Threaded tool joint for connecting large tubes

A connector for connecting large tubes of the type used in oil well tubing, casing and the like, without resorting to welding, to form a pipe string for use in drilling. The connector comprises a pin and box threaded connection having guiding surfaces integral with the pin and box members to facilitate the threaded connection of large tubes. The connection is designed so that large tubes, for example tubes comprising an external diameter of two or more feet, may be joined by threaded connection facilitated by the integral guiding surfaces of the pin and box threaded members. The integral guiding surfaces allow large tubes, which are otherwise too cumbersome to position and manipulate for threaded connection to be practical, to be quickly and safely connected by threaded engagement of the pin and box members without the use of a weld between the tubes.

FIELD OF THE INVENTION 
The present invention relates to pipe connectors, particularly, but not 
exclusively, for use in connecting sections of a pipe string for use in 
drilling. More specifically, it relates to the design of a pin and box 
connection of the type used in oil well tubing, casing, and the like. The 
invention provides a driveable threaded joint with dual mating shoulders 
and nose faces on the pin members and box members. The dual mating 
shoulders substantially improve the joint's ability to withstand the 
intense axial compression loading that occurs when driving the pipe into 
the ground. A significant advance in the art is achieved by providing a 
threaded joint for large tubes of the types described which permits the 
large tubes to be connected one to another by threaded means without the 
use of a weld bead between the tubes, and yet still provide a reliable 
connection capable of withstanding the axial compression loads of drilling 
or other industrial scale uses. 
DESCRIPTION OF RELATED ART 
Connecting large diameter tubes, such as metal well bore tubes which are 
lowered down hole in oil wells, has been problematic in the art. Large 
tubes, for example tubes comprising an external diameter of two feet or 
greater, are cumbersome and generally have to be handled using powerful 
machines to grip and position the tubes. Connecting such tubes by means of 
threaded connection has been impractical because of the precision required 
in positioning and turning the cumbersome large tubes. As a consequence of 
the difficulty and impracticality of threadedly connecting large tubes, 
such tubes are generally connected by way of welding. Even where an 
initial connection is made by threaded engagement, it is a common practice 
in the art to reinforce or secure the connection with weld beads. 
To connect large tubes by welding, the tube members may be stationary, so 
no manipulation, turning, or precision placement of the tubes is required. 
However, welding is very time consuming, expensive, and requires highly 
skilled personnel to perform. 
A connector for rapidly and securely connecting large tubes by threaded 
engagement without the use of welding would be useful to avoid the time, 
expense and expertise required for a welded connection. Such a threaded 
connection should be designed so that the axial compression loads acting 
on the connection joint are not born solely by the threads of the 
connection, but rather, the loads should be born as much as possible by 
the shoulders and nose faces of the connector so that the integrity of the 
threads is maintained. Further, a threaded connection should be able to be 
made up quickly, for example is as few as approximately one and one-half 
turns, so that handling of the tubes is minimized to enhance the safety of 
making-up the connection. 
Threaded connections between pipe members are typically made by providing 
one end of one pipe member with a male connector in the form of an 
externally threaded pin member, providing one end of a second pipe member 
with a female connector in the from of an internally threaded box member 
which receives the pin member. The pin and box members may be integral 
parts of their respective pipe members, or may be added thereto by welding 
or threaded engagement. 
In the past, several different types of threaded connections have been 
designed to manage the extreme compressive, tensile, and bending forces to 
which the connection is exposed. Several prior art design incorporate 
internal and/or external matting shoulders and end faces on the pin and 
box members. As used in this description, the terms "end face" and "nose 
face" are interchangeable. In several designs, the mating shoulders are 
used as torque shoulders to stop axial advancement of the pin and box 
members during make up of the joint. In many designs, the shoulders are 
also used to provide resistance to axial compression during pile driving. 
