Abstract:
A subsea wellhead comprises a generally cylindrical body including a lower extension for forming an annular space for a cement column between the lower extension and an outer conductor. The extension carries a multiplicity of interfitting rings forming a sleeve which facilitates the flexure of the extension in the presence of the cement column. Each ring may comprise an inner flange for disposition adjacent the outside of the extension and an outer flange for fitment over the inner flange of an adjacent ring. The outer flange may have a radial through-bore.

Description:
[0001]    This application claims priority from GB patent applications No. 1006158,8 filed Apr. 14, 2010 and No. 1007974.7 filed May 13, 2010, the entire contents of which applications are hereby incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to subsea wellheads. 
       BACKGROUND TO THE INVENTION 
       [0003]    Currently all wellhead systems comprise a rigid extension, normally of steel, welded onto a shaped lower part of the wellhead body. Such an extension is exposed to an injected column of cement in an annular space between the extension, and any casing string that it supports, and an outer conductor housing and any casing string that it supports. 
         [0004]    Wellhead systems are exposed to cyclic forces from, usually, the drilling rig, the marine riser, motion of the blow-out preventer (BOP) and from other causes pressures in the well bore. Repeated forces will, if sufficiently large or extended over sufficient time, produce a liability to fatigue damage and the possible failure of the wellhead system. The aforementioned column in the annular space is important to ensure control of the well. However, there is no established means of controlling the final height of the cement and there is a presumption that the cement will extend up to the level of the circulation ports. Cement at this high level on the outside of the wellhead extension reduces the freedom of the extension to flex. This loss of freedom is liable to produce repetitive stress which is potentially great enough to cause fatigue failure. 
       THE STATE OF THE ART 
       [0005]    It is known from U.S. Pat. No. 5,029,847 to provide the exterior of the extension with a continuous elastomeric sleeve which is about 3 to 6 mm thick and prevents bonding of the cement to the extension. However, such a thin unitary sleeve is easily damaged and has a predetermined longitudinal extent. 
       SUMMARY OF THE INVENTION 
       [0006]    The invention provides a subsea wellhead comprising a generally cylindrical body including a lower extension for forming an annular space for a cement column between the lower extension and an outer conductor, in which the extension carries a multiplicity of interfitting rings forming a sleeve which facilitates the flexure of the extension in the presence of the cement column. 
         [0007]    Each ring may comprise an inner flange for disposition adjacent the outside of the extension and an outer flange for fitment over the inner flange of an adjacent ring. The outer flange may have a radial through-bore. 
         [0008]    One example of the present invention will be described in detail with reference to the accompanying drawing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0009]    The single  FIG. 1  is a sectional view of a wellhead according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]      FIG. 1  illustrates in section a subsea wellhead assembly. The particular assembly illustrated is designed for use with a riser system of 13 ⅝″ (346 mm) inside diameter within either a standard 30″ (762 mm) or 36″ (914.4 mm) diameter outer conductor. The comparatively slender bore through the wellhead allows a greater thickness for the wellhead than is usual. However, the invention is not necessarily limited to the stated dimensions of the riser system or other components. 
         [0011]    The major components of the assembly are a generally cylindrical wellhead body  1  and a generally cylindrical conductor housing  2  within which the body  1  is received. The conductor housing  2  has a lower annular weld preparation profile  3  and, by means of a weld  4 , supports an outer cylindrical conductor casing  5  that extends downwardly from the conductor housing  2  into a (pre-drilled) hole in the seabed (not shown). 
         [0012]    The lower part  6  of the body  1  tapers inwardly to a slim weld preparation profile  7  abuts and, by means of a weld  8 , supports a casing extension  9 . In this example the casing extension  9  has an outside diameter of 14″ (355.6 mm) and an inside diameter of 13 ⅝″ (346 mm). 
         [0013]    Typically the casing extension  9  extends down at least as far as the level of the seabed and preferably somewhat further. The conductor casing  2  has lateral vent ports  10  in communication with the annular space  11  between the outer conductor  5  and the casing extension  9 . 
         [0014]    A column of cement is formed in the space  11  between the outer conductor  5  and the casing extension  9  (and any casing components suspended from it). Cement is pumped down the well and rises up the annular space  11  up to at least the lower part  6  of the body  1  and possibly as far as the ports  10 . 
         [0015]    Wellhead systems are exposed to cyclic forces which will, if great enough, lead to potential fatigue damage and integrity failure of all or part of the wellhead system. Cement on the outside of the wellhead extension will reduce the freedom for the extension to flex and so the repetitive stresses may be high enough for potential fatigue failure. 
