Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DTH-Hammer Drilling Device And Overburden Drilling Method
The present invention relates to a drilling device, in
particular by use of a down-the-hole hammer, and a drilling
method in particular for overburden drilling.
Drilling devices denoted "down-the-hole hammer" have a rotating
percussion drilling head which is lowered into the borehole.
Other terms used therefor are "Inhole-hammer" and "DTH-hammer",
which term will be used in the following.
In the context of this application, overburden drilling is
intended to denote a drilling method wherein a tube providing a
casing for the borehole is dragged along at least in the region
of the bottom of the borehole.
In the published German patent application DE 43 10 726 Al a
drill rig with a DTH-hammer is described wherein compressed air
is supplied as a flushing medium from a peripheral conduit of
the drill rig to a centre of the drill bit using a distributor,
and the cuttings are lifted through a conduit arranged centrally
in the drill rig.
The known device further has a skirt open at its lower end in
the area of the hammer, through which the compressed air
enriched with the cuttings is fed to the distributor.
In US patent application US 2005/0103527 Al a dual wall drill
pipe is described, the inner part of which is formed as a hose
through which the cuttings are transported upwardly, while
drilling mud is urged downwardly through the space between inner
and outer hoses.
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In the International application WO 02/081856 Al a drilling head
is described, in which the flushing medium is supplied outwardly
to a rim of the drill bit, and the cuttings are transported from
a central part of the drill bit to a periphery of the drilling
head.
It has been found that the known drilling devices on the one
hand have a need for high flushing medium flow rates, and that
on the other hand, in some applications the drill depth is
insufficient, the drill speed is partly not satisfactory, or the
provided borehole diameter is small.
It is therefore an object of the present invention to provide a
drilling device and method overcoming the drawbacks of the prior
art; e.g., by having an increased efficacy, i.e. provide a
greater drill depth or a larger borehole diameter faster or with
lesser flushing medium flow rates.
According to a first aspect, the drilling device according to
the invention has a DTH-hammer with a percussion drill bit, a
drilling tube with a waste conduit for the cuttings and at least
one supply conduit for the flushing medium, and an overburden
tube connected to the drill bit.
According to an embodiment of the invention, above a
distributor, a ring is mounted preferably rotatably around the
drill pipe, on which ring at least one, preferably two to five
disks are arranged as a seal. In particular, rubber disks
separated by smaller diameter plastic disks are used, where an
outer diameter of the rubber disks corresponds to, i.e. is about
equal to or slightly larger than, an inner diameter of the
overburden tube. The tile of rubber and plastic disks is
preferably held together by two steel disks of a lesser
diameter, which are screwed together. In an axial direction, the
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seal assembly of the disks and a ring supporting same is
preferably secured by a flange or the like.
By this measure, it is achieved that flushing medium pressure
losses through gaps in the rock are reduced. An additional
pressure loss due to the borehole volume's increase during
drilling is avoided. As a result, the energy transfer medium
throughput necessary to drive the hammer is sufficient to serve
as a flushing medium after leaving the hammer, largely
independently from the nature of the surrounding rock (gaps,
water).
Furthermore, the invention provides a method of overburden
drilling using air lifting. According to this aspect, the method
according to the invention comprises supplying the flushing
medium, in particular peripherally to the carrying away of the
cuttings, at least over a predominant part of the drilling
depth, and dragging along an overburden tube adjacent the
surrounding rock. Herein, it is preferred that the overburden
tube does not rotate, or rotates more slowly relative to the
surrounding rock than a centrally arranged drill pipe for the
supply and carrying away of the flushing medium.
In a preferred embodiment, the method according to the invention
further comprises the sealing of the borehole bottom against the
borehole opening, and in particular the dragging along of the
sealing packing. The packing rotates relatively to the
surrounding rock at most with the speed of the drill pipe, and
at least with the speed of the overburden tube.
