Abstract:
The invention relates to improved metal/composite interfaces, particularly for risers used in offshore oil and gas production to connect a subsea oil well to a surface drilling/production facility. The invention provides a composite hoop structure configured to a bearing surface of a metal groove of metal liner over which axial fibers are laid. The composite hoop structure provides for improved transition of axial, torsional and compressive forces between composite fibers and metal surfaces of the metal liner.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefits of priority under 35 U.S.C. §119 and 120 of U.S. Provisional Patent Application No. 61/857,478 filed Jul. 23, 2013, the entire contents of which are incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to improved metal/composite interfaces, particularly for risers used in offshore oil and gas production to connect a subsea oil well to a surface drilling/production facility. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the oil and gas industry, risers are a well known and widely used piece of equipment for offshore drilling and production. Risers are generally described as individual conduits that can be axially interconnected to provide a continuous conduit between a subsea oil well to a surface drilling/production facility. 
         [0004]    Risers are used in both the drilling phases of an offshore drilling well as well as the production phase. During drilling, interconnected risers are used to surround a drilling string as the drilling string moves up and down and rotates during the downholedrilling and upholetripping phases. As such, the riser provides a means of encasing and protecting the drill string as well as to also contain circulating drilling fluid across the distance between the seafloor and surface drilling rig. During production, a riser will primarily serve as the conduit between the well and the on-surface production vessel. Risers may also provide a primary structural interconnection between the drilling platform on the sea surface and the ocean floor. 
         [0005]    As such, risers are engineered to withstand the various loads required by the foregoing functions. That is, a riser must withstand substantial internal pressures from the internal pressure of drilling or production fluids being pumped within the drill string and within the riser. Risers must also withstand axial tension loads from the interconnected weight of a string of assembled risers that are being moved into and out of position prior to and after drilling. In addition, risers must also withstand axial compressive and bending loads that are imparted to a string of risers during use as a drilling rig moves with respect to the ocean floor.  FIG. 1  shows the general construction of an offshore drilling rig (described in greater below). 
         [0006]    Increasingly, offshore drilling rigs are also being operated in deeper water than in the past, and as a result, the length of riser strings are similarly increasing. Accordingly, the above described loading forces on the risers are also increasing with the need for risers to withstand greater loads. 
         [0007]    As offshore drilling rigs were initially developed, first generation risers were manufactured as unitary steel structures and simply comprised a hollow steel tube having an outer wall and with appropriate threaded or flanged connectors at either end to enable interconnection of individual risers. The main problems with steel risers included the relative weight of individual risers for the required loading such that as the loading on the risers increased, they became progressively more unwieldy in terms of weight. That is, with increasing weight, there were associated costs and complexities of handling large and heavy tubes particularly in an offshore environment. As a result, later generation composite risers have been developed and have been in use for many years. 
         [0008]    In the typical composite riser design, the riser includes a relatively thin internal steel liner surrounded by an outer composite jacket. At either end of the riser, industry standard steel fittings are continuous with and/or are connected to the thin steel liner to enable adjacent risers to be connected to one another. As such, the internal steel portion of the riser includes a relatively thin central portion that either tapers towards or is connected to a thicker end portion that comprises the connectors. Similarly, the composite outer jacket has a thicker central portion that surrounds the thin central steel portion that tapers to a thinner end portion that is engaged with the connector portions. 
         [0009]    The junction between the steel and composite material is engineered to ensure that the structural loads between the steel and composite materials are effectively transmitted between the two materials over time. More specifically, as a result of differences in the stiffness of steel and composite structures, the forces are transmitted over a smaller area as compared to materials that may have similar stiffnesses. This mismatch in stiffnesses can be addressed by effectively increasing the areas of contact between the two materials. As shown in  FIG. 2  and as described in U.S. Pat. No. 6,719,058 (incorporated herein by reference) a typical metal/composite interface (MCI) is shown and is often referred to as “traplock” design. In this design, the tapering metal end is provided with one or more integral rings or grooves transverse to the longitudinal liner axis (i.e. in a sagittal plane). When the internal steel components are wrapped with composite material, which may include a combination of axial, transverse and angled resin-impregnated fibers, the axial forces (compressive and tensile) are more effectively transferred across the interface. 
         [0010]    More specifically, axially oriented reinforcing fiber is held in the trap by circumferentially wound fiber. The tapered interface of the metal/composite fitting transfers load through bearing pressure. The load is distributed from the metal/composite interface to the axial fibers by shear. Generally, a high hoop stiffness and strength is needed in the area behind the metal/composite interface to prevent the axial fibers from being pulled out of the laminate or the composite from expanding and sliding over the top of the tapered interface. 
         [0011]    While the foregoing has been an effective design, there remain limits regarding the ultimate strength of such systems. For example, as shown in  FIG. 2 , finite element analysis of a typical junction reveals that there remain significant stress points at particular positions on the grooves as the axial compressive or tensile loads increase. 
         [0012]    As a result, there has been a need for improved metal/composite interfaces particularly for risers. 
       SUMMARY OF THE INVENTION 
