Note: Descriptions are shown in the official language in which they were submitted.
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High frequency fluidpressure driven drill hammer for percussion drilling
in hard formations
The present invention relates to a fluidpressure driven, high frequency
percussion hammer for drilling in hard formations, which percussion hammer
comprises a housing, which in one end thereof is provided with a drill bit
designed to act directly on the hard formation, which percussion hammer
further
comprises a hammer piston moveably received in said housing and acts on the
drill bit, which hammer piston has a longitudinally extending bore having
predetermined flow capacity, and the bore being closeable in the upstream
direction by a valve plug that partly follows the hammer piston during its
stroke.
Hydraulically driven percussion hammers for drilling in rock have been in
commercial use for more that 30 years. These are used with jointable drill
rods
where the drilling depth is restricted by the fact that the percussion energy
fades
through the joints, in addition to the fact that the weight of the drill rod
becomes
too heavy such that little energy finally reaches the drill bit.
Downhole hammer drills, i.e. hammer drills installed right above the drill
bit, is
much more effective and are used in large extent for drilling of wells down to
2-
300 meter depth. These are driven by compressed air and have pressures up to
approximately 22 bars, which then restricting the drilling depth to
approximately
20 meters if water ingress into the well exists. High pressure water driven
hammer drills have been commercial available more than 10 years now, but
these are limited in dimension, which means up to about 130mm hole diameter.
In addition, they are known to have limited life time and are sensitive for
impurities in the water. They are used in large extent in the mining
industries
since they are drilling very efficiently and drill very straight bores. They
are used
in a limited extent for vertical well drilling down to 1000 ¨ 1500 meters
depth,
and then without any directional control.
It is desired to manufacture downhole drill fluid driven hammer drills which
can
be used together with directional control equipment, which have high
efficiency,
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can be used with water as drill fluid and can also be used with water based
drill
fluid having additives, and having economical lifetime. It is expected great
usage both for deepwater drilling for geothermic energy and for hard
accessible
oil and gas resources.
In percussion drilling, drill bits are used having inserted hard metal lugs,
so
called "indenters". These are made of tungsten carbide and are typically from
8
to 14mm in diameter and have spherical or conical end. Ideally viewed, each
indenter should strike with optimal percussion energy related to the hardness
and the compressive strength of the rock, such that a small crater or pit is
made
in the rock. The drill bit is rotated such that next blow, ideally viewed,
forms a
new crater having connection to the previous one. The drilling diameter and
the
geometry determine the number of indenters.
Optimal percussion energy is determined by the compressive strength of the
rock, it can be drilled in rock having compressive strength over 300 MPa. The
supply of percussion energy beyond the optimal amount, is lost energy since it
is not used to destroy the rock, only propagates as waves of energy. Too
little
percussion energy does not make craters at all. When percussion energy per
indenter is known and the number of indenters is determined, then the optimal
percussion energy for the drill bit is given. The pull, or drilling rate, (ROP
¨ rate
of penetration) can then be increased by just increasing the percussion
frequency.
The amount of drilling fluid pumped is determined by minimum necessary return
rate (annular velocity) within the annulus between the drill string and the
well
bore wall. This should at least be over 1 m/s, preferably 2 m/s, such that the
drilled out material, the cuttings, will be transported to the surface. The
harder
and brittle the rock is, and the higher percussion frequency one is able to
provide, the finer the cuttings become, and the slower return rate or speed
can
be accepted. Hard rock and high frequency will produce cuttings that appear as
dust or fine sand.
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The hydraulic effect applied to the hammer drill is determined by the pressure
drop multiplied with pumped quantity per time unit.
The percussion energy per blow multiplied with the frequency provides the
effect. If we look into an imaginary example where drilling into granite
having
260 MPa compressive strength and drilling diameter of 190mm is performed,
water is pumped by 750 l/min (12,5 liters/second) from the surface. It is
calculated that approximately 900 J is optimal percussion energy.
