signal reaching the tubing wall section uphole of the hanger structure is substantially weakened. In the case of a subsea well application, this weakened signal may then have to travel thousands of feet upwardly through the tubing wall above the hanger structure to reach the signal receiving location. Thus, the through-tubing acoustic transmission of downhole well parameter signals to a signal receiving location uphole of the well completion hanger structure has proven difficult, and in many applications unfeasible, to implement. A need thus exists for an improved acoustic-based signal transmission system in a well completion. A need additionally exists to transmit acoustical signals downwardly past the hanger structure, to a downhole location, to actuate devices and reconfigure acoustic transmission devices for better communications. 
   SUMMARY 
   In carrying out the principles of the present invention, in accordance with an embodiment thereof, a subterranean well completion is provided which comprises a wellbore extending into the earth, a tubular structure extending into the wellbore, and an acoustic energy dissipating well structure, representatively a hanger structure, which engages the tubular structure, with an upper portion of the tubular structure extending upwardly from the hanger structure, and a lower portion of the tubing structure extending downwardly from the hanger structure and through the wellbore. 
   The well completion, which may be a subsea completion or a surface-based completion, further comprises a specially designed signal transmission system operable to transmit an acoustic signal, representatively a downhole well parameter signal, upwardly through the lower tubing structure section toward the hanger structure from a downhole location, convert the acoustic signal to a non-acoustic signal at a location on the lower tubing structure section below the hanger structure, and transmit the converted, non-acoustic signal from an output section of the signal transmission system through a signal path structure coupled between the output section and a signal receiving location disposed above the hanger structure. Since the initially acoustic downhole well parameter signal is converted to a non-acoustic signal below the hanger structure, the substantial acoustic dissipation characteristic of the hanger structure does not appreciably weaken the signal eventually reaching the signal receiving location. 
   In an illustrated embodiment thereof, a signal transmission apparatus portion of the overall signal transmission system includes a lower transceiver structure connected in the lower tubing structure section below the hanger and operative to acoustically transmit the predetermined well parameter signal upwardly through the lower tubing structure section toward the hanger structure, and an upper transceiver structure, having a transceiver portion and a signal converting portion, disposed in the lower tubing structure portion between the hanger structure and the lower transceiver structure. The upper transceiver structure is representatively of a tubular configuration and has an axial bore with a diameter substantially equal to that of the lower tubing structure section, and receives the acoustic signal, converts it to a non-acoustic form, and outputs the converted signal to the signal path structure. The converted signal may, for example but not by way of limitation, be a digital or analog electric signal, a photoelectric signal, or an electromagnetic signal. 
   In one version of the well completion, the signal path includes a signal cable structure extending through the hanger structure and routed upwardly along the upper tubing structure section and through and/or around various well components mounted in the upper tubing structure section. In another version of the well completion, the signal path structure extends externally around the hanger structure and representatively includes a portion of the earth adjacent the upper transceiver structure. In this version, incorporated in a subsea embodiment of the well completion, the upper transceiver structure outputs electromagnetic wave signals which are propagated through the earth and received by a transmitter disposed on the sea bed and having an output cable for transmitting the received signal upwardly through the water to the signal receiving location. 
   Preferably, the signal transmission system is also capable of downwardly transmitting control signals, via the signal path structure and the tubing structure, to the lower transceiver structure to modify various aspects of the signal transmission system, including but not limited to changing the predetermined sensed downhole well parameter, changing the parameter value range associated with the downhole well parameter, changing the type of data transmitted by the lower transceiver structure, and changing the type of data transmitted by the lower transceiver structure. In addition, the downward transmission of control signals could be utilized to actuate downhole actuators such as valves or pumps to modify well test parameters. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic cross-sectional view through a portion of a representative subsea subterranean well completion having incorporated therein a specially designed acoustic transmission system embodying principles of the present invention; 
       FIG. 2  is an enlargement of a portion of the  FIG. 1  well completion; 
       FIG. 3  is a schematic cross-sectional view of a portion of a first alternate embodiment of the  FIG. 1  well completion; 
       FIG. 4  is a schematic cross-sectional view of a portion of a second alternate embodiment of the  FIG. 1  well completion; 
       FIG. 5  is a schematic cross-sectional view of a portion of a third alternate embodiment of the  FIG. 1  well completion; and 
       FIG. 6  is a schematic partly cross-sectional, partly elevational view of a non-subsea version of the  FIG. 1  subterranean well completion. 