       [0013]    In accordance with the invention, there is provided a composite riser having a metal/composite interface comprising: a tubular metal liner having at least one tubular metal end connected to the tubular metal liner, the tubular metal end having at least one external groove having at least one bearing surface, and a base surface; a composite hoop structure within the at least one external groove and configured against the at least one bearing surface; and a plurality of axial fibers laid over the composite hoop structure and the base surface, the axial fibers secured by at least one layer of hoop fibers wound over the axial fibers within the external groove. 
         [0014]    In one embodiment, multiple layers of alternating axial and helical fiber layers, axial and hoop fiber layers or combinations of axial and helical and hoop fiber layers are set over the composite hoop structure. 
         [0015]    In one embodiment, the at least one external groove has a volume defined by the bearing surface, the base surface, an outer surface and a wall surface and wherein the composite hoop structure fills 10-25% of the volume or 10-15% of the volume. 
         [0016]    In one embodiment, the tubular metal end comprises at least three grooves and each groove has a composite hoop structure. 
         [0017]    In another embodiment, the composite hoop structure is wound to include outer transition surfaces having a rounded surface. 
         [0018]    In yet another embodiment, a first layer of axial fibers is laid over the composite hoop structure and the base surface of a groove. 
         [0019]    In one embodiment, a groove includes a wall surface and the ends of the axial fibers extend against the wall surface. 
         [0020]    In another aspect, the invention provides a method of manufacturing a composite riser having a tubular metal liner having at least one tubular metal end connected to the tubular metal liner, the tubular metal end having at least one external groove having at least one bearing surface, and a base surface comprising the steps of: a) winding a composite hoop structure against a bearing surface and base surface of each external groove; b) laying axial fibers over the composite hoop structure and within a first external groove; c) securing the axial fibers against the composite hoop structure and within the first external groove by winding at least one layer of hoop or helical fibers over the axial fibers; and d) repeating steps b) and c) for each groove of the tubular metal end. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention is described with reference to the accompanying figures in which: 
           [0022]      FIG. 1  is a schematic diagram of an offshore drilling and production rig in accordance with the prior art; 
           [0023]      FIG. 2  is a representative example of metal/composite interface in accordance with the prior art; 
           [0024]      FIG. 3  is a schematic diagram of an MCI in accordance with one embodiment of the invention showing a hoop insert in each traplock; 
           [0025]      FIG. 4  is a schematic diagram showing the layout of hoop wound and axial fibers against a hoop insert in accordance with one embodiment of the invention; 
           [0026]      FIG. 4A  is a schematic diagram showing the layout of axial fibers in accordance with one embodiment of the invention; 
           [0027]      FIGS. 5A and 5B  are plots showing the hoop stress of a finite element analysis (FEA) study for an MCI in accordance with the prior art ( 5 A) (no hoop insert) and in accordance with the invention ( 5 B) (with hoop insert) for a 1250 kips axial load; 
           [0028]      FIGS. 6A and 6B  are plots showing the hoop stress of a finite element analysis (FEA) study for an MCI in accordance with the prior art ( 6 A) (no hoop insert) and in accordance with the invention ( 6 B) (with hoop insert) for a 1250 kips compression load; 
           [0029]      FIGS. 7A and 7B  are plots showing the hoop stress of a finite element analysis (FEA) study for an MCI in accordance with the prior art ( 7 A) (no hoop insert) and in accordance with the invention ( 7 B) (with hoop insert) for a 1250 kips shear load. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    With reference to the figures, an improved metal/composite interface is described. 
         [0031]      FIG. 1  is a schematic diagram of a conventional offshore drilling and production assembly  10  that illustrates the context of the present invention. An offshore platform  10  supports derrick  12  which is a conventional apparatus for drilling or working over a borehole and producing hydrocarbons from the borehole. Offshore platform  14  is supported by pontoons  16 . A subsea platform  18  is provided on the seafloor  20  and a borehole  22  extends downward from the seafloor  20 . 
         [0032]    A string  24  of risers  24   a  extends between borehole  22  and platform  14 . The riser string  24  includes a tieback connector  26  proximate borehole  22  and flex or taper joints  28  and telescoping section  30 . Flex joints  28  and telescoping joint  30  accommodate the movement of platform  14  relative to the subsea platform  18  and borehole  22 . The elongated riser sections  24   a  are coaxially secured to one another. Each riser section  44   a  must accommodate the pressure of the fluid or gas within the section, as well as the tensile load which is caused by the suspension of additional riser sections  24   a  below the section, the tensioner load and the bending moments imposed by the relative movement of the platform  14  with respect to the subsea platform  18 . 
         [0033]    As shown in  FIG. 2 , a typical traplock design within a composite riser is shown. In a composite riser, a metal liner  50  having metal connectors  52  form a liner assembly which is subsequently and sequentially wrapped or covered with a plurality of fiber layers typically including an elastomeric shear ply  54 , composite layers  56 , an external elastomeric jacket  58  and an outerwrap  60 . The composite layers generally consist of small diameter fibers (6 to 10 microns) of high strength and modulus embedded in a polymer matrix material, e.g., resins or glues. The resin materials have bonded interfaces which capture the desirable characteristics of both the fibers and the matrix. The fibers carry the main load in the composite material while the matrix maintains the fibers in the preferred orientation. The matrix also acts to transfer load into the fibers and protects the fibers from the surrounding environment. The composite material properties depend upon the two major components, the fibers and the polymeric matrix. Preferably, thermoset or thermoplastic polymeric resins are used for the matrices including vinylesters and epoxies. A preferred fiber is a low cost, medium modulus (33 msi or 44 msi) polyacrylonitrile (PAN) carbon fiber. In addition, a hybrid of glass and carbon fibers incorporated in the matrix is acceptable. The fibers can also include glass fibers such as E-glass fibers. 