Although many prior art designs use a combination of external and internal 
shoulders, these designs are usually configured such that only one of the 
shoulders will mate with its corresponding nose face upon initial makeup 
of the joint. These designs rely on either the external or the internal 
shoulder alone to mate with its corresponding nose face at initial make-up 
of the joint, with the other shoulder remaining axially spaced from its 
corresponding nose face at initial make-up of the joint, and some designs 
may never mate, or only make contact with its corresponding nose face 
after the threads or other portions of the joint begin to yield. It is one 
object of the present invention to provide a threaded connection design 
that uses dual mating shoulders in which both external and internal 
shoulders mate with their corresponding nose faces during initial make-up 
of the joint. By providing dual mating shoulders, the shoulders share 
axial compression loads and provide the joint with improved performance in 
resisting the extreme axial compression loads encountered during pile 
driving. 
In addition to providing resistance to axial compression loading, the dual 
mating shoulders in the present invention also function as torque 
shoulders to stop axial advancement of the pin and box members during 
make-up of the joint. In several prior art designs, the threaded 
connections use converging or wedge-type thread flanks rather than 
shoulders to act as a torque stop. As used in this description, the terms 
"converging" and "wedge-type" are interchangeable. In general, the pin and 
box threads in a converging thread flanks connection have progressively 
changing axial widths. The axial thread width of the pin member 
progressively decreases in the direction of the mouth of the pin member 
over the length of the thread structure. The axial thread width of the box 
member, on the other hand, progressively decreases in the opposite 
direction, such that a pair of pin and box members in the fully made up 
condition have a mutual wedging interfit. When converging threads are 
screwed together and wedging between the flanks takes place, the torsional 
resistance of the connection increases as the thread flanks act as torque 
stop to axial advancement of the pin and box members. Several other thread 
connection designs use tapered buttress-type thread forms that rely on 
radial interference to stop axial advancement of the pin and box members 
during make-up. In a tapered threads configuration, the radial 
interference fit forms as the crests and roots of the pin and box threads 
converge upon make-up of the joint. 
Although these thread form designs may succeed in providing a torque stop 
to halt axial advancement of the pin and box members during make-up, and 
also allow the threads to provide resistance to axial compression loading, 
taking pressure off any pin and box shoulders that may be used in the 
design, such use of an interference fit in the thread form has its 
drawbacks. Such uses of interference fits in the form may create high 
surface contact stresses on the threads, which can cause galling and other 
localized thread damage that can severely limit the number of times the 
connection can be made up. In addition to limiting the repetitive use of 
the threads, the areas of high surface contact stress are susceptible to 
stress corrosion cracking, known as sulfide stress cracking, that occurs 
in petroleum well conduits. It is one object of this invention to provide 
a threaded joint connection that uses the shoulders of the pin and box 
members rather than the threads to function as a torque stop. 
Conventionally, the pin member of the joint is tapered inwardly from the 
proximal end of the threaded portion to the distal end to mate with a 
similarly tapered female threaded box member. The taper facilitates entry 
of the pin member in to the box member. Although the taper facilitates 
entry of the pin member, the wall thickness at the nose face end of a 
tapered thread form is often very small, especially in the flush joint 
configuration. Although the wall thickness at the shoulder of the pin and 
box member may be a substantial portion of the pipe wall thickness, with 
the shoulder occupying only a small portion of the wall, the wall 
thickness at the nose face end may be very small. This tapered 
configuration leaves the nose face end with a reduced wall thickness that 
must withstand the extreme axial compression during pile driving, as well 
as the extreme tensile, compressive, and bending forces to which the pipe 
is exposed downhole. It is an object of the present invention to provide a 
threaded pin and box joint in which the thread form is straight rather 
than tapered, to allow substantially the one-half thickness of the wall of 
the pin and box members for sustaining compressive, tensile, and bending 
forces to which the pipe is exposed. 
Although a tapered thread form may facilitate entry of the pin and box 
members during make-up of the joint, tapered threads are still susceptible 
to cross-threading if the pin and box members are not properly aligned at 
the point of threaded engagement. One example of an apparatus designed to 
prevent cross-threading is found in U.S. Pat. No. 4,407,527, issued to Mr. 
