BACKGROUND OF INVENTION

Field of the Invention

[0001] The invention relates generally to threaded connections on tubular pieces.

More specifically, the invention relates to threaded connections on tubular pieces that are adapted to be plastically radially expanded.

Background Art

[0002] Casing joints, liners, and other oilfield tubulars are often used in drilling, completing, and producing a well. Casing joints, for example, may be emplaced in a wellbore to stabilize a formation, to protect a formation against elevated wellbore pressures (e.g., wellbore pressures that exceed a formation pressure), and the like. Casing joints may be coupled in an end-to-end manner by threaded connections, welded connections, and other connections known in the art. The connections may be designed so as to form a seal between an interior of the coupled casing joints and an annular space formed between exterior walls of the casing joints and walls of the wellbore. The seal may be, for example, an elastomeric seal (e.g., an o-ring seal), a metal-to-metal seal formed proximate the connection, or similar seals known in the art.

[0003] In some well construction operations, it is advantageous to radially plastically expand threaded pipe or casing joints in a drilled ("open") hole or inside a cased wellbore. In a cased wellbore, radially expandable casing can be used to reinforce worn or damaged casing so as to, for example, increase a burst rating of the old casing, thereby preventing premature abandonment of the hole. In open hole sections of the wellbore, the use of radially expandable casing may reduce a required diameter of a drilled hole for a desired final cased hole diameter, and may also reduce a required volume of cement required to fix the casing in wellbore.

[0004] Deformations occurring proximate threaded tubular connections, such as those found on oilfield casing and tubing, on the ends of radially expandable casing after radial plastic expansion can be problematic. Problems are encountered in at least three areas. First, internal metal-to-metal seals, which rely on radial

contact stresses to form the seal, may unload (i.e., lose sealing engagement) if a pin nose formed on a pin connection deforms in a direction away from a seal surface formed on a box connection. Second, gross deformation of the pipe ends proximate the region of the threaded connection can cause some of the threads to move radially and, as a result, unload. This adversely affects a load carrying capacity of the threaded connection. Third, local deformations in the region proximate each individual thread may "twist" the individual threads, thereby adversely affecting a load distribution between individual threads.

[0005] When a cold-forming expansion process is used (e.g., when a cold-forming expansion tool or "pig" is moved through a casing string so as to radially plastically expand the casing string), the casing string is usually run into the hole "box-down" (e.g., the "box" or female threaded connection is run into the hole facing downhole so that the expansion tool ("pig") does not deform the pin nose of each connection when the expansion tool is forced upward through the casing string). Note that tubular strings such as drill pipe, casing, or similar tubular members are normally run into the well "pin-down" because it is easier to make up the threaded connections in the tubular string.

[0006] Prior art analysis of expandable threaded tubular connections has concentrated on an analysis of the post-expansion stresses in the connection, with an implicit assumption that a relationship between stress and strain is relatively linear, even within the plastic deformation regime. However, thin-walled carbon steels typical of those used in oilfield tubular goods exhibit a drastically non-linear relationship between stress and strain during plastic deformation, particularly in a near-plastic region immediately following a yield point where plastic yielding is characterized by the formation of Lϋders bands. For this reason, it is desirable that expandable threaded tubular connections be designed to control and focus the plastic strains resulting from expansion rather than be designed to achieve a desired stress field in the connection so that threaded connection and seal integrity may be maintained after radial plastic expansion of the threaded connection (and the associated tubular member on which the threaded connection is formed).

[0007] One example of a tubular connection designed to control and focus the plastic strains resulting from expansion is shown in U.S. Patent No. 6,607,220 B2, which is assigned to the assignee of the present invention and incorporated herein

by reference in its entirety. In the '220 patent, a circumferential groove is placed at one or more locations on the outside surface of the box member to control the distortion of the threaded connection during the radial plastic expansion. The '220 patent also discloses placing a helical groove on the inside or outside surface of the pin member at an axial position substantially equal to the root of the external thread.

SUMMARY OF INVENTION

[0008] In one aspect, the present invention relates to a tubular connection including a box member that includes an internal thread and a pin member that includes an external thread. The external thread is adapted to threadedly engage the internal thread. A substantially helical groove is formed on an outer surface of the box member.

[0009] In another aspect, the present invention relates to a tubular connection including a box member that includes an internal thread, a first seal surface, and a second seal surface and a pin member that includes an external thread, a third seal surface, and a fourth seal surface. The external thread is adapted to threadedly engage the internal thread, the first and third seal surfaces are adapted to form a seal against an external pressure, and the second and fourth seal surfaces are adapted to form a seal against an internal pressure upon engagement thereof. A substantially helical groove is formed on an outer surface of the box member. The substantially helical groove is positioned substantially in-lead with the internal thread.

