Stress reduction groove for tubular connection

A stress reduction groove for tubular connections which relates to a threaded pipe connection, and includes a box member, a pin member, and a continuous groove formed in the thread of one of the members. The box member has a tapered, internal, generally dovetail-shaped thread with stab flanks, load flanks, roots, and crests. The internal thread has a beginning and an end and increases in width in one direction. The pin member also has a tapered, external, generally dovetail-shaped thread with stab flanks load flanks, roots, and crests. The external thread increases in width in the other direction so that the complementary flanks of the respective threads move into engagement during rotational make-up of the connection and form sealing surfaces that resist the flow of fluids between the threads upon rotational make-up of the connection. A section of one of the internal and external threads has the continuous groove formed in the roots thereof. The groove extends from either the beginning or the end of the one thread to a point between the beginning and end of the one thread, thereby reducing the pressure that develops between the sealing surfaces during rotational make-up in a thread lubricant applied to the threads.

FIELD OF THE INVENTION 
The present invention involves threaded tubular joints usable in oil and 
gas well drilling and production, such as tubing, casing, line pipe, and 
drill pipe, commonly known collectively as oilfield tubular goods. More 
particularly, the invention relates to a tubular joint for connecting male 
(pin) and female (box) members. 
BACKGROUND OF THE INVENTION 
The use of threaded tubular connections for joining flow conduits in an 
end-to-end relationship to form a continuous flow path for transporting 
fluid under pressure is well known. Oilfield tubular goods all use 
threaded connections for connecting adjacent sections of conduit or pipe. 
Examples of such threaded end connections designed for use on oilfield 
tubular goods are disclosed in U.S. Pat. Nos. 2,239,942; 2,992,019; 
3,359,013; RE 30,647; and RE 34,467, all of which are assigned to the 
assignee of the present invention. 
In U.S. Pat. No. RE 30,647 by Blose, a particular thread form is disclosed 
for a tubular connection that provides a strong joint while controlling 
the stress and strain in connected pin and box members within acceptable 
levels. The pin member is equipped with at least one generally 
dovetail-shaped external thread whose width increases in one direction 
along the pin, while the box member is equipped with at least one matching 
generally dovetail-shaped internal thread whose width increases in the 
other direction. In this manner, the mating set of helical threads 
provides a wedge-like engagement of opposing pin and box flanks that 
limits the extent of relative rotation between the pin and box members and 
defines a forcible make-up condition that completes the connection. In 
this thread structure, the flank shoulder angles, as well as the thread 
width, can be used to control the stress and strain preload conditions 
induced in the pin and box members for a given make-up torque. Thus, by 
tailoring the thread structure to a particular application or use, the 
tubular connection or joint is limited only by the properties of the 
materials selected. 
During make-up of a threaded connection in accordance with the teachings of 
the Blose reissue patent, it has been observed that both liquid and 
paste-like thread lubricants can temporarily become trapped in the helical 
clearance or gap formed between the roots and crests of the respective pin 
and box member threads. The trapped lubricant (also known as thread dope) 
can, under certain circumstances, produce a torque reading between the 
relatively sliding pin and box threads indicative of the torque at the 
made up condition, thereby providing a false indication that the joint has 
been fully made up. Thereafter, the temporarily trapped thread lubricant 
can bleed off through the helical clearance between the roots and crests 
reducing the preload stress and strain to such an extent that the 
anticipated performance level or strength of the joint cannot be achieved. 
