CA1164855A - Rolling cutter drill bit - Google Patents

Abstract

ABSTRACT OF THE INVENTION

This invention discloses a rolling cone drilling bit having a plurality of cutters which have inserted therein hard metal cutting elements preferably formed of tungsten carbide alloy; and in which bit the rolling cone cutters are located in such a manner that their rotational axes are greatly offset from the rotational axis of the drill bit. In addition, a fluid jetting system is provided in the invention that directs a pres-surized fluid spray across the main cutting inserts and against the formation face so that when the drill bit is used in its most advantageous areas, such as the soft, medium-soft and plas-tic formations, the jetting system prevents "balling up" of the cutters and greatly increases the drilling efficiency of the bit.

Description

1 ~6~85S

ROLLING CUTTE~ DRILL BIT
BACKGROUND OF T~IE I NVENTIO~l In the drilling of boreholes through underground for-mations for the purposes of locating and producing oil and gas, ~1 S and for the purposes of mining and production of steam energy through thermal wells, the most common type of drilling appara-tus used today is the tri-cone rolling cuttcr drill bit. This bit generally comprises a central body scction having thrce legs extending downwardly thercfrom. Eacll leg has an in;;.lrdl~

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projecting bearing journal upon which is rotatably mounted a frustoconical cutter. General]y, the most prevalent type of cutting structure utilized in the tri-cone bit is tlle tungsten carbide insert cutting structure. Tungsten carbide cutting elements are press-fit in holes drilled in the frustoconical cutters and protrude outwardly to provide a dig~ing, crushing and gouging action on the bottom of the borehole as the bit is rotated.

The tungsten carbide insert bit has generally been known and used for approximately the last 30 years. For the first 20 years (1950 to about 1970), those in the art felt that the cutting structure of the insert bit should be of the non-offset or "true rolling cone" type. The offset, which is de-fined as the amount by which the rotational axes of the rolling cutters is offset from the rotational axis of the main bit, was a feature found in milled tooth bits but believed to be detri-mental to insert bits because of the breakage problem in the tungsten carbide inserts when the additional drag forces were introduced through the use of offset.

In February, 1970, a new bit design was patented by P. W. Schumacher, Jr. (U. S. Patent No. 3,495,668) in whicll, for the first time, an insert bit successfully incorporated offset axis cutters to achieve greater gouging and scraping action in the borehole. A subsequent patent, U. S. 3,696,876, issued to Ott in October, 1972, also disclosed a similar invention wherein offset axis cutting elements were incorporated into an insert bit.

Drilling bits incorporating the novel combination of offset cutters and tungsten carbide inserts were successfully introduced by the assignèe of the present invention, Reed Rock Bit Company, in 1970, ancl have become the most prevalent type of drill bits in the drilling industry over t}lC ~ast tel~ ycars.
This second generation of drill bits utilizing offset axes and tungsten carbide inserts are particularly advantageous in soft to medium-soft formations by reason of their introduction of a gouging and scraping action which enhances the drilling efficien-cy and rate of penetration of the bit in these Eormations. The amount of offset utilized in these bits ranc~es on the order of from about 1/64 to about 1/32 inch offset per inch of drill bit diameter. For instance, a 7-7/8 inch bit having offset would have from 1/8 inch to 1/4 inch total offset in the cutters.

Conventional drilling bits currently on the market are limited in the a~ount of offset introduced into the cutters to about 1/32 inch of offset per inch of diameter. Thus, the maximum amount of offset utilized in these soft formation bits currently runs about 1/4 inch in a 7-7/8 inch diameter bit.
During this ten year period when offset axis insert bits have been made commercially successful, those skilled in the art of drill bit technology generally have followed the principle that any additional offset in the cutters above about 1/32 inch per inch of bit diameter would not add any sic~nificant efficiency or increased drilling r~te to the bit to justify the increased breakage that such increased offset would introduce; In faet, drilling tests conducted utilizing insert bits with offset some-what greater than 1/32 inch per inch of bit diameter have indi-eated insignificant gains in rate of penetration, but larger incidences of insert breakage. Thus, those skilled in the art have restricted their insert bit designs to havill-3 an offset range of from zero to 1/32 inch per inch of bit diameter.

