Note: Descriptions are shown in the official language in which they were submitted.
I
TOOL JOINT
Background of the Invention
____,
Field of the Invention
_,
This invention relates to new and useful improvements in
threaded tool joints for connecting pipe together and more
particularly to a tool joint having low stress, and reduced
susceptibility to fatigue failure and to over-torquing during
use.
Brief Description of the Prior Art
.. ..
Threaded tool joints for connecting pipe together are
commonly used in drilling operations in the petroleum incus-
try. These tool joints consist of a male pin member which
makes up into a female box member. One ox the major problems
with tool joints is the failure of the joint as a result of
fatigue after an extended period of use.
"Tool joint", as used herein, is any threaded connection
I in a drill string used for petroleum or mining purposes, such
as intercJal or welded on joints, drill collars, stabilizers,
reamers r etc.
Previous methods of solving the fatigue problem with
threaded tool joints have involved principally the use of
surface preening methods and also stress relief grooves intro-
duped near the shoulder of the pin. These methods have some
limited success but generally are concerned with the effect of
high stress concentrations in such regions as the thread root
and shoulder to land interface.
Higbee U. S. Patent 658,087 discloses a tapered threaded
coupling which produces a jamming union when coupled with
other threaded members.
Van Don Wesley U. S. Patent 3,050,318 discloses a box and
pin type threaded -joint in which the bearing face of the
threads changes along the length of the threaded joint.
Hjalsten U. S. Patent 3,388,935 discloses a threaded
drill rod element having a shallow thread which varies in the
amount of force applied to the threads along the length
thereof.
Mud U. S. Patent 4,113,290 shows a threaded connection
for large diameter casing in which the thread taper and the
thread profile changes the point of application of pressure
along the length of the thread.
Holmberg U. S. Patent 4,295,751 discloses still another
threaded connection in which the threads are designed to vary
the amount of force applied on the individual threads. This
threaded connection is designed for use in a percussion drill.
Over-torquing o-f the tool joint in use is a problem in
some formations. It has been suggested to utilize a second
pair of shoulders which abut at approximately full make up of
the joint to greatly increase the torque needed to over-torque
the joint. See Arthur U. S. Patent 2,532,632. Lucy no-
Coors careful machining of a second pair of shoulders at
close tolerance.
Summary of the Invention
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One of the objects of this invention is to provide a new
and improved threaded tool joint characterized by low stress
and reduced susceptibility to fatigue failure when made up
tight.
Another object of this invention is to provide an imp
I proved tool joint having a tapered pin with a relieved thread
root which results in lower local stresses and reduce suscep-
tabulate to fatigue failure.
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Another object of this invention is to provide a new and
improved threaded tool joint having a tapered thread con fig-
unction which distributes the stresses over a larger number of
threads.
Still another object of this invention is to provide a
new and improved tool joint having a tapered threaded pin with
a thread configuration providing for more even distribution of
load and reduced susceptibility to fatigue failure and which
fits a standard APT box thread.
Another object is to provide a new and improved threaded
tool joint in which over-torquing during use is prevented by
creating a radial interference of the pin threads into the box
threads during make up of the joint.
Other objects of this invention will become apparent from
time to time throughout the specification and claims as
hereinafter related.
Brief Description of the Drawings
Figure 1 is a view in longitudinal section of a typical
prior art tool joint;
Figure 2 is an enlarged view in cross-section of the
threaded connection of the prior art tool joint shown in
Figure l;
Figure 3 is a reproduction of a photograph of a fatigue
break in a prior art tool joint as seen from the side;
Figure 4 is a reproduction of a photograph of the fatigue
break of the tool joint shown in Figure 3 as seen from the
end;
Figure 5 is a detail, enlarged sectional view of a prior
art threaded tool joint connection shown in exaggerated fox
the tendency to load the leading edge and unload the trailing
edge of the thread when fully made up;
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Figure 6 is an enlarged view of the pin thread shown in
Figure 5 with a theoretical graph or plot of the distribution
of stresses linearly along the threads;
Figure 7 is a view in section of a basic APT thread form
for purposes of establishing nomenclature;
Figure 8 is a theoretical graph or load distribution
curve showing the distribution of load by thread number
starting from the shoulder of the pin;
Figure 9 is a sectional view of a pin thread modified in
accordance with this invention to provide stress relief at the
thread root;
figure 10 is a sectional view of a pin thread, sub Stan-
: tidally as shown in Figure 9, illustrating the maximum desired
amount of relief at the thread root;
Figure 11 is a sectional view of a box and pin threaded
connection showing the fitting of the threads at the beginning
of the loading conditions.
Figure 12 is a theoretical graph of stress distribution
curve showing the distribution of stresses by pin thread
number as measured from the pin shoulder for the standard APT
thread and the thread produced in accordance with the prefer-
red embodiment of this invention;
Figure 13 is a view in elevation of a pin blank and
fragment of a box blank showing the different taper of the
threads; and
Figure 14 is a sectional view on a threaded pin showing
both the taper and the relieved root construction of the
thread.
Description of the Preferred Embodiment
This invention relates -to new and useful improvements in
tool joints for connecting pipe together, end particularly
pipe used in the drilling operations in the petroleum incus-
try. A tool -joint connector consists of a male pin meter
which makes up into a female box member The separate combo-
newts of the tool joint may be welded to drill pipe and
provide the collections for assembling the pipe together. The
invention is particularly concerned with an improved design
which reduces the susceptibility of the tool joint pins to
fatigue failure and prevents over-torquing of the joint during
use.
Referring to the drawings by numerals of reference, and
more particularly to Figure 1, there is shown a typical prior
art tool joint 10 consisting of a tubular pin member 11 and a
tubular box member 12. The pin member 11 has a tapered,
threaded pin portion 13 which makes up into a tapered threaded
box portion 14 on box member 12. The threaded connection is
customarily made up tight so that the shoulder 15 on pin
member 11 makes a tight engagement with the end face 16 of the
box member 12. Additional details of the prior art pipe tool
joint connection are shown in Figures 2, 3, 4, 5, 6, 7 and a.
In a tightly made up tool joint as shown in Figure 1,
most fatigue failures occur at the last enraged thread of the
pin approximately three-fourths inch from the shoulder In
Figure 1 the fatigue break line would start at about 17 on the
thread of the pin. In Figures 3 and 4 pin member 11 is shown
with a fatigue break along the line 18 from about thread 17.
Typically, a fatigue failure begins at the root radius of the
last engaged thread and over a period of time, depending upon
the severity of use, a crack will propagate from this root
region toward the bore 19 of the pin until failure occurs. In
Figures 3 and 4 the fatigue crack and break line is shown in a
photographic view of the broken pin.
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Current theory with respect to fatigue failures suggests
that such failures result from a cyclic plastic flow. The
highly loaded thread develops hairline cracks in a high stress
region at the root of the threads and the cracks are propagate
Ed to lower stress regions over a period of time as a result
of periodic applications of stresses during the normal opera-
lion of the tool. It would seem then that the susceptibility
of fatigue failure could be lowered by reducing the local
stress intensity at the thread roots of the pin thread In
the prior art methods of solving the problem of fatigue
failure in tool joints took a substantially different apt
preach. Prior art methods involved the use of surface paneling
methods (e.g. Hi Flex threads) and also stress relief grooves
provided near the shoulder of the pin. These approaches have
had only limited success since they generally deal with the
effect of high stress concentration in such regions as the
thread root and shoulder to land interface. The preferred
embodiment o-f this invention is directed to solving the cause
of the stress concentratiorl~ and reduces the stress concentra-
lions drastically by specific geometric changes in the thread
structure and engagement of the threads which not only aids in
reducing stress concentration, but also inhibits over-torquing
during use.
In APT specification for Rotary Drilling Equipment IAPI
Spew 7 Thirty-Third Edition December 1981), issued by the
American Petroleum Institute, Production Department, 211 North
Ervay, Suite 1700, Dallas, Texas 75201, there are set forth
the industry standards and specifications for rotary drilling
equipment and various components thereof. Section 9, pages
23-25, of the APT specification covers Rotary Shouldered
Connections. This section deals with the specification for
threaded connections as used in tool joints for drill pipe.
Sections 10-12, pages 26-32, cover Gaging Practice, Gage
Specification and Gage Certification for Rotary Shouldered
Connections. Section 9 of the APT specification gives the
detailed dimensions of thread size and taper for APT threaded
connections for rotary shouldered connections.
n Figure 2 there is shown a typical oil field tool joint
thread form which is the APT numbered connection with V.038R
thread form having a taper of two inches per foot and four
threads per inch. In the standard APT thread the thread form
geometry is equally distributed about a centerline through the
thread root which is perpendicular to the thread axis and no
compensation is made for the load side of the thread. This is
seen in Figure 2. In the thread form shown in Figure 2 (APT
standard thread V.038R~ the pin 11 has threads 13 shown fitted
into the threads 14 of box member 12 (roots and crest shim in
phantom). The root 20 between pin threads 13 is shown to be
at the base of a sixty degree angle between the adjacent
thread walls. The thread walls are distributed uniformly
thirty degrees on either side of a vertical line (thread
centerline) running through the center of the thread. The
root of this particular thread has a radius of 0.038 inches
about a center lying on the thread centerline. The root is
trUIlcated Ox 038 inches The root 21 of the box threads 14 has
a radius which is substantially the same as the root 20 of the
pin threads 13. Pin threads 13 are truncated as indicated at
22 with edge or corner radiuses 23 having a length of about
0.015 inches.
