A threaded pipe connection

Abstract

The present invention relates to a threaded pipe connection. A threaded pipe connection comprising a first pipe (100) terminating in a pin (110) having an external male thread (120); a second pipe (200) terminating in a box (210) having an internal female thread (220), wherein the pin (110) is configured for engagement with the box (21), wherein: the male thread (120) comprises a male varying-thread-width zone (122) throughout which the load flank lead (124) of the thread differs from the stab flank lead (126) of the thread; the female thread (220) comprises a female varying-thread-width zone (222) throughout which the load flank lead (224) of the thread differs from the stab flank lead (226) of the thread; the male varying-thread-width zone (122) is arranged to engage the female varying-thread-width zone (222) when the pin (110) is engaged with the box (210); and the load flank lead and the stab flank lead vary throughout at least one of the male varying-thread-width zone (122) or the female varying-thread-width zone (222).

Description

[0001] A THREADED PIPE CONNECTION

[0002] FIELD

[0003] The present invention relates to a threaded pipe connection. In particular, the threaded pipe connection utilises a varying-width thread.

[0004] BACKGROUND ART

[0005] Varying-width threads are well known in the art. A varying-width thread is a thread that increases in width with increasing distance from the terminal end of the pipe. As a connection having complementary male and female varying-width threads is made up, the opposing load and stab flanks come into contact and reach a state of interference upon rotational engagement. The interference increases as the two pipes are screwed further together. A made-up position is reached when the torque has increased sufficiently to achieve a desired make-up torque.

[0006] It is known to achieve wedging of varying-width threads by designing the threads to have a wedge ratio. The wedge ratio for each member (male or female) is the difference between the load flank lead and the stab flank lead. These load and stab flank leads are held at a constant value. Known exceptions to this are at transitions between constant lead portions, where the lead may change from one constant value to another, or at the very end of the connection. These transitions are typically only present over very small portions of the thread such that the majority of the compressive and tensile loads to which the connection is subjected are carried by portions of thread having constant load and stab flank leads.

[0007] SUMMARY

[0008] The inventors have recognised that beneficial performance of a connection can be obtained by varying the leads continually (that is, without interruption over a number of turns, or without a portion with constant leads) over a zone that represents a substantial portion of the threaded connection. That is, there may be provided a zone that carries a substantial proportion of the compressive and tensile loads of the connection throughout which there is a continual variation in load and / or stab flank leads.

[0009] According to the invention, there is provided a threaded pipe connection comprising: a first pipe terminating in a pin having an external male thread; a second pipe terminating in a box having an internal female thread, wherein the pin is configured for engagement with the box, wherein: the male thread comprises a male varying-thread-width zone throughout which the load flank lead of the thread differs from the stab flank lead of the thread; the female thread comprises a female varying-thread-width zone throughout which the load flank lead of the thread differs from the stab flank lead of the thread; the male varying-thread-width zone is arranged to engage the female varying-thread-width zone when the pin is engaged with the box; and the load flank lead and the stab flank lead vary throughout one of the male varying- thread-width zone or the female varying-thread-width zone.

[0010] The load flank leads of the threads differ from the stab flank leads of the threads throughout the respective varying-thread-width zones so that the width of the threads varies across the entirety of the varying-thread-width zones.

[0011] The male varying-thread-width zone is arranged to engage the female varying-thread-width zone when the pin is engaged with the box so that in a made-up connection, the entirety of the male varying-thread-width zone is axially aligned with the entirety of the female varying- thread-width zone such that both zones extend of the same axial range.

[0012] In the varying-thread-width zone, the thread has a wedge ratio. This does not mean that the thread must result in engagement at both flanks of the thread along the entirety of the varying-thread-width zones, but merely that there is a difference between the load flank lead and the stab flank lead. In some cases, that difference will lead to engagement at both flanks of the thread. Where this occurs, the engagement of the flanks can generate torque during the make-up process.

[0013] The continual variation in the leads of the stab flanks and load flanks enables a thread that is both varying in width, but also varying in pitch.

[0014] The variation of the load and stab flank leads throughout one or both of the male and the female varying-thread-width zones can enable numerous advantageous improvements in performance. This is at least in part because, in contrast to a conventional varying-width thread in which the flanks start to engage simultaneously at every point along their full length, in threads having such variation in leads, the engagement of flanks can progress along the length of the thread as the connection is made up.

[0015] One example advantage is that the progressive engagement can allow a preferred distribution of stresses within the connection. For example, the variation of the load and stab flank leads throughout one or both of the male and the female varying-thread-width zones can result in a connection in which the load flanks of the pin and box engage with progressively reducing load from the end of the pin towards the end of the box. In this way, the greatest tensile force experienced in the connection can be experienced where the box is thicker with the smallest tensile force experienced in the connection where the box is thinner. This can enable the overall outer dimension of the coupling to be smaller than would be needed for a connection utilising a conventional varying-width thread.

[0016] Another example advantage is that the problem of trapping dope experienced by conventional varying-width threads can be avoided. In conventional varying-width threads, dope can be trapped between the flanks of the thread during make-up because the flanks come together simultaneously at every point along their full length, limiting the ability of the dope to be evacuated from the gap between the threads. The trapping of the dope can lead to the appearance of a higher torque than is actually carried by the engagement of threads. Subsequent flowing of the dope after make-up can then leave gaps between the threads of the connection allowing leakage. In the disclosed thread, the progressive nature of the make-up of the flanks of the threads can lead to a beneficial flow of dope along the threads, which can aid in the evacuation of the dope. This can lead to a better make-up of the threads and also avoid the problems of poor sealing caused in conventional threads by continued evacuation of dope post-make-up.

[0017] Advantageously, the final turn of the thread on one of the members, nearest the terminal surface of that member, can be wider than would otherwise be achievable with constant leads of the stab and load flanks. This can provide a stronger connection overall, whilst maintaining a preferred wedge ratio.

[0018] In the other of the male varying-thread-width zone and the female varying-thread-width zone at least one of the load flank lead or the stab flank lead may vary throughout the varying- thread-width zone.

[0019] As is known in the art, a conventional varying-width thread is a thread in which the load flank lead of the thread differs from the stab flank lead of the thread by a wedge ratio. Ordinarily, the wedge ratio is constant throughout the thread. In the varying-thread-width zones of the present application, that need not be the case. Varying wedge ratios are envisaged.

[0020] Optionally, the varying-thread-width zone comprises at least 3 turns of the respective thread. Since the properties extend over at least 3 turns of the thread, the inventive features cannot be confused with prior art transitions in lead values, which typically extend over a fraction of a turn. The number of turns of thread is an important feature of a threaded connection, since it defines the area of engagement of the threads in the axial direction, and so the connection efficiency. With a varying-width thread, the variation in width determines the rate at which relative rotation of the pin and box increases the interference between contacting load and stab flanks. A conventional connection will be designed to have a desired number of threads to provide a desired connection efficiency, and a desired wedge ratio to provide a desired make-up torque.

[0021] Pitch is the axial distance by which the thread advances along a pipe per turn. A conventional thread will have a pitch that is constant along the entirety of the thread. In a varying-width thread the pitch is defined by the midpoint of the load and stab flanks. Thus, although the load and stab leads may be different, the pitch of a varying-width thread is conventionally constant along the entirety of the thread. This is because the pitch is the average of the two leads, and since neither lead varies (despite being different), nor does the pitch.

[0022] As such, a conventional thread will have a constant thread pitch and a constant wedge ratio over the entirety of the thread. When such a thread is formed over an available axial length of a pipe, the difference in width of the thread at either end of that axial length is determined by the wedge ratio and pitch. The minimum width of the thread and the maximum width of the thread are not independent.

[0023] In accordance with the present invention, however, the load flank lead and the stab flank lead vary throughout one of the male varying-thread-width zone or the female varying- thread-width zone. The varying load flank lead and varying stab flank lead can result in a varying pitch of the varying-width thread (the average of the two varying leads also varies). Because pitch can vary along the length of the varying-thread-width zone, the minimum width of the thread and the maximum width of the thread have a degree of independence. Varying the pitch of the thread varies the number of turns within the available axial length on the pipe. For a given wedge ratio, increasing the number of turns increases the absolute difference between the minimum width of the thread and the maximum width of the thread, while decreasing the number of turns decreases the absolute difference between the minimum width of the thread and the maximum width of the thread.

[0024] Accordingly, for a desired wedge ratio over an available axial length of pipe, the pitch can be varied to achieve a more desirable width of thread. The minimum width of the thread defines the weakest turn of the thread, which is most likely to fail under tensile or compressive load. The independence provided by the ability to vary the pitch along the thread allows the designer to increase the minimum width of the thread, whilst maintaining the same wedge ratio without increasing the maximum width of the thread. This is in contrast with a conventional thread having the same wedge ratio, where increasing the minimum width of the thread will also increase the maximum width of the thread.

[0025] In other words, the present invention can enable the minimum width of the thread to be increased independently from the wedge ratio, making the thread more able to carry axial loads.

[0026] In accordance with the present invention, preferably the load flank lead and the stab flank lead decrease in a direction away from a terminal surface of the pin throughout the male or female varying-thread-width zone. In such cases, the final turn nearest the terminal end of the pin, which is subjected to great stress, can be wider than a conventional varying-width thread having the same wedge ratio and the same number of turns.

[0027] Optionally, both the load flank lead and the stab flank lead decrease in a direction away from a terminal surface of the box throughout the male or female varying-thread-width zone. In such cases, the final turn on the box, which is subjected to great stress, can be wider than a conventional varying-width thread having the same wedge ratio and the same number of turns.

[0028] Preferably, both of the load flank lead and the stab flank lead increase throughout the male varying-thread-width zone. Preferably, the wedge ratio is constant throughout one of the male and female varying-thread-width zones. In other words, the wedge ratio has the same value at any point within one of the male and female varying-thread-width zones. Such features can enable a stronger connection.

[0029] However, it is even more preferred that in the other of the male and female varying-thread- width zones the wedge ratio is constant through a first portion of the varying-thread-width zone and the wedge ratio varies throughout a second portion of the varying-thread-width zone (the second portion may be, for example, at least three turns, preferably at least four turns, more preferably at least five turns). This can assist the process of stabbing the pin into the box as discussed below. The first and second zones are contiguous axial portions of the thread.

[0030] Moreover, the presence of the second portion can be used to obtain a reduction in engagement between the threads at desirable locations, such as at the distal end of the female thread, where the thread is of minimum width and more prone to fatigue. Thus, the provision of the second portion can lead to a more robust threaded connection.

[0031] Even more preferably, in the second portion the stab flank lead increases at a lower rate than the load flank lead. This is one way of achieving a variation in wedge ratio.

[0032] Yet even more preferably, in the second portion the stab flank lead is constant. This can achieve a variation in wedge ratio with an easy manufacturing step.

[0033] The second portion is preferably at the end of the varying-th read-width zone nearest a terminal surface of the box. The inventors have identified that this region of the varying- thread-width zone is where unwanted engagement between the threads of the pipes during stabbing can occur and is greatest and so is the preferred location for the second portion. Specifically, when inserting the pin into the box, the grooves of the female thread can be more able to accommodate the crests of the male thread.

[0034] Preferably, the wedge ratio of the female thread is a constant value through a first portion of the female varying-thread-width zone and increases from that constant value through the remainder of the female varying-thread-width zone, the remainder being at the end of the female varying-thread-width zone closest to the terminal end of the box (the remainder may be, for example, at least three turns, preferably at least four turns, more preferably at least five turns). The inventors have identified that the end of the female varying-thread-width zone closest to the terminal end of the box is where engagement during stabbing can occur and is greatest and so increasing the wedge ratio on the female thread in this location improves stabbing.

[0035] Preferably, the load flank lead varies throughout the female varying-thread-width zone; and a first portion of the stab flank lead is constant in the female varying-thread-width zone, the first portion being at the end of the female varying-thread-width zone closest to the terminal end of the box.

[0036] Optionally, when the pin is engaged with the box and the male varying-thread-width zone is arranged to engage the female varying-thread-width zone, the region of the male varying- thread-width zone that engages the first portion of the female varying-thread-width zone that has a constant female stab flank lead includes male teeth of reduced height. It is preferable at make-up for the second portion to coincide to some degree with the teeth of reduced height of the male thread, since the variation of the wedge ratio at this part of the female member is therefore less deleterious to the overall performance of the threaded connection.

[0037] The wedge ratio of the male thread may be constant value throughout the male varying- thread-width zone. It is preferred that the variation in wedge ratio be implemented only on the female thread.

[0038] Preferably, the varying-thread-width zones extend over at least 65% of the length of the respective thread. Since the varying-thread-width zones extend over at least 65% of the length of the thread, the varying-thread-width zones provide the majority of the connective strength of the connection.

[0039] Preferably, the varying-thread-width zones extend over at least the central 20%, more preferably the central 50% of the length of the respective thread. Such embodiments can achieve variation in the widths of the narrowest turns of the thread, even if the varying- thread-width zones are present only in the centre of the connection.

[0040] In embodiments in which, in at least one of the threads, the wedge ratio is constant through a first portion of the varying-thread-width zone and the wedge ratio varies throughout a second portion of the varying-thread-width zone, the second portion may extend over at least 65% of the length of the respective thread and / or the second portion may extend over at least the central 20%, preferably the central 50% of the length of the respective thread.

[0041] Preferably, the crests of the male thread and / or female thread are angled relative to the pipe axis. That is, the crests of the male thread and / or female thread in a longitudinal crosssection through the respective pipe are not parallel to the axis of that pipe. This known option to assist with the stabbing process has been found to be particularly beneficial in the present case, since the variation of the load flank lead or the stab flank lead throughout one or both of the male varying-thread-width zone or the female varying-thread-width zone can increase the difficulty of stabbing.

[0042] In such cases, it is also preferred that the roots of the male thread and / or female thread in a longitudinal cross-section through the respective pipe are not parallel to the axis of that pipe, most preferably complementary to the crests.

[0043] The male and female threads are preferably dovetail threads for greater engagement in the made up connection. LIST OF FIGURES

[0044] For a better understanding of the invention, and to show how the same may be put into effect, reference will now be made, by way of example only, to the accompanying drawings in which:

[0045] Figure 1 shows a half-cross-section of a tubular coupling forming part of a first embodiment of a threaded pipe connection in accordance with the invention;

[0046] Figure 2 depicts an example of a thread in accordance with the present invention;

[0047] Figure 3 shows a graph of stab flank lead and load flank lead for the male and female threads of the first embodiment;

[0048] Figure 4 shows a graph of stab flank lead and load flank lead for the male and female threads of a second embodiment;

[0049] Figure 5 shows a graph of stab flank lead and load flank lead for the male and female threads of a third embodiment;

[0050] Figure 6 shows a graph of stab flank lead and load flank lead for the male and female threads of a fourth embodiment;

[0051] Figure 7 shows a graph of stab flank lead and load flank lead for the male and female threads of a fifth embodiment;

[0052] Figure 8 shows a graph of stab flank lead and load flank lead for the male and female threads of a sixth embodiment.

[0053] DETAILED DESCRIPTION

[0054] As shown in Figure 1 , a first embodiment of the invention is a first threaded pipe connection comprises a first pipe 100 and a second pipe 200. This may be an integral-type connection or, as shown, a coupling-type connection.

[0055] The first pipe 100 terminates in a pin 110 having an external male thread 120. The pin 110 has a terminal surface 112. The second pipe 200 terminating in a box 210 having an internal female thread 220. The box 210 has a terminal surface 212.

[0056] The terminal surface 112 of the pin 110 is preferably a planar surface perpendicular to the longitudinal axis of the first pipe 100.

[0057] The terminal surface 212 of the box 210 is preferably a planar surface perpendicular to the longitudinal axis of the second pipe 200. The male thread 120 comprises a male varying-thread-width zone 122 throughout which the load flank lead 124 of the thread differs from the stab flank lead 126 of the thread by a wedge ratio 128.

[0058] The female thread 220 comprise a female varying-thread-width zone 222 throughout which the load flank lead 224 of the thread differs from the stab flank lead 226 of the thread by a wedge ratio 228.

[0059] The male load flank lead 124 is greater than the male stab flank lead 126 and the female load flank lead 224 is greater than the female stab flank lead 226.

[0060] The connection is shown in a made up state in which the pin 110 engages the box 210 and the male varying-thread-width zone 122 engages the female varying-thread-width zone 222. As such, in Figure 1 , the male and female varying-thread-width zones 122, 222 extend over the same radial extent in the made-up state.

[0061] The male and female varying-thread-width zones 122, 222 may each comprise at least three turns, preferably at least four turns, more preferably at least five turns.

[0062] Figures 3 to 8 show graphs in which the Y-axis 10 is the lead value, and the X-axis 12 is the number of threads from the start of the varying-thread-width zones 122, 222. The X-axis 12 extends from the terminal end of the pin / body end of the box on the left to the body end of the pin / terminal end of the box on the right.

[0063] Figure 2 depicts an example of a thread in accordance with the present invention. As can be seen, since the load flank lead and the stab flank lead vary throughout one of the male varying-thread-width zone or the female varying-thread-width zone, the engagement of the threads varies in the axial direction. For example, the final thread turn near the terminal end of the pin engages the opposing female thread in tension by a greater amount than the next depicted turn, and so on. In this way, the contact stress between the threads can vary along the length of the thread.

[0064] FIRST EMBODIMENT

[0065] Figure 3 shows a graph of the leads of the load and stab flanks 124, 224, 126, 226 of the male and female threads 120, 220 throughout the varying-thread-width zones 122, 222 in a first embodiment. Figures 4 to 8 show alternative lead graphs, which may be used in place of the leads depicted in Figure 3, for example, with the other features of the pipes (for example, the pipes 100, 200 of Figure 1 ) being kept the same.

[0066] As can be seen from Figure 3, in the first embodiment, both of the load flank lead 124 and stab flank lead 126 vary throughout the male varying-thread-width zone 122, and both of the load flank lead 224 and stab flank lead 226 vary throughout the female varying-thread-width zone 222.

[0067] Although drawn as separated lines, in fact, the load flank lead 124 of the male thread 120 and the load flank lead 224 of the female thread 220 are identical, as are the stab flank lead 126 of the male thread 120 and the stab flank lead 226 of the female thread 220. The small separation is merely to assist visualisation.

[0068] As can be seen, the wedge ratio 128, 228 (the difference between stab flank lead and load flank lead) of each of the male and female threads is preferably constant within the male and female varying-thread-width zones 122, 222.

[0069] The leads 124, 126 of the male thread 120 increase in a direction away from the terminal surface 112 of the pin 110, while the leads 224, 226 of the female thread 220 increase in a direction towards the terminal surface 212 of the box 210.

[0070] In this way, the thread width of the final turn of the female thread 220 nearest the terminal surface 212 of the box 210 can be greater than would otherwise be achievable with a constant wedge ratio.

[0071] As can be seen from Figure 3, the leads 124, 126 of the male thread 120 increase linearly in a direction away from the terminal surface 112 of the pin 110, while the leads 224, 226 of the female thread 220 increase linearly in a direction towards the terminal surface 212 of the box 210. (Here, “linearly” means that the increase per turn is constant throughout the varying- thread-width zone 122, 222 i.e., the graph of lead has a constant gradient in the figures).

[0072] Moreover, the constant wedge ratio is achieved in this example because the load and stab leads 124, 126 of the male thread 120 vary at the same rate (here, the term “rate” means rate of change of the lead with respect to the distance along the thread, i.e., the gradient in the figures). Similarly, the load and stab leads 224, 226 of the female thread 220 vary at the same rate. Most preferably, the rate of change of the lead with respect to the distance along the thread is constant and equal for each of the four leads 124, 126, 224, 226.

[0073] In the first embodiment, upon make-up, the threads of the varying-thread-width zones 122, 222 preferably engage at both load and stab flanks along the entire length of the varying- thread-width zones 122, 222.

[0074] SECOND EMBODIMENT

[0075] Figure 4 shows a graph of the leads of the load and stab flanks 124, 224, 126, 226 of the male and female threads 120, 220 throughout the varying-thread-width zones 122, 222 in a second embodiment.

[0076] Although drawn as separated lines, in fact, the load flank lead 124 of the male thread 120 and the load flank lead 224 of the female thread 220 are identical, as are the stab flank lead 126 of the male thread 1 0 and the stab flank lead 226 of the female thread 220. The small separation is merely to assist visualisation.

[0077] As can be seen from Figure 4, in the second embodiment, both of the load flank lead 124 and stab flank lead 126 vary throughout the male varying-thread-width zone 122, and both of the load flank lead 224 and stab flank lead 226 vary throughout the female varying-thread- width zone 222.

[0078] As can be seen, the wedge ratio 128, 228 (the difference between stab flank lead and load flank lead) of each of the male and female threads is preferably constant within the male and female varying-thread-width zones 122, 222.

[0079] The leads 124, 126 of the male thread 120 decrease in a direction away from the terminal surface 112 of the pin 110, while the leads 224, 226 of the female thread 220 increase in a direction away from the terminal surface 212 of the box 210.

[0080] In this way, the thread width of the final turn of the male thread 120 nearest the terminal surface 112 of the pin 110 can be greater than would otherwise be achievable with a constant wedge ratio.

[0081] In the second embodiment shown in Figure 4, the leads 124, 126 of the male thread 120 decrease linearly in a direction away from the terminal surface 112 of the pin 110, while the leads 224, 226 of the female thread 220 increase linearly in a direction away from the terminal surface 212 of the box 210.

[0082] The constant wedge ratio is achieved in this example because the leads 124, 126 of the male thread 120 decrease at the same rate in a direction away from the terminal surface 112 of the pin 110, while the leads 224, 226 of the female thread 220 increase at the same rate in a direction away from the terminal surface 212 of the box 210.

[0083] Most preferably, the rate of change of the lead with respect to the distance along the thread is constant and equal for each of the four leads 124, 126, 224, 226.

[0084] In the second embodiment, upon make-up, the threads of the varying-thread-width zones 122, 222 preferably engage at both load and stab flanks along the entire length of the varying-thread-width zones 122, 222.

[0085] THIRD EMBODIMENT

[0086] Figure 5 shows a graph of the leads of the load and stab flanks 124, 224, 126, 226 of the male and female threads 120, 220 throughout the varying-thread-width zones 122, 222 in a third embodiment.

[0087] Although drawn as separated lines, in fact, the load flank lead 124 of the male thread 120 and the load flank lead 224 of the female thread 220 are identical, as are the stab flank lead 126 of the male thread 120 and the stab flank lead 226 of the female thread 220 until they diverge. The small separation is merely to assist visualisation.

[0088] As can be seen in Figure 5, in the third embodiment, both of the load flank lead 124 and stab flank lead 126 vary throughout the male varying-thread-width zone 122, and the load flank lead 224 varies throughout the female varying-thread-width zone 222.

[0089] In contrast to the first embodiment, the stab flank lead 226 varies in a first portion 222A of the varying-thread-width zone, but is constant in a second portion 222B. In the example shown, the first portion 222A is the portion from the end of the female varying-thread-width zone 222 nearest the pipe body end of the box 210 to a point (in this non-limiting example, the fifth turn) along the female varying-thread-width zone 222, while the second portion 222B is the portion from a point (in this example, also the fifth turn) to the end of the female varying-thread-width zone 222 nearest the terminal end of the box 210. In this embodiment, the two portions 222A, 222B collectively define the female varying-thread-width zone, but in some embodiments other portions may separate the first and second portions 222A, 222B.

[0090] As can be seen, the wedge ratio 128 (the difference between stab flank lead and load flank lead) of the male thread is preferably constant within the male varying-thread-width zone 122. Moreover, the wedge ratio 228 of the female thread is preferably constant within the first portion 222A of the female varying-thread-width zone 222. However, because of the divergence between the load flank lead 224 and the stab flank lead 226 of the female thread 220 in the second portion 222B, the wedge ratio of the female thread 220 increases towards the terminal end of the box 210.

[0091] It should be noted that the stab flank leads 126, 226 of the male and female threads 120, 220 are the same in the first portion 222A of the female varying-thread-width zone 222 (i.e., when made up with the male varying-thread-width zone 122 and female varying-thread-width zone 222 coinciding). However, the stab flank lead 226 of the female thread 220 is lower than the stab flank lead 126 of the male thread 120 in the second portion 222B. The result of this difference in stab flank lead 126, 226 is that during the stabbing process (when inserting the pin 110 into the box 210) the grooves of the female thread 220 are more able to accommodate the crests of the male thread 120. Specifically, the extra space can avoid contact between the crest of the male thread 120 and the crest of the female thread 220 enabling easier stabbing than would be achieved in the first embodiment.

[0092] In the third embodiment, upon make-up, the threads of the varying-thread-width zones 122, 222 preferably engage at both load and stab flanks along the entire length of the first portion 222A.

[0093] FOURTH EMBODIMENT

[0094] Figure 6 shows a graph of the leads of the load and stab flanks 124, 224, 126, 226 of the male and female threads 120, 220 throughout the varying-thread-width zones 122, 222 in a fourth embodiment.

[0095] Although drawn as separated lines, in fact, the load flank lead 124 of the male thread 120 are identical and the load flank lead 224 of the female thread 220 are identical until they diverge. The stab flank lead 126 of the male thread 120 and the stab flank lead 226 of the female thread 220 are identical. The small separation is merely to assist visualisation. As can be seen in Figure 6, in the fourth embodiment, both of the load flank lead 124 and stab flank lead 126 vary throughout the male varying-thread-width zone 122, and the stab flank lead 226 varies throughout the female varying-thread-width zone 222.

[0096] In contrast to the second embodiment, the load flank lead 224 of the female thread 220 varies in a first portion 222A of the varying-thread-width zone, but is constant in a second portion 222B. In the example shown, the first portion 222A is the portion from the end of the female varying-thread-width zone 222 nearest the pipe body end of the box 210 to a point (in this non-limiting example, the fifth turn) along the female varying-thread-width zone 222, while the second portion 222B is the portion from a point (in this example, also the fifth turn) to the end of the female varying-thread-width zone 222 nearest the terminal end of the box 210. In this embodiment, the two portions 222A, 222B collectively define the female varying- thread-width zone, but in some embodiments other portions may separate the first and second portions 222A, 222B.

[0097] As can be seen, the wedge ratio 128 of the male thread is preferably constant within the male varying-thread-width zone 122. Moreover, the wedge ratio 228 of the female thread is preferably constant within the first portion 222A of the female varying-thread-width zone 222. However, because of the divergence between the load flank lead 224 and the stab flank lead 226 of the female thread 220 in the second portion 222B, the wedge ratio of the female thread 220 increases towards the terminal end of the box 210.

[0098] It should be noted that the load flank leads 124, 224 of the male and female threads 120, 220 are the same in the first portion 222A of the female varying-thread-width zone 222 (i.e., when made up with the male varying-thread-width zone 122 and female varying-thread-width zone 222 coinciding). However, the load flank lead 224 of the female thread 220 is greater than the load flank lead 124 of the male thread 120 in the second portion 222B. The result of this difference in load flank lead 124, 224 is that during the stabbing process (when inserting the pin 110 into the box 210) the grooves of the female thread 220 are more able to accommodate the crests of the male thread 120. Specifically, the extra space can avoid contact between the crest of the male thread 120 and the crest of the female thread 220 enabling easier stabbing than would be achieved in the second embodiment.

[0099] In the fourth embodiment, upon make-up, the threads of the varying-thread-width zones 122, 222 preferably engage at both load and stab flanks along the entire length of the first portion 222A. FIFTH EMBODIMENT

[0100] A fifth embodiment is presented in Figure 7.

[0101] The fifth embodiment is a variation of the third embodiment. In the fifth embodiment, instead of maintaining a constant stab flank lead 226 in the second portion 222B of the female varying-thread-width zone 222, the stab flank lead 226 increases, but at a lower rate than in the first portion 222A.

[0102] In preferred versions of the fifth embodiment, and as depicted in Figure 7, the stab flank lead 226 increases towards the terminal end of the box 210 at a first rate in the first portion 222A and at a second rate in the second portion 222B, with the second rate being smaller than the first rate.

