US 20080073123A1

(19) United States

(12) Patent Application Publication (10) Pub. No.: US 2008/0073123 A1
Mullins et al. (43) Pub. Date: Mar. 27, 2008

(54) DUAL-MEMBER AUGER BORING SYSTEM Publication Classi?cation

(76) Inventors: H. Stanley Mullins, Perry, OK (51) Int. Cl.

(US); Jerry W. BeckWith, Perry, E21B 7/04 (2006.01)
OK (US); Kelvin P. Self,
StillWater, OK (US); Brent G. (52) U.S. c1. ......................... .. 175/62; 175/162; 175/323
Stephenson, StillWater, OK (US);
Floyd R. Gunsaulis, Perry, OK
(Us) (57) ABSTRACT
Correspondence Address: A system for boring horizontal boreholes and installing
TOMLINSON & O’CONNELL, P.C. products using a dual member drill string. The system is
TWO LEADERSHIP SQUARE, 211 NORTH comprised of a boring machine With a frame and a rotary
drive supported on the frame, a doWnhole tool, and a dual
ROBINSON, SUITE 450
OKLAHOMA CITY, OK 73102 member drill string. The drill string comprises an inner
member disposed Within a tubular outer member such that
(21) Appl. No.: 11/106,205 the inner member is rotatable independent of the outer
member. The outer member has at least one helical projec
(22) Filed: Apr. 14, 2005 tion supported on an exterior surface of the outer member.
The projections on the outer member function as an auger to
Related US. Application Data
clear spoils and support the bore. The auger arrangement
(60) Provisional application No. 60/562,029, ?led on Apr. With the dual member drill string can be used in forWard
14, 2004. reaming or backreaming operations.
Patent Application Publication Mar. 27, 2008 Sheet 1 0f 11 US 2008/0073123 A1
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US 2008/0073123 A1 Mar. 27, 2008

DUAL-MEMBER AUGER BORING SYSTEM of helical projections supported on an outer surface of the
drill string. The helical projections contact a Wall of the
CROSS REFERENCE TO RELATED
borehole during the boring.
APPLICATION BRIEF DESCRIPTION OF THE DRAWINGS

[0001] This application claims the bene?t of US. Provi [0007] FIG. 1 is a diagrammatic representation of a drill
sional Application No. 60/562,029, ?led on Apr. 14, 2004, ing machine and system having a dual-member drill string
the contents of Which are incorporated herein fully by built in accordance With the present invention.
reference. [0008] FIG. 2 is a fragmented, side elevational, partly
sectional vieW of a dual-member pipe section suitable to be
used in a dual-member drill string.
FIELD OF THE INVENTION [0009] FIG. 3 is a fragmented, side elevational, partly
sectional vieW of a dual rotary drive usable With boring
[0002] The present invention principally relates to the ?eld machine of the present invention.
of horiZontal directional drilling. More particularly, the [0010] FIG. 4a and FIG. 4b are diagrammatic side vieWs
present invention is directed to improved apparatus and of alternative embodiments of doWnhole directional tool
methods for creating substantially horiZontal near-surface assemblies for use With the present invention.
boreholes useful for such purposes as the installation of [0011] FIG. 5 is a diagrammatic side vieW of a dual
underground utility services, such as pipes and/or cables. member drill string With ?exible, helically-arranged proj ec
Although preferably implemented as part of a conventional tions disposed along an exterior surface of the outer drill
dual-member drill string equipped horiZontal directional string member.
drilling (“HDD”) system, some of the features described [0012] FIG. 6 is a sectional side vieW of the entry and
herein may also be usefully employed in a conventional horiZontal segments of a borehole being created by an
single-member drill string HDD systemias Well as in alternative embodiment of the present invention.
non-directional drilling systems. [0013] FIG. 7 is an enlarged sectional vieW of the drill
string of FIG. 6 Within the curved borehole.
[0014] FIG. 8 is an enlarged, fragmented side vieW depict
SUMMARY OF THE INVENTION ing another embodiment of the ?exible projections of the
present invention.
[0003] The present invention is directed to a dual member [0015] FIG. 9 is an enlarged, fragmented, partially sec
drill pipe for use in horiZontal directional drilling. The drill tional side vieW of yet another embodiment of the ?exible
pipe comprises an elongate holloW outer member having an projections of the present invention.
inner surface and an exterior surface and having a pin end [0016] FIG. 10 is a fragmented side vieW of the drilling
and a box end, an elongate inner member, and at least one machine of FIG. 1 shoWing an adaptation to a front Wrench.
helical projection on the exterior surface of the outer mem [0017] FIG. 11 is a series of diagrammatic, partially sec
ber. The inner member is disposed Within the outer member tional side vieWs depicting utiliZation of the helically
such that it is rotatable independent of the outer member. arranged projections of the present invention during the
Each helical projection makes at least one rotation around forWard-reaming upsiZing of the borehole.
the outer member. [0018] FIG. 12 is a diagrammatic representation of an
[0004] In another embodiment the invention comprises a backreaming operation utiliZing the present invention.
system for use in horiZontal boring. The system comprises a [0019] FIG. 13 is an enlarged, fragmented side eleva
boring machine comprising a frame and a rotary drive tional, partly sectional vieW of the backreaming doWnhole
supported on the frame, a doWnhole tool, and a drill string tool assembly of FIG. 12 Wherein the spoil (cuttings) con
having a ?rst end connectable to the rotary drive and a veying arrangement is depicted as ?exible, auger-like
second end connected to the doWnhole tool. The drill string appendages along a shaft or tube internal to the casing or
comprises an outer tubular shaft having an exterior surface,
product pipe.
an inner member disposed Within the outer member such
DETAILED DESCRIPTION OF THE
that the inner member is rotatable independent of the outer PREFERRED EMBODIMENTS
member, and at least one helical projection supported on the
exterior surface of the outer shaft. [0020] Turning noW to the ?gures in general and to FIG.
