Abstract:
A pipe is wrapped with a composite to form a pre-stressed interface to facilitate load sharing between the materials. Prior to being wrapped, the pipe is placed in tension via hydraulic devices or the like. Alternatively, a combination of cured and uncured composite wraps is used in proximity to each other to apply compressive stress to the pipe. In addition, the pipe may be subjected to cold temperatures to change its dimensions prior to being wrapped with a composite material.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present invention relates in general to reinforced pipes and, in particular, to an improved system, method, and apparatus for pre-tensioning riser pipes to share load with composite covers. 
         [0003]    2. Description of the Related Art 
         [0004]    In the prior art, the internal pressure capacity of steel alloy risers may be increased by wrapping the outer diameters (OD) of the risers with a composite material, such that the pressure capacity is shared by the steel pipe and the composite wrap. For example, in one application a pipe is wrapped with a carbon-fiber and epoxy matrix composite that is applied to the OD. The composite is strictly limited to increasing the hoop stress capacity of the pipe. In addition, a fiberglass covering can be applied over the top of both the composite and the pipe to provide damage protection. 
         [0005]    During service, the pipe is subjected to high axial tension and bending moments, extremely high internal pressures, and environmental fatigue loads. Consequently, one manufacturing consideration is how to prevent the pipe from yielding prior to the internal pressure load being taken by the composite. 
         [0006]    Several methods that address this issue have been suggested and include the following: (a) pre-yielding the composite-wrapped pipe by pressuring the inner diameter (ID) past the yield strength of the pipe to ensure load sharing (also known as “auto-frettage”); (b) letting the load sharing occur via deflection (with or without yielding) during normal service of the product; and (c) Developing hoop stress by applying high tension to the carbon fibers during winding of the pipes (also known as “frettage”). Although these solutions are workable for some applications, improved solutions having even higher performance capabilities would be desirable. 
       SUMMARY OF THE INVENTION 
       [0007]    Embodiments of a system, method, and apparatus for pre-stressing the interface between a pipe and composite wrap on the pipe are disclosed. The invention improves the load sharing between the two materials and thereby prevents yielding of the materials. In one embodiment, the pipe is put in tension prior to being circumferentially wrapped with a composite material, such as a carbon fiber-based product. After the pipe is wrapped and the pre-tension is relieved, a pre-stressed interface between the pipe and composite is achieved. 
         [0008]    The composite wrap may be circumferentially applied from a spooling machine that is tangent to the OD of the pipe. The composite is cured for some applications to pre-stress the interface between the pipe and composite. Alternatively, axially-oriented composite fibers may be applied as well, or a combination of axial and circumferential fibers. The fibers may be wrapped in bundles or as a continuous, pre-manufactured tape. 
         [0009]    In addition, the invention comprises various methods of applying the tension to the pipe segments. For example, one or more annular hydraulic pistons may be inserted into an interior of the pipe section prior to winding the carbon fibers. End connections are welded to the pipes and are formed (e.g., machined) with reaction features against which the ends of the piston react. The hydraulic piston applies tension to the pipe and the pipe is wound with the composite. After the composite is wound and cured, the hydraulic piston pressure is relieved. The hydraulic tensioning equipment is reusable for each pipe segment. The internally-tensioned pipe advantageously does not interfere with the external wrapping of the pipes. The pipe can be spun with a stationary fiber spool, or the fiber spool may wind around a stationary pipe. 
         [0010]    Alternatively, the pipe may be externally tensioned with a tensioning frame having one or more hydraulic cylinders for applying force. The frame reacts against an upset end piece on the pipe, such as a feature on the welded end connections. The hydraulic cylinder reacts against the frame to supply the tension. The external system can apply more tension to the pipe than an internal system. The frame does not interfere with the winding process and has sufficient space around the frame to add the fibers to the pipe. In one embodiment, the pipe rotates with respect to the spooling machine and the tension frame, and uses high-capacity bearings between the pipe reaction shoulder and frame. 
         [0011]    In another embodiment, a combination of cured and uncured composite wraps is used in proximity to each other. For example, a dry fiber assembly or pre-cured composite assembly is first wrapped onto the pipe under tension to apply a compressive stress to the pipe. The pipe is then rotated in the opposite direction while a wet composite is simultaneously applied to the pipe directly adjacent to the unrolling dry fiber assembly. In this way, the wet composite takes advantage of the pre-compressed condition beneath the dry composite. After the wet composite has completely replaced the dry fiber, the wet composite is allowed to cure on the compressed pipe. 
