Abstract:
A composite marine riser may be formed, in one embodiment, of a pair of threaded metallic sections having threaded external surfaces. The threaded external surfaces may taper from a larger to a smaller diameter. The composite material may be formed over the threaded external surfaces, which surfaces serve to transfer force from the metallic sections to the composite material.

Description:
BACKGROUND  
       [0001]     This invention relates generally to drilling for oil and gas from a floating drilling rig.  
         [0002]     In marine applications, the marine riser connects a rig on the water&#39;s surface to a well bore at the seabed. Drilling risers or production risers may be employed for offshore applications. A marine drilling riser is designed to facilitate a closed drilling fluid system, where fluid pumped down the drill pipe returns back to the drilling rig through a system from seabed to surface. Production risers, used in the completion phase of a subsea well, provide access to the well bore through the completion tubing deep into the well with return of the completion fluids or mud remaining within a closed pumping system from the deepest circulating position through the seabed to surface.  
         [0003]     Generally, risers undergo severe tensile, bending and/or torsional loading while in operation. Currently and conventionally, risers are made of very high strength steel.  
         [0004]     Tension is applied from the floating rig to the riser through the riser tensioner system on the drilling rig. The amount of applied tension is a function of the depth of water between the rig and the seabed floor. The deeper the drilling operation, the longer the riser that must be provided. The longer the riser, the greater the weight and the required tension that is applied to the floating drilling rig. Thus, the deck load on the rig and the applied tension are determined by the length of the riser which, in turn, is determined by the depth of the water over the drilling operation.  
         [0005]     Risers are assembled in sections called joints which vary in length, but generally are about 70-75 feet long. While casing or drill pipe joints are threaded and screwed together, riser joints are connected using a high pressure flanges and bolts. Usually steel or titanium connectors or end fittings are used, especially so in deep water drilling operations. The joints need to absorb not only axial loads but also bending stresses and high vibrations that exist in hostile marine environments.  
         [0006]     The use of composite materials to reduce the weight of risers has been proposed for some time. However, the use of composites has not been well received in the industry for a variety of reasons. One of those reasons is the need to bond the composite material to the metal connectors while maintaining the mechanical integrity of tensile steels or equivalent. Thus, a critical composite riser design element is the load transfer mechanisms between the integrated steel/composite tube body and the end fittings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a schematic depiction of one embodiment of the typical subsea drilling set up in respect to an offshore environment, utilizing a floating drilling rig;  
         [0008]      FIG. 2  is a partial, enlarged, front elevation view of a portion of the embodiment shown in  FIG. 1  in accordance with one embodiment of the present invention;  
         [0009]      FIG. 3  is a partial, enlarged, cross-sectional view taken generally along the line  3 - 3  in  FIG. 2 ; and  
         [0010]      FIG. 4  is a partial, enlarged, cross-sectional-view taken generally along the line  3 - 3  in  FIG. 2  in accordance with another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0011]     Referring to  FIG. 1 , a subsea convention includes a subsea blowout preventer (BOP) stack  12  on the seabed SF connected to the floating drilling rig  66  through a marine riser  22 . The riser  22  is connected to a telescopic joint  58  that takes into account the drilling rig  10  heave.  
         [0012]     The rig  66  has a tension ring  14  at the top of the marine riser  22  to be tensioned using ring tensioners  16 . The tensioners  16  are coupled by pulleys or hydraulic systems  54  to hydraulic cylinders  56  to create a tensioning system  50 .  
         [0013]     The telescopic joint  58  (which is not under tension) allows the upper portion  60  of the apparatus to telescope in and out relative to the lower portion  62  in the event that the rig  66  is heaving relative to the tension ring  14 . Seals  64  prevent fluid escape. The system  50  allows this relative movement and adjustment of relative positioning while maintaining tension on the riser  22 , which extends from the floating rig  66  downwardly to the subsea BOP stack  12 .  
         [0014]     A lower marine riser package (LMRP) emergency disconnect enables the riser  22  to be disconnected from the subsea BOP  12 . The wellhead  22  is also coupled to the subsea BOP  12 .  
         [0015]     Referring to  FIG. 2 . The riser joint  112  includes an upper joint  112   a  joined to a lower joint  112   b  through flanges  114  and bolts  116 . The flanges  114  and bolts  116  may be made of high tensile steel or equivalent.  
         [0016]     Referring to  FIG. 3 , the upper and lower flanges  114  include integral end fittings  115 . The fittings  115  are covered by a composite material  120  woven over the connection between the end fittings  115  and an inner metallic tube  118 .  
