Patent Publication Number: US-2011049879-A1

Title: Clamp suitable for increasing the fatigue life of the butt welds of a pipe pressure vessel which is subsequently bent

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of U.S. Provisional Patent Application No. 61/238,852 entitled “CLAMP SUITABLE FOR INCREASING THE FATIGUE LIFE OF THE BUTT WELDS OF A PIPE PRESSURE VESSEL WHICH IS SUBSEQUENTLY BENT,” filed Sep. 1, 2009, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a pipe clamp. More particularly, the invention relates to a pipe clamp for locating over a butt weld in a length of pipe to be subjected to bending forces to protect against excessive strain in the weld area. 
     BACKGROUND OF THE INVENTION 
     The application area is the storage of natural gas (CNG) or other fluids or gases under pressure in pressure vessels comprised of very long lengths (generally over 1 km and under 30 km) of steel pipe. For practical management, these lengths of pipe are coiled on a steel spool by a simple winding process. The result is termed a “Coselle®”. Many Coselles® may be installed in the hold of a ship. A Coselle® carrying ship loads gas in a first port and discharges in a second port. A Coselle® carrying ship may make ˜60 cycles per year. The pressurizing and de-pressurizing associated with the loading and unloading of gas induces major stress changes in the steel of the Coselles®, which could possibly result in fatigue cracking. The long lengths of pipe are necessarily constructed by welding shorter lengths of pipe together. The welds are more sensitive to fatigue problems than the parent metal of the pipe. 
     It has become apparent during prototype Coselle® testing where 6″ pipe was wound about a hub of ˜9 meters diameter, that higher than average strains were being experienced in the outside wall of the pipe at the heat affected zone (HAZ) interface of the circumferential joining welds (butt welds) contained in the pipe. These high localized strains, (approximately 6% as opposed to approximately 1.8% average) shortened the fatigue life of the weld and are responsible for limiting the ultimate fatigue life of the Coselle®. As a result it was necessary to operate at a lower working pressure than would be allowed if the fatigue life were extended, which reduced the gas volume and raised the overall cost of gas transport. 
     In addition to exceptional strain, the weld properties may also be adversely affected by ovalization of the pipe during the bending process. At the four points of high and low curvature of the oval, stress is increased during the pressurization process and this will reduce fatigue life, particularly for the butt welds. 
     In summary it is economically important to ensure a long fatigue life for the butt welds in the Coselle® pipe pressure vessel, to protect against both excessive strain of the heat affected zone and ovalization, both induced by the coiling process. This is achieved by the following clamp apparatus. 
     SUMMARY OF THE INVENTION 
     The solution chosen to the excessive strain problem is to severely limit the bending of the pipe and to prevent ovalling in the region of butt welds and simultaneously to largely prevent strain in the outside of the pipe. This may be accomplished by applying a clamp to a pipe over a butt weld. The following describes the clamp that was developed and tested successfully. However, other embodiments of a clamp could be derived to accomplish the desired results. 
     An example clamp has a length of 300 mm. Preferably, the clamp has a length of 1.75 external diameters of pipe, but larger or smaller clamps could also be used. The clamp functions as a reinforcing brace composed of two semi-circular, straight half-cylinders of steel bolted together on both sides of the pipe. The clamp is made up of a first half-cylinder located on the outside of the bend of the pipe, and is made up of a second half-cylinder located on the inside of a bend of the pipe. The inside radius of curvature of both clamp half-cylinders is preferably equal to the outside radius of the pipe. The thickness of the clamp is such that the clamp is not plastically bent by the bending forces. 
     The rigidity of the clamp limits bending of the pipe in the region of the weld and, with its special features described below, ensures that the localized axial strains in the outside wall of the pipe remain low during and after winding. Simultaneously, the clamp ensures that ovalization is minimized. 
