Note: Descriptions are shown in the official language in which they were submitted.
~ ~8~3~
'I'l-ll\US'I' C;l';;~ r~l\ rol~ )r~ L~oi\-r NG rl~OO[.IS
IiACK~,RG ID OF r~E m~:'J.'`~rrION
FIELD OF INVENTION
The present invention ;elates to the provision oE a
system for providing forward thrust for a fluid im~ersed boring
tool. More particularly, the invention provicles the necessary
thrust, a dynamic force, for the operation o-f a rotary boring
tool in situations, such as th~ drilling of generally horizon-
tal bore holes, where the force of gravity does not ac-t eEfec-
tively to provide forward tnrust. The invention is especially
efflcaceous with flexible drill pipe or conduit such as may be
used in drilling bore holes havlng a small radius of curvature.
ESCRIPTION OF THE PRIOR AR~
~ ecently, Esso Resources Canada L-td., ln seeking to re-
cover heavy oil Lrom the deposits at Cold Lake, Alberta drilled
what it claimed to be "the first horizontal well to be drilled
in North America" using conven,lonal drilling equipment in a
special manner described in Oilweek, November 12, 1979, pages
68 to 70. A milled tooth bit rotated by a Dyna-Drill, offered
by the Dyna-Drill Company, a division of Smith International,
Inc., of Irvine, California, was used 'c make hole. Bent subs
provided the angle build up. 'l'he drill collars, rather than
being positioned just above thc- bit, were located in the "more
vertical portlon of the hole to provide weight on bit." Special
"Hevi-wate pipe was used betwee.. collars and bi-t assembly be-
cause ordinary drill pipe cannot be used in compression". Oil
base mud was used to minimize the tendenc~ of the drill pipe to
drag and to reduce the tendency of the curved pipe to stick
against the sicle oE the hole.
7~9
The technology emplo~ l, however, was time-consuming
and costly. Also, there would appear to be limitations as to
how far one could proceed in a hori~ontal direction after it had
been obtained by pushiny a heavv drill:;ng assembly with heavy
drill pipe.
~ Iydraulic power has been used to rotate boring tools
in drilling vertical and devia~ing bore holes Eor many years.
Typical of such tools is the Dyna-Drill. The power generated by
the Dyna-Drill is used, and only used, to rotate the drilling
bit. The system is used with conventional drill pipe and drill
collars to provide the desired weight on bit or thrust.
In U. S. Patent No. 2,251,916 there is disclosed a
system for solution mining of salts occurring in thin layers
where forced circulation of the solvent is necessary to effec~
tively contact and dissolve th. material to be mined. Generally,
the patentee, Roy Cross, discloses the use of horizontally
directed nozzles to direct a stream of fresh solvent, specificall~,
water, against the face of the materiai to be mined, specifically
potash salts. In one form of i,is invention, shown in his Figure
5, Cross schematically shows an electrically driven device
which purportedly will produce horizontal circulation and at the
same time cut away residue salts not dissolved by the solvent.
Rotation of an electric motor shaft is supposed to rotate a
drill bit on one end thereof and a propeller generating forward
thrust on the other end. Since the motor housing is freely sus-
pended and has no resistance to ro-tation, as soon as any torque
is imposed on the shaft by the ;~ropeller or the drill bit, the
housing rather than the shaft wo~ld rotate. Thus, the device
shown is inoperative.
In U. S. Patent 3,~ o2 there is disclosed a device
for cleaning conduits or boriny holes having a rotatably mounted
boring head and containing ports ~ro~ which f~uicl is ejectecl to
LJro~Jide ~orward thrust and to p;ovid~ rota~.ve torque.
I ~81~39
SU~iARY OF T~IE INVE~rrIO~
The instant inventior ~tilize~ ~he principles of a
marine screw propeller to derive thrust forces for the operation
of an earth boring tool. The rnarine screw propeller is normally
used to de~elop the thrust needed to move a -vessel through water.