Larry E. Reimert. The Reimert patent discloses a guide surface axially 
spaced from the internal threads of the box member to constrain the 
relative orientation between the pin and box members prior to threaded 
engagement. Although the Reimert design may be successful in preventing 
cross-threading, we have found that the guiding means may also integrate 
into a mating shoulder configuration by axially spacing the nose face from 
the thread on the pin and box members. It is therefore an object of this 
invention to provide a guiding means for preventing cross-threading that 
is integrated into the shoulders and nose faces of a pin and box 
connection. 
Several further objects of the present invention include providing means 
for preventing separation of the pin and box members, providing a thread 
form configuration that allows quick make-up of the joint, as well as 
several other objects and advantages that will become apparent from a 
reading of the attached claims and description of the preferred 
embodiments. 
SUMMARY 
These and other objects of the invention are attained by providing one end 
of one pipe member with a male connector in the form of an externally 
threaded pin member, and providing one end of a second pipe member with a 
female connector in the form of an internally threaded box member which 
receives the pin member. The pin and box members may be integral parts of 
their respective pipe members, or may be added thereto by welding or 
threaded engagement. In the preferred embodiment of the present invention 
the pin and box members are integral parts of their respective pipe 
members, but it should be understood that the inventive design may also be 
used by mounting the pin and box members on their respective pipe members, 
or could be used in any of the various forms of collars or nipples known 
in the art featuring combinations of the two box ends, two pin ends, or a 
box end with a pin end for threaded connection to appropriate ends of two 
pipe members sought to be mutually connected. 
A threaded connector for connecting large diameter tubes is disclosed, the 
connector comprising: 
a first large diameter tube comprising a pin member with external threads, 
and a guiding surface integral with the external threads; and 
a second large diameter tube comprising a box member with internal threads 
and a guiding surface integral with the internal threads and complementary 
to the guiding surface of the pin member to facilitate entry of the pin 
member threads into the box member threads, wherein upon threaded 
engagement of the pin member of the first tube and the box member of the 
second tube, the first and second tubes may be connected in as few as 
approximately one and one half turns, whereby large diameter tubes may be 
connected one to another without a weld between the tubes. 
The threaded connection has dual mating shoulders in which both the 
internal and the external shoulder mates with its corresponding nose face 
during initial make-up of the joint. By providing dual mating shoulders, 
the shoulders share axial compression loads and provide the joint with 
improved performance in resisting the extreme axial compression loads 
encountered during pile driving. In addition to providing resistance to 
axial compression loading, the dual mating shoulders in the present 
invention also function as torque shoulders to stop axial advancement of 
the pin and box members during make-up of the joint. The thread form on 
the pin and box members is straight, rather than tapered, and does not 
have converging thread flanks, so the threads do not act as a torque stop, 
nor do they provide and substantial portion of the resistance to the 
extreme axial compression loading encountered during pile driving. 
By providing dual mating shoulders that share the axial compression loads, 
and by using a thread form having straight threads with uniform axial 
thread widths, the compressive loads on the pin and box members are 
transferred substantially through the shoulders rather than through the 
thread form. This configuration allows the shoulders to take the brunt of 
the axial compression loading and spare the threads. This configuration 
avoids high surface contact stress on the threads to prevent galling and 
other localized thread damage that severely limit the number of times the 
connection can be made up. This configuration also helps to prevent stress 
corrosion cracking that occurs in areas of high surface contact stress 
that are exposed to sulfide in petroleum wells. The use of a straight 
thread form, rather than tapered, provides substantially the one-half 
thickness of the wall of the pin and box members for sustaining 
compressive, tensile, and bending forces to which the pipe exposed. 
The straight thread form provides substantially the full one half thickness 
of the wall of the pin and box members for sustaining the force to which 
the pipe is exposed, but the ideal design of the pin and box members 
results in the wall thickness of the pin and box members being not 
precisely one-half the connector thickness. The optimal design provides 
that the pin and box members will be of equal strength. In order to design 
the pin and box members to be of equal strength, the pin and box members 
are configured to have equal annular cross-sectional areas. Because the 
inner diameter of the box member is aligned with the outer diameter of the 
pin member, the medial diameter of the box member is larger than the 
medial diameter of the pin member. To design the pin and box members to be 
of equal strength, the wall thickness of the pin member (the member with a 
smaller medial diameter) is increased to slightly greater than one-half 
the total wall thickness of the connection, and the wall thickness of the 
box member (the member with a larger medial diameter) is decreased to 
slightly less than one-half the total wall thickness of the connection. 