     The outer half of the clamp contains scoring on its inside to ensure bonding with the outside surface of the pipe. The primary mechanism by which the clamp works to limit strain in the outside of the pipe is inter-particle static friction on the surface of the pipe, and this is the reason for providing scoring. Scoring (which preferably resembles threading) has a pitch such that the ridges will bite slightly into the pipe and an overall contact area so as not to damage the pipe by compression during the bending process. The inside half of the clamp is not scored and in fact, slippage on the inside of the pipe is not undesirable. 
     In the vicinity of the pipe weld, the pipe is almost completely surrounded by the rigid clamp. Therefore, ovalization of the pipe is almost completely prevented. However, the presence of the clamp can induce serious ovalling, even kinking, of the pipe at both ends of the clamp as the pipe is wound. To address this problem, two shape modifications to the inner half of the clamp are made. First, the two ends of the inside half of the clamp are ground away in a specific pattern to ease the concentration of force on the pipe at the ends of the clamp. The preferred shape of the metal removed is a semicircle of radius equal to the radius of the pipe, but approximations to this semi-circle may be used. Secondly, ovalization in the vicinity of the clamp may be further reduced by grinding away some of the thickness of the inside half of the clamp so that it is significantly thinner at its outside edges than at its center (roughly half the thickness is preferred). The reason is to further minimize the transverse force on the pipe when the clamp first hits the underlying pipes beyond the relief provided by the removed semicircles. 
     While both halves of the clamp have their radii of curvature identical to the outside of the pipe, the two halves of the clamp do not comprise a complete 360° turn. When bolts are fully tightened, the two halves of the clamp preferably do not touch each other so that the full force of the clamp halves is impressed on the pipe. 
     When the bending of the pipe is complete, the clamp is removed as follows: the inward force of the pipe is reduced by a very slight reverse turn of the spooling hub. The clamp is then disassembled. The outside half of the clamp is removed and then the inside half of the clamp is rotated around the pipe and lifted off. To facilitate the rotation, the bottom half of the clamp preferably has a reasonably uniform or reduced thickness profile in the rotary sense. 
     In use, the clamp halves are very strongly tightened up on the pipe by a series of bolts. The scoring and the tightening threads in the bolt holes wear down with use so that the clamp will have to be replaced after a number of uses. 
     Both halves of the clamp have a small channel of metal removed from the inside centers of the halves in the hoop direction to accommodate the weld bead of the butt weld. This obviates the need to grind the weld. 
     The clamp is preferably made of steel with a Young&#39;s Modulus closely equal to that of the steel of the pipe. The yield stress of the steel used in the clamp should be at least that of the pipe and up to 40% greater. 
     A first embodiment of the invention is rigid and heavy (about 200 lbs weight). This embodiment totally protects the weld and even with 1,000,000 cycles no fatigue damage was found to have occurred to the weld. The first embodiment is, however, cumbersome to manipulate. A second embodiment is a lighter version (about 40 lbs). 
     The clamp described above in detail is a preferred embodiment of a device to minimize the strain in the outer edge of a pipe and to minimize ovalling in the region of a butt weld while being bent around a coil ( FIG. 9 ). Other embodiments may also be used. 
     For example, the clamping force may be provided not by bolts, but by external hydraulic pressure. The halves of the clamp may be held together by welding or binding with wire. Alternatively the clamp halves may be significantly heated before being applied and welded. The clamping force would be due to metallic contraction as the clamp cools. Alternatively, the two clamp halves could be drawn together by a buckling mechanism, with or without the assistance of a hydraulic press. 
     Alternative means of creating friction may be developed to eliminate the need for the threading, such as metal to metal adhesives, low temperature metal alloy solders, a layer of sharp particles of a substance harder than steel, such as silicon carbide, etc. 
     Strips of steel, centered on the butt weld may be welded axially on the pipe. 
     The butt weld and adjacent areas may be wound with a strong material that is bonded to the pipe. The material may be steel wire, e-glass, Kevlar or other material that could be permanently bonded to the pipe, which would reduce the induced winding strains and/or reduce ovalization. 
     In a further embodiment, a section of pipe whose ID is the same as the OD of the main pipe may be slipped onto the main pipe before the welding operation. The section of pipe may then be centered over the butt weld. This pipe section resembles the clamp but may remain in place after the main pipe is bent. 
