Note: Descriptions are shown in the official language in which they were submitted.
WO 2022/180226
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Coupling for connecting downhole tubulars with reduced stress
Field of invention
The present disclosure generally relates to a cambered thread for a drill
string coupling and
in particular, although not exclusively, for a drill string utilized for
percussive rock drilling.
Background
EP 1 511 911 / US 8,066,307 discloses a screw joint for a drill run or drill
string for
percussive rock drilling including male and female screw threads on the
elements to be
joined together to form a drill string, characterized in that the male thread
and the female
thread have a trapezoidal shape; that the threads have a conical inclination
along the length
of the threads with a cone angle smaller than 7 degrees, and in that the flank
angles
between the flanks of the threads and the line that is tangential with the
apices of the
threads is smaller than 45 degrees.
US4121862 discloses a tubular connection having a tapered pitch diameters at
the entrance
and exit of the thread. US2006/118340 discloses a screw joint having a
trapezoidal shape
and slight conical inclination.
The conical thread of the EP '911 patent is not optimal for distributing
bending load evenly
across the length thereof nor does the conical thread result in expedient
coupling and
uncoupling time. The wear resistance of the conical thread of the EP '911
patent leaves
room for improvement. EP3536894 discloses a cambered thread wherein the crest
and the
root of the thread are each cambered about a respective first and second
camber radius
along the entire length of the thread-form, and each camber radius is greater
than an outer
diameter of the coupling, which provides improved coupling characteristics and
stiffness
when subject to bending loads. However, there is a requirement to provide a
coupling with
a reduced level of stress, has a reduced risk of premature breakage and
increased the
lifetime.
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Summary of the invention
The present disclosure generally relates to a cambered thread for a drill
string coupling and
in particular, although not exclusively, for a drill string utilized for
percussive rock drilling.
In one embodiment, a coupling for connecting downhole tubulars includes: a
tubular body;
a female coupling part; a male coupling part; and at least one of: a male
screw thread
formed on an outer surface of the body, and a female screw thread formed in an
inner
surface of the body. The at least one thread has a thread-form including a
crest, a root, and
a pair of flanks. The crest and the root are each cambered about a respective
first and
second camber radius. Each camber radius is greater than an outer diameter of
the
coupling characterized in that each camber radius (Rb, Rb-T) is between 700 -
1900 mm,
preferably between 800 - 1700 mm, more preferably between 900 - 1500 mm, even
more
preferably 1050 ¨ 1400 mm, even more preferably 1100 ¨ 1300 mm
Advantageously, this provides a threaded coupling wherein the stress is
reduced as low as
possible as an average along the length of the thread. Consequently, the wear
along the
length of the thread occurs more evenly due to an even distribution of contact
pressure
while bending so that premature failures are less likely. If the radius is too
small the stress
concentrations become too large and will result in premature failures/reduced
tool life, and
if the camber is too small it will not couple the threads tight enough for it
to stay together
while drilling.
Preferably, the male screw thread has a camber radius, Rbm, and the female
screw thread
has a camber radius, Rbf, and the ratio of the male thread camber radius to
the female
thread camber radius, Rbm/ Rbf, 1S > 1.0 and <1.1, preferably between 1.01 and
1.05, even
more preferably between 1.01 to 1.03. Advantageously, this provides a threaded
coupling
wherein the stress is reduced as low as possible as an average along the
length of the
thread. Further, within this ratio the stress is most evenly distributed along
the length of the
thread and the presence of concretion peaks of high stress in localised
regions are avoided.
Consequently, the wear along the length of the thread occurs more evenly and
so
premature failures are less likely to happened and the lifetime of the parts
are therefore
increased.
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In one aspect of the embodiment, each flank is straight, and each flank is
connected to an
adjacent crest and/or root by a respective arc.
In another aspect of the embodiment, a centerline of the thread-form
perpendicular to an
arc of each camber radius is inclined relative to a longitudinal axis of the
coupling by an
acute and nearly perpendicular first angle adjacent to a start of the at least
one thread and
inclined by a second angle adjacent to an end of the at least one thread, and
the second
angle is less than the first angle.
In another aspect of the embodiment, each camber radius is at least 5 times
greater than an
outer diameter of the coupling. In another aspect of the embodiment, the
thread-form is
asymmetric. In another aspect of the embodiment, the thread-form is
trapezoidal. In
another aspect of the embodiment, a sweep angle of the at least one thread
ranges between
one and 10 degrees. In another aspect of the embodiment, the root and the
crest are
concentric. In another aspect of the embodiment, an arc length of the root and
an arc
length of the crest are equal. In another aspect of the embodiment, an arc
length of the root
and an arc length of the crest are not equal.