   

   DETAILED DESCRIPTION 
   Representatively and schematically illustrated in  FIGS. 1 and 2  are longitudinal portions of a subsea subterranean well completion  10  which embodies principles of the present invention. In the following description of the well completion  10  and other apparatus and methods described herein, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used only for convenience in referring to the accompanying drawings. Additionally, it is to be understood that the various embodiments of the present invention described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present invention. 
   With reference to  FIGS. 1 and 2 , the well completion  10  includes a representatively vertical wellbore  12  extending downwardly from the sea bed  14  into the underlying earth  16 , the wellbore  12  being lined with a tubular metal casing  18  extending downwardly form the sea bed  14 . A smaller diameter metal tubing structure  20  extends centrally through the casing  18  and forms therewith an annulus  22  laterally circumscribing the tubing  20 . As illustrated, the tubing  20  has an upper section that extends upwardly from the sea bed  14  sequentially through an undersea wellhead/blowout preventer structure  24  and a tubular riser  26  extending upwardly from the structure  24  through the water  28  to a rig floor  30 . 
   Operatively mounted in the section of the tubing  20  above the sea bed  14 , and of conventional construction, are (from bottom to top as viewed in  FIGS. 1 and 2 ) a longitudinally ported tubular slick joint  32 , a subsea test tree  34 , and an electrohydraulic module  36 , the structures  32  and  34  being disposed within the wellhead/BOP (blow out preventer) structure  24 , and the structure  36  being in the riser  26  above the wellhead/BOP  24 . Disposed within the wellhead/BOP  24  are conventional ram and shear ram sets  38 , 40  that respectively oppose the slick joint  32  and a section of the tubing  20  between the test tree  34  and the electrohydraulic module  36 . 
   Operatively disposed at sea bed level beneath the slick joint  32  is a conventional metal fluted tubing hanger structure  42  that includes a metal hanger member  44  anchored to the tubing  20 , and a metal wear bushing structure  46  complementarily engaged by the metal hanger member  44 . In a manner subsequently described herein, downhole well parameters (such as, but not limited to, pressures and temperatures within the tubing  20  and the annulus  22 ) are sensed and acoustic signals indicative of the sensed downhole well parameters are responsively transmitted upwardly through the metal wall of the downhole section of the tubing  20 . 
   Conventional attempts to utilize acoustic well parameter indicating signals transmitted through the tubing, and ultimately received at an uphole signal converting station, have typically been frustrated by the presence of the hanger structure  42  which, due to its metal-to-metal contact with the tubing  20 , substantially dissipates an acoustic signal traveling through the tubing upwardly through the hanger structure. Simply stated, the attenuated acoustic signal exiting the hanger structure via the tubing section above the hanger structure tends to be too weak to be useful. 
   To overcome this problem, the present invention incorporates in the well completion a specially designed acoustic-based signal transmission system which, as will now be described, generates acoustic well parameter signals in the wellbore below the hanger structure  42 , transmits the acoustic signals upwardly through the tubing  20  to a conversion point therein downhole of the hanger structure  42  at which the acoustic signals are converted to a non-acoustic form, and then transmits the converted signals to a signal receiving location uphole from the hanger structure  42 . In this manner the undesirable acoustic attenuation properties of the hanger structure  42  do not adversely affect the quality and strength of the well parameter signals ultimately reaching the signal receiving location. 
   With continuing reference to  FIGS. 1 and 2 , the acoustic transmission system includes a first acoustic transceiver structure  48  (see  FIG. 1 ) which is of a suitable conventional construction and is representatively secured to the lower end of the tubing  20  within the cased wellbore  12 . Transceiver or well tool structure  48  functions to monitor at least one downhole well parameter and responsively transmit an acoustic signal, which is indicative of the value of the sensed parameter, upwardly through the metal wall of the tubing  20  toward the hanger structure  42 . 
   The acoustic transmission system also includes a second acoustic transceiver structure  50  which is secured in-line in the tubing  20  above the transceiver  48  and somewhat below the hanger structure  40 . In a simplified uplink system, the second transceiver structure could consist of a suitable acoustic wave measurement sensor, and a signal amplifier, and a suitable packaging structure. The acoustic measurement sensor would convert the acoustic signals into non-acoustic signals, preferably electrical signals. The electrical signals could be amplified and transported to the surface by the signal amplifier. Equipment at the surface would decode the signals to obtain the downhole well parameters. 
   The transceiver structure  50  schematically depicted in  FIGS. 1 and 2  representatively includes an acoustic transceiver  52  and an associated signal converter section  54 . Transceiver  52  representatively has a resonant stack construction similar to a transceiver construction illustrated in U.S. Pat. No. 6,137,747 which is hereby incorporated herein by reference. A central circular bore  56 , having a diameter substantially identical to that of the interior of the tubing  20 , axially extends through the acoustic transceiver structure  50  between its upper and lower ends. Representatively, a suitable conventional acoustic signal repeater  58  (see  FIG. 1 ) is mounted in the tubing  20  between the first and second acoustic transceiver structures  48 , 50 . 
   During operation of the acoustic transmission system, at least one sensed well parameter signal is transmitted, in acoustic form, upwardly from the first acoustic transceiver structure  48 , through the metal wall of the tubing  20 , to the repeater  58  which, in turn, sends a corresponding acoustic signal through the tubing wall to the transceiver portion  52  of the upper acoustic transceiver structure  50 . 
   According to a key aspect of the present invention, the signal converter section  54  of the upper transceiver structure  50 , which is disposed below the hanger structure  42 , receives these acoustic signals and converts them to non-acoustic signals such as, for example, digital electrical signals, analog electrical signals or photoelectric signals. These converted, non-acoustic signals are then transmitted to a remote signal receiving location (not illustrated) disposed, for example, on the rig (offshore) or wellsite (onshore). As illustrated in  FIGS. 1 and 2 , these converted, non-acoustic signals are routed upwardly from the signal converter portion  54  of the upper transceiver structure  50  to the signal receiving location via a signal transmission cable structure  60 . Because acoustic signals are not passed upwardly through the hanger structure  42  (which, as previously discussed herein, is a structure which would otherwise greatly dissipate tubing-carried acoustic signals passing upwardly therethrough), the hanger structure  42  does not appreciably weaken well parameter and audio signals ultimately reaching the signal receiving location. 
   From its connection to the signal converter portion  54  the cable  60  sequentially passes upwardly through the hanger member  44 , upwardly through a vertical sidewall port in the ported tubular slick joint  32 , upwardly around the exterior of the subsea test tree  34 , and upwardly along the exterior of an adjacent section of the tubing  20  to a cable connection portion  62  of the electrohydraulic module  36 . From the electrohydraulic module  36  the converted signals are routed to the signal receiving location via electrohydraulic cabling  64  wrapped around an upper end portion of the tubing  20  and operatively connected to an electrohydraulic reel  66  (see  FIG. 1 ) disposed on the rig. From the reel  66  the converted signals are routed to the signal receiving location via a schematically depicted electrical wire connection  68  coupled to the reel  66 . Thus, as to the acoustic downhole well parameter and audio signals there is an acoustic signal transmission path disposed beneath the hanger structure  42 , and a non-acoustic signal path which extends upwardly past the hanger structure  42  and forms at least a portion of the remaining signal path routed to the signal receiver location. 
   While this non-acoustic signal transmission path has been representatively depicted herein as being a cabled path, extending clear to the surface and carrying electric or photoelectric converted signals, other types of non-acoustic signal transmission paths could alternatively be provided above the hanger structure or other source of substantial attenuation of through-tubing acoustic signal strength. For example, as subsequently discussed herein, this non-acoustic signal transmission path extending above the hanger structure could include an electromagnetic path emanating from the signal converter  54 . Alternatively, once the converted non-acoustic signal path upwardly passes the hanger structure  42 , the non-acoustic signal could be re-converted to acoustic form and transmitted through an upper portion of the tubing  20  (as indicated by the dashed arrow “A” in  FIG. 2 ) to the surface. 