According to "Principles of ~aval Architecture", Vol. II, edited
by ~ossell and Chapman, and pub,ished by the Society of Naval
Architects and Marine Engineering, "propellers derive the propul-
sive thrust by accelerating the fluid in which they work". The
term "marine screw propeller" as used herein includes any device
rotation of which develops thrust relative to the axis of rotation
by accelerating the fluid in which it works.
Thrust derived from a marine screw propeller in accord-
ance with my invention provides the 'weight on bit' necessary for
earth boring. This thrust ma~ also provide the force required
to advance a conduit, preferably neutrally buoyant in the drilling
fluicd, through which drilling fluids and energy needed for the
boring operation are supplied.
In accordance with the present invention the shaft or
shafts upon which the propeller or propellers are mounted are
caused to rotate either by an electric motor or by hydraulic
'corce derived from the circulating drilling mud. The electric
motor may be either an alternating or a direct current motor.
Either the field or the armature or both of the motor may rotate
and be fixed to a hollow rotating shaft. In the simplest form
of my invention where a single shaft is employed and rotated by
an electric ~iotor, the drilling bit will be fixed to the forward
end of the shaft. Where a plurality of shafts are employecl, the
bit will be fixed to the forw,rd end of the innermost and longest
shaft which usually, though not necessarily, will carry a marine
~crewpropcll~r to c~enerate ad-~-tiorlal tnL~St. Where the rotative
7 3 ~
~orce is hydraulically generat~d a mud ~,otor, such as the Dyna-
Drill, may be used or a portiG-~ oE thc- circulating drilling fluid
may be discharged through jet nozzles ne,ar the propeller blacle
tips to provide rotative torc~ue through the reactive Eorce oE
the fluid ejected at high velocity.
The shaft to which the bit is attached is hollow to
allow the passage of drilling Eluid therethrough to the bit for
discharge therefrom. The drilling fluid serves -to cool the bit
and remove cuttings from the newly formed bore hole. The amount
of drilling fluid circulated should be sufficient to discharge
the functions oE bit cooling and cutting removal but not so great
as to significantly nhibit bit advance. However, the reaction
force of the ejected fluid will be c~reat enough to push the bit
backward if rotation, and hence thrust, ceases. Where bit and
propeller or propellers dre mounted on a single shaft, the bit is
self non-stalling, since as the bit tends to stall forward thrust
rapidly approaches zero and the reactive forces of the discharged
drilling fluid retract the bit. In cases wnere the thrust gener-
ating propeller is on a diEferent shaft than the bit, the rotation
of the bit may be monitored from the surface and, should the bit
stall, the flow of power to the motor may be adjusted to reduce
or terminate propeller rotation and to allow the reactive forces
generated by the dischar~ing drLlling fluid to retract the bit.
Since the drilling system of my invention is primarily
intended for use in drilling substantially horizontal holes, the
conduit for conducting drilling fluid dnd electric power to the
motor and bit combination is so constructed as to be flexible and
capabl~ of conforming to the curvatures of the well bore. On
the other hand it must have sufi-icien-t resis-tance to twisting so
that it will resist and substantially prevent free rotation oE
the motor housing. The use oi ~ neu~ral1v L~uoyant drillinq system
173~
is taught and claimed in my co,~ellding application Serial ~
371,274, filed February 19, l~il. The ~erm "neutrally buo~ant"
as used herein means that the UtillSi ty of a mass immersed in the
drilling fluid is from 70 to 133 per cent oE the density of the
fluid.
BRIEF DF RIPI`ION OF THE D~AWl~GS
Figure 1 is a view, partly in cross-section and partly
schematic, of a hydraulically powerec; propeller bit form of the
invention.
Figure 2 is a frontal view of the propeller bit of
Figure 1 showing the fluid conduits and jet nozzles carried by the
propeller blades in phantom.
Figure 3 is an enlarged cross-sectional view of a pro-
peller blade tal<en on the line 3-3 of Figure 2.
Figure 4 is an enlarged view of the fluid conduit in a
propeller blade in cross-section showing the jet nozzle and the
fluid inlet port in the wall of the hollow shaft on which the
propeller blade is mounted.
