Note: Descriptions are shown in the official language in which they were submitted.
CA 02303710 2000-04-17
STRAIGHT HOLE DRILLING SYSTEM
The present invention relates to drill strings.
Drill strings are used to transmit rotary and percussive forces from a surface
drilling
s machine to a drill bit. The holes bored may be relatively deep, typically 50
feet, and
accordingly, the string is made from a series of drill rods that are connected
by couplings.
Typically, the ends of each rod are threaded and the coupling is an internally
threaded sleeve
which receives the ends of adjacent rods.
When the strings are used to bore blast holes, for example, it is important
that the holes
1 o are drilled as straight as possible to preserve the required pattern of
the blast holes. Deviation of
the hole during drilling will produce a different pattern at the bottom of the
holes to that at the
surface and the results of the blast will therefore be inconsistent. Deviation
of the drill string is
caused primarily by bending induced by the vertical loads caused by the feed
of the string and
the percussive forces on the string. In some cases, the deviation can be
extreme, resulting in a
i5 deviation of several feet over a 50-foot hole. The effect of this is to
increase the production
costs as the spoil or "muck pile" created from blasting is of a non-uniform
size which may
require additional secondary breaking to accommodate the size of hauling
equipment. It is a
goal of the drilling industry to maintain the deviation to within 1.0% but
this has not been
achieved on a consistent basis.
a o One approach to reducing the deviation is to utilize tube drilling. In
tube drilling, a thin
walled tube that has an outside diameter close to the drilled hole size is
utilized in an attempt to
provide the requisite bending stiffness. However, the thin walled tubing
limits the form of
thread that can be used to connect the tubes with the result that an
unsatisfactory rod life is
obtained.
2 5 An alternative approach is to avoid percussive loads on the threads by
placing the
percussive hammer down inside the hole (DTH systems) and not transmitting the
percussive
shock wave through the connecting string. However, this approach tends to be
more expensive.
It is therefore an object of the present invention to obviate or mitigate the
above
disadvantages.
3o According to one aspect of the invention, a drill string includes a
plurality of drill rods for
CA 02303710 2000-04-17
connection in series and each having at least one thread formed at one end
thereof for connection
to an adjacent rod. Each drill rod includes a cylindrical body having an outer
surface and an
internal bore extending between the ends of the rod. The diameter of the rod
is greater than or
equal to 65 mm and the ratio of the diameter of the body to that of the bore
is at least 2:1 and
s preferably equal to or greater than 3:1.
By increasing the outer diameter of the rod, the bending stiffness of the rod
is increased
but by maintaining the ratio of the bore within the prescribed limits, an
appropriate form of
thread can be provided.
An increase in rod diameter generally has been considered to be detrimental to
the life of
i o the thread used to interconnect the rods as the larger root area
associated with the increased
diameter results in an increased region of high stress concentration and thus
reduces the fatigue
life of the rod. The longitudinal loads are significantly increased if the
string is "rattled" during
withdrawal to assist in loosening the couplings. Rattling imposes rapidly
reversing vertical
loads on the string and the stresses induced may cause failure of the threads,
particularly the
i5 initial thread which is more heavily loaded than the other threads.
Previous attempts have been made to reduce the stress concentrations by
adopting
specific profiles for the threads. In USP Nos. 5,056,611; 5,060,740; and
5,163,523, the thread
is formed with a part elliptical root which, according to these patents,
provides a larger effective
root diameter and therefore reduced stress concentration. However, in each
case, the thread
z o profile is asymmetric and to maintain an adequate wear volume, the
ellipses used to define the
roots of the male and female threads are of different sizes or configurations.
This unduly
complicates the production of the thread. Moreover, the fatigue life is still
determined by the
highest stress concentration in the threaded structure and so a high stress
concentration at one
root will effectively limit the life of the joint.
z 5 There is therefore a need to provide a threaded joint in which the above
disadvantages are
obviated or mitigated.
According to another aspect of the present invention, there is provided a
threaded
coupling for a drill string having a pair of cooperating male and female
threaded members. Each
of the members has a respective thread with a pair of opposed flanks
converging to a root region.
CA 02303710 2000-04-17
The root region is circular with the flanks tangential to the root radius with
a crown on the
cooperating thread spaced from the root. The female threaded member has a
tapered outer
surface which reduces in diameter towards the distal end of the female member
and extends over
at least one pitch of the thread.
Preferably, the tapered portion has a half angle of between 3° and
10° and, more
preferably, between 4° and 6°.
One factor associated with the life of the thread is the wear volume which is
the volume
of material between adjacent flanks of the thread that can be sacrificed
before the flanks intersect.
This volume can be increased by utilizing a single start thread as the spacing
between adjacent
i o flanks is then increased. However, wear will inevitably occur and can lead
to premature
replacement of the rod.
