Abstract:
The invention is directed to a double-wall steel pipe joint. Each section of double-wall steel pipe has a steel outer casing and a steel inner carrier pipe. The completed joint comprises a steel joint ring fitted within the annular space at the end of each double-wall steel pipe section. The combination of locking serration and epoxy bonding join and seal the inner steel pipe to the bore of the joint ring, without welding. Welding joins the ends of the sections of outer casing.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to methods and apparatus for joining sections of double-wall steel pipe. Double-wall steel pipes are typically used in specialty pipeline systems to provide an additional measure of protection for containment of the material being transported. For instance, double-wall steel pipelines are typically installed in subsea operations and are often used for the transportation of corrosive or hazardous material. The annulus between the two “walls” may be filled with an insulation material to provide thermal protection for temperature-sensitive materials, such as crude oil or heavy fuel oil, where external pipe insulation methods are not suitable. In some applications, the combination of both leak containment and thermal protection are desirable. 
     A double-wall steel pipe typically comprises a steel outer casing and a steel inner carrier pipe. The outer casing provides bending, and tensile strength to the pipe, in addition to providing containment of the material being transported. The inner carrier pipe provides bending and tensile strength, as well as radial strength. 
     Historically, sections of double-wall steel pipes have been joined by welding both the inner carrier pipe and the outer casing. The inner carrier pipe must extend beyond the outer casing on both ends of adjacent double-wall sections in order to provide access for full circumference welding and weld inspection. Welding is done either by a machine affixed to the inner carrier pipe or by hand. After the sections of inner carrier pipe are welded, a two-piece steel joint is used to join the steel outer casing. This two-piece casing joint is comprised of two half-rings of sufficient length to join adjacent sections of outer casing. The half-rings are fabricated from a section of steel pipe with the same outside diameter, wall thickness and steel grade as the steel outer casing. The steel pipe section is cut to fit the open area remaining between adjacent outer casing sections. The two half-rings are formed by cutting the steel pipe section axially 180° apart. One half-ring forms the bottom portion of the casing joint, the second half-ring the top portion. Two axial welds join the half-rings to form the outer casing joint. Two full circumference welds join adjacent sections of steel outer casing to the steel casing joint. This method of joining two sections of double-wall steel pipe is both time-consuming and labor-intensive. 
     The present invention eliminates the step of welding in order to join the inner steel pipe. In addition, the present invention succeeds in joining the steel outer casing of adjacent double-wall pipe sections with one weld instead of four. Reducing the number of welds required saves time and increases production. 
     The disadvantages of the prior art are overcome with the present invention, and novel methods and apparatus are hereinafter described for efficiently and reliably joining together sections of double-wall steel pipelines. 
     SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the present invention, convenient lengths of double-wall steel pipe may be formed in a manufacturing plant, comprising a steel outer pipe or casing, a steel inner pipe or carrier, and an optional annular foam or other insulating material. If foam or another insulating material is used, both the casing and the carrier will extend a short distance beyond the annular insulating material, leaving an annular void at both ends of each pipe section. The double-wall steel pipe joint is formed in the general shape of a ring fabricated according to the diameter of the double-wall steel pipe. The joint ring is a steel band having a serrated inner circumference, or bore, which is slightly tapered at both ends. The outer circumference of the ends of the steel inner pipe is also serrated. Serration geometry provides resistance to axial tension and pullout during installation and during subsequent heating and cooling cycles. A groove may be cut in the end of each section of inner pipe to hold a hydraulic O-ring. 
