Abstract:
A pressure mitigating device is used to reduce pressure in a void within a wellhead housing. In one embodiment, the pressure mitigating device includes two plates that define a void between them. Increased pressure in the wellhead housing causes the plates to elastically displace towards each other. The plates contact each other, limiting the displacement prior to plastic deformation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates in general to a method and apparatus to mitigate trapped pressure in a wellhead and in particular to a compressible pressure limiting device for limiting pressure from a void typically located between two crown plugs in a wellhead tree system. 
         [0003]    2. Brief Description of Related Art 
         [0004]    A horizontal subsea tree has a production outlet extending generally horizontally, in relation to the wellbore, and a bore that is axially aligned with the wellbore. A tubing hanger lands in the horizontal tree and supports a string of tubing extending into the wellbore. The tubing hanger has a vertical passage and a lateral passage extending from the vertical passage and registering with the production outlet of the tree. In some installations an internal tree cap lands in the tree above the tubing hanger, the tree cap normally having a vertical passage that aligns with the vertical passage in the tubing hanger. As a dual safety barrier, a wireline deployed crown plug is installed in the vertical passage of the tubing hanger and another crown plug is installed in the vertical passage of the tree cap. In other installations, the internal tree cap is omitted. In that case, the vertical passage of the tubing hanger is typically plugged with two crown plugs to meet requirements of having dual safety barriers. 
         [0005]    Fluid, such as, for example, completion fluid, may be trapped in the vertical passage between the two plugs. The fluid may be relatively cold when it is trapped because the subsea temperature is relatively cold. During production, the well fluid flowing through portions of the wellhead is at a higher temperature and subsequently heats the subsea wellhead. As the fluid trapped between the crown plugs heats up and is restricted from expanding, the trapped fluid pressure can potentially increase above the working pressure of the crown plugs and, thus, damage the integrity of the crown plugs. It is thus desirable to limit the pressure in the void between the crown plugs, without releasing the fluid trapped between the plugs into the environment. 
       SUMMARY OF THE INVENTION 
       [0006]    A pressure compensating device can be used to mitigate the pressure increase that can occur when fluid tries to thermally expand in a confined space. In one embodiment, the pressure compensator is located in a wellhead assembly that has a cylindrical bore, a first plug located in and sealingly engaging the cylindrical bore and a second plug located in and sealingly engaging the cylindrical bore. The second plug can be spaced axially apart from the first plug, and thus the cylindrical bore, the first plug, and the second plug define a cavity. Trapped fluid can be retained in the cavity. To mitigate the pressure increase, a pressure compensator having a pair of plates (a first plate and a second plate) can be located within the cavity. The pair of plates can define a void between them and a compressible fluid can be located within the void. When the volume of the wellbore fluid in the cavity increases, it can cause the plates to deflect inward, toward each other. 
         [0007]    The inward deflection of the first plate, into the void, can be limited by the second plate such that the first plate does not plastically deform prior to being so limited by the second plate. In one embodiment, the inward deflection of the second plate, into the void, is limited by the first plate such that the second plate does not plastically deform prior to being so limited by the first plate. A cylindrical ring can connect the first plate and second plate and thus define an outer diameter of the void. One or both plates can have a concave surface in relaxed state. Alternatively, one or both plates can have a generally flat surface in its relaxed state. The plates can be made of any of a variety of materials including, for example, metal, polymer, or elastomer. The void between the plates can be filled with a compressible fluid including, for example, a gas such as air, nitrogen, or argon. In one embodiment, the void can be at negative pressure, less than atmospheric pressure, when the plates are in their relaxed state. 
         [0008]    The compensator assembly can be located in a frame, or cage, that can be placed in the cavity. The frame can have a sidewall with an aperture so that wellbore fluid can pass through the aperture, into the frame, and thus reach the surface of the compensator plates. More than one compensator can be located in the cavity and, indeed, more than one compensator can be located in a single frame. If more than one compensator is used, a gap can exist between the plates of the two compensators so that wellbore fluid can reach the exterior surfaces of those plates. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    So that the manner in which the features, advantages and objects of the invention, as well as others which will become apparent, are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and is therefore not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
           [0010]      FIG. 1  is a sectional view of a subsea horizontal tree having an exemplary embodiment of a pressure compensating device. 
           [0011]      FIG. 2  is a sectional view of an embodiment of the pressure compensating device of  FIG. 1 , showing the plates of the pressure compensating device in a relaxed state. 
           [0012]      FIG. 3  is a sectional view of an embodiment of the pressure compensating device of  FIG. 1 , showing the plates of the pressure compensating device in a compressed state. 
           [0013]      FIG. 4  is a sectional view of another embodiment of the pressure compensating device of  FIG. 1 , showing concave plates of the pressure compensating device in a relaxed state. 
           [0014]      FIG. 5  is a sectional view of the pressure compensating device of  FIG. 1 , showing an embodiment having a frame and a plurality of pressure compensating devices. 
           [0015]      FIG. 6  is a sectional view of another embodiment of the pressure compensating device of  FIG. 1 , showing an embodiment having a frame and a plurality of pressure compensating devices. 
           [0016]      FIG. 7  is a partial sectional view of another embodiment of the pressure compensating device of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings which illustrate embodiments of the invention. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and the prime notation, if used, indicates similar elements in alternative embodiments. 
         [0018]    Referring to  FIG. 1 , Christmas tree  100  is of a type known as a horizontal tree. It has a tree block  101  with a vertical or axial tree bore  102  extending completely through it. A set of grooves  104  is located on the exterior near the upper end for connection to a drilling riser (not shown). A removable corrosion cover  106  fits over the upper end of tree  100 . Tree  100  has a lateral production passage  108  that extends generally horizontally from bore  102  and is controlled by a valve  110 . Tree  100  will be landed on top of a wellhead housing (not shown), which supports casing extending into a well. 
         [0019]    The tree  100  has an inner wellhead assembly  111  housed within the axial bore  102  of the tree  100 . A tubing hanger  112  lands sealingly in bore  102 . Tubing hanger  112  is secured to tree  100  by a lock down mechanism  114 . A string of production tubing  116  extends through the casing hangers (not shown) into the well for the flow of production fluid. Production tubing  116  is secured to tubing hanger  112  and communicates with a vertical passage  122  that extends through tubing hanger  112 . A lateral passage  124  extends from vertical passage  122  and aligns with tree lateral passage  108 . 
         [0020]    A lower wireline retrievable plug  126 , or crown plug, will lock in vertical passage  122  above lateral passage  124 , sealing the upper end of vertical passage  122 . Seals can form a seal between plug  126  and tubing hanger  112 , and dogs, or other types of locking devices, may be used to lock plug  126  in place. In this example, a tree cap  128  inserts sealingly into tree bore  102  above tubing hanger  112 . Tree cap  128  has a downward depending isolation sleeve  130  that is coaxial. Sleeve  130  fits within a receptacle  132  formed on the upper end of tubing hanger  112 . The interior of sleeve  130  communicates with an axial passage  144  that extends through tree cap  128 . Axial passage  144  has approximately the same inner diameter as tubing hanger passage  122 . 
         [0021]    An upper wireline retrievable crown plug  146  inserts into tree cap passage  144 . Various seals can provide sealing between components within tree  100  including, for example, metal seal  148  on crown plug  146 , which can engage a surface in passage  144 . Dogs, or other types of locking mechanisms, can be used to lock upper crown plug  146  in place. Upper crown plug  146  is a redundant plug for further sealing passage  144 , the primary seal being formed by lower plug  126 . Upper crown plug  146  and lower plug  126 , thus, form dual safety barriers against gas or liquids that may pass up through vertical passage  122 . Any type of upper and lower plug can be used to form such safety barriers. 
         [0022]    Cavity  150  is a space within tree  100  having a circumference defined by passage  144  and ends defined by lower plug  126  and seal  148  of crown plug  146 . Cavity  150  may also include the volume associated with bores or recesses on the top of lower plug  126  or the bottom of crown plug  146 . Completion fluids can be trapped in cavity  150  when upper crown plug  146  is sealed in place, which is after tree  100  is installed subsea. Once upper crown plug  146  is installed, the fluid pressure of cavity  150  will not necessarily remain at the hydrostatic pressure of the seawater surrounding tree  100 . 
         [0023]    The trapped wellbore fluids can thermally expand within cavity  150 , causing an increase in pressure. Compensator  152  can be located within cavity  150  to mitigate such pressure increases. Tree  100  is one example of a wellhead assembly. Compensator  152  can be used in any type of wellhead assembly having a cavity which can contain fluids. 
         [0024]    Referring to  FIG. 2 , compensator  152  ( FIG. 1 ) can include plate assembly  154 . In one embodiment, plate assembly  154  can have a deformable member, such as upper plate  156 , and a support member, such as lower plate  158 . In one embodiment, the support member, such as lower plate  158 , can be deformable. Similarly, in one embodiment, the deformable member, such as upper plate  156 , can act as a support member. Each plate  156 ,  158  can be made of any of a variety of materials including, for example, metal, plastic, or polymer. Perimeter  160  can separate plate  156  from plate  158 , thus defining void  164 . Plates  156 ,  158  and perimeter  160  can be made of a unitary material or can be individual pieces that are connected to one another. Compensator  152  can be constructed such that void  164  is generally sealed and, thus, does not permit ingress or egress of gas or liquids. 
