Abstract:
In a pipe joint where two pipe sections ( 14, 16 ) have threadably engaged end potions ( 17, 18 ), joint ends ( 32, 34 ) are substantially sealed to each other by a sealing surface ( 40, 50 ) on one pipe section that substantially abuts a sealing surface ( 42, 52 ) on the other pipe section. In one joint, a ring-shaped cutout( 121 , FIG.  7 ) is formed in a first pipe section and a ring-shaped spacer ( 120 ) is located in the cutout. The spacer is chosen from several that have slightly different lengths so the spacer ends contacts the corresponding sealing surface( 130 ).

Description:
CROSS-REFERENCE 
     This is a Division of U.S. patent applicaton Ser. No. 13/013,739 filed Jan. 25, 2011 which claimed priority from US. provisional patent application Ser. No. 61/316,371 filed Mar. 22, 2010. 
    
    
     BACKGROUND OF THE INVENTION 
     Pipe strings consisting of many pipe sections connected in tandem, are widely used in the sea to transfer crude oil and other hydrocarbons between the sea floor and a floating body, or between floating bodies. One common type of pipe string includes many steel pipe sections, each of a length such as 30 meters, that are connected together by threaded end portions lying at opposite ends of each pipe. Applicant notes that one type of thread is a helical thread, and another type includes axially-spaced circular threads. Each end of a pair of adjacent pipe sections is preferably sealed to the other pipe section by a sealing surface on the end of one pipe section that presses against a sealing surface on the end of the other pipe section as the pipe section are threaded together. Previously, the opposite ends of a pipe sections could not both be tightly scaled, unless very high precision were used in the manufacture which results in prohibitive cost. 
     Many fluids to be carried by the pipe string are highly corrosive. A protective coating or cladding can be applied to the inside surface of the pipe sections, and to adjacent pipe surfaces. The cost of applying such coating can be minimised by minimizing the area to be coated, especially areas that include sharp angles. 
     A pipe joint that includes a pipe with threaded portions and with sealing surfaces at the ends, often includes tongue and groove joints that lie beside sealing surfaces. There is occasional fracture of the steel pipe end at a side of the groove. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, a corrosion resistant pipe string is provided with threaded pipe joints that are constructed to substantially seal to one another. To assure that the sealing surfaces at opposite ends of a pipe section abut corresponding sealing surfaces, applicant shortens one of the abutments to create a cutout. Applicant also provides a ring-shaped spacer that lies in the cutout. Several spacers are available of very slightly different lengths, and during pipe string assembly a spacer is chosen that just fills the cutout. 
     To protect the pipe sections against corrosion by corrosive fluid, the inside of each pipe section is coated, as by a cladding of corrosion resistant material. Where there is a rounded corner between the inside of the pipe and a sealing surface, the coating is extended around the corner and along the sealing surfaces, but no further. To avoid extending the coating around a rounded corner, the sealing surfaces each can be provided with a sharp (small radius of curvature) corner so the two corners abut one another. 
     The pipe ends include tongue and groove connections. To avoid one of the pipes fracturing at the outer side of the groove, the groove bottom is provided with a small radius of curvature. This allows the outer side of the groove to be provided with a large radius of curvature so that stresses are better resisted. 
     To prevent the threaded end portions of the pipe sections from turning to loosen the threaded connection, at least one pipe thread is knurled to generate small ridges that resist loosening. 
     The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a joint potion of a pipe string, showing the threadably connected adjacent ends of two pipe sections and showing the sealing surfaces at the opposite ends of the threads. 
         FIG. 2  is a sectional view of area  2 - 2  of  FIG. 1 . 
         FIG. 3  is a sectional view of area  3 - 3  of  FIG. 1 . 
         FIG. 4  is a sectional view similar to that of  FIG. 3 , but with a corrosion-resistant cladding an the entire straight inside portion of the pipes. 
         FIG. 5  is sectional view similar to that of  FIG. 4 , but with a corrosion-resistant cladding on only the inside portion that lies adjacent to the sealing surfaces. 
