Patent Publication Number: US-2006017287-A1

Title: Tie-back connection for subsea well

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is related to and claims benefit of U.S. Provisional Application Ser. No. 60/409092 filed Sep. 9, 2002 and assigned to the Assignee of the present application. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to connectors for securing riser pipes to subsurface wellheads. More specifically, the present invention relates to an improved connection between a riser stress joint and a tie back connector.  
     BACKGROUND OF THE INVENTION  
      An external string of pipe is frequently used to communicate fluids and well products between a subsea wellhead and a drilling or production structure located at the water surface. This string of pipe, frequently referred to as a riser, is subjected to strong lateral and longitudinal stresses caused by water currents and motion between the surface structure and the wellhead. Because the wellhead is stationary relative to the riser, these stresses concentrate in the connection between the riser and the wellhead.  
      Substantial effort has been exerted in the design of the structure used to connect the lower end of the riser string to the wellhead to minimize the effects of the stresses acting through the riser. A stress compensating tie-back connector may be used to tie back a subsea well to a floating surface structure such as a SPAR or tension leg platform. In these installations, the lower end of the riser connects to the top of a stress joint that is designed to connect to the wellhead to absorb part of the stresses exerted by the riser. The riser, stress joint and tie back connector are all run as a unit that is lowered through the water from the surface structure and secured to the subsea wellhead. Conventional prior art assemblies frequently construct the upper tie back connector body and the stress joint as a single piece component.  
      Where the upper tie back connector body and the stress joint are configured as a single piece component, difficulties can be encountered in the assembly of hydraulic actuation components or other mechanisms forming part of the tie back connector. This problem may be encountered, for example, where hydraulic lock actuation mechanisms used for securing the riser to the wellhead must be assembled over the top of the tie-back connector. Moreover, optimum stress reaction characteristics of the stress joint may not be attainable when the upper tie back connector body and the stress joint are constructed as a single piece component.  
      Constructing the upper tie back connector body and the stress joint of different materials, which could improve the stress capacity of the connection, introduces the problem of increased corrosion caused by the galvanic action resulting from the joinder of dissimilar materials in a saltwater environment.  
     SUMMARY OF THE INVENTION  
      A connection structure is provided between a stress joint in a riser and a tie-back connector of a subsea wellhead. The connection structure joins a stress joint of one material with a wellhead tie-back connector of a different material. The connection structure allows components of the tie-back connector to be assembled over the tie-back connector without first having to be maneuvered over a long stress joint.  
      High stress capacity materials are used in the construction of the stress joint to minimize the effects of riser generated stresses in the wellhead connector. The connection fixture provides electrical insulation between the engagement points of the stress joint and the tie-back connector to minimize corrosive galvanic action.  
      In the preferred embodiment, the connection structure provides an upwardly facing flange extending from the top of the tie-back connector adapted to mate with and secure to a downwardly facing flange at the bottom of the stress joint. The upwardly facing flange is preferably threaded to the top of the tie-back connector.  
      In a modified form of the Invention, mechanically actuated radial dogs or C-ring locking members are used to secure the upwardly facing flange to the top of the tie-back connector.  
      From the foregoing, it will be appreciated that a primary object of the present invention is to provide a connection structure to join a stress joint to a tie-back connector of a subsea wellhead.  
      Another object of the present invention is to provide a connection structure to join a stress joint of one material to a tie-back connector of a different material.  
      Yet another object of the present invention is to provide a connection structure that permits components of a tie-back connector to be assembled without the obstruction provided by a stress joint integrally connected to the tie-back connector.  
      It is also an object of the present invention to minimize corrosion in an assembly of dissimilar metals in a connection between a subsea well and a stress joint.  
      The foregoing features, advantages and objects of the present invention will be more fully understood and better appreciated by reference to the following address vacation and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a vertical sectional view of a connection structure of the present invention secured to the upper body (indicated in dotted line) of a tie-back connector.  
       FIG. 2  is an enlarged vertical sectional view of the connection structure of  FIG. 1 .  
