Abstract:
An offshore composite enhanced metallic drilling riser is equipped to enable preloading of the composite shell and the metallic riser. A riser has steel end connectors and a continuous metallic inner liner, encased in a composite shell. A segmented hyperboloid shaped profile is located near each of the end fittings for preloading of the composite and the metallic riser. In one version, both halves of the hyperboloid shape are capable of axial movement by adjusting jack bolts connected to one of the hyperboloid halves. One of the halves is limited in axial movement while the other half moves axially away from the restricted half, the movement simultaneously generates the composite pre-load and the metallic riser pre-load. The other version uses fluid pressure between mating faces of the segments to push them apart.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to offshore drilling, and in particular to a method and apparatus for preloading a composite enhanced metallic drilling riser assembly. 
     BACKGROUND OF THE INVENTION 
     As floating production platforms are moving to deeper waters, lower weight drilling risers are required. A drilling riser is a large diameter string of pipe made up of sections that are secured together, typically by flanged connections. Metallic drilling and productions risers need to be 30% to 50% lighter than metallic risers used in standard depth platforms. A composite overwrap on a metallic tubular improves the hoop characteristics and allows the riser weight to be reduced by approximately 30%. However, a further reduction to 50% requires a unique method to not only support the hoop loading, but also to carry a larger portion of the axial loading. Problems exist in transferring axial loading from the metallic tubular to the composite in a composite enhanced metallic drilling riser system. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, embodiments of the present invention beneficially provide an offshore composite enhanced metallic drilling riser equipped to enable preloading of the composite shell and the metallic cylinder. The composite enhanced metallic drilling riser system as comprised by the present invention comprises steel end connectors and a continuous metallic cylinder, encased in a composite shell. A segmented hyperboloid shaped profile is located near each of the end fittings for preloading of the composite and the metallic riser. A ring is mounted to the metallic cylinder between the halves of the hyperboloid. Both halves of the hyperboloid shape are capable of axial movement by adjusting jack bolts connected to one of the hyperboloid halves. The other end of the jack bolts are secured to connector flanges on the metallic cylinder. One of the halves is limited in axial movement by the ring surrounding the metallic cylinder. As the other hyperboloid half moves axially away from the restricted half, the movement simultaneously generates the composite pre-load and the metallic cylinder pre-load. 
     Embodiments of the present invention also provide an alternate embodiment segmented hyperboloid shaped profile located near each of the end fittings for preloading of the composite and the metallic cylinder. In one embodiment, one half of the hyperboloid shape is moved axially to generate the composite pre-load. Pressure is introduced by a radial port between the metallic cylinder and the composite and enters at the vertical plane of the two hyperboloid halves, to drive the two axially apart. A ratcheting thread is located on the horizontal interface between the hyperboloid half and the metallic cylinder, to maintain the axial position of the hyperboloidal profile while pre-loading the composite. An inwardly biased “C-ring” is located at the vertical plane of the two hyperboloids, and moves radially into the axial gap created between the hyperboloid halves. The width of the “C-ring” allows the calculated pre-load to be maintained, prohibiting the hyperboloid halves from moving closer to one another. 
     In view of the foregoing, the present invention provides an apparatus and method which utilizes the movement of hyperboloid shaped halves in order to provide a reliable method of pre-loading the composite material and the cylinder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a composite enhanced drilling riser assembly constructed in accordance with this invention. 
         FIG. 2  is a sectional view of the composite enhanced drilling riser assembly of  FIG. 1 , during a first portion of a process for preloading the composite enhanced drilling riser assembly. 
         FIG. 3  is a sectional view of the composite enhanced drilling riser assembly of  FIG. 1 , after the process of preloading the composite enhanced drilling riser is completed. 
         FIG. 4  is a sectional view of the ring attachment taken along the line  4 - 4  of  FIG. 2 . 
         FIG. 5  is a sectional view of the inner tail piece taken along the line  5 - 5  of  FIG. 1 . 
         FIG. 6  is a sectional view of a composite enhanced drilling riser assembly constructed in accordance with an alternate embodiment of this invention. 
         FIG. 7  is a schematic sectional view of the composite enhanced drilling riser assembly of  FIG. 6  after preloading. 
