Abstract:
Processes for manufacturing flexible pipes having extruded layers made of crosslinked polyethylene, in which the polyethylene is crosslinked by electron beam irradiation. A flexible pipe for use in sub-sea and land-based applications having an innermost core of crosslinked polyethylene, in which the crosslinked polyethylene is cured by electron beam irradiation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/441,788, filed Jan. 22, 2003, the entire disclosure of which is hereby incorporated herein by reference. 
     
    
     
       BACKGROUND  
         [0002]    The present embodiment relates generally to processes for curing crosslinked polyethylene for use in flexible pipes formed of multiple layers of different materials and suitable for use in subsea and land-based applications. The present embodiment also relates to such flexible pipes that include at least one layer made of a crosslinked polyolefin.  
           [0003]    It is known to produce flexible pipes that include at least one layer of plastic and plies of armoring to provide the compressive and tensile strength necessary for subsea and land-based applications intended for oil production or other services from offshore or onshore deposits. The high strength flexible pipes with a structure having several layers, which are particularly advantageous, have diameters varying from 50 mm to 500 mm, and allowable internal pressures which may vary depending on the diameter having a minimum value of 50 bar and which may reach 1000 bar. Various plastics are used for the layers of such flexible pipes. Polyolefins, principally polyethylene, and in particular high density polyethylene (HDPE), however, undergo physicochemical attack from live crude oil.  
           [0004]    It is known that certain properties of polyethylene can be improved by crosslinking to produce crosslinked polyethylene (PEX). Three methods are known by which polyethylene can be crosslinked, namely by irradiation, chemically by silane, or chemically by using peroxide chemistry.  
           [0005]    For instance, U.S. Pat. No. 4,455,406 discloses that the mechanical behavior of polyethylenes for large diameter pipes can be improved by chemical crosslinking with peroxides. The chemical crosslinking method, however, requires a temperature that can prevent the polyethylene from supporting their own weight during the crosslinking process.  
           [0006]    Also, U.S. Pat. No. 5,514,312 discloses a process for curing silane grafted crosslinked polyethylene by circulating hot water through the interior of a pipe.  
           [0007]    Also according to U.S. Pat. No. 5,514,312, it is disclosed that the flexible pipe itself could be used to convey a fluid for supplying the heat and moisture making it possible to increase and to maintain the temperature necessary for obtaining the desired reaction in a reasonable time. According to this process, the plies of metallic wires that constitute the strength armoring of the pipe were electrically conductive and may be used as heating resistance elements.  
           [0008]    It is difficult, however, to use these methods for crosslinking polyethylene on an industrial scale to produce flexible polyethylene pipes in part because of the large size of the pipes. Therefore, there is a need for simplified processes for manufacturing flexible pipes that include a crosslinked polyethylene layer.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0010]    [0010]FIG. 1 illustrates an electron-beam accelerator system for producing a tubular shaped conduit having a crosslinked polyethylene layer according to one of the present embodiments.  
         [0011]    [0011]FIG. 2 is a perspective view of one embodiment of a flexible pipe having an innermost layer prepared according to the process illustrated by FIG. 1.  
         [0012]    [0012]FIG. 3 is a perspective view of another embodiment of a flexible pipe having an innermost layer prepared according to the process illustrated by FIG. 1  
     
    
     DESCRIPTION  
       [0013]    The present embodiment is directed to processes for manufacturing flexible pipes having extruded layers made of crosslinked polyethylene in which the polyethylene is crosslinked by electron beam irradiation, which is also referred to as X-ray curing of polyethylene. Polyethylene that is crosslinked by electron beam irradiation is also referred to as PEX-C. The present embodiment is further directed to a flexible pipe design having an innermost layer of PEX-C.  
         [0014]    It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the disclosed technology. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.  
         [0015]    Referring now to FIGS. 1 and 2, an embodiment of a process for manufacturing a flexible pipe having at least one layer made of polyethylene that was crosslinked by electron beam radiation is illustrated.  
         [0016]    According to this process, a tubular shaped conduit  100  of a crosslinkable material, preferably polyethylene, is manufactured according to known methods. For example, a tubular shaped conduit of a crosslinkable material can be extruded through a single screw, twin screw, or other extrusion and die assembly. As tubular shaped conduit  100  exits an extrusion assembly (not illustrated), a conveyor  72  provides the tubular shaped conduit  100  to an enclosure  76 , by way of an opening  74 . Preferably, the enclosure is a concrete room, although other materials are suitable as long as they provide protection against exposure outside of the enclosure to the radiation occurring inside the enclosure. To assist the feed of tubular shaped conduit  100  through the enclosure  76 , a feeder pipe  77  is attached to one end of the tubular shaped conduit  100 . For example, the feeder pipe  77  can be fed through opening  74  and positioned to exit the enclosure  76  at an exit  79 , where a take-away conveyor  73  is operable to convey the feeder pipe  77  through the exit  79 . According to one embodiment, with at least a portion of the feeder pipe  77  through the exit  79 , the take-away conveyor  73  operates to convey the tubular conduit  100  through the enclosure  76 , and ultimately through exit  79 .  