The present invention utilizes a unique insert bit design havin~ ~reat amounts of offset in the cuttillg structure far exceeding those ranges utilized in conventional offset-axis insert bits. It was found by this inventor that when offset equal to or greater than 1/16 inch per inch of bit diameter was introduced into a tri-cone insert bit, that greatly significant increases in rate of penetration and bit uerformance can be ob-tained. For some reason unknown to the inventor, the penetration rate and drilling efficiency of an offset insert bit does not increase significantly from about 1/32 inch offset per inch of bit diameter (upper range of conventional insert offset bits) up to about 1/16 inch offset per inch of bit diameter. It was discovered though that beginning at about 1/16 inch offset per inch of bit diameter a significant jump in the rate of penetra-tion and drilling efficiency was noted.

The use of large amounts of offset in milled-tooth rolling cutter drill bits may not in itself be a novel concept.
For instance, see U. S. Patent No. 1,388,456 to H. W. Fletcher, dated August 23, 1921, in which a two-cone rolling cutter drill bit having milled tooth cutters apparently incorporated a large amount of offset in the two cutters. The patent discloses no specific amount of offset to be utilized and,-as far as this in-ventor is aware, no commercial embodiment of the Fletcher design ever became successful. The conventional milled tooth drill bits which have been available for the last 40 years have generally utilized offset in the range of 1/64 to 1/32 inch per inch of bit diameter and have been tri-cone bits. It was not until 1970, and the issuance of the Schumacher patent, that the indus-try was intro~luced to the use o~ insert type bits ut:ilizillg t~leoffset already present in milled tooth bits. The reason that the hiyh offset cutters were not thought practicaL ~as that in-creases in offset above the 1/32 incll limit previously mentioned would ~ain very little in cutting efficiency, but increased the amount of breakage of tungsten carbide inserts in the insert type bits. Also, increasing the offset necessarily requires re-ducing the size of the cutter cones to prevent interferellce be-tween the inserts on adjacent cones. Smaller cones mean smaller bearing areas and/or thinner cone shells, both of which add to earlier bit failure. Also, greater offset means less efficient intermeshing of inserts on adjacent cones which in turn reduces the amount of self-cleaning of the inserts and increases "balling-up" .

Conventional jetting systems are generally made up of two different types. The oldest type is the regular drilling fluid system where large, relatively unrestricted fluid openings are provided in the bit body directly above the cutter cones to allow a low pressure flow of the drilling fluid to fall on the cones and move around the cones to the bottom of the borehole.By necessity, this is a low-volume, low-velocity flow since the fluid stream impinges directly upon the cutter face, and abra-sion of the cones is a serious problem under these circumstances.
The second type of conventional bit fluid system comprises the "jet" bits. In a jet bit a high pressure jet of fluid is gene-rated from the bit body directly against the formation face with-out impinging on any cutting elements or any yortion of the bit.
In some instances, the so-called jet bits have fluid nozzles ex-tending from the bit bodies all the way downward to a point only a fraction of an inch above the formation ~ace to maximize 1 1~4855 hydraulic ellergy of thc fluid strc;~ im~ ing tllc ~oLI~la~ioll face. The conventional jet bits do not emit fluid against any eutting elements because of the adverse effeet of erosion from the h:Lc3h-pressure drillinc3 fluid. The present invention differs S from these two convclltional ty~es in that it uses a directed jet spray which i~pinges directly upon the cutter inserts.

The present invention discloses an insert ty~L~e bit, as opposed to a milled tooth bit, which insert bit utilizes rol-ling eone cutting elements rotatably mounted on lugs having ro-tational a~es ~ith large offset from the rotational axis of thedrill bit. The amount of offset ranges between 1/16 and 1/8 ineh per ineh of bit diameter. The resulting invention produces greatly inereased rates of pelletration and clrilLing efficienc~
when utilized in soft to medium-soft formations. It should bc noted that the present invention, when embodied in a tri-eone oilwell drilling bit, suffers a greater amount of erosion and breakage of the hard metal eutting inserts in the eones, but the total gain in drilling effieieney and rate of penetration far offsets the inereased wear and breakage of the eutting ele-ments.