According to a further aspect of the invention, the coupling between the rods
of the string
includes a sleeve having a female thread for engagement with the thread on the
rod. The wear
volume on the female thread is less than that on the male thread so that
replacement of the sleeve
i s will be required prior to replacement of the rod.
Preferably, the thread is formed such that the longitudinal dimension of the
crest of the
female thread is less than the longitudinal dimension of the crest of the male
thread to provide an
increased wear volume for the male thread. It is preferred that the female
thread has a wear
volume of between 80% and 95% of the male member, more preferably 89% to 95%
and
z o typically, 92%.
According to a still further aspect of the invention, there is provided a
threaded coupling
for a drill string having a pair of co-operating male and female threaded
members. Each of said
threaded members has a single start helical thread with a helix angle of
between 7° and 10° with
a pair of opposed flanks intersecting in a root region. The root region is
circular with the flanks
2 5 tangential to the root. Each flank is delimited by a crest extending
generally parallel to the
longitudinal axis of the thread. The depth of each thread measured radially is
greater than 5 mm
and flanks of the male thread and female thread overlap radially greater than
3.7 mm.
Preferably, the axial dimension of the crest on the female and male threads
differs and as
a further preference, the axial dimension of the female crest is less than
that of the male crest.
CA 02303710 2003-12-11
4
An embodiment of the invention will now be described by way of example only
with
reference to the accompanying drawings, in which:
Figure 1 is a front elevation of a drill string;
Figure 2 is a sectional view of a joint in the string of Figure 1;
s Figure 3 is an enlarged view of the threads in the joint of Figure 2;
Figure 4 is a view similar to Figure 3 with the components separated;
Figure 5 is a sectional view of a sleeve shown in Figure 2; and
Figure 6 is a sectional view of one end of a rod shown in Figure 2.
Referring therefore to Figure 1, a drill string 10 includes a number of drill
rods 12
to connected end to end at joints 13 by sleeves 14. The string 10 is connected
at one end to a drill
machine (not shown) by means of a striker bar 15 and at the opposite end to a
drill bit 11. The
string 10 transmits rotation and percussive loads from the machine to the bit
11 to drill a hole.
As can be seen more clearly in Figures 2 and 6, each of the drill rods 12 is
cylindrical
having an outer diameter dl and a male thread 16, 18 formed at opposite ends
17, 19 for
is connection to the sleeve 14. Each of the ends 17, 19 is identical and,
accordingly, only one will
be described in detail with reference to Figure 6. The thread 16 is formed on
the outer surface
and extends from a planar end face 21 to a waist 23. The end face 21 projects
axially beyond the
thread 16 to provide a cylindrical button 25 of diameter less than the thread
16. The waist 23 has
a diameter less than the thread and is uniformly radiused with tangential
flanks extending to the
2 o rod surface and the thread 16. A bore 27 extends internally through the
rod 12 and has an inner
diameter d,.
Referring to Figure S, the sleeve 14 has a complementary pair of female
threads 20, ?? to
receive the threaded ends 17, 19 of adjacent rods 12. The female threads 20,
2? are formed on
the inner wall 26 of the tubular body 24 of the sleeve 14. The threads 20, 22
terminate at spaced
as annular recesses 28, 30 that are separated by a land 32 that projects
inwardly into the body ?=1.
The outer surface 34 of the body 24 has a central cylindrical section 36 and,
in the preferred
embodiment, tapered sections 38, 40 at opposite ends that reduce in diameter
from the central
section 36 to the distal ends of the sleeve 14.
The thread form can best be seen in Figures 3 and 4 and has a similar though
not identical
CA 02303710 2003-12-11
form for the male and female threads so that like reference numerals will be
used to denote
similar components. The thread is a single start thread and each of the
threads 16, 18, 20, 22
(Figure 2) has a root 42 defined by a part circular generatrix with linear
flanks 44 extending
tangentially from the root 42 at an angle a to the longitudinal axis of the
rod 12. The linear
s flanks 44 are truncated by a flat crest 46 that extends between adjacent
flanks 44 and is smoothly
radiused to merge with the flanks 44. It will be noted from Figure 3 that when
assembled, the
crest 46 is spaced from the root 42 so that only the linear flanks 44 of the
male and female
threads are in contact to transfer the loads. Each of the threads has a wear
volume 48, indicated
by chain dot lines, defined between adjacent flanks 44 as the minimum
longitudinal spacing
to between the portions of the flank in contact with the other thread and the
opposition directed
flank and which defines the volume of material that may be removed before the
flanks 44
intersect.
In the specific example shown, the outside diameter d, of the rod 12 is 68 mm
and more
generally should be greater than 65mm to reduce the deflection resulting from
the vertical loads
imposed. The bore 27 has an internal diameter of 22 mm to provide a ratio of
outside to inside
diameter of greater than 3:1. More generally, the ratio should be greater than
2:1 to provide the
requisite stiffness and allow formation of the thread 16.