         [0017]    Inside enclosure  76 , a plurality of electron beam accelerators are radially spaced apart and arranged so that the entire surface of the tubular shaped conduit  100  is equally exposed to electron beam irradiation therefrom. According to the embodiment illustrated by FIG. 1, three electron beam accelerators  78   a ,  78   b , and  78   c , are radially spaced apart, and are arranged about 120 degrees apart with respect to each other. Electron beam accelerators  78   a ,  78   b  and  78   c  are illustrated in FIG. 1 as disposed against the ceiling and opposite facing walls of enclosure  76 . Electron beam accelerators  78   a ,  78   b  and  78   c  can be affixed in place or supported in place by methods known to those of ordinary skill in the art, as long as the electron beam accelerators are radially spaced apart and arranged so that the entire surface of a conduit will be equally exposed to electron beam irradiation therefrom.  
         [0018]    As the tubular shaped conduit  100  passes through the enclosure  76  on the conveyor  72 , the entire surface of the tubular shaped conduit  100  is equally exposed to electron beam irradiation from the electron beam accelerators  78   a ,  78   b , and  78   c . According to one embodiment, each electron beam accelerator is operated in a range of about 150 kilovolts to about 6 megavolts, with a power output capable of supplying the desired voltage. The voltage can be adjusted to appropriate levels which may be, for example, 100,000, 300,000,1,000,000, 2,000,000, 3,000,000 or 6,000,000, higher or lower. The irradiation is usually carried out at a voltage between about 3 megarads to about 35 megarads, preferably between about 8 to about 20 megarads. Further, the irradiation can be carried out conveniently at room temperature, although higher and lower temperatures, for example 0° C. to about 60° C., may also be employed.  
         [0019]    Using electron beam irradiation as described in the present embodiments, crosslinking of the crosslinkable material comprising the tubular shaped conduit  100  takes place while the conduit  100  is exposed to the radiation generated inside the enclosure  76  by the electron beam accelerators. Accordingly, the crosslinking process according to the present embodiments is quick, as compared to peroxide-based crosslinking or silane grafted crosslinking as discussed above. Generally, it is preferred that the desired level of crosslinking is obtained in a relatively short operating duration, approximately two to three minutes, although, the time required for crosslinking is dependant on the intensity of the radiation, the length and diameter of the tubular shaped conduit, and the thickness of the layer to be crosslinked. However, when the tubular shaped conduit  100  exits the enclosure  76 , it has at least some level of crosslinking. Using the present embodiment, the tubular shaped conduit  100  can be crosslinked to a gel content of at least 50%, and up to as high as 94%, which provides excellent aging properties to the tubular shaped conduit  100 . The tubular shaped conduit  100  can be passed through the electron beam accelerators multiple times, if necessary, in order to increase the gel content of the crosslinked polyethylene to a desired level. In addition, more than three electron beam accelerators can be used, as long as they are arranged in a configuration such that the entire surface of the tubular shaped conduit  100  is equally exposed to the radiation.  
         [0020]    The process for manufacturing flexible pipes having extruded layers made of crosslinked polyethylene as illustrated in FIG. 1 can also be used to crosslink layers other than polyethylene, such as poly(vinylidene fluoride) (PVDF) and other crosslinkable materials.  
         [0021]    Once a satisfactory level of crosslinking has been achieved in the crosslinkable material comprising the tubular shaped conduit  100 , a flexible pipe can be manufactured with the tubular shaped conduit  100  comprising the innermost layer of the flexible pipe. According to the present embodiments, a flexible pipe having a crosslinked material as its innermost layer is a high strength flexible pipe, having a diameter of between 50 mm to 500 mm, and allowable internal pressures of between 50 bar and 1000 bar.  