In addition to the aforementioned unique drill biteonstruetion, the present invention also embodies a new and unique nozzle jetting system for delivering drilling fluid to the eut-ting elements and the faee of the formation as it is bcing drilled.
This je~ting system utilizes direeted noæzles wilieh ereate a spray of pressurized drilling fluid and dirccts this s"ray across the protruding tungstcn carbide inserts and against the formation face. The new jetting system provides a dual function of clean-ing material from the inserts and also sweepinc3 the euttings , ~~

1 lS~8~
from the borehole face. This system is particularlv advantageous when drilling through those certain types of formations which, because of their softness or ductility, become very plastic during drilling operations, and tend to "ball up" in the spaces between the inserts on the cutters. This "balling up" greatly reduces the rate of penetration and the cutting efficiency of drill bits when penetrating such plastic formations. The new jetting system provides a plurality of fluid jets directed at preselected angles to spray drilling fluid across the inserts without impinging the cutter cone surfaces, with the spray also being d;rected against the formation face to further flush and clean the cuttings as they are gouged and scraped out of the formation.
In one broad aspect, the invention pertains to a rotary drill bit for drilling a well bore. The drill bit com-prises a bit body having a threaded pin at its upper end adapted to be detachably secured to a drill pipe for rotating the bit, a chamber adapted to receive drilling fluid under pressure from the drill pipe, a plurality of depending legs at its lower end with each leg being spaced from the other legs and having an inwardly and downwardly extending generally cylindrical bearing journal at its lower end, and a plurality of nozzles in flow communication with the chamber for exit of the drilling fluid from the bit body. The drill bit further comprises a plurality of roller cutters, one for each leg with each roller cutter having a generally conical cutter body rotatably mounted on the bearing journal of the respective leg and having a plurality of cutting elements on the body. Each of the nozzles has a nozzle orifice above the central axis of the bearing journal of an adjacent roller cutter at its inner end with respect to the bit ~ody, and each nozzle directs the drilling fluid to flow downwardly and in the direction opposite to the direction of rotation of the bit, with the fluid flowing in a stream gener--6a-1 164~55 generally tangent to the cutter body of the adjacent roller cutter and thereafter impinging portions of the bottom of the well bore closely adjacent to, but spaced apart from the points of engagement of the cutting elements of the a~jacent roller cutter with the bottom of the bore, whereby the drilling fluid engages and cleans at least some of the cutting elements and well bore bottom immediately prior to the engagement of the portions of the well bore bottom by the cutting elements for enhanced drill bit cutting action.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a side view of one em~odiment of the pre-sent invention comprising a three-cone bit. Fiaure 2 is an axial bottom view of the three-cone bit of Figure 1. Figure 3 is a schematic representation of the three cutter cones of the bit of Figures 1 and 2, showing the concept of offset cutter axes. Figure 4 is a diagram of the cutter configuration in one embodiment of the invention illustrating the location and placement of the inserts in the cutter and also indicating the offset of the cutters. Figure 5 is a schematic diagram showing an overlay of the insert pattern of all three cutters of Figure 4 to show bottom hole coverage of the bit. Figure 6 is a schematic illustration of one embodiment of this invention indicating the directed nozzle system and its interraction with the cutter and the formation. Figures 7 and 8 are illustrations of a particular embodiment of the directed nozzle system shown schema-8 5 ~

tically in Figulc 6; L~ urc 7 is an a~iaL cnd-view o~ a ccntra1 nozzle systcm, ancl Figurc 8 is a partial cross-sectional side view of the nozzle of Figure 7. Figures 9 through 11 are different views of a second cmbodiment of the dirccted nozzle J S system utilizing an intermediate jet. Fi~urcsl2 through 14 - illustrate a.Yial bottom views of a third embodiment of the pre-sent invention which utilizes a peripheral directed nozæle sys-tem.