With reference to known data for corresponding drilling, but with smaller
diameters, a drilling rate (ROP) of 22 m/h (meters per hour) with a percussion
frequency of 60 Hz, can be expected. It is here assumed to increase the
percussion frequency to 95 Hz, consequently ROP then become 35 m/h.
Required net effect on the drill bit then becomes: 0,9 kJ X 95 = 86 kW. We
assume the present hammer construction to have a mechanical-hydraulic
efficiency of 0,89, which then provides 7,7 MPa required pressure drop over
the
hammer.
This hammer drill will then drill 60% quicker and by 60% less energy
consumption than known available water propelled hammer drills.
This is achieved by a percussion hammer of the introductory said kind, which
hammer is distinguished in that the valve plug is controlled by an associated
valve stem slideably received in a valve stem sleeve, said valve stem
comprises
stopping means able to stop the valve plug by a predetermined percentage of
the full stroke length of the hammer piston and separates the valve plug from
a
seat seal on the hammer piston, said bore thus being opened and allows the
bore fluid to flow through the bore.
Preferably the stopping means comprises a stop plate at the upstream end of
the valve stem, and a cooperating internal stop surface in the valve stem
sleeve.
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In one embodiment the predetermined percentage of the full stroke length of
the
hammer piston can be in the order of magnitude 75%.
Conveniently, it is the inherent tension spring properties of the valve stem
that
returns the valve plug, which valve stem being long and slender.
Preferably, the percussion hammer can further be provided with an inlet valve
assembly, which is not opening for operation of the hammer piston until the
pressure is build up to approximately 95% of full working pressure, which
inlet
valve assembly being adapted to close off a main barrel, and a side barrel
within the hammer housing can pressurize an annulus between the hammer
piston and the housing elevating the hammer piston to seal against the valve
plug.
Conveniently, the hammer piston and the valve assembly can be returned by
recoil, where both the hammer piston and the valve assembly are provided with
hydraulic dampening controlling the retardation of the return stroke until
stop.
Conveniently, the hydraulic dampening takes place with an annular piston which
is forced into a corresponding annular cylinder with controllable clearances,
and
thus restricts or chokes the evacuation of the trapped fluid.
Further, an opening can be arranged in the top of the valve stem sleeve, into
which opening the stop plate of the valve stem is able to enter, and the
radial
portions of the stop plate can seal against the internal side of the opening
with
relatively narrow radial clearance.
Further, an annular backup valve can be arranged in a ring groove underneath
the opening, which backup valve being able to open and replenish fluid through
bores in the valve stem sleeve.
The percussion hammer housing can be divided into an inlet valve housing, a
valve housing and a hammer housing.
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The hammer drill construction according to the present invention is of the
type
labeled "Direct Acting Hammer", i.e. that the hammer piston has a closing
valve
thereon, which valve in closed position enables the pressure to propel the
5 piston forward, and in open position enables the hammer piston to be
subjected
to recoil. The previous variant of a hydraulic hammer has a valve system that
by
means of pressure propels the hammer piston both ways. This provides poor
efficiency, but more precise control of the piston.
The key to good efficiency and high percussion frequency, is in the valve
construction. The valve needs to operate with high frequency and have well
through flow characteristics in open position.
With great advantage, the hammer drill construction can also be used as
surface mounted hydraulically driven hammer for drilling with drill rods, but
it is
the use as a downhole hammer drill that will be described in detail here.
Other and further objects, features and advantages will appear from the
following description of preferred embodiments of the invention, which is
given
for the purpose of description, and given in context with the appended
drawings
where:
Fig. 1 shows in schematic view a typical hydraulic hammer drill according to
the
invention,
Fig. 2A shows an elevational view of a downhole hammer drill with drill bit,
Fig. 2B shows the hammer drill of fig. 2A turned about 900
,
Fig. 2C shows a view in the direction of the arrows A-A in fig. 2A,
Fig. 2D shows a view in the direction of the arrows B-B in fig. 2A,
Fig. 3A shows a longitudinal sectional view of the hammer drill shown in fig.
2A
where the internal main parts are shown,
Fig. 3B shows a transversal cross sectional view along the line A-A in fig.