         [0022]    Referring now to FIG. 2, a perspective view of one embodiment of a flexible pipe  70 , which is illustrated in cut-away to show various layers comprising the pipe  70 , is illustrated. As seen in FIG. 2, a first supporting layer  102  is wrapped around the tubular shaped conduit  100 , which has been crosslinked as described with respect to FIG. 1, to provide resistance to internal pressure. According to one embodiment, the layer  102  is formed by helically wrapping a continuous metal strip, with adjacent windings being interlocked, to form a flexible layer that provides significant hoop and axial strength. One example of such a layer is found in flexible pipe commercially available from Wellstream, Inc., where the layer is referred to under the “Flexlok” trademark.  
         [0023]    A second supporting layer  103  extends over the layer  102 . According to one embodiment, second supporting layer  103  comprises an extruded polymer layer.  
         [0024]    A third supporting layer  104  extends over the second supporting layer  103 . According to one embodiment, third supporting layer  104  comprises a wrapped wire assembly, and consists of a series of wires helically wrapped around the exterior of the second layer  103  to form a first tensile armoring layer, and an additional series of wires wrapped around the first series of wires to form a second tensile armoring layer extending over the first tensile armoring layer. A suitable wire assembly is described in U.S. Pat. No. 6,363,974, the entire disclosure of which is incorporated herein by reference.  
         [0025]    A fourth supporting layer  106  extends over the third supporting layer  104 . According to one embodiment, fourth layer  106  is a protective insulative sheath formed of a foamed, or blown, polymer.  
         [0026]    A fifth supporting layer  108  extends over the fourth supporting layer  106 . According to one embodiment, fifth layer  108  is a tape that is helically wrapped over the fourth supporting layer  106 . The tape can be formed of plastic or metal, and can be reinforced with glass, metal or a different type of plastic.  
         [0027]    A sixth supporting layer  110  extends over the fifth supporting layer  108  and provides additional resistance to hydrostatic collapse and crush. According to one embodiment, the sixth layer  110  comprises a plurality of helically wrapped, corrugated and/or interlocked strips to provide additional collapse and radial compression resistance.  
         [0028]    A seventh supporting layer  112  extends over the sixth supporting layer  110 . According to one embodiment, the seventh layer  112  is an outer protective sheath extruded over the sixth layer  110  in a conventional manner.  
         [0029]    Preferably, at least second supporting layer  103  is sealed at its ends to sustain imposed loads and maintain a fluid-tight seal. More layers, less layers, and other materials beyond those illustrated in FIG. 2 can be used to make a pipe comprising an innermost crosslinkable layer  100 . The number and types of layers will vary depending at least in part on the desired application for the finished pipe. Those of ordinary skill in the art can select and apply, without unreasonable experimentation, suitable layers and materials to make a pipe such as pipe  70 . The method of forming a pipe  70  can be any known to those of ordinary skill in the art, as long as the innermost layer of the pipe  70  is formed of a crosslinkable material, such as PVDF or polyethylene.  
         [0030]    According to another embodiment illustrated in FIG. 3, a crosslinkable layer  54  is extruded onto a metal carcass  52  according to methods known to those of ordinary skill in the art. The crosslinkable layer  54  can be crosslinked by electron beam irradiation according to the process described above with respect to FIG. 1. After the crosslinkable layer  54  has been crosslinked, one or more supporting layers, such as layers  56  and  58 , can be formed, layered or wrapped around layer  54  according to methods known to those of ordinary skill in the art. Such supporting layers can be any material for any purpose, including but not limited to thermosetting materials, thermoplastic materials, non-plastic materials, metal strips, tape or wires.  
       Variations and Equivalents  
       [0031]    It is understood that various methods of co-extrusion, other than those shown and described herein, may also be used to extrude a tubular shaped conduit  100  as described herein.  
         [0032]    It is understood that shapes other than tubular can be suitable for use in the manufacture of flexible pipe, and that tubular conduit  100  can be extruded or formed in other such suitable shapes.  
         [0033]    Additional tensile layers of wires can be provided in addition to a series of wires as described in one embodiment of third layer  104 .  
         [0034]    The relative thicknesses of the layers extending over tubular shaped conduit  100  are shown only for the purpose of example, it being understood that these thicknesses can be varied within the scope of the invention.  
         [0035]    More or less layers than those illustrated herein, as well as materials other than those illustrated herein, can be used to make a flexible pipe comprising a tubular conduit  100  prepared according to the present embodiments.  
         [0036]    Spatial references, such as “under”, “over”, “between”, “outer”, “inner” and “surrounding” are for the purpose of illustration only and do not limit the specific orientation or location of the layers described above.  
         [0037]    Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many other modifications, changes, and substitutions are possible and intended in the foregoing disclosure. Accordingly, all such modifications, changes, and substitutions are intended to be included within the scope of this invention.