         FIG. 8  is an enlarged sectional view of the threaded interface of the composite enhanced drilling riser assembly of  FIGS. 6 and 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a drilling riser assembly, represented generally by reference numeral  20 , is presented. The drilling riser assembly comprises a metallic cylinder  21  made up of sections of riser pipe secured together. In this embodiment, the various pipe sections are secured together by flanges  22  and bolts (not shown), but other means are feasible, such as by radially moving dogs. Flange sections  22  are welded onto each end of metallic cylinders  21 . 
     A ring  23  is placed around and welded to metallic riser  21 . Ring  23  contains a set of milled slots  25  in its outboard face ( FIG. 4 ). Two segments, inner tail piece  26  and outer tail piece  27 , form a hyperboloid shaped profile when positioned together around metallic cylinder  21 . Inner tail piece  26  is machined with a shoulder  28  on its interior surface, allowing it to pass over and move relative to ring  23 . Tabs  29  are securely attached to inner tail piece  26  and align with milled slots  25  on ring  23  ( FIG. 5 ). Milled slots  25  and matching tabs  29  ensure that inner tail piece  26  does not rotate about the axis of metallic cylinder  21 . 
     Ring  23  is captured between inner tail piece  26  and tabs  29 . The axial movement of inner tail piece  26  is limited in range by shoulder  28  and tabs  29 . Outer tail piece  27  is placed around metallic cylinder  21  and positioned in abutment with inner tail piece  26  and tabs  29 , forming a hyperboloid shaped profile. The end of outer tail piece  27  closest connector flange  22  is machined with a plurality of threaded holes  31  capable of receiving jack bolts  33 . Jack bolts  33  extend through apertures on flange  22  and screw into threaded holes  31  on outer tail piece  27 . Once ring  23 , inner tail piece  26 , and outer tail piece  27  are assembled on cylinder  21 , a mold release agent is placed over these components, ensuring that the composite layer  37  does not bond to the components during the application process. 
     A composite layer  37  is then formed over the metallic riser  21 , inner tail piece  26 , and outer tail piece  27 . The composite fabrication process may be accomplished by a variety of processes including, for example, filament winding, tape laying, roll wrapping, and hand layup. Once the composite has cured, the assembly is ready to be preloaded. 
     Referring generally to  FIGS. 1-3 , the riser assembly  20  comprises a preloading system that is adapted to apply a tensile load to the composite layer  37  and apply a compressive load to the cylinder  21 . As jack bolts  33  are tightened, outer tail piece  27  is moved axially toward flange  22 . As outer tail piece  27  moves closer to flange  22 , the movement simultaneously causes composite layer  37  to move axially toward flange  22 . As outer tail piece  27 , and composite layer  37  move, inner tail piece  26  also moves closer toward flange  22 , while ring  23  remains fixed to cylinder  21 . The axial movement of the composite layer  37  with respect to cylinder  21  results in a tension preload in composite layer  37  which is balanced by a compression preload in cylinder  21  ( FIG. 3 ), as represented by arrows. The preload of the composite  37  against cylinder  21  relieves cylinder  21  of some portion of the externally applied tensile load borne by the riser joint assembly  20  when it is placed in service within a riser string. The riser assembly  20  allows the apportionment of the applied load carried between the cylinder  21  and the composite  37  to be controlled and optimized. 
     Inner tail piece  26  is able to move axially toward flange  22 , but is limited in range by shoulder  28  contacting ring  23 . When shoulder  28  comes into contact with ring  23 , inner tail piece  26  can no longer move axially. As illustrated by  FIG. 3 , jack bolts  33  may be turned even further, resulting in increased axial movement of outer tail piece  27 , and an increased distance between inner tail piece  26  and outer tail piece  27 . The movement forces inner tail piece  26  and outer tail piece  27  into greater contact with the inner surfaces of layer  37 , increasing pre-loading of the composite to metal joint. 
     The axial movement of outer tail piece  27  away from inner tail piece  26  increases the contact pressure between tail pieces  26 ,  27  and composite  37 . This increased contact pressure creates an internal preload between metallic components  26 ,  27  and composite  37  of the composite to metallic interface. The preload prevents looseness or relative motion between composite  37  and components  26 ,  27 , increasing fatigue performance. 