[0005] In yet a further embodiment, the present invention 1 in particular, shoWn therein is a conventional HDD system
is directed to a method for boring a horiZontal borehole 1 comprising a boring machine 10, suitable for drilling a
using a dual member drill string. The method comprises borehole 12 and near-horiZontal subsurface placement of
rotating a cutting tool using a ?rst member of the dual utility services, for example under the roadWay denoted by
reference numeral 14. The boring machine 10 comprises a
member drill string and clearing spoils and drilling ?uids by
frame 16 and drives a doWnhole tool assembly 18 connected
rotating an auger using a second member of the dual member
at the end of a drill string 20. The doWnhole tool assembly
drill string. 18 preferably comprises a directional assembly for guiding
[0006] In still another embodiment, the present invention the direction of the borehole 12. The typical HDD borehole
comprises a method for boring a horiZontal borehole. The 12 begins from the ground surface as an inclined segment
method comprises boring a ?rst portion of a borehole using that is gradually leveled off as the desired product installa
a single member drill string and boring a next portion of the tion depth is neared. This near horiZontal path 12 may be
borehole using a dual member drill string having a plurality maintained for the speci?ed length of the product installa
US 2008/0073123 A1 Mar. 27, 2008

tion. Generally, the doWnhole directional assembly 18 is sections 34 are rotatable independently of the interconnected
then directed upWard to exit the ground surface in a drilling outer members 36. An annular space 40 betWeen the inner
segment of similar or steeper incline to that of the entry members 38 and outer members 36, or a holloW tubular
segment. Drilling ?uids are typically pumped doWn the drill construction for the inner member (not illustrated), may be
string 20 to lubricate the drilling process, aid the stabiliZa useful for conveyance of drilling ?uid doWnhole for pur
tion of the borehole 12, cool the doWnhole electronics, and poses outlined above. One or the other of these longitudinal
to ?oW any non-recompacted cuttings out of the hole. The cavities may also be useful for conveyance of slurried drill
present invention is suitable for use With an HDD system 1 cuttings uphole for disposal. It Will be appreciated that any
as described above, but may also have additional applica dual-member drill string having an outer member and an
tions. For instance in the removal of spoil (cuttings), air may inner member, the inner member disposed Within the outer
be substituted for bentonite-style drilling ?uids. A pit member and independently rotatable, may be used With the
launched style of HDD system is also readily adaptable to present invention. Embodiments for suitable dual member
the present invention, and may be preferred in applications drill strings 20 are described in Us. Pat. No. 5,490,569, U.S.
Where negotiating a sharply curved entry-to-horiZontal bore Pat. No. 5,682,956 and its reissue RE38,418, the contents of
path transition poses difficulty. Which are incorporated herein by reference.
[0021] The drilling machine 10 of the HDD system 1 is [0024] Referring again to FIG. 3, a preferred embodiment
operatively connected by the drill string 20 to the directional for the rotary drive system 28 is shoWn. This rotary drive
doWnhole tool assembly 18. The tool assembly 18 may be system 28 has separately driven spindles 40 and 42 for
any one of several possible doWnhole tool assemblies suit individually controlling the rotation of the outer member 36
able for either creating the borehole 12 or later upsiZing its and inner members 38 (FIG. 2) of the dual-member drill
original diameter sufficiently to accept pullback installation string 20. A ?uid sWivel 44 is shoWn connected to the inner
of the desired utility services or product pipe (not shoWn). spindle 42, useful for instance to pump drilling ?uid doWn
The HDD system 1 may further comprise a tracking receiver hole through holloW inner members of the drill string 38.
22 positioned at one of several reference placement stations Alternatively, the ?uid sWivel 44 may be located at the front
and a doWnhole transmitter or beacon 24 containing one or or rear end of the outer spindle 40. The dual-spindle rotary
more sensors, such as a pitch sensor and a roll (tool face) drive system 28 further comprises tWo drive groups 46 and
sensor. The progression of borehole 12 along a desired path 48 for independently driving the respectively interconnected
is facilitated by information communicated betWeen track outer members 36 and interconnected inner members 38 of
ing receiver 22 and a control station 26 of the HDD system the dual-member drill string 20. The outer members 36 and
1. The operation of HDD system 1 and its drilling machine inner members 38 are separately rotationally controlled by
10 may be controlled manually through a system of levers, independent operation of the outer 50 and inner 52 drive
sWitches or similar controls at the control station 26. Alter motors. For instance, as is advantageous With the present
natively, the control station 26 may comprise a system that invention, one of the drill string members 36 or 38 can be
automatically operates and coordinates the various functions held Without rotation or rotated at a substantially different
comprising the drilling operation. speed than the other of the tWo drill string members are
[0022] Referring noW to FIGS. 2 and 3, the drill string 20 rotated.
may be a dual-member drill string as shoWn, comprised of [0025] The rotary drive system 28 is slidably mounted on
an outer drill string surrounding an inner drill string. For use the inclined frame 16 of drilling machine 10 by Way of a
With a dual member drill string 20, the drilling machine 10 carriage 30. For purposes later described, it may also be
is equipped With a rotary drive system 28. The rotary drive advantageous for the inner 40 and outer drive spindles 42 to
system 28 is movably supported by a carriage 30 on the be supported in a manner that they can be displaced axially
frame 16. Movement of the carriage 30 and rotary drive With respect to each otherithereby advancing or retracting
system 28, by Way of an axial advancement means (not the inner drill string members 38 With respect to the outer
shoWn), betWeen a ?rst position and a second position along drill string members 36. This relative motion may be accom
the frame 16 axially advances the drill string 20 and direc plished, for instance, by slidably supporting the outer mem
tional doWnhole tool assembly 18 to create borehole 12. The ber drive group 48 upon the carriage 30 and displacing it
drill string 20 transmits the thrust of carriage 30 and torque axially With respect to the inner member drive group 46 by
of rotary drive system 28 to doWnhole tool assembly 18 for Way of a linear actuator such as the hydraulic cylinder 54.
drilling subsurface borehole 12. Reactionary forces on the Thus, in a particular operational mode, the carriage 30 may
drilling machine 10 may be resisted by machine Weight advance the inner member drive group 46 While the slidably
supplemented by earth anchors 32 (FIG. 1). As is necessary supported outer member drive group 48 could be held With
at times, the doWnhole tool assembly 18 may be disengaged little or no forWard movement (for only a short interval) by
from the earth at the distant end of the borehole 12 by retraction of the hydraulic cylinder 54. In that Way, the inner
retraction of the drill string 20 through reverse movement (to drill string 38 is advanced With respect to the outer drill
that described above) of the carriage 30. string 36. One skilled in the art can readily envision other
[0023] With continued reference to FIG. 2, there is shoWn inner member 38 and outer member 36 relative translation
therein a dual-member pipe section 34 suitable for assembly modes made possible, such as one or more telescopic drill
into a dual-member drill string 20. The pipe section 34 is pipe segments.
comprised of an outer member 36 With male and female [0026] A dual-spindle rotary drive system 28 adaptable to
matingly threaded ends for threading to correspondingly the above purposes is disclosed in previously referenced
threaded ends of adjacent pipe sections. The pipe section 34 U.S. Pat. No. 5,682,956. Additional details on the operation
is further comprised of an inner member 38 With corre of a HDD system 1 equipped With a dual-spindle rotary drive
sponding male and female geometrically shaped ends. lnter 28 are given in commonly oWned application Ser. No.
connected inner members 38 of adjacent dual-member pipe 10/724,572, incorporated herein by its reference.