3A,
Fig. 3C shows a transversal cross sectional view along the line B-B in fig.
3A,
Fig. 3D shows a transversal cross sectional view along the line C-C in fig.
3A,
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Fig. 3E shows a transversal cross sectional view along the line D-D in fig.
3A,
Fig. 3F shows a two times enlarged, encircled detail view H in fig. 3A,
Fig. 30 shows a two times enlarged, encircled detail view H in fig. 3A,
Fig. 3H shows a five times enlarged, encircled detail view F in fig. 3A,
Fig. 31 shows a five times enlarged, encircled detail view G in fig. 3A,
Fig. 4A shows correspondingly to that shown in fig. 3A, but at the end of an
acceleration phase,
Fig. 4B shows an elevational view of the valve assembly shown in section in
fig.
4A,
Fig. 4C shows a transversal cross sectional view along the line B-B in fig.
4A,
Fig. 4D shows a five times enlarged, encircled detail view A in fig. 4A,
Fig. 4E shows a five times enlarged, encircled detail view C in fig. 4A,
Fig. 5A shows correspondingly to that shown in fig. 3A and 4A, but in that
moment when the hammer piston strikes against the impact surface in the drill
bit,
Fig. 5B shows a five times enlarged, encircled detail view A in fig. 5A,
Fig. 5C shows a four times enlarged, encircled detail view B in fig. 5A,
Fig. 6A shows correspondingly to that shown in fig. 3A, 4A and 5A, but when
the hammer piston is in full return,
Fig. 6B shows a section along the line E-E in fig. 6C,
Fig. 6C shows a five times enlarged, encircled detail view A in fig. 6A,
Fig. 6C' shows a 20 times enlarged, encircled detail view D in fig. 6C,
Fig. 6D shows a 20 times enlarged, encircled detail view C in fig. 6E,
Fig. 6E shows a four times enlarged, encircled detail view B in fig. 6A,
Fig. 7A shows correspondingly to that shown in fig. 3A, 4A, 5A and 6A, but
when the hammer piston is in the final part of the return,
Fig. 7B shows a 20 times enlarged, encircled detail view B in fig. 7C,
Fig. 7C shows a four times enlarged, encircled detail view A in fig. 7A,
Fig. 8 shows curves that illustrates the working cycle of the hammer piston
and
the valve,
Fig. 9A shows the curve that illustrates the abrupt closing characteristic of
the
valve relative to pressure drop, and
Fig. 9B illustrates flow and pressure drop over the gradually closing valve.
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Fig. 1 shows a typical hydraulic hammer drill for attachment on top of
jointable
drill rods where the hammer mechanism is located internal of a housing 1
constructed by several house sections, where a rotary motor 2 rotates a drill
rod
via a transmission 3 rotating an axle having a threaded portion 4 to be
screwed
to the drill rod and a drill bit (not shown). The hammer machine is normally
equipped with a fixation plate 5 for attachment to a feeding apparatus on a
drill
rig (not shown). Supply of hydraulic drive fluid takes place via pipes and a
coupling 6 and hydraulic return via pipes with a coupling 7.
Fig. 2A and 2B show a downhole hammer drill with drill bit. These will be used
in the following description. The illustrated housing 1 has a first house
section 8
that receives what later on will be described as the inlet valve, while a
second
house section 9 contains a valve, a third house section 10 contains a hammer
piston and the reference number 11 denotes the drill bit. Drill fluid is
pumped in
through an opening or main run 12, and a threaded portion 13 connects the
hammer to the drill string (not shown). A flat portion 14 is provided for use
of a
torque wrench to screw the hammer to/from the drill string. A drain hole 15 is
required for the function of the later on explained inlet valve, outlet hole
16 is
present for return of the drill fluid in the annulus between the drill hole
wall and
the hammer drill housing (not shown) back to the surface. Hard metal lugs 17
are those elements that crush the rock being drilled. Fig. 2C shows a view in
the
direction of the arrows A-A in fig. 2A, and fig. 2D shows a view seen towards
the drill bit 11 in the direction of the arrows B-B in fig. 2A.