Figure 5 shows another form of hydraulically powered
propeller bit, partly in cross-section and partly schematic, in
position for drilling a horizontally deviated well bore with
separate means for rotating the propeller and drilling bit.
Figure 6 shows the details of the spline connection
shown in Figure 5 linking the propeller shaft and the motor
shaft.
Figure 7 shows a form of electric motor driven pro-
peller bit, partly in cross section and partly schematic, in
position for drilling a horizontal extens:ion of a deviated well
bore.
Figure 5 is an enlarged view OL- ~he motor, propeller
and drill bit of FiguLe / showir~ decal;s oE the hollow motor
shat and the mounting of the blt ancl propeller thereon.
i
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73~
Figure 9 shows anoth-L- Eorm oE motor, propeLler
and bit combination employincJ ~wo co-axial motor driven shafts
wherein ~he motor field s a~Eixed to one shaft, the armature
to the other shaft and the two shafts rotate in opposite direc-
tions.
Figure 10 shows a form of my invention w~erein both
the inner shaft and the housing are fixedly connected to a ro-
tating flexible conduit carrying the drilling fluid and electric
power and the rotor of the motor drives the outer shaft carrying
the propeller in a direction independent of the direction of
rot~tion of the conduit and inner shaft.
Figure 11 is a graphic illustration of the relationship
between the speed of rotation of the propeller and the delivered
thrust and useful torque.
In the various figures of the drawings, like parts are
designated by like reference characters.
DESCRIPTION OF P~EFERRED EMBODIMENTS
One form of hydraulically advanced and rotated boring tool
indicated in general by the numeral 11 is shown in Figure 1. The
tool has the general shape of a multibladed marine screw propeller.
Blades 13 and blade tips 15 are connec-ted by a circular tubular
ring 17. Blades and ring are fitted wi~h hardened cutting elements
19 as is the surface around water course 21. Blades 13 may have
the usual marine propeller conEiguration or may be formed into an
asymmetrical hydrofoil shape as indicated in Figure 3, which in-
creases lift/drag ratio. Dri:Lling Eluid passes through hollow
conduit shaEt 23 having fluid passage 25 and through rotary union
27 to propeller shaft33 to which blades 13 are fi~ed, thence into
--6--
1 ~8~73~
passaye 29, I;iguLe 2, ~o be eJ~ eci tanc~entially at high velocity
through jet nozzle 31 to impar~ rotational Eorce to propeller bit
11. Ejected fluid is preventecl fro~ i~pinging upon, and possibly
damaging, ring 17 by anglincJ je-, no~zl~s 31 slightly toward the
rear or by making appropriate notches in ring 17.
Figure ~ shows a cross-sectioll of a fluicl passage ~9 con-
tained in a hydrofoil shaped propeller blade 13, shown in cross-
section in Figure 3. Nozzle 31 is replaceable as is common practice
in oil well drilling. The propeller blade 13 is Eastened to pro-
peller shaft 33, which, as described above, is connected to the con-
duit shaft 23 by rotary union 27, permitting relative rotation
therebetween. Drilling fluid flowing ~hrough passage 25 into the
hollow propeller shaft flows in part throùgh ports 35 into the
fluid passages 29 and exits as a jet stream through nozzles 31.
The remaining porlion of the drilling fluid exits through water
course 21 where it func~ions to lubricate the cutting elements
and wash away the cuttings.
The propeller bit illustrated in Figure~l may be con-
structed from any suitable material. For example, the blades 13
and ring 17 may be of alloy steel while the cutting elements 19
and water courses 21 may be made of a hardened material such as one
of the carbides.