[0010] In another aspect, the present invention relates to a method of manufacturing a box member of a tool joint. The method includes selecting an outer diameter for a box member that causes the tool joint to have a greater than a selected bending strength ratio. A helical groove is formed on the outer diameter of the box member such that the helical groove reduces the bending strength ratio to be about the selected bending strength ratio. An internal thread is formed on an internal surface of the box member.

[0011] In another aspect, the present invention relates to a method of modifying a box member that includes an internal thread. The method includes forming a

substantially helical groove on an outer surface of the box member. The substantially helical groove is positioned substantially in-lead with the internal thread.

[0012] In another aspect, the present invention relates to a method of manufacturing a box member. The method includes forming an internal thread on an internal surface of a cylinder and forming a substantially helical groove on an outer surface of the cylinder. The substantially helical groove is positioned substantially in-lead with the internal thread.

[0013] Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 shows atypical radial expansion tool.

[0015] Figure 2A shows an unexpanded joint of plain-end pipe.

[0016] Figure 2B shows a of the plain-end pipe joint shown in Figure 2A after expansion.

[0017] Figure 3 A shows a graph of strains along the length of the radially-expanded pipe shown in Figure 2B proximate a first expanded end.

[0018] Figure 3B shows a cross-sectional view of the radially-expanded pipe shown in Figure 2B.

[0019] Figure 4A shows a graph of strains along the length of the radially-expanded pipe shown in Figure 2B proximate a second expanded end.

[0020] Figure 4B shows a cross-sectional view of the radially-expanded pipe shown in Figure 2B.

[0021] Figure 5 shows a cross-sectional view of a tubular connection in an unexpanded condition in accordance with an embodiment of the present invention

[0022] Figure 6A shows a cross-sectional view of a box member in accordance with an embodiment of the present invention.

[0023] Figure 6B shows a cross-sectional view of a pin member adapted to connect to the box member shown in Figure 6A in accordance with an embodiment of the present invention.

[0024] Figure 7 A shows a cross-sectional view of a prior art tool joint.

[0025] Figure 7B shows a cross-sectional view of a tool joint in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

[0026] In one aspect, the present invention relates to tubular connections. More specifically, the present invention relates to managing stress in tubular connections.

[0027] An expansion tool is typically used to plastically radially expand a string of casing or tubing disposed inside a wellbore from an initial condition (e.g., from an initial diameter) to an expanded condition (e.g., with a larger diameter). One common prior-art expansion process shown in Figure 1 uses a conically tapered, cold-forming expansion tool 101 (commonly referred to as a "pig") to expand casing in a wellbore. The expansion tool 101 is generally sealed inside of a pig launcher (not shown), which is a belled section attached to a lower end of a casing string that is run into the wellbore. Because the pig launcher must pass through the parent casing already installed in the wellbore, the pig launcher has an outer diameter that is less than a "drift diameter" of the parent casing. As used herein, the "drift diameter" is the maximum external diameter that can pass through a wellbore. Generally, the drift diameter is somewhat smaller than the internal diameter of the wellbore due to the wellbore not being perfectly straight. Because of this, a tool having exactly the internal diameter of the wellbore would be unable to move freely through a wellbore.

[0028] Typically, after running the casing string into the wellbore, the casing string is suspended inside the wellbore using slips (not shown). Then, drill pipe (not shown) is run into the wellbore and latched onto the expansion tool 101. After connecting the drill pipe, the weight of the casing string is supported by the expansion tool 101. The drill pipe is then used to further lower the casing string to the selected location in the wellbore. The expansion tool 101 includes a tapered

section 98A having a taper angle 98B that is generally between 5 degrees and 45 degrees. The expansion tool 101 is generally symmetric about a longitudinal axis

97 thereof. The expansion tool 101 also includes a cylindrical section 96 having a diameter that corresponds to a desired expanded inner diameter of a casing string (not shown) that is followed by a tapered section 95.

[0029] The next step in this particular expansion process is to pump cement through the drill pipe and out of a cement port on the pig. The cement flows between the outside of the casing string to be expanded and the inside of the wellbore. After the selected amount of cement has been pumped, the cement port is sealed off, typically by a dart designed to seat in the cement port. The dart is usually pumped with drilling fluid through the drill pipe. Continuing to pump drilling fluid pressurizes the pig launcher, which drives the expansion tool 101 forward (i.e. upward toward the surface) and the casing further into the wellbore. As the expansion tool 101 moves forward, the casing string expands. Expansion continues until the entire casing string has been expanded. In many instances, the casing string will include a length of casing that remains inside the parent casing after expansion. The extra length of casing can be designed to act as a liner hanger for the casing string and to seal between the parent casing and the expanded casing string.

