Abstract:
A tensioner assembly for applying tension to a tubular member, such as a riser, can include an upper latch connected to the tubular member, a platform with a bore, and a lower latch ring. After applying tension to the tubular member, the lower latch ring can be closed around the tubular member so that when the tension is released, the upper latch lands on and engages the lower latch. The assembly can include a locking mechanism that prevents axial movement of the upper latch, relative to the lower latch, after engagement. The upper latch can self-center on the lower latch as it is moved into the latching position.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to mineral recovery wells, and in particular to an apparatus and method for supporting a tensioned tubular assembly. 
     2. Brief Description of Related Art 
     Tubular members such as wellbore risers are often placed under tension. A riser, for example, can extend from a subsea wellhead upward to a drilling platform. It is often necessary to place a certain amount of tension on the riser. The tension can be applied by, for example, latching the riser into place on the wellhead, and then drawing it upward through an opening in a drilling platform until the riser is subject to the desired amount of tension. The riser can then be latched into place by a latching mechanism on the drilling platform to maintain the tension. Conventional methods of tensioning and latching a riser have numerous problems. 
     For example, it can be difficult to center the riser assembly within the opening of the drilling platform or within the latching mechanism. If the riser is offset within the opening, then it can be difficult, or even unsafe, to latch the riser in position with conventional latching mechanisms. Those conventional latching mechanisms can include segmented dogs that can engage the riser assembly. It is difficult to engage in the riser with segmented dogs when the riser is offset. Engaging the riser with the segmented dogs can also require personnel to be present on the drilling platform to operate heavy equipment. Safety can be an issue any time personnel are operating heavy equipment, especially in close proximity to a tensioned riser. Furthermore, heavy equipment must be lifted and operated in order to engage the riser with the segmented dogs, which can further present safety issues. Additionally, the conventional latching mechanisms have a large number of moving parts. Those moving parts can be expensive and can have mechanical failures. 
     Another problem with conventional latching techniques is that they are not able to prevent upward movement of the riser assembly. Under some circumstances, risers can be subjected to upward force that can cause the riser assembly to thrust upward from the drilling platform. Conventional risers are not suited to provide downward support to prevent a riser assembly from thrusting upward. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention include a method and apparatus for applying tension to a tubular conductor, such as a riser for subsea well drilling operations. Specifically, a tension latch can sit atop a conductor, such as a riser assembly, or on a deck of an offshore platform. As the riser is made up, all segments of the riser system must pass through a rotary or a spider. One constraint for the riser is that the greatest outer diameter (“OD”) on the riser must be less than the inner diameter (“ID”) of the spider. The same limitation is also present at the tensioner; the largest OD must be able to pass through the tension latch. In the past the tension latch is a segmented ring that pivots backwards inside a housing and leaves an opening to allow the largest member of the riser to pass. Once the riser has moved to the proper location, then the segmented latches can be rotated into position and made up to complete the tensioner system. The segmented latch design in the past has also presented some make up obstacles, such as making up with an offset on the riser due to loading. 
     In embodiments of the present design, the latch ring includes two separate components. There is a lower latch that can be a segmented ring design that is configured as a single piece component. The upper latch is a solid ring latch that is run on the tension joint. As the riser is run, the lower latch ring and housing assembly are retracted by a spider like device so it does not interfere with the riser running. This allows the riser to pass with no ID limitations once it is through the spider. The tension joint is run with a solid piece latch pre-installed at a predetermined position. Once the riser is close to the landed position the lower latch ring and housing assembly is actuated into place by, for example, a hydraulic powered system (similar to a spider) and fixed in the final position. A c-ring is installed on the upper latch ring, which can provide retaining force should there be an upward force on the tension latch. The lower latch ring and housing assembly can now accept the solid upper latch ring, as it is lowered into place. As the upper latch lands out on the lower latch it compresses the c-ring; once it is fully landed the c-ring will snap back inward into a groove in the lower latch. This c-ring can provide the capability to support an upward force. 
     The method of operating the system can include inserting a c-ring into a solid upper tension latch and installing the upper tension latch on the tension joint (prior to welding). The tension joint can be passed down through the tensioner with a centralizer ring attached to keep the tension joint (riser) in the correct position. Once the exact location of the upper tension latch is determined, the latch can be rotated on the threads on the tension joint to determine the exact position and be brought to that position. The upper tension latch outer diameter is small enough to pass through the rotary or spider. The lower tension latch is actuated, for example hydraulically, outward while the riser is being (using a device similar to a spider), which allows the riser to pass through easily. Once the tension joint is in the appropriate location the upper tension latch is in place), the lower tension latch is actuated into the proper position. The geometry of the upper tension latch allows it to self-center as it is lowered over the lower tension latch, regardless of initial offset. This will centralize even when the tension joint is at the maximum offset allowed by the tension ring. The upper tension latch lowers over the lower tension latch and compresses the c-ring attached to the upper tension latch and the upper tension latch lands out on the lower tension latch. At the same time, the c-ring snaps into a groove in the lower tension latch. The c-ring provides the necessary area to prevent axial movement of the upper latch, relative to the lower latch, in response to an upward force in the tension joint. 
