Abstract:
A connector for joining a segment of composite pipe is disclosed. The connector includes an end connector having at least one fiber trap on its outer surface thereof. The end connector is attached to a liner portion of the segment of composite pipe. Fibers forming an outer surface of the segment of composite pipe are wound around the at least one trap under tension. The connector includes a binder which impregnates the fibers. The tension is maintained on the fibers in the trap during cure of the binder.

Description:
This application claims priority based on U.S. provisional application No. 60/155,328, filed on Sep. 22, 1999. 
    
    
     STATEMENT AS TO FEDERALLY SPONSORED RESEARCH 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Award #70NANB5H1053 awarded by the Department of Commerce, National Institute of Standards and Technology-Advanced Technology Program. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to composite tubing and more particularly to couplings for composite tubing. 
     BACKGROUND OF THE INVENTION 
     Fiber reinforced composite materials are known in the art and desirable for various applications due to their light-weight, high strength characteristics. One application for composite materials is pipe that is made in tubular form with a fiber reinforced plastic material. Segments of the composite pipe have a significant use in the petroleum industry. However, in typical petroleum industry applications, the composite pipe will be subjected to high loads. Ideally, couplings which join the segments of the pipe should have the ability to withstand the same pressures and loads that are exerted on the pipe itself. 
     Composite pipe is commonly manufactured by winding or braiding reinforcing composite fibers that are impregnated with resin over a mandrel and/or an interior liner made of a thermoplastic or elastomeric material. The reinforcing fibers may be glass, carbon or other suitable material. The resin is later cured to form hard tubing. The fibers are typically in the form of filaments or “tows” which are wound around the interior plastic liner or the mandrel to form the pipe. 
     Composite pipe is commonly manufactured in discrete lengths, usually up to about 30 feet in length, by the filament winding process where the mandrel is rotated within the filaments. Alternately, the tube may be manufactured as a continuous tube by either braiding or filament wrapping over a non-rotating winding mandrel which becomes an integral liner of the finished tube. FIG. 1 shows an example of a segment of composite pipe as it is being manufactured. The composite pipe  10  is formed as fibers  14  are wound around a plastic liner  12 . FIG. 2A illustrates a type of filament winding machine  16  that is commonly used to manufacture composite pipe  10 . The plastic liner  12  or a mandrel is drawn through several filament spool frames  20 . These frames  20 , as shown in FIG. 2B, rotate around the liner  12  while filament spools  18  unwind to extend fibers  14  which are then wound onto the liner  12  to form the composite pipe  10 . When the desired length of the segment of pipe is reached, a connector must be added so that the segment can be attached to other segments of pipe. Consequently, it is advantageous for a connector for composite pipe to provide similar strength characteristics as the composite pipe when the two segments of pipe are attached together. 
     Prior art connectors for high-strength composite pipe for petroleum industry applications include the following types: (1) pinned joints that carry loads through radially oriented pins; (2) bonded joints that carry loads through the shear strength of an adhesive layer; (3) mechanically locked wedge-type joints that carry loads through a mechanical wedge; and (4) trap-type joints. The trap-type joint carries loads from the composite pipe to the connector by means of the composite fibers. The composite fibers are wound into grooves in the end of the end connector affixed to the composite pipe and are trapped in the groove by subsequently applied “hoop” or circumferential fiber windings. The trap-type joints are generally considered to provide the highest load-carrying capacity of the known composite connector types. 
     In the prior art, the fibers used with trap-type connectors are commonly “wound into” the composite tube itself during the manufacturing process. The strongest prior art trap-type joint is most commonly provided by a discrete length filament winding. Alternatively, trap-type joints may be attached to an already cured composite pipe by applying additional fiber windings that are adhesively bonded to the cured pipe. 