US 2008/0073123 A1 Mar. 27, 2008

[0027] With reference noW to FIGS. 4a and 4b, shown creation of a straight borehole segment is desired. Such a
therein are alternative embodiments for directional doWn directionally deployable doWnhole tool 18 (Whether
hole tool assembly 18 for use With a dual-member drill string deployed by relative motion of a dual-member drill string,
20. The directional doWnhole tool assembly 18 of FIG. 4a is by hydraulically actuated doWnhole radial motion, or oth
suitable for creating boreholes in normal soils, Whereas the erWise) frees up the rotational mode of the outer drill string
directional doWnhole tool assembly 1811 of FIG. 4b is useful 38 to be dedicated for yet to be described purposes of the
for drilling boreholes in hard soil or soft to medium hardness present invention. When utiliZing a directionally deployable
rock. These and other types of directional doWnhole tool doWnhole tool 18 actuated by one of the methods described
assemblies may be utiliZed With the present invention. above, it may be advantageous to equip the doWnhole end of
[0028] The directional doWnhole tool assembly 18 of FIG. the outer drill pipe 36 With a second beacon (not shoWn), the
411 comprises a plate-style bit 110 removably attached to the ?rst beacon 24 being installed in the deployable doWnhole
inclined nose of a forWard housing assembly 112 containing tool mounted at the doWnhole end of the inner drill string 38.
the doWnhole transmitter or beacon 24. The housing assem Such a dual-beacon tracking system is disclosed in com
bly 112 is rotationally ?xed to the doWnhole end of the inner monly oWned application Ser. No. 10/724,572, previously
drill string 38 by Way of an inner drive member 114 incorporated by reference.
bearingly supported inside (not shoWn) a bearing housing [0032] Most HDD doWnhole tool assemblies 18 rely on
assembly 116ithe latter being ?xedly attached to the doWn the ?oW of drilling ?uid to convey non-recompacted cuttings
hole end of the outer drill string 36. Thus the inner drill out of the borehole 12 as it is being created or upsiZed.
string 38 is utiliZed to rotate the bit 110 While the outer drill Certain soil types and/or operational situations adversely
string 36 may be independently rotated or held substantially affect the hole-cleaning effectiveness of this process. This
Without rotation. Rotating the bit 110 While it is advanced could endanger successful completion of the project utiliZ
creates straight segments of the borehole 12. Orienting the ing conventional HDD systems. For instance, larger cuttings
slanted face of the bit 110 in the desired directioniWith the of clay soils can be di?icult to break doWn into particles
aid of the beacon 24 roll (tool face) sensor (not shoWn)iand small enough to be suspendable in the drilling ?uid, Whereas
advancing Without rotation of the inner drill string 38 granular cuttings can quickly settle out of drilling ?uid
initiates a borepath directional change. The tool 18 and its suspension Within the substantially horiZontal borehole 12.
operation are described in more detail in commonly assigned The present invention addresses the needs of enhanced
US. Pat. No. 6,827,158, incorporated herein by its refer particle suspension and of improved conveyance of drill
ence. cuttings out of the borehole 12. The invention also addresses
[0029] The directional doWnhole tool assembly 1811 of these needs for special types of HDD systems, such as those
FIG. 4b comprises a tri-cone bit 120 (or other suitable bit) utiliZing particle-creating dry boring techniques and high
attached to the doWnhole end of the inner drill string 38 by volume (velocity) air for spoil (cuttings) conveyance.
Way of an adapter end 124 of an inner drive member [0033] Turning noW to FIG. 5, shoWn is a side vieW
bearingly supported (not shoWn) inside a bearing housing depicting the present invention. Pipe sections 34 of the
assembly 126. The housing assembly 126 is ?xedly attached dual-member drill string 20 are connected to each other and
to the doWnhole end of the outer drill string 36 by Way of a to the doWnhole directional tool assembly 18 by Way of tool
“bent” end cap 128. The tWo components 126 and 128 joints 60. The dual-member drill string 20 preferably has
comprise a “bent” housing assembly, Which may be inde substantially helically-arranged projections 62 disposed
pendently rotated or held substantially Without rotation by along an exterior surface of the outer member 38. As Will
the outer drill string 36 While the inner drill string 38 is become clear later, the projections 62 may encompass a
utiliZed to rotate the bit 120. The bearing housing assembly broad range of substantially helically-arranged projections
126 further contains the doWnhole transmitter or beacon 24. not limited to the “auger-like” ?exible ?ighting depicted in
[0030] In typical operation of the doWnhole tool assembly FIG. 5. More preferably, at least some of the projections 62
18a, the inner drill string 38 continuously rotates the bit 120 exhibit ?exural ability under axial and radial loading. Such
While it is being advanced to create the borehole 12. To loadings may occur at points of contact or along arcs of
create straight segments of the borehole 12, the outer drill contact With the Wall of the borehole 12 or With cuttings
string 36 either sloWly rotates the “bent” housing assembly located therein. Depending upon the direction of load appli
126 or alternates the position of its bend betWeen an up cation and factors related to the design of those ?exible
doWn or left-right orientation every feW feet of advance. projections 62, ?exure Will likely change their shape (pro
Orienting this bend in a desired directioniWith the aid of ?le) and height (radial component of the outer diameter,
the beacon 24 roll (tool face) sensor (not shoWn)iand OD.). Preferably, a nominal OD. of these projections falls
advancing Without rotation of the outer drill string 36 Within a range bounded approximately by an inside diameter
initiates a borepath directional change. (ID.) of the borehole 12 and an OD. of the connective tool
[0031] With reference again to FIG. 3, the relative motion joints 60 betWeen sections of the outer members 36. More
feature of advancing or retracting the inner drill string 38 preferably, the loWer end of the range is greater than the tool
With respect to the outer drill string 36 may be utiliZed to joint OD., While the upper end of the range is smaller than
activate and deactivate redirection of a purpose-built doWn the ID. of the borehole 12. This narroWer range provides
hole tool assembly 18. Use of relative motion alloWs the improved conveyance of drill cuttings across the tool joints
directional capability of a doWnhole tool 18 to be deployed 60 of the outer drill string 36, and also reduces the amount
selectively only When needed; e.g., to create a curved of rotational and axial drag created by the presence of the
borehole or to correct an unplanned deviation from the projections 62.
desired path. This is advantageous in that the doWnhole tool [0034] The projections 62 may vary in type and/or in their
18 has no continual bias having to be overcomeiby one of OD. along the length of a particular segment 34 of the outer
the previously described operating techniquesiWhenever drill pipe 36. The projections 62 may also be continuous
US 2008/0073123 A1 Mar. 27, 2008

along a full length of a pipe segment 34 or the drill string. of the bore. The severity of curvature for either transition is
The drill string 20 may also be an assembly of pipe segments generally limited to an alloWable bend radius of the drill
34 di?ering With respect to their projections 62. It may be string 20 or of the product (not shoWn) to be installed in the
advantageous for these di?ering pipe segments 34 to be resulting borehole 12, Whichever is larger. A portion of a
arranged in an ordered assembly so as to yield a repetitive curved transition of the borehole 12 is shoWn in FIG. 7 as an
pattern change (of projections) along the length of the drill enlargement. This interval is the focal point of the present
string 20. Also, the type and arrangement of projections 62 discussion.
at the doWnhole end of the drill string 20iin one or more [0038] As best seen in FIG. 7, the ?exible projection 64
pipe segments 34 near or immediately adjacent to a given may be a series of spring steel shapes arranged in one or
doWnhole tool assemblyimay differ from other pipe seg more roWs spirally Wrapped around the exterior surface of
ments comprising the drill string. Further, a particular shape the outer members 36. Preferably, the projections 64 are
and/or arrangement of projections 62 may preferably be constructed of “strap-like” material formed into a “boW”
associated With a given doWnhole tool assembly 18 and/or shape and tWisted axially to coincide With the desired helix
soil condition. angle of their sequential arrangement. Each strap-like pro
[0035] In certain instances it may be advantageous for at jection 64 may be attached to the exterior surface of the
least some intervals of the projections 62 to have a larger outer member 36 of the drill string 20 by a variety of
OD. (e.g., by 10% or so) than the ID. of the borehole 12. techniques, so long as the desired maximum OD. of their
This results in the outer drill string 36 (because its projec assembly is not exceeded and their inherent radial ?exibility
tions 62 are integral thereto) being an interference ?t Within is not unduly impeded. For instance, proper positioning and
the borehole 12. By a controlled coordinated rotational siZing of a slotted connection to one end of the strap-like
advance (i.e., the proper number of revolutions per unit projection 64 can accommodate these tWo aspects. The outer
advance) of the outer drill string 36 in approximate direct extent of the slotted connection serves to restrain the
relationship With the helical pitch of the projections 62, arrangement of projections to a desired maximum O.D.,
threaded engagement of the projections into the undisturbed While the inner end serves as a ?rst stop in the radial collapse
soil surrounding the borehole 12 may then be accomplished. of each ?exible boW-shaped projection 64. Additional radial
Where the inner drill string 38 can be advanced and retracted collapse at a point of contact may occur if the contact
a short distance separately from the outer drill string 20 loading is su?icient to distort the boW shape.
(FIG. 3), this threaded engagement may be utiliZed to [0039] The strap-like projections 64 may be constructed
augment the earth anchors 32 in resisting the thrust and/or from approximate rectangular cross-section material, such
pullback force of the drilling machine 10. Altemately, once as ?at bar stock having appropriate spring steel metallurgy.
the outer member 36 is so threadedly engaged, an increase HoWever, for purposes such as extended Wear life and
in its rate of rotation per unit advance can be utiliZed to apply improved function, the cross-sectional material thickness
additional thrust force (or, for reverse rotation, apply addi may desirably be non-uniform (not illustrated). For example,
tional pullback force) to the doWnhole tool assembly 18. constructing the ?exible projections 64 from oval or half
This increased “thrust on bit” better enables penetration of oval cross-section bar stock Would reduce rotational drag on
“tough” spots encountered along the borepath, While less the drill string 20. Other bar stock cross-sections may also
ening the risk of buckling the drill string 20ias might be the be utiliZed Without detracting from the spirit of the present
case if greater thrust Were applied to the uphole end of the invention. Material thickness of the “boWed strap” may also
drill string 20 by the drilling machine 10. This capability be purposefully non-uniform lengthWise. For instance, Wear
may be particularly advantageous When attempting to life may bene?t by gradually increasing material thickness
advance the bit 110 Without rotation, to make directional toWard its mid point of length, or by having a rounded
changes in the borepath 12. To avoid the above-mentioned enlargement at that location (not illustrated). A lengthWise
drill string buckling concern, an interval of borehole engag mid point of the strap-like projection 64 should be consid
ing projections 62 is preferably positioned near the doWn ered equivalent to a central point of its greatest radial o?fset
hole end of the outer drill pipe 36. from the outer member 36.
[0036] Although the projections 62 shoWn in FIG. 5 are [0040] The ?exible projections 64 along the outer drill
arranged in a helix of much shorter pitch than the length of string member 36 can be in contact With portions of the
the drill pipe segment 34, larger helical pitches are also borehole 12 Wall that may vary according to particular
envisionedito include pitches so large as to yield only tWo operating modes of the HDD system 1. For instance during
to three or so turns around the typical 10 to 15 foot length a directional change With the doWnhole tool assembly 18,
of a pipe segment 34. Also, the apparent absence of projec the drill string 20 might be advanced With little or no rotation
tions 62 in the tool joint 60 intervals along the outer drill of its outer member 36 and no rotation of its inner member
string 36 is not limiting upon the present inventionirather 38. The thrusting action necessary to initiate this directional
it is an illustrative simpli?cation to improve visualiZation of change may “boW” the outer member 36 and its projections
a segmented drill string. If so desired, the tool joint intervals 64 against the bottom of the curved transition4opposite to
60 may be bridged in various Ways With projections that illustrated in FIGS. 6-7. Whereas, the con?guration
62isuch as projections in the form of spirally placed Welds shoWn (i.e., contact With the top of the curved transition)
or spiral-like appurtenances machined onto (i.e., beyond) the may occur during rotational advance of the directional
nominal tool joint outer diameter. doWnhole tool assembly 18 in order to drill a straight
[0037] With reference noW to FIG. 6, shoWn therein is a segment of the borehole 12. In this operational situation, the
sectional side vieW of a borehole 12 being created by another thrust requirement on the drill string 20 is generally reduced,
embodiment for the dual-member drill string 20 of the so the ?exural sti?fness (bending resistance) of the drill
present invention. As previously mentioned, the borehole 12 string may tend to hold it in contact With the top of the
may be drilled With curved portions at the beginning and end curved transition of the borehole 12. The illustrated contact
US 2008/0073123 A1 Mar. 27, 2008

may also be expected during pullback upsiZing of the rounding soil into the desired borehole alignment are often
borehole 12. If the outer members 36 of the drill string 20 abrupt enough to be intimately contacted by the sequential
Were rotated one revolution in the direction of the helix per series of ?exible projections 64 later passing by such loca
an incremental advance equal to the helical pitch of the tions. In effect, the protrusions become “point load supports”
projections 64, helical line segments of contact Would be for the drill string 20 in a Weight-supportive and/or ?exural
created betWeen the projections and the top of the bore manner. Protrusions not at the “bottom” of the borehole are
holeisimilar to the previously described case of threaded supportive in a ?exural manner. Point loading creates much
engagement. The rotational speed of the outer member 36 of higher radial contact forces betWeen the projections 64 and
the drill string 20 Will generally be faster or sloWer than this the protrusions than does the uniform contact inherent in a
specialty situation. In these situations, the projections 64 in straight borehole. Repetitive axial and rotary sliding contact
effect slide axially With respect to the Wall of the borehole of the ?exible projections 64 against the radial protrusions
12. This sliding creates axial forces (shoWn by “A” in FIG. under high radial loading aids reduction of their radial o?fset.