     The “Self centering” feature makes installation and running the equipment easier and safer. For example, embodiments of the design do not include dogs or dog teeth to center and engage the riser and, thus, do not require rig personnel to be in the immediate vicinity of the latch and riser during tensioning. The operation is also safer because there is no need for manual labor to move dogs and the lower tension latch is not actuated hydraulically when the riser is under tension. In embodiments having hydraulic actuators, they can be actuated before the riser is placed under tension. Additionally, the self-centering function can center the upper latch and riser more quickly and more consistently than conventional tensioning systems. 
     Furthermore, embodiments of the tension latch assembly can handle a large load if the tension joint were to generate an upward force, which was not previously possible. In addition to being safer and handling upward force, embodiments of the tension latch assembly use fewer parts than conventional latch designs. 
     An embodiment of an apparatus for providing tension to a riser includes a platform having a bore therethrough, a tubular member extending through the bore, an annular upper latch member connected to an outer diameter of the tubular member, the upper latch member having a downward facing latch recess on a bottom surface, and a retractable lower latch ring connected to the platform, the lower latch ring being movable from an open position to a latch position. The open position allowing the upper latch member to pass through and the latch position stopping downward axial movement of the upper latch member, the lower latch ring having a cylindrical guide extending upward in an axial direction and having an outer diameter that is less than an inner diameter of the latch recess when the lower latch ring is in the latch position so that the cylindrical guide can fit inside the latch recess. 
     Embodiments of the apparatus include a downward and inward facing tapered surface extending downward from the latch recess. The tapered surface can center the upper latch member on the lower latch ring when the cylindrical guide enters the latch recess. Embodiments can include an annular lock ring recess on each of an outer diameter surface of the cylindrical guide and an inner diameter surface of the upper latch member, and a resilient lock ring initially positioned in one of the annular lock ring recesses, the lock ring expanding to engage the other annular lock ring recess when the cylindrical guide is positioned inside the upper latch member. The resilient ring can be a c-ring. The lock ring can be initially positioned in the annular lock ring recess of the upper latch member. The resilient ring can engage the latch recess and, thus, prevent the upper latch member from moving axially upward. 
     In embodiments of the apparatus, the upper latch member threadingly engages the outer diameter of the riser. In embodiments, the upper tension latch is a solid member free of moving parts. Embodiments include a hydraulic actuator connected to the lower latch ring, the hydraulic actuator causing the lower latch ring to move between the open and the closed positions. 
     In embodiments of a method for tensioning a riser, the method includes the steps of connecting an upper tension latch to a tension joint, the tension latch having a downward facing annular receptacle and the tension joint being a segment of a riser assembly; passing the tension joint downward through an inner diameter of a lower latch assembly to determine the desired amount of tension, then tensioning the riser assembly by drawing the tension joint upward through the lower latch assembly; moving the lower latch assembly from an open position to a latch position, the inner diameter of the lower latch assembly being less than an outer diameter of the upper tension latch when the lower latch assembly is in the latch position; and lowering the tension joint onto the lower latch assembly until a portion of the lower latch assembly occupies the annular receptacle and engages a downward facing surface at the uppermost portion of the annular receptacle to prevent further downward movement of the lower latch assembly. 
    
    
     
       BRIEF DESCRIPTION OF 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 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. 
         FIG. 1  is an environmental view of an embodiment of the tension latch assembly. 
         FIG. 2  is a partial environmental view of the tension latch assembly of  FIG. 1 , showing the latch support and lower latch in the closed position. 
         FIG. 3  is a partial sectional side view of the tension latch assembly of  FIG. 1 . 
         FIG. 4  is a partial sectional side view of the tension latch assembly of  FIG. 1  showing an offset condition. 
         FIG. 5  is a partial sectional side view of the tension latch assembly of  FIG. 1  showing partial engagement of the lower and upper latch assemblies. 
         FIG. 6  is a partial sectional side view of the tension latch assembly of  FIG. 1  showing the upper latch landed on and lockingly engaged to the lower latch. 
     
    
    
     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, and the prime notation, if used, indicates similar elements in alternative embodiments. 