     Alternatively, a small pipe section or other steel structure may be inserted inside the main pipe and restrained so as to be centered on the butt weld. The small section of pipe remains in place after welding and bending. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of the pipe weld clamp of the invention including an inside clamp segment and an outside clamp segment; 
         FIG. 2  is a partial cross-sectional view of the pipe weld clamp of  FIG. 1  shown installed within a pipe; 
         FIG. 3  is a perspective view of the inside pipe clamp segment of  FIG. 1 ; 
         FIG. 4  is a schematic view of an outside surface of the inside pipe clamp segment of  FIG. 1 ; 
         FIG. 5  is a schematic view of an inside surface of the inside pipe clamp segment of  FIG. 1 ; 
         FIG. 6  is a perspective view of the outside pipe clamp segment of  FIG. 1 ; 
         FIG. 7  is a schematic view of an outside surface of the inside pipe clamp segment of  FIG. 1 ; 
         FIG. 8  is a schematic view of an inside surface of the inside pipe clamp segment of  FIG. 1 ; 
         FIG. 9  is a perspective view of the pipe of  FIG. 2  wound on a core. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIGS. 1 and 2 , shown is pipe clamp designated generally  10 . Pipe clamp  10  is for affixing to pipe  12  ( FIGS. 2 ,  9 ). Pipe  12  consists of first pipe segment  14  and second pipe segment  16 . First pipe segment  14  and second pipe segment  16  are joined together via butt weld  18 . Pipe clamp  10  is provided to protect butt weld  18  from excessive strain of a heat affected zone and ovallization, both induced by a coiling process of pipe  12 , as shown in  FIG. 9 . 
     Pipe clamp  10  consists of inside clamp segment  20  ( FIGS. 1 ,  3 - 5 ). Inside clamp segment  20  has a semi-cylindrical shape and has a first end  22  and a second end  24 . Inside clamp segment  20  additionally has an inside surface  26 , an upper lengthwise mating surface  28 , and a lower lengthwise mating surface  30 . First end  22  defines first stress relief area  32 . Second end  24  defines second stress relief area  34  ( FIGS. 3-5 ). 
     Inside clamp segment  20  preferably defines radial groove  36  for receiving butt weld  18  when inside clamp segment  20  is positioned on pipe  12 . Inside surface  26  of inside clamp segment  20  is preferably substantially smooth for permitting slippage of an outside surface of pipe  12  with respect to inside surface  26  of inside clamp segment  20 . 
     In one embodiment, first stress relief area  32  defines a first semi-circle and second stress relief area  34  defines a second semi-circle. The first semi-circle defined by first stress relief area  32  and a second semi-circle defined by the second stress relief area  34  preferably each have a radius equal to the radius of pipe  12 . 
     In one embodiment, as best seen in  FIG. 4 , a thickness of inside clamp segment  20  proximate first end  22  and second end  24  is less than a thickness of inside clamp segment  20  at a center of inside clamp segment  20 . 
     Pipe clamp  10  additionally consists of outside clamp segment  40  ( FIGS. 1 ,  2 ,  6 - 8 ), having a semi-cylindrical shape. Outside clamp segment  40  has a first end  42  and a second end  44 . Outside clamp segment  40  additionally defines an upper lengthwise mating surface  46  and a lower lengthwise mating surface  48 . Outside clamp segment  40  additionally has an inside surface  50 . Preferably, inside surface  50  defines a friction element  52 . Friction element  52  is for gripping an outside surface of pipe  12 . 
     Outside clamp segment  40  additionally preferably defines radial groove  54  ( FIGS. 6 ,  8 ) for receiving butt weld  18  when outside clamp segment  40  is located on pipe  12 . In a preferred embodiment, friction element  52  is made up of scoring on inside surface  50 . More preferably, scoring is made up of threads. An example thread has a depth of 0.77″ and a peak to peak measurement of 0.166″. Friction element  52  may also be created through use of metal adhesives, low temperature metal alloy solders, or by a layer of hard particles affixed to inside surface  50 . 