7) on the ?exible projections 64, While contact against the This “borehole straightening” capability may be particularly
borehole Wallisuch as along the top of the curved borehole advantageous Where the borehole 12 is intended for the
transitionicreates radial loading (shoWn by “R” in FIG. 7) installation of on-grade storm drainage or seWerage pipes.
on them. Of course, the particular projections 64 Which are Critical alignments such as these cannot accept more than
experiencing these loads change in relation to the rotation minor deviations from the planned borepath.
and translation (advance or retraction) of the outer drill [0044] Many other con?gurations and shapes than shoWn
string members 36. As earlier mentioned, such loadings can in FIGS. 5-7 could suitably serve as the ?exible projections
change the shape and radial height of the ?exible projections 62 and 64, including: tines, bristles, paddles, ribbon auger,
64. These changes are a desirable feature of the present coiled spring segments, and outlying spiral lobes. The
invention; otherWise there may be greater tendency to “Wal borehole contacting points of several such projections may
loW out” the curved transition of the borehole 12 resulting in be desirably modi?ed in various Ways to enlarge the contact
a potentially unacceptable shift in its alignment. area at this interface and thereby provide improved radial
[0041] Rotation of the outer drill string members 36 support. In the speci?c instance of a tine, this may be
creates another component of loading on the projections 64 accomplished by bending its end into a supportive shape or
Whenever they are in contact With the borehole 12 or With by adding a rounded enlargement thereto. As previously
cuttings therein. This loading (not shoWn in FIG. 7) Would discussed, a rounded shape reduces rotational drag.
be directed in or out of the plane de?ned by the A and R [0045] Flexible projections comprised of coiled spring
forces. The load is directed approximately transversely segments 66 are illustrated in the embodiment of FIG. 8.
tangential to the boWed strap projection 64. The resulting These segments 66 comprise at least one Wrap and may
tWisting action must be properly considered in the design of comprise multiple helical pitches coiled around the outer
the projection and its attachments to the outer members 36 member 36 of the drill string 20. To accommodate speci?c
of the drill string 20. soil conditions, the cross-sectional shape of the coiled spring
[0042] Control of the outer drill string 36 by the dual 66 may be other than the circular shape illustrated in FIG. 8.
spindle rotary drive system 28 alloWs its rotational speed to For instance, an oblong or ?attened circular cross-sectional
be adjusted for optimum spoil removal for a given arrange shape may be more suitable in softer soils, While a rectan
ment of projections 64 in particular soil conditions. Opti gular shape may be more suited to harder soil conditions.
miZation is more readily accomplished by a control system (Note that a coiled rectangular shape Would be similar in
26 that automatically operates and coordinates rotational appearance to the ?ighting of a ribbon auger.) Each coiled
speed of the outer members 36 of the drill string 20 With the segment 66 is mounted approximately concentric With the
various other functions comprising the drilling operation. In outer member 36 by Way of supporting attachment points 68
this Way, rotation can be held to the loWest speed that at either end of the segment. For a multiple-Wrapped coiled
effectively aids spoil (cuttings) removal. This minimiZes any segment 66, additional supporting points (not shoWn) could
tendency the ?exible projections 64 may have to laterally be spaced betWeen the ends of the segments. HoWever,
“WalloW out” the borehole 12 and unacceptably shift its shorter overlapping coil segments 66isuch as shoWn in
alignment. An automated control system adaptable to the FIG. 8iprovide enhanced support Without undo complica
present invention is disclosed in commonly assigned U.S. tion. The coiled projections 66 and their supporting ends 68
patent application Ser. No. 10/617,975, the contents of may be constructed of spring steel or other appropriate
Which are incorporated herein by reference. As used herein, ?exible and long-Wearing materials.
automatic operation is intended to refer to operations that [0046] Turning noW to FIG. 9, illustrated therein is yet
can be accomplished Without operator intervention and another embodiment of the present invention. Paddle-like
Within certain predetermined tolerances. ?exible projections 70 are attached to the outer member 36
[0043] Not Withstanding their ?exibility and controlled of the drill string 20. The projections may be arranged in one
rotation, the projections 64 purposefully retain inherent or more helical roWs 72 along and spiraling around the drill
capability of at least partially correcting undesired path string. (The dashed helical line 72 shoWn in FIG. 9 repre
variances that may occur along short intervals of an intended sents the location Where a second roW 1800 out of phase With
straight borehole. These undesired borepath variances might the ?rst roW Would be placed, if present). This arrangement
result When the doWnhole tool 18 encounters a soil variation is intended to aid the movement of spoil (cuttings) uphole
(strati?cation, cobble, etc.) or because of inappropriate steer Whenever the outer member 36 of the drill string 20 is
ing decisions (over-steering, improper timing, etc.). Many of rotated clockWise as vieWed from uphole looking doWnhole.