In the form of my invention shown in Figure 5 the rotative
force for propeller blades 13 is derived from a hydraulically
powered motor 61. Flow of pressurized drilling fluid from conduit
shaft 23 through hydraulic -turbine motor 61 causes the turbine
blades 63 and power shaft 65 to rotate, rotating the propeller
blades and drilling bit 67. Suitable hydraulic turbodrill motors
are manufactured and marketed by Neyrfor Alsthom - Atlantique,
Grenoble, France. Thrust generated by the propeller forces the bit
against formation 69 extending the well bore. Drilling flui{l exits
-- 7 --
39
the turbine throuc3h port 7:L ii the turbine shaft wher,ce it flows
forward through the turbine an~l propeller shaft and exits througr,
water courses 21 in the bit. ~esirabl~y, the propeller shaft 33
and power shaft 65 are joinecl by a splined connection 73, shown
in detail in ~igure 6. The male spline 75 àt the end of shaft 65
mates with the female splinecl yrooves 77 in the rear end of pro-
peller shaft 33. Stop 79 at the forward end of the turbine shaft
will engage stop 81 on the aft end of the propeller shaft holdin~
the spline connection together as the propeller advances. Shoul~
the bit 67 and propeller 13 stall, drilling Eluid jetted from the
forward facing water courses 21 will drive the bit and propeller
back until the rear side of stop 81 rests against stop 83. Suf-
ficient retraction of the bit ~o allow it to resume rotation will
thus be obtained without requiring the backing up of the entire
system.
As stated above, one way to overcome a downward trend o.
a well bore being drilled with the drilling sys-tem of the present
invention is to employ a tool and fluid conduit neutrally buoyant
in the drilling fluid. For use in a 10 pounds per gallon drilling
fluid boring tool 11 may be cast from a mix comprised of cycloali-
phatic epoxy 61 parts, ceramic microballoons 31 parts and glass
fibers 5 parts by volume with s-teel trim, additional epoxy cement
and tungsten carbide or other hard material comprising the re-
maining 3 parts by volume. Ring 17 is essentially neutrally
buoyant when made from a length of aluminum tubing hermetically
sealed against fluid entry. Eor example, a one-inch outside
diarneter, round, aluminum tubing, closed at the ends, weighing
0.28 pounds per foot would have a density of 0.83 grams per milli-
liter. ~ 1.5-inch tube weighing 0.635 pounds per foot would have
the same density. Such tubular rings may be roughened and coatecl
with an epoxy with a hard surEace material added to Eorm a cutting
eleMent 19. Other cutting elements 19 are set into small dlameter
1 ~8~3~
holes bored into the epoxy casling a~ter it has hardened using an
epoxy cement. Such neutrally buoy-int tools ~end to be fragile and
mainly are suited to boring sol~ L, shallow strata.
A suitable, low density drilliny fluicl sha~t conduit may
be formed from a glass reinforced polyethylene plastic, having,
for example, a density of 1.20 grams pec milliliter, a tensile
modulus of 25,000 pounds per square inch and a tensile strength,
determined in burst, of 3,600 pounds per square inch.
One form oE electrically powered thrust generator and
boring tool is shown in F~igure 7. Thc- tool is shown in position
to drill a horizontal extension oE well bore 91 in earth formation
93. As shown weli bore 91 is deviated Erom vertical portion 95,
which may be drilled in the conventional manner and, as shown, is
provided with the usual casing 97, generally throughout its vertical
depth, and an upper surface casing 99.
F'lexible conduit 23 provides communication betweer. the
surface and the electric motor, generally indicated by the numeral
24, for -the transmission of drilling fluid and electric power.
Conduit 23 must have flexibility to conform to the curved well
bore, must withstand the pressure differential between the drilling
fluid within it and the returns on the outside, and must be re-
sistant to twisting. It is also desirable that it be neutrally
buoyant in the drilling fluid circulating in the well bore so
that it is essentially weightless and its advance offers minimum
impedance to the thrust generated by the motor-driven propeller
as described hereinafter~ A polyoleEin sucn as polyethylene or
other plastic material, such as epoxy resins or polyamides, re-
inforced with glass, steel or carbon Eibers may be used. Flexible
conduit 23 need not extend all the way to ~he surface but rather
it may be connected to the lower encl o~ conventional drill pipe
26 ~nd lowerecl into the well bo;e ~sing ~ coilventional drilling
7.'~
rig, not shown. The drill pi~e and the conduit are provic]ed
internally with electrlcal con~ ctors 28, as shown in Figure 8,
Eor the transmission of electr1c power from the surEace to motor
24. In operation motor 2~ rotates propeller 13 and drill bit ll
which are mounted on the forward end of ho]low shaft 33. ~lollow
motor shaEt 33 transmits the drilling fluid from conduit 23
through mo-tor 24 to bit ll from whence it elxits via ports 21.