     Prior art connectors include multiple grooves or traps for the stronger connections. Each fiber layer of the composite pipe typically carries the load to a selected trap for that particular layer. For example, where the pipe has five distinct fiber layers, the trap-type connector may have five separate traps or grooves (i.e., one for each layer). After each composite layer is completed, a hoop wrap is applied over the trap. The hoop wrap completely fills the trap while holding the fibers in place. The excess fiber extending beyond the trap may be trimmed at the distal end of the trap. The shape of the trap may be designed with various angles. This design allows the windings of each layer to lay against the bottom of the trap. This avoids “bridging” the fibers across the trap as they are wound. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is a method for making a connection for composite pipe. The method includes attaching a connector having at least one trap to a segment of composite pipe comprising a plurality of filament fibers, winding the plurality of filament fibers across the end connector, wherein tension is continuously maintained on the filament fibers so that the filament fibers bridge across the at least one trap. The plurality of filament fibers that bridge across the at least one trap are compressed, and a binder interspersed in the filament fibers is then cured while tension on the fibers is maintained. In one embodiment, the connector has a plurality of traps. In one embodiment, the fibers in each trap are wrapped with a hoop wrap. In one embodiment, the hoop wrap in each trap has a modulus related to the position of the trap with respect to the end of the connector. In one embodiment, a flank angle of each trap is related to the position of the trap with respect to the end of the connector. In one embodiment, the width of each trap is related to the position of the trap with respect to the end of the connector. In one embodiment, the depth of the trap is related to the position of each trap with respect to the end of the connector. 
     Other aspects of the invention will be apparent from the description which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a segment of composite pipe with a thermoplastic liner being manufactured. 
     FIG. 2A shows an embodiment of a filament winding machine that is used to manufacture composite pipe. 
     FIG. 2B shows a filament spool winding frame that winds fibers onto the composite pipe. 
     FIG. 3 shows a cross-sectional view of a threaded connector. 
     FIG. 4 shows a cross-sectional view of the composite trap and the liner trap sections of a connector with an attached segment of composite pipe. 
     FIG. 5 shows one embodiment of a connector with “stair-stepped” traps. 
     FIG. 6 shows a prior art method of winding truncated fibers across a trap. 
     FIG. 7 shows a prior art method of dwell winding non-truncated fibers across a trap. 
     FIG. 8 shows a prior art method of winding truncated fibers across a hyperboloid-shaped trap. 
     FIG. 9 shows fibers on a composite tube that exhibit “micro-waviness”. 
     FIG. 10 shows an example of “bridging” across a trap in accordance with one embodiment of the present invention. 
     FIG. 11 shows filament fibers that are held under tension and bridge across a trap in accordance with one embodiment of the present invention. 
     FIG. 12 shows a view of a segment of a connector with filament fibers bridging across a trap of the connector in accordance with one embodiment of the present invention. 
     FIG. 13 shows filament fibers that are held under tension and compressed in accordance with one embodiment of the present invention. 
     FIG. 14 shows a reduced axial angle of filament fibers after compression in accordance with one embodiment of the present invention. 
     FIG. 15 show s a tensioning ring used to maintain tension on the fibers during cure. 
     FIG. 16 shows an example of an ANSI Raised-Face Lap Joint flange connection which can be made using the method of the invention. 
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the invention will be described with reference to the accompanying drawings. Like items in the drawings are shown with the same reference numbers. 
     A coupling or connector for joining segments of fiber reinforced composite pipe and a method for manufacture of the same has been developed that offer improved performance by increased resistance to bursting, collapsing, and compression, tension, and differential pressure loads than prior art connections are able to withstand. FIG. 3 shows a cross-section of a typical threaded connector  22  for composite pipe. While a connection which uses threads is a common configuration, other embodiments may include flanged and hub couplings or other specialized couplings. The connector  22  shown in FIG. 3 can be used with both rotating and non-rotating mandrel manufacturing processes. 
     As shown in FIG. 3, the connector  22  has three distinct sections: the threaded section  24 ; the composite trap section  26 ; and the liner trap section  28 . The threaded section  24  of the connector  22  is shown with a metallic split ring shoulder  30  which is made up of a split ring  32  and a lock ring  34 . The composite trap section  26  is shown with several composite traps  40   a-d  that are used to secure the composite layer of the tubing to the connector. The liner trap section  28  is shown with a liner bump  36  that attaches the liner of the composite pipe section to the connector  22 . While four separate traps  40   a-d  are shown, any number of traps could be used to provide the desired characteristics of the connector  22 . It is also contemplated that the length, depth and angular profile of each trap  40   a-d  may be varied to provide better performance of the connection. FIG. 5 shows an alternative design that uses “stair-stepped” traps  40   a-d  for the connector  22 . 
     FIG. 4 shows a section of a connector  22  that has been “wound-in” to a composite tube  10  with a thermoplastic liner  12 . Where the tube  10  has the thermoplastic liner  12 , the first step in attach the connector  22  according to the invention is to install a seal  38  on the end of the connector  22 . O-rings  38 , including high durometer back-up rings, are preferred where the maximum pressure differential will exceed 5000 pounds per square inch. The next step is to swage or “bell out” the end of the liner  12  using a heated, cone-shaped swaging tool. The swaging tool should be heated to approximately 50-60% of the thermoplastic material&#39;s characteristic softening temperature. The effect is to expand the end of the liner  12  enough to allow the end of the connector  22  to fit inside. The next step is to heat the connector  22  and push it into the liner  12  until the liner  12  contacts the shoulder  42  of the connector  22 . After the liner  12  has cooled, it will relax to its original shape and fill in over the liner trap bump  36 . Next, liner hoop wraps  46  are wound over the liner  12  and the liner trap section  28 . The liner hoop wraps  46  are preferably impregnated with a quick-curing resin which has the same or higher glass-transition temperature as the resin used in the composite pipe. A fiber sold under the trade name KEVLAR by E. I. duPont de Nemours &amp; Co., Wilmington, Del., is a preferred material for the hoop wraps  46  because it has a negative coefficient of thermal expansion (i.e. it shrinks when heated). Finally, the hoop wraps  46  are overwrapped with a shrink-wrap tape (not shown) and quickly heated to the cure temperature of the resin. The hoop wraps  46  and the tape (not shown) will shrink and consequently tighten when heated, thus ensuring a tight installation of the liner  12  on the liner trap section  28  of the connector  22 . 
     Vulcanize-in-place elastomeric liners can be used in lieu of thermoplastic liners, but they require an internal mandrel (usually steel or aluminum) to form the interior elastomeric liner. With vulcanize-in-place liners, the connector  22  is cleaned and may be primed with an epoxy-based metal primer and then coated with a standard epoxy adhesive. The connector  22  is secured to the mandrel (not shown) and a strip of the elastomeric material is wound over the connector, usually with about 50% overlap. The elastomeric strip can be vulcanized-in-place before or after the composite is wrapped. 
     After the liner  12  is installed on the liner trap section  28  of the connector  22 , the composite layer  50  is wound across the composite trap section  26  using the same fibers, resin, winding angles, and curing technique used to manufacture the adjoining composite tubing  10 . After the fibers are wound across the traps  40   a-d , they must be compressed into the traps  40   a-d  before the composite layer  50  is cured. The fibers can be compressed in several ways: (1) with hoop wraps; (2) with a split die machined to the desired trap profile; or (3) with shrink-wrap tape; (4) overlaying a fiber layer using a subsequent fiber layer having a higher lay angle. FIG. 4 shows a compression method using composite trap hoop wraps  52  with each trap  40   a-d . Hoop wraps have the advantage that their application is an automatic process, while the split dies and the shrink-wrap tape are manual operations. 
     It is advantageous, especially in petroleum industry applications, to have a connection at the end of a composite tube which carries internal/external pressure loads effectively and has a very thin wall design. Ideally, the connection will be externally and internally flush with the composite tube body. Alternatively, a flush outside diameter (OD) or a flush inside diameter (ID) connection is advantageous. The connection is designed for the minimum wall thickness possible according to the following parameters: (a) the cross sectional area of the connector  22  at the trap thickness  44  must have sufficient strength to carry 100% of the load carried by the composite tube; (b) the liner trap bump  36  should be between 50-100% of the liner  12  thickness; (c) the OD of the liner trap bump  36  should be greater than the ID of the liner  12  so that the liner can be “swaged” onto the liner trap bump  36 ; (d) the trap thickness  44  should be 50-100% of the composite layer  10  thickness. If the trap thickness  44  is equal to the liner bump  36  height and the trap bump height  48  is equal to or less than the thickness of the liner  12 , then the wall thickness of the connection will be the sum of the trap thickness  44 , the trap bump height  48 , and the thickness of the composite layer  50 . Since the section of the composite layer  54  which extends beyond the last trap will be ground down after curing, the minimum thickness of the connection will be limited only by the thickness of the liner trap bump  36  and the trap bump height  48 . An additional method for further reducing the minimum thickness of the end connection, particularly if using a very thick liner, is thinning the liner  12 . This thinning of the liner  12  is done at the liner trap section  28  by machining away the liner material after the liner  12  is joined to the connector  22 . 
     For high pressure applications, one embodiment of the present invention will be between 50-65% thicker than the composite tube upon which it is installed. The dimensions for such an embodiment, a flush OD connection for composite coiled tubing (CCT), are: 
     
