This invention relates generally to a pipe joint for use with oil country tubular goods such as tubing, casing, and drill pipe used in oil, gas, geo- thermal, and other wells to threaded tubular members adapted to be connected to another threaded member to form the joint, and to the method of making the threaded members. In particular, the invention relates to pipe joints and to members for forming pipe joints having mated threads that when coupled or made-up, are torqued to specified limits to effect performance of the joint. The invention is particularly useful in connections having contact surfaces such as torque shoulders to facilitate make-up.

All joints of this type include a box or female member having internal threads and a pin or a male member having external threads that mate with the in¬ ternal threads on the box and hold the joint together when the joint is made-up. The threads are,usually of a modified buttress type that are designed to provide clearance between the flanks of the threads to allow the joints to be made-up with little torque required until the sealing and shouldering contact surfaces engage initially. There may be just one pair of contact sur¬ faces that not only act as torque shoulders to limit the distance the pin can move into the box but also de¬ fine sealing surfaces to effect sealing of the joint. Alternatively, there may be two or more pair of engaging contact surfaces where one or more act to seal the joint

and one or more act as torque or stop shoulders. Or the joint may have separate set ^' s of engaging sealing surfaces and separate torque shoulders tha can also provide a seal. In any event, after the joint is ade- up hand-tight, the torque required to completely make-up the joint and to provide sufficient torque to develop desired performance serves to -force the contacting sur¬ faces of the joint together with the compressive force required to form the desired performance. In the prior art, the mating threads on the box and the pin have the same pitch or lead so that in the hand-tight position, all of the threads are sub¬ stantially in engagement at their load flanks. As make up torque is applied, the threads on the pin and the box adjacent the engaging contact surfaces carry substantially the full reaction to the compressive load imposed on the adjacent engaging surfaces, while the threads farther from these surfaces carry little or none of the reaction load. This results in localized stresses that are intensified when the pipe is placed in a pipe string and subjected to the substantial tensile load of the pipe it is supporting below it. This condition can result in localized yielding within the connection when the loads approach maximum levels. The object of the present invention is to provide a means and method for preloading or pre- stressing the joint so that when the joint is placed under load, the reactive forces on the threads in the joint are more uniformly distributed throughout the joint.

The present invention provides a threaded tubular member for use in a pipe joint and adapted for connecting to another threaded member to form a threaded connection between the two members, comprising a tubular body, a projecting helical rib on the body forming screw

threads having load flanks shaped to have clearance between the flanks of the thread and the load flanks of the threads of the threaded member to which the mem¬ ber is adapted to be connected and a contact surface on the body to. engage a contact surface on the other threaded member to limit the distance one of the members can enter the other for a given make-up torque, char¬ acterized by said threads being formed on the body so that selected threads will have less clearance between them and the mating threads on the other member than do the other threads on the member and the mating threads on the other member so that the selected threads will engage the mating threads on the other member before the other threads and the mating threads on the other member engage to cause the selected threads to be stres¬ sed initially and to more uniformly stress the threads when the connection is made-up and additional external loads are applied.

The present invention also provides a method of machining a threaded tubular member adapted for connecting to another threaded member to more uni¬ formly stress the threads where there is clearance between the load flanks of the threads and the mem¬ bers have contact surfaces that engage to limit the distance one of the members can enter the other for a given make-up .torque characterized by the steps of ma¬ chining a tubular body to form helical screw threads on the body having selected threads positioned so that these threads and the mating threads of the other mem- ber will engage before the other threads of both members when the two members are screwed together.

The present invention minimizes the presence of high stress points that may cause localized yielding of the joint, thereby increasing the overall utility and functionality of the threaded connection without requiring basic design configuration changes.

This is advantageous for at least two reasons. First, by modifying only the pin or the box in ac¬ cordance with the invention, the improved load dis¬ tribution is- achieved when the novel.pin or box is. made- up with a compatible prior art thread. Second, the well- accepted,, successful connections of the prior art do not have to be drastically re-configured in order to achieve the advantages of the present invention.

In essence, the invention permits the trans- figuration of the prior art rigid thread designs into flexible, more stable, connections that are predictably prestressed during makeup to permit more uniform reaction to load. By understanding the load requirements of the connection, selective preloading techniques can be used to achieve maximum use of thread designs for particular applications.

Assuming, as described above, that a threaded pin and box have one or more contact surfaces or shoulders for providing torque and sealing implementation, then the initial reaction load at makeup will be primarily carried through the shoulders by the threads. Under tensile load, the connection will stretch and eventually the reactive load will be distributed throughout the connection, with the primary components being carried by the threads. Stated simply, any rigid connection, no matter how rigid, will flex and begin to react as a spring under loading conditions.

The present invention recognizes this con¬ dition and responds to it by taking the reactions to load into consideration in the design stage, prior to make-up. By altering and/or varying thread pitch of the mating threads within prescribed limits, the resulting spring can be made to perform and react in a desirable, uniform, and predictable manner during load. In ad- dition, where connections include multiple thread sets, the threads on the individual sets may be shifted or

offset relative to one another.

The advance provided by the present invention can be compared with basic mechanical structure design such as, by way of example, bridge design. As bridge loads increase, bridges may increase in size and rigidity until they become so cumbersome and unwieldy, that they are prohibitive to build. By adapting pre-stressed designs, where loading actually enhances the design, bridges are lighter weight, less costly, and more ef- fective.

The present invention recognizes the value of preloading to achieve similar ideals in oilfield thread technology. By using built-in stresses to advantage, instead of rigidly building around them, a better end product is achieved.

While the following description primarily uses the well-known two-step, non-tapered, shouldered con¬ nections of the Hydril Company to illustrate the features of the invention, it will be understood that the invention described herein is readily applicable to any prior art thread design where it is desirable to distribute the reactive load in a predictable manner during use of the joint.

Other advantages and features of this in- vention will, be apparent to those skilled in the art from a consideration of this specification including the attached drawings and appended claims. In the drawings:

Figure 1A (prior art) shows a typical two-step type of pipe joint having cylindrical threads made-up to Where the threads and the inner sealing surfaces located at the lower end of the pin and the bottom.of the box have made contact yet prior to contact between the outer sealing surfaces and torque shoulders located at the upper end of the box and pin, i.e., a gap exists between 18 and 20.

Figure IB (prior art) shows the joint of Figure.1A made-up whereby surfaces 18 and 20 are loaded together by compression resulting from assembly make¬ up together with the threaded, surfaces. Figure 2 is a sectional view through a joint of the type to which this invention relates-having mat¬ ing sealing surfaces at both ends of the threads and torque shoulders intermediate the ends of the threads. Figure 2A is an enlarged sectional view of a modified torque shoulder arrangement for use with the joint of Figure 2.

Figure 3 is a sectional view through a pipe joint of the type to which this invention relates hav¬ ing tapered threads and combined sealing surfaces and torque shoulders at the end of the pin and the base of the box only.

Figure 4 is an enlarged view of the sealing surface and first thread adjacent the sealing surfaces of the joint of Figure 3. Figure 5 is an enlarged view of the thread form used in this type of joint.

Figure 6 shows the load distribution on the threads of a two-step joint of the type shown in Fig¬ ures 1A and IB when the joint is fully made-up, when it is subjected to a tensile load, and when it is subjected to a tensile load and internal pressure.

Figure 7 shows the load distribution on the threads of the same type joint as shown in Figures 1A and IB where the threads of the small step on either the box or the pin, but not both, have been shifted .0076 cm. (.003 inch) relative to the threads of the large step.

Figure 8 shows"the load distribution on the threads of the same type joint as shown in Figures 1A and IB where the threads of the small step on either the box or the pin, but not both, have been shifted .0102 cm. (.004 inch) relative to the threads of the large step.

Figure 9 shows the thread load distribution in a joint wherein the large step thread load flank clearance increases .0025 cm. (.001 inch) pitch per threads, i.e., the first clearance gap is 0, the se- cond is .0025 cm. (.001 inch), the third gap is .0050 cm. (.002 inch), and so on.

Figure 10 shows the thread load distribution in a joint wherein the large step thread load flank clear¬ ance increases .0012 cm. (.0005 inch) per thread, i.e., the first clearance gap is 0, the second is .0012 cm. (.0005 inch), the third gap is .0025 cm. (.0010 inch) and so on.

Figure 11 shows the thread load distribution where there has been a shift in the threads between steps of either the pin or box plus a variable pitch on each step of either the box or the pin to produce sel¬ ected clearances between the flanks of the threads in the hand-tight position.

