Abstract:
In accordance with certain embodiments, the present invention provides a connector for attaching to a multi-toothed profile on a wellhead features a tooth profile that staggers loading preferably starting at a loading surface furthest from the connector body sitting on the wellhead and moving toward the connector body. The staggered loading more evenly distributes stresses on the matching loading surfaces as compared to the result of using a tooth profile on the connector that nearly exactly matches the profile on the wellhead. The joint can then take advantage of an increased preload and exhibit improved stress characteristics when operating at high loading conditions.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    Not Applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       BACKGROUND 
       [0003]    In accordance with certain embodiments, the present invention relates to the field of connectors that attach to multi-toothed profiles on subsea wellheads and, more particularly, to connector profiles that better distribute stress among the teeth to strengthen the connection. 
         [0004]    Connectors are employed to attach certain types of equipment to wellhead housings. One common example provides attaching blowout preventer equipment to a subsea wellhead. Bodies that house a blowout preventer are connected to a wellhead. Early designs of such a connection involved a generally C-shaped clamp that was forced to move radially to capture a pair of spaced flanges on the wellhead and the body of the blowout preventer. One example of this single contact surface for this type of collet connector is shown in U.S. Pat. No. 3,096,999. Another form of engagement uses a series of contact surfaces performing a similar connecting function as single surface, but the loading is now distributed on the multiple surfaces available. A common example of this connection kind is the Vetco H4 wellhead. Connector designs in the past may have varied in actuation techniques or size and shape of locking dogs, but one thing they all had in common was that the tooth profile was designed to match the wellhead profile for the size and spacing of engaging teeth. Some examples of such closely matched connector profiles to the wellhead profiles can be seen in DX series connectors offered by Drill Quip, H-4 connectors from ABB Vetco Gray and similar products from Cameron. These products featured a group of radially moving dogs where the tooth profile on the dog matches the wellhead tooth profile, and an angled ring drove the profiles together to connect a body to the wellhead. 
         [0005]    This practice has gone on for years without recognition of a limitation of such mirror image tooth profile designs in wellhead connector art. The problem not heretofore realized and addressed by the present invention is that using a mirror image tooth profile on the locking dog results in an unequal distribution of stress and contact forces on the loading surfaces, with the loading surface closest to the connector body on the locking collet and wellhead bearing a disproportionately large percentage of the stress and contact force among the loading surfaces. This occurs because from a common reference line on the locking collet the loading surface closest to the reference line experienced the lowest percentage elongation and thus carried more of the stress than loading surfaces progressively further from a common and stationary reference line. The elongation of the dog and compression of the wellhead makes the loads progressively lower for each tooth profile further from a common reference line. 
         [0006]    The present invention, exemplary embodiments of which are discussed below, provides various benefits and abates various concerns, such as the concerns addressed above. 
       SUMMARY OF THE INVENTION 
       [0007]    In accordance with certain embodiments, the present invention puts forward a staggered contact design where contact is first established at the lowermost end of the collet or dog and on the wellhead at a location furthest from the preventer body. Then, as the collet or dog is powered to move radially inwardly, additional loading surfaces come into contact in a direction approaching the connector body. 
         [0008]    As further exemplary embodiments, the present invention provides a connector for attaching to a multi-toothed profile on a wellhead, the connector featuring a tooth profile that initially staggers loading starting at a loading surface furthest from the preventer body sitting on the wellhead and moving toward the preventer body. The staggered loading more evenly distributes stresses at the preloaded condition on the matching loading surfaces as compared to the result of using a tooth profile on the connector that nearly exactly matches the profile on the wellhead. The joint can then handle higher operating pressures and external loads with reduced risk of connection failure. Of course, the foregoing are just examples of the present invention and are not intended to limit the appended claims to the embodiments described. 
         [0009]    These and other features of the present invention will be more readily understood by those skilled in the art from a review of the drawings and the description of the exemplary embodiments provided below. Finally, the claims that later appear are indicative of the full scope of the present invention. 
     
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
         [0010]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0011]      FIG. 1  is a section view of an exemplary connector in the fully open position; 
           [0012]      FIG. 2  is the view of the connector of  FIG. 1  in the closed position; 
           [0013]      FIG. 3  is the view of the connector of  FIG. 2  in the full preload position; 
           [0014]      FIG. 4  is an exemplary close up view of the initial tooth contact; 
           [0015]      FIG. 5  is the view of  FIG. 4  showing the start of radial movement of the collet; 
           [0016]      FIG. 6  is the view of  FIG. 5  illustrating additional radial collet movement; 
           [0017]      FIG. 7  is the view of  FIG. 6  with radial collet movement completed; and 
           [0018]      FIG. 8  is a detail view of an exemplary connector assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIGS. 1-3  show the basic structure of an exemplary embodiment in 3 positions. When the body  10  is lowered onto the wellhead  12  the actuator piston  14  is abutting the surface  16  on body  10 . The body  10  may facilitate connection of any number of components to the wellhead  12 . Indeed, the body  10  may facilitate connection of a production tree, a blow-out-preventer, drilling-tools, among various kinds of tubular devices for oilfield use, to the wellhead. A taper  18  on collet segments  19  engages extending point  20  to retract the lower teeth  22  away from mating teeth  24  on the wellhead  12 . This allows the body  10  to be lowered without the weight of it being supported on teeth  24 . The top  26  of the wellhead  12  has a shape that, in this embodiment, conforms to the lower end  28  of body  10  so that when they go together, as shown in  FIG. 2 , the interface between surfaces  26  and  28  can be sealed by a seal  30 . Piston  32  resides in housing  34  which defines two compartments  36  and  38  that are isolated from each other and sealed to accept hydraulic pressure for urging the collets  19  between the positions in  FIGS. 1-3 . Tapered surfaces  40  and  42  ride on each other as piston  32  moves down to force the collets  19  to move radially toward centerline  44 . 