         [0025]    Void  164  can contain a compressible fluid. The fluid can be, for example, a gas such as air, argon, or nitrogen. Alternatively, the fluid can be a liquid. In one embodiment, the liquid has a high boiling point so that it does not expand significantly when heated. Alternatively, void  164  can contain a mixture of different types of fluids including, for example, multiple gases or combinations of gas and liquid. In another embodiment, void  164  can be evacuated such that the initial pressure is below ambient pressure. Plates  156 ,  158  can be sufficiently rigid that they generally maintain their shape when void  164  is evacuated. Fill valve  165  can be used to evacuate fluids from void  164  and introduce fluids into void  164 . 
         [0026]    Plates  156  and  158  can be generally flat and parallel to each other. In one embodiment, plates  156  and  158  can remain generally flat and parallel to each other at a first external pressure within cavity  150 . For example, the initial pressure in cavity  150 , prior to thermal expansion, may be insufficient to alter the shape of plates  156  and  158 , even though such initial pressure is greater than atmospheric pressure. The pressure of the fluid in void  164  or the rigidity of plates  156  and  158  can contribute to the plates remaining generally flat up to a certain external pressure. In one embodiment, the first external pressure can be the hydrostatic pressure of the seawater at the tree  100 . 
         [0027]    Referring  FIG. 3 , when the external pressure reaches a second pressure, plates  166  and  168  can move toward each other, thereby compressing the fluid in void  169 . In the embodiment shown in  FIG. 3 , a portion of upper plate  166  has moved toward lower plate  168 . A portion of lower plate  168  has also moved toward upper plate  166 . The travel distance  170  of upper plate  166  is limited by contacting the interior surface  172  of lower plate  168 . Lower plate  168  can stop the movement of upper plate  166  before upper plate  166  plastically, or permanently, deforms. Thus, the deformation of upper plate  166 , through travel distance  170 , is limited to elastic deformation. Likewise, a portion of lower plate  168  can move toward upper plate  166 . Upper plate  166  can limit the movement of lower plate  168  to elastic deformation. Fluid in void  164  can be compressed when plates  166 ,  168  deflect inward toward each other. The pressure of the compressed fluid in void  164  can limit the deformation of plates  166  and  168 , thereby allowing only elastic deformation. 
         [0028]    Referring to  FIG. 4 , in one embodiment of compensator  174 , upper plate  176  and lower plate  178  can have a generally concave shape in their relaxed state. As with other embodiments, void  180  can be located between plates  176  and  178 , and can be filled with a fluid. External pressure within cavity  150  ( FIG. 1 ) on plates  176  and  178  can cause either or both plates to deflect inward, compressing any fluid located in void  180 . A cylindrical ring (not shown) can be located between plates  176  and  178  to increase the volume of void  180 . As with other embodiments, the movement of plates  176  and  178  can be limited to elastic deformation. In one embodiment, outer diameter  182  can increase as plates  176  and  178  are compressed. A frame, such as frame  186  ( FIG. 5 ) or an inner diameter of cavity  150  ( FIG. 1 ) can limit the radial expansion of outer diameter  182 , thus limiting the movement of plates  176  and  178 . 
         [0029]    Referring to  FIG. 5 , compensator assembly  184  can include frame  186  and one or more compensators  188 . Frame  186  can be an apparatus that holds one or more compensators  188 . Frame  186  can be, for example, a cylinder having annular retainer rings  190  for retaining compensators  188 . Compensators  188  can be spaced apart within compensator assembly  184 , thereby creating gaps  192  between compensators  188 . Gaps  192  can allow wellbore fluid to flow between compensators  188  and, thus, the wellbore fluid can act on the outer surfaces  194  of each compensator  188 . A variety of techniques can be used to establish gaps  192 . For example, spacer ring  196  can be an annular ring located between each compensator  188 . In one embodiment (not shown), spacers can be connected to or formed into the inner diameter of frame  186 . Apertures  198  can allow wellbore fluid to pass into gaps  192 . Apertures  192  can be, for example, vent holes, slots, or large openings through the sidewall rings  190  of frame  186 . 
         [0030]    Referring to  FIG. 6 , in one embodiment, frame  200  can include a cage for retaining compensators  202 . In this embodiment, frame  200  can be wire or metal rods configured to support compensators  202  but still allow wellbore fluid to pass into gaps  204 . 
         [0031]    Referring back to  FIG. 2 , in operation of one embodiment, compensator  154  is assembled by connecting plates  156  and  158  to perimeter ring  160 . Void  164  can be evacuated to subatmospheric pressure through valve  165 , or void can remain filled with air at atmospheric pressure. Void  164  can be filled with a compressible gas. Compensator  154  can be placed directly into cavity  150  ( FIG. 1 ), or one or more compensators  154  can be placed in frame  186  ( FIG. 5 ), and then the assembly  184  ( FIG. 5 ) can be placed into cavity  150 . Because the tree ( FIG. 1 ) can be located on the sea floor, the pressure inside cavity  150  can be greater than atmospheric pressure. Cavity  150  is exposed to hydrostatic pressure while crown plug  146  is being installed. In one embodiment, this initial higher pressure inside cavity  150  is not sufficient to cause significant deflection of plates  156  and  158 . 