DESCRIPTION OF TlIE PREFI:RRED E~'~lBODI``lLNTS

Referring to Figure l, a first embodiment of the in-vention shown in isometric view, this embo~iment comprises a tri-cone drilling bit 10 having a central main body section 12 with an upwardly extended threaded pin end 14. The threaded pin 14 comprises a tapered pin connection adapted for threadedly engag-15 ing the female end of a section of drill stem. The body section 12 has three downwardly extending legs 1~ formed thereon, each of which contains a rotatably mounted frustoconical cutter 16.
A plurality of nozzles 20 may be located in the periphery of the body section 12 aimed downward past cutters 16. In Figure 2, 20 which is an a~ial view looking up from the borehole toward the bottom of the bit, the cutters 16 of bit 10 are shown with hard metal cutting elements 22 projecting from raised lands 24 formed on the surfaces of the cones. In a typical e~odiment the in-serts generally would comprise three differcnt categories, thc 25 gauge row inserts 26, intermediate row inserts 28, and nose in-serts 30. As is well ~nown in the indust:ry, tlle inserts are sc-cured in the cones ~y drilling a hole in thc conc for each in-sert with the hole having a slightly smaller diameter than the 1 16~8$~

insert dialllc~tcr, thus resultin;3 in an intcLfcrence fit. The in-serts are then presse(l under relatively hic~h pressure into the holes and the press fit insures that the inserts are securely held in the cones.

Although not shown in the drawings, eacil cutter 16 is rotatably mounted on a cylindrical bearing journal machined on each lcg 8, as is well known in the art. ~s is also well known in t'ne art, bearings such as roller bcarings, ball bear-ings, and/or sleeve bearings are located between the cutter and the bearing journal to provide the rotational mounting. In one preferred embodiment, cutters were mounted on bearing journals with sleeve bearings and ball bearings therebc~wecn as illustra-ted in the ~lenry W. Murdoch patents, U. S. 3,990,751 and U. S.
4,074,922, granted November 9, 1976, and February 21, 1978, re-spectively, and assigned to Reed Tool Company of ~louston, Texas.

In Figure 3, the cutters 16 are illustrated schematic-ally as simple frustoconical figures. ~ach cutter cone 16 has an axis of rotation 32 passing substantially through the center of the frustoconical figure. The central rotational axis of the bit 10 is illustrated as point 34 in Figure 3 since Figure 3 is taken from a view looking directly along t`nc rotational axis of the bit. From Figure 3, it can be seen that because of the offset of axes 32, none of the axes intersect a~is 34 of the bit. In this flat projection, the intersection of the axcs 32 forms an equilateral triangle 36. The amount of offset mcasurc~
in a linear distance for any yarticular bit can be determined from a full scale ~iayram similar to Figure 3 for that bit by measuring the distance from axis 3~ to tlle mi~-point of any sidc of triangle 36.

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Referring now to Flyure il, in whic)l a cutter layout is illustrate~, the profilcs or cross-sectiolls o~ eacll of tlle cutters on thc tri-cone bit of the preferrc~ em~odiment are layed out in relation to cach othcr to show tlle intermesh of the cut-ting elements or inscrts 22. Generally, each cutter in a tri-cone bit is of a slightly different profile in order to allow optimum spacinc~ o~ the inserts for tlle entirc blt. In Figure 4, the threc cutters are labe1cd ~, 13 and C. Tlle C cutter has been divided to illustrate its intermesh with both cutters ~ and B. It should be noted that the projections have been flattened out, an~ becausc of the two-dimensional aspect of this relation-ship, a distortion in the true three dimensional relationship of the cutters is necessary. In Figure 4, the central axis of rotation 34 of the bit is indicated. Each cutter ~, B and C, has a rotational axis 32 wllicll is o~sct b~ a ~list~ncc ~ Erc~m an imaginary axis 32' which is parallel to the actual axis 32 and passes through point 34 which is the bit rotational axis.

Figure 5 is a cutter profile which is an overlay of one-half of each of the cutters A, B and C to indicate the place-ment of all of the inserts with respect to bottom hole coverayc.Each insert in the ~rofile of Figure 5 is 1a~ele-l accol~ to the particular cutter cone in which the insert is located. The angle X is indicated to show the journal angle of thc bit. The journal angle is the angle that the bearing journal axis, which coincides with the rotational axis 32 of the cutter, makes with a plane normal to the bit rotational axis 34.

In this particular embodilnellt it wa, foul~ llat ~lle preferred range of insert protrusion above thc cuttcr sur~a~c 1 16~$

shoulcl ~-e greater than or e(~u.ll to al~out c~ne-half the diametcr of the insert. ~T1Y PrO~rUSiOn SiC3nifiCant1Y 1eSS t:h.ln one-l~alt the ~iameter would make the gouging ancl seraping aetion result-ing from the larcJe amount of offset ineffective. The preferred ranc3e of insert protrusion is from one-half to one times th~
insert cliameter. The preferred shape vE the protruding portion of the insert is eonieal or chisel. ~ece~)table alterllate sha~es are the hcmispherieal ancl the sharpelled hemispllerieal inserts.