When this standard APT thread is loaded by tightening the
joint, the load flank 24 is placed under compression against
the flank 25 of box thread 14 and the unloaded flank 26 of the
pin thread 13 becomes relatively unloaded. This is shown in
Figure 5 in a somewhat exaggerated showing of the clearance
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I
between the unloaded flank 26 and the adjacent unloaded plank
of the box thread 14.
In Figure 6 the calculated consequences ox this nonuni-
form loading of the cross-section of the first two pin threads
becomes apparent In the upper portion of Figure 6 the
application of load to -the pin load flank 24 it shown along
load lines 27. In the stress distribution curve or graph set
forth at the bottom of Figure 6 the maximum calculated surface
stress intensity is plotted for various points along the
surface of the pin thread. From the stress distribution
curve, it is seen that the stress ranges from 0 or less on the
unloaded side 26 of the thread up to a maximum at the root 20
of the thread. The loading of the thread cross-section
develops a highly asymmetric stress distribution at the thread
root 20. With these asymmetric stress distributions on the
threaded root, the cyclic application of forces to the thread-
Ed joint in the normal use of the drill string will ultimately
cause cracks to start and propagate from the thread root.
This ultimately results ion a fatigue break as shown in Figures
3 and 4.
As a result of the study of stress distributions in the
pin threads and the fact that the loading flank of the pin
threads becomes substantially loaded while the unloaded flank
becomes unloaded, it was concluded that certain modifications
could be made in the thread form to produce a thread design
which would have a more uniform distribution of stresses and
of thread loading.
In a standard APT thread figure 7) the pin member 11 has
threads 13 which have a crest diameter indicated by the dotted
line 28 and a root diameter indicated by the dotted line 29.
The pitch diameter is indicated by dotted line 30. The radius
is indicated by the line 31 which represents the radius of the
or
root 20 of the thread 13. Roy center of radius line 31 lies
on the thread centerline 40 which is perpendicular to the
thread axis and to the central axis of the pin. The crest,
pitch, and root diameter line are parallel and indicate the
taper of the threads. The root is truncated the distance from
point 42 to root diameter 29. When this standard thread is
fully made up with the shoulder 15 of the pin member 11
engaging the end face 16 of the box member 12, a highly
asymmetric distribution of load is carried on the pin threads.
In Figure 8 the theoretical percentage of total load carried
by the threads is plotted against the thread number starting
from the shoulder. The thread loading distribution is nonillion-
ear and almost sixty percent of the load is carried on the
first three threads and the remainder of the load is carried
on the next eleven threads. With this exceptionally high
loading near the shoulder 15, it is not surprising that the
region of the first and second thread is the point where most
fatigue failure occurs.
In modifying the APT thread to make a low stress thread,
several changes are preferably made. First, the modified
thread on the pin preferably is capable of mating with a
standard APT box thread since the modified pin thread prefer-
ably is compatible with the box threads on equipment already
present in the field. Next, it is preferred to modify the
radius of the pin threads to relieve the stress at the root of
the threads. Third, it is preferred to modify the thread
taper to create an artificial pitch difference between the pin
and box member and provide for a more even distribution of
loading of the thread when fully made up. Fourth, this new
geometry preferably creates a radial interference starting
from the threads remote from the shoulder to prevent over-
tonguing.
go
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The first modificatiorl of the thread structure to be
considered is the modification of the root radius. this is
illustrated in Figures 9 and 10 of the drawing and to some
extent in Figure 14~ In the modified thread structure the
root radius indicated by arrow 31 at the root 20 of the pin
threads 13 is increased approximately fifty percent. Where
the root radius 31 of a typical standard APT thread for four
threads per inch and two inches taper per foot may be 0.038
inches, the root radius in the modified thread structure shown
in Figure 9 is preferably 0.057 inches. This enlarged and
gentler curve 32 in the root portion 20 may continue or may
blend smoothly into another curve 33 of substantially shorter
radius 34. In this case radius 31, which is 0.057 inches,
blends into radius 34, which is 0.032 inches. The shorter
radius curve or the single radius curve 33 is terminated at a
flat surface 35 which diverges outwardly at an angle of about
five degrees from a plane normal to the thread axis to a point
where it engages or intersects the normally sloping wall of
the unloaded thread.