     In the field, the double-wall steel pipe may be joined by installing the pipe joint of the present invention according to the following procedures. Prior to joining, the joint ring may be heated, causing it to expand. Epoxy is applied to the ends of the steel inner pipe, which have circumferential serrations on their outer circumferences. Epoxy is also applied to the serrated bore of the joint ring. The joint ring is then slipped into the annular space as the two pipe sections it connects are pulled together. As the joint ring cools, it contracts, locking onto the ends of the sections of inner pipe it joins. To ensure performance, taper in the bore of the steel joint ring must not be great enough to cause the steel inner pipe to yield. Inner pipe deformation should be in the elastic range. The elevated temperature of the joint ring increases the cure rate of the epoxy, reducing the time required for the epoxy to achieve its requisite strength. The sliding engagement and the contraction of the joint ring onto the serrated inner pipe ends, along with the epoxy film therebetween, seal the joint&#39;s bore to the inner pipe, without welding. 
     Low operating pressure systems and/or small pipe diameter systems may be joined without preheating the joint ring. These systems will typically require less insertion force for full depth insertion of the inner pipe into the joint ring. Additionally, less than optimum epoxy strength may be sufficient for form a fully-sealed joint for low operating pressure system requirements. For example, an epoxy may develop a lap shear of 2,500 psi at low temperatures and 3,000 psi or more when cured above 150° F. If the force required for insertion is not excessive and if the epoxy lap shear strength required is obtainable at lower temperatures, preheating will not be required. 
     After the ends of the inner pipe are joined, adjacent ends of the sections of steel outer casing are welded together, completing the joining operation. As the outer casing is welded, the joint ring acts as a heat sink. The heat absorbed by the joint ring keeps the epoxy, which is located where the joint ring contacts the surface of the inner pipe, in the desired temperature range for both minimum cure time and maximum strength development. 
     The joint ring is fabricated to have higher radial and tangential strength than the steel inner pipe, allowing the joint ring to form a tight seal when inner pipe is under high pressure. Sealing surfaces are axially aligned. Therefore, high pressure forces the inner pipe wall to press against the joint, sealing the joint tightly. 
     Double-wall steel pipe assemblies are suitable for many pipeline applications in which the use of single-wall pipe is not acceptable due to environmental, safety, thermal, negative buoyancy, and other considerations. In addition to its reliability with respect to sealing the fluids carried by the steel inner pipe, the pipe joint described herein offers cost effectiveness due to the efficiency of its installation. Therefore, its use in the installation of double-wall steel pipelines is preferable to the use of pipe joints which require welding to join both the inner pipe and the outer casing of sections of double-wall steel pipe. 
     The present invention may also be applied as a bell and spigot. Welding one side of a joint ring onto an end of a steel inner pipe forms the bell. The other end of the steel inner pipe, fabricated with full circumferential serrations, is the spigot. The procedure used to join the sections is similar to that described above. However, in the field, the joint ring requires no handling, and epoxy application is reduced by half. 
     The present invention may also be applied joining single-wall steel pipe when welding is not possible due to an explosive atmosphere or the presence of a pipe coating and/or lining that cannot withstand the heat of welding. 
     Accordingly, it is an object of the present invention to provide a novel joint for joining sections of double-wall steel pipe. 
     Another object of the present invention is to provide a double-wall steel pipeline joint which reliably seals the pipeline fluids carried in an inner steel pipe, under high pipeline pressure. 
     Yet another object of the present invention is to provide a method and apparatus for joining sections of double-wall steel pipe in a time-efficient and cost-effective manner. 
     Still another object of the present invention is to provide a method and apparatus for joining the inner carrier pipe, or a single-wall steel pipe, when it is internally coated or lined with materials which cannot withstand the heat resulting from joining by weld. 
     These and other features and advantages will become apparent in the following detailed description, wherein reference is made to the figures in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified fractional longitudinal cross-sectional view through the pipe joint of the present invention, showing two sections of double-wall steel pipe which have been joined together, and further showing the serrations on the exterior surface of the inner pipe. 
     FIG. 2 is a fractional longitudinal cross-sectional view of the pipe joint assembly of the present invention, showing the locking serrations on the bore of the joint ring. 
     FIG. 3 is a fractional longitudinal cross-sectional view of an end of a steel inner carrier pipe, showing the serrations on its exterior circumference. 