Figure 12 shows the thread load distribution for the same thread adjustment as in Figure 11 but ar¬ ranged to produce slightly different clearances between the thread flanks.

Figure 13 shows the surface hoop stress im- ^' posed on a joint of the type shown in Figures 1A and IB when subjected to tension and internal pressure.

Figure 14 shows the substantial reduction in the surface hoop stresses on the pin I.D. of the joint of the type shown in Figure 11 when subjected to the same tension and internal pressure. Figure 15 compares the load distribution ac¬ ross the 30 shoulder at the outer end of the box of the joint of the type shown in Figure 1A and IB and that of a joint embodying this invention shown in Figure 12, when the joints are subjected to a tensile load which il- lustrates the more uniform load across the shoulder that results from the pretension effects even when high

tensile loads are imposed.

Figure 16 is the same as Figure 15 except here the joints are in compression.

Figure 17A-17E are graphs of the circum- ferential distance along the joint that each thread is located for various embodiments of this invention as compared to the conventional thread of this type for joints of the two-step straight or cylindrical thread type. Figure 18 is a graph of torque versus degrees of rotation of the pin relative _* to -the box of the joint of the type shown in Figures 1A and IB and a joint em¬ bodying this invention.

Figures 19 is a comparison of load versus deflection graphs of a joint of the type shown in Fig¬ ures 1A and IB and a joint embodying this invention.

The prior art joint shown in Figures LA and IB include box 10 and pin 12. The box and pin are at¬ tached to or formed on the ends of tubular members such as casing, tubing, and drill pipe. The box has in¬ ternal threads and the pin has external threads that mate with the threads on the box to allow the pin to be • screwed into the box to make up the joint and connect the ends of the tubular members. In this joint, the threads are straight or cylindrical and they are stepped so that half of the threads have a smaller major and minor diameter than the other threads. The larger diameter threads on the box and pin are referred to as the large step (L.S.) and the smaller diameter threads are refer- red to as the small step (S.S.). The actual distance between the threads, L, called pitch, is the same for both steps. The threads are shown in the-drawing as being square, but in practice they are usually a modi¬ fied buttress of the type shown in Figure 5. With the lead of the threads in both steps

the same and one being a continuation of the helix of the other, i.e., the distance between the two steps is some variance-of the lead, the flanks of' he threads will all move into engagement as shown in Figure 1A when the joint is made-up hand-tight. The hand-tight condition occurs when conical sealing surface 14 on the end of pin 12 makes initial contact with and engages the matching conical sealing surface 16 at the base of the box. Usually these sealing surfaces are inclined from the longitudinal axis of the joint about 14°. At the upper end of box 10 when the joint is made-up hand- tight, sealing surface 18, which is the upper end of the box, is spaced from the mating sealing surface provided by shoulder 20 on pin 12. Surfaces 18 and 20 will be moved into sealing engagement when the joint is com¬ pletely made-up as shown in Figure IB. In this joint they also act as torque shoulders to limit the distance pin 12 can be forced into box 10 for a given make up torque. Sealing surfaces 14 and 16 at the other end of the threads provide some limiting effect to the move¬ ment of the pin into the box, but at 14 they are not as positive a stop as is the engagement of the 30 sealing surfaces 18 and 20.

The pin seal surface 16 slides along box sur- face 14 as surface 18 is moved into engagement with pin shoulder 20, thus energizing the seal. This requires very little torque, usually considered inadequate to prevent accidental unscrewing of the connection in ser¬ vice. Adequate break out resistance is developed by ad- ditional rotation of the pin relative to the box after box surface 18 contacts pin shoulder 20. This additional rotation results in elongation of the pin adjacent to shoulder 20 and compression of the box adjacent to surface 18. The forces required to achieve this additional ro- tation are directly related to the full make-up torque applied to the connection.

The major portion of this force is imposed on thread 22 on the box and thread 24 on the pin. The other threads take some of the load but substantially less than the two.engaging threads adjacent the engaging surfaces. The stress lines in Figure IB show the con¬ centration of stress in the box and pin between these two threads and the engaging shoulders.