     As outer tail piece  27  moves closer to flange  22 , the movement simultaneously causes composite layer  37  to move, placing the composite structure in increased tension. As composite layer  37  is placed in increased tension, metallic cylinder  21  is placed in increased compression. The result is simultaneous pre-loading of composite structure  37  and metallic cylinder  21 . The end of cylinder  21  opposite the end shown may have a similar arrangement to apply tension and enhance bonding of composite layer  37 . 
     Referring generally to  FIGS. 6-8 , an alternate embodiment of a drilling riser is presented. Referring to  FIG. 6 , the riser assembly  41  includes a metallic cylinder  43  made up of sections of riser pipe secured together. In this embodiment, the various pipe sections are secured together by flanges  45  and bolts (not shown). Flange sections  45  are welded onto each end of metallic cylinders  43 . Floating segment  47  and fixed segment  49  form a hyperboloid shaped profile when positioned together around metallic cylinder  43 . Segment  49  is shaped as half of a hyperboloid, and is fixed to cylinder  43 ; segment  49  may be formed integrally as part of cylinder  43 . Segment  47 , the other half of the hyperboloid, is connected to the metallic cylinder  43  by way of a ratchet interface arrangement  51 . The two segments  47 ,  49  are positioned in abutment with one another to form a hyperboloid shaped profile near the end segments of each cylinder  43 . 
     Referring to  FIG. 8 , ratchet interface  51  may include a split ring  53  with external teeth  55 . Split ring  53  is carried in a recess  56  of segment  47 . Split ring  53  is biased inward into engagement with thread or grooves  57  formed on the exterior of cylinder  43 . Teeth  55  are saw-toothed in shape. As segment  47  moves in the direction of the arrow, ring  53  expands and contracts, with teeth  55  moving over grooves  57 . 
     An angled shoulder  59  is located on the outer diameter of face  61  of segment  47 . Face  61  is perpendicular to the axis of cylinder  43  and initially abuts a similar face  63  on segment  49 . An inwardly biased C-ring  65 , of a predetermined width is held in shoulder  59 . An access port  67  is located radially outwards from cylinder  43 , extends axially along the length of cylinder  43 , passes through segment  47 , and ends at the abutting faces  61 ,  63  of the two hyperboloid halves  47 ,  49 . 
     A mold release agent is placed over the riser  43 , segments  47 ,  49 , C-ring  65 , and port  67 , ensuring that the composite material does not bond to these components during the application process. A composite layer  71  is then formed over the metallic riser  43  and the hyperboloid shaped profile. The composite fabrication process may be accomplished by a variety of processes including, for example, filament winding, tape laying, roll wrapping, and hand layup. Once the composite has cured, the assembly is ready to be preloaded. 
     As illustrated by  FIG. 7 , air or another fluid is introduced through pressure port  67 , and enters between faces  61 ,  63  of the two hyperboloid halves  47 ,  49 , driving the two axially apart as pressure increases. Ratcheting thread arrangement  51  maintains the axial position of the hyperboloidal profile  47  while pre-loading the composite  71 . As pressure is supplied to port  67 , the pressure build up between faces  61 ,  63  of segments  47 ,  49  causes segment  47  to move away from segment  49  and toward flange  45 . Inwardly biased C-ring  65  slides on tapered shoulder  59  and moves radially into the axial gap created between the faces  61 ,  63  of hyperboloid halves  47 ,  49 . The width of C-ring  65  allows the calculated pre-load to be maintained, prohibiting the hyperboloid halves  47 ,  49  from moving closer to one another. As segment  47  moves axially, composite layer  71  is placed in tension. The movement of segment  47  also causes pre-loading of the composite to metal joint due to the increased contact between the composite layer  71  and segments  47 ,  49 . 
     Alternatively, the positions of segments  47 ,  49  could be switched so that segment  49  move axially away from flange  45 . This arrangement would provide a means for pre-loading the composite to metal joint. However, the arrangement would not place the entire composite structure  71  in tension as the previous arrangement. 
     While the invention has been shown in only two of its forms, it should be apparent to those skilled in the art that it is not so limited but susceptible to various changes without departing from the scope of the invention.