Fig. 3A shows a longitudinal section of the hammer drill where the internal
main
parts are: an inlet valve assembly 18, a valve assembly 19 and a hammer
piston 20. The drilling fluid is pumped in through the inlet 12, passes the
inlet
valve 18 in open position through bores 21 shown on section A-A in fig. 3B,
further through bores 22 in section B-B in fig. 3C to a valve plug 23 that is
shown in closed position in section C-C in fig. 3D against the hammer piston
20
and drives the piston to abutment against the bottom portion 24 of the drill
bit.
Section D-D in fig. 3E shows a longitudinally extending spline portion 25 in
the
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drill bit 11 and the lowermost part of the hammer housing 10 that transfer the
torque at the same time as the drill bit 11 can move axially within accepted
clearances determined by a locking ring mechanism 26. This because by blows
of the hammer piston 20 against the drill bit 11, it is only the mass or
weight of
this that is displaced in concert with penetration of the hard metal lugs 17
into
the rock. This is to obtain that as much as possible of the percussion energy
shall be transferred to the crushing of the rock and as little as possible to
be lost
to mass displacement of the relatively light drill bit 11.
The detailed section in fig. 3F showing the inlet valve 18 in closed position
is
taken from H in fig. 3A. When the hammer function is to be initiated, the
pumping operation of the drill fluid in the inlet 12 is commenced. A side, or
branch off, bore 27 through the wall of the valve house 8 has hydraulic
communication with a pilot bore 28 in the mounting plate 29 of the inlet valve
18. The mounting plate 29 is stationary in the valve house 8 and contains a
pilot
valve 30 that is retained in open position by a spring 31. The drill fluid
flows
freely to a first pilot chamber above a first pilot piston 32, the diameter
and area
of which are larger than the area of the inlet 12. During pressure buildup, a
limited moveable valve plug 33 will be forced to closure against a valve seat
34
in the housing 8. Under pressure buildup against closed inlet valve 18, an
annulus 35 between the housing 10 and the hammer piston 20 is pressurized
through the side bore 27, which via longitudinally extending bores 36 in the
valve housing 9 feed an inlet 37, see detailed view F.
The detailed sections in fig. 3H and fig. 31 are taken from F and G in fig. 3A
and
show the abutment of the hammer piston 20 against the inner wall of the
hammer housings 9, 10. The diameter of a piston 38 is somewhat larger than
the diameter of a second piston 39. By the use of the hammer drill to drill
vertically downwards, the hammer piston 20 will in unpressurized condition,
due
to the gravity, obviously creep towards the strike or impact surface 24 in the
drill
bit 11. In this condition there will be clearance between the valve plug 23
and its
seat 40 (see detailed view F) in the hammer piston 20. Accordingly the drill
fluid
will flow freely through the valve at the plug 23, through a bore 41 in the
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hammer piston 20 and the bores 16 (see fig. 2A), and therefore too little
pressure buildup takes place to start the hammer.
The arrangement shown in detailed section in fig. 3F, having closed inlet
valve
18 and pressure buildup in the annulus 35, elevates the hammer piston 20 to
seal against the valve plug 23. Due to the required clearance between the
surface of the piston 38 and the inner wall of the housing 9, drilling fluid
leaks
out in the space above the valve plug 23 through lubrication channels 42 and a
bore 43 such as an arrow shows in detailed view F. In order to prevent that
this
leakage volume shall provide pressure buildup in the space above the valve
plug 23, this is drained through a bore 44 in the valve mounting plate 29 and
an
opening 45 that the pilot valve 30 in this position allows, and further out
through
the drain hole 15. When the pressure has increased to over 90% of the working
pressure the hammer is designed for, the piston force in a second pilot
chamber
46 exceeds the closing force of the spring 31 and the pilot valve 30 shifts
position such as illustrated in fig. 30.