Bit ll is shown as being prov:ided with hardened abrasive inserts
l9, such as tungsten carbide or polycrystaline diamonds to facili-
tate the drilling action, especially in hard earth formations.
Figure 8 shows in more detail the arrangement of the
components that comprise the drilling system of Figure 7 of my
invention. The electric motor comprises a housing 39 which encloses
the field or stator ~l of the motor and within which the armature
or rotor 43 is affi~ed to and rotates shaft 33 in operation.
Annular extensions at the forwdrd end 45 and rear end ~17 of the
housing serve as journal boxes for the hollow shaf-t 33. Rotary
seals 49 and bearings 51 permit the shaft to rotate freely while
preventing drilling fl~id introduced through conduit 23 and
surrounding the motor housing fLom entering the mo~or housing.
~ear housing extension ~7 is externally threaded to receive in-
ternally threaded female coupling 53 fixed to the end of conduit
23- Thus, conduit 23 will resist the tendency of the stator and
motor housing to rotate in a direction counter to the rotation of
shaft 33 as a tor~ue load is imposed throuyh the action of the
drill bit and the propeller. Electrical conductor 28 which is
carried by conduit 23 passes through coupling 53 and is connected
to the motor by means of fluid-proof connectors 550 Thrust bear-
ings 57 transmit the forward thrust of propeller 29 to housing 39
which, in turn, transmits a pu11ing Eorce on conduit 21 via coup-
ling 53.
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1 ~173~
In the form of my inv~l,tion shown in F`iyures 7 ancl ~3
the drilling bit and pLopeller rotate at the same speed. The
forward thrust generated by roL~.ti~n o. the marine screw propeller
provides weight on bit and via l:hrust bearing 57 provides the
necessary forward thrust to aclvance the motor and concluit as
drilling progresses.
Another form oE my invention employing concentric
hollow rotating shafts is shown in Fiqure 9. In this embodiment
the field 41 of the motor is fi~ed to one shaft and the armature
43 to the other. Both field and armature are Eree to rotate
within the outer casing 39 of t'le motor housing. The field and
the armature will rotate in opposite directions and hence inner
shaft 33 and outer shaft 59 will likewise rotate in opposite
directions. The absolute speecl of rotation of field and armature
will be dependent upon the load imposed upon shafts 33 and 59.
since it is usually desirable that more horsepower be utilized
for generating thrust than for rotating the bit, it is desirable
that each shaEt carry a propeller. As shown propeller 13 is
mounted on shaft 33 and propeller 113 on shaft 59 with rotary
seal 69 preventing leakage of drilling fluid into the annulus
between the shafts and bearings 71 maintaining alignment. Since
shaft 33 and shaft 59 will rotate in opposite directions and
since it is desired that the rotation of each generate forward
thrust, the pitch of the blades on the two propellers will
necessarily be opposite in direction.
One form of motor wherein both field and armature rotate
in opposite directions is disclosed in U. S. Patent No. 2,462,182
to D. A. Guerdan et al assigned to Westinghouse Electric Corporation.
Forward thrust generated by the rotation of propeller
113 is transmitted to housing 39 via thrust bearing 57 for
739
advancing the housin~; allcl conduit and will be availa~le to s~lppLe-
ment the thrus~ ~ener~ted by ~ropel:Ler 13 to provide adclitional
weight on the bit 11. The rei~tive amounts of forward thrust
generated by propellers :L3 and 113 will be dependent upon the
absolute speed of rotation of each, which in turn will depend upon
the load imposed restraining rotation of each shaft. Because of
variations in the loacl imposed on t}~le drill bit and turbulence
of the fluids in which the pro2ellers are operating, the relation-
ship between the thrust generated by each will vary. In view of
this, collar 65 on inner shaft 59 is suitably recessed to receive
thrust bearings 73 Eor receiving thrust Erom housing 39 and
thrust bearings 75 for transmitting thrust to outer shaft 59
and hence to housing 39~
Several advantages are obtained by using the embodiment
of Figure 9. Counter rotation oE ~he propellers, particularly
where closely spaced, results in more eEEicient generation of
forward thrust. Also, importantly, counter rotation of the motor
parts and shaft~, which are linked to the motor housing only via
rotating seals and bearings, results in the transmittal of mini-
mal rotative torque to the housing and hence to the conduit. This,
in turn, reduces the amount of twist resistance that must be built
into the conduit.