The tapered sections 38, 40 of the sleeves 14 each extend over several pitches
of the
thread 16 and, preferably, extend at least one pitch and typically over 2.5 to
3.5 pitches. The
z o angle of the taper, i.e. half the cone angle, is selected to distribute
the loads among the threads.
The taper 38, 40 matches the longitudinal elasticity of the sleeve 14 more
closely to that of the
threads 16 so that the maximum stress on the thread profiles is mitigated. In
one example, a
taper of 4.2° has been found beneficial and more generally tapers in
the range of 3° to 10° may
be utilized, preferably in the range of 4° to 6°. The taper
angles referred to above are the ankle
zs of the surface to the longitudinal axis, i.e. are half the included cone
angle. Although a tapered
section on the sleeve is preferred for optimum stress mitigation, the tapers
may be omitted anti
satisfactory operation of the coupling obtained.
The thread form selected may be of any suitable form, but a flank angle of
3~° with a
pitch of 1.179 inches (approximately 3.0 mm) and a helix angle of between
7° and 10°,
CA 02303710 2000-04-17
6
preferably 8.9°, has been found satisfactory with a nominal major
diameter of 68 mm.
The major diameter of the male thread was 2.660 inches and a circular root
profile of
appropriate radius was utilized. A thread depth from crest to root of greater
than 0.2 inches (S
mm) has been found satisfactory.
s It will be noted from Figures 3 and 4 that the longitudinal dimension of the
crests 46
indicated at l, and 12 differ between the male and female threads. The crest
46 of the female
threads 20, 22 (1z) is shorter than the male thread by up to 20% of the crest
on the male thread.
The crest 46 of the female threads 20, 22 may be between 80% and 100% of the
crest 46 of male
thread 16 (1,) typically between 89% and 95%, preferably 92%. The difference
in lengths of the
to crests 46 proportions the available wear volume between the sleeve 14 and
rod 12 so that there is
less wear volume on the sleeve 14. In this manner, the sleeve will require
replacing prior to the
rod, thereby preserving the life of the rod 12. A thread depth of greater than
5 mm
(approximately 0.2 inches) is preferred and preferably greater than 5.3 mm
(i.e. 0.21 inches). To
provide the requisite wear volume, the male and female threads should
preferably overlap greater
15 than 3.8 mm (i.e. 0.15 inches) and more preferably, greater than 3.8 mm
(i.e. 0.15 inches) and,
more preferably, greater than 0.43 mm (i.e. 0.17 inches).
To connect a pair of rods 12, the sleeve 14 is threaded onto the thread 16 at
end 17 until
the end of the thread abuts the side of the land 32. In this position, the
button 25 overlaps the
land 32. The end 19 of the next rod 12 is threaded in to the sleeve 14 until
it abuts the side of
a o land 32. In that position, the end faces 21 are in contact. As the torque
is applied to the string
10, the joint 13 tightens and imposes a longitudinal stress upon the threads.
Upon mating of the male and female threads, the radiused root 42 is spaced
from the crest
46 of the engaging thread and so out of the contact area between the flanks
44. The root 42
provides an area of maximum stress concentration which is spaced from the
contact area and
a s accordingly is considered beneficial. The tapering of the sleeve 14 also
contributes to the
reduction in maximum stress by matching the longitudinal elasticity of the
sleeve 14 to the stress
distribution in the threads.
The string 10 as exemplified above has been found to provide significant
advantages over
those utilizing thread forms presently widely available such as that known as
HM51. In
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7
particular, the wear volume 48 is increased 2.9 and 3.5 times respectively and
the maximum
tensile stress at the critical location is 10% less than that of the joint
provided by available thread
and sleeve combinations for a given impact load. Finite element analysis
indicates that even
with an impact force that is 50% greater than that experienced in the HM51
joint, the maximum
s tensile stress at the critical location is still 22% less than that
previously encountered. The
reduction in stress is, of course, indicative of an increased life of the
joint 10.
It will be seen, therefore, that the provision of the tapered sleeve to match
the longitudinal
elasticity to the stress distribution in the threads does provide a reduction
in the stress levels at
the critical locations and that the spacing of the crown and radiused root
ensure that the
i o maximum stress concentration is remote from the contact area. Accordingly,
a larger diameter
rod can be provided to increase the stiffness and enhance the straight line
drilling performance of
the rod.
It will be appreciated that whilst the joint 13 has been illustrated with a
sleeve as the
female member of the female thread could be formed integrally with one end of
the rod.
i5 However, the provision of a separate sleeve enables it to be replaced
without sacrificing the
entire rod at the end of its useful life.
