Abstract:
A stiffened mandrel used in the manufacture of a composite tubular. The mandrel is placed on a filament winding machine and is comprised of an outer and inner shell. A pressure source is connected to the mandrel to exert a tensile force on the outer shell to stiffen the mandrel and thereby prevent sagging and bending of the mandrel. The stiffened mandrel allows for the manufacture of a composite tubular without undesirable flexing, wrinkling, or contamination of the uncured composite.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to provisional application 61/160,161 filed Mar. 13, 2009. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates in general to fabricating composite pipes using a filament winding process, and in particular, to preventing sag or bending of the mandrel used in filament winding by pressurizing a portion of the mandrel&#39;s interior. 
       BACKGROUND OF THE INVENTION 
       [0003]    Composite materials are used to replace metal in many applications. For example, composite materials are used is in the fabrication of tubulars, such as pipes and pipelines used in oilfield exploration, drilling and production operations. 
         [0004]    The construction of composite tubulars may take a variety of forms, depending upon the intended application of the tubular. Generally, composite tubulars comprise a plurality of layers of material, the selection of the layers being determined by the intended application. Wolfe et al. U.S. Pat. No. 5,261,462 and Wolfe et al. U.S. Pat. No. 5,435,867 disclose a tubular structure such as a pipe or a pressure vessel. Commonly, the composite pipes are prepared by winding the fibers impregnated with epoxy resin using a filament winding process around a mandrel. The filaments can be carbon fiber or other types of materials. As an alternative, if it is required to have a metallic liner present in the finished pipe, the filament winding process may be carried out around the liner itself. The epoxy is allowed to cure in order to complete the composite structure. The selection of the materials, the number, arrangement and thickness of the individual layers, together with the orientation of the fibers in the individual layers, are determined by the type of application. 
         [0005]    During construction of composite tubulars, sagging and bending of the mandrel can result in unwanted flexing and wrinkling of the uncured composite. This is especially true when a long, slender mandrel is used to fabricate very long tubulars having small diameters. 
         [0006]    Several different approaches have been taken to address the problems associated with sagging and bending of the filament winding mandrel. For example, special filament winding machines that apply tension and/or a bending moment to the ends of the mandrel have been used in an effort to prevent sagging in the mandrel. These special filament winding machines are expensive, requiring significant support structure and bearings. Another approach to avoid sagging or bending in the mandrel has been to inflate or pressurize the entire interior length of the mandrel. However, pressurizing the entire interior length of mandrel, which is typically metallic, is potentially dangerous to workers. This is because the significant volume presented by the mandrel&#39;s interior results in a large amount of stored energy in the pressurized mandrel, thereby increasing the destructive potential if the mandrel ruptures. Still another approach has been the use of rollers to support the mandrel during the filament winding process. Although the rollers may prevent sagging and bending, the rollers pinch and contaminate the uncured composite. 
         [0007]    The fabrication of quality long, slender composite tubulars requires that mandrel sagging and bending be controlled. Current methods are expensive or do not prevent the unwanted wrinkling or contamination of the uncured resin. A need exists for a technique to control the sagging and bending in the filament winding mandrel in a cost-effective manner. 
       SUMMARY OF THE INVENTION 
       [0008]    In an embodiment of the present invention, a mandrel comprising an outer cylindrical shell and a concentric inner cylindrical shell sealed against the inner surface of the outer cylindrical shell, can be used to manufacture a composite tubular. A chamber within the mandrel defined by the inner surface of the outer shell and sealed head of the inner cylindrical shell is connected to a pressure source via a test port that communicates with the chamber. The chamber is pressurized until a compressive force is exerted on the inner shell to stiffen and thereby prevent sagging and bending of the mandrel. 
         [0009]    Once the mandrel is stiffened, it is mounted on a filament winding machine and filaments coated in resin are wound onto the mandrel and cured to form a composite tubular. The stiffened mandrel allows for the manufacture of a composite tubular without undesirable flexing, wrinkling, or contamination of the uncured composite. 
     
    
     
       BRIEF 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  illustrates a filament winding machine on which a mandrel is placed, in accordance with an exemplary embodiment of the present technique; 
           [0012]      FIG. 2  illustrates a mandrel assembly in accordance with an exemplary embodiment of the present technique; 
           [0013]      FIG. 3  illustrates the stiffening of the mandrel assembly through pressurization of the mandrel assembly in accordance with an exemplary embodiment of the present technique. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Referring generally to  FIG. 1 , a filament winding machine for producing composite tubulars is presented, and represented generally by reference numeral  9 . The filament winding machine  9  is adapted to secure and rotate a mandrel  10  to enable composite filaments  11  to be wound on the mandrel  10  to produce a composite tubular. In the illustrated embodiment, the filament winding machine  9  comprises a hollow mandrel  10  that may be pressurized internally to increase the rigidity of the mandrel to control sagging during the filament winding process. Filaments  11  can be unwound from a creel system  8  and may be wound onto the mandrel  10  as a drive portion of the filament winding machine  9  rotates the mandrel  10 . The creel system  8  may move back and forth along the axis of the mandrel  10 . 