U.S. Pat. No. RE 34,467 by Reeves discloses an improvement to the thread 
structure disclosed in the Blose reissue patent. Specifically, the 
potential for false torque readings in the joint resulting from trapped 
thread lubricant in the clearance between the roots and crests of the 
threads is addressed. Reliance on the torque readings developed by the 
forcible make-up of the connection is necessary to insure that the design 
stress and strain preload conditions actually exist in the connection. The 
Reeves reissue patent discloses a thread structure wherein the box and pin 
threads are tapered, in addition to having thread widths that increase in 
opposite directions, so that the roots, crests, and flanks of the threads 
are moved into engagement as the joint is made up. The threads are 
particularly designed so that the complementary roots and crests move into 
engagement before both of the opposing stab and load flanks move into 
engagement, whereby the volume of lubricant in the clearance between the 
roots and crests is substantially reduced. In this manner, most of the 
thread lubricant is displaced to the helical clearance between the 
opposing load flanks and forms a long, very thin ribbon that has little, 
if any, effect on the proper make-up of the connection or the ability of 
the thread surfaces to form seals as they are moved together. 
Because of imperfections in the machined, seal forming thread surfaces, 
like those described in the Reeves reissue patent, thread lubricant can 
become isolated between sealing surfaces within the tubular connection. 
Once rotation between the pin and box members has advanced until the 
thread lubricant entirely fills the isolated volume between the pin and 
box members, additional rotation will produce an increase in the pressure 
of the lubricant. This increased pressure can result in higher tangential 
(hoop) and radial stresses in the connection, particularly in harsh cold 
weather environments, such as the North Sea, which cause the lubricant to 
become hardened and more viscous. 
SUMMARY OF THE INVENTION 
Advantages of the invention may include one or more of the following. A 
thread lubricant relief path is provided that can be formed in existing 
oilfield tubular goods without a need to recut the threads. 
A threaded pipe connection is provided having a box member and a pin 
member. The box member has a tapered, internal, generally dovetail-shaped 
thread with stab flanks, load flanks, roots, and crests. The internal 
thread has a beginning and an end and increases in width in one direction. 
Also, the pin member has a tapered, external, generally dovetail-shaped 
thread with stab flanks, load flanks, roots, and crests. The external 
thread increases in width in the other direction so that the complementary 
flanks of the respective threads move into engagement during rotational 
make-up of the connection and form sealing surfaces that resist the flow 
of fluids between the threads. A continuous groove is formed in roots of a 
section of one of the internal and external threads. The groove extends 
from either the beginning or the end of the one thread to a point between 
the beginning and end of that thread, thereby reducing the pressure that 
develops between the sealing surfaces during rotational make-up in a 
thread lubricant applied to the threads. 
A continuous groove may be formed in roots of a section of each of the 
internal and external threads. The groove extends from either the 
beginning or the end of the each thread to a point between the beginning 
and end of the one thread, thereby reducing the pressure that develops 
between the sealing surfaces during rotational make-up in a thread 
lubricant applied to the threads. 
The present invention provides a means for preventing the undue buildup of 
thread lubricant pressure between pin and box members by providing a path 
through which the lubricant can vent. In some embodiments, the present 
invention provides such venting means are along only a portion of the 
thread length, whereby the remaining length of the thread can be employed 
for creating a pressure seal. A thread lubricant relief path is provided 
that does not change its shape or size with wear in the tubular connection 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings wherein like reference characters are used for 
like parts throughout the several views, FIGS. 1, 2, and 3 illustrate a 
wedge-like thread connection or pipe joint, generally referenced by 
numeral 10, in accordance with the Reeves reissue patent. As shown in FIG. 
2, connection 10 includes a pin member 11 and a box member 12. Box member 
12 has tapered, internal, generally dovetail-shaped thread structure 14 
formed thereon and adapted for engaging a complementary tapered, external, 
generally dovetail-shaped thread structure 15 formed on pin member 11 to 
mechanically secure the box and pin members in a releasable manner. 
Internal thread 14 of box member 12 is formed with stab flanks 18, load 
flanks 16, roots 20, and crests 24, and increases in width progressively 
at a uniform rate in one direction substantially the entire helical length 
of thread 14. External thread 15 of pin member 11 is formed with stab 
flanks 19, load flanks 17, roots 21, and crests 25, and increases in width 
progressively at a uniform rate in the other direction substantially the 
entire helical length of thread 15. The oppositely increasing thread 
widths and the taper of the threads 14 and 15 cause the complementary 
flanks, roots, and crests of the respective threads to move into forcible 
engagement during rotational make-up of the connection and form sealing 
surfaces that resist the flow of fluids between the threads. 