   Since the signal transmission components  48 , 50  are both transceiver structures they are, of course, capable of both transmitting and receiving signals. In the well completion  10  representatively depicted in  FIGS. 1 and 2 , various control signals may also be transmitted (from the signal receiving location) through the overall illustrated signal path downhole to the lower transceiver structure  48 . These control signals are sequentially transmitted in non-acoustic form through the cabling  62 , 60  through the hanger member  44 , and then converted to acoustic form by the signal converter  54  and acoustically transmitted downwardly through the tubing wall, via the repeater  58 , to the lower transceiver structure  48 . The control signals sent in this manner to the transceiver structure  48  may be utilized in a variety of manners including, for example but not by way of limitation, to change in the lower transceiver structure the sensed downhole well parameter(s), the ranges of parameter value(s) sensed, the transmission frequency, or the type of data transmitted. 
   The representative signal transmission system just described may be incorporated in a variety of well completions having configurations different than that shown in  FIGS. 1 and 2 . For example, the subsea well completion embodiment  10   a  shown in  FIG. 3  does not have an electrohydraulic module such as the electrohydraulic module  34  shown in  FIG. 2 . Accordingly, above the subsea test tree  34 , the cable  60  is wrapped around the tubing  20  and extended to the surface for routing to the signal receiving location. 
   The subsea well completion embodiment  10   b  shown in  FIG. 4  is similar to that shown in  FIG. 3 , with the exception that the subsea test tree  34  has a built in electrical feed-through portion  70  to which portions of the cable  60  above and below the feed-through portion  70  are operatively connected. 
   As previously mentioned herein, the converted signal path which, in effect, “bypasses” the undesirable acoustic attenuation of the hanger structure  42  is not limited to a wholly or partly electrical or photoelectric nature. For example, in the subsea well completion embodiment  10   c  shown in  FIG. 5 , the signal converter portion  54  of the upper transceiver structure  50  is operative to convert its received acoustic signals to electromagnetic waves  72  which are transmitted through the earth  16  to a suitable transceiver structure  74  located on the sea bed  14  and coupled to a cable structure  76  extending upwardly through the water  28  to the signal receiving location. Upon receiving the electromagnetic signals  72 , the transceiver structure  74  converts them to suitable electrical form for upward transmission through the cable structure  76 . Of course, signals may also be transmitted downwardly through this overall transmission path to the upper transceiver structure  50  for transmission therefrom to the lower transceiver structure  48 . 
   The signal transmission system of the present invention may also be incorporated in a land-based well completion such as the well completion embodiment  10   d  schematically depicted in  FIG. 6 . In this well completion, in which a rig floor  78  is disposed above the earth&#39;s surface  80 , and the tubing  20  extends upwardly from the ported tubular slick joint  32  to schematically depicted surface equipment  82 , the acoustic-attenuating hanger structure is defined by metal slips  84  which engage the slick joint  32 . In well completion  10   d , the portion of the cable  60  upwardly exiting the slick joint  32  is appropriately routed to the signal receiving location. 
   The foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims.

Summary:
In a subterranean well completion a bi-directional signal transmission system includes an in-line acoustic transceiver mounted in a tubing string extending through the wellbore, the transceiver being disposed beneath a hanger structure engaging the tubing string. Via the tubing string the transceiver receives acoustic signals from well parameter sensing apparatus further downhole and converts the received acoustic signals to non-acoustic signals. The resulting non-acoustic signals are then transmitted upwardly through the hanger structure, to a signal receiving location, via cabling. In this manner, the hanger structure does not adversely affect the strength of either upwardly or downwardly transmitted signals traversing it. Alternatively, the acoustic well parameter signals received by the transceiver are converted to electromagnetic signals which pass through the earth, are picked up by a receiver external to the well completion, and then relayed to the receiving location.