According to another aspect, a rotative-percussive drilling
method is provided, which comprises the supplying of energy by
means of an energy transfer medium to a hammer arranged in the
borehole concurrently with rotating the hammer.
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According to a still further aspect, a drilling device according
to the invention has a reduced free cross-section around the
hammer at most ten times as large as the free cross-sectional
area within the waste conduit.
Further advantageous embodiments and details are disclosed in
the dependent claims, the subsequent description and the
enclosed drawings. The claims are intended to be a non-limiting
approach to defining the invention in general terms.
The drawings show:
Figure 1 a drilling device according to the invention in a
lengthwise sectional view;
Figure 2 a drilling device according to the invention in
cross-sectional view;
Figure 3 a plug as used with the invention in cross-sectional
view;
Figure 4 a sketch overview of a drilling device according to
the invention;
Figure 5 another drilling device according to the invention in
a partial lengthwise sectional view.
In the embodiment according to Figures 1 and 2, the drill rig 1
comprises a drill pipe 2 with a central waste conduit 3 and a
peripherally arranged air supply conduit 4. To the lower end of
the drill pipe 2, a connecting piece 5 is welded, in which a
DTH-drill head 6 is screwed with its central connector 7, which
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comprises a compressed air supply opening 8 through which the
DTH-drill head 6 is supplied with compressed air for driving its
percussion drill bit 9. The diameter of the drill bit 9 is
larger than the diameter of the drill pipe 2 and is larger than
the diameter of the drive part of the DTH-drill head 6. In the
connecting piece 5, a supply conduit section 10 leads from the
centrally arranged waste conduit 3 of the drill pipe 2 obliquely
downwards to a laterally arranged inlet opening 11. An air duct
section 13 leads from the peripheral air conduit 4 of the drill
pipe 2 via a connection annulus 111 to a central area of the
connector 7 of the drill head.
The percussion drill bit 9 is driven with compressed air, which
in the example is provided by a compressor station 60 located
adjacent the borehole opening, to perform a hammering action
while being rotated by the drill rig 1. The drill bit 9 includes
a central pilot bit 9a and a surrounding reaming bit 9b. The
pilot bit 9a and the reaming bit 9b are connected to one another
by a releasable bayonet coupling. The reaming bit 9b is
rotatably, and preferably coaxially, connected to an overburden
tube 12 via a casing shoe 15, thereby dragging along the
overburden tube 12 during drilling. The air flowing from the
borehole bottom back to the borehole opening, aided by water
already present in the borehole or additionally supplied, serves
as a flushing medium and maintains a flow velocity in the
central waste conduit 3 sufficient to carry away the cuttings.
The flow velocity exceeds 30 m/s, and is preferably 70-80 m/s,
in particular about 75 m/s.
In the embodiment according to Figure 1, the drill bit 9 further
comprises a supply duct section 41 for compressed air 42 flowing
from the hammer 6 into the drill bit 9, a supply duct 43, a
waste duct 44, a bypass (choke shunt) 45 and a waste discharge
duct 46 for compressed air 47 flowing back towards the surface.
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At the face of the drill bit 9, drill elements 50a and 50b of
the pilot and reaming bit, respectively, are arranged. The
reaming bit 9b is, on the one hand, releasably reconnected to
the pilot bit 9a via the securing elements 51, 52, 53, and on
the other hand is rotatably connected to the casing shoe 15, so
that the reaming bit 9b is able to drag along the casing shoe 15
as well as the overburden tube 12 welded thereto. Thereby, the
overburden tube 12 does not rotate, or rotates much more slowly
relative to the surrounding rock than the drill bit 9, depending
on the friction provided by the borehole wall and the torque
applied through the drill bit 9.
Compressors 60 having a combined throughput capacity of 30 to
80 m3/min, preferably 50 to 70 m3/min, serve as the compressed
air supply for an 18"-DTH-hammer and a drill pipe having 200 mm
outer tube diameter and 125 mm inner tube diameter. With
increasing drilling depth and grain size of the cuttings,
accordingly larger air supply rates are used. Also, the supply
rates scale with hole diameter.