         [0032]    Crown plug  146  ( FIG. 1 ) can be placed in tree  100  ( FIG. 1 ), thereby trapping fluid in cavity  150 . As the temperature of the fluid in cavity  150  increases, the fluid can thermally expand, thereby increasing its volume and increasing the pressure within cavity  150 . The expansion of the fluid, and the corresponding increase in pressure, can cause elastic deformation of plates  156  and  158  from a first position to a second position. The second position can put upper plate  156  axially nearer to lower plate  158 . As upper plate  156  is deflected, it can compress the compressible fluid located in void  164 . The space previously occupied by upper plate  156  can now be occupied by the now-expanded wellbore fluid. 
         [0033]    Similarly, the lower plate  158  can elastically deform, toward upper plate  156 , thereby compressing the fluid in void  164  and allowing the now-expanded wellbore fluid to occupy space previously occupied by lower plate  158 . Because the deformation of either or both plates  156 ,  158  is elastic, the plates can return to their original, relaxed state when the wellbore fluid cools and contracts. Thus, the pressure within cavity  150  does not drop to a pressure significantly lower than its initial pressure. 
         [0034]    Referring to  FIG. 7 , in another embodiment, compensator  210  can include shell  212  and bell  214 . Shell  212  can be a deformable member, and bell  214  can be a support member. Shell  212  can have a bell shape in its relaxed state, wherein one end is closed and generally rounded, and the body gradually becomes larger toward the other end. Bell  214  can be generally solid and have a contour on its exterior surface that is similar to the contour on the interior surface of shell  212 . Bell  214  can be coaxially nested within shell  212  to define gap  216  between them. Gap  216  can be filled with a compressible fluid. Port  220  can be used to introduce fluid into gap  216 . In one embodiment, passage  222  can communicate the fluid from port  220  to gap  216 . Plug  224  can be inserted into port  220  to seal port  220  from fluid located on the exterior of compensator  210 . In one embodiment, plug  224  can be a check valve that can be used to introduce the compressible fluid into gap  216 . 
         [0035]    Shell  212  and bell  214  can be joined by any of a variety of techniques. In one embodiment, joint  226  can be a weld, wherein shell  212  and bell  214  are welded together to form a seal. In other embodiments (not shown), joint  226  can include, for example, adhesive seals, threaded connections, and elastomeric seals. In the welded embodiment, port  220  can remain unsealed during the welding process to allow fumes from gap  216  to escape. 
         [0036]    Compensator  210  can be introduced into cavity  150  ( FIG. 1 ) by any technique. In one embodiment, compensator  210  can be lowered on a wireline or a running tool. In another embodiment, compensator  210  can be connected to one of the crown plugs  126 ,  146  ( FIG. 1 ) and run into cavity  150  when the crown plug  126 ,  146  is used to seal an end of cavity  150 . For example, threads  228  can be located on an inner diameter surface of shell  212  and can be connected to threads (not shown) on upper crown plug  146  ( FIG. 1 ). Set screw  230 , or grub screw, can be used to prevent compensator  210  from rotating relative to the member to which it is attached, such as crown plug  126  or  146 . Alternatively, compensator  210  can be integrally formed with a crown plug (not shown) or connected by another technique such as bolts, pins, or welding. 
         [0037]    In operation of one embodiment of compensator  210 , gas can be introduced into gap  216 , through plug  224 , which can be a check-valve plug, to pressurize gap  216  to a pressure that is greater than atmospheric pressure. The pressure can be selected to support shell  212 , such that shell  212  does not deform due to the hydrostatic pressure in cavity  150 , but still allow shell  212  to elastically deform when the pressure in cavity  150  increases to a predetermined level above hydrostatic pressure. 
         [0038]    Compensator  210  can then be connected to upper crown plug  146  via threads  228 , secured against rotation by set screw  230 , and lowered into cavity  150  ( FIG. 1 ) when upper crown plug  146  is set into place. As increased temperatures within wellhead  100  cause the completion fluid pressure to increase, the completion fluid can cause shell  212  to elastically deform inward, toward bell  214 . Shell  212  can deflect to be nearer bell  214 . In one embodiment, the deflection can include an axial deflection, a radial deflection, or both an axial and a radial deflection, depending on the shape of shell  212 . The fluid in gap  216  can be compressed when shell  212  deforms inward. Bell  214  provides support to shell  212 , thereby limiting its travel distance and preventing plastic deformation of shell  212 . 
         [0039]    While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.