Abstract:
A pipe string component that connects an offshore platform with a sea bed is provided. The pipe string component includes a threaded tubular connection. The threaded tubular connection includes a male tubular element including a tapered male threading, and a female tubular element including a tapered female threading that cooperates with the male threading by makeup to produce a rigid mutual connection of the tubular elements with radial interference between radial load transfer zones of the threadings. The male and female threadings each have a load flank extending substantially perpendicularly to an axis of the male and female threadings. The radial load transfer zones are at a radial distance from envelopes of thread roots of the male and female threadings. The radial load transfer zones of the threadings comprise at least one surface substantially parallel to an axis of the connection.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of and claims the benefit of priority under 35 U.S.C. §120 from U.S. Ser. No. 10/581,360, filed Jun. 2, 2006, which is a National Stage application of PCT/EP04/13743, filed Dec. 2, 2004 and claims benefit of priority under 35 U.S.C. §119 from French Application No. 0314527, filed Dec. 11, 2003, the entire contents of each of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for improving the fatigue resistance of a threaded tubular connection subjected to stress variations, said connection comprising a male tubular element including a tapered male threading, and a female tubular element including a tapered female threading which cooperates with the male threading by screwing to produce a rigid mutual connection of said tubular elements with radial interference between radial load transfer zones of said threadings. 
         [0003]    That type of threaded connection is primarily intended for the production pipe strings for hydrocarbon or the like wells. 
         [0004]    Said radial interference is primarily intended to prevent breakout of the threaded connections in service—which would be catastrophic—, and it also renders the threaded connection far more monolithic. 
       DISCUSSION OF THE BACKGROUND 
       [0005]    Threaded connections of that type are known in which radial interference is obtained by contact between thread crest and corresponding thread root, in particular between the crest of the female thread and the root of the male thread. 
         [0006]    Such contact zones between corresponding thread crests and roots then constitute radial load transfer zones for the threadings. 
         [0007]    It has been established that, when such a threaded connection is subjected to stress variations, in addition to cracking by fatigue in stress concentration zones, for example at the foot of the load flank, micro-cracks appear in contact zones at the thread root, which tend to develop if high and variable tensile stresses exist in that zone, compromising the fatigue resistance of the connection. 
         [0008]    Such phenomena primarily occur in rotary drillpipe strings and have required for such products threadings cut in very thick attached elements termed “tool joints” comprising triangular threads of great depth with rounded crests and roots. There is no contact between those thread roots and crests, nor in general any radial interference. Even if such interference were implemented, the radial loads would be transferred to the thread flanks where the tensile stresses are much lower than at the thread root. The load flanks which, it will be recalled, are the flanks directed towards the side opposite to the free end of the tubular element under consideration, make an angle of 60° with respect to the axis of the threaded connection. The stabbing flanks are disposed symmetrically, making the same angle with the axis. 
         [0009]    These phenomena also occur in pipe strings connecting an offshore platform with the sea bed, under the action of waves, wind, tides and sea currents, which induce variable tensile or bending loads on the string. 
       SUMMARY OF THE INVENTION 
       [0010]    However, with that type of connection, it is not always possible to produce threads with a large thread depth and triangular threads run the risk of disengaging or jumping out from the tubular elements in service in the well. 
         [0011]    The invention aims to overcome these disadvantages. 
         [0012]    The invention aims in particular at a method of the type defined in the introduction and provides that the threadings each have a load flank extending substantially perpendicular to the axis of the threadings, and provides that said radial load transfer zones are at a radial distance from the envelopes of the thread roots of the male and female threadings and form an angle of less than 40° with the axis of the threadings. 
         [0013]    The term “envelope of the thread root” means the tapered surface which envelops the thread roots which is furthest from the thread crests. 
         [0014]    Due to the radial separation of the radial load transfer zones with respect to the envelopes of the thread roots, the micro-cracks which can form therein are not affected by the tensile stresses existing in the material beyond the thread root envelope and thus do not deleteriously affect the fatigue resistance of the connection. 