The first pilot chamber above the pilot piston 32 is drained and the inlet
valve 18
opens up. At the same time the opening 45 is closed such that drainage through
the bore 44 is shut off so that pressure is not lost through this bore in
operating
mode. The pressure in the chamber above the hammer piston 20 and the
closed valve plug 23 results in start of the working cycle with instant full
effect.
The arrangement with a backup valve 47 and a nozzle 48 is provided to obtain
a reduced drainage time of the second pilot chamber 46 for thereby achieve
relatively slow closure of the inlet valve 18. This to obtain that the inlet
valve 18
remains fully open and is not to make disturbances during a working mode
since the pressure then fluctuates with the percussion frequency.
Fig. 4A shows the hammer drill at the end of an accelerating phase. The
hammer piston 20 has at this moment arrived at max velocity, typically about 6
m/s. This is a result of available pressure, as an example here just below 8
MPa, the hydraulic area of the hammer piston, here for example with a diameter
of 130mm, and the weight of the hammer piston, here for example 49 kg. The
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valve plug 23 is kept closed against the seat opening of the hammer piston
since the hydraulic area of the valve plug 23, here for example with a
diameter
of 95mm, is a bit larger, about 4%, than the annular area of the hammer piston
shown in section B-B in fig. 4C as 23 and 24 respectively. At this moment the
5 hammer piston has covered about 75% of its full stroke, about 9mm. The
clearance between the hammer piston 20 and the strike surface 24 of the drill
bit is about 3mm, shown in enlarged detailed view C in fig. 4E.
A moveable valve stem 49 having a stop plate 50 now lands on the abutment
10 surface of a stationary valve stem sleeve 51 in the housing 9 and stops
the
valve stem 49 from further motion, as shown in enlarged detailed view A in
fig.
4D, after which the valve plug 23 is separated from the seat 40 in the hammer
piston 20 and thereby being opened. The moveable valve assembly 23, 49, 50
is shown in elevational view in fig. 4B.
The kinetic energy of the valve plugs 23 momentum will by the abrupt stop
thereof marginally elongate the relatively long and slender valve stem 49, and
thereby transform to a relatively large spring force that very quick
accelerates
the valve in return. The marginal elongation of the valve stem 49, here as an
example calculated to be about 0,8mm, needs to be lower than the utilization
rate of the material, which material in this case is high tensile spring
steel. The
mass of the valve plug 23 should be as small as possible, here as an example
made of aluminum, combined with the length, the diameter and the properties of
the material of the valve stem 49, determines the natural frequency of the
valve
assembly.
For practical usages, this should be minimum 8-10 times the frequency it is to
be used for. The natural frequency is determined by the formulas:
k
=-- where k =
2. TT
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The mass and the spring constant have most significance. The natural
frequency for the shown construction is about 1100 ¨ 1200 Hz and therefore
usable for a working frequency over 100Hz.
The shown construction has in this example a recoil velocity that is 93% of
the
impact or strike velocity.
Fig. 5A shows the position and the moment for when the hammer piston 20
strikes against the strike or abutment surface 24 within the drill bit 11. The
valve
plug 23 including the stem 49 and the stop plate 50 are in full return speed,
see
detailed view A in fig. 5B, such that relatively fast a large opening between
the
valve plug 23 and the valve seat 40 on the hammer piston 20 is created, such
that drilling fluid now flows by relatively small resistance through the
longitudinal
bore 41 in the hammer piston 20, see detailed view B in fig. 5C.
The kinetic energy of the hammer pistons 20 momentum is partly transformed
into a spring force in the hammer piston 20, since the piston is somewhat
compressed during the impact. When the energy wave from the impact has
migrated through the hammer piston 20 to the opposite end and back, the
hammer piston 20 accelerates in return. The return velocity here at the start
is
calculated to be about 3,2 m/s, about 53% of the strike or impact velocity,
this
because a portion of the energy has been used for mass displacement of the
drill bit 11, while the rest has been used to depress the indenters into the
rock.
Fig. 6A shows that moment when the hammer piston 20 is in its full return
speed. The valve plug 23 has at this point of time almost returned to the end
stop where the detailed view A in fig. 6C shows the stem 49 including the stop
plate 50 entering an opening 52 in the top of the valve stem sleeve 51.