     Preferably, upper lengthwise mating surface  28  and lower lengthwise mating surface  30  of inside clamp segment  20  define a plurality of orifices  60 . Additionally, in a preferred embodiment, upper lengthwise mating surface  46  and lower lengthwise mating surface  48  of outside clamp segment  40  defines a plurality of orifices  62 . Plurality of bolts  64  ( FIGS. 1 ,  2 ) are provided for locating in plurality of orifices  60  defined by upper lengthwise mating surface  28  and lower lengthwise mating surface  30  of inside clamp segment  20 . Bolts  64  additionally locate within plurality of orifices  62  defined by upper lengthwise mating surface  46  and lower lengthwise mating surface  48  of outside clamp segment  40 . Bolts  64  secure inside clamp segment  20  to outside clamp segment  40 . 
     In a preferred embodiment, inside clamp segment  20  and outside clamp segment  40  have a thickness such that pipe clamp  10 , i.e., inside clamp segment  20  and outside clamp segment are not plastically deformed by bending forces when pipe  12  is bent after pipe clamp  10  is secured to pipe  12 . 
     In a preferred embodiment, inside clamp segment  20  and outside clamp segment  40  have a radius of curvature substantially equal to a radius of curvature of pipe  12 . Preferably, inside clamp segment  20  and outside clamp segment  40  are sized such that when inside clamp segment  20  and outside clamp segment  40  are fully tightened about pipe  12 , upper lengthwise mating surface  28  of inside clamp segment  20  and upper lengthwise mating surface  46  of outside clamp segment  40  do not touch one another. Similarly, it is preferred that lower lengthwise mating surface  30  of inside clamp segment  20  and lower lengthwise mating surface  48  of outside clamp segment  40  do not touch one another so that the full compressive force of inside clamp segment  20  and outside clamp segment  40  is impressed upon pipe  10  when bolts  64  are tightened. 
     Other methods of compressing pipe  12  within pipe clamp  10  are also contemplated, including forcing inside clamp segment  20  and outside clamp segment  40  together with hydraulic pressure. Inside clamp segment  20  and outside clamp segment  40  may also be forced together with tightened wire. Further, inside clamp segment  20  and outside clamp segment  40  may be forced together by metallic cooling of inside clamp segment  20  and outside clamp segment  40 . 
     It is also contemplated that a strong material bound to pipe  12  may be wound around pipe  12  to prevent ovalling. Contemplated materials include wire, e-glass, and Kevlar®. It is further contemplated that a pipe segment may be centered over butt weld  18 . It is additionally contemplated that pipe segment may be centered under butt weld  18 . 
     When the bending of pipe  12  is complete, clamp  10  is removed as follows: the inward force of pipe  12  is reduced by a very slight reverse turn of the spooling hub  66 . Clamp  10  is then disassembled. Outside clamp segment  40  is removed and then inside clamp segment  20  of the clamp  10  is rotated around pipe  12  and lifted off. To facilitate the rotation, the bottom half of the clamp  10  preferably has a reasonably uniform or reduced thickness profile in the rotary sense. 
     In use, clamp segments  20 ,  40  are very strongly tightened up on pipe  12  by a series of bolts  64 . Other clamp mechanisms may also be used. Friction element  52 , e.g., scoring, and the tightening threads in the bolt holes  60  wear down with use so that clamp  10  will have to be replaced after a number of uses. 
     Both segments  20 ,  40  of clamp  10  have a small channel or radial groove  36  of metal removed from the inside surface  26  of clamp segment  20  and inside surface  50  of clamp segment  40  in the hoop direction to accommodate the weld bead of weld  18 . This obviates the need to grind the weld  18 . 
     The clamp  10  is preferably made of steel with a Young&#39;s Modulus closely equal to that of the steel of pipe  12 . The yield stress of the steel used in the clamp  10  should be at least that of pipe  12  and up to 40% greater. 
     Thus, the present invention is well adapted to carry out the objectives and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those of ordinary skill in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the specification.