The joint in Figure 2 includes box 26 and pin 28. This joint differs from the joint of Figures 1A and IB by having 14° degree sealing surfaces on both sides of the threads and separate torque shoulders located between the larger step and smaller step. These are shoulders 30 on the pin and 32 on the box. They can be perpendicular to the longitudinal axis of the joint or inclined to form a hooked shoulder, as shown in Figure 2A, where shoulders 30' and 32* are in¬ clined about 15 from the transverse axis of the joint. When the pin and box are made-up hand-tight, there will be a gap between torque shoulders 30 and 32 and both the 14 sealing surfaces at either end of the joint will be in engagement or approximately so. The gap be¬ tween the torque shoulders is made-up or is closed by additional rotation of the pin relative to the box. This will produce the compressive forces between the sealing surfaces required to obtain the desired seals against both internal and external pressures and increase the resistance to unscrewing of the connection.

.The joint of Figure 3 employs tapered threads. The threads in the joint shown in the drawings _, is a modified buttress thread. The invention, however, is as readily applicable to buttress threads, reverse load flank (hooked) threads, vee threads and other thread forms. As. best seen in Figure 5, a clearance is provided between the flanks of the threads to allow the threads to be free running and to require a minimum amount of tor¬ que to be made-up hand-tight. At that point, with this

particular joint, the sealing surfaces on the lower end of pin 34 are in engagement with the sealing surfaces provided on box 36. In this embodiment, both the box and the pin have two sealing surfaces that are at dif- ferent angles to each other. As shown in Figure 4, pin 34 has sealing surface 38 at a substantially flat angle and sealing surface 40 at a steeper angle that engage matching surfaces on box 36. The sealing sur¬ faces- are forced together with sufficient compressive force to provide the seal for the joint. Here again with the pitch of the threads being the same, thread 42 on the pin and thread 44 on the box will be stressed substantilly higher than the other threads of the joint. An example of how high the stress on the threads adjacent the contacting surfaces can be as com¬ pared to the other threads of the joint is shown in Fig¬ ure 6. The graph shows the thread load distribution on a 19.25 cm. (7 inch) (29 lb./ft/43 kg/m.) casing joint equipped with a joint of the type shown in Figures 1A and IB. When the joint is initially made-up using

1493 kg. m (10,800 ft. lbs.) of torque, thread No. 1 of the large step (thread 22 of Figure 1A) has already ex¬ ceeded the calculated thread yield load of 29683 kg. (65,4.40 lbs.) by about 6800 kg. (15,000 lbs.). The load on the other threads of the large step drop off rapidly with threads 4 through 8 carrying substantially smaller loads.

The made-up joint was then subjected to a ten¬ sile load of 231,336 kg. (510,0001bs. ). This nearly doubles the load on the No. 1 thread moving it far above the thread yield load limit and increases the load on thread No. 2 up to the thread yield load limit. The load on the remaining threads of large step follow a U- shaped curve with threads 6, 7 and 8 having a substantial increase in load, but they still remain below the yield

- 12 - point. The threads of the small step show what is a typical U-shaped load distribution with threads 1 and 8 having the highest load. When the tensile load of

231,336 kg. (510,000 lbs.) was coupled with an internal

2 pressure of 574 kg./cm (8160 p.s.i.), the load on the

No. 1 and No. 2 threads.of the large step actually dropped with practically no change in the load on the other threads and.subs antially no change in the load on the threads of the small step. As stated above, this invention is concerned with providing a pipe joint of this type where the force required to provide the compressive force desired be¬ tween the sealing surfaces of the joint and resultant forces from applied loadings are more evenly distri- buted among the threads of the joint. In accordance with one embodiment of this invention, this objective is attained by shifting the threads on one or both steps of either the pin or the box relative to the mating threads on the box or the pin. Figures 7, 8, 9 and 10 show the thread load distribution for various combinations of thread offsets on the threads of a typical two step connection. In all cases, except Figure 9, simply making up the joint did not overly stress any of the threads with the exception of the joint with a .0025 cm. (.001 inch) pitch dif¬ ference, which produced a large increase in the load on thread 8 in the large step. This is a good indication of how.sensitive the load distribution is to slight changes in pitch relative to the mating threads. When the joints were placed in tension and subjected to internal pressure, several of the threads were loaded above the yield point. But as the mismatch or relative pitch dif¬ ference was decreased, the threads that were exceeding yield were those located away from the sealing surfaces and therefore, should cause less reduction in the com¬ pressive force between ^" the two contacting surfaces due

to the tensile load.