         [0034]    As shown in  FIG. 2 , each metal connector  52  includes a number of traps that enable the effective transfer of shear, bending, compressive and axial loads between the metal connectors and the composite fibers and matrix. A typical metal connector will include at least three generally symmetrical grooves  70   a,    70   b,    70   c  (also referred to herein as “traplocks”) around the metal connector adjacent the metal end of the connector. As shown, the diameter of each groove may be different where the smallest diameter groove  70   a  is adjacent the central section of the riser and the largest diameter groove  70   c  is adjacent the metal end of the riser. Each groove includes a generally flat inner surface  72  and two outwardly tapering flat sides  74   a  (medial side; towards middle of riser),  74   b  (lateral side; towards end of riser). The angles and dimensions of the sides of each traplock are designed in accordance with the specific performance requirements of the riser as known to those skilled in the art. In a typical design, the traplock may be 1-1.5 inches wide and 0.25-0.5 inches deep. Chamfers and/or fillets at surface junctions will be preferably included to avoid sharp transitions between the different surfaces. 
         [0035]    Generally, the composite layers are a combination of axially oriented fibers, hoop wound fibers and axially wound fibers. 
         [0036]    As shown in  FIG. 3 , an improved MCI in accordance with the invention is described. As described herein, it has been determined that a “hoop insert”  80  forming a component of the trap lock against the liner assembly can significantly improve the strength of the riser particularly under axial load. In addition, the positioning and placement of axial fibers within the trap lock can also significantly improve the strength of the riser. 
         [0037]    A hoop insert  80  is a ring of composite fiber and matrix abutting the medial surface  74   a  and flat surface  72  of each traplock  70   a,b,c.  The hoop insert is generally wound to fill approximately 10-15% of the volume of the traplock as calculated by outer dimension x shown in  FIG. 3 . The hoop insert may fill upwards of about 25% of the volume of the traplock in some designs. More specifically, the hoop insert is formed from fiber wound circumferentially against the medial surface and flat surface from resin-impregnated fiber strands. It is preferred that each hoop insert includes transition surfaces  80   a,    80   b  that provide smooth transitions between adjacent uncovered surfaces. 
         [0038]    Importantly, as discussed in greater detail below, the hoop insert provides an effective transition between the metal liner assembly  50  and the longitudinal fibers where the hoop insert distributes the effect of the relative difference in stiffness of the metal liner and the fiber layers across a greater volume. As such, the various loads (torsion, bending, axial and compression) that may be applied to the riser are transferred more evenly around the metal/composite interfaces which significantly improves the strength of a riser for a particular design of inner liner. 
         [0039]    After winding, the axial, helical and hoop layers are applied over the hoop insert. The hoop insert may be cured prior to the application of axial, helical and hoop layers if there are manufacturing advantages to doing this. 
         [0040]    As shown in  FIG. 4 , the placement, position and orientation of the fiber layers in accordance with one embodiment is described. As shown, each traplock includes a hoop insert set against the medial surface  80   a  of the traplock. 
         [0041]    During assembly, each of the axial, helical and hoop layers will be sequentially laid around the liner assembly in a sequence that provides support for underlying layers and that optimizes the performance characteristics of the riser for a particular design. 
         [0042]    For the purposes of clarity,  FIG. 4  principally shows the position of the axial fibers  100  relative to the traplocks and metal liner assembly. As shown, a plurality of layers of axially oriented fibers  100   a,    100   b,    100   c  are laid into and secured within each traplock. For example, within first traplock  74   a,  a first layer of axial fiber  100   a  is positioned within the traplock. As shown, the first axial fiber layer  100   a  is laid against flat surface  80   a  over the hoop insert  80  and extending to wall surface  80   c  of the traplock. When positioned, the first axial layer  100   a  is secured to the metal liner assembly by hoop wound fiber  82   a  (shown as white space) wound over top of the axial fiber layer  100   a  thus securing that axial fiber layer  100   a  within the traplock. It should be noted that the medial portion of the axial fiber  100   a  is also supported by hoop or helically wound fiber. 
         [0043]    Thereafter, successive layers of hoop wound fiber  82   b,    110   a  are added to the riser to support a further layer of axial fiber  100   b.  As with the first axial layer, the second axial layer is laid over top of the hoop insert  80  and extends in an orientation generally parallel to the flat surface  80   b  to the wall surface  80   c.  The process is repeated such that a desired number of axial (eg. layer  100   c  and so on) and hoop and/or helically wound fiber layers are built up for each traplock. 
         [0044]    As shown, it is generally preferred that each layer of axial fibers are separated from one another by hoop or helically wound layers as each axial fiber layer transitions away from the trap lock. That is, the axial fiber layers will generally be in close contact with adjacent axial fiber layers over the hoop insert  80  but be separated from one another towards the medial section of the riser. This separation of fibers away from the trap lock also assists in the transitioning or distribution of forces between the fibers that have different orientations. 
         [0045]    In an alternate embodiment as shown in  FIG. 4 , the ends of an axial fiber layer  100   a,    100   b  and  100   c  may also extend up wall surface  80   c  as shown in traplock  74   c.    
       Testing Results and Discussion 
       [0046]    Table 1 compares a finite element analysis (FEA) of the axial load level for a riser design with and without a hoop insert with axial load level at 15.7 ksi shear stress (=3% of shear strain).  FIGS. 5A-7B  also show an FEA study of the designs with and without a hoop insert. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 In axial fibers 
                 In hoop insert 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 No hoop insert - 
                 1 275 kips 
                   