The detailed view D in fig. 6C' shows how the radial portion of the stop plate
50
seals, with relatively narrow radial clearance, against the internal side of
the
opening 52. A small negative pressure is created in the chamber underneath
the stop plate 50 when the stop plate 50 moves the last 2mm until stop. An
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annular backup valve 58 opens and replenishes liquid through the bore 59. The
confined or trapped volume under the stop plate 50 prevents that the valve
plug
23 performs a recoil motion and remains in position until next cycle starts.
The backup valve 58 of the type "annular backup valve", which in this
embodiment is an annular leaf spring, is chosen since this has little mass and
relatively large spring force and accordingly is able to work with high
frequency.
The detailed view B in fig. 6E shows the relatively large opening between the
valve plug 23 and the valve seat 40 in the hammer piston 20, in order that the
flow of drilling fluid there through takes place with a minimum of resistance.
The
underside of the valve stem sleeve 51 is formed as an annular cylinder pit 53
shown in detailed view C in fig. 6D. The top of the valve plug 23 is formed as
an
annular piston 54, which by relatively narrow clearances fits into the annular
cylinder pit 53. The confined fluid volume is, as the valve returns all the
way to
the end stop, evacuated in a controlled way through the radial clearances
between the annular piston 54 and the annular cylinder 53 plus an evacuation
hole 55. This controlled evacuation acts as a dampening force and stops the
return of the valve in such a way that the valve does not perform recoil
motions.
The same type of dampening arrangement is present on the hammer piston 20.
On the detailed view B is an annular piston 56 shown on top of the hammer
piston 20, in addition to an annular cylinder groove 57 in the lower part of
the
valve housing 9.
Fig. 7A shows the last part of the return of the hammer piston 20. The
termination of the return stroke is dampened in a controlled way until full
stop at
the same time as the valve seat 40 meets the valve plug 23, shown in detailed
view A in fig. 7C. The detailed view B in fig. 7B illustrates how the confined
or
trapped fluid volume within the annular cylinder pit 57 is displaced through
the
radial clearances between the annular piston 56 and a drain hole 60.
The gap between the valve seat 40 and the valve plug 23 needs not to be
closed completely in order that the pressure to build up and a new cycle
starts.
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Calculations show that with an opening of 0,5mm the pressure drop is
approximately the same as the working pressure. This results in that the
surface
pressure on the contact surface between the valve plug 23 and the seat 40
becomes small and the components can experience long life time.
Fig. 8 shows curves that illustrate the working cycle of the hammer piston 20
and the valve. Curve A shows the velocity course and curve B the position
course through a working cycle. For both curves the horizontal axis is the
time
axis, divided into micro seconds.
The vertical axis for curve A shows the velocity in m/s, stroke direction
against
the drill bit 11 as + upwards and ¨ downwards, here the return velocity.
The vertical axis for the curve B shows distance in mm from the start
position.
The curve section 61 shows the acceleration phase, where the point 62 is the
moment when the valve is stopped and the return thereof is initiated. The
point
63 is the impact of the hammer piston 20 against the drill bit 11.
The curve section 64 is the displacement of the drill bit 11 by progress into
the
rock, 65 is the acceleration of the recoil, 66 is the return velocity without
dampening and 67 is the return velocity with dampening. The curve section 68
is the recoil acceleration for the valve, 69 is the return velocity for the
valve
without dampening and 70 is the slow down dampening phase for the return of
the valve.
Fig. 9A shows a curve 71 that illustrates the abrupt closing characteristics
for
the valve with regard to the pressure drop and opening between the valve plug
23 and the seat 40 in the hammer piston. This situation is shown in fig. 9B.
The
horizontal axis is the opening gap in mm and the vertical axis the designed
pressure drop in bar at nominal rate of pumped drilling fluid, which, as an
example here, is 12,5 l/sec. As shown, the closing gap needs to get under
1,5mm before a substantial pressure resistance is received.