The borehole is clad with the overburden tube 12 at least in the
area of the hole bottom and beyond the hammer 6. Above the
connector 5, a plug 18 (Figure 3) is set as a sealing packing
which tightly fits between the overburden tube 12 and the drill
pipe 2 and is carried downwards during drilling along with the
drill pipe 2. The plug 18 in the embodiment shown comprises
three resilient disks 20 supported in a spaced apart manner by
two rigid disks 22 of a lesser diameter. Two to four or five
rubber disks are preferred, spaced apart by a number of plastic
disks one less. The pile of rubber disks and plastic disks is
held on both sides by steel disks 24 of the same diameter as the
plastic disks 22, and six threaded bolts 28 passing through
bores 26 in the disks, and nuts. The stack of disks is arranged
on a mounting ring 30 and fastened with bolts 32 accessible
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through a flange hole 31, whereby the mounting ring 30 is
rotatably mounted on the drill pipe 2 and is fixed in axial
direction by a fixing ring 34 welded to the drill pipe 2. By
this arrangement, the mounting ring 30 does not rotate with the
drill pipe 2, or possibly rotates slowly, while the rubber disks
20 contact the inner wall of the overburden tube 12. Relative to
the surrounding rock, the sealing disks of the plug 18 rotate
with a speed of at least that of the overburden tube and less
than that of the drill pipe 2. In an alternative embodiment (not
shown), instead of the disk pile, an inflatable tube ring
("packer") is arranged on the mounting ring 30. In another not-
shown variant, no overburden tube is present, and the rubber
disks accordingly seal against the borehole wall.
As the overburden tube 18, for example a pipe of dimensions
711 (28")x8 mm is used, into the inner diameter of which of
695 mm, the rubber disks 20 of the plug are fit. The rubber,
steel and plastic disks of the plug 18 each have a thickness of
15 mm. These disks are mounted on a bronze ring 30 as shown in
Figure 3, the bronze ring 30 in turn being rotatably sled onto
the drill pipe 2 of outer diameter 200 mm, and secured in axial
direction by a fixing ring 34.
Above the connector 5, a drill collar 36 (see Figure 4) is
arranged for damping the upward hammer action and for thereby
directing hammer action downwardly. The drill collar 36 is, for
example, formed as a part of the drill pipe 2 with an outer
sleeve filled with lead. The plug 18 is arranged above or
preferably, below the drill collar 36.
In the above described example, the overburden tube is a
contiguous pipe string, which is continuously appended during
drilling and may be left in the borehole after drilling. In an
alternative drilling device, the overburden tube is not appended
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beyond the plug, but is closed at its upper end by a e.g.
frustoconical lid piece. In this case, alternatively the lid
piece is formed as a sealing packing, so that the plug is
dispensible. Herein, the lid piece rotatably seals against the
drill pipe.
In another embodiment (see Figure 5), a sleeve tube 14
surrounding the hammer 6 is arranged below the lateral inlet
opening 41, wherein the sleeve tube is closed below the inlet
opening 41, preferably at its lower end, and preferably is
double-walled. In this example, the sleeve tube 14 extends
upwardly somewhat beyond the inlet opening 41, namely up to the
upper rim of the connector 5, wherein a duct 115 is formed in
the sleeve tube 14 for the lateral inlet opening 11. Thereby,
the flushing medium is forced to flow through the constricted
annular space (outer shaded area in Figure 2) between the sleeve
tube 14 and the overburden tube 12. Due to the constricted free
cross section, the flow speed at the height of the hammer 6 is
increased so as to secure the blowing out of the cuttings.