[0030] In this expansion process, a rate of radial expansion is determined by, for example, a total plastic strain required to expand the casing string, the taper angle

98 A, and a rate of axial displacement of the expansion tool 101 through the casing string. Consistency of the expansion process is controlled by transitions along the expansion tool 101 and a cross-sectional area of, for example, lengths of casing that form the casing string, threaded connections that couple the length of casing, and the like.

[0031] The above expansion process is only one example of expansion processes known in the art. The expansion tool 101 may be started at either the bottom or the top of the casing string depending on the tool design and the application. Radial expansion may be performed at rates of, for example, 25 to 60 feet per minute. Other expansion processes, such as expansion under localized hydrostatic pressure, or "hydroforming," are known in the art, but are generally not used as much as cold-forming expansion processes. Other expansion tools for cold-forming the

casing also exist. One of ordinary skill in the art will appreciate that the expansion process to be used in some embodiments of the invention may vary without departing from the scope of the present invention.

[0032] While the tapered expansion tool method has proven to work quite well on tubular bodies of casing strings, the expansion of made-up threaded connections has exposed structural sealing problems in the expanded threaded connections. Threaded connections that undergo radial plastic expansion have a tendency to exhibit a non-uniform axial elongation and react differently to residual hoop stresses remaining after expansion. Specifically, male (pin) threaded members and female (box) threaded members deform differently during radial expansion. The box member will generally move away from the pin member during radial expansion. This differential displacement phenomenon results in a loss of preload in axially-engaged seals, making the use of conventional metal-to-metal seals (including, for example, shoulder seals) generally ineffective for plastically radially expanded casing and tubing.

Radial Plastic Expansion of Plain-End Pipe

[0033] When a joint of casing or tubing is radially plastically expanded, a wall thickness of the casing joint and an overall axial length of the casing joint are reduced by a process commonly referred to as "Poissoning," and residual stresses are retained in the casing joint. Proximate a middle of the casing joint, the casing joint will maintain a substantially uniform diameter and wall thickness because each discrete region experiences support from adjoining discrete regions. However, deformation proximate ends of the casing joint depends on a direction in which the expansion tool travels.

[0034] Figure 2A shows ajoint of unexpanded pipe 100 with a nominal diameter-to- wall thickness ratio (d/t) of about 20. The unexpanded pipe 100 is formed from, for example, 80,000 psi yield carbon steel. No connection is formed on the unexpanded pipe 100 in Figure 2A. The unexpanded pipe 100 has an inner diameter (ID) 10OA, an outer diameter (OD) 10OB, and a wall thickness lOOC. The unexpanded pipe 100 is designed to be expanded with an expansion tool 101 that is generally moved through the unexpanded pipe 100 in a direction 101A from an unexpanded first end 102 toward an unexpanded second end 103.

[0035] Figure 2B shows the joint of pipe 104 after it has been expanded. The expanded pipe 104 has an ID 104A, an OD 104B ₅ and a wall thickness 104C. The expanded pipe 104 is shorter than unexpanded plain-end pipe (100 in Figure 2A) (note that the extent of the shortening is exaggerated in Figure 2B for clarity), and the wall thickness 104C of the expanded pipe 104 is generally less than the wall thickness of the unexpanded pipe (lOOC in Figure 2A). Ends 105, 106 of the expanded, plain-end pipe 104 show deformations characteristic of casing and tubing which has been expanded by a conical ly-tapered cold-forming "pig," such as the expansion tool 101 shown in Figure 2A. Expanded first end 105 shows radial "belling-out" at the end of the pipe and an axial "wrinkling" or "rippling" proximate a pipe section near the end. Expanded second end 106 shows a radial "belling- in" proximate the end. Note that the radial displacements of the pipe wall are exaggerated for clarity.

[0036] Figure 3 A shows a plot of strains in the expanded plain-end pipe (104 in

Figure 2B) along the length of the pipe (104 in Figure 2B) proximate the first expanded end (105 in Figure 2B). The strain at first expanded end (105 in Figure 2B) is shown at chart end 108 A, while the strain proximate the pipe body is shown toward chart end 108B. Note that Figure 3B shows a cross-sectional view of the pipe that may be viewed as a physical corollary to the plots shown in Figure 3 A.

[0037] In Figure 3 A, strains at the expanded ID (104A in Figure 3B) are shown as an ID hoop strain curve 107 A, an ID axial strain curve 107C, and an ID radial strain curve 107E. Strains at the expanded outer diameter (104B in Figure 3B) are shown as an OD hoop strain curve 107B, an OD axial strain curve 107D, and an OD radial strain curve 107F. Note that the axial strains (107C and 107D) and radial strains (107E and 107F) are substantially constant and negative in the pipe body, as shown near chart end 108B, confirming that the pipe (104 in Figure 3B), as expected, becomes shorter and has a reduced wall thickness as it is expanded by the expansion tool (101 in Figure 2A).