     FIGS. 4 a  and  4   b  are details of the complementary serrations and the epoxy bond line surface. 
     FIG. 5 is a detailed fractional longitudinal cross-sectional view of the pipe joint assembly of the present invention, showing two sections of double-wall steel pipe which have been joined together. 
     FIG. 6 is a fractional longitudinal cross-sectional view of another embodiment of the pipe joint assembly, showing a bell and spigot connection. 
     FIG. 7 is a fractional longitudinal cross-sectional view of yet another embodiment of the pipe joint assembly, showing its use to join single-wall steel pipe. 
    
    
     NOTE: Certain features of the preferred embodiment, i.e., size of serrations, the amount of taper, and the deformation of the inner pipe, have been exaggerated for purposes of illustration. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the pipe joint assembly  1  connects a first section of double-wall steel pipe  2   a  to a second section of double-wall steel pipe  2   b . Each section of double-wall pipe  2   a ,  2   b has a steel outer casing  3   a ,  3   b  and a steel inner pipe  4   a ,  4   b  (the carrier). The drawing shows the ridges of the circumferential serrations  5   a ,  5   b  on each of the adjoining ends of the inner pipe  4   a ,  4   b . The outer diameter  6   a ,  6   b  of the steel inner pipe  4   a ,  4   b  may be 1″ n to 2″ (or less) smaller than the inner diameter  7   a ,  7   b  of the steel outer casing  3   a ,  3   b , leaving an annulus  8   a ,  8   b  between the “walls” of the double-wall steel pipe  2   a ,  2   b . With the exception of the part of the annulus  8   a ,  8   b  occupied by the joint ring  10 , the annulus  8   a ,  8   b  generally may be filled with a non-metallic material such as insulation foam or another insulating material  9   a ,  9   b.    
     The joint ring  10  is a ring-shaped band having a diameter less than the inner diameter  7   a ,  7   b  of the outer casing  3   a ,  3   b , such that a portion of its collar  11   a ,  11   b  fits within the annulus  8   a ,  8   b  of each section of the double-wall steel pipe  2   a ,  2   b . Because the steel joint ring  10 , by itself, is able to withstand the radial and tangential stress components of a pressurized pipe without bowing or distorting, it does not have to interface with the outer casing  3   a ,  3   b . The joint ring  10  may be fabricated from carbon steel. If the double-wall pipe  2   a ,  2   b  will be used to transport corrosive materials, another alloy may be used for fabricating the joint ring  10 . The present invention utilizes a joint ring  10  which can be fabricated from virtually any metal alloy. Exotic alloys such as Hastelloy®, Inconel®, Incoloy®, Monel®, Nickel and 300 Series Stainless Steel have exceptional resistance to corrosion. Specific applications will depend on the corrosive characteristics of the material being transported. Typically, the specific alloy selected for fabrication of the joint ring  10  will be the same alloy as the inner pipe  4   a ,  4   b . In any event, the joint ring  10  must be fabricated from a metal that is compatible with the line pipe it will join. Otherwise, if the metal of the joint ring  10  is reactive with the line pipe metal, the resulting corrosion will weaken the joint pipe assembly  1 . 
     Hydraulic O-rings  20   a ,  20   b  may be fitted into slots near the ends of inner pipe  4   a ,  4   b  to provide secondary seals which are not essential to the integrity of the pipe joint assembly  1 . The O-rings are made from neoprene or a similar material. 
     The ends of outer casing  3   a ,  3   b  are joined by full circumference weld  25 . 
     FIG. 2 shows the joint ring  10  prior to installation. The joint ring  10  has been fabricated with locking serrations  12   a ,  12   b  on the interior circumference of the joint ring  10 . The joint ring ends  13   a ,  13   b  have been slightly beveled to the joint bore  14   a ,  14   b  to form bevels  19   a ,  19   b . From each joint end  13   a ,  13   b  toward the joint center shoulder  15 , the joint bore  14   a ,  14   b  is parallel to the outer circumference  16  of the joint ring  10  for approximately {fraction (3/4+L )} to  1{fraction (1/2+L )}. Then the joint bore 14   a ,  14   b  is slightly tapered. For example, the diameter of the joint bore  14   a ,  14   b  may decrease 0.012 inches over a  5{fraction (1/2+L )} distance to form a taper 17.    