these variances occur Within a distance interval shorter in (The doWnhole direction is toWard the right in all Figures
length than one or tWo drill pipe segments 34 (typically less herein.) Other arrangements may also be acceptable toWard
than 20-30 feet). The resulting radial protrusions of sur this purpose. The illustrated approximate radial orientation
US 2008/0073123 A1 Mar. 27, 2008

of the projections 70, i.e., perpendicular With respect to the arrangement of spokes 82. The outer member 36 of the drill
axial orientation of the drill string 20, may be desirable string 20 may thus be rotated Without causing rotation of the
Where the outer member 36 of the drill string has bi outer rim 78 of the support 76. The outer rim 78 is preferably
directional rotational (clockWise and counter-clockWise) of contoured cross-section and diametrically siZed to be a
capability and/or is expected to undergo axial pullback (i.e., slip ?t Within the borehole 12, and preferably has a smooth
bi-directional axial motion). Canting the paddle-like proj ec or scalloped (not illustrated) circumferential surface. The
tions 70 uphole (toWard the left in FIG. 9) may be bene?cial outer rim 78 is preferably purposefully constructed to ?ex
in certain soil conditions, or Where pullback of the drill ibly distort from circular engagement With the borehole 12
string 20 is not planned. Although the paddle-like ?exible to an oblong or point-Wise indented shape as may be
projections 70 are shoWn in FIG. 9 as being arrayed tangent necessary to pass over protrusions from the Wall of the
to their particular helical line 72, other relationships are borehole or to pass through out-of-round intervals of bore
acceptable. For instance, arranging the projections 70 trans hole. The rim 78 is also preferably ?exible to shift uphole or
versely to their respective helical lines 72 may be bene?cial doWnhole (left or right in FIG. 9) With respect to its hub 80
When more than tWo lines 72 of projections 70 are ?tted under axial loadings associated With such protrusions and
around the outer drill string 36. out-of-roundness. This and other features of the rim 78
[0047] Some or all of the paddle-like projections 70 may reduce the likelihood of it becoming Wedged in the borehole
have enlargements 74 at their outer ends. The enlargement 12. The relative movement and point-Wise indention capa
74 may have increased thickness (not illustrated) as Well as bility is accommodated by ?exibility inherent in the material
an extended arc length at points of potential contact With the and shape comprising the spokes 82. The spokes 82 are
borehole 12 Wall. As previously mentioned, enlargements at preferably constructed of spring steel bent into a shape that
the outer end of certain types of ?exible projections may be purposefully aids the above-desired ?exibility of the outer
useful for Wear resistance and for enhanced support of the rim 78.
drill string 20 Within the borehole 12. The latter feature Will [0050] The various ?exible helical projections 62, 64, 66,
mitigate the tendency that some non-augmented projection and 70 described herein are particularly bene?cialiby Way
shapes could have to unduly rede?ne the borepath (WalloW of their agitating effect while being rotated by the outer drill
out the borehole in one or more radial directions). End string 36iin keeping drill cuttings (spoil) in suspension
enlargements 74 also may be bene?cial in reducing the Within the drilling ?uid being injected during the drilling
amount of rotational and axial drag created by the presence process. For conventional HDD drilling machines, periods
of the ?exible projections 70. This is particularly the case of non-rotation of the drill string occur Whenever a neW
When the projections 70 have a gradation in their radial segment 34 of drill pipe must be added to the drill string 20.
height along the drill string 20, as illustrated in FIG. 9. Taller FloW of drilling ?uid into the borehole 12 also ceases during
(larger OD.) intervals of the projections 70 are shoWn in this time. This period of inactivity equates to stagnation
FIG. 9 toWard the left and right of center for the segment 34 Within the borehole 12, Which may be particularly detrimen
of the drill string 20. These projections 70 have end enlarge tal toWard holding larger or heavier cuttings in ?uid sus
ments 74 to provide supportive contact With the borehole 12 pensionisuch as those created While drilling through rock
Wall, thus freeing the shorter projections from this contact. formations. The drilling machine 10 of the present invention
[0048] When its propelling drill string 20 is held in has been adapted to eliminate this period of stagnation by
approximate alignment With the borehole 12 centerline, the instituting capability for rotation of the outer drill string
doWnhole tool 18 (as Well as other doWnhole tools described member 32 after it has been disconnected from the rotary
herein) often has less di?iculty holding a given borehole drive 24.
alignment. Thus the centraliZed support offered by the larger [0051] With reference noW to FIG. 10, the drilling
OD. projections 70 is a feature particularly useful toWard machine 10 further comprises a front Wrench 200. The front
holding alignment in on-grade boring applications. In cer Wrench 200 of the present invention is adapted to rotate the
tain situations, including softer soil conditions, it may be outer member 36 of the drill string 20 While maintaining a
advantageous to augment this supportive feature by the grip on the upper most tool joint 60 at the uphole end of the
placement of centraliZers or bearing supports 76 at intervals drill string. The front Wrench 200 comprises a pipe grip 202
along the drill string 20. The centraliZer supports 76 may be and a rotary drive 204.
positioned Within the bands of taller, end-enlarged projec [0052] The front Wrench 200 operates When the drill string
tions 70ias shoWn in FIG. 9. Altemately, they may be 20 has been advanced su?iciently that the upper most tool
placed in the midst of the shorter projections. Depending joint 60 is positioned at the front Wrench (or the drill string
upon the positional placement of a drill pipe segment 34 has been retracted su?iciently that an outer member tool
Within the drill string 20 and the length of the segments, it joint is positioned at the front Wrench and the breakout
may be bene?cial to have as many as three bearing supports Wrench (not shoWn)). The rotary drive system 28 of the
76 spaced along a segment of the drill string 20. HoWever, drilling machine 10 is disconnected from the drill string 20
adjacent pipe segments 34 Within the drill string 20 may be to alloW the addition (or removal) of another pipe segment
?tted With differing numbers of supports 76 and some pipe 34. [Note: For clarity, the breakout Wrench is not shoWn in
segments may have none at all. Thus various supportive FIG. 10. It Would preferably be located immediately to the
arrangements of the drill string 20 can be con?gured by left of the front Wrench 200.] Typically this disconnection is
sequential addition of appropriately equipped drill string accomplished byiamong other actionsigripping the tool
segments 34. For instance, closer spacing of the supports 76 joint 60 located at the uphole end of the drill string 20 to
may be more bene?cial at the doWnhole end of the drill prevent its rotation. The pipe grip 202 of the Wrench 200 is
string 20 for reasons mentioned earlier. adapted to secure the tool joint. Preferably, the pipe grip 202
[0049] The centraliZer supports 76 may be comprised of comprises a set of engagable vise jaWs. The rotary drive 204
an outer rim 78 ?exibly supported on a bearing hub 80 by an Within the front Wrench 200 of the present invention may be
US 2008/0073123 A1 Mar. 27, 2008

comprised of a gear arrangement (not shown) supporting the behind the guide segments is a dual-member drill string. The
vise jaWs 202. The gear arrangement is preferably driven by ?rst guide segments 300 serve as a guide rail, forcing the
a hydraulic motor. Such an arrangement Would be purpose outer drill string member 36 to folloW the same borepath 12
fully designed With su?icient rotational torque capacity to direction. A cutting head 302 suitable for borehole upsiZing
turn the outer member 36 of the drill string 20 Within a is placed at the doWnhole end of the outer drill string 36. The
borehole 12 and, by Way of the ?exible projections 62 cutting head 302 may be rotationally coupled to the outer
thereon, agitate the drill cuttings. A brake (not shoWn) may drill string 36, or alternately driven. An arrangement of gears
be actuated to lock the rotary drive 204 of the Wrench 200 (not shoWn) could comprise the rotational coupling to the
during the conventional process of making and breaking tool outer drill string 36, enabling it to be rotated at a different
joint 60 connections. The outer member 36 of the drill string
speed (faster or sloWer in relation to the gear ratio) than the
20 could alternatively be rotated by a rotary drive separate
cutter head 302. Suitable arrangements for such gearings are
from the front Wrench 200. Such a separate drive Would be
located directly ahead of a conventional front Wrench. The shoWn in commonly assigned US. Pat. Publ. No. 2005
outer member 36 of the drill string 20 could then be rotated 0029016. In operation, the spoil (cuttings) conveying capa
upon release of the front Wrench. bility of the auger-like projection 62 attached to the external
surface of the outer member 36 of the drill string 20 can
[0053] Once the initial borehole 12 has been completed, it
more readily be matched to the excavation rate of the cutter
is often necessary to upsiZe the hole diameter to accept the
product being installed. The present invention has utility in head 302.