         [0016]    In this example the casing extension  9  is provided with a segmented resilient sleeve  12  made of rubber of other suitable polymeric material. The sleeve extends all the way round the extension and extends from just above wells between the profile  7  at the lower end  6  of the wellhead body  1  for a suitable distance part of, or all, the way down the casing extension  9 . The thickness of the sleeve  12  needs to be selected such that it allows some flexure of the casing extension and allows sufficient circulation flow-by but does not prejudice the structural support that the extension requires. Such flexure will reduce the stresses occurring in the weld  8  between the profile  7  and the extension  9  and in the extension  9  itself. 
         [0017]    The sleeve  12  is, in accordance with the invention, composed of a multiplicity of annular segments embodied by the inter-fitting rings  13   a ,  13   b . These rings, except for the uppermost ring  13   a,  each have an upper outer flange  14  and an inner lower flange  15  so that each upper flange  14  fits over a shoulder formed by the lower flange  15  on the adjacent upper ring. Each of the upper flanges  14  has a radial through-bore  16  which facilitates the close fitting of the rings  13 , acting as a vent for any fluid trapped when the rings are fitted together. The rings thereby fit snugly together to form a substantially continuous resilient sleeve on the outside of the casing extension  9 . The segmented sleeve  12  may therefore be provided to any desired depth on the casing extension. The uppermost ring  13   a  overlaps the profile  7  and is shaped on its inside to conform to the profile  7 . The inner flanges  15  may be bonded to the extension  9 . 
         [0018]    The thickness of the sleeve may be in the range of 15-35 mm. In the example, the sleeve  12  has an outside diameter of 18″ (457 mm) and an inner diameter of 14″ (356 mm). 
         [0019]    The conductor housing  2  is pre-tensioned by means of a tensioning device  17  in which, as described in GB patent No. 2393990 and U.S. Pat. No. 7,025,145, movement of an operating member  18  causes outward oblique movement of a driving ring  19  and thereby tensioning of the housing  2 . 
         [0020]    Within the casing extension  9  is disposed a production casing  20  extending downwardly from and supported by a casing hanger  21 . In this example the production casing has a 10.75″ (273.05 mm) outside diameter. 
         [0021]    The annular space  22  defined at its inner periphery by the production casing and at its outer periphery by the casing extension  9  (and the casing string depending therefrom) is usually called the ‘B’ annulus. Normally the B annulus is sealed by cement at its lower end and sealed by means of a ‘pack-off’ at the production casing hanger. Monitoring of the pressure within the B annulus enables the detection of for example a leak in a casing string. Such a leak is liable to cause collapse or other damage to the production casing. The described wellhead provides a system in which an access to the B-annulus may be controlled by way of a production tree, avoiding penetration of or valves in the casing hanger and other complexities. 
         [0022]    Extending obliquely upwardly from the inner surface of the lower part  6  of the wellhead body  1  are passages  23  in communication with an annular gallery  24 . Extending upwardly within the body  1  from the gallery  24  is a vertical passageway  25  (shown by a chain line in  FIG. 1 ) which leads to an annular gallery  26  on the outside of a slide valve  27  disposed about a sleeve  28  that fits into the upper part of the bore  29  which extends axially through the wellhead body  1 . The sleeve defines with the wall of the bore  29  a chamber for the valve  27 . The valve  27  is biased to a closed (lower) position by means of one or more springs  30  between the top shoulder of the valve and a radial flange  31  of the sleeve  28 . 
         [0023]    The valve  27  can be moved between its open (upper) position and closed (lower) position by the application of fluid pressure either above or below the valve by way of passages not shown in  FIG. 1 . When the valve  27  is in its open position the vertical passageway  25  from the B annulus is in communication by way of the gallery  26  in the valve  27  with an isolation sleeve (not shown in  FIG. 1 ) disposed above the sleeve  28 . The valve  27  and the sleeve  28  have lateral seals adjacent the inner wall of the bore  29 . 
         [0024]    The passageway  25  is a bore which extends from a shoulder  37  around the top aperture of the wellhead vertically through the body  1  to the gallery  24  which is in communication with the region of the B annulus  22 . The passageway  25  can extend along and within the wall of the body in this manner owing to the comparative thickness of the wall and the comparatively slender bore of the wellhead. The passage  25  is blocked at its top  38  after it has been formed. 
         [0025]    The production casing hanger  21  carries a split ring  32  which is forced laterally into a recess in the bore when the production casing hanger is landed. The recess has oblique load bearing surfaces so that load can be transferred from the hanger  21  to the body  1 . 
         [0026]    The casing hanger  21  has an upper pack-off  34  which is forced into a profile in the bore by an activating sleeve  36 . Within the sleeve  36  is a resilient barrel-shaped ring (collet)  35  which, in a manner not relevant to the present invention, aids the release of the pack-off from a running tool (not shown) and maintains the pack-off in position after it has been set in place. 
         [0000]    Also shown in  FIG. 1  is a slot  37  which allows by-pass of a tubing hanger (not shown) into the body  1  of the wellhead. This feature is not relevant to the claimed invention and will not be described further.