Abstract:
A system for determining the location of a movable element within a container is provided in which a linear variable differential transformer (“LVDT”) is formed with the container and the movable element therein. The LVDT includes a coil assembly including a primary or excitation winding, a secondary or output winding, and a movable element or core that is magnetically permeable. Measurement of an output signals allows for precise determination of the movable element location relative to the container. The system can be utilized to determine fluid volumes in accumulators used for controlling subsea equipment by monitoring the location of a movable element, e.g., a piston, within a hydraulic fluid accumulator.

Description:
BACKGROUND 
       [0001]    In most offshore drilling operations, a wellhead at the sea floor is positioned at the upper end of the subterranean wellbore lined with casing. A blowout preventer (“BOP”) stack is mounted to the wellhead, and a lower marine riser package (“LMRP”) is mounted to the BOP stack. The LMRP is connected to a drilling vessel located at the sea surface via a drilling riser that, in some cases, may be thousands of feet long. The drilling riser provides a conduit to extend a drill string from the surface vessel into the LMRP, the BOP stack, the wellhead and, ultimately, the wellbore. To accommodate movement of the vessel, LMRPs typically include a flex joint coupled to the lower end of the drilling riser. 
         [0002]    During drilling operations, drilling fluid, or mud, is pumped from the sea surface down the drill string and into the wellbore. The mud is circulated back to the surface in the annulus between the drill string and drilling riser. The mud facilitates drilling operations and provides a barrier against undesired formation-fluid release into the environment. In the event of a rapid invasion of formation fluid into the wellbore, commonly known as a “kick,” the BOP stack and/or LMRP may help seal wellbore and control the kick. In particular, the BOP stack typically includes closure members designed to help seal the wellbore and prevent the release of high-pressure formation fluids from the wellbore. Thus, the BOP stack functions as a pressure control device. 
         [0003]    In many subsea drilling operations, hydraulic fluid for operating the BOP stack and the LMRP is provided using a hydraulic fluid supply physically located on the surface drilling vessel. However, access to that supply may be lost, reducing the operability of the BOP stack. As a backup, or even possibly a primary means of operation, hydraulic fluid accumulators—located at the sea surface or subsea—are filled with pressurized hydraulic fluid. The amount and size of the accumulators depends on the anticipated operation specifications for the well equipment and the depth at which such equipment or accumulators will be located. 
         [0004]    One common type of accumulator is a piston accumulator. As the name suggests, a piston accumulator has a movable piston that separates a charged-gas section filled with an inert gas (e.g., nitrogen) and a hydraulic-fluid section filed with hydraulic fluid. The charged gas is pressurized and, thus, acts as a spring against the piston to maintain the hydraulic fluid under pressure. The fluid section is connected to a hydraulic circuit so that the hydraulic fluid may be used to operate the well equipment. As the fluid is discharged, the piston moves within the accumulator under pressure from the gas to maintain pressure on the remaining hydraulic fluid until full discharge. Thus, as fluid is discharged, the piston moves, making the gas section larger and the fluid section smaller. 
         [0005]    The ability of the accumulator to operate a piece of equipment depends on the amount of hydraulic fluid in the accumulator and the pressure of the charged gas. Thus, it is beneficial to know the volume of the hydraulic fluid remaining in an accumulator so that control of the well equipment may be managed. Measuring the volume of hydraulic fluid in the accumulator over time can also help identify if there is a leak in the accumulator or hydraulic circuit or on the gas side of the piston. 
         [0006]    Currently, the ability of an accumulator to power equipment is estimated by measuring the pressure in the hydraulic circuit downstream of the accumulator. However, pressure is not a complete indicator of the overall capacity of an accumulator to operate equipment, because the volume of hydraulic fluid remaining in the accumulator is not known. That is, the accumulator may have hydraulic fluid under sufficient pressure but not enough fluid to effectuate actuation of the system. Also, accumulators are typically arranged in banks of multiple accumulators all connected to a common hydraulic circuit, therefore, the downstream pressure measurement is only an indication of the overall pressure in the bank, not per individual accumulator. 
         [0007]    A possible way of determining the volume of hydraulic fluid remaining in the accumulator is to use a linear position sensor such as a cable-extension transducer or linear potentiometer that attaches inside the accumulator to measure the movement of the internal piston. However, these electrical components may fail and because the discharge of hydraulic fluid may be abrupt, the sensors may not be able to sample fast enough to obtain an accurate measurement. 
         [0008]    Another method of determining the volume of hydraulic fluid is through the use of physical position indicators that extend from the accumulator. These indicators only offer visual feedback though and are insufficient for remote monitoring and pose a significant challenge to maintaining the integrity of the necessary mechanical seals under full operating pressures. 
         [0009]    Through-the-wall sensors (e.g., Hall effect sensors) have also been considered. However, the thickness and specifications of an accumulator wall is such that these types of sensors are not always able to penetrate the material. 
       SUMMARY 
       [0010]    In accordance with certain embodiments of the invention, a system for determining the location of a movable element within a container is provided, this system provides a linear variable differential transformer (LVDT) formed with the container and the movable element therein. 
         [0011]    The exemplary measurement system includes a coil assembly including a primary or excitation winding, a pair of secondary or output windings (each wound differentially) coupled in series, and a movable element or core that is magnetically permeable. The excitation and output windings can be disposed in the interior of the container, in the walls of the container, on the exterior of the container, or proximate to but not in physical contact with the container. 
         [0012]    When the excitation winding is electrically excited by an excitation signal supplied from an excitation source, such as by a constant amplitude alternating current source, the output winding is inductively coupled to the excitation winding and produces an output signals that is based on the position of the movable core. A controller is configured to receive the output signals and produce a measurement signal. The measurement signal is indicative of the movable element&#39;s location relative to the container. 
         [0013]    In commercial embodiments, the invention can be utilized to determine fluid volumes in accumulators used for controlling subsea equipment by monitoring the location of a movable element within the accumulator, e.g., a piston, within a hydraulic fluid accumulator. This invention overcomes prior art systems because, among other reasons, it enables remote monitoring, maintains system integrity, and functions irrespective of the container wall thickness. 
         [0014]    This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
     
    
     
       DRAWINGS 
         [0015]    For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which: 
           [0016]      FIG. 1  shows a schematic view of an offshore system for drilling and/or producing a subterranean wellbore with an embodiment of a measurement system; 
           [0017]      FIG. 2  shows an elevation view of the subsea BOP stack assembly and measurement system of  FIG. 1 ; 
           [0018]      FIG. 3  shows a perspective view of the subsea BOP stack assembly and measurement system of  FIGS. 1 and 2 ; 
           [0019]      FIG. 4  shows a cross section view of an embodiment of a LVDT system for measuring the position of a movable element in a container; 
           [0020]      FIG. 5  shows a cross section view of another embodiment of a LVDT system for measuring the position of a movable element in a container; and 
           [0021]      FIG. 6  shows a cross section view of another embodiment of a LVDT system for measuring the position of a movable element in a container. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    The following discussion is directed to various embodiments of the invention. The drawing figures are not necessarily to scale. Certain features of the embodiments may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce the desired results. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment. 
         [0023]    Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness. 
         [0024]    In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices, components, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis. 
         [0025]    Referring now to  FIG. 1 , an embodiment of an offshore system  10  for drilling and/or producing a wellbore  11  is shown. In this embodiment, the system  10  includes an offshore vessel or platform  20  at the sea surface  12  and a subsea BOP stack assembly  100  mounted to a wellhead  30  at the sea floor  13 . The platform  20  is equipped with a derrick  21  that supports a hoist (not shown). A tubular drilling riser  14  extends from the platform  20  to the BOP stack assembly  100 . The riser  14  returns drilling fluid or mud to the platform  20  during drilling operations. One or more hydraulic conduits  15  extend along the outside of the riser  14  from the platform  20  to the BOP stack assembly  100 . The one or more hydraulic conduits  15  supply pressurized hydraulic fluid to the assembly  100 . A casing  31  extends from the wellhead  30  into the subterranean wellbore  11 . 
         [0026]    Downhole operations are carried out by a tubular string  16  (e.g., drill string, tubing string, coiled tubing, etc.) that is supported by the derrick  21  and extends from the platform  20  through the riser  14 , through the BOP stack assembly  100  and into the wellbore  11 . A downhole tool  17  is connected to the lower end of the tubular string  16 . In general, the downhole tool  17  may comprise any suitable downhole tools for drilling, completing, evaluating and/or producing the wellbore  11  including, without limitation, drill bits, packers, cementing tools, casing or tubing running tools, testing equipment, perforating guns, and the like. During downhole operations, the string  16 , and hence the tool  17  coupled to it, may move axially, radially and/or rotationally relative to the riser  14  and the BOP stack assembly  100 . 
         [0027]    Referring now to  FIGS. 1-3 , the BOP stack assembly  100  is mounted to the wellhead  30  and is designed and configured to control and seal the wellbore  11 , thereby reducing the likelihood of a release of undesired hydrocarbon fluids (i.e., liquids and gases) into the environment. In this embodiment, the BOP stack assembly  100  comprises a lower marine riser package (LMRP)  110  and a BOP or BOP stack  120 . The BOP stack  120  is releasably secured to the wellhead  30  as well as the LMRP  110 . The LMRP  110  is releasably secured to the BOP stack  120  and the riser  14 . 
         [0028]    In this embodiment, the BOP stack  120  comprises an annular BOP  113  as previously described, choke/kill valves  131  and choke/kill lines  132 . A main bore  125  extends through the BOP stack  120 . In addition, the BOP stack  120  includes a plurality of axially stacked ram BOPs  121 . Each ram BOP  121  includes a pair of opposed rams (e.g., shear rams, blind rams, variable-bore rams) and a pair of actuators  126  that actuate and drive the matching rams. In other embodiments, the BOP stack  120  may include a different number of rams, different types of rams, one or more annular BOPs or combinations thereof. As will be described in more detail below, the control pods  114  operate the valves  131 , the ram BOPs  121  and the annular BOPs  113  of the LMRP  110  and the BOP stack  120 . The control pods may be located at the sea surface on the vessel, or subsea near or mounted to the BOP stack. 
         [0029]    As shown in  FIG. 3 , the BOP stack  120  also includes a set or bank  127  of hydraulic accumulators  127   a  mounted on the BOP stack  120 . While the primary hydraulic pressure supply is provided by the hydraulic conduits  15  extending along the riser  14 , the accumulator bank  127  may be used to support operation of the rams  121   a, c  (i.e., supply hydraulic pressure to the actuators  126  that drive the rams  121   a, c  of the stack  120 ), the choke/kill valves  131 , the connector  50   b  of the BOP stack  120  and the choke/kill connectors  130  of the BOP stack  120 . As will be explained in more detail below, the accumulator bank  127  may serve as a backup means to provide hydraulic power to operate the rams  121   a, c,  the valves  131 , the connector  50   b,  and the connectors  130  of the BOP stack  120 . However, the accumulators could be designed to serve as the primary operators for the subsea equipment. 
         [0030]    As previously described, in this embodiment, the BOP stack  120  includes one annular BOP  113  and four sets of rams (one set of shear rams  121   a,  and three sets of pipe rams  121   c ). However, in other embodiments, the BOP stack  120  may include different numbers of rams, different types of rams, different numbers of annular BOPs (e.g., annular BOP  113 ) or combinations thereof. Further, although the LMRP  110  is shown and described as including one annular BOP  113 , in other embodiments, the LMRP (e.g., LMRP  110 ) may include a different number of annular BOPs (e.g., two sets of annular BOPs  113 ). Further, although the BOP stack  120  may be referred to as a “stack” because it contains a plurality of ram BOPs  121  in this embodiment, in other embodiments, BOP  120  may include only one ram BOP  121 . 
         [0031]    A container and measurement system  400  are shown in  FIG. 4 . It should be appreciated by those of skill in the art that the container may be any type of container with an internal volume and an element movable within the internal volume (e.g., piston or bellows type accumulators). In the embodiment illustrated in  FIG. 4 , the container  420  is a hydraulic accumulator that includes an element  401  movable within its internal volume, or cavity,  402 . The hydraulic accumulator  420  body is composed of an outer layer and an inner layer. The outer layer  409  of the accumulator  420  may include a metal, metal alloy and/or composite material (e.g., carbon fiber reinforced plastic). Composite materials are lighter than steel counterparts and possess high strength and stiffness, providing high performance in deep water, high pressure applications. The inner layer  410  of the accumulator  420  may include a metal and/or alloy. 