       
         
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 End Con- 
                   
                   
               
               
                   
                   
                   
                 nection/ 
                   
                   
               
               
                   
                   
                   
                 Composite 
                   
                   
               
               
                   
                   
                   
                 Tube 
                 Trap 
                 Liner 
               
               
                   
                 Composite 
                 End 
                 Thickness 
                 Thick- 
                 Trap 
               
               
                 Liner 
                 Layer 
                 Connection 
                 Ratio 
                 ness 
                 Bump 
               
               
                   
               
             
             
               
                 1.000″ ID 
                 1.200″ ID 
                 0.675″ ID 
                   
                   
                   
               
               
                 1.200″ OD 
                 1.500″ OD 
                 1.500″ OD 
                 1.65 
                 0.112″  
                 0.162″ 
               
               
                 0.100″ thick 
                 0.150″ thick 
                 0.412″ thick 
               
               
                   
               
             
          
         
       
     
     This embodiment has been burst tested to 19,500 psi. In this test, a failure occurred in the composite tube body but not at the connection. The 0.675′ ID dimension is required for the strength of the “Stub Acme” thread used on the particular end connection. It is not required for the strength of the composite joint. 
     CCT is a special case since it is typically very small diameter. Additionally, it does not rotate in service and it requires low-profile torque connections. In order to maximize the ID of a CCT connection fitted with Stub Acme threads while maintaining a flush OD, a removable split ring shoulder  30  (shown in FIG. 3) may be added to the connector  22 . This increases the through-bore ID to 0.750′ as compared to the 0.675′ ID of the flush-shoulder connection. 
     The design and number of the traps is also an important factor in determining the performance of the connection. Multiple traps may be required for high axial loads or improved fatigue performance with cyclical loading. In order to efficiently share the loads between the traps, it is advantageous to minimize the load on the first trap (the trap nearest the composite tube) and maximize the load on the later traps (the traps closer to the end of the connector). There are several variables that can be modified to accomplish this: 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Variable 
                 First Trap 
                 Later Traps 
               
               
                   
               
             
             
               
                 Load Flank Angle 
                 Low Angles (e.g. 30°) 
                 Higher Angles 
               
               
                   
                   
                 (e.g. 50-60°) 
               
               
                 Hoop Wrap Fibers 
                   
                 High Modulus (e.g. 
               
               
                   
                 Lower Modulus 
                 Carbon Fiber) 
               
               
                   
                 (e.g. Kevlar, Glass, 
               
               
                   
                 Hybrids) 
               
               
                 Height &amp; Depth of Trap 
                 Short, Shallow Trap 
                 Longer, Deeper 
               
               
                   
                   
                 Trap 
               
               
                 Shear Ply between Trap 
                 Low Shear-Strength 
                 Higher Shear- 
               
               
                 and Composite 
                 Shear Ply (e.g. Rubber) 
                 Strength Shear 
               
               
                   
                   
                 Ply (e.g. Primer &amp; 
               
               
                   
                   
                 Adhesive) 
               
               
                   
               
             
          
         
       
     