In another aspect of the embodiment, a connection includes: a female coupling
part having
the female thread; and a male coupling part having the male thread and screwed
into the
female thread. One of the flanks is a contact flank and the other flank is a
non-contact
flank when the couplings are in compression. Each flank is straight. Each
thread-form has
a centerline perpendicular to an arc of each respective camber radius. Each
flank has a
flank angle inclined relative to the respective centerline. Each contact flank
angle is
greater than the respective non-contact flank angle.
Optionally, each non-contact flank angle is less than 45 degrees. Since the EP
'911 patent
defines flank angle as being measured from the thread apex, then the EP '911
teaching
translates to flank angles being greater than 45 degrees. Minimizing the non-
contact flank
angle facilitates uncoupling and facilitates transmission of the shockwave
during
uncoupling.
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In another aspect of the embodiment, each coupling is made from a metal or
alloy. The
male coupling part has an outer diameter portion, a reduced diameter portion
having the
male thread, and a shoulder connecting the two portions. The shoulder is
engaged with an
end of the female coupling part to form a metal to metal seal.
In another aspect of the embodiment, a drill rod for percussive drilling
includes: a rod
body; a female coupling part having the female thread and welded to a first
end of the rod
body; and a male coupling part having the male thread and welded to a second
end of the
rod body. Optionally, an outer diameter of the couplings ranges between 5 and
20
centimetres, and each camber radius is greater than one meter.
In another aspect of the embodiment, a drill string includes a drill rod.
Brief description of drawings
A specific implementation of the present invention will now be described, by
way of
example only, and with reference to the accompanying drawings in which:
Figure 1 illustrates a drill rod having a male coupling and a female coupling,
each coupling
including a cambered screw thread, according to one embodiment of the present
disclosure;
Figures 2A illustrates a cambered helix for designing the cambered threads.
Figure 2B
illustrates parameters of the cambered threads;
Figures 3A-3G illustrate formulas for the cambered helix;
Figure 4 illustrates a profile of the male cambered thread;
Figures 5A and 5B are enlargements of portions of Figure 4;
Figure 6 illustrates a profile of the female cambered thread;
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Figures 7A and 7B are enlargements of portions of Figure 6;
Figure 8 illustrates the male and female couplings screwed together.
Figure 9a-c show safety factor images captured using the Dang van criterion
using rotating
bending as the load case. Figure 9a show the Dang van criterion when the
camber radius is
inside the preferred range while Figure 9b and 9c shows the Dang van criterion
when the
radius is above and below the preferred range respectively.
Detailed description
Figure 1 illustrates a drill rod 1 having a female coupling 2 and a male
coupling 4, each
coupling including a respective cambered screw thread 2t, 4t, according to one
embodiment of the present disclosure. The drill rod 1 may be made from a metal
or alloy,
such as steel. The drill rod 1 may also be case hardened, such as by
carburization. Each
coupling 2, 4 may be attached, such as welded 5, to an intermediate rod body 3
so as to
form longitudinal ends of the drill rod 1. Each weld 5 may be seamless, such
as a friction
weld. The drill rod 1 may have a flow bore formed therethrough. The drill rod
1 may have
a length of 6 meters.
A drill string (not shown) may be formed by screwing together a plurality of
drill rods 1
together (Figure 8) along with a drill bit at one end and a shank adapter at
the other end.
The drill bit and shank adapter may also have either of the cambered screw
threads 2t, 4t.
The drill string may be used for percussive rock drilling with a top hammer
(not shown) or
downhole hammer (not shown). If a downhole hammer is used, the hammer may have
each of the cambered screw threads 2t, 4t for assembly as part of the drill
string.
Alternatively, the drill rod 1 may have a pair of male couplings 4 and a
sleeve (not shown)
having a pair of female couplings 2 may be used to connect a pair of drill
rods together.
Alternatively, the cambered screw threads 2t, 4t may be used to connect other
types of
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downhole tubulars, such as oilfield drill pipe, oilfield casing or liner,
oilfield production
tubing, or oilfield sucker rod.
The male coupling part 4 may have a tubular body with an outer diameter upper
portion for
connection to a lower end of the rod body 3, a reduced diameter lower portion
having the
external male thread 4t formed in an outer surface thereof, and a shoulder 4s
connecting
the upper and lower portions. The upper portion of the male coupling part may
have a
plurality of wrench flats formed in an outer surface thereof. The flow bore in
the upper
portion may include a nozzle and a portion of a throat. The throat may extend
through the
shoulder 4s and the lower portion.