In the method according to the invention, the drill bit 9 is
rotated, while an energy transfer medium, e.g. a portion of the
flushing medium, is supplied through the drill pipe 2 to the
hammer 6 for transmitting a percussive action onto the rotating
drill bit 9. Along with the drill bit 9, an overburden tube 12
and a sealing packing 18 between the overburden tube 12 and the
drill pipe 2 is dragged, and the cuttings are flushed away
through the drill pipe 2. The overburden tube may be
continuously supplemented at its upper end at the borehole
opening, or may the closed at its top after mounting the seal.
In the former case, the overburden tube may be left in the
borehole after conclusion of the drilling, or may be taken out
of the hole. In the latter case, there is the advantage that the
frictional resistance provided by the dragged-along overburden
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-9-
tube remains independent of the drill depth, and therefore does
not set a substantial limit thereto.
The drilling methods of the invention have the advantage that by
carrying along the plug 18 above the air distributing connector
5 for redirecting the flushing medium, and by dragging along an
overburden tube 12, a sufficient pressure gradient between the
borehole bottom and the borehole opening for blowing out the
cuttings, is maintained: On the one hand, the flushing medium
cannot escape into gaps in the surrounding rock, and on the
other hand, the volume to which the pressure is applied remains
constant during drilling, and is relatively small.
In another preferred embodiment, the flow-through cross-section
for the cuttings is constricted in the area of the hammer, if
necessary by means of a sleeve tube surrounding the hammer, in
such a manner that the cross-section is at most 10 times as
large as the inner cross-section of the waste conduit of the
drill pipe (inner shaded area in Figure 2), on the other hand
preferably not less than same, in particular is at least four
times as large, and particularly preferably is six to eight
times as large, such that even coarse cuttings can pass through.
The necessary constriction may also be achieved by using an
oversized hammer, which is preferably choked below its nominal
operation pressure (e.g. <6.9 bar) so as not to damage the drill
bit due to its nominally excessive energy output. Also, the
cross-sectional shape of the constricting device may be other
than cylindrical, e.g. may have an undulating circumference
providing a number of alternating more and less constricted
portions, respectively.
It is preferred to form the overburden tube, the waste conduit
of the drill pipe and the optional sleeve tube circular-
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cylindrically, in particular concentrically. In this case, the
three lengths (see Figure 2):
A outer diameter of the hammer, or of the sleeve tube
arranged around the hammer if present;
n=B = 1.5 to 4 times, preferably 2.5 to 3 times the inner diameter
B of the waste conduit for the flushing medium above the
distributor; and
C inner diameter of the overburden tube, or of the bore hole
if no overburden tube is present in the bottom hole section,
when arranged to a geometrical shape, form an acute-angled
triangle or, preferably, a right-angled triangle with sides A,
n=B und C. Also, e.g. C > 5.5 B, preferably C > n=B.
By these measures it is ensured that the cuttings are blown
peripherally past the hammer into the drill pipe, and do not
accumulate in the borehole.
The optional sleeve tube is preferably single or dual walled and
may comprise a duct or a cut-out connecting the inlet opening of
the distributor with the annular space surrounding the sleeve
tube. Further, the sleeve tube may extend at its upper end up to
the drill pipe or the distributor, and at its lower end down to
the lower end of the hammer. Preferably, the lower end of the
device or tube is closed, but it suffices if the closure is
located below the distributor inlet.
Thereby, it is achieved that the cuttings entrained in the
flushing medium flow around the sleeve tube to the inlet opening
of the distributor with a velocity sufficiently high so as to
accelerate the cuttings to its lifting speed.
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In a preferred embodiment, the drilling method is carried out in
a first section, at a relatively larger hole diameter, using
such a sleeve tube; and after reaching a certain depth of the
borehole, a second section is drilled with a smaller hole
diameter without the sleeve tube, or with a smaller diameter
sleeve tube. Into the upper, wider hole section, there can e.g.
be installed a detention casing. In this manner, the entire
drilling operation can be accomplished with largely the same
components and with accordingly reduced requirements in terms of
space, time and cost.