         [0015]    Optional characteristics of the invention, which may be complementary or substitutional, will be given below:
       said radial load transfer zones are constituted by i) the crest of at least one helical protuberance formed on the thread root of at least one threading with respect to the envelope of the thread root and ii) the facing zone located on the thread crest of the corresponding threading;   the protuberance or protuberances is/are disposed on the male thread root;   the crest of the protuberances is convexly domed;   the protuberances are connected to the thread root via one or more concave rounded portions;   said protuberances are each constituted by the crest of a helical rib formed on the thread root of the threading under consideration;   said radial load transfer zones comprise the crests of at least two helical ribs which are in axial succession along the thread root of the male threading;   said radial load transfer zones comprise the crest of a boss extending from the foot of the load flank to the foot of the stabbing flank on the thread root of the threading under consideration;   said radial load transfer zones comprise the crest of a boss bearing on one of the flanks of the threading under consideration;   said facing zones located on the thread crest of the corresponding threading each have a recessed helix partially enveloping each protuberance;   said radial load transfer zones are constituted by respective intermediate regions of the stabbing flanks of the male and female threadings, said intermediate regions forming a smaller angle with the axis of the threadings than the neighbouring regions of said flanks;   the angle between said intermediate regions and the axis of the threadings is substantially zero;   said radial load transfer zones are ramps constituting the stabbing flanks of the male and female threadings over the major portion of the radial height thereof;   the angle between said ramps and the axis of the threadings is in the range 20° to 40°;   the angle between said ramps and the axis of the threadings is about 27°;   the invention is implemented in a zone of full height threads termed perfect threads;   the invention is implemented both in a zone of perfect threads and in a zone of imperfect threads, in particular in a zone of run-out threads;   the profile of the male threading comprises a first concave rounded portion defining the thread root and tangential to said ramp;   the profile of the male threading comprises a second concave rounded portion with a smaller radius of curvature than the first rounded portion and tangential thereto and to the load flank;   a groove defining the female thread root extends axially from a first wall constituted by the load flank to a second wall which is connected to the ramp of the female threading;   the profile of said groove comprises a central concave rounded portion framed by first and second rounded concave portions respectively tangential to said first and second walls and with a smaller radius of curvature than the central rounded portion;   the profile of the female threading comprises a convex rounded portion tangential to a second rounded portion and to said ramp, the zone of inflexion between the convex rounded portion and the second rounded portion constituting the second wall.       
 
         [0037]    The invention also relates to a threaded tubular connection for implementing the above-defined method, comprising a male tubular element including a tapered male threading, and a female tubular element including a tapered female threading which cooperates with the male threading by screwing to produce a rigid mutual connection of said tubular elements with radial interference between radial load transfer zones of said threadings. 
         [0038]    The threaded connection comprises in accordance with the invention at least one of the following particularities:
       said radial load transfer zones are constituted by i) the crest of at least one helical protuberance formed on the thread root of at least one threading with respect to the envelope of the thread root and ii) the facing zone located on the thread crest of the corresponding threading;   said radial load transfer zones comprise the crest of a boss extending from the foot of the load flank to the foot of the stabbing flank on the thread root of the threading under consideration;   said radial load transfer zones comprise the crest of a boss bearing on one of the flanks of the threading under consideration;   said radial load transfer zones are constituted by respective intermediate regions of the stabbing flanks of the male and female threadings, said intermediate regions forming a smaller angle with the axis of the threadings than the neighbouring regions of said flanks;   said radial load transfer zones are ramps constituting the stabbing flanks of the male and female threadings over the major portion of the radial height thereof, and the profile of the male threading comprises a first concave rounded portion defining the thread root and tangential to said ramp;   said radial load transfer zones are ramps constituting the stabbing flanks of the male and female threadings over the major portion of the radial height thereof, and a groove defining the female thread root extends axially from a first wall constituted by the load flank to a second wall which is connected to the ramp of the female threading.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]    The characteristics and advantages of the invention will now be described in more detail in the following description made with reference to the accompanying drawings. 
           [0046]      FIGS. 1 to 6  are partial views in axial cross section of the threadings of different tubular connections of the invention. 
           [0047]      FIG. 7  shows an application of the threads of  FIG. 1  on a male tubular element. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0048]    The threaded tubular connection shown in part in  FIG. 1  comprises a male tubular element  1  and a female tubular element  2  respectively including a tapered male threading  3  and a tapered female threading  4 . The female threading  4  has a conventional trapezoidal profile, defining a load flank  5  which extends substantially perpendicular to the axis of the threadings, i.e. vertically in the figure, the axis being horizontal, a stabbing flank  6  forming a different angle which is, however, close to 90° with the axis of the threadings, a thread root  7  and a thread crest  8  substantially parallel to the axis, the root  7  and crest  8  being connected to flanks  5  and  6  via rounded portions. The direction of the inclination of the flank  6  is such that the helical groove formed by the female threading shrinks in the direction of the root  7 . 
         [0049]    The profile of the female threads  4  can in particular correspond to a profile designated in the American Petroleum Institute&#39;s specification API 5CT as a “buttress” profile. 
         [0050]    The “buttress” threading has a taper of 6.25% ( 1/16), 5 threads per inch of length, a load flank angle of +3° and a stabbing flank angle of +10°. 
         [0051]    Other threadings, in particular derived from the “buttress” threading type, can be used. 
         [0052]    The male threading  3  has a load flank  10 , a stabbing flank  11  and a thread crest  12  located facing flanks  5  and  6  and the thread root  7  respectively and orientated in the same manner thereas, as well as a thread root  13  located facing the thread crest  8  and which extends parallel to the axis but which is interrupted by two helical ribs  14 , the height of which with respect to the thread root  13  is advantageously in the range about 0.2 to 0.4 mm. The crest  12  and root  13  are connected to flanks  10  and  11  via rounded portions. The two ribs  14  with identical profiles and the same pitch as threadings  3  and  4  are offset with respect to each other in the axial direction to leave a fraction of flat bottom  13  between them, and two other fractions either side of the ribs. The ribs  14  have a rounded crest  15  defining a helical contact line between the rib and the female thread crest  8 . They are also connected to the bottom of the male thread  13  via rounded portions. 
         [0053]    Because of the disposition of the invention, when threadings  3  and  4  are made up one into the other so that load flanks  5 ,  10  bear on each other and a radial interference fit is obtained between the elements  1  and  2 , the radial loads transferred between elements  1  and  2  are transferred via the contact lines  15  which are at a radial distance from the thread root  13 , so that microcracks which may form there because of stress variations or slight relative movements cannot develop, the tensile stresses only existing beneath the threading roots inside the envelope E of the thread root  13  (i.e. below this envelope in  FIG. 1 ). 
         [0054]    It should be noted that after makeup, a radial clearance subsists between the crest of the male thread  12  and the root of the female thread  7 . An axial clearance also subsists between the stabbing flanks  6 ,  11 , which axial clearance should advantageously be minimized. The radial clearance between the male thread crest  12  and the female thread root  7  is in particular a function of the rounded portion between this thread root and the female load flank  5 . The radius of curvature of this rounded portion should be maximized to limit stress concentrations which are deleterious to the fatigue resistance. This is the same for the rounded portion between the male load flank and the male thread root  13 . 
         [0055]      FIG. 2  shows part of a male tubular element  1   a  and a female tubular element  2   a  provided with respective threadings  3   a  and  4   a.  Reference numerals  5 ,  7 ,  8 ,  10  and  12  designate elements that were already described above with reference to  FIG. 1  and will not be described again. In contrast to  FIG. 1 , the male thread root  13   a  extends continuously parallel to the axis of the threadings facing the female thread crest  8 . The stabbing flank of the male threading is in three portions, namely a portion  20  having substantially the same inclination as flanks  6  and  11  of  FIG. 1  and connecting via a rounded portion to root  13   a,  a portion  21  with the same inclination as portion  20 , connecting via a rounded portion to the thread crest  12 , and an intermediate portion  22  extending parallel to the axis and connecting to portions  20  and  21  via rounded portions. Similarly, the stabbing flank of the female threading comprises three portions, namely portions  24  and  25  with the same inclination as portions  20  and  21 , located respectively facing them and connected via rounded portions to the thread crest  8  and to the thread root  7  respectively, and an axially extending intermediate portion  26  facing the portion  22  and connected to portions  24  and  25  via rounded portions. When the threadings  3   a  and  4   a  are made up one into the other to obtain radial interference, the radial loads are transferred via portions  22  and  26  of the stabbing flanks, which are radially distanced from the thread root  13   a  of the male threading and the envelope E of the male thread root, thus producing the effect described with reference to  FIG. 1 . 