this process as Well. The auger-like ?exible projections 62, [0057] A casing 304 is slidably ?tted over the ?exible
64, 66, and 70 of the present invention are particularly ?ighted 62 outer drill pipe 36. The casing 304 may be steel
helpful in aiding the transport of reamer cuttings out of the pipe or other product desired to be installed. Alternative
borehole 12. Several different system con?gurations may be casings of a more ?exible material can be utiliZedisuch as
utiliZed for upsiZing the borehole 12 and installing product. high-density polyethylene (HDPE), polyvinyl chloride
For instance, the borehole 12 is typically upsiZed by a (PVC) or similar materials. Flexible casing is bene?cial
backreaming process Wherein a reaming device is pulled When the desired borehole 12 contains curvilinear segments.
back toWard the drilling machine 10 after the initial borehole A ?ange 306 connected to a doWnhole end of the casing 304
has been completed. HoWever, the borehole 12 may also be serves to protect the casing from the rotating cutting head
upsiZed by a forWard reaming process Wherein a reaming 302. The ?ange 306 also prevents cuttings from plugging off
device is pushed in the opposite direction by a drill string 20. a narroW relief annulus 308 betWeen the casing and the
ForWard reaming is particularly suitable for “blind bore upsiZed borehole 12. Were this plugging to occur, advance
holes”ias illustrated in FIG. llithat cannot have an of the casing 304 may be impeded. Preferably, the cuttings
ending (target) pit or a surface exit point. Such boreholes 12 are conveyed uphole in an annulus betWeen the casing 304
are frequently desirable in environmental monitoring and and the exterior surface of the outer member 36 of the drill
remediation applications, as Well as in other applications string 20ibeing substantially aided by rotation of its ?ex
Where sensors or equipment are to be placed underground. ible ?ighting 62. The large volumes of drilling ?uid asso
[0054] With reference noW to FIG. 11, three stages of a ciated With typical borehole upsiZing processes may thus be
blind hole forWard reaming installation are shoWn. In this substantially reduced, and in some cases essentially elimi
example, a pit-launched HDD drilling machine 10 is uti nated. This aspect is particularly advantageous for the
liZed. HoWever, a surface-launched drilling machine as above-mentioned environmental applications.
described previously is also suitable for this purpose. The [0058] The protective ?ange 306 may have provisions (not
borehole 12 may be initiated (FIG. 11a) in one of the shoWn) to transfer a toWing force to the doWnhole end of the
manners previously described, Wherein the drilling machine casing 304iutiliZing a portion of the thrust applied to the
10 is adapted for utiliZing a dual-member drill string 20. The cutter head 302 by the outer drill string member 36. In
inner drill string member 38 is equipped With a directional addition (or alternately), the drilling machine 10 may have
doWnhole tool assembly 18 to direct the borehole 12 along provisions (not shoWn) for applying a pushing force on an
the desired path. As Will noW be explained, the leading end uphole end of the casing 304 to move it into the reamed
of the inner drill string member 38 in effect serves as a guide borehole 12 in concert With the drill string 20. Casing
rail for the forWard reaming outer drill string member 36. pushing techniques are Well-known and need not be
Alternatively, a conventional single member drill string can described herein.
be used to drill the initial portion of the borehole. [0059] With continued reference to FIGS. 11b and 110, it
[0055] In this ?rst operational stage, shoWn in FIG. 11a, as should be clear that the doWnhole directional tool 18 must
little as one segment 300 or up to several segments of the advance beyond the desired end point of the ?nal cased or
inner drill string member 38 are preferably drilled along the lined borehole 12. Where the drilling machine 10 to continue
desired path. The length drilled before adding the outer to advance the dual-member drill string 20 in lock-step, the
member 36 is dependent upon the lateral support offered by directional tool 18 Will then be located the length of the
the local soil conditions. This may be enhanced by selecting guide segment(s) 300 beyond the desired end point. In some
an inner drill string member 38 of OD. nearly as large as applications it may be undesirable to disturb the soil for such
that of the doWnhole tool 18, and/or by the addition of a distance beyond the end of the installed casing 304. The
centraliZers (not shoWn) along these initial segments 300 of feature described in respect to FIG. 3, for axially advancing
the inner drill string. the outer members 36 of the drill string 20 With respect to the
[0056] Referring noW to FIG. 11b, the outer drill string 36 inner members 38, may be utiliZed to move the cutter head
having auger-like projections 62 is slidably ?tted around and 302 toWard the directional tool 18 once the tool has cleared
sequentially advanced in conjunction With the guide seg the desired end point. The resulting disturbed interval Will
ments 300 of the inner drill string 38. Thus the drill string 20 then be limited to the length of the directional tool 18 alone.
