Patent Publication Number: US-2013248199-A1

Title: Wellhead assembly having a sinusoidal sealing profile and a method to assemble the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to subsea wellheads and, in particular, to a metal sealing system for subsea wellheads and related subsea equipment disposed in the subsea wellhead and a method to assemble the same. 
     2. Brief Description of Related Art 
     In hydrocarbon production wells, a wellhead housing is located at the upper end of the well. The wellhead housing is a large tubular member having an axial bore extending through it. Casing will extend into the well and will be cemented in place. A casing hanger, which is on the upper end of the casing, will land within the wellhead housing. The exterior of the casing hanger is spaced from the bore of the wellhead housing by an annular clearance which provides a pocket for receiving an annulus seal. Annular spaces within concentric members, such as the wellhead housing and the casing hanger, may be exposed to high pressures downhole that require isolation from within the wellhead housing and/or production tree. One manner of isolation involves setting seals within the annular spaces to form a pressure barrier between the downhole pressure and the ambient pressure to the wellhead housing. 
     There are many types of annulus seals, including rubber, rubber combined with metal, and metal-to-metal. One metal-to-metal seal in use has a U-shape, having inner and outer walls or legs separated from each other by an annular clearance. An energizing ring, which has smooth inner and outer diameters, is pressed into this clearance to force the legs apart to seal in engagement with the inner surface of the outer wellhead member and with the exterior of the inner wellhead member. 
     Some annular seals utilize wickers. Wickers may be located on the exterior of the inner wellhead member, in the bore of the outer wellhead member, or both. The outer leg of the seal embeds into the wickers of the outer wellhead member while the inner leg of the seal embeds into the wickers of the inner wellhead member. This provides the function of both locking the annulus seal in place and axial restraint to the inner wellhead member, as well as forming a seal between the outer wellhead member and the inner wellhead member. Lockdown is the term used for the capacity and capability of the inner wellhead member and seal assembly to stay in place vertically in the wellhead when a pressure or other force is applied from below. This force may be, for example, the result of annulus pressure build-up or from thermal growth of the casing attached to a bottom of the hanger or a combination of both. A sufficient lockdown capacity is needed to ensure that the seal integrity is maintained and the inner wellhead member and seal remain static. 
     The sealing wickers are machined directly into the bore of the outer wellhead member or landing subs and the neck of the inner wellhead member. The annulus seal is made of a sufficiently deformable metal to allow it to deform against the wickers. The deformation occurs as the wickers “bite” into the annulus seal. In order to cause the seal to deform without damaging the wickers, the annulus seal is made of a metal that is softer than the steel used for the inner and outer wellhead members. The wicker bite resists the lockdown force from the inner wellhead member as a shear resistance. The higher the wicker bite, the higher the lockdown capacity. The lower the wicker bite, the lower the lockdown capacity. 
     An energizing ring is pressed into the seal between the inner and outer legs to deform the seal against the wickers. This causes the seal to form in a top down manner, forming sequential sealing bands at progressively lower axial locations as each wicker is engaged by the annulus seal. As these wellhead assemblies are disposed in subsea locations, fluid will fill the annulus into which the annulus seals are disposed prior to setting of the seal. As the sealing bands are formed, fluid may be trapped between each wicker. The trapped fluid builds up pressure quickly due to its high bulk modulus. This pressure buildup causes hydraulic lock that limits total engagement or “bite” between the wickers and the annular seal. The limited engagement of the wickers reduces the total lockdown and damage tolerance capabilities of the annulus seals. Therefore, there is a need for an improved sealing assembly that decreases instances of hydraulic lock, thereby increasing lockdown capacity and damage tolerance of the annulus seals. 
     SUMMARY OF THE INVENTION 
     These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention that provide a wellhead assembly having a sinusoidal sealing profile and a method to assemble the same. 
     In accordance with an embodiment of the present invention, a wellhead assembly is disclosed. The wellhead assembly includes an outer tubular wellhead member with a bore having an axis and a sealing surface extending an axial distance along an inner diameter surface of the bore. Wickers are formed in the sealing surface, the wickers circumscribing the bore along an axially undulating path. The wellhead assembly also includes an inner tubular member in the outer tubular wellhead member, and a seal element between the inner tubular wellhead member and the outer tubular wellhead member. The seal element may be urged against the wickers to form a sealing interface. 
     In accordance with another embodiment of the present invention, a wellhead assembly is disclosed. The wellhead assembly includes an outer tubular wellhead member with a bore having an axis and a sealing surface extending an axial distance along an inner diameter surface of the bore. Wickers are formed in the sealing surface and circumscribe the bore along an axially undulating path. Partial wickers are also formed in the sealing surface. Each partial wicker extends along a portion of the circumference of the bore and follows an axially undulating path. The wellhead assembly also includes an inner tubular member in the outer tubular wellhead member, and a seal element between the inner tubular wellhead member and the outer tubular wellhead member. The seal element may be urged against the wickers to form a sealing interface. 
     In accordance with yet another embodiment of the present invention, a method of forming a wellhead assembly is disclosed. The method provides an outer tubular wellhead member having a bore, an axis, and an inner surface of the outer tubular wellhead member having wickers that circumscribe the bore and vary axially in position around the circumference of the bore. The method inserts an inner tubular wellhead member in the outer tubular wellhead member and an annular seal in an annulus between the outer tubular wellhead member and the inner tubular wellhead member. The method urges the seal against the wickers so that the wickers deform the seal and create a barrier in the annulus. 
     An advantage of a preferred embodiment is that it provides a wellhead assembly that may seal with multiple independent sealing surfaces with reduced risk of hydraulic lock. This is accomplished by providing a flowpath for fluid residing between wickers to move out of the wicker area during setting of the wellhead assembly. In addition, the disclosed embodiments provide increased lockdown capacity by the seal, thereby limiting instances of upward movement of a casing hanger sealed to a high pressure housing or wellhead. Still further, the disclosed embodiments provide a wellhead assembly with increased tolerance for wellbore damage when installing the wellhead assembly in subsea locations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 embodiments thereof which are illustrated in the appended drawings that form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and are therefore not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
         FIG. 1  is a sectional view of a high pressure housing of a subsea wellhead in accordance with an embodiment of the present invention. 
         FIG. 2  is an enlarged sectional view of a plurality of wickers of the high pressure housing of  FIG. 1  in accordance with an embodiment of the present invention. 
         FIG. 2A  is an enlarged view of a portion of the plurality of wickers of the high pressure housing of  FIG. 2 , in accordance with an embodiment of the present invention. 
         FIG. 3  is a sectional view of the high pressure housing of  FIG. 1  having additional components of a subsea wellhead assembly disposed therein in accordance with an embodiment of the present invention. 
         FIG. 4  is a schematic sectional view of a portion of the wellhead assembly of  FIG. 3 , illustrating a seal assembly in an unenergized or unset position between the high pressure housing and a casing hanger in accordance with an embodiment of the present invention. 
         FIG. 5  is a schematic sectional view of the portion of the wellhead assembly of  FIG. 4 , illustrating the seal assembly in an energized or set position between the high pressure housing and the casing hanger in accordance with an embodiment of the present invention. 
         FIG. 6  is a schematic view of a portion of the plurality of wickers of  FIG. 2 , illustrating fluid movement during energization of the seal assembly of  FIGS. 4 and 5  in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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. 
     In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. Additionally, for the most part, details concerning wellhead placement, construction, operation, and the like have been omitted inasmuch as such details are not considered necessary to obtain a complete understanding of the present invention, and are considered to be within the skills of persons skilled in the relevant art. As used herein, terms such as above and below are used to describe relative position of components of the invention as illustrated and are not intended to limit the disclosed embodiments to a vertical or horizontal orientation. 