The female coupling part 2 may have a tubular body with a lower portion for
connection to
an upper end of the rod body 3 The female coupling part 2 may have the
internal female
thread 2t formed in an inner surface thereof adjacent to the flow bore
thereof. The flow
bore may be sized to receive the reduced diameter lower portion of the male
coupling part
4 of another drill rod (Figure 8). The male coupling part 4 may be screwed
into the female
coupling part 2 until the shoulder 4s abuts a top 2p of the female coupling,
thereby creating
a metal-to-metal seal for isolating the flow bore and fastening the two drill
rods together.
The flow bore of the female coupling part 2 may include a diffuser located
adjacent to a
lower end of the female thread 2t.
Alternatively, the male coupling part 4 may be connected to an upper end of
the rod body 3
and the female coupling part 2 may be connected to a lower end of the rod
body. In this
alternative, the nozzle of the male coupling part 4 would be a diffuser and
the diffuser of
the female coupling part 2 would be a nozzle.
Figures 2A illustrates a cambered helix 6 for designing the cambered threads
2t, 4t. Figure
2B illustrates parameters of the cambered threads 2t, 4t. Figures 3A-3H
illustrate formulas
for the cambered helix 6. To design the cambered threads 2t, 4t, one or more
thread
parameters, such as a start diameter Do, an end diameter Di, and a (linear)
length L, may be
specified utilizing dimensions of the drill rod 1. Once the thread parameters
have been
specified, a camber radius Rh may be calculated utilizing the formula of
Figure 3A. The
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camber radius Rb may extend from a centerpoint Cp and may define crests of the
male
thread 4t and roots of the female thread 2t. The thread parameters may be
specified such
that the camber radius Rb is greater than, such as 5 or 10 times greater than,
an outer
diameter of the coupling parts 2, 4. The outer diameter of the coupling parts
2, 4 may
range between 5 and 20 centimeters and the camber radius Rb may be greater
than one
meter, such as ranging between 1,05 meters and 1,7 meters.
Once the camber radius Rb has been calculated, a sweep angle y may be
calculated utilizing
the formula of Figure 3B. The sweep angle y may range between one and ten
degrees.
Once the sweep angle y has been calculated, the cambered helix 6 may be
generated using
the parametric formulas of Figures 3C-3G. The cambered helix 6 may be used to
define an
outline of the cambered threads 2t, 4t. In the parametric formulas, R(t) may
be a radial
coordinate of the cambered helix about a longitudinal axis GL of the drill rod
1 The
convention of the formulas of Figures 3E-3G may be negative (shown) for a left-
handed
thread and positive for a right-handed thread.
The female 2t and male 4t threads may be complementary such that the male
thread of one
drill rod 1 may be screwed into the female thread of another drill rod (Figure
8). To
facilitate screwing and unscrewing of the threads 2t, 4t, the male 4t and
female 2t threads
may be similar but not be identical mirror images of each other. The above
discussed
design process may be performed once for the female thread 2t and again for
the male
thread 4t. Each of the female 2t and male 4t threads may be double threads.
To reduce the stress in the threaded coupling each camber radius Rb, Rb-T
should be
between 700 - 1900 mm, preferably between 800 - 1700 rum, more preferably
between 900
- 1500 mm, even more preferably 1050 ¨ 1400 mm, even more preferably 1100 ¨
1300
mm..
Preferably, the ratio of the camber radius on the male thread Rbm to the
camber radius on
the female thread Rbf should be between >1.0 and <1.1, preferably between 1.01
and 1.05,
even more preferably between 1.01 to 1.03.
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If the ratio is <1.0, then a high concentration of stress in the region of a
first male thread 20
will be present. If the ratio is above 1 .1 then a high concentration of
stress in the region of
the endmost male thread 22 will be present.
Alternatively, the cambered threads 2t, 4t may be right-handed threads.
Alternatively, each
of the female 2t and male 4t threads may be a single thread or triple threads.
Figure 4 illustrates a profile 7m of the male cambered thread 4t. Figures 5A
and 5B are
enlargements of portions of Figure 4. Once the outline of the male thread 4t
has been
generated, the profile 7m may be determined. The profile 7m may start at a
standoff
distance X0 from the shoulder 4s. The profile 7m may end at a point where the
crest of the
profile intersects an axis Gt parallel to the longitudinal axis CTL and offset
to the end
diameter Di. The sweep angle 7 may define the arcuate extent of the profile 7m
from start
to end and may range between one and 10 degrees.