[0038] Proximate inflection point 109, the OD axial strain 107D and the ID axial strain 107C diverge, wherein the OD axial strain 107D increases (i.e., becomes less negative) and the ID axial strain 107C decreases (i.e., becomes increasingly negative), indicating that the pipe (104 in Figure 3B) is beginning to "bulge" in a

radially outward direction. When viewing from left to right at inflection point HOA, the OD axial strain 107D begins to decrease, while ID axial strain 107 C begins to increase, thereby indicating that the pipe (104 in Figure 3B) is contracting in a radially inward direction. At inflection point 11OB, the axial strain curves 107 C, 107D intersect. At inflection point 111, the pipe (104 in Figure 3B) begins outward radial expansion and, at inflection point 112, the ID axial strain 107C begins to decrease while the OD axial strain 107D continues to increase (e.g., a rate of radial expansion decreases proximate the end (105 in Figure 3B) of the pipe (104 in Figure 3B)).

[0039] Referring to Figure 3B, a nominal expanded outer diameter 104D of the expanded pipe 104 is shown as a reference for the radial displacements of the pipe 104 at the inflection points 109, HOA, HOB, 111, 112. Note that radial displacements of the pipe wall are slightly exaggerated for clarity. Note that a maximum displacement occurs at inflection point 111 and that a wall thickness 104C is slightly decreased proximate the end 105 of the pipe 104.

[0040] The "wrinkles" or "ripples" proximate the end of the pipe resemble, for example, a heavily-damped sinusoidal function. Based on Finite Element Analysis (FEA) and physical experimentation with the expansion process, the present inventors have hypothesized that the amplitude, period, and damping of the "ripples" depend on the total plastic expansion required in the pipe, the diameter- to-wall thickness ratio (d/t), the design of the expansion tool (particularly the taper angle), and a coefficient of friction between the expansion tool and the unexpanded pipe.

[0041] Figure 4 A shows a plot of strains in the pipe 104 along the length of the pipe proximate the second expanded end 106. The strains at the second expanded end 106 are shown at chart end 114A, while the strains toward the pipe body are shown toward chart end 114B. Note that Figure 4B shows a cross-sectional view of the pipe that may be viewed as a physical corollary to the plots shown in Figure 4A.

[0042] Strains at an expanded inner diameter (ID) 104A are shown as an ID hoop strain curve 113A, an ID axial strain curve 113C, and an ID radial strain curve 113E. Strains at an expanded outer diameter (OD) 104B are shown as an OD hoop strain curve 113B, an OD axial strain curve 113D, and an OD radial strain curve

113F. Note that the axial strains (113C and 113D) and the radial strains (113E and 113F) are substantially constant and negative in the pipe body, as shown near chart end 114B, confirming that the pipe (104 in Figure 4B), as expected, becomes shorter and has a reduced wall thickness as it is expanded by the expansion tool (101 in Figure 2A). Also note that strain levels in the body of the pipe (104 in Figure 4B) (e.g., strain values at chart end 114B) shown in Figure 4 A correspond to strain levels in the body of the pipe shown in Figure 3 A (e.g., strain levels at chart end 108B).

[0043] Referring to Figure 4A, viewing from right to left at inflection point 115A, the OD axial strain curve 113D is increasing (i.e., becomes less negative) while the ID axial strain curve 113C is decreasing (i.e., becomes increasingly negative), indicating that the pipe (104 in Figure 4B) is beginning to bulge radially outward. At inflection point 115B, the OD axial strain curve 113D is decreasing while the ID axial strain curve 113C is increasing, indicating that the pipe (104 in Figure 4B) is belling radially inward. Note that the "inward belling" trend continues all the way to the second expanded end (106 in Figure 4B), represented at chart end 114A.

[0044] Figure 4B shows a nominal outer diameter 104D of the expanded pipe body as a reference for radial displacements of the pipe 104 at the inflection points 115A, 115B. Note that radial displacements of the pipe wall are slightly exaggerated for clarity and that a wall thickness 104C remains substantially constant. At inflection point 115A, there is the predicted outward belling of the pipe. As discussed with respect to Figure 4A above, the pipe 104 begins inward radial belling at inflection point 115B and continues to bell radially from the inflection point 115B to the second expanded end 106.