     Referring to  FIG. 1 , a tension latch system  100  is shown. Tension latch system  100  can be used in a variety of applications requiring tension to be applied to a tubular member including, for example, the application of subsea well drilling operations. As shown in  FIG. 1 , tension latch system  100  is used to apply tension to riser  102 , which is a riser extending from a wellhead (not shown) at the ocean floor up to a drilling platform  104  and through bore  106  of drilling platform  104 . Riser  102 , which can be conventional, is an assembly made up of tubular riser segments. Tension joint  108  is installed as one or more segments of riser  102 . Tension joint  108  is a tubular member having threads  110  on an outer diameter surface. Upper latch  112  is installed on tension joint  108  by way of threads  114  ( FIG. 3 ) on an inner diameter surface which threadingly engage threads  110 . Upper latch  112  can, thus, be positioned anywhere along the threaded portion of tension joint  108  by rotating upper latch  112 . Other techniques can be used to engage and position upper latch  112  on tension joint  108 . For example, upper latch  112  can have a ratcheting mechanism (not shown) which can engage threads or wickers (not shown) on tension joint  108 . Upper latch  112  has an outer diameter that is smaller than the inner diameter of bore  106  so that upper latch  112 , as well as riser  102  and tension joint  108 , can pass through bore  106 . 
     Lower latch  116  is a segmented annular ring having segments  118  and  120 . In the embodiment shown in  FIG. 1 , lower latch  116  includes two such segments  118  and  120 , each of which is semi-circular. Embodiments can have a greater number of segments which can be assembled to create an annular lower latch assembly. Lower latch  116  is connected to latch support  122 . Latch support  122  can be any structure and mechanism that can support segments  118  and  120  as they move between the open and latched position. In the open position, segments  118  and  120  are spaced apart such that upper latch  112  can pass between segments  118  and  120 . Segments  118  and  120  move linearly or pivotally between the open and the latch position. The movement can be in response to, for example, a hydraulic actuator, an electric actuator, or any other type of mechanism sufficient to move latch support  122  and latch segments  118  and  120 . 
     Referring now to  FIG. 2 , lower latch  116  is shown in the latched position. In the latched position, the segments of latch support  122  have moved toward each other so that segments  118  and  120  are brought together to form lower latch  116 . Latch  116  has an inner diameter  126 , which is larger than the outer diameter of riser  102  so that riser  102  can extend through latch  116  when latch  116  is in the latch position. 
     Referring now to  FIG. 3 , lower latch  116  has a guide  128  extending upward to define the uppermost portion of lower latch  116 . Guide  128  is a cylinder and having the same inner diameter  126  as the rest of tower latch  116 . Top surface  130  defines the uppermost portion of guide  128 . Top surface  130  can be generally flat or can have a profile. Shoulder  132 , the transition from the outer diameter of guide  128  to top surface  130 , has an upward and outward facing tapered surface. Guide  128  is shown as a cylindrical guide having a solid cylindrical body, but other configurations of cylindrical guide can be used guide upper latch  112  into concentric alignment with lower latch  116 . For example, a plurality of posts or a plurality of arc-shaped segments (not shown) can be spaced apart around lower latch  116 , each of the posts or segments (not shown) extending upward from lower latch  116  and having a generally vertical portion for engaging upper latch  112 . 
     The surface of outer diameter  134  of lower latch  116  includes an annular groove  136 , which can be located somewhere between the upper and lower boundaries of guide  128 . The body of lower latch  116  also includes support groove  142 . As shown in  FIG. 3 , support groove  142  is an upward facing annular groove. Support groove  142  has a v-shaped cross section so that the axial depth increases from the deepest part of the groove when moving radially inward and radially outward. 
     Still referring to  FIG. 3 , upper latch  112  has a generally frustoconical shape with an outer surface that generally faces outward and upward, and has a bore therethrough. As discussed above, threads  114  are located on the inner surface of the bore. Upper latch  112  is not limited to a frustoconical shape. The outer surface can be, for example, cylindrical, octagonal, or a variety of other profiles. In embodiments, upper latch  112  can be a solid member free of moving parts. 
     Latch recess  146  faces downward from the bottom end of upper latch  112 . Latch recess  146  is a bore having a bore sidewall  148 , the diameter of which is the same is or slightly greater than the outer diameter of guide  128 . The opening of latch recess  146  includes a downward and inward facing taper  150 . In embodiments, taper  150  can extend at an angle of about 10-80 degrees relative to the axis of upper latch  112 . In embodiments, taper  150  can extend at an angle of about 30 degrees to about 60 degrees relative to the axis of upper latch  112 . In embodiments, taper  150  can extend at an angle of about 45 degrees relative to the axis of upper latch  112 . Outward taper  152  faces downward and outward and is located at the bottom of upper latch  112 , proximate to taper  150 . The profile of taper  150  and outward taper  152 , combined, can be an inverse of the profile of support groove  142 . 
