Abstract:
An expansion loop for an above-ground pipeline that utilizes cold bent pipe sections having a bend angle of less than 90 degrees and a bend rate (in degrees per foot) of relatively low severity.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to above-ground pipelines. In another aspect, the invention concerns expansion loops utilized in elevated hydrocarbon pipelines.  
           [0003]    2. Description of the Prior Art  
           [0004]    It is well known that land-based pipelines are commonly used to transport hydrocarbons (e.g., oil and/or gas) from remote production areas to processing and/or end use facilities. Due to aesthetics, economics, and other reasons, these pipelines are typically buried in the ground whenever practical. However, in some regions, buried pipelines cannot be used because of the nature of the terrain through which the pipeline passes. Examples of terrain that necessitates the use of above-ground pipelines include permafrost regions, regions of rugged terrain, and regions with active seismic faults. In these types of terrain, elevated pipelines are often used to transport hydrocarbons to their destinations.  
           [0005]    For example, in building conventional elevated pipelines across the permafrost terrain of North Slope, Ak., a plurality of vertical supports (e.g., 5 feet tall) are installed across the terrain at approximately 45 to 65 foot intervals. A long section of pipeline is welded together and then raised on to these vertical supports. Sliding surfaces are provided between the pipeline and each of the vertical supports to thereby allow slight relative movement therebetween. At spaced intervals (e.g., 1500 feet), the pipeline is fixed to a vertical and horizontal anchor. “Expansion loops” are spaced (e.g., every 1500 feet) within the pipeline to compensate for any substantial thermal expansion/contraction of the pipeline between any two adjacent fixed anchors.  
           [0006]    [0006]FIGS. 1A and 1B schematically illustrate a conventional pipeline  10  supported above the ground  12  by a plurality of vertical supports  14  and including a conventional expansion loop  16 . The conventional expansion loop  16  includes a plurality of 90° bends  18  with a straight pipe section  20  extending between the 90° bends  18 . Typically, the conventional 90° bends  18  used in the expansion loop  16  have a bend radius of about 3 D, where “D” is the nominal diameter of the pipe employed in the 90° bend  18 . Due to the severity of the curvature of the conventional 90° bends  18  (e.g., bend rates of 10 to 20 degrees per foot), the conventional 90° bends  18  are typically made by induction bending a “mother” pipe that has a special metallurgy and/or wall thickness different from that of the straight pipe sections  20 , 22  used in the pipeline  10 .  
           [0007]    One disadvantage of using 90° bends with relatively severe bend rates in the above-ground pipeline is the high pressure drop and slugging forces associated with the 90° bends. A further disadvantage of using conventional 90° bends in above-ground pipelines is that only a few facilities have the capability of induction bending large diameter (e.g., greater than 12 inch nominal diameter) mother pipes. These induction bending facilities capable of forming the 90° bends are typically located a great distance (e.g., thousands of miles) from the pipeline location where the 90° bends will ultimately be installed. Thus, the 90° bends must be shipped a substantial distance from the bending location to the installation location. Further, due to their shape, the conventional 90° bends must typically be shipped separate from the straight pipe sections used for the rest of the pipeline. This separate fabrication and shipping of the 90° bends can be expensive and can cause time delays.  
           [0008]    In a conventional below-ground pipeline used to transport corrosive hydrocarbons, high density polyethylene (HDPE) liners are typically used to provide corrosion and erosion resistance for the metallic pipe sections. However, in above-ground pipelines the 90° bends of a conventional expansion loop typically do not allow the pipeline to be lined with HDPE because doing so would require the incorporation of a flange at every bend. Incorporating a flange at every 90° bend is impractical because of the cost and leakage risk associated with each flange. Thus, unlined conventional 90° bends tend to corrode and erode at a greater rate than the lined straight pipe sections of the pipeline.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0009]    It is, therefore, an object of the present invention to provide an expansion loop for an above-ground pipeline that does not require the curved pipe sections of the expansion loop to be induction bent from a mother pipe having a different metallurgy and/or wall thickness than the straight pipe sections of the pipeline.  