         [0015]    Referring generally to  FIG. 2 , an exemplary embodiment of the mandrel  10  assembly is illustrated. The illustrated embodiment of the mandrel  10  may be quite long. For example, the illustrated embodiment of the mandrel  10  may be seventy-five feet long or greater. However, the techniques described herein may be used with mandrels of shorter lengths, as well. In the illustrated embodiment, the mandrel  10  is comprises an outer shell  12  and an inner shell  14 . Both shells,  12 ,  14 , may be cylindrical in shape and preferably of steel construction. However, materials other than steel may be used. The inner shell  14  is concentrically located within the outer shell  12 . The outer shell  12  has spindles  20  on both ends for securing it onto a filament winding machine, as shown in  FIG. 1 . The clearance between the outer wall of the inner shell  14  and the inner wall of the outer shell  12  is sufficient to allow the inner shell  14  to move axially relative to the outer shell  12 . The outer shell  12  has a port  18  on one end that communicates through the outer shell&#39;s  12  wall into a chamber  22  defined by the outer shell&#39;s  12  interior and the head  24  of the inner shell  14 . The head  24  of the inner shell  14  preferably has a pair of annular seals  16  to seal one end of the chamber  22  and, thereby, define the volume of the chamber  22 . Annular seals  16  seal between the outer diameter of inner shell  14  and the inner diameter of outer shell  12 . Head  24  of inner shell  14  is spaced inward from the head of outer shell  12 . In addition, the end  26  of the inner shell  14  opposite head  24  is open and contacts the interior end  28  of the outer shell  12 . The interior end  28  of the outer shell  12  is opposite the end with the pressure port  18 . Head  24  serves as a piston to push abutting end  26  against interior end  28 . Although inner shell  14  is shown to be tubular, it could be a solid rod having a cylindrical piston as head  24  and the length of inner shell  14  could be considered to be a shaft of the piston. The term “inner shell” as used herein means not only a hollow tubular member, but also a solid tubular member. In this embodiment, the only seals between the outer diameter of inner shell  14  and the inner diameter of outer shell  12  are seals  16  at head  24 , thus only head  24  needs to be cylindrical and closely spaced to the inner diameter of outer shell  12 . Moreover, if only a single chamber  22  is employed closed end  28  need not seal the interior of outer shell  12 , although it is preferred that inner shell  14  be tubular and have a constant outer diameter. 
         [0016]    As will be discussed in more detail below, the mandrel  10  is pressurized to increase the rigidity of the mandrel  10 . However, rather than pressurizing the entire length of the interior of the mandrel  10 , only the volume defined by the chamber  22  is pressurized. This is a much safer condition than having the entire interior length of a hollow mandrel pressurized. Chamber  22  has a length much shorter than the length of outer shell  12 . 
         [0017]      FIG. 3  illustrates the mandrel  10  stiffening process. This process is preferably performed before any resin and filament is wound onto the mandrel  10 . A compressor or other pressure source  30  is attached to port  18  and operated to pressurize the chamber  22  created by the outer shell&#39;s  12  interior and the sealed head  24  of the inner shell  14 . The chamber  22  can be pressurized hydraulically or with a gas, such as air. The chamber  22  is pressurized until the internal pressure in the chamber  22  induces axial compression (Fc) of the inner shell  14  and corresponding axial tension (Ft) in the outer shell  12 , thereby stiffening the mandrel  10  sufficiently to control sagging or bending. The axial compression (Fc) on the inner shell  14  places a compressive pre-load on the inner shell  14 , which opposes a resulting tensile pre-load (Ft) placed on the outer shell  12 . The tensile pre-load on the outer shell shell  12  increases the effective stiffness of the mandrel  10 , reducing sag and bending. Abutting end  26  of inner shell  14  abuts and exerts a force against interior end  28 , which creates the compression in inner shell  14  and tension in outer shell  12 . Once the chamber  22  is pressurized, the port  18  may then be sealed and the stiffened mandrel  10  can be placed on the filament winding machine  9 , as shown in  FIG. 1 . 
         [0018]    Filaments  11  coated with resin may be wound directly onto outer shell  12 . Once the filament winding process has concluded and the resin has cured, the mandrel  10  is depressurized and extracted from the cured composite pipe by conventional means such as by a hydraulic ram. Although the preceding example relates to fabricating composite pipes for the oil drilling and production industry, the apparatus and process could also be used in other industries such as the converting, paper mill, and membrane industries. 
         [0019]    In another embodiment (not shown), a screw rather than fluid pressure is utilized to axially compress the inner shell  14  and place corresponding axial tension on the outer shell  12 . The screw would be located in chamber  22 . 
         [0020]    Another embodiment (not shown) would use pressure ports on both ends to axially compress the inner shell  14  and place corresponding axial tension on the outer shell  12 . A second chamber  22  similar to chamber  22  would be located on the opposite end of the mandrel  10 . 
         [0021]    The system reduces the problems associated with fabricating long, slender, composite tubulars by stiffening the mandrel. The stiffened mandrel  10  controls sagging and bending and thus reduces wrinkling and flexing of the uncured composite material. Furthermore, the system controls these problems without pinching or contaminating the uncured composite material and without dangerous pressurization of the entire mandrel  10 . The mandrel  10  assembly thus eliminates costly defects in composite tubular fabrication and the need for expensive, specialized machinery. 
         [0022]    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.