Figures 11 and 12 show the load distribution on the threads of this.same joint when there has been a thread set shif on one step relative the other plus a variable pitch for both steps on either the box or pin to obtain, the clearances shown on the section of the joint shown below the distribution diagram.. This more complicated arrangement achieved a reactive load where the distributed load on all threads is at or below the thread yield load lines for both steps.

The spacing between the load flanks ^" of the threads at hand-tight make-up that would produce a uni¬ form load on the threads under working conditions can be calculated. Thus introducing the appropriate gaps between load flanks of selected threads when the joint is hand-tight can reduce the thread loads due to power make-up and imposed external loads.

The joint of this invention provides advan¬ tages other than more uniform distribution of the load on the threads of the joint. Figures 13 and 14 show a comparison of the surface hoop stresses on a joint of the type shown in Figure LA and a joint of the type shown in Figure 12, respectively. The stress numbers are in thousands of p.s.i. and indicate a substantial reduction in the surface stresses at the pin I.D. when the pipe is subjected to a tensile load of 231,336

2 (510,000 lbs.) and an internal pressure of 574 kg./cm

(8,160 p.s.i.g.). The reduction results because the threads of the joint in Figure 12 are under a substan- tailly uniform load. Therefore, there is a more uni¬ form frictional force between the flanks of the teeth and this force will help the pin resist expansion due to internal pressure. In other words, the joint acts more as a unitary structure than the joint of Figure 1A. Figures 15 and 16 show the load distribution over the 30 shoulders of a Figure 1A type joint and a

- 14 - joint embodying this invention. As shown in Figure 16, when these joints were subjected to 231,336 kg. (510,000 lbs.) of tensile force, the load distribution across the shoulders is nearly flat with the joint of this in- vention whereas.with the joint of the type shown in Fig¬ ure 1A, the load distribution drops rapidly to zero adjacent the outside of the joint. This further shows the high concentrations of load adjacent the inside sur¬ face of the joint because of the short distance between the adjacent highly loaded threads and the shoulders.

Figure 16 confirms this. When the ^" joints are under compression, a compressive load of 153,317 kg. (338,000 lbs.) in the joint of this invention produced the same load distribution across the shoulders as 193, 687 kg. (427,000 lbs.) did with the joint of Figure 1A. Thus, an improvement in the load distribution in one di¬ rection, for example a tensile load, can have a negative effect on the load distribution for a compressive load. Therefore, the effects should be balanced so that both are within acceptable limits. Here, the modification that results in a compressive load of 153,317 kg. (338, 000 lbs. ) producing the same load distribution as pre¬ viously obtained with a 193,687 kg. (427,000 lb.) com¬ pressive load is acceptable when balanced with the im- pro ement in the load distribution for a tensile load, which is the condition most likely found in the field.

Figures 17 shows how the thread locations are altered from those of a traditional two-step thread with a uniform pitch and zero offset between steps by shift- ing the steps, by varying the pitch, and by doing both. The thread locations are for joints where the stop shoulders are near pin thread No. 1 on the large step.

Figure 18 is a graph of percentage of make up torque versus degrees of rotation of a joint of the type shown in Figure 1A, which is shown in the solid line, and

that of a joint embodying this invention, shown in the dash lines. Both joints move along the same line with very little torque until they reach point A where both joints are made-up hand-tight. From there to total make up of the joint, the degrees of rotation for the joint of Figure 1A are between 8 to 10 , whereas the joint of this invention increases the degrees of rotation to make up to approximately 15 . This results in a fifty to one- hundred percent improvement. The 15° rotation is more easily observed than an 8° to 10° movement between the pipe in final make up and the same is true when the joint is broken out.

Figure 19 shows the advantage of distributing the load uniformly among the threads of the joint so that the entire joint is- stressed uniformly rather than having a high concentration of stress adjacent one end only.

The graph of Figure 19 plots load in lbs. against deflection with the type joint shown in Fig- ure 1A with very short distance between the threads adjacent the engaging shoulders and the first threads that take the load. The deflection curve for both the pin and the box is relatively steep, therefore, when you add tension to the pin, it may quickly move to the point where there is no compression left in the shoulder of the box. as shown by the dotted lines. Whereas with the joint embodying this invention, the deflection for the same tensile and compressive load in the box and the pin is substantially greater. This means that the same tension on the joint will still have substantial com¬ pression in the box, thereby insuring that the sealing surfaces remain in engagement under sufficient compres¬ sion to maintain a satisfactory seal for the joint.