         [0032]    In the embodiment in  FIG. 4 , the movable element  401  is a piston separating a hydraulic fluid  403  from a gas  404  stored in the internal volume of the accumulator  420 . It should be appreciated by those of ordinary skill in the art that the movable element could be any device movable in an internal volume of a container that is capable of separating fluids. The piston  401  includes a magnetic core, including a magnetically permeable material, such as, for example, any magnetically permeable metal or alloy capable of inductively coupling with the excitation and output windings. The magnetic core of the piston  401  can constitute the entire piston, discrete surface areas of the piston, or any portion therebetween. 
         [0033]    Referring again to  FIG. 4 , the accumulator  420  further includes a linear variable differential transformer (“LVDT”) measurement system  400 . The measurement system includes an excitation winding  412  centered between a pair of output windings  414  within the wall, or between the outer and inner walls, of the accumulator  420 . In alternative embodiments, the windings may be positioned on the interior of the accumulator  420 , on the exterior of the accumulator  420 , or proximate to but not in physical contact with the accumulator  420 . In the embodiment illustrated in  FIG. 4 , the output windings  414  are symmetrically and sequentially spaced about the excitation winding  412 . In alternative embodiments, the excitation winding  412  and output windings  414  can be spaced concentrically with at least part of the exciting winding  412  and output winding  414  overlapping, or can have varying distances from the excitation winding. 
         [0034]    A power source  416  is coupled to the excitation winding  412  and adapted to supply an excitation signal to the excitation winding. The power source supplies alternating current power at a constant amplitude. The power source may include signal conditioning equipment. The excitation winding  412  is inductively coupled with the piston  401  as a result of the excitation signal, thereby generating a magnetic flux. The magnetic flux is coupled by the piston  401  to the output windings  414 . The piston  401  is movable along the longitudinal axis of the accumulator  420 . If the piston  401  is half way between the output windings  414 , equal magnetic flux is coupled to each output winding  414  so the voltage differential is zero. However, movement of the piston  401  along the longitudinal axis of the accumulator  420 , and relative to the excitation winding  412  and the output windings  414 , causes variations in the voltage differential across the output windings  414 . The variations in the voltage across the output windings  414  results in output signals that are converted by a controller ( 430 ) into a measurement signal that is indicative of the position of the piston  401  within the accumulator  420 . 
         [0035]    In the illustrated system, the location of the piston  401  can be determined based on measuring the voltage differential between the output signals supplied from the output windings  414 . The output signals supplied from the output winding  414  may be measured and analyzed by any device commonly understood in the art to measure such characteristics, such as current and/or voltage. For example, the system  400  may comprise a controller  430  that is coupled to the power source  416  and to the excitation and output windings. The controller  430  directs the power source to provide the excitation signal to the excitation winding. Subsequent movement of the piston changes the induced signals in the output windings, facilitating calculation of the piston&#39;s position by the controller  430 . With a pair of windings, the differential voltage between the output windings is measured by the controller  430  which produces a measurement signal that is used by the controller  430  to calculate the position of the piston in the accumulator. 
         [0036]    A container and measurement system  500  are shown in  FIG. 5 . It should be appreciated by those of skill in the art that the container may be any type of container with an internal volume and an element movable within the internal volume. In the embodiment illustrated in  FIG. 5 , the container can be a hydraulic accumulator  520  that includes an element  501  movable within its internal volume, or cavity,  502 . The hydraulic accumulator  520  body is composed of an outer layer and an inner layer. The outer layer  509  of the accumulators  520  may include a metal, metal alloy and/or composite material. Composite materials are lighter than steel counterparts and possess high strength and stiffness, providing high performance in deep water, high pressure applications. The inner layer  510  of the accumulator  520  may include a metal and/or metal alloy. 
         [0037]    In the embodiment in  FIG. 5 , the movable element  501  is a piston separating a hydraulic fluid  503  from a gas  504  stored in the internal volumes of the accumulators  520 . It should be appreciated by those of ordinary skill in the art that the movable element could be any device movable in an internal volume of a container that is capable of separating fluids. The piston  501  includes a magnetic core, including a magnetically permeable material, such as for example a metal. The magnetic core of the piston  501  can constitute the entire piston, discrete surface areas of the piston, or any portion therebetween. 