       FIG. 1  illustrates a partial sectional view of a high pressure housing  13  of a subsea wellhead assembly  11 . Subsea wellhead assembly  11  may be any suitable wellhead assembly disposable in a subsea environment to drill and produce fluid from a subsea location. In the illustrated embodiment, high pressure housing  13  provides a pressure vessel interface between a blowout preventer (not shown) and well casing (not shown) disposed in high pressure housing  13 . High pressure housing  13  may include one or more of the following: a dual seal profile at the wellhead connector interface, a blowout preventer isolation test tool seal area, a running tool cam profile, and one or more hanger seal profiles  15 . Seal profiles  15  may be formed in a bore  17  of high pressure housing  13 . Bore  17  and high pressure housing have a common axis  19 . A person skilled in the art will recognize that one or more casing hangers and associated casing strings as well as one or more tubing hangers and associated tubing strings may be disposed within high pressure housing  13 . In some embodiments, a lockdown bushing or lockdown hanger may be disposed within high pressure housing  13  to provide additional pressure capacity for the system. A person skilled in the art will also recognize that a separate seal profile  15  may be formed in bore  17  for every hanger or bushing expected to be disposed within high pressure housing  13 . A person skilled in the art will understand that other equipment, such as subsea trees and the like, may include sealing profiles  15  as described herein. 
       FIG. 2  illustrates a detailed view of seal profile  15  of high pressure housing  13 . Seal profile  15  may be formed of a plurality of wickers  21 . As shown in  FIG. 2 , wickers  21  are grooves formed in bore  17  of high pressure housing  13 . Referring to  FIG. 2A , each wicker  21  may have a triangular cross sectional profile having a crest  23 , an upper flank  25 , and a lower flank  27 . In an embodiment, crests  23  of adjacent wickers  21  are separated by a distance  29 . In an embodiment, distance  29  may be approximately one-eighth of an inch. A person skilled in the art will understand that distance  29  may vary as needed for the particular application to which high pressure housing  13  is put to use. For example, distance  29  may be increased or decreased as needed to increase or decrease lockdown capacities and damage tolerance capabilities of wellhead assembly  11 . Upper flank  25  and lower flank  27  of adjacent wickers  21  join at a valley  31 . A person skilled in the art will recognize that valleys  31  may be spaced by a distance equivalent to distance  29  and may vary as needed for the particular application of high pressure housing  13 . Each wicker  21  may have a depth  33  from crest  23  to valley  31 . In the illustrated embodiments, depth  33  is uniform for any given axial cross section of high pressure housing  13 . Similarly, distance  29  may be uniform for any given axial cross section of high pressure housing  13 . In an embodiment wickers  21  are machined into bore  17  of high pressure housing  13  with a computer numerical controlled lathe or a cam driven manual machine. In other embodiments, high pressure housing  13  may be cast with wickers  21 . A person skilled in the art will understand that any suitable method may be used to form wickers  21 . 
     Each crest  23  circumscribes bore  17  so that a crest  23  may be traced from a circumferential location, around bore  17  to return to the circumferential location where the trace began. For example, each crest  23  may form a continuous line around the circumference of bore  17  so that the trace starts and finishes at the same axial height and radial position as it started. Similarly, upper flank  25 , lower flank  27 , and valley  31  of each wicker  21  may be traced from a circumferential location, around bore  17  to return to the circumferential location where the trace began. Crests  23  of each wicker  21  may vary axially around the circumference of bore  17  as shown in  FIG. 2 . For example, if location  22  is designated as a baseline height for the upper most wicker  21  of the plurality of wickers  21 , locations  24 ,  28 , and  30  may all have different axial positions relative to location  22 . In this example, a trace of the upper most wicker  21  beginning at location  22  would be continuous around bore  17  and return to location  22 . As shown, some locations of uppermost wicker  21  may be positioned axially higher relative to location  22 . For example locations  28  and  30  have higher axial positions in bore  17  relative to location  22 . Similarly, some locations of upper most wicker  21  may be positioned axially lower than location  22 . For example location  24  has a lower axial position in bore  17  than location  22 . Still further, some locations may be positioned at the same axial height as location  22 . For example, location  26  is has an equivalent axial height as location  22 . In the illustrated embodiment, crests  23  have multiple locations both axially higher and axially lower than location  22  while maintaining a continuous line circumscribing bore  17 . In the illustrated embodiment, the axial variation of crests  23  of wickers  21  oscillates in a sinusoidal manner. Thus, the axial position of crest  23  varies with an equivalent amplitude both axially above and axially below initial location  22 . Similarly, upper flanks  25 , lower flanks  27 , and valleys  31  vary axially around the circumference of bore  17  with crest  23  as described above. In this manner, each wicker  21  may have locations of higher, lower, and equivalent axial position relative to an initial position selected at an arbitrary circumferential location of wickers  21 . Optionally, wickers  21  may follow a path of approximately any waveform, whether or not the waveform is sequential, symmetric, or curved. A person skilled in the art will understand that axial location of a particular wicker or plurality of wickers  21  may vary with a variety of different shapes and patterns to vary the height of a seal band formed by seal profile  15  as described in more detail below. 