This optimal design provides substantially the full one-half thickness of 
the wall of the pin and box members for sustaining the forces to which the 
pipe is exposed, but also provides that the wall thickness of the pin and 
box members will be slightly other than precisely one-half the connector 
thickness, in order to provide that the pin and box members will be of 
equal strength. 
The present invention also provides an integrated guiding means to 
facilitate entry of the pin in to the box member. This integrated guiding 
means also functions as a self-centering means to align the pin and box 
members upon threaded engagement to avoid cross-threaded. The integrated 
guiding and self-centering means is achieved by providing a design in 
which the shoulders and nose faces of the and box members are axially 
spaced from their most adjacent thread flanks. This configuration 
facilitates entry of the pin and box members at the point of threaded 
engagement, thus avoiding cross-threading. 
Another feature of the present invention is the use of trapped thread 
flanks to prevent separation of the pin and box members. Conventional pin 
and box connections are susceptible to separation, often called "jumpout", 
when the connection is subjected to extensive axial tension and/or bending 
type loads. Under axial loading in tension, the members will shrink due to 
the "Poisson's" effect, and the box member will expand or "bell out", a 
condition known as "belling". To counteract these conditions, the thread 
form is provided with reverse angle load flanks, often referred to as 
"trapped" or "hooked" thread flanks. When the connection is subjected to 
axial loads in tension, the trapped load flanks cause the pin member to be 
pulled radially inward toward the pin member. This feature secures the pin 
and box members together and prevents jumpout that could otherwise cause 
failure of the joint. By placing the box member in a state of hoop 
compression and the pin member in hoop tension, the trapped load flanks 
also serve to counteract induced assembly stresses and improve the 
joints's strength in sulfur environments that could otherwise make the 
joint susceptible to stress corrosion or hydrogen fracture. 
In addition to providing trapped thread flanks to prevent jumpouts, the 
present invention provides trapped nose faces as well. Some prior art 
designs provide mating shoulders and nose faces having dissimilar angles 
so that the shoulder the nose face. One example is found in U.S. Pat. No. 
4,822,081, issued to Thomas L. Blose. The Blose patent discloses a 
shoulder and nose face having dissimilar angles so that the shoulder traps 
the nose face and the nose face will not slip out upon the application of 
axial driving force. The present invention improves on this type of 
feature by providing a trapped nose face that is radially balanced to 
provide a radially balanced resistance to axial loading compression. The 
radially balanced nose face efficiently distributes compressive forces and 
allows the nose face to withstand increased compressive loading without 
yielding. 
Another feature of the present invention is a thread form configuration 
that provides a quick make-up of the joint. As can be seen in the drawings 
more fully described below, the preferred embodiment provides complete 
make-up of the joint in approximately one and one-half turns, a feature 
which offers great advantages in the field. The connection of the present 
invention can be made-up in less than ten minutes, in part due to the fact 
that very little turning is required. Welding, in contrast, typically 
requires about an hour to complete. 
Also disclosed is a method for connecting large diameter tubes without a 
weld between the tubes, the method comprising: 
providing a first large diameter tube comprising a pin member with external 
threads, and a guiding surface integral with the external threads; 
providing a second large diameter tube comprising a box member with 
internal threads and a guiding surface integral with the internal threads 
and complementary to the guiding surface of the pin member to facilitate 
entry of the pin member threads into the box member threads; 
aligning the guiding surfaces of the pin member and the box member; and 
connecting the first and second tubes in as few as approximately one and 
one-half turns by turning one of the tubes in relation to the other so 
that the pin member threads enter the box member threads to form a 
connection between the first tube and the second tube. 
The present invention will be more fully understood from the following 
description of the preferred embodiments, given by way of example only, 
with reference to the drawings.