[0103] The rate of increase in stab flank lead 226 in the second portion 222A of the female thread 220 determines the rate of increase of the wedge ratio of the female thread 220 towards the terminal end of the box 210. By increasing the rate of increase of the wedge ratio (equivalent to lowering the gradient of the stab flank lead 226 in the figure) of the female thread 220, the clearance following make-up between the opposing stab flanks of the male and female threads 120, 220 in this region can be increased. By decreasing the rate of increase of the wedge ratio (equivalent to lowering the gradient of the stab flank lead 226 in the figure) of the female thread 220 in the second portion 222B, the clearance following make-up between the opposing stab flanks of the male and female threads 120, 220 in this region can be decreased or removed or interference introduced.

[0104] In the fifth embodiment, upon make-up, the threads of the varying-thread-width zones 122, 222 preferably engage at both load and stab flanks along the entire length of the first portion 222A.

[0105] SIXTH EMBODIMENT

[0106] A sixth embodiment is presented in Figure 8.

[0107] The sixth embodiment is a variation of the fourth embodiment. In the sixth embodiment, instead of maintaining a constant load flank lead 224 in the second portion 222B of the female varying-thread-width zone 222, the load flank lead 224 decreases, but at a lower rate than in the first portion 222A. In preferred versions of the sixth embodiment, and as depicted in Figure 8, the load flank lead 224 decreases towards the terminal end of the box 210 at a first rate in the first portion 222A and at a second rate in the second portion 222B, with the second rate being smaller than the first rate.

[0108] The rate of decrease in load flank lead 224 in the second portion 222A of the female thread 220 determines the rate of increase of the wedge ratio of the female thread 220 towards the terminal end of the box 210. By increasing the rate of increase of the wedge ratio (equivalent to increasing the gradient of the load flank lead 226 in the figure) of the female thread 220, the clearance following make-up between the opposing stab flanks of the male and female threads 120, 220 in this region can be increased. By decreasing the rate of increase of the wedge ratio (equivalent to lowering the gradient of the stab flank lead 226 in the figure) of the female thread 220 in the second portion 222B, the clearance following make-up between the opposing stab flanks of the male and female threads 120, 220 in this region can be decreased or removed or interference introduced.

[0109] In the sixth embodiment, upon make-up, the threads of the varying-thread-width zones 122, 222 preferably engage at both load and stab flanks along the entire length of the first portion 222A.

[0110] GENERAL

[0111] In the embodiments of Figures 3 to 8, the male varying-thread-width zone 122 includes the entirety of the male thread 120, and the female varying-thread-width zone 222 includes the entirety of the female thread 220.

[0112] However, this is not essential, and embodiments are considered in which the male varying- thread-width zone 122 and female varying-thread-width zone 222 form a lesser, but significant proportion of the length of the respective thread.

[0113] A significant proportion in this context may mean that the zones carry a substantial proportion of the compressive and tensile loads of the connection. For example, the male varying-thread-width zone 122 may form at least 65% of the length of the male thread 120 and the female varying-thread-width zone 222 may form at least 65% of the length of the female 220 thread. More preferably, the male varying-thread-width zone 122 may form at least 80% of the length of the male thread 120 and the female varying-thread-width zone 222 may form at least 80% of the length of the female 220 thread.

[0114] When the male varying-thread-width zone 122 and female varying-thread-width zone 222 form less than 100% of the respective thread, there may also be a distal thread zone and a proximal thread zone provided.

[0115] The varying-thread-width zones may extend over at least the central 20%, preferably the central 50%, of the length of the respective thread.

[0116] For example, the thread height of the male and female threads 120, 220 may be constant within the male and female varying-thread-width zones 122, 222. The distal and proximal thread zones may include so-called thread run outs in which the height of the thread is reduced in the male and / or female thread.

[0117] In alternative embodiments, a run out may be provided in a run out portion of the male varying-thread-width zone 122, so as to overlap with the second portion 222B. In some cases the second run out portion may be coincident (exactly overlapping) with the second portion 222B.

[0118] A further benefit of the thread of the first embodiment is that the increase in the load and stab leads 124, 126 with increasing distance from the terminal surface 112 of the pin 110 results in the thread crests defining an outer envelope lying on a curved surface that has increasing taper towards the pin terminal surface 112. The inventors have discovered that this can assist with the process of initially inserting the pin 110 into the box 220.

[0119] It has been found that this is particularly beneficial in combination with sloping thread crests. That is, although not essential, it is preferable that the crests of the male and female threads are angled relative to the longitudinal axis of the connection (the longitudinal axis of the connection being the longitudinal axis of each pipe 100, 200 when coupled). That is, the height of the thread crest in cross-section preferably reduces towards the terminal end of the pin 110 or box 120.