         FIG. 6  is a sectional view of only one side of the pipe joint of a pipe string similar to that of  FIG. 1 , bat with a spacer lying in a cutout. 
         FIG. 7  is a sectional view of the cutout and spacer region of the joint portion of  FIG. 6 . 
         FIG. 8  is a sectional view similar to that of  FIG. 2 , but with a tongue and groove connection constructed with a gradual curvature at one side of the groove. 
         FIG. 9  is a view similar to that of  FIG. 3 , showing the gradual curvature at one side of the groove. 
         FIG. 10  is a sectional view of a pair of thread lengths of the threadably joined end portions of the pipe sections of  FIG. 1 , showing knurling. 
         FIG. 11  is an isometric view of a portion of one of the thread lengths of  FIG. 10 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a pipeline, or pipe string  10  that includes many steel pipe sections, each of a length such as 30 meters, which are threadably connected in tandem. Applicant notes that one type of threaded joint includes a helical thread, and another type includes axially-spaced (with respect to the pipeline axis) circular threads that lie on an imaginary cone.  FIG. 1  shows a pipe joint  12  where adjacent end portions  17 ,  18  of two pipe sections  14 ,  16  are connected. The joint includes threads  20   22  on the two pipe sections that term a threaded connection  24 . The threads are tightened to press the two pipe ends close together at sealing surfaces  40 ,  42 ,  50 ,  52  that lie at axially (A) opposite joint ends  33 ,  34 . To minimize thread chafing during a threadable connection applicant forms a port  51  in the radially (with respect to axis  36 ) outer pipe end  18 . The joint can be pressurized by a fluid (liquid or gas) directed through the port  51 , which compresses radially inner pipe end  17  and which expands radially outer pipe end  18 , thereby easing the threads  20 ,  22  over one another. 
     Joint end  32  has upper and lower abutments, or sealing surfaces  40 ,  42  that both lie at the inside I of the pipeline. Joint end  34 , which is shown located at the upper end of the joint  12 , has radially outer (with respect to the axis  36 ) sealing surfaces. 
     Of the two joint ends  32 ,  34 , usually only one, which is the upper and outer abutment location  34  has its abutment faces  50 ,  32  forcefully abutting one another when the threads are fully tightened. It would require extremely close dimensional tolerances (e.g. less than about 0.02 mm, or 0.001 inch) to assure forceful abutment of the abutment surfaces at both joint end  32 ,  34 . To manufacture the pipe section ends to these tolerances would be difficult and costly. 
     Applicant causes the inner sealing surfaces  40 ,  42  and the outer sealing surfaces  50 ,  52  to forcefully abut one another and form a fluid seal thereat that prevents the ingress and egress of fluids. This also preloads the connector threads to thereby reduce fatigue stresses and stabilize metal-to-metal seal movement at the pine joint ends  32 ,  34  shown in  FIG. 7 . 
     The inside I ( FIG. 1 ) of the pipe joint is exposed to fluids  60  lying in the pipe, which may include corrosive chemicals when the pipeline is used to carry a mixture of hydrocarbons and corrosive components such as is often found in crude oil. The outside O of the pipe joint is exposed to seawater or atmospheric conditions. 
     In recent years, offshore oil production has been conducted in ever deeper waters, with the present maximum depth being about 7000 to 10,000 feet. In such depths, steel pipelines connected to floating production units are usually employed to transport well fluids from the seabed to the sea surface. The high seawater pressure at large depths requires large pipe wall thickness to resist collapse, although the pipe inside diameter must be large enough for economic hydrocarbon production rates. This leads to relatively stiff pipes and high bending stresses, especially where a pipeline hanging in a catenary shape makes contact with the seabed. This bending is repetitive as it is the result of the heave of the surface vessel which is constantly being excited by surface waves. This repetitive bending leads to pipe fatigue. This fatigue can be accelerated by the presence of H 2 S, CO 2  or other chemicals in the well fluids flowing through these pipes, which can lead to a very limited pipe fatigue life. To minimize this chemical effect, Applicant prefers to apply a CRA (corrosion resistant alloy) cladding (a type of coating), shown in phantom lines at  70 ,  72  in  FIG. 3 , on the inside surface  44  of the pipeline, with portions  74 ,  76  of the cladding lying on the inside sealing surfaces  40 ,  42 . Since the inside joint end  32  is closed during use, the cladding portions  74 ,  76  at the abutting surfaces need not be thicker than on the rest of the inside of the pipe. The corners  82 ,  84  between the radially inside surfaces  44  and the sealing surfaces  40 ,  42  have a large radius of curvature of at least 0.05 inch so the cladding extends over them. 