     As shown in  FIG. 2 , sealing profile  15  may include partial wickers  32 . Partial wickers  32  are formed in bore  17  so that wickers  21 ,  32  substantially populate a total height  34  of sealing profile  15 . Partial wickers  32  have a similar depth  33 , pitch or height  29 , and component parts, upper flanks  25 , lower flanks  27 , crests  23 , and valleys  31  as each wicker  21 . Partial wickers  32  may only be partially formed so that a trace of a partial wicker  32  is not continuous around the circumference of bore  17 . Partial wickers  32  may vary in height axially in a manner similar to wickers  21  creating partial fluid channels for the flow of fluid described in more detail below. In the example of  FIG. 2 , partial wickers  32  each extend a distance less than the circumference of bore  17  and are spaced apart from each adjacent partial wicker  32 . Although shown as being substantially equidistant, partial wickers  32  can have varying lengths and be space apart at regular or irregular distances. 
     Illustrated in a side sectional view in  FIG. 3  is one example of a wellhead assembly  35  and a running tool  37  inserted in wellhead assembly  35 . Running tool  37  includes an inner mandrel  39 , having a threaded upper end for connection to a raising and/or lowering device, such as a drill pipe (not shown). The drill pipe, which in an example depends downward from a drilling platform (not shown), can be used for raising, lowering, and operating running tool  37 . Wellhead assembly  35  of  FIG. 3  includes high pressure housing  13 , also referred to as an outer annular wellhead housing. Running tool  37  is shown landing a casing hanger  41  within high pressure housing  13 . After landing casing hanger  41  in high pressure housing  13 , an annular space is formed between respective portions of casing hanger  41  and high pressure housing  13 . Additional hangers of similar configurations are illustrated coaxially within high pressure housing  13  and below casing hanger  41 . The additional hangers include, a bridging hanger  43  and another casing hanger  45  beneath the bridging hanger  43 . Seal assemblies  47  are illustrated set in the annular spaces between each of the hangers  41 ,  43 ,  45  and high pressure housing  13  as described in more detail below. At each seal assembly  47  location, a sealing profile  15  may be formed in high pressure housing  13 . 
     Referring to  FIG. 4 , high pressure housing  13  is a large tubular member located at the upper end of a well, such as a subsea well. Casing hanger  41  is a tubular conduit secured to the upper end of a string of casing (not shown). Casing hanger  41  has an upward facing shoulder  51  on its exterior. An exterior wall  53  of casing hanger  41  is parallel to the wall of bore  17  but spaced inwardly. This results in an annular pocket, clearance, or annulus  52  between casing hanger exterior wall  53  and bore  17 . Wickers  55  are located on exterior wall  53  of casing hanger  41 . Wickers  55  are similar to wickers  21  of  FIGS. 2-2A  adapted to circumscribe exterior wall  53  of casing hanger  41 . Wickers  55  may have a similar sinusoidal axial positional variation. When disposed in high pressure housing  13 , wickers  55  of casing hanger  41  may be radially adjacent to wickers  21  of high pressure housing  13 . 