           [0010]    A further object of the invention is to provide an expansion loop that reduces pressure drop and slugging forces associated therewith.  
           [0011]    Another object of the invention is to provide an expansion loop that allows a HDPE liner to be installed therein without requiring a flange for every bent section of pipe.  
           [0012]    It should be understood that the above-listed objects are only exemplary, and not all the objects listed above need be accomplished by the invention described and claimed herein.  
           [0013]    Accordingly, in one embodiment of the present invention, there is provided an above-ground pipeline that comprises a first cold bent pipe section having a nominal diameter (D) and a bend radius in the range of from about 10 D to about 100 D.  
           [0014]    In another embodiment of the present invention, there is provided an expansion loop of an above-ground pipeline comprising a first bent pipe section and a first substantially straight pipe section. The first bent pipe section presents first and second ends, with the first substantially straight pipe section being coupled to the first end of the first bent pipe section. The wall thickness of the first bent pipe section and the first substantially straight pipe section are substantially identical.  
           [0015]    In a further embodiment of the present invention, there is provided a method of constructing an above-ground pipeline that includes the step of cold bending a first substantially straight pipe section to thereby form a first bent pipe section having a nominal diameter (D) and a bend radius in the range of from about 10 D to about 100 D.  
           [0016]    In still another embodiment of the present invention, there is provided a method of constructing an above-ground pipeline comprising the steps of: (a) fabricating a first substantially straight pipe section at a fabrication location, said first substantially straight pipe section having a length in the range of from about 30 to about 100 feet and a nominal diameter (D) in the range of from about 0.5 to about 6 feet; (b) transporting the first substantially straight pipe section to a bending location at least 50 miles from the fabrication location; and (c) cold bending the first substantially straight pipe section at the bending location to thereby form a bent pipe section having a bend radius in the range of from about 10 D to about 100 D and a bend angle in the range of from about 10 to about 60 degrees. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
       [0017]    A preferred embodiment of the present invention is described in detail below with reference to the attached drawing figures, wherein:  
         [0018]    [0018]FIG. 1A is an elevation view of a prior art above-ground hydrocarbon pipeline employing a conventional expansion loop that utilizes 90° bends having a relatively severe bend rate;  
         [0019]    [0019]FIG. 1B is a top view of the prior art hydrocarbon pipeline shown in FIG. 1A; and  
         [0020]    [0020]FIG. 2 is a top view of an above-ground pipeline including an expansion loop constructed in accordance with the principles of the present invention, particularly illustrating the use of cold bent pipe sections having a bend angle of less than 90 degrees and a bend rate of relatively low severity. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    Referring to FIG. 2, an above-ground pipeline  30  is illustrated as including a plurality of vertical supports  32  and an expansion loop  34 . Preferably, pipeline  30  is an elevated pipeline used to transport hydrocarbons (e.g., oil and/or gas). Alternatively, pipeline  30  is a water line. Expansion loop  34  includes a first cold bent pipe section  36 , a second cold bent pipe section  38 , and a substantially straight pipe section  40  fluidly disposed between first and second cold bent pipe sections  36 , 38 . As used herein, the term “cold bent” shall be descriptive of an object, such as a section of pipe, that was initially formed in a substantially straight configuration, but later bent from that substantially straight configuration without significant heating of the object. As used herein, the term “above-ground pipeline” shall denote a pipeline that is supported above the surface of the earth by a plurality of vertical support members spaced along the pipeline. As used herein, the term “expansion loop” shall denote a portion of an above-ground pipeline that uses lateral offsetting to accommodate for axial expansion/contraction of the pipeline.  
         [0022]    As shown in FIG. 2, first cold bent pipe section  36  can be defined by a bend angle (α), a bend radius (r), and a length (l). It is preferred for the bend angle (α) of first cold bent pipe section  36  to be in the range of from about 10 to about 60 degrees, more preferably about 20 to about 50 degrees, still more preferably about 25 to about 45 degrees, and most preferably 30 to 40 degrees. It is preferred for the bend radius (r) of first cold bent pipe section  36  to be in the range of from about 10 D to about 100 D, where “D” is the nominal diameter of first cold bent pipe section  36 . More preferably, the bend radius (r) of first cold bent pipe section  36  is in the range of from about 15 D to about 75 D, still more preferably about 20 D to about 50 D, even more preferably about 25 D to about 45 D, and most preferably 30 D to  40 D. It is preferred for the length (l) of first cold bent pipe section  36  to be in the range of from about 10 to about 200 feet, more preferably about 30 to about 100 feet, still more preferably about 40 to about 80 feet, and most preferably 50 to 70 feet.  