[0120] As can be seen from Figures 3 to 8, a threaded pipe connection forming an embodiment of the invention may have at least three leads (any three of: the load flank lead of the male thread; the stab flank lead of the male thread; the load flank lead of the female thread; and the stab flank lead of the female thread) that vary throughout the male and female varying- thread-width zones.

[0121] Any thread form is contemplated for the disclosed threads. However, preferably any of the embodiments may use dovetail threads. Indeed, the third to sixth embodiments are particularly preferred for connections in which the thread is formed as a dovetail thread. An assembly (or string, as it is known) of pipes may be formed from a plurality of threaded pipes, wherein the pipes are connected together to form a plurality of connections according to the present disclosure.

Claims

CLAIMS:1 . A threaded pipe connection comprising: a first pipe terminating in a pin having an external male thread; a second pipe terminating in a box having an internal female thread, wherein the pin is configured for engagement with the box, wherein: the male thread comprises a male varying-thread-width zone throughout which the load flank lead of the thread differs from the stab flank lead of the thread; the female thread comprises a female varying-thread-width zone throughout which the load flank lead of the thread differs from the stab flank lead of the thread; the male varying-thread-width zone is arranged to engage the female varying-thread- width zone when the pin is engaged with the box; and the load flank lead and the stab flank lead vary throughout at least one of the male varying-thread-width zone or the female varying-thread-width zone.

2. The threaded pipe connection of claim 1 , wherein at least one of the load flank lead or the stab flank lead varies throughout the other of the male varying-thread-width zone and the female varying-thread-width zone.

3. The threaded pipe connection of any preceding claim, wherein the varying-thread-width zone comprises at least 3 turns of the respective thread.

4. The threaded pipe connection of any preceding claim, wherein the load flank lead and the stab flank lead increase throughout one or both of the male or female varying-thread-width zone.

5. The threaded pipe connection of any preceding claim, wherein the load flank lead and the stab flank lead increase in a direction away from a terminal surface of the pin throughout the male or female varying-thread-width zone.

6. The threaded pipe connection of any preceding claim, wherein the load flank lead and the stab flank lead increase in a direction away from a terminal surface of the box throughout the male or female varying-thread-width zone.

7. The threaded pipe connection of any preceding claim, wherein both of the load flank lead and the stab flank lead increase throughout the male varying-thread-width zone.

8. The threaded pipe connection of any preceding claim, wherein the variation in lead is linear in relation to distance along the longitudinal axis of the connection.

9. The threaded pipe connection of any preceding claim, wherein the wedge ratio of the thread is constant throughout one of the male and female varying-thread-width zone.

10. The threaded pipe connection of claim 9, wherein in the other of the male and female varying-thread-width zones the wedge ratio is constant throughout a first portion of the varying-thread-width zone and varies throughout a second portion of the varying-thread- width zone.11 . The threaded pipe connection of claim 10, wherein in the second portion the stab flank lead is constant or increases at a lower rate than the load flank lead.

12. The threaded pipe connection of claim 10 or claim 11 , wherein the second portion is at the end of the varying-thread-width zone nearest a terminal surface of the box.

13. The threaded pipe connection of any one of claims 1 to 9, wherein the wedge ratio of the female thread has a constant value throughout a first portion of the female varying-thread- width zone and increases from that constant value throughout the remainder of the female varying-thread-width zone, the remainder being at the end of the female varying-thread- width zone closest to the terminal end of the box.

14. The threaded pipe connection of any preceding claim, wherein: the load flank lead varies throughout the female varying-thread-width zone; and the stab flank lead is constant in a second portion of the female varying-thread-width zone, the second portion being at the end of the female varying-thread-width zone closest to the terminal end of the box.

15. The threaded pipe connection of claim 14, wherein when the pin is engaged with the box and the male varying-thread-width zone engages the female varying-thread-width zone, the external male thread has reduced height relative to the remainder of the thread in the region of the male varying-thread-width zone that engages the first portion of the female varying- thread-width zone.

16. The threaded pipe connection of any preceding claim, wherein at least three of the load flank lead of the male thread, the stab flank lead of the male thread, the load flank lead of the female thread and the stab flank lead of the female thread vary throughout the male and female varying-thread-width zones.

17. The threaded pipe connection of any preceding claim, wherein the varying-thread-width zones extend over at least 65% of the axial length of the respective thread.

18. The threaded pipe connection of any preceding claim, wherein the varying-thread-width zones extend over at least the central 20%, preferably the central 50%, of the axial length of the respective thread.

19. The threaded pipe connection of any preceding claim, wherein the crests of the male thread and / or female thread in a longitudinal cross-section through the respective pipe are not parallel to the axis of that pipe.

20. An assembly comprising a string of pipes having a plurality of threaded pipe connections in accordance with any preceding claim.

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