Radial Plastic Expansion of Threaded Tubing Joints

[0045] When designing threaded connections for radially expandable tubing or casing joints, the present inventors have discovered methods for compensating for the aforementioned axial and radial displacements that occur in the tubing joints proximate the threaded connections. Accordingly, embodiments of the invention are designed to compensate for the axial and radial displacements proximate ends of threaded pipe joints, casing joints, and tubing joints after radial plastic

expansion thereof. In some embodiments, selectively placed grooves are used to maintain the integrity of the threaded connections after radial plastic expansion thereof.

[0046] It has been determined from experimentation that when a length of pipe comprising made-up vee-type or square-shouldered threaded connections (such as API "round" or "buttress" threads) is radially plastically expanded, plastic deformation of the made-up connections results in axial and radial displacements that are similar to the aforementioned axial and radial displacements experienced during radial plastic expansion of plain-end pipe because the threads do not provide significant resistance to relative radial deformation of the pin and box members. For example, when a radial expansion tool is moved in an axially upward direction through a typical string of casing (which is usually run into a wellbore with the pin connection ends pointed down ("pin-down")), a pin end of a made-up connection comprising API buttress threads would be deformed in a manner similar to the expanded first end (105 in Figure 2B) shown in Figure 2B, while a box end would be deformed in a manner similar to the expanded second end (106 in Figure 2B) shown in Figure 2B. Because the pin and box ends (in this example comprising API buttress threads) are radially unrestrained with respect to each other, the box end will generally radially "bell-out" and the pin end will generally radially "bell-in" as described in detail above.

[0047] However, it has been determined through experimentation and Finite

Element Analysis (FEA) that if the threaded connection resists radial deformation, as in the case when generally dovetail-shaped threads are used to form the threaded connection, then unsupported ends (e.g., portions of the threaded connection extending past the last engaged thread) of both the box and pin connections will generally radially bell inward in a fashion similar to the expanded second end (106 in Figure 2B) shown in Figure 2B. Accordingly, an external metal-to-metal seal (e.g., a seal formed between the pin connection and the box connection proximate an outer shoulder formed by the end of the box connection) can be formed with a generally dovetail-shaped thread form because of increased contact stresses between the box end and a pin shoulder proximate the box end generated when the box end bells radially inward after radial plastic expansion thereof. However, an internal metal-to-metal seal (e.g., a seal formed proximate a pin nose on the pin

connection) is difficult to maintain, even with a generally dovetail-shaped thread, because the pin nose end tends to bell inward (away from the box connection) after radial plastic expansion thereof. Other sealing mechanisms for tubular connections, such as elastomeric seals, encounter similar difficulties.

[0048] U.S. Patent No. 5,423,579, issued to Blose at el., and assigned to the assignee of the present invention and incorporated by reference herein in its entirety, discloses a metal-to-metal seal design for threaded tubular connections. To form a reliable metal-to-metal seal, the pin and box connections generally contact each other so as to form a thin, cylindrical contact area (which may be achieved by, for example, using mismatched angles on corresponding pin and box elements), and the connection must generally be tightened so as to form a selected minimum contact stress at the seal contact area (e.g., so as to seal against internal and/or external pressure). Accordingly, in order to form a reliable metal-to-metal seal proximate threaded connections disposed on radially plastically expanded casing strings, the connection should be designed so as to relieve expansion stresses to prevent damage to the connection and maintain a seal contact area.

[0049] In certain embodiments of the invention described herein, threads used to form threaded connections are "wedge" threads comprising generally dovetail- shaped threads with converging thread widths. Wedge threads are known in the art and are disclosed in, for example, U.S. Patent No. RE30,647 issued to Blose, U.S. Patent No. RE34,467 issued to Reeves, U.S. Patent No. 4,703,954 issued to Ortloff et al., and U.S. Patent No. 5,454,605 issued to Mott, all assigned to the assignee of the present invention. Each of these patents is incorporated herein by reference. Wedge threads used in these specific embodiments of the invention comprise generally dovetail-shaped threads in which, for example, a crest of a thread is wider than a root of the thread and wherein two thread flanks comprise different but constant leads. A width of the wedge thread form generally changes along the length of the thread (e.g., along a helical thread form) so that, as the connection is "made-up" (screwed together), thread flanks formed on connecting pin and box members get closer together. Moreover, some embodiments of the invention may comprise wedge thread forms in which both flanks have a corresponding negative angle (with respect to an the axis of the tubular member on which they are formed), in which one flank has a negative angle and one flank has a positive

angle, and the like as long as a crest of the thread form is generally wider than a root of the thread form. Further, wedge threads may also be formed with non- dovetailed thread forms.