         [0023]    First cold bent pipe section  36  can also be defined by its “bend rate,” which measures the severity of the bend in first cold bent pipe section  36  in degrees per foot. Preferably, first cold bent pipe section  36  has a bend rate in the range of from about 0.1 to about 10 degrees per foot, more preferably about 0.25 to about 5 degrees per foot, and most preferably 0.5 to 2 degrees per foot. First cold bent pipe section  36  preferably has a nominal diameter in the range of from about 0.25 to about 10 feet, more preferably about 0.5 to about 6 feet, still more preferably about 0.75 to about 4 feet, even more preferably about 1 to about 3 feet, and most preferably 1.5 to 2.5 feet. The wall thickness of first cold bent pipe section  36  is preferably in the range of from about 0.005 D to about 0.25 D (where “D” is the nominal diameter of pipe section  36 ), more preferably about 0.005 D to about 0.2 D, and most preferably 0.005 D to 0.15 D.  
         [0024]    It is preferred for second cold bent pipe section  38  to have a substantially similar configuration to that of first cold bent pipe section  36 . Thus, second cold bent pipe section  38  preferably has the same bend angle (α), bend radius (r), length (l), bend rate, nominal diameter, and wall thickness as first cold bent pipe section  36 . However, as shown in FIG. 2, when expansion loop  34  is assembled, it is preferred for first and second cold bent pipe sections  36 , 38  to be curved/bent along a common (typically horizontal) plane, but in substantially opposite directions.  
         [0025]    A significant advantage of expansion loop  34  is its utilization of cold bent pipe sections  36 , 38  which can be bent “in the field” at a location near the final assembled location of bent pipe sections  36 , 38  in pipeline  30 . In a preferred embodiment, prior to bending, pipe sections  36 , 38  can be fabricated and transported in a substantially straight configuration along with straight pipe sections  40 , 42  of pipeline  30 . Preferably, prior to bending, the configuration and metallurgy of bent pipe sections  36 , 38  is substantially the same as that of straight pipe sections  40 , 42 . Thus, it is preferred for the nominal diameter and wall thickness of bent pipe sections  36 , 38  to be within about 10 percent of the nominal diameter and wall thickness of straight pipe sections  40 , 42 , more preferably within about 5 percent of the nominal diameter and wall thickness of straight pipe sections  40 , 42 , and most preferably within 2 percent of the nominal diameter and wall thickness of straight pipe sections  40 , 42 . The in-the-field cold bending of bent pipe sections  36 , 38  allows the distance from the bending location of bent pipe sections  36 , 38  to the final assembly location of bent pipe sections  36 , 38  to be minimized. Preferably, the distance from the bending location of bent pipe sections  36 , 38  to the final assembly location of bent pipe sections  36 , 38  is less than about 10 miles, more preferably less than about 1 mile, and most preferably less than 0.5 miles. Further, the in-the-field cold bending of bent pipe sections  36 , 38  allows for easy transportation of pipe sections  36 , 38  (in their pre-bent straight configuration) over the typically large distance between the fabrication location of pipe sections  36 , 38  and the bending location of pipe sections  36 , 38 . Typically, the distance between the fabrication location of pipe sections  36 , 38  and the bending location of pipe sections  36 , 38  is at least about 10 miles, more typically at least about 50 miles, even more typically at least about 100 miles, and frequently at least 500 miles.  
         [0026]    The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Obvious modifications to the exemplary embodiments, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention.  
         [0027]    The inventor hereby states his intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as it pertains to any apparatus or method not materially departing from but outside the literal scope of the invention as set forth in the following claims.