         [0012]    In still another embodiment, the length and diameter of the pipe is reduced prior to wrapping by significantly reducing a temperature of the pipe. The cooling operation may be performed by extreme freezing in a chamber, or a continuous supply of a cold medium to the ID of the pipe (such as liquid nitrogen). The cold temperatures may reduce curability of some composite matrix materials (e.g. heat-cured epoxy). However, matrix materials other than heat-cured epoxies (e.g., UV-cured epoxies) may be used. In addition, the pipe axially expands upon re-warming, causing an axial pretension in the composite wrap. 
         [0013]    The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the present invention, taken in conjunction with the appended claims and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    So that the manner in which the features and advantages of the present invention, 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 embodiments thereof that are illustrated in the appended drawings which form a part of this specification. It is to be noted, however, that the drawings illustrate only some embodiments of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
           [0015]      FIG. 1  is an isometric view of one embodiment of a pre-tensioned pipe constructed in accordance with the invention; 
           [0016]      FIG. 2  is an isometric view of a pipe illustrating the effect of tension being applied thereto; 
           [0017]      FIGS. 3A-3C  are isometric views of another embodiment of a pre-tensioned pipe at initial, intermediate, and final phases of assembly and are constructed in accordance with the invention; 
           [0018]      FIG. 4  is a sectional side view of a third embodiment of pre-tensioning a pipe and is constructed in accordance with the invention; 
           [0019]      FIGS. 5A and 5B  are sectional side and isometric views of a fourth embodiment of pre-tensioning a pipe and is constructed in accordance with the invention; 
           [0020]      FIG. 6  is an isometric view of a fifth embodiment of pre-tensioning a pipe and is constructed in accordance with the invention; 
           [0021]      FIG. 7  is a high level flow diagram of a sixth embodiment of pre-tensioning a pipe and is constructed in accordance with the invention; 
           [0022]      FIG. 8  is a schematic isometric view of another embodiment of pre-tensioning a pipe and is constructed in accordance with the invention; and 
           [0023]      FIG. 9  is a schematic isometric view of still another embodiment of pre-tensioning a pipe and is constructed in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    Referring to  FIG. 1 , one embodiment of an assembly  11  comprising a composite-wrapped pipe  13  is shown. The letter “T” represents tension being applied to the pipe section along a longitudinal axis thereof. Although the pipe is schematically illustrated without end connections, some applications of the pipe may be fitted with connectors or other means of joining pipe sections together along their axes. The pipe also includes a composite fiber-matrix material  15  that is overlaid on top of the pipe&#39;s outer diameter (OD). 
         [0025]      FIG. 2  illustrates the effects of applying axial tension T to pipe  13 . As the pipe is placed in tension it stretches in the direction that the load T is applied. Because the volume of material cannot change, the pipe deflects inward for a reduction in diameter. This is known as Poisson&#39;s effect. Thus, when a pipe of original length L and diameter D is placed under axial tension T, the pipe expands to length L T  and reduces in diameter to D T , where L T &gt;L and D&gt;D T . If the stress in the pipe is below yield strength, the pipe will elastically return to the original dimensions D and T when the tension is removed. 
         [0026]    If the pipe  13  has a restriction (e.g., a composite covering  15  as shown in  FIG. 1 ) placed on the OD while the pipe is under axial tension T, the covering  15  restricts the capability of the pipe to return to its original shape after the tension is removed. This restriction produces a pressure at the interface between the pipe  13  and the covering  15 , which develops a tensile hoop stress in the covering  15  and a compressive hoop stress in the pipe  13 . Relatively speaking, if the elastic modulus of the covering is much higher than the pipe, the stress will be higher, whereas if the elastic modulus of the covering is low compared to the pipe, the stress will be lower. 
         [0027]    The magnitude of this pre-stress will determine how much load sharing will occur between the pipe and the covering. This value may be adjusted by, for example: (a) altering the elastic modulus of the pipe (e.g., using different materials of steel, titanium, etc.); (b) altering the elastic modulus of the composite fiber; and/or (c) altering the amount of initial pipe deflection through the magnitude of the tension. 