         [0017]     As used herein, composite material is a material that includes fibers in a binding matrix. Composite fibers may be glass, carbon, aramid, or other materials. The fiber may be encased in a polymer resin matrix that binds the fibers together and spreads loads across the fibers.  
         [0018]     Resins may be generally thermoset or thermoplastic resins. Both resin types may include molecular polymer chains. Thermosets are cross-linked so they are fixed in one shape. Thermoplastic molecular chains may be processed at higher temperatures and may take a particular shape through molding. Unsaturated polyester resins are commonly used as thermosets.  
         [0019]     Among the useful thermoset materials include polyesters, orthopolyesters, isopolyesters, vinyl esters, epoxies, and phenolics. Examples of suitable thermoplastics include engineered thermoplastics such as polyphenylene sulfide, polyvinylidene fluoride, polyetheretherketone, nylons, polypropylene, and polyethylene.  
         [0020]     The composite material  120  may be made up in a variety of fashions. Among the suitable applications include pultrusion, filament winding, and molding.  
         [0021]     Filament winding is an automated, high volume process. Machine set ups can include two axis mechanical chain drive operations, computer control, multi-axis, and multi-spindle systems capable of producing multiple lines of product at once. A wet winding machine may pull dry roving or tow from creoles or racks through a resin bath. As mandrel rotates on a spindle, the roving or tow delivery system reciprocates along the length of the mandrel, laying down helical ply after helical ply of material. A winding may also be performed using tow pre-impregnated with resin.  
         [0022]     Returning to  FIG. 3 , the fitting  115  includes an externally threaded portion  134 . The portion  134  may taper diametrically outwardly as it extends over the internally threaded riser liner reduced diameter section  136 . The riser liner reduced diameter section  136  and the end fitting  115  have metal-to-metal seal regions  132  at opposed ends of their mating threads to form a threaded, torqued, pressure tight metal-to-metal sealing connection between the liner  118  and the flanges  114 .  
         [0023]     Thus, the length of the joint  112  is primarily taken up by the liner  118  that acts as a substrate on which the composite material  120  is helically wound. For example, the composite material  120  may be wound up using the liner  118  as a rotating mandrel in one embodiment of the present invention.  
         [0024]     The external surface of the liner reduced diameter  136  has an external thread which forms a threaded joint  134  with the fitting  115 . Thus, the liner  118  is simply rotated into threaded, sealing, torque loaded engagement with the end fitting  115 . A good seal is formed by the seal regions  132  and the torsional integrity established through metal-metal seals regions  132 .  
         [0025]     The external surface of the fitting  115  includes a plurality of large helical screw threads  140 . Similar helical screw threads  138  are formed on the exterior surface of the liner  118  proximate to the end fitting  115 . The threads  138  and  140  effectively transfer loads to the composite material  120  and, particularly, transfer tension loads to the composite material  120 . Thus, a substantial portion of the load on the joint  112  is carried by the composite material  120  and, particularly, by the thicker portion of a composite material  120  spaced from the end fittings  115 .  
         [0026]     The threaded surfaces of the fitting  115  and linear  118  taper diametrically inwardly as they extend away from the flange  114 . This provides room for the composite material  120  in one embodiment of the present invention.  
         [0027]     The threads  138  and  140  include a flat upper horizontal surface  142  facing the closest fitting  115  and extending generally transversely to the length of the joint  112 . The threads  138  or  140  also have a slanted lower surface  148  which extends downwardly away from the horizontal surface  142  at an acute angle (for example approximately 30 to 40 degrees). The threads  138  and  140  may be much larger than the threads used for making the joint  134 .  
         [0028]     A metallic load ring  124  may be threaded onto the fitting  115  before the fitting  115  is made up with the liner  118 . Thus, the ring  24  may be shaped to include a face with a thread groove  144  formed therein which matches the pointed end  146  of the threads  140 . Thus, the ring  124  can simply be rotated and screwed upwardly along the helical threads  140  to a desired position along the length. Because the threads  140  expand radially as they extend away from the liner  118 , eventually the ring  124  is frictionally locked in place on the fitting  115 .  
         [0029]     Once the composite material  120  is formed over the load ring  124 , the load ring  124  is effective to transfer both rotary and tensile stress from the fitting  115  to the composite material  120 . In one embodiment of the present invention, the load ring  124  may be positioned above the joint  134  to improve the strength of the overall structure.  
         [0030]     Referring to  FIG. 4 , in one embodiment, the ring  124  is not used but the structure otherwise operates similarly to the embodiment of  FIG. 3 .  
         [0031]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.