Thus the radius preferably does not extend beyond a plane
41 (Figure 9) parallel to the thread centerline and passing
through the unloaded flank on the root side of the pitch
diameter of the unloaded flank. To provide for wear of tools
the line 35 between the root and unloaded flank preferably
diverges outwardly at about a five degree angle to the thread
centerline.
It should be noted that any substantial increase in root
radius of the root 20 of an APT thread will have some effect
in reducing the localization of force or stress at the root of
the thread.
In the conventional APT thread planes forming extensions
of the opposing flanks intersect the thread centerline at 42
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(figure 7). APT specifications call for truncation TV the
root by providing a root radius equal to the distance between
the root diameter I and point 42. In other words, the
selected root radius and root truncation art equal and the
center of the root radius lies on the thread centerline 40.
For instance, in the V.038R thread form, both the root trunk
lion and root radius are 0~038 inches.
In accordance with this invention the root radius 31 is
greater and preferably substantially greater than the root
truncation. Further, to provide that the load flank be
tangent to the root radius, the center of the radius will lie
on the unloaded flank side of the thread centerline. Any
increase of the root radius over root truncation will result
in a decrease in maximum stress and a substantial increase
such as 0.057 inches for the V~038R thread form is preferred
to obtain a substantial decrease in maximum stress. This
larger root radius should extend to at least the thread
centerline 40 and preferably extends to the root diameter 29
of the thread as shown in Figure 10.
The thread form described could only be used for the last
enraged thread on the pin or for any one or more complete
threads. It is preferred that the entire pin thread be of
this form. There are, however, limitations to the amount of
increase in root radius which is possible
The root radius may not be increased to the extent that
it reduces the distance from the root of the thread to the
bore of the tool joint substantially since this will result in
a substantial weakening of the tool joint from the loss of
metal. Using the criteria of this invention no more than a
ten percent reduction in pin cross-section under the last
engaged thread will result. This is acceptable. Also, the
root radius should not be increased to a point which is
lo
greater than a distance which would intersect or exceed the
pitch diameter. This condition is shown in Figure 10 and
represents the limiting condition of enlargement of root
diameter. I've limit of enlarged root diameter, which is shown
in Figure 10, is difficult to use in practice since it would
tend to produce an undercut area 36 which would make it
difficult -to use standard cutting tools which move in a plane
normal to the thread axis. It would be possible, of course,
to use this limit of radius with standard tools if the under-
cut area were eliminated and a flat surface introduced, as
indicated by the dotted line 37. It should be noted that
one half the distance along a line extending normally to the
load flank and passing through the pitch diameter at the
unloaded flank is considered to be the limit of increase in
root radius since a further increase in root radius would
change the creakily configuration above the pitch diameter and
thus make the threads incompatible with a standard APT box
thread The pitch diameter represents the depth of punter-
lion of the standard APT thread gage end as long as the thread
configuration above the pitch diameter matches an APT gage,
the thread will mate with a standard APT box thread. Also,
the original thread shear area (base width of the thread form)
preferably is not reduced by more than twen~y-five percent.
Joining the unloaded flank to the thread root by the surface
35 will accomplish this objective. The modification of the
thread below the pitch diameter line has the effect of reduce
in stress concentrations in the thread root 22 without
interfering with the thread fit with the standard APT box
thread.
The thread configuration of this invention may be used at
any point along the pin threads. Any full or complete thread
formed in accordance with this invention should chive improved
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results. As the stress failure usually occurs in the last
engaged thread it is preferred that the last engaged thread
employ the new thread form. It is further preferred that the
entire pin thread system utilize the new thread form as this
should reduce the maximum stresses occurring in the thread
root along the entire pin. Further, as the base width of the
thread form is reduced the threads are not as stiff as stank
dart APT threads. This permits relief of the more highly
loaded threads and redistribution of the load among the
several threads. Thus the high loading of the first threads
may be reduced.