[0050] Wedge threads are preferred for use in some embodiments of the invention because, for example, they do not require separate torque shoulders. They do not require separate torque shoulders because converging load and stab flanks disposed on the wedge threads react against one another at make-up. Moreover, the generally dovetail-shape of the thread form provides resistance to radial separation of the box and pin during radial plastic expansion of the connection.

[0051] Note that wedge threads may also provide some advantages with radially plastically expandable casing strings and the like because traditional "shouldered" threaded connections may experience severe wear and/or disintegration of the shoulders because axial shrinkage of the casing string (e.g., due to "Poissoning") during radial plastic expansion may raise the contact stresses at the shoulders beyond an ultimate compressive strength of a material from which the casing string is formed. However, for moderate radial plastic expansion (e.g., for substantially less than 10% expansion), a shouldered connection comprising generally dovetail- shaped threads may provide adequate resistance to radial separation of the connection during expansion and a sufficient contact stress at the shoulder. It will be appreciated by those skilled in the art that a "semi-dovetailed" thread form (e.g., a thread form wherein one of the thread flanks is at an approximately 90 degree angle with respect to the pipe axis) may also provide resistance to radial separation of the connection during expansion. In other embodiments, no dovetail may be present at all. Accordingly, the invention is not intended to be limited by embodiments described herein that include a "dovetailed" thread form.

[0052] Figure 5 shows an embodiment of the invention in a substantially unexpanded state. The connection comprises a pin member 130 having external threads 153, and abox member 131 having internal threads 132. Moreover, the pin member 130 and the box member 131 may comprise corresponding adjacent seal surfaces 134 A, 134B, respectively, adapted to form a metal-to-metal seal 134 upon make-up of the connection. In some embodiments, the threads 153 and 132 may be wedge threads as shown in Figure 5. In this embodiment, the threads 153 and 132 are generally dovetail-shaped, which helps prevent the pin member 130 and

the box member 131 from deforming radially relative to one another during radial plastic expansion. A direction of travel 133 of an expansion tool (not shown) used to radially plastically expand the connection, in view of the discussion above, determines that the pin connection 130 will behave similarly to the second expanded end of plain end pipe (106 in Figure 3A) in a region of the seal 134 defined by contact between adjacent seal surfaces 134A, 134B (Le., a distal end 135 of the pin 130 will tend to bell inwards after radial plastic expansion thereof).

[0053] In the embodiment shown in Figure 5, a helical groove 150 having a rectangular cross section is formed on the outside of the box member 131. One of ordinary skill in the art will appreciate that the cross section of the helical groove 150 may vary in size and shape without departing from the scope of the invention. For example, in other embodiments, the helical groove 150 may have a V-shaped, parabolic, U-shaped, or semi-circle cross section. The helical groove 150 is at substantially the same axial position as the crest 152 of the internal thread 132. This positioning of the helical groove 150 may be referred to as being "in-lead" above the internal thread 132. Because the expansion tool does not contact the box member, only the pin member (130 in the embodiment in Figure 5) directly experiences axial force imposed by the expansion tool. This causes the pin member 130 to elongate, which forces the box member 131 to elongate a lesser amount due to the connection between the two members. The wedge thread 132 on the box member resists the elongation because of the thickness at the crest 152. By locating the helical groove 150 in-lead above the crest 152 of the internal thread 132, the wall thickness of the box member is reduced where it normally has an increased thickness. The reduction in wall thickness resulting from the helical groove 150 relieves the axial stress experienced by the internal thread 132. At the same time, the overall axial and hoop strength of the connection is not significantly reduced because the minimum wall thickness (occurring at the root 155 of the internal thread 132) of the box member is not reduced.

[0054] Turning to Figures 6A and 6B, a box member 215 and pin member 216 in accordance with an embodiment of the present invention are shown. The box member 215 and pin member 216 have an internal thread 211 and an external thread 212, respectively, and are adapted to threadedly engage with each other to connect two tubular members. A helical groove 207 having a trapezoidal cross

section is formed on the outside surface of the box member 215. The in-lead location of the helical groove 207 is illustrated by dashed lines drawn from the helical groove 207 to the internal thread 211. The placement and helical nature of the helical groove 207 does not significantly reduce the critical section 210 of the box member 215, which determines the maximum tension and compression that the connection box member 215 will withstand. The "critical section" is located at the smallest cross section of the box member 215 that must withstand the full axial load. The area of the critical section 210 is measured transverse to the axis (not shown) of the tubular. In this particular embodiment, the location of the critical section 210 coincides with the circumferential groove 205. In embodiments that do not have a circumferential groove 205, the critical section 210 may be located at a thread relief groove (not shown). The critical section 210 is commonly a failure point for a box member during compressive and tensile loads. Because the helical groove 207 removes only a small portion of material from any axial location on the box member, there is little effect on the critical section 210.