     In FIG. 3, the end of a single steel inner pipe  4  is shown. It has been fabricated with circumferential serrations  5  which are complementary to the locking serrations  12   a ,  12   b  shown on the joint ring  10  in FIG. 2. A slot  18  has been cut near the end of the inner pipe  4  for installation of an O-ring  20  (not shown). The position and the depth of the slot  18  are critical. The outside diameter of an O-ring  20  insertion in the slot  18  must be less than the inside diameter of the bottom of the circumferential serrations  5 . 
     FIG. 4 a  shows a typical geometry of the complementary serrations of the present invention. In the pipe joint assembly  1 , the locking serrations  12  of the joint ring  10  mesh with the circumferential serrations  5  of the inner pipe  4  to join sections of inner carrier pipe. To achieve an optimum seal, epoxy has been applied to the contact surfaces of the circumferential serrations  5  and the locking serrations  12 . An example of the epoxy used is Magnobond 6398, supplied by Magnolia Plastics. Magnobonde® 6398 is a thixatropic epoxy paste formulated specifically to bond to steel. As shown in FIG. 4 b , the geometry of the serrations is designed so that insertion of the inner pipe  4  into the joint ring  10  encounters little resistance; on the other hand, pullout of the inner pipe  4  encounters maximum resistance. For example, in the detail shown, the angle  21  may be 60°, with a height  22  of 0.027″ and a pitch  23  of 0.0625″. 
     The complete procedure for joining sections of double-wall steel pipe may best be understood by referring to FIG.  5 . First, the joint ring  10  is heated, whereby it expands. Epoxy is applied to the locking serrations  12   a ,  12   b  on the interior circumference of the joint ring, and to the circumferential serrations  5   a ,  5   b  on the ends of the steel inner pipe  4   a ,  4   b . The slight bevel  19   a ,  19   b  on the joint ring ends  13   a ,  13   b  provides a guide the steel inner pipes  4   a ,  4   b  will follow as two sections of double-wall steel pipe  2   a ,  2   b  are axially aligned and slip-fit together. As the steel inner pipe  4   a ,  4   b  is advanced into the straight-bore portion of the joint ring  10 , final axial alignment is achieved. As the inner pipe  4   a ,  4   b  advances further into the joint bore  14   a ,  14   b , the inner pipe  4   a ,  4   b  contacts the tapered portion  17  of the joint ring  10 , compressing the end of the inner pipe  4   a ,  4   b . The joint ring  10  may be fabricated with a joint center shoulder  15  to ensure full depth insertion of the inner pipes  4   a ,  4   b  of the sections of double-wall steel pipe  2   a ,  2   b . The two layers of epoxy form a seal and bond the steel inner pipe  4   a ,  4   b  to the joint ring  10 . 
     After the beveled ends of the steel outer casing  3   a ,  3   b  have been fitted over the joint ring  10 , full-circumference welding permanently joins the sections of double-wall steel pipe  2   a ,  2   b  at weld  25 . The welding operation supplies additional heat to the joint ring  10 , thereby maintaining or increasing its temperature. Keeping the joint ring  10  at the optimum curing temperature range for a longer period of time further ensures that the epoxy will fully cure to its maximum strength in the shortest possible time. As the joint ring  10  cools and contracts, it locks and seals the ends of the steel inner pipes  4   a ,  4   b  into place in the joint ring  10 . 