The pin member 11 or the box member 12 defines the longitudinal axis 13 of 
the made-up connection 10. The roots and crests of the box and pin members 
are flat and parallel to the longitudinal axis of the connection and have 
sufficient width to prevent any permanent deformation of the threads when 
the connection is made up. 
As used herein, and as conventionally understood where tubular joints are 
being connected in a vertical position such as when making up a pipe 
string for lowering into a well bore, the term "load flank" designates the 
side wall surface of a thread that faces away from the outer end of the 
respective pin or box member on which the thread is formed. The term "stab 
flank" designates that side wall surface of the thread that faces toward 
the outer end of the respective pin or box member and supports the weight 
of the upper tubular member during the initial make-up of the joint. 
An angle alpha is formed between stab flank wall 18 and root wall 20 of 
thread structure 14. The angle alpha is also formed between stab flank 
wall 19 and root wall 21 of thread structure 15. Flank wall 16 and root 
wall 20 of thread 14, as well as flank wall 17 and root wall 21 of thread 
15, form a second angle beta. The angles alpha and beta are preferably 
acute angles to provide dovetail shaped threads. However, it should be 
realized that a semi-dovetail shaped thread can be provided by defining 
one of the angles, alpha or beta, as ninety (90) degrees, thus making the 
appropriate flank wall perpendicular or normal to longitudinal axis 13 of 
connection 10. 
Roots 21 and crests 25 on pin member 11 are dimensioned to eliminate radial 
clearance with complementary crests 24 and roots 20 of box member 12 
during make-up of connection 10. In other words, thread roots 21 and 
thread crests 25 of pin member 11 come into interference contact with the 
corresponding thread surfaces of box member 12, upon making the joint hand 
tight. The elimination of radial clearance between the respective roots 
and crests of threads 14 and 15 substantially reduces the likelihood of 
creating chambers between the roots and crests during make-up. These 
chambers can temporarily trap the thread lubricant or dope and produce 
false torque readings indicating that the joint has been made up, i.e., 
that the flanks are in engagement as well as the root/crest engagement. 
In practice, however, it has been found that imperfections in the machining 
of threads 14 and 15 can produce regions wherein the thread lubricant 
becomes isolated between sealing surfaces in the connection. For example, 
a preferred embodiment described in the Reeves reissue patent contemplates 
the engagement of stab flanks as the pin is introduced into the box, 
followed by the engagement of the corresponding roots and crests at the 
hand tight position achieved by rotation of pin 11 relative to box 12. 
Pipe thread dope or lubricant can become isolated between regions of 
engagement in the root/crest interface at the hand tight position, and 
thus cannot flow into the clearance that exists between load flanks at 
that time. Continued rotation of pin 11 relative to box 12 reduces the 
volume of the lubricant in the trapped regions, due to plastic deformation 
of the thread surfaces, and causes an increase in the thread lubricant 
pressure. The increased pressure in the isolated regions results in higher 
stress and strain preloads than intended for optimal preload conditions in 
connection 10. 
An embodiment of the present invention is shown in FIGS. 4 and 5 in 
accordance with the wedge thread described in the Reeves reissue patent. 
Like reference characters to those used in FIGS. 1-3, increased by 100 in 
magnitude, are used throughout FIGS. 4 and 5 to simplify and shorten the 
written descriptions. An important feature of the present invention is 
exemplified by groove 130, which provides a pathway through which the 
thread lubricant can vent to prevent the buildup of excessive pressure in 
regions where the lubricant has become trapped between external thread 114 
of pin 111 and internal thread 115 of box 112. As the connection is being 
made up, excess thread lubricant is squeezed into the groove and expelled 
from the connection, enabling the proper stress and strain preload 
condition on the tubular connection at make-up. 