         [0056]    The above observations concerning the radial clearance between the male thread crest  12  and the female thread root  7 , and the rounded portions between the load flanks and the thread roots are also applicable to the connection of  FIG. 2 . There is also an axial clearance between portions  21 - 25  and between portions  20 - 24  of the stabbing flanks. 
         [0057]      FIG. 3  partially shows a male tubular element  1   b  and a female tubular element  2   b  provided with respective threadings  3   b,    4   b.  As with the embodiments described above, the load flanks  5 ,  10  of the female and male threadings extend substantially radially and their thread crests  8 ,  12  extend substantially axially. Regarding the thread roots and stabbing flanks, their profiles are defined by a combination of straight lines and rounded portions which is described below, the values for the radii of curvature being indicated by way of example for a tubular connection belonging to a pipe string with an external diameter of 177.8 to 339.73 mm (7″ to 13″ ⅜). 
         [0058]    Opposite to the male load flank  10  perpendicular to the axis of the threaded connection, the rectilinear axial profile of the male thread crest  12  connects via a convex rounded portion  30  to the stabbing flank constituted by a straight line  31  which forms an angle of 27° with the axis and which moves away from the flank  5  in the direction of the axis. At the opposite end to the crest  12 , segment  31  is tangential to a concave rounded portion  32  with a large radius of curvature, more than 1 mm, for example of the order of 1.5 mm, which defines the male thread root, a further concave rounded portion  33  with a radius of curvature of 0.3 mm being tangential to the rounded portion  32  and to the radial rectilinear profile of the load flank  10 . 
         [0059]    The double rounded portions  32 + 33  minimize stress concentrations at the foot of the load flank  10 . 
         [0060]    Opposite to the load flank  5 , the axial rectilinear profile of the female thread crest  8  connects via a large radius of curvature convex rounded portion  35  to the stabbing flank constituted by a straight segment  36  with the same inclination as the segment  31 . Opposite to the rounded portion  35 , the segment  36  is tangential to a convex rounded portion  37  with a low radius of curvature which is itself tangential to a concave rounded portion  38 , also with a low radius of curvature, the common tangent of the rounded portions  37  and  38  forming a zone of inflexion being inclined in the same direction as segments  31  and  36  and forming an angle of 70° with the axis. The rounded portion  38  is followed by two other concave rounded portions  39  and  40  the radii of curvature of which are more than and less than 1 mm respectively, the rounded portion  40  connecting to the load flank  5 . The common tangent to the rounded portions  38  and  39  is orientated axially and defines the female thread root. 
         [0061]    The set of rounded portions  37 ,  38 ,  39 ,  40  constitutes a kind of groove. The double rounded portions  39 - 40  minimize the stress concentrations at the foot of the load flank  5 . 
         [0062]    The zone of inflexion between the rounded portions  37 ,  38  constitutes one of the walls of said groove; the other wall is constituted by the load flank  5 . 
         [0063]    When threadings  3   b  and  4   b  are made up into each other, in addition to axial bearing between load flanks  5 ,  10  and between stabbing flanks  31 ,  36 , radial interference is obtained between the stabbing flanks defined by the inclined segments  31  and  36 , which are at a radial distance from the envelope E of the male thread root, producing the advantages described with respect to  FIG. 1 . 