Referring specifically to Figure 5A, a thread-form of the profile 7m may
include a first
crest Al. The thread-form may have a trapezoidal shape. The first crest A1 may
be an arc
with the (outer) camber radius Rb and may extend to a second arc A2. The
centerline CL
may be inclined relative to the offset axis Gi at an acute and nearly
perpendicular first
angle 60. The second arc A2 may have a radius less than one percent of the
outer camber
radius Rb. The second arc A7 may extend from the first crest Ai to a non-
contact flank Ei .
The second arc Az may be tangential to the first crest Ai and the non-contact
flank El.
The non-contact flank El may be a straight line inclined at a first flank
angle a relative to
the centerline CL. The first flank angle a may range between 35 and 55 degrees
or the first
flank angle may be less than 45 degrees. The non-contact flank El may extend
from the
second arc A2 to a third arc A3. The third arc A3 may have a radius less than
one percent of
the outer camber radius Rb. The third arc A3 may extend from the non-contact
flank El to
a first root A4. The third arc A3 may be tangential to the non-contact flank
Et and the first
root A4. The thread-form may have a height T between the first root A4 and a
second crest
A7. The first root A4 may be an arc with an inner camber radius Rb-T and may
extend from
the third arc Az to a fifth arc As. The height T may be less than one percent
of the outer
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camber radius Rb such that the inner camber radius Rb-T is also greater than
the outer
diameter of the male coupling part 4, as discussed above for the camber
radius. The first
root A4 may be concentric with the first crest Ai. The centerline CL may be
perpendicular
to an arc of each camber radius Rb, Rb-T.
The fifth arc A5 may have a radius less than one percent of the camber radius
Rb. The fifth
arc A5 may extend from the first root A4 to a contact flank E2. The fifth arc
A5 may be
tangential to the first root A4 and the contact flank E2. The contact flank E2
may be a
straight line inclined at a second flank angle 13 relative to the centerline
CL. The second
flank angle 13 may range between 40 and 45 degrees. The first flank angle a
may be less
than the second flank angle 13, thereby resulting in an asymmetric thread-
form. The contact
flank E2 may extend from the fifth arc A5 to a sixth arc A6. The sixth arc A6
may extend
from the contact flank E7 to the second crest A7. The sixth arc A6 may be
tangential to the
contact flank E2 and the second crest A7. The second crest A7 may be an arc
with the outer
camber radius Rb =
The thread-form may have an (arc length) pitch P between a start of the
profile 7m and a
center of the second crest A7. The first crest Ai may have an arc length Xi
which may also
be equal to one-half the arc-length of the second crest A7. The first root A4
may also have
an arc length equal to twice that of the arc length Xi.
Alternatively, the crests and roots may have different arc lengths.
Alternatively, the
second flank angle may be less than 45 degrees.
Referring specifically to Figure 5B, due to the camber of the profile 7m about
each camber
radius Rb, Rb-T, the centerline CL of the thread-form adjacent to the end of
the profile 7m
may be inclined relative to the offset axis Gi at a second acute angle 61
which is less than
the first angle 60.
Figure 6 illustrates a profile 7f of the female cambered thread 2t. Figures 7A
and 7B are
enlargements of portions of Figure 6. Once the outline of the female thread 2t
has been
generated, the profile 7f may be determined. The profile 7f may start at a
standoff distance
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X0 from the top 2p. The profile 7f may end at a point where the root of the
profile
intersects an axis GI parallel to the longitudinal axis GL and offset to the
end diameter Di.
The standoff distance Xo of the female profile 7f may differ slightly from the
standoff
distance of the male profile 7m. The sweep angle 7 may define the arcuate
extent of the
profile 7f from start to end and may range between one and 10 degrees.
Referring specifically to Figure 7A, a thread-form of the profile 7f may
include a first root
Ai. The thread-form may have a trapezoidal shape. The first root Ai may be an
arc with the
outer camber radius Rh and may extend to a second arc A2. The outer camber
radius Rb of
the female profile 7f may differ slightly from the outer camber radius of the
male profile
7m. The centerline CL may be inclined relative to the offset axis Gi at an
acute and nearly
perpendicular first angle 60. The second arc Az may have a radius less than
one percent of
the outer camber radius Rb The second arc A7 may extend from the first root Ai
to a non-
contact flank El. The second arc A2 may be tangential to the first root A1 and
the non-
contact flank El. The non-contact flank Ei may be a straight line inclined at
a first flank
angle a relative to the centerline Cr. The first flank angle a may range
between 35 and 55
degrees.
The non-contact flank El may extend from the second arc A2 to a third arc A3.
The third
arc A3 may have a radius less than one percent of the outer camber radius Rb.
The third arc
A3 may extend from the non-contact flank Ei to a first crest A4. The third arc
2^v3 may be
tangential to the non-contact flank El and the first crest A4. The thread-form
may have a
height T between the first crest A4 and a second root A7. The first crest A4
may be an arc
with an inner camber radius Rb-T and may extend from the third arc A3 to a
fifth arc A5.
The inner camber radius Rb-T of the female profile 7f may differ slightly from
the inner
camber radius of the male profile 7m. As shown by the pair of phantom lines
extending
from endpoints of the first crest A4, the centerline Cr may extend through a
midpoint of the
first crest A4. The centerline Cr may be perpendicular to an arc of each
camber radius Rb,
Rb-T. The height T may be less than one percent of the outer camber radius Rb
such that
the inner camber radius Rb-T is also greater than the outer diameter of the
female coupling
2 as discussed above for the camber radius.
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The fifth arc A5 may have a radius less than one percent of the outer camber
radius Rb.
The fifth arc A5 may extend from the first crest A4 to a contact flank E2. The
fifth arc A5
may be tangential to the first crest A4 and the contact flank E2. The contact
flank E2 may
be a straight line inclined at a second flank angle p relative to the
centerline CL. The
second flank angle 13 may range between 40 and 45 degrees. The first flank
angle a may be
less than the second flank angle ft thereby resulting in an asymmetric thread-
form. The
asymmetric thread-form is further illustrated by proj ections of the flanks
El, E2 intersecting
at a point offset from the centerline CL. The second flank angle 13 of the
female profile 7f
may differ slightly from the second flank angle of the male profile 7m. The
contact flank
E2 may extend from the fifth arc A5 to a sixth arc A6. The sixth arc A6 may
extend from the
contact flank E2 to the second root A7. The sixth arc A6 may be tangential to
the contact
flank E2 and the second root A7. The second root A7 may be an arc with the
outer camber
radius Rb
The thread-form may have an (arc length) pitch P between a start of the
profile 7m and a
center of the second root A7. The first root Ai may have an arc length Xi
which may also
be equal to one-half the arc-length of the second root A7. The arc length Xi
of the female
profile 7f may differ from the arc length of the male profile 7m. The first
crest A4 may
also have an arc length less than twice that of the arc length Xi.
Alternatively, the roots and crests may have the same arc lengths.
Referring specifically to Figure 7B, due to the camber of the profile 7m about
each camber
radius Rb, Rb-T, the centerline CL of the thread-form adjacent to the end of
the profile 7m
may be inclined relative to the offset axis Gi at a second acute angle 61
which is less than
the first angle 60.
Reference to the contact flanks E2 and the non-contact flanks El is for the
context of
drilling when the drill string is in compression. When tripping the drill
string from the
drilled hole and unscrewing the drill rods, the drill string is in tension and
the contact
flanks E2 become the non-contact flanks and the non-contact flanks El become
the contact
flanks, as shown in Figure 8.
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Figure 8 illustrates the male 4 and female 2 coupling parts screwed together.
Once the
thread profiles 7m,f have been generated, each profile may be adapted to
create the
geometry of the respective cambered thread 4t, 2t, such as by truncation
thereof. The
cambered curvature of each thread 2t, 4t along the longitudinal axis GL may
result in an
frusto-ogive shape.
Figure 9a-c show safety factor images captured using the Dang van criterion
with rotating
bending as the load case for couplings having different camber radius sizes,
as described in
Table 1 below:
Figure Rb (mm) Dang van
9a (inventive) 1050 7.0
9b (comparative) 2000 6.5
9c (comparative) 666 4.8
Table 1: Dang van criterion
The images were captured using the Dang van criterion. Using implicit analysis
in LS-
Dyna the Dang van criterion is extracted using the nCode software. The risk
for failure is
increased as the value of the Dang van criterion in decreased. Thus, darker
colours mean
higher risk for failure. For the camber radius that is inside the preferred
range it is possible
to balance the Dang van criteria and reach a high value in the whole female
thread. For
both the the radius that is above the preferred range in 9b and when the
radius is below the
preferred range in 9c it is not possible to balance the Dang van critera and a
lower value is
reached and thus a higher risk of failure.
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