       FIG. 4  shows one alternative construction of a pint end  32 B, where CRA cladding portions  70 B,  72 B have been applied on the inside surface of the pipeline, but not on the sealing surfaces  40 B,  42 B. The corners  86  have a small radius of curvature of less than 0.05 inch so the cladding portions lie facewise adjacent to each other and do not extend around the corners,  FIG. 5  shows another alternative, where the cladding extends only a limited distance M, N on either side of the inside  80  of the abutment, where M and N are no more than half each pipe section length. 
       FIG. 6  shows another pipe joint  100  along a pipeline  102  in which adjacent ends  104 ,  106  of two pipe sections  110 ,  112  are threadably connected, which provides sealing at axially spaced inside and outside locations, or joint ends  114 ,  116  at the bottom and the top of the pipe joint. The sealing is made by parts that are precisely spaced along the pipeline axis, but which can be made with high precision at moderate cost. At the upper and outside seal location  116 , sealing surfaces  50 ,  52  of the two pipe sections directly abut one another in the same manner as in  FIG. 1 . However, at the lower and inside seal location  114 , applicant provides a spacer  120  ( FIG. 7 ) that lies in a gap  121  that results in a short end ( 125 ). The spacer abuts both a shoulder  122  on the upper pipe section end and a shoulder  124  on the lower pipe section end. 
     The spacer  120 , which is in the form of a ring, is preferably formed of metal in order to withstand the high compressive forces applied when the two pipe sections are tightly threaded together. Applicant may fabricates multiple spacers with slightly different heights, or axial thicknesses T. A storage container containing many spacers of slightly different thicknesses H are provided near the location where the pipes are to be joined. In one example, for steel pipes of 12 inches diameter (at the inside), multiple spacers are provided whose thicknesses vary in increments of 0.02 mm (0.001 inch). The lengths of the pipe section ends are preferably constructed with precision, with an accumulated tolerance and therefore variation in gap height H such as 0.2 mm (0.1 inch). Recent studies show that several spacers may not have to be taken into the field, and therefore the spacer ring may be pressed into place before equipment is taken into the field. 
     Before assembling pipe sections, applicant calculates and\or measures the size of the gap H to be filled by the spacer, for two pipe ends that are to be joined. This may be done with a laser range finder or a more conventional caliper. Applicant measures the distance D ( FIG. 6 ) between the two sealing surfaces of each pipe joint end. The spacer height H is chosen to equal the difference. A spacer of the proper thickness is selected to fill the gap when those two pipe section ends are to be joined. The pipe sections and spacer are preferably numbered and kept together so that the particular spacer can be used when those two pipe section ends are threadably joined. It would be possible to join two pipe sections and measure the gap before disassembly to insert the spacer. However, such a process consumes time and idles high cost workers and equipment which would be most efficiently used to join pipe sections. 
     The spacer  120  ( FIG. 7 ) is shown as having a rectangular cross section with slightly rounded corners  126  and with one beveled corner  128  that lies at an inside corner of a pipe section end. The sealing, or abutting faces  130 ,  132  of the spacer are shown as flat. However, it is possible to form a sealing face with one or more ridges, as shown at  134 , to enable greater axial compression of the spacer at a given compression force. It also is possible to use a material other than steel such as one that is more easily compressed, if a suitable material is found. 
     Applicant, prefers to place the spacer  120  at the inside of the pipeline, where the spacer cannot be dislodged if the pipeline is hit, it would also be possible to move the spacer  120  to the outer abutment surface  34  in which case the inner abutment could be machined to be without a ring spacer  120 . It would also be possible to simplify and also rectify the machining of the connector in such a way that one uses spacers  120  at both joint ends  32  and  34 . 