Groove 130 may be a round bottom groove formed in the roots of pin member 
thread 114, such as shown in FIG. 5, and may be of a depth between 0.015" 
and 0.020". Groove 130 can also be formed at the intersection of the root 
and load flank, as seen in FIG. 4, or can be similarly formed in either of 
these ways in box member thread 115 with similar results. The use of a 
round bottom groove machined in root 121 of pin member 111 minimizes the 
stress concentration at the groove, which in turn reduces peak stress at 
the groove and maximizes the fatigue strength of connection 110. The 
locations and shapes of the groove 130 in FIGS. 4 and 5 are shown for 
illustrative purposes only; those skilled in the art will recognize that 
the groove 130 may take any shape and be disposed in any location as may 
be appropriate for a particular application without departing from the 
spirit and scope of this invention. 
Groove 130 is machined in a section of external thread 114 in continuous 
fashion, but in some embodiments is formed only over a limited portion 
(not shown) of the thread. Thus, the groove is only of a length that 
extends from either the beginning or the end of the thread to a point 
between the beginning and end of the thread, providing a limited pathway 
for venting excessive pressure of the thread lubricant that develops 
between the sealing surfaces during rotational make-up. Because the 
pressure reduction path may be employed over a limited portion of the 
thread, the remaining portion of the thread length can be used to form an 
internal and external pressure seal, e.g., in the manner disclosed by the 
threaded tubular connection of the Reeves reissue patent. 
Those skilled in the art will appreciate that the present invention is 
equally applicable to threaded connections wherein slight clearance exists 
between complementary roots and crests, such as described in the Blose 
reissue patent. In such thread structures, e.g., those having clearances 
of 0.003" (0.006" total for both sides of connection) up to 0.006" (0.012" 
total), the thread lubricant can itself form a seal under high pressure 
that resists the flow of fluids across the connection. When tubular joints 
are exposed to severe cold weather environments, such as in the North Sea 
and Canada during the winter months, the thread lubricant becomes somewhat 
incompressible and very difficult to displace from clearances between the 
pin and box members. When the lubricant is unable to flow freely, it is 
subjected to increased pressure by the rotational make-up of the joint. 
The use of pressure reduction groove 130 provides a pathway for the highly 
pressurized lubricant to escape, ensuring that the stress and strain 
resulting from forcible make-up of the connection will not substantially 
exceed the designed preload condition. 
In this regard, it is well known that wear develops in thread surfaces 114 
and 115 as the connection is made up and disassembled several times, and 
such action will gradually permit additional travel of pin member 111 into 
box member 112 and eventually eliminate any radial clearance between the 
corresponding roots and crests. Such wear, however, will have no 
detrimental effect on the operation of groove 130 since the relief path 
will not substantially change shape or size with typical wear in the 
threaded joint. Furthermore, the fact that the relief path is machined in 
the threads independently of the threads themselves makes the present 
invention well suited for application to existing oilfield tubular goods 
with minimal cutting requirements. 
From the foregoing it will be seen that this invention is one well adapted 
to attain all of the ends hereinabove set forth, together with other 
advantages which are obvious and which are inherent to the disclosed 
apparatus and structure. 
It will be understood that certain features and subcombinations are of 
utility and may be employed without reference to other features and 
subcombinations. This is contemplated by and is within the scope of the 
claims. 
Because many possible embodiments may be made of the invention without 
departing from the scope thereof, it is to be understood that all matter 
herein set forth or shown in the accompanying drawings is to be 
interpreted as illustrative and not in a limiting sense. 
For example, although described with regard to a single thread set, the 
present invention is similarly applicable to a tubular connection having 
more than one thread set, such as a two-step thread seal connection. 
While the present invention has been described with respect to a limited 
number of preferred embodiments, those skilled in the art will appreciate 
numerous modifications and variations therefrom. The appended claims are 
intended to cover all such modifications and variations which occur to one 
of ordinary skill in the art.