         [0038]    The accumulator  520  further includes a linear variable differential transformer measurement system  500 . The measurement system includes an excitation winding  512  which is centered between a pair of output windings  514  outside the outer layer  509  of the accumulator  520 . In the embodiment illustrated in  FIG. 5 , the output windings  514  are symmetrically and sequentially spaced about the excitation winding  512 . In alternative embodiments, the excitation winding  512  and output windings  514  can be spaced concentrically with at least part of the exciting winding  512  and output winding  514  overlapping. 
         [0039]    A power source  516  is coupled to the excitation winding  512  and adapted to supply an excitation signal to the excitation winding. A controller  530  is coupled to the output windings. The piston  501  can be inductively coupled to the excitation winding  512  and/or the output windings  514  when the excitation winding  512  is in an excited state. In a particularly preferred embodiment, the piston  501  is inductively coupled to both the exciting winding and the output winding  514 . The piston  501  is movable along the longitudinal axis of the accumulator  520 . Movement of the piston  501  along the longitudinal axis of the accumulator  520 , and relative to the exciting winding  512  and the output winding  514 , causes variations in the output signals supplied from the output winding  514 . 
         [0040]    The location of the piston  501  can be determined based on measuring the output signals supplied from the output winding  514 . The output signals supplied from the output winding  514  may be measured and analyzed by any device commonly understood in the art to measure such characteristics, such as current and/or voltage. 
         [0041]    A container and measurement system  600  are shown in  FIG. 6 . It should be appreciated by those of skill in the art that the containers may be any type of container with an internal volume and an element movable within the internal volume. In the embodiment illustrated in  FIG. 6 , the containers are hydraulic accumulators  620  that include an element  601  movable within their internal volume, or cavity,  602 . The hydraulic accumulator  620  body is composed of an outer layer and an inner layer. The outer layer  609  of the accumulators  620  may include a metal, metal alloy and/or composite material. Composite materials are lighter than steel counterparts and possess high strength and stiffness, providing high performance in deep water, high pressure applications. The inner layer  610  of the accumulators  620  may include a metal and/or metal alloy. 
         [0042]    In the embodiment in  FIG. 6 , the movable element  601  is a piston separating a hydraulic fluid  603  from a gas  604  stored in the internal volumes of the accumulators  620 . It should be appreciated by those of ordinary skill in the art that the movable element could be any device movable in an internal volume of a container that is capable of separating fluids. The piston  601  includes a magnetic core, including a magnetically permeable material, such as for example a metal. The magnetic core of the piston  601  can constitute the entire piston, discrete surface areas of the piston, or any portion therebetween. 
         [0043]    Referring again to  FIG. 6 , the accumulator  620  further includes a linear variable differential transformer measurement system  600 . The measurement system includes an excitation winding  612  which is centered between a pair of output windings  614  inside the inner layer  610  of the accumulator  620 . In the embodiment illustrated in  FIG. 6 , the output windings  614  are symmetrically and sequentially spaced about the excitation winding  612 . In alternative embodiments, the excitation winding  612  and output windings  614  can be spaced concentrically with at least part of the exciting winding  612  and output winding  614  overlapping. 
         [0044]    A power source  616  is coupled to the excitation winding  612  and adapted to supply an excitation signal to the excitation winding. A controller  630  is coupled to the output windings. The piston  601  can be inductively coupled to the excitation winding  612  and/or the output windings  614  when the excitation winding  612  is in an excited state. In a particularly preferred embodiment, the piston  601  is inductively coupled to both the exciting winding and the output winding  614 . The piston  601  is movable along the longitudinal axis of the accumulator  620 . Movement of the piston  601  along the longitudinal axis of the accumulator  620 , and relative to the exciting winding  612  and the output winding  614 , causes variations in the output signals supplied from the output winding  614 . 
         [0045]    The location of the piston  601  can be determined based on measuring the output signals supplied from the output winding  614 . The output signals supplied from the output winding  614  may be measured and analyzed by any device commonly understood in the art to measure such characteristics, such as current and/or voltage. 
         [0046]    Although the present invention has been described with respect to specific details, it is not intended that such details should be regarded as limitations on the scope of the invention, except to the extent that they are included in the accompanying claims.