     The pipeline shown in the figures has a diameter (outside) of 20 inches and a wall thickness of one inch (along much of its length). The spacer  120  has a radial width of 0.33 inch and an axial height of 0.75 inch. The pipes and spacer are each fabricated of steel. 
     In the tongue and groove joint portion of  FIG. 7 , one pipe end portion  104  forms a tongue  140  that is received in an interference fit in a groove  142  of the other pipe end portion  112 . In  FIG. 7  the groove end  144  has about the same radius of curvature as the tongue end, and the radius R is half the distance 2R across the groove. Applicant has found that the walls of the groove  142  sometimes would crack in the outer curved part of the groove due to repetitive bending stress. Applicant found that by forming the outside  154  ( FIG. 8 ) of the groove at  142 A that lies adjacent to sealing surface  124 A with a moderate radius of curvature S instead of a straight side, applicant avoided such cracking. The groove side  154  has a radius of curvature S at least 50% greater than the distance R which is half the groove width, but less than 10R. In  FIG. 8  the radius of curvature is centered at  160  and is four times the distance R. The bottom  150  of the groove has a small radius of curvature that is less than half the width 2R, but lies away from the tongue. In making the wide outer curvature in groove  142  ( FIG. 7 ) the groove becomes asymmetric and ends up with a curvature at the inside of the connector, as shown in  FIGS. 8 and 9 . The upper groove  182  shown in  FIG. 9 , is similarly curved. 
       FIG. 1  shows that the adjacent pipe end portions  17 ,  18  are connected by concentric parallel threads. Steps should be taken to be sure that after the threads  20 ,  22  are threadably connected, they do not slip by torque applied to one of the pipe end portions relative to the other. Applicant prefers to do this by knurling the threads.  FIGS. 10 and 11  show knurling  180 . In knurling, small grooves are formed by pressing a knurling tool against a location on the threads. The pressure of knurling displaces the material that forms the threads to form depressions separated by slight projections  182 . The displaced material prevents the threads from turning relative to each other unless a large torque is applied. Applicant provides knurling at one side of the threads in a thread groove wall  184  rather than in the cylindrical wall  186 , of a height H of 0.004 inch and width of 0.030 inch. Applicant could resist relative turning by roughening surfaces that turn relative to each other and that are not part of the threads, as by shot peening. However threads are precisely cut surfaces, and the knurling formed in them is more precise than roughening of a smooth (non-threaded) surfaces. 
     In the case of axially spaced concentric threads, there can be cases where the torsional resistance of the knurling  180  may not be sufficient to keep the connector from slipping. Applicant prefers to make the surfaces of the tongue  140  ( FIG. 7 ) and groove  142  of materials that will allow slippage without causing damage to these surfaces. This can be aided with the use of a anti galling coatings or lubricants or by the use of different materials of these contacting surfaces. In the event these surfaces cannot be kept from damage by slippage, applicant inserts a key into grooves machined into the threads of connector halves to prevent slippage. 
     Thus, the invention provides pipe joints for connecting the threaded ends of a pass of pipe sections, which minimizes corrosion resulting from corrosive fluids, which enables an abutment at each end of the point to abut a sealing surface at moderate cost, and which avoids cracking of a pipe at a groove. Corrosion is avoided by coating the inside end of each pipe section where it lies adjacent to an inside end of the other pipe section. Where there is a large radius of curvature lot more than 0.05 inch) at the adjacent corners of the two pipe inside ends, the corners and facing surfaces are coated, preferably by a corrosion-resistant cladding. Where the inside ends of the two pipes abut each other at sharp corners (radius of curvature less than 0.01 inch), no coating is applied beyond the corners to lie on the facing surfaces. Sealing surfaces at opposite ends of each pipe can be made to abut one another by making one pipe end short and placing a spacer at that end, with the spacer chosen to precisely fill the gap to the adjacent sealing surface. Where a tongue and groove joint connects the pipe ends, cracking at one side wall of the groove is avoided by making a portion of that groove wall with a large radius of curvature. 
     Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.