         [0064]    The embodiment shown in  FIG. 3  has a certain number of advantages:
       a) the pre-stress generated by the threads bearing both on the load flanks and on the stabbing flanks reduces the geometrical stress concentration factor at the thread root;   b) bearing at the stabbing flanks  31 ,  36  eases any possible axial abutment (shown in  FIG. 7 ) under axial compression and bending loads.   c) The angle of 27° with respect to the axis of the stabbing flanks  31 ,  36  (i.e. an angle of 63° with respect to the normal to the axis) can minimize the torque generated by axial bearing of said flanks with respect to that generated by radial interference.       
 
         [0068]    An angle for the stabbing flank with respect to the axis of more than 40° renders the contribution of axial bearing on the makeup torque preponderate and prejudicial. That angle is preferably kept below 30°. 
         [0069]    Further, too great an angle requires a substantial reduction in the tolerances on the thread width, which is detrimental to production costs for the threadings. Similarly, a sufficiently small angle produces a certain flexibility in the thread crest, which distributes the load over the load flank better. 
         [0070]    A stabbing flank angle of less than 20° with respect to the axis, in contrast, results in too much axial hindrance in the threads. 
         [0071]    Modifications can be made to the embodiments described and shown without departing from the scope of the invention. Thus, the two ribs  14  in  FIG. 1  can be replaced by a single rib or by three or more ribs. The crest of the ribs, instead of being a point in axial cross section, can have a certain extent in the axial direction, resulting in a contact surface and not in contact line with the female thread crest. 
         [0072]    In the embodiment shown in  FIG. 4 , elements  1   c,    2   c,    3   c,    4   c  and  8   c  correspond to elements  1 ,  2 ,  3 ,  4  and  8  of  FIG. 1 . The ribs  14  are replaced by a boss  45  which extends between the foot of the male load flank  10  and the foot of the male stabbing flank  11  and which connects with the male thread root  13   c.    
         [0073]    In the embodiment shown in  FIG. 5 , elements  1   d,    2   d,    3   d  and  4   d  correspond to elements  1 ,  2 ,  3  and  4  of  FIG. 1 . A boss  55  is connected on one side to the male load flank  5  and bears against it, and on the other side to the male thread root  13   d.    
         [0074]    In the embodiment shown in  FIG. 6 , elements  3   e,    4   e  and  65  correspond to elements  3   d,    4   d  and  55  of  FIG. 5 . A rib  14   e  is pre- sent on the male thread root  13   e  and the female thread crest  8   e  has a recessed helix partially enveloping the rib  14   e  after making up the tubular elements  1   e,    2   e  such that a radial clearance exists between the remaining portions of the female thread crest and the male thread root. 
         [0075]    In the embodiment shown in  FIG. 2 , the intermediate regions  22  and  26  of the stabbing flanks are not necessarily orientated axially, but can be slightly inclined with respect to the axis. 
         [0076]    In the embodiments shown in  FIGS. 1 ,  2 , and  4  to  6 , the angle of the load flank can be slightly negative as described, for example, in International patent application WO-A-84/04352 or in the VAM TOP threaded connection sold by the Applicant (catalogue no. 940, publication date July 1994). 
         [0077]    The angle of the stabbing flank can be less than 10° or more than 10°. 
         [0078]      FIG. 7  shows the application of the invention as shown in  FIG. 1  to a threaded connection the male threading  3  of which includes a portion with perfect threads  43  of full height and similar to those shown in  FIG. 1  and a portion of run-out threads  44  of truncated height which progressively reduce from the full height at the junction with the portion  43  to zero when the envelope line E of the thread roots reaches the outer surface of the tube where the male threaded element is formed. 
         [0079]    The ribs  14  at the male thread root can advantageously be implanted both in the perfect thread zone  43  and in the run-out thread zone  44 . 
         [0080]    The embodiment of  FIG. 7  can also be applied to the threadings of  FIGS. 2 to 6 . 
         [0081]    The invention can be applied to many types of radially interfering threads, with a single threaded portion or with a plurality of axially distinct threaded portions disposed on the same tapered surface or on a plurality of radially distinct tapered surfaces. 
         [0082]    The taper of the threadings can vary widely, for example between 5% and 20%.