[0055] In the embodiments shown in Figures 6A and 6B, there are four separate seals. The box member 215 has two sealing areas 201 A and 203A that are adapted to form seals with sealing areas 20 IB and 203B, respectively, located on the pin member 216. The box member 215 and pin member 216 have grooves 205 and 206, respectively, formed thereon that are adapted to contain separate sealing rings (not shown), such as an elastomeric O-ring. Sealing areas 201A and 201 B and the sealing ring located in groove 206 seal against external pressure. Sealing areas 203A and 203B and the sealing ring located in groove 205 seal against internal pressure. The properties of embodiments of the present invention help to maintain these seals during the radial plastic expansion of the connection.

[0056] While the above embodiments illustrate helical grooves having a constant cross section, the scope of the present invention is not limited to constant cross sections. In some embodiments, the helical groove may vary in width at a rate equal to the varying of the width of the crest of a wedge thread on the box member. In some embodiments, the depth of the helical groove may vary along the axial length of the box member. For example, it may be desirable to have a shallower helical groove on the distal end of the box member where the cross section is thinnest. One of ordinary skill in the art will appreciate that the shape and size of

the cross section of the helical groove may vary without departing from the scope of the present invention. Furthermore, the helical groove may have a length less than the length of the internal thread. One of ordinary skill in the art will appreciate that the helical groove may occur on only a portion or the entire length of the box member without departing from the scope of the present invention.

[0057] While the helical grooves discussed in the above embodiments are useful for maintaining strength and seal integrity of connections on tubular members that are intended to be radially plastically expanded, the helical grooves also help to resist the effects of damage on the external surface of the box member. Severe scratches and gouges occur on the external surface of the box member during surface handling (e.g., making up of connections) and when running the tubular members into the well. Additional scratches and gouges may occur while passing the tubular members through a casing window located in the well. These scratches and gouges act as stress risers that can cause the splitting of the box member during the expansion process. When severely stressed, these scratches and gouges can form cracks that will propagate. Cracks resulting from axial scratches and gouges, such as those that occur during the sliding of the tubular member in a well, typically propagate along the axis of the box member when exposed to severe hoop stress. Cracks resulting from circumferential scratches and gouges, such as those that occur during the making up of connections, typically propagate in the circumferential direction when exposed to severe axial stress. A helical groove may help reduce the propagation of those cracks in both the axial and circumferential directions. This advantage of the helical groove formed on the outside surface of the box member may also be useful for connections that are not intended for expansion.

[0058] Forming a helical groove on the OD of a box member has been discovered by the present inventors to be particularly useful for connections of tubulars, such as drill pipe and drill collars. Drill collars are typically manufactured from one piece of pipe having a substantially constant OD. Connections for drill collars generally have the same OD as the body of the drill collar. Drill pipe is typically manufactured by welding a tool joint on each end of a section of tubular. The tool joint typically has an OD greater than the drill pipe. The tool joint includes a connection having either an internal thread (box) or external thread (pin). A

standard section of drill pipe would have a pin member on one end, and a box member on the opposing end.

[0059] Figure 7 A shows a cross section of a prior art tool joint. The tool joint has a pin member 701 and a box member 702, which are adapted to connect to each other. The pin member 701 is attached to a section of drill pipe 706 A. The box member 702 is attached to a section of drill pipe 706B. The OD 703 of the tool joint is larger than the OD of the drill pipe 706 A and 706B. The ID 705 of the tool joint is typically smaller than the ID of the drill pipe 706A and 706B.

[0060] Continuing with Figure 7, the OD 703 of a tool joint experiences wear during use. Most of the wear results from rotating the drill pipe in the wellbore. The rate of wear varies by application, depending on the conditions of the particular wellbore. As the OD 703 wears, most connections experience a reduced torque and tensile strength because of the reduced cross-sectional area. Wedge threads 707 in Figure 7, which do not depend on shoulder area 708 for strength, maintain substantially full strength up to the bevel 709 diameter. Other connections that require making up to a shoulder experience gradually reduced strength during the wear of the OD 703. After wear reduces the OD 703 past the bevel 709, the drill pipe is replaced.

[0061] To reduce wear, tool joints may have hardbanding 704 applied to portions of the tool joint. Hardbanding 704 may be added using carbide particles in a weld- metal matrix. Besides wear on the OD 703 of the tool joint, hardbanding 704 also helps to reduce the wear of the wellbore, in particular metal casing. Some situations cause wear at particularly high rates despite the use of hardbanding 704. One such situation is basaltic rock, which may be encountered while drilling in certain areas. Geothermal wells are a common location where basaltic rock may exist. The basaltic rock has a high abrasiveness that rapidly erodes the OD 703 of drill pipe, as well as heavy weight drill pipe and drill collars. Drilling in basaltic rock formations has been known to reduce the useful life of drill pipe to as little as three wells, which is a small portion of the typical expected useful life.