     Seal assembly  47  lands in annulus  52  between casing hanger exterior wall  53  and bore  17 . Seal assembly  47  may be made up entirely of metal components. These components may include a generally U-shaped seal member  57 . Seal member  57  has an outer wall or leg  59  and a parallel inner wall or leg  61 , the legs  59 ,  61  being connected together at the bottom by a base and open at the top. The inner diameter of outer leg  59  is radially spaced outward from the outer diameter of inner leg  61 . This results in an annular clearance  63  between legs  59 ,  61 . The inner diameter and the outer diameter are smooth cylindrical surfaces parallel with each other. Similarly, the inner diameter of inner leg  61  and the outer diameter of outer leg  59  are smooth, cylindrical, parallel surfaces. 
     An energizing ring  65  is employed to force legs  59 ,  61  radially apart from each other and into sealing engagement with wickers  21 ,  55  as shown in  FIG. 5 . Wickers  55 ,  21  bite into inner leg  61  and outer leg  59 , respectively, of seal assembly  47  as energizing ring  65  forces outer and inner legs  59 ,  61  against wickers  21 ,  55 . Energizing ring  65  has an outer diameter that will frictionally engage the inner diameter of outer leg  59 . Energizing ring  65  has an inner diameter that will frictionally engage the outer diameter of inner leg  61 . The radial thickness of energizing ring  65  is greater than the initial radial dimension of the clearance  63 . 
     In the illustrated embodiments, bore  17  may be filled with fluid during running, landing, and setting of casing hanger  41  and seal assembly  47 . As energizing ring  65  moves axially downward into clearance  63 , so that the seal is formed by engaging wickers  21  from top to bottom of sealing profile  15 . Upper wickers  21  such as wicker  21 A of  FIG. 6  bite into and deform the sealing surface of seal member  57  to create a sealing band at wicker  21 A. As U-shaped seal member  57  engages successive wickers  21   1-n ,  55   1-n , circumferential seal bands may be formed at each wicker  21 ,  55  sequentially from an upper end of seal member  57  to a lower end of seal member  57 . U-shaped member  57  engages each wicker  21 ,  55  proximate to an inflection point  67  where the wickers change direction, illustrated in  FIG. 6 . Continued axial downward movement of energizing ring  65  causes U-shaped seal member  57  to engage successively more of each wicker  21 ,  55 , deforming seal member  57  into valleys  31 . As this occurs, fluid residing between adjacent crests  23  of each wicker  21 ,  55 , for example between crest  23   1  and crest  23   2  of wickers  21   2  and  21   2 , respectively, flows from inflection points  67  toward inflection points  69  where wickers  21 ,  55  change direction opposite inflection point  67  as shown by arrows  71 . For example, as shown in  FIG. 6 , fluid may flow from higher relative locations  26 ,  28 , and  30  toward location  24 . In some embodiments, a portion of the fluid residing between adjacent crests  23  flows out of wickers  21 ,  55 . In this manner, the volume of #fluid that may be trapped by U-shaped seal member  57  between adjacent wickers  21 ,  55  may be reduced. Reduction of fluid volume between U-shaped seal member  57  and wickers  21 ,  55  increases engagement between U-shaped seal member  57  and wickers  21 ,  55 , deforming more of U-shaped seal member  57  into wickers  21 ,  55  to create higher lockdown capacities and reduce instances of hydraulic lock during seal setting. Curving the wickers increases their effective length and thus their total volume. 
     Accordingly, the disclosed embodiments provide numerous advantages. For example, the disclosed embodiments provide a wellhead assembly that may seal with multiple independent sealing surfaces with reduced risk of hydraulic lock. This is accomplished by providing a flowpath for fluid residing between wickers to move out of the wicker area during setting of the wellhead assembly. In addition, the disclosed embodiments provide increased lockdown capability by the seal, thereby limiting instances of upward movement of a casing hanger sealed to a high pressure housing or wellhead. Still further, the disclosed embodiments provide a wellhead assembly with increased tolerance for wellbore damage when installing the wellhead assembly in subsea locations. 
     It is understood that the present invention may take many forms and embodiments. Accordingly, several variations may be made in the foregoing without departing from the spirit or scope of the invention. Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Many such variations and modifications may be considered obvious and desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.