DETAILED DESCRIPTION 
FIG. 1 shows a cross-sectional view of a threaded connection according to 
the present invention with the pin and box members in a fully made-up 
condition. FIG. 1 shows upper pin member 10 secured into lower box member 
11 to form a connection designated generally as 12 along axis 13. In a 
preferred embodiment, the threaded connection 12 has mating pin and box 
members having outside diameters and inside diameters substantially 
identical for each of the two members. This is commonly referred to as a 
flush connection when assembled. The flush connection is preferred in 
practice to avoid irregularities on the outer surface of the joint that 
cause resistance when driving the casing into the ground or when running 
the pipe through the well bore. Although the flush connection is 
preferred, the present invention is not limited to flush connections. Nor 
is the invention limited to the pin and box members being integral parts 
of their respective pipe members. The pin and box members may be integral 
of their respective pipe members, or may be added thereto by welding or 
threaded engagement. Still referring to FIG. 1, the threaded connection 12 
includes pin member threads 16 that are adapted to be made-up with box 
member threads 17. Also shown in FIG. 1 are pin member nose face 18, box 
member shoulder 19, box member nose face 20 and pin member shoulder 21. 
FIG. 2 shows a partial cross-section of the box member 11. The box member 
17 includes box member threads 17 having box thread crests 26 and roots 
27. The box member threads 17 also include stab flanks 28 and load flanks 
29. The term stab flank refers to the side of the thread facing inwardly 
towards the joint, and term load flank refers to the side of the thread 
facing away from the joint. 
FIG. 3 shows a partial cross-section of the pin member 10. The pin member 
10 includes pin member threads 16, which have pin thread crests 22 and 
roots 23. Also shown are pin member stab flanks and load flanks 25. 
FIG. 4 shows a partial cross-section of the threaded connection prior to 
final make-up. The figure shows the connection at the point of threaded 
engagement at which the first stab flank 30 on the pin member contacts the 
first stab flank 31 on the box member. In this position, one can see that 
the axial spacing between the nose face 18 and the first stab flank 30 on 
the pin member, and the axial spacing between the nose face 20 and the 
first stab flank 31 on the box member, form guiding surface 32 on the pin 
member and 33 on the box member. These guiding surfaces facilitate entry 
of the pin into the box member and function as self centering means to 
align the pin and box members upon threaded engagement to avoid 
cross-threading. This configuration prevents cross-threading by 
constricting the relative orientation of the pin and box members at the 
point of threaded engagement. 
FIG. 5 shows the threaded connection in a fully made-up condition. The 
tolerances of the thread form are designed so that when the joint is fully 
made-up, although the load flanks are in intimate contact, the clearances 
remain between the stab flanks to ensure that compressive loads on the pin 
and box members are transferred substantially therethrough the pin and box 
shoulders rather than through the thread form. FIG. 5 shows stab flanks 
24, 28, 30, and 31 as substantially square. Load flanks 25 and 29 form 
"nonpositive" or "reverse" angle, or may form a trapped angle, to create a 
trapped or hooked thread. In this configuration, the thread crest extends 
over the thread root. The nonpositive angled load flanks help ensure that 
the thread do not slip out and become disengaged during axial loading 
tension. 
FIG. 6 shows the lower end of pin member 10. FIG. 7 shows a cross-section 
of the upper end of box member 11. Seal groove 44 is identified in FIGS. 6 
and 7. Seal groove 44 on pin member 10 is located proximate the shoulder 
of pin member 10 and seal groove 44 on box member 11 is located proximate 
the shoulder of box member 11. Each of these seal grooves 44 may be used 
to contain an elastomer ring or metal seal to seal pin 10 and box 11 
members from leakage. The connection may be designed to include one or 
both of these seal grooves or may be configured to not include either seal 
groove. Regardless of whether a seal groove is included in the design, the 
annular shoulder region 47 of pin member 10 function as a seal against the 
annular end region 48 of box member 11 and the annular shoulder region 49 
of box member 11 seals against the end region 50 of pin member 10. As 
described above, the annular shoulder region function as a guiding surface 
as well as a sealing surface. 
The above disclosure and description is illustrative and explanatory of the 
present invention, and it understood that various changes in the method 
steps as well as the details of the illustrated apparatus may be made 
within the scope of the following claims without departing from the spirit 
of the invention.