         [0020]    The relation of the parts and the movements to secure the body  10  to the wellhead  12 , in general, is by way of background to the invention, as the invention is addressed to the relation between the teeth  22  and  24 . Those skilled in the art will know that most wellheads feature a tooth pattern  24  that has become an industry standard. The collet tooth pattern  22  thus forms a relationship to this industry standard pattern  24 . The industry standard pattern  24  features a series of parallel ridges  25 ,  27  and  29 . These generally are at a common fixed distance as between adjacent ridges. That said, embodiments of the present invention envision connecting to a variety of profiles in wellheads  12  that are commercially available or will be available in a manner that better distributes stress and contact forces as compared to currently available connector designs that emphasize a mirror image of the wellhead pattern on the collet that engages to it. Thus reference to teeth or engaging surfaces is not intended to be limited to particular existing wellhead patterns. Rather, such references relate to designs of interacting multiple surface assemblies that engage each other to attach a body such as a blowout preventer body to a wellhead. 
         [0021]    Referring now to  FIG. 4 , the initial contact is by surface  46  on surface  48 . At that point there are preferably gaps  50 ,  52  and  54  that are progressively larger as they are positioned closer to the upper end  56  of wellhead  12 . As the collets move radially to start to apply preload,  FIG. 5  illustrates that gap  50  has disappeared while gaps  52  and  54  still exist. Further radial movement of collets  19  shown in  FIG. 6  shows only gap  54  remains. Finally in  FIG. 7 , all the gaps are gone as the radial movement of the collets  19  is finished. One reason this happens is that the spacing between adjacent ridges  31 ,  33 ,  35  and  37  on the collets  19  is not uniform. In the exemplary embodiment this spacing decreases as between adjacent ridges in a direction going toward upper end  56 . 
         [0022]    There are variations to the pattern in the  FIGS. 4-6 . For example, initial contact can leave only gaps  52  and  54  which then close up in series in a direction toward upper end  56 . Alternatively, only gap  54  can be present at initial contact. To get stress distribution that is more equalized between or among loading surfaces the contact is preferably sequenced in at least two steps with the first being an initial contact location and the next being contact at another load surface preferably spaced between the initial contact location and the upper end  56  of the wellhead  12 . 
         [0023]    In the loading shown in  FIGS. 4-7 , when surfaces  58  and  60  begin contact, surfaces  46  and  48  have already been in contact and have had relative sliding movement between them. When surfaces  62  and  64  begin to contact, surfaces  58  and  60  have increased the stress level from their initial contact and surfaces  46  and  48  now also have greater stress than when they initially contacted and when surfaces  58  and  60  made initial contact. This pattern continues as surfaces  66  and  68  make contact. 
         [0024]    The end result of this sequential contact is the stress and load distribution on the mating tooth profiles  22  and  24  is more balanced from top to bottom instead of being more concentrated toward the upper end  56  of wellhead  12 . The prior designs featuring symmetrical tooth patterns for the collets and the wellhead stressed the uppermost teeth in the profile significantly more than the teeth closer to the collet lower end, where, for example surfaces  46  and  48  are located. By staggering the contact in a pattern using a plurality of pairs of contact surfaces from the downhole to the uphole direction, the resulting stress distribution is more uniform, improving the preload and increasing the integrity of the connection at higher loading conditions. 
         [0025]    Turning to  FIG. 7 , this figure illustrates in detail view an exemplary collet  19  in relation to, for example, a wellhead  12  it secures to. As illustrated, the mating teeth  24  on the wellhead  12  engage with the teeth  22  on the connector  19 . The teeth  22  on the connector comprise a lower tooth  90 , a lower intermediate tooth  92 , an upper intermediate tooth  94 , and an upper tooth  96 . The number of teeth may be increased or decreased as desired. Moreover, although the lower tooth  90  is illustrated as initiating contact with the wellhead, the tooth of initial contact may be one of the other teeth, depending on the particular mechanics of the system, for instance. For example, the lower intermediate tooth  92  may be the tooth of initial contact. 
         [0026]    With respect to these exemplary teeth, and incorporating any slope relationship that may be present with respect to these teeth, certain profile characteristics are present. For example, the distance from a given point on a ridge of a tooth to a corresponding point on a ridge of the same slope-polarity on the adjacent tooth decreases when progressing from a lower tooth to an upper tooth. For instance, in the illustrated embodiment, the distance represented by “Y” is greater than the distance represented by “Z”, and the distance represented by “Z” is greater than the distance represented by “A.” As another characteristic, the intermediate lower tooth  92  is thicker (distance “F”: the distance from a point on a ridge to the corresponding point on the opposite ridge on the same tooth) than upper intermediate tooth  94  (distance “E”). Moreover, upper intermediate tooth  94  is thicker than upper tooth  96  (distance “D”). 
         [0027]    As a result of the arrangement presented in this figure, the gap represented by “J” is larger than that represented by “K”, and the gap represented by “K” is larger than “L”. Conversely, the distances represented by “X” are constant. Advantageously, an arrangement as such, as but one example, provides for the staggered engagement discussed above. 
         [0028]    The above description is illustrative of the exemplary embodiments, and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below. Again, the above description is illustrative of exemplary embodiments, and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below.