[0062] Figure 7B shows a cross section of a tool joint in accordance with an embodiment of the present invention. The tool joint in Figure 7B has substantially the same OD 703 as the tool joint in Figure 7 A, which makes it suitable for use in

the same size wellbore as Figure 7 A A helical groove 715 has been formed on the OD 703 of the tool joint The helical groove 715 does not need to be in-lead with the thread 707 to expeπence one or more of the advantages of the present invention, however, the box member 702 is stronger if the helical groove 715 is in- lead with the thread 707

[0063] The following comparison of Figure 7A to Figure 7B is intended as a specific example and should not be considered to limit the scope of the invention As a specific example, the tool joint in Figure 7A is a WT 50 tool joint sold by Hydπl Company LP The WT 50 tool joint commonly has an OD 703 of 6-3/4 inches and an ID 705 and is adapted to be attached to 19 50 lb/foot dπll pipe having an OD of 5 inches and an ID of 4-1/4 inches The bevel 709 diameter may be 6-11/32 inches The tool joint in Figure 7B is similar to a WT 46 tool joint sold by Hydπl Company LP, and, therefore, would typically have a smaller OD than the WT 50 tool joint in Figure 7A, and would have less strength than the WT 50 tool joint if the ID 705 is kept the same as that of a WT 50 tool joint If reduced strength is not allowable, then the ID 705 may be reduced A reduced ID, however, results in an increased pressure drop when drilling fluid is pumped through the dπll pipe In the embodiment shown in Figure 7B, however, the OD of the tool joint has been increased to be equal to the OD of the WT 50 tool joint, while the ID 705 has been kept the same as that of the WT 50 tool joint As a result, the pin member 701 on the tool joint has a reduced strength compared to the WT 50 tool joint

[0064] Because the OD of the tool joint shown in Figure 7B is larger than normal, the strength of the box member 702 is increased relative to the pin member 701 The ratio of box member strength to pin member strength is commonly known as the bending strength ratio (BSR) A BSR of 1 (i e 1 1) means that the box member and pin member have equal strength A BSR greater than 1 means that the box member is stronger than the pm member If a box member is too strong (stiff) in bending relative to the pm member, the pin member may be damaged when the tool joint expeπences bending The desirable BSR vanes for different threads For example, a wedge thread tool joint (as m the WT 46 and WT 50) generally has a desired BSR between 1 0 and 1 5 For most American Petroleum Institute (API) tool joints, the BSR should be about 2 5 For most API

connections, the relative flexibility of the pin member causes the pin member to stretch during make-up of the connection, which preloads the connection. One of ordinary skill in the art will appreciate that the desired BSR may vary depending on the particular tubular connection. Accordingly, embodiments of the present invention are not limited to particular values for the BSR.

[0065] Maintaining the OD 703 and ID 705 of the WT 46 tool joint to be equal to the WT 50 tool joint would result in a higher BSR than may be desired. This is because the box member 701 has been strengthened relative to the pin member 702.To reduce the BSR, a helical groove 715 may be formed in the OD 703. The helical groove 715 removes material from the box member 702, which causes it to be more flexible. The depth, width, and pitch of the helical groove 715 may be varied depending on the required reduction of the BSR. Removing more material reduces the BSR, but results in less material for wear resistance. In one embodiment, the helical groove 715 is in-lead with the thread 707 on the box member 702, and has a depth equal to the bevel 709. One of ordinary skill will appreciate that the balance of BSR and wear resistance may vary without departing from the scope of the present invention. The excess material on the box member 702 provides more material to be worn during usage. Thus, it provides a longer useful life for the same rate of wear. Additionally, the helical groove 715 provides an easily observable wear indicator. If the depth of the helical groove 715 is near the minimum useful OD of the tool joint, one would know to replace the drill pipe when the helical groove 715 is no longer visible.

[0066] While Figures 7A and 7B provide specific examples, one of ordinary skill in the art will appreciate the same principles can be applied to other connections to achieve one or more of the advantages of the present invention. In summary, a smaller tool joint than normal may be selected, while maintaining the OD of a larger tool joint. The addition of a helical groove removes material from the box member, which results in a more flexible box member to achieve a desired BSR. A helical groove may also be formed on box members of drill collars in a similar manner to provide a useful wear indicator and to increase the flexibility of the box member. Further, a helical groove may be formed on connections in any application where OD wear is a concern to achieve one or more of the advantages of the present invention.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.