     The elastic modulus of the hoop wrap fibers can also be varied by using varied ratios of material. For example, the first trap can use hoop wraps of 80% fiberglass and 20% carbon, while the last trap can use hoop wraps of 80% carbon and 20% fiberglass. The depth of the trap in the above table can be adjusted both by selecting the height of the trap walls, and by adjusting the thickness of the connector body below the trap floor. 
     It has been determined that an important factor in the performance of the connection is the tension of the fibers running through the traps before the composite is cured. FIG. 6 shows one example of a prior art winding method where the fibers  14  are truncated or cut and laid in the bottom of the trap  40  before they are compressed and held in place by hoop wraps. FIG. 7 shows an alternative prior art method of winding with uncut fibers. Instead of cutting the fibers  14  in the bottom of the trap  40 , the fibers  14  are wound into the trap  40  and a series of dwell (stationary) wraps  56  are completed before the direction of the winding is reversed. In this prior art method, the fibers  14  are not kept in tension nor do they “bridge” across the trap. FIG. 8 shows still another prior art method of winding the fibers. In this method, the trap  40  is formed in a hyperboloid shape in order to allow the truncated fibers  14  to lay flat against the bottom of the trap  40 . This prevents the fibers  14  from “bridging” during the winding. 
     In the invention, it is preferred that as the fibers that are laid across the traps they remain in tension throughout the manufacturing process so that prior to compression into the traps, the fibers tend to “bridge” the trap. FIG. 10 shows an example of “bridging” where the fiber  14  is held under tension during the winding across the trap  40 . As a result, the fiber  14  does not contact the bottom of the trap  40 . This allows, after proper compression of the fibers into the trap, the various loads to be fully transferred to the connector through the fibers. It is preferred that the fibers not be truncated before the cure of the composite. If the fibers are truncated before the cure, they will not have a tension sufficient to carry the load to the connector when compressed into the traps. The fibers that are not properly tensioned will tend to exhibit what is referred to as “microscopic waviness” within the fiber layer, which indicates an inability to carry a sufficient load. FIG. 9 shows an example of fibers that exhibit “microscopic waviness”  58  as compared with fibers  14  that are properly tensioned and do not exhibit microscopic waviness. Additionally, it is preferable to wind the fibers of all layers of the composite through all of the traps. If a particular layer is truncated at a particular trap, the addition of a hoop wrap is insufficient to transfer the load between the traps. FIG. 11 shows the results of the preferred winding method of the present invention wherein the fibers  14  are kept in tension during the winding so that they bridge the trap  40  before compression. FIG. 12 shows an alternative view of the present invention with the fibers  14  bridging the trap  40  on the connector  22  before compression. FIG. 13 shows the results of the preferred winding method after compression of the fibers  14  into the bottom of the trap  40 . 
     Continuous manufacturing techniques of composite tubing allow tension to be maintained across the traps as the fiber is wound in one direction only. Filament winding methods require that the fiber winding direction be reversed with every traverse of the mandrel. In order to maintain the required fiber tension during filament winding, it is preferable that the fibers be wound onto the mandrel for a distance of ten mandrel diameters past the end of the composite traps. Two dwell (stationary) wraps should be made at this point before the winding direction is reversed. 
     Alternatively, tension on the fibers can be maintained during cure by winding the fibers on to a tensioning ring at the point at which the wind direction is reversed. Referring to FIG. 15, a tensioning ring  152  is positioned at the distal end of the connector  22 . As the fibers (not shown in FIG. 15) are applied to the exterior of the connector  22 , eventually they will reach the axial position of the ring  152 . The ring  152  includes a plurality of azimuthally spaced apart pins  150 . As the fibers are wound past the pins, the winding direction is reversed. The fibers will at that point wrap around the pins  150 . Wrapping the fibers can then proceed in the reverse direction. After winding is complete, the ring  152  can be pulled axially away from the composite tube (not shown in FIG. 15) by means of an hydraulic retraction mechanism  154  or any similar retraction device so as to put the fibers in tension. Then the fibers need to be pulled down into the traps while tension is maintained thereon. The ring  152  can then be returned to its original position after resin cure is complete, and the fibers can be truncated. 
     It is preferred that the tension of the fibers be increased just before the cure by compressing all of the fibers down into the traps. Compressing the fibers into the traps will make the wound-fiber angles more shallow as the tension is increased by compression. FIG. 14 shows the change in the axial angles of the fibers  14  in the trap  40  after compression. The shallower angles will actually improve the tensile load carrying capacity of the connection. Fibers that are not compressed into the traps will not carry as much load. Further, they will also exhibit some degree of “micro-waviness” even if they are not truncated. It is preferable to wind perforated polyester shrink-wrap tape over the traps  40  to compress the fibers  14 . Typical applications may include three layers of tape that provides pressure of about 60 psi when the tape is shrunk. In the invention, shrink wrap compressed traps alone, without the use of hoop wraps, have provided burst test pressures within 80-90% of the burst test pressures attained using connections with hoop wrapped traps. 
     Another type of connection between a composite pipe and an end fitting or connector is shown in FIG.  16 . The connection shown in FIG. 16 is one example of an ANSI Raised-Face Lap Joint flange connection. The connection shown in FIG. 16 can be made according to the following procedure. An already cured composite tube with a thermoplastic liner therein is cut-of to have a squared-off end. The composite reinforcing layer is cut back, exposing several inches of the underlying liner material. The liner is swaged (as previously described) to fit over the end of a connector  160  which includes a raised face  160 A thereon. One or more resin-impregnated composite over-wraps (typically, but not exclusively formed from braided tube, composite tapes, or woven composite cloth or the like), are placed around the end connector  160  and tightly clamped to the connection adjacent the rear of the flange  166 . The composite over-wrap is pulled tight over the already-cured composite tube, and the distal end(s) are clamped to the composite tube. The braided tube, tapes or cloth are over-wrapped with shrink-wrap tape over their entire length and are then heat-cured. The heat of curing of the shrink wrap tape will cause the composite overwrap to be pulled-down into the traps  162 , where the composite overwrap it will be cured under tension. The traps  162  can be filled with composite hoop wraps, which are then cured in the normal manner. The connector  160  may have an o-ring groove  164  in its surface to seal a liner, similar to the connector shown in FIG.  4 . 
     The advantages of the disclosed invention may include a connection for composite pipe with improved load bearing performance due to continuous tension being maintained on the composite fibers during manufacture. Other advantages may include reduced wall thickness of the connection. 
     While the invention has been disclosed with reference to specific examples of embodiments, numerous variations and modifications are possible. Therefore, it is intended that the invention not be limited by the description in the specification, but rather the claims that follow.