       FIG. 3  is a vertical sectional view of a modified form of the connection structure of the present invention.  
       FIG. 4  is a vertical sectional view of yet another modified form of the connection structure of the present invention. 
    
    
     DETAILED DESCRIPTION OF INVENTION  
       FIG. 1  illustrates a preferred embodiment of the connection structure of the present invention, indicated generally at  10 , for connecting a stress joint  12  to a subsea wellhead assembly (not illustrated). An upper body of a tie-back connector, indicated generally at  14 , extends toward the water surface from the subsea wellhead assembly. An example of the tie-back connector  14  is disclosed in U.S. patent application Ser. No. 09/954,998, now published in Pub. No. U.S. 2002/0096878, incorporated herein by reference for all purposes.  
      The tie-back connector  14  includes an elongate tubular tie-back body  16  and a tie-back flange  18  mechanically secured to the tie-back body  16 . The tie-back flange  18  has a mating face  22  facing upwardly from the tie-back connector  14  for connecting to a flange  12   a  at the base of the stress joint  12 .  
      The mechanical connection securing the connection structure  10  and the tie-back body  16  may comprise a threaded connection  28  between the tie-back flange  18  and the tie-back body. The threaded connection  28  may be a square thread or a buttress thread, and is preferably comprised of substantially the same material as the tie-back body  16 . The stress joint flange  12   a  has a mating face  26  facing downwardly from the stress joint  12  for mating with the mating face  22  of the tie-back flange  18 . Threaded fasteners  31 , such as threaded studs  30  and nuts  32 , may secure the stress joint flange  12   a  to the tie-back flange  18 . The studs are threadably received within tapped bores formed in the tie-back flange  18 . A gasket  36  provides a seal between the stress joint flange  12   a  and the tie-back flange  18 .  
      The stress joint  12  including stress joint flange  12   a  is preferably constructed of a material, such as a titanium alloy, that exhibits high resistance to structural failure caused by fatigue damage. The top of the stress joint portion shown in the figures may be welded to an extended portion of stress joint. The tie-back body  16 , including the flange  18 , may be constructed of a much less expensive low alloy steel. The physical connection of the steel and titanium alloys in the presence of saltwater can produce damaging electrical currents that are conducive to rapid corrosion and destruction of the connection. The effects of this phenomenon are minimized by providing electrical insulators to separate the engagement points of the two metals. To this end, an electrically non-conductive bent insulating ring  34  is disposed between the tie-back body  16  and the stress joint flange  12   a . The insulation ring  34  may comprise an upper sealing surface for sealing with the gasket  36  and for insulating between the gasket  36  and the riser flange  12   a . The insulation ring may further serve as a spacer to space apart portions of mating faces  22  and  26  to prevent electrical contact between them (as shown, outer portions of mating faces  22  and  26  are angled to create a gap between them. In other embodiments, the insulation ring  34  may extend further between mating faces  22  and  26 , to insulate greater area. Additionally, insulation washers  38  are positioned around the threaded studs  30 , between the nuts  32  and the stress joint flange  12   a , to complete the electrical isolation between the titanium stress joint  12  and the low alloy steel tie-back body  16 . The insulators  34  and  38  may be constructed from any suitable material providing the desired isolation to electrical current flow between the dissimilar metals while simultaneously providing the necessary structural strength to withstand the compression imposed during the bolting together of the flanged components. Examples of such insulation material include ceramic or ceramic-coated steel. Additionally, an OD of the threaded studs  30  is preferably smaller than the mating hole in the flange  12   a  through which they pass, to create a gap around threaded studs  30  and prevent electrical contact between threaded studs  30  and flange  12   a.    