               
               
                   
                 Hoop insert with 34 msi 
                 2 800 kips 
                 1400 kips 
               
               
                   
                 fibers only 
               
               
                   
                   
               
             
          
         
       
     
         [0047]    As shown in Table 1, in the case of the riser having a hoop insert, the shear stress in the hoop insert reaches 15.7 ksi (110 MPa) shear stress atan axial load of about 2800 kips. In the case with no hoop insert, the shear stress reaches 15.7 ksi already at only 1275 kips. The significant improvement in axial load capacity has been proven by testing. 
         [0048]    It is believed that high shear stresses in the hoop insert will not immediately lead to a delamination and an axial load failure as high shear stresses in the axial fibers are more critical. With hoop inserts the axial fibers reach 15.7 ksi (110 MPa) shear at an axial load of more than 2800 kips. The shear stress in the hoop insert at this load level is around 33 ksi (230 MPa). The compression load in the hoop insert at 2800 kips is at the most 90 ksi (620 MPa). This is a high value, however, taking 3-D effects into account, the insert has been proven to be able to take such a highloading. This is currently evaluated applying the so-called Puck failure criteria. It is noted however, that to predict the exact load capacity is very difficult from a FE analysis alone. 
         [0049]      FIGS. 5A ,  5 B,  6 A,  6 B,  7 A and  7 B show the results of FEA studies of an MCI design with and without a hoop insert. 
         [0050]    As shown in  FIGS. 5A  (no hoop insert) and  5 B (with hoop insert), with simulated 1250 kips axial load, the hoop insert shows lower stress development around each trap lock.  FIGS. 6A and 6B  show a similar result under compression and  FIGS. 7A and 7B  show a similar result under shear. 
         [0051]    Although the present invention has been described and illustrated with respect to preferred embodiments and preferred uses thereof, it is not to be so limited since modifications and changes can be made therein which are within the full, intended scope of the invention as understood by those skilled in the art.