A further change in thread design relates to the duster-
button of loading linearly along the threads. As previously
noted, the load carrying distribution of a standards APT
thread form in a fully made up condition localizes about sixty
percent of the load on the first three threads. As previously
noted, this is the region where most of the fatigue failure
occurs. In this invention, the taper of the base cone 113 on
which the male thread of pin 11 is formed is decreased rota-
live to the box, as shown in Figure 13. The taper of pin at
113 is less than the taper 114 of the box. This results in an
artificial change in pitch which causes the threads remote
from the shoulder to become loaded before the threads near the
shoulder become loaded when the joint is fully made up. In
the standard APT thread used as an example (NC46), the pin
taper is 2.000 inches (plus .030l minus .000) per foot. In
the preferred modified thread design of this invention, the
taper is 1.925 inches (plus .006, minus .000) per foot. In
this modified taper, the end threads it at the tapered end)
of the pin member become loaded before the threads near the
shoulder when the joint is made up. The initial loading
condition is shown in Figure 11 where the threads at the
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I
extreme end away from the shoulder are in a loaded condition
and the threads carry a reduced load for some distance back
toward the shoulder of the pin member The clearance between
pin threads 13 and box threads 14 at the left end of Figure 11
is exaggerated for clarity of illustration. The actual
clearance is so small that it would not show in a drawing of
this type but is sufficient to produce a reduced load condo-
lion on those threads. While the pin taper could be reduced
to a value just above the taper which would result in plastic
flow, a reduction of no more than ten percent, i.e., 1.80
inches per foot in the above example, is believed sufficient
to obtain the objects of this invention.
In Figure 12 there is shown a calculated load duster-
button curve for the modified thread of this invention super-
imposed over the load distribution curve for a standard APT
thread. The standard APT thread is shown in solid line where
the thread of this invention is shown in dotted line labeled
Reed thread. It is seen that the modified thread form has a
lower proportion of load carried on the first six threads than
is carried on the corresponding threads of an APT thread form.
The threads ranging from thread number seven to the end of the
pin carry a substantially higher load than the corre~poncllng
APT thread. This obviously results in a much more even
distribution of stresses along the lencJth of the thread. The
modified thread is shown in Figure 14 which illustrates both
the relieved root structure of the thread and the taper of the
threaded pin.
It will be apparent from the above that top modified
geometry of FicJures 9 and 10 may be utilized with and without
the use of different tapers of the pin and box. Also, the
different tapers may be utilized without the modified thread
profile cJeometry. The use of the modified tapers will prove
I
useful in formations which usually give problems of over-
tonguing joints. For instance, in -tests of standard APT and
the illustrated joint utilizing a two inch taper box and
comparing a two inch tapered pin with a 1.925 inch tapered pin
it was found that the decreased taper pin required a minimum
of four thousand foot pounds of torque to reach shoulder
engagement as compared to one thousand for the two inch
tapered pin. Thus the joint is able to withstand more severe
conditions without over-torquing as this force must be over-
come in addition to the usual forces induced by making up the
shouldered joint. For example, one standard joint may be made
up with seventeen thousand foot pounds of force. With the
1.925 tapered pin the joint may be made up with approximately
an additional three thousand foot pounds of force to provide
the same shoulder engagement. Additionally, the joint will be
improved by redistributing the load through the threads to
reduce top stress load at the last engaged thread of the pin.
Theoretical studies indicated that the maximum stress
load at the last engaged thread should be reduced about thirty
percent with a minimum of about twelve percent of the reduce
lion due to the new thread profile geometry. Actual test with
the pin of Figure 9 having a reduced taper of 1.925 and a root
radius of 0.057 inches made up with 17,000 foot pounds of
torque showed a decrease in maximum stress of about twenty
percent as compared to the standard APT V.038 thread form.
This test was made with strain gauges and measured an average
value in the small root area covered by the gauge. This
confirmed the calculated values.
The modified thread design describec1 above has used a
standard APT thread for purposes of illustration. It should
be obvious to those skilled in the art that the principles of
modification of the thread can be applied to other standard or
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~3425~ `
nonstandard thread designs. The basic elements of the thread
design which result in lower stress at the thread root and
more uniform stress distribution are the enlargement of the
root radius and -the reduction of the pin taper to cause an
artificial change in pitch and a radial interference starting
from the threads remote from the shoulder. In any particular
case the thread has the root radius enlarged as far as posse-
bye without removing sufficient metal to weaken the pin and
without enlarging the radius to the point that the thread
configuration is modified above the pitch diameter. The taper
of the thread is slightly reduced to produce the desired
loading effect. If the principles of thread design discussed
above are to be applied to other types of threads, it is
merely necessary to use those principles in designing the
thread cutting die.
While this invention has been described fully and come
pletely with special emphasis upon a single preferred embody-
mint, it should he understood that within the scope of the
appended claims the inventiorl may }ye practiced otherwise then
as specifically described herein.
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