Abstract:
A tensioner anti-rotation device and method that transfers rotational force from an offshore platform deck to a riser in response to waves and currents. The device includes a bracket mounted on the deck, and a rib that transfers force from the deck through the bracket, to the rib, and into the riser such that the tensioners on the riser are not subjected to torque.

Description:
[0001]    This application claims priority from the provisional application Ser. No. 60/988,188. filed Nov. 15, 2007 entitled “Tensioner Anti-Rotation Device,” which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field of Invention This invention relates in general to devices for tensioning risers for offshore well production, and in particular to a device that causes the upper portion of the riser to rotate along with the vessel if the vessel rotates due to waves and currents. 
         [0003]    2. Description of Related Art 
         [0004]    Offshore production platforms must support production risers from oil or gas wells which extend to the platform from subsea wells. For platforms that are fixed to the ocean floor this is readily accomplished and is well known in the art. However, for subsea completions in deep water that require the use of floating platforms, such as tension leg platforms or semi-submersible platforms, supporting risers present significant problems. These platforms move under the influence of waves, wind, and current and are subjected to various forces, including rotational forces. Thus, the riser tensioning mechanism must permit the platform to move relative to the riser and care must be taken that the tensioning mechanism can withstand rotational forces. 
         [0005]    The riser tensioning mechanism must also maintain the riser in tension so that the entire weight of the riser is not transferred to the wellhead and so that the riser does not collapse under its own weight. The tensioning mechanism must therefore exert a continuous tensional force on the riser. Also, this force must be maintained within a narrow tolerance. The use of a hydraulic cylinders attached between a platform and a riser to support the weight of the riser is well known in the art. The tensioner cylinders of the prior art are exposed to torque resulting from vessel rotation in response to environmental forces such as waves, currents, and wind. Improvements to protect the tensioner cylinder from this torque are desired. 
       SUMMARY OF INVENTION 
       [0006]    The present invention disclosure comprises a tensioner anti-rotation device and method for transferring rotational force from the deck of a floating platform to a riser, thereby protecting tensioner cylinders from torque produced by vessel rotation in response to environmental forces such as waves, currents, and wind. 
         [0007]    The apparatus comprises a key adapted to be mounted either to the riser or the deck; with a slot adapted to be located on the other of the riser or the deck, in vertical sliding engagement with the slot. The apparatus preferably includes two brackets mounted on the vessel&#39;s deck. Each bracket can have either a key or a slot on its inner side and is preferably located 180° apart from the other. Each bracket is in proximity to the riser and tensioner cylinders. An axially extending key or slot is located on the riser, or on an attached sleeve. The key and slot arrangement between the riser and deck-mounted bracket loosely engage, allowing axial movement of the arrangement with respect to the riser. The system allows for axial movement that is greater than the stroke length of the tensioner cylinders. 
         [0008]    Where the key comprises a rib adapted to be mounted to the riser, the rib has a greater longitudinal length than slot. However, if the slot is adapted to be located on the riser, the slot then has a greater longitudinal length than the key. 
         [0009]    When the vessel rotates, the key and slot arrangement on the riser and bracket engage, with the key contacting the inner portion of the slot. The contact between the key and slot transfers the torque from the deck, the bracket, through the key and slot arrangement, and to the riser. The tensioner cylinders are thus protected from torque that could disturb their alignment or result in damage. 
         [0010]    The apparatus could also include a guide roller adapted to engage riser in lieu of brackets. The guide roller is mounted to the deck and has a slot, with a key adapted to be mounted to the riser. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]    Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
           [0012]      FIG. 1  is a perspective view illustrating a riser tensioner constructed in accordance with this invention 
           [0013]      FIG. 2  is a side elevational view of the riser tensioner of  FIG. 1 . 
           [0014]      FIG. 3  is a sectional view of a portion of the riser of  FIG. 2 , illustrating the attachment of a sleeve. 
           [0015]      FIG. 4  is a sectional view of the riser tensioner of  FIG. 2 , taken along the line  4 - 4  of  FIG. 2 . 
           [0016]      FIG. 5  is a side elevational view of an alternate embodiment of the riser tensioner constructed in accordance with this invention. 
           [0017]      FIG. 6  is a sectional view of the riser tensioner of  FIG. 5 , taken along the line  6 - 6  of  FIG. 5 . 
           [0018]      FIG. 7  is a side elevational view of another embodiment of a riser tensioner constructed in accordance with this invention. 
           [0019]      FIG. 8  is a sectional view of the riser tensioner of  FIG. 7 , taken along the line  8 - 8  of  FIG. 7 . 
           [0020]      FIG. 9  is a side elevational view of still another alternate embodiment of a riser tensioner in accordance with this invention. 