       FIG. 3  illustrates a modified embodiment of the present invention indicated generally at  110 . As with the embodiment of  FIGS. 1 and 2 , the connection structure  110  includes a tie-back connector indicated generally at  114  having an elongate tubular tie-back body  116  and a tie-back flange  118  mechanically connected to the tie-back body. Components of the embodiment of  FIG. 3  that correspond with those of the embodiment of  FIGS. 1 and 2 , and function in the same or similar manner, have been accorded reference characters that are greater by 100 than the corresponding reference characters used in  FIGS. 1 and 2 . The tie-back body  116  terminates in a threaded male connection  128  that is threadably engaged with the tie-back flange  118  to form an upwardly facing flange face at the upper body of the tie-back connector  116 . A riser flange  112   a  is formed at the base of a riser joint  112 .  
      The stress joint flange  112  has a mating face  126  facing downwardly from the riser  112  to a sandwich flange  134  held between the stress joint flange  112   a  and the tie-back flange  118 . Threaded fasteners  131  including studs  130  extend through flange openings in the flange  112   a  and sandwich flange  134  into threaded receptacles in the tie-back flange  118 . Nuts  132  are tightened onto the studs  130  to secure the components together. Compression seals  136  and  136   a  prevent leakage in the connection structure  110 .  
      The sandwich flange  134  is constructed of an electrical insulating material such as described above to prevent the generation of galvanic currents between the stress joint  112  and the tie-back body  116 . The insulating washers  138  prevent current flow through the studs  130 .  
       FIG. 4  illustrates another embodiment of the connection structure of the present invention indicated generally at  210 . As with the embodiment of  FIGS. 1-3 , the connection structure  210  includes an elongate tubular tie-back body  216  and a tie-back flange  218  mechanically connected to the tie-back body  216 . Components of the embodiment of  FIG. 4  that correspond with those of the embodiment of  FIGS. 1-3 , and function in the same or similar manner, have been accorded reference characters that are greater by 100 than the corresponding reference characters used in  FIG. 3 . In contrast to the threaded connections  28  and  128  used to connect the tie-back bodies  16 ,  116  with the tie-back flanges  18 ,  118 , respectively, in  FIGS. 1, 2 , and  3 , the connection structure  210  employs a radially movable latch member  228  for connecting the tie-back body  216  and the tie-back flange  218 . The latch member  228  is constructed with upper and lower sets of circumferentially extending teeth  230  and  232 , adapted to respectively engage grooves  231  and  233  about adjacent ends of the tie-back body  216  and the tie-back flange  218  when the latch member  228  is moved radially inward. The latch member  228  may comprise a plurality of mechanically actuated dogs or a one-piece C-ring locking member.  
      The latch member  228  may be retained in the radially inward position illustrated in  FIG. 4  by a tubular retaining member  250 . The retaining member is secured with retaining member  251  including threaded studs  252  and nuts  254 . The threaded studs  252  are received in tapped receptacles formed in the flange connector  218 .  
      Radial movement of the latch member  228  into the radially inward position may be caused by axial movement of the retaining member  250  relative to the latch member  228 , such as occurs when the bolts  254  are tightened on the studs  252 . Mating cam surfaces  260 ,  261  on the latch member  228  and the retaining member  250  slidably engage during the axial movement of the retaining member  250  to cause the radial movement of the latch member  228 .  
      The tie-back flange  218  has an upwardly facing mating face  222  for connecting to a stress joint  212 . A stress joint flange  212   a  at the base the of the joint  212  has a mating face  226  facing downwardly for mating with the mating face  222  of the tie-back flange  218 . Studs  230  and nuts  232  secure the joint  212  to the tie-back flange  218 . An insulation ring  234  between the opposing faces and  222  and  226  provides electrical insulation between the engagement points of the stress joint  212  and the tie-back flange  218 . Insulating washers  238  complete the electrical insulation of the dissimilar metals of the connection structure from each other.  
      Although specific embodiments of the invention have been described herein in some detail, it is to be understood that this has been done solely for the purposes of describing the various aspects of the invention, and is not intended to limit the scope of the invention as defined in the claims which follow. Those skilled in the art will understand that the embodiments shown and described are exemplary and various other substitutions, alternations, and modifications, including but not limited to those design alternative specifically discussed herein, may be made in the practice of the invention without departing from the spirit and scope of the invention.