A form of my inventior wherein the electric motor is
used only to provide the Eorward thrust ~or the system is dis-
clo~ed in Figure 10. In this embodiment the conduit 23 is rotated
at the speed at which it is desired to rotate shaft 33 and bit 11.
Referring back to Figure 7, the drill pipe 26 may be rotated from
the surface by the rotary table on a convent:ional drilling rig.
This, in turn, will rotate concluit 23. In this form the conduit
is fixedly connected not only to motor housing 39 but also to
shaEt 33 and bit 11 will rotate with the conduit. As shown the
~ ~1739
end o~ shaEt 33 extencls slicJiltLy beyond the rear end of motor
housing 3~ and is welded 73 th Le~o. Rear housing extension is
externally threaded to receive ,nt~Lnally threaded female coup-
ling 53 and fixed to the end ol conduit 23. Sealing rubber ring
75 may be inserted between the mating faces of coupling 53 ancl
the end of the shaft. Electriccll conductor 2~, shown as carried
externally of conduit 23, is connected to connector 55 on the
coupling and thence through the coupling to a connector 55 on the
motor housing.
The field or stator ~1 of the electric motor is fixed
to the housing and rotates with it. Rotor armature ~3 is fixed
to the outer concentric shaft wnich carries propeller 13, rota-tion
of which generates the necessary forward thrust to provide weight
on bit-and to advance the system. As in the embodiment shown in
Figure 9, thrust bearings 57 transmit the forward thrust to the
housing, inner shaft and oit and conduit. The embodiment of Figure
10 is particularly suitable where the drilling characteristics are
such that a high weight on bit is desired along with a relatively
low speed of bit rotation.
Figure 11 illustrates the relationship between propeller
RPM, useful torque and thrust. At zero RPM thrust is zero and
torque is a maximum. Thrust increases with increasing rate of
rotation while torque available to rotate the propeller and the
bit where both are on the same shaft decreases. Factors affecting
the speed of rotation and hence, the balance between torque and
thrust delivered by a rotating marine screw propeller include the
density and viscosity of the drilling fluid in which it is immersed.
Also where propeller and bit are on the same shaft, the nature of
the formation being drilled will affect the speed of rotation.
~13-
739
Where p~rt or all oE l-he torque Eor rotating the bit is
derived from a propeLler o~ th~ same shaft as the bit the system o~
my invention has the inherent a~ility ~o acljust the balance between
torque and thrust. When the L"~ount oE torque required to rotate
the bit increases, the bit slows clown, and the thrust deliverecl
by the propeller decreases, sl~,wing the rate of penetration unti.
the system comes into balance. Conversely, when the ~orque required
diminishes, the resultant decrease in rotational load allows the
bit and propeller to speed up increasing the thrust and the rate
oE drilling, again until the system comes into balance. ~lso, and
importantly, should the bit tend to stall, as, for example, caused
by suddenly entering a difficult to drill Eormation, the concurrent
slowing oE the propeller would rapidly reduce the thrust to or
substantiaily to ~ero allowing the reactive forces generated by
the clischarge of fl~lid from the fluid courses in the drill bit tc,
retract the bit typically as shown in Figures 5 and 6.