     The above-described pipe assembly can be efficiently performed in the field. According to the present invention, convenient lengths of double-wall steel pipe (e.g. 40′, 30′ or 160 40   lengths) may be completely formed at a manufacturing plant. The ends of the steel inner pipes will be serrated, and O-ring slots will be cut before the inner pipe is placed inside the outer casing. If insulating foam or other insulating material is used, it too will be installed in the manufacturing plant so that the outer casing and inner pipe extend beyond it, forming an annulus. Similarly, properly-sized joint rings may be prefabricated at a plant, with serrations on the joint bore. 
     Assembly of the sections of double-wall pipe is not dependent on extensive care or elaborate joint and pipe field operation techniques. The epoxy may be easily applied to the serrated areas at the installation site. The inner steel pipe may also be heated to maintain the optimum curing temperature of the epoxy. A temperature differential, with the inner steel pipe having the lower temperature, must be maintained. The temperature differential must be calculated to provide acceptable clearance between the joint ring and the steel inner pipe as the joint ring is slip-fit onto the inner pipe. The slip fitting of the joint over the end of the inner steel pipe is not difficult, since the steel joint acts as a guide to properly align the inner steel pipe within the joint; it also keeps the joined sections in place until the welding operation is complete. The only time-consuming operation is the welding of the outer casing of the sections of double-wall pipe. 
     FIG. 6 shows the present invention as applied to a bell and spigot configuration. In this application, specially-fabricated sections of double-wall steel pipe  30   a ,  30   b  are used. On one end of each section, one side of a specially-fabricated joint ring  31  has been joined to an end of steel inner pipe  32   a  by weld  33  in a manufacturing facility, together forming the bell  34 . The inner pipe  32   b  on the other end of each section, called the spigot  35 , has circumferential serrations  36 . The bell  34  has complementary locking serrations  37  on its inner circumference. in the field, joining the bell and spigot configuration will be similar to the process described above, for FIG.  5 . However, this configuration requires the application of epoxy to and slip-fit joining of only one end of adjacent line pipe sections as opposed to two. Also, there will be no need for additional handling of the joint ring  31  in the field, as it will already be affixed to one end of a double-wall or single-wall line pipe section to form the bell  34 . This configuration requires an application of epoxy to both the locking serrations  37  on the inner circumference of the bell  34  and to the circumferential serrations  36  on the outer circumference of the spigot  35 . The bell  34  is heated, causing its inner circumference to increase for insertion of the spigot  35 . When the bell  34  cools, it contracts, locking onto the spigot  35 . The beveled edges of the outer casing  38   a ,  38   b  are then joined by weld  39 . 
     It has been shown that epoxy of the type previously referenced, when cured at an elevated temperature, has a higher resistance to lap shear, greater compressive strength and reduced curing time. Therefore, heating the bell  34  also acts to affect development of these desirable properties of the epoxy. 
     FIG. 7 shows the application of the present invention to join sections of single-wall steel pipe. Use of the present pipe assembly is desirable when welding is not possible due to an explosive atmosphere, or when weld joints, flange joints, or other pipe joints are not applicable or acceptable. The steel joint, by itself, is capable of withstanding the axial, radial, and tangential stress components of a pressurized pipeline. The pipe joint assembly  50  connects a first section of single-wall steel pipe  51   a  to a second section of single-wall steel pipe  51   b . The drawing shows the ridges of circumferential serrations  52   a ,  52   b  on each of the adjoining ends of single-wall steel pipe  51   a ,  51   b . The joint ring  55  is fabricated in the same configuration as the joint ring  10  described above for FIG.  2 . The joint ring  55  has locking serrations  56   b ,  56   b  on the inner circumference of the collar  57   a ,  57   b . The joint ring ends  58   a ,  58   b  have bevels  59   a ,  59   b . O-rings  60   a ,  60   b  are optionally installed into slots  61   a ,  61   b  in the ends of single-wall steel pipe  51   a ,  51   b . The ends of sections of single-wall steel pipe  51   a ,  51   b  have been inserted to rest against the joint center shoulder  62 . As in FIG. 2, the inner circumference of the joint ring  55  has a tapered section  63 .