     The upper portion of latch recess  146  includes a downward facing shoulder  156 . Shoulder  156  can be generally flat or can have a profile. The shape of shoulder  156  can be the inverse of the shape of top surface  130 . The axial length from the uppermost portion of taper  150  to shoulder  156  is about equal to or greater than the axial length from the uppermost portion of the inner leg of support groove  142  to top surface  130  of guide  128 . In embodiments wherein that axial length is the same, tapers  150  and  152  can land in and be supported by support groove  142 , and downward facing shoulder  156  can land on top surface  130 , when tension joint  108  lands on lower latch  116 , as best shown in  FIG. 5 . 
     Annular lock ring recess  154  is a groove located on bore sidewall  148 , such that the diameter of lock ring recess is greater than the diameter of bore sidewall  148 . The axial height of lock ring recess  154  is approximately the same as the axial height of groove  136 . A resilient lock ring  138  is installed in groove  136 . In embodiments, lock ring  138  can be a c-ring. Lock ring  138 , in its relaxed state, has an outer diameter greater than the outer diameter of guide  128  and in inner diameter greater than the outer diameter of groove  136 . The cross-sectional width of lock ring  138  is less than or equal to the depth of groove  136 . Lock ring  138  is installed in groove  136  so that it protrudes outward from the surface of guide  128  but can be compressed into groove  136  until it is flush or nearly flush with the outer diameter surface of guide  128 . The upper and outer shoulder  1140  of lock ring  138  is a tapered surface. In some embodiments (not shown), the lock ring can initially be installed in an annular groove on the lower latch such that it expands and engages a corresponding groove on the upper latch when the upper latch lands on the lower latch. 
     Access ports  158  are passages from the exterior of upper latch  112  to the outer diameter surface of lock ring recess  154 . As best shown in  FIG. 5 , when tension joint  108  is landed on lower latch  116 , lock ring recess  154  is axially aligned with groove  136 . When latch  112  is positioned on lower latch  116 , lock ring  138  expands outward to permit outer diameter  134  of lower latch  116  to pass into latch recess  146 . Latch  112  moves downward onto lower latch  116  until lock ring recess  154  is aligned with annular groove  136 , at which time lock ring  138  collapses inward to engage annular groove  136 . When engaging annular groove  136 , lock ring  138  still partially resides in lock ring recess  154  and, thus, prevents axial movement of latch  112  relative to lower latch  116 . 
     Referring to  FIG. 4 , in the event that riser  102  is offset in bore  106 , lower latch  116  functions as a centralizer to center latch  112 , and thus riser  102 , as it is latched into place.  FIG. 4  illustrates an offset condition. As latch  112  moves downward, taper  150  contacts shoulder  132 . Due to the angle of taper  150 , taper  150  slidingly engages the contact point of shoulder  132 , thereby forcing latch  112  into concentric alignment with lower latch  116  as latch  112  moves downward. 
     Referring now to  FIG. 5 , as upper latch  112  is lowered onto lower latch  116 , taper  150  urges lock ring  138  inward into annular groove  136 . Upper latch  112  moves axially downward so that guide  128  of lower latch  116  enters lock recess  146 . In embodiments having other configurations of guide  128 , such as spaced apart upward extending posts or arc-shaped segments, the posts or arc-shaped segments enter lock recess  146 . Referring now to  FIG. 6 , continued downward movement of latch  112 , relative to lower latch  116 , causes upper latch  112  to land on lower latch  116 . Tapers  150  and  152  land in support groove  142 . In embodiments, shoulder  156  can also land on top surface  130 . The landed surfaces prevent further downward movement of upper latch  112  relative to lower latch  116  and, thus, prevent downward movement of riser  102  relative to platform  104 . Upon landing, lock ring  138  radially expands outward to engage both lock ring recess  154  and annular groove  136 , thereby preventing upward movement of upper latch  112  relative to lower latch  116 . 
     Furthermore, the v-shape profile of support groove  142  reduces or eliminates lateral movement of upper latch  112  relative to lower latch  116 , thus centralizing riser  102  in bore  106 . For example, downward and inward facing taper  150  can engage support groove  142  to prevent lateral movement of riser  102  toward the axis of bore  106 , and outward taper  152  can engage support groove  142  to prevent lateral movement of riser  102  away from the axis of bore  106 . Because the interlocking surfaces are annular, they prevent lateral movement of riser  102  in any direction relative to bore  106 . 
     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.