US 2008/0073123 A1 Mar. 27, 2008

[0060] Once the casing 304 has been advanced to the No. l0/8l3,824, incorporated herein by its reference. The
desired point in the borehole 12, a collapsible feature built backreaming apparatus 602 may comprise a cutting face 610
into the cutter head 302 allows it to be retracted into the and a support barrel 612, Wherein the cutting face is rota
casing. Such collapsible features are commonly knoWn and tionally disconnected from the support barrel. At the junc
need not be described herein. Alternately, a sacri?cial cutter tion betWeen the cutting face 610 and the barrel 612, a ?ange
head 302 could be ?tted to the outer drill string 36 then 614 on the support barrel of approximately the same diam
released and abandoned in the borehole 12. In stage 3 of the eter as the upsiZed borehole 12 prevents more than an
process, shoWn in FIG. 110, the outer 36 and inner members inconsequential amount of the reamer cuttings from entering
38 of the drill strings are WithdraWn from the boreholei the annulus betWeen the upsiZed borehole 12 and the support
leaving the casing 304 in place. barreliand later the casing 606. Alternately the support
[0061] Referring noW to FIG. 12, there is shoWn therein barrel 612 itself may be a close ?t in the borehole 12, thereby
the more typical borehole upsiZing process knoWn as “back eliminating need of the ?ange 614. Either of these tech
reaming”iWherein the borehole 12 is enlarged as the drill niques minimiZes the possibility for the casing 606 to
string 20 is draWn back toWard the drilling machine 10. For become bound up before it has been completely draWn into
a backreaming operation, a doWnhole tool assembly 18 the borehole 12. The support barrel 612 preferably com
preferably comprises a backreaming assembly 600 con prises provisions at an opposite endisuch as connection
nected to the doWnhole end of the dual-member drill string collar 616 and fasteners 618ifor an axially aligned and
20 after the drill string has exited the distal end of the substantially sealed connection to the casing 606. The slur
initially created borehole 12. As shoWn in FIG. 12, the drill ried reamer cuttings are thus encouraged to ?nd other
string 20 is being WithdraWn from the borehole 12. In effect, pathWays to the surface. Some cuttings may ?oW toWard the
the backreaming assembly 600 is advancing toWard the drilling machine 10, for instance through an annulus
drilling machine 10 as it upsiZes the borehole 12 to a larger betWeen the drill string 20 and the borehole 12 or through an
diameter. annulus betWeen the inner 38 and outer members 36 of the
[0062] The backreaming assembly 600 may comprise a drill string. This direction of ?oW may be encouraged by
backreaming apparatus 602 and a spoil (cuttings) conveying utiliZing the previously described ?exible projections 62
arrangement 604. The conveying arrangement 604 is pref attached to the outside of the outer member 36. Altema
erably disposed Within a casing 606 or product to be tively, projections may be attached to the inside (not shoWn)
installed in the borehole 12. The casing 606 may be a of the outer member of the drill string if the ?oW path for
temporary or permanent liner for the upsiZed borehole 12. spoil is the inter-pipe annulus.
Altemately, in certain supportive soil conditions such as [0065] An alternate and usually preferred ?oW path for the
continuous rock, the assembly 600 may be utiliZed Without reamer cuttings is in the opposite direction. Cuttings pass
a casing. The casing 606 may be steel pipe. Alternately, a through purposeful openings (not shoWn) in the cutting face
more ?exible material can be utiliZedisuch as high-density 610 and enter the support barrel 612. An auger-like arrange
polyethylene (HDPE), polyvinyl chloride (PVC) or similar ment 620 rotating Within the support barrel 612 and the
materials. The casing 606 and its internal conveying casing 606 enables the cuttings to be readily moved toWard
arrangement 604 may be pre-assembled into one continuous the distal end of the casing or of the borehole 12. The auger
length prior to initiation of the borehole 12 upsiZing process. arrangement 620 comprises a drive shaft 622 adapted to be
Where available space is limiting, pre-assembly may alter connected to the drill string 20, a bearing arrangement 624,
nately be as tWo or more segments to be interconnected as and an auger shaft 626. One member of the dual-member
the ?rst and subsequent segments are draWn into the upsiZed drill string 20 is utiliZed to rotate the auger arrangement 620,
borehole 12. While the other member rotates the cutting face 610. Pref
[0063] Drilling ?uid is typically pumped doWnhole erably, the outer member 36 of the drill string 20 is used to
through the drill string 20 to aid the removal of su?icient rotate the cutting face 610, While the inner members 38
cuttings (spoil) 608 to create diametrical space for installa rotate the auger arrangement 620 by Way of a geometrical
tion of the casing or product pipe 606. The present invention connection 628 to the drive shaft 622. The drive shaft 622 is
is particularly suited for aiding this removal process. It is supported in the barrel 612 by the bearing arrangement 624.
also suitable for this same purpose With particle-creating dry The bearing arrangement 624 alloWs the auger shaft 626 to
boring techniques, such as those that utiliZe high volume air rotate Within and independent of the barrel 612. The auger
for cuttings removal. For larger diameter casings or pipes shaft 626 has ?exible helical ?ighting projections 628 simi
606 the borehole 12 upsiZing process may involve multiple lar to those previously described and is preferably extended
passes through the borehole to increase its siZe in a step-Wise into and along the casing 606. Alternately, the auger shaft
manner. Sequentially larger diameter backreaming assem 626 could be a ribbon auger With or Without a shaft or central
blies 600 Would be utiliZed in that case. The conveyed spoil tube.
608 may be discharged from the trailing end of the pipe 606 [0066] Torque-multiplying arrangements of gears (not
and deposited in a WindoW along the ground surface as the shoWn) could comprise either or both rotational connections
pipe advances into the borehole 12. Alternately, a spoil of the dual-member drill string 20 to the backreaming
collection apparatus (not shoWn) such as a vacuum truck assembly 600. This may be particularly bene?cial for the
may be adapted to the discharge point to minimiZe site auger arrangement 620 being driven by the inner drill string
disturbance. 38 and its drive group 52 (FIG. 3), Which typically have a
[0064] Turning noW to FIG. 13, and With continued ref higher speed and a loWer torque capability than the outer
erence to FIG. 12, the assembly 600 is comprised of a drill string 36 and its drive group 50. For instance, the inner
backreaming apparatus 602. The backreaming apparatus 602 members 38 may have up to tWo (2) times greater rotational
may be a steerable or non-steerable reamer. Steerable ream speed capability but might deliver less than one-third (1/3)
ers are disclosed in Us. Provisional Patent Application Ser. the torque capability of the outer members 36.