         [0028]    The pre-stressing of the pipe places an unloaded pipe in hoop compression and axial tension. Conversely, the composite on the pipe is in hoop expansion and axial compression. Consequently, when the product is in service, internal pressure in the pipe is shared by the composite, such that a pipe with a relatively thinner wall may be used to take the same amount of internal pressure without yielding. Without the pre-stress applied by the composite wrap, thinner walled pipes would be in danger of yielding prior to the composite sharing enough of the load. In addition, a circumferential composite wrap is less likely to separate from the pipe when environmental tension is applied in service, depending on the magnitude of the pre-tension applied. 
         [0029]    In one embodiment ( FIGS. 3A-C ), the invention comprises a method of forming a pipe assembly. A pipe  13  has an axis, an inner diameter and an outer diameter. A force, such as tension T, is applied to the pipe in the axial direction such that the outer diameter is reduced to a tension diameter. As shown in  FIG. 3A , a covering  15  is applied to the outer diameter of the pipe, such as from a composite manufacturing/spooling machine  21 . After the pipe  13  is wrapped as shown in  FIG. 2B  (e.g., except for connector portions  23 ), the covering may be cured by heat, UV radiation, etc. Thereafter, tension on the pipe is released ( FIG. 3C ) such that stress is developed at the interface  25  between the pipe and covering. The stress improves load sharing between the pipe and the covering when the inner diameter of the pipe is pressurized under operating conditions. 
         [0030]    The covering may comprise a composite that is circumferentially wrapped on the pipe from a spooling machine as shown in  FIGS. 3A and 3B . For example, the covering may comprise carbon fibers, at least some of which are axially-oriented  81  ( FIG. 8 ) and at least some of which are circumferentially-oriented  83  relative to the pipe  81 . Alternatively, the covering may comprise bundles or a continuous, pre-manufactured fabric or tape (e.g., pre-pregnated tape). 
         [0031]    In other embodiments ( FIGS. 4-5 ), tension may be applied to the pipe with one or more hydraulic devices. For example, as shown in  FIG. 4 , the piston  41  and cylinder  43  of a hydraulic device  45  may be located in the interior  47  of the pipe  49 . End connections  48  may be welded to the pipe  49  and are formed with reaction features (e.g., shoulders) against which the hydraulic device  45  reacts. 
         [0032]    As shown in  FIGS. 5A and 5B , a hydraulic device  51  is located external to the pipe  53 . The pipe  53  is mounted in a tensioning frame  55  (e.g., with or without bearings  57 ) having at least one hydraulic device  59  for applying tension to the tensioning frame  55  which reacts against upset end pieces on the pipe end connections  58  to supply the tension. The pipe  53  may be rotated such that the covering  61  is wrapped from a stationary spool  63  that only rotates about an axis of the spool. Alternatively, the covering  61  may be moved around the pipe  53  while the pipe is stationary, or a combination of rotating the pipe and moving the spool about the rotating pipe. 
         [0033]    Referring now to  FIG. 6 , a pre-cured composite  71  may be applied to the pipe  73  under tension to exert a compressive stress on the pipe  73 . The pre-cured composite  71  is then unrolled from the pipe  73  while simultaneously applying an uncured composite  75  to the pipe  73  directly adjacent to the unrolling pre-cured composite  71 , such that the uncured composite  75  takes advantage of the compressed condition of the pipe  73  beneath the pre-cured composite  71 . After the uncured composite  75  has completely replaced the pre-cured composite  71 , the uncured composite  75  is cured on the compressed pipe  73 . 
         [0034]    As shown in  FIG. 9 , another embodiment of the invention comprises applying another source of compressive stress on the pipe. The source of compressive stress on the pipe may comprise a roller assembly or a bearing assembly  91  for compressing the pipe  93 . The assembly  91  is axially moved relative to the pipe while simultaneously applying the uncured composite  95  to the pipe directly adjacent to the moving assembly  91 . In this way, the uncured composite  95  takes advantage of the compressed condition of the pipe beneath the assembly  91  as described above for  FIG. 6 . 
         [0035]    Another alternate embodiment is depicted in  FIG. 7  as a high level flow diagram. The method begins as indicated at step  81 , and comprises reducing a temperature of the pipe (step  83 ), wrapping the pipe in a covering (step  85 ), and then warming the pipe to an ambient temperature to provide an axial tension in the covering (step  87 ), before ending as indicated at step  89 . The temperature of the pipe may be reduced by, for example, freezing the pipe in a chamber, supplying a cold fluid to the inner diameter of the pipe, etc. The covering may comprise a UV-cured epoxy that is cured with UV radiation prior to warming the pipe. 
         [0036]    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.