           [0021]      FIG. 10  is a sectional view of the riser tensioner of  FIG. 9 , taken along the line  10 - 10  of  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0022]    Referring to  FIG. 1 , riser tensioner assembly  11  provides tension to a riser  13  that has its lower end secured to subsea equipment such as a subsea wellhead assembly (not shown). Riser  13  extends upward through an opening  15  in a deck  17  of the vessel. Although moored, typically the vessel will move relative to riser  13  in response to current and wave motion. A plurality of cylinder assemblies  19  are supplied with hydraulic fluid and gas under pressure to provide an upward force to riser  13  to maintain a uniform tension in riser  13  as deck  17  moves relative to riser  13 . Cylinder assemblies  19  are conventional, each having a piston and piston rod. A lower end of each cylinder assembly  19  is mounted to deck  17  and an upper end is mounted to a clamp assembly  21 , which clamps to riser  13 . A plurality of guide rollers  23  may be employed to centralize riser  13 . Guide rollers  23  are mounted circumferentially around riser  13  for rolling engagement with riser  13 , or a component mounted to riser  13 . 
         [0023]    In this embodiment, a cylindrical sleeve  25  is mounted around riser  13 . Sleeve  25  is rigidly attached to and surrounds an inner cylinder of the riser  13  so that it will not move axially or rotationally relative to riser  13  and thus may be considered as part of the riser  13 . Sleeve  25  has a length greater than the maximum stroke of cylinder assemblies  19  from the contracted to the extended positions so that rollers  23  remain in engagement with sleeve  25 . 
         [0024]    In this example, sleeve  25  has an inner diameter larger than an outer diameter of riser  13 , defining an annular clearance between them. Sleeve has a flange  27  at its upper and lower ends that extends radially outward. An axially extending key or rib  29  is mounted on the exterior of sleeve  25  and extends from the lower flange  27  ( FIG. 2 ) to the upper flange  27 . Rib  29  may be attached either by welding or fasteners. Rib  29  may have a rectangular or other configuration in cross-section. 
         [0025]    Sleeve  25  may be secured to riser  13  in a variety of manners so as to be rigidly attached. In this embodiment, as shown in  FIG. 3 , flange  27  has a tapered inner diameter  31 . A split bushing  33  has a tapered surface  35  that faces outward and mates with tapered surface  31 . Split bushing  33  is placed around riser  13  then clamped tightly to flange  27 , such as by fasteners. Split bushing  33  has an inner diameter slightly smaller than the outer diameter of riser  13  so that when secured to flange  27 , it wedges sleeve  25  into tight engagement with riser  13 . A similar arrangement for securing sleeve  25  will be located on the lower flange  27 . 
         [0026]    Referring to  FIG. 4 , a bracket  37  is mounted to deck  17  (shown schematically) for each rib  29 . In the embodiment of  FIG. 4 , there are two keys or ribs  29 , each located 180° apart from the other; however, fewer or more ribs  29  could be employed. Each bracket  37  has a slot  39  on its inner side that fits loosely around three sides of one of the ribs  29 . As deck  17  moves upward and downward relative to riser  13 , brackets  37  will slide up and down ribs  29 . If a rotational force occurs, such as from current or yawing of the vessel, this rotational force is transferred from deck  17  through brackets  37 , ribs  29 , sleeve  25  and to riser  13 . The upper portion of riser  13  will rotate with the vessel. The engagement between brackets  37  and ribs  29  causes the upper portion of riser  13  to twist or rotate in unison with deck  17 . This arrangement avoids a twisting force or torque being transferred through cylinder assemblies  19 . 
         [0027]    Other devices may be employed to cause riser  13  to twist if the vessel rotates. For example, the embodiment in  FIGS. 5 and 6  does not employ a sleeve  25 . Rather, key or rib  41  is attached directly to riser  13 , such as by cap screws  43 . As shown in  FIG. 6 , brackets  45  may be identical to brackets  37  of  FIG. 4 . Brackets  45  have slots that receive ribs  41 . 
         [0028]    In  FIG. 7 , rather than a key or rib, a slot  47  is formed, and in this example, slot  47  is formed directly in riser  13 , although it could be formed within a sleeve. Slot  47  extends axially along riser  13  in the same manner as ribs  41  ( FIG. 5 ) and ribs  29  ( FIG. 2 ). As shown in  FIG. 8 , brackets  49  are mounted to deck  17 . Each bracket  49  has a key or tongue  51  that locates within one of the slots  47 . Tongue  51  is able to slide up and down within slot  47 , and will transmit any rotational force from deck  17  to riser  13 . Rollers  23  engage riser  13 . Alternatively, one or more rollers having a central rib that is inserted into slot  47  may be used. 
         [0029]    Referring to  FIG. 9 , in this embodiment, a sleeve  53  is rigidly attached to riser  13 . Sleeve  53  may be attached by flanges  55  and a split bushing (not shown) in the same manner as sleeve  25  of  FIG. 2 . Keys or ribs  57  are secured to the outer diameter of sleeve  53 , either by welding or fasteners. As shown in  FIG. 10 , in this embodiment, there are two ribs  57 , each spaced 180° apart. Rather than a separate bracket, such as bracket  37  ( FIG. 4 ) or bracket  45  ( FIG. 6 ), each rib  57  is engaged by a central portion  61  of one of the rollers  59 . In this example, two of the rollers  59  have one of the central portions  61 , each central portion  61  being of smaller diameter than the outer end portions. The outer end portions engage the outer diameter of sleeve  53 . Each central portion  61  engages one of the ribs  57  to transfer any torque imposed by rotational movement of the vessel through rollers  59  into riser  13 . 
         [0030]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. These embodiments are not intended to limit the scope of the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.