In lateral boring operations the power required may be
divided into two parts: first, the power needed to rotate the
drilling tool, and second, the power needed to generate the
advancing force, or thrust, required to cause the system to move
forward which, of course, inclucles the "weight on bit" necessary
to achieve penetration. Studies of rotary drilling practive
show that relatively little power is required at the bit to rotate
it, usually in the range of from a few horsepower up to one hundred
or more depending upon the diameter of the hole, the kind of bit,
the type of formation being drilled, the weight on the bit or
forward thrust and the speed of rotation. In normal drilling
practice the weight on bit, which results from the mass of drill
dollars used, may vary from a thousand pounds up to fiEty thousancl
pounds or more. To translate weiyht on bit, a static gravity force,
~ ~8~'~3~
into horsepower, marine practic. shows that bollard pull of a tuc3-
boat amounts to 15 to 40 or more pouncls per shaft horsepower.
Using a figure o~ 30 pounds o~ tilrust per horsepower, 300 horse-
power would deliver a penetratlng Eorce of 9,000 pounds (weight
on bit) on the bit. Rotative power required could be of the
order of lO horsepower. The ratio of dynamic, advanc;ng thrust
to rotative power in this example is 30. This ratio may vary as
noted above. In consideration of the above factors it has been
found that the ratio of dynamic thrust horsepower to rotative
horsepower may vary from as low as one to 220. Additionally,
since the dynamic thrust must not only provide the equivalent of
weight on bit but also the force for advancing the drilling
system through the horizontal portion of the well bore, the ratio
is usually greater than one. Speeds of propeller rotation in
excess of the speed of bit rotation are especially useful for
attaining the higher ratios of dynamic thrust horsepower to rota-
tive horsepower. ~he more easily penetrated, softer formations
would utilize the :Lower ratios while the harder formations would
require the higher ratios.
The forms of my invention shown in Figures 9 and 10 are
especially suitable where the higher ratios of dynamic thrust
horsepower to rotative horsepower are desiredO
In drilling ordinary earth formations using conventional
drilling bit, the bit rotation speed is preferably of the order
of from 40 to 400 r.p.m. To ob-tain optimum thrust in such situa-
tions, the propeller or propellers preferably should rotate at
considerably higher speeds. In such situations a plurality of
shafts are required with the outer shaft or shafts rotating at
the higher speeds. Any marine screw propeller on the innermost
shaft, of course, would rotate at the same speed as the bit.
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~ ~73~1
In the case oE the so:E~er ancl ea.sier to clri:Ll ~ormations sufEicient
thrust may be oh~ained :i.rom a single propeller rotating at the same
speed and on the same s~.aft as ~..he bit. Also, and especial.ly
where drilling is hard formatl~,ns using speci.al bits such as poly~
crystalline diamond compact ('~) bits, also known as Stratapax
bits, incorporating Stratapa~ crystals manufacturecl by the General
Electric Company or regular diamond bits, high bit rotation speeds,
a thousand or more rpm, are employed and again aclequate thrust may
be obtained from a single propeller.
As stated previously, since my invention is in-tended for
use in the non-vertical portions of bore holes where thrust forces
other than or in addition to the force of gravity are needed to
provide adequate weight on bit, the conduits for ^onducting drill-
ing fluid and power should be flexible. On the other hand, since
there is provided some form of motor Eor generating rotative force
at the end of the conduit, it must have some resistance to twisting.
Such resistance may be providecl by suitable design of the reinforc-
ing fibers.
In the Eorm of invention shown in Figures 1 to 4, where the
hydraulic motor resembles a turboprop, very little rota-tive force
is transmitted to conduit 23 via bearing 27. On the other hand, in
the forms of the invention shown in Figure 5 and in Figure 7 the
conduit must develop sufficient resistance to further twisting
without kinking to balance the rotative torque generated by the
motor. Similarly, in the form oE my invention shown in Figure 10,
since the rotative torque for the bit is transmitted by the conduit
directly, it must be highly resistant to twisting~ As mentioned
previously, relative low twist resistance is required for the
conduit where the form oE motor shown in Figure 9 is usecl.
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~ 1~17~
Since r"~lny mociifications and possible embocliments anduses may be macle of the apparatus oE this invention without de-
parting from the scope thereof, it i5 to be understood that all
matter herein set forth or shown is to be interpretecl as .i11us-
trative not as limit1n~.
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