Whereas the insert ean be made of any hard metal alloy sueh as titanium earbide, tantalum earbide, or chromium carbide, in a suitable rnatrix, one particular range of embodiments uti-lizes tungsten earbide in a eobalt matrix. The cobalt eontent ranges from about 5~ to about 20~ by ~ei(lht of the insert mlteri-al, ~ith the remainder of tlle mc~tal l~eillcl eitl~el~ silll;e~ l Ol cast tungsten car~ide, or both. The hardness of thc inserts is eon-trolled by varying the eobalt eontent and by other well-knowll methods. The hardness ranges from about 85 Roekwell A to about 90 Roekwell ~. In one partieular embodiment, eonieal inserts having a protrusion greater than one-half of their diameter were used, with the inserts being made of tungsten earbide-eobalt alloy, having a cobalt eontent of aroullcl l2 anCI a l1aL(IneSS Of about 86.5 Roekwell A.

Referring now to Figure 6, a schematie sketch of the directed nozzle fluid system of the invention is illustrated.
In Figure G, a generally eylindrieal jet no%zle ~0 is ShOWIl conneeted to bit body 12 and eommunicating witll a hiclh pressure drillinc3 fluid passage 42 passincJ therctllrv~ . t~vzz~e 40 h.
an exit jet 44 from which high pressure drillillg fluicl 46 i5 emitted in a tight directecl spray. ~it leg 1~ is illustrated h~vin~ conical cutter 16 located tiereQn. A direction arrow 48 is drawn on le~ 18 to indicate the direction of movement of the bit leg in the borehole as the drill bit is rotated. Likewise, a second rotation arrow 50 is drawn on cutter 16 to indicate the simultaneous rotation of cutter 16 with movement of bit 10 in the borehole. The high-pressure drilling fluid stream 46 is directed in a closely controlled direction such that the fluid stream is either eYactly tangent with the surface of cutter 16 or slightly displaced therefrom as shown in the drawing. The placement of stream 46 in a tangential relationship with cutter 16 allows effective cleaning of inserts 22 as they move through stream 46, but also prevents abrasive erosion of the cutter shell 16 which would occur if 46 impinged squarely thereon. Although the preferre~ embodiment is to have stream 46 either tangential to or slightly displaced from cutter shell 16, a slight impinge-ment of 46 with cutter shell 16 would not be highly detrimental due to the very slight angle of incidence of stream 46 against the cutter surface. As fluid stream 46 passes over inserts 2~
and close to cutter shell 16, it dislodges material built up be-tween inserts 22 and drives it downward with the motion of the cutter 16. After the fluid passes the inserts it impinges the bottom 52 of the borehole and travels along the bottom picking up cuttings as they are chipped and gouged from the formation by inserts 22. The drilling fluid then passes below the cutter 16 and moves back upward outside the drill bit and up through the borehole in the conventional manner.

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Referring now to Fiyures 7 and ~, one embodiment of the directed jetting system is disclosed. This embodiment 1 lG48~5 utili7.c; .~ mult-i-ori~i.ce jct n~zzlc WiliC21 L~rOtlA-lCIes dOWrltiarCIly from the ccntr;ll arca o~ thc bit bo(ly towards t ~ ' ccntral arca betwcen the three conical cutters. Fi~ure 7 is a partial a~ial encl-viet~ c E the bit 10 partially illa:lstratinc3 two cutters 16 and thc location of the multi-orificl jet 5f. Jet 56 is gene-rally cy].incl--ical in nature having a ~evcllccl cdc3c 5~ at thc downward projectin(3 end therco and hclving three noz7.1e openinc3s 60 formecl through the bevellecl surface 58. .~ ~lat, closed cncl 62 is locatcd at tllc bottom of the noz.zle. .~ ~luicl sl~ray 64 is shown emanating from one of the o~)eninc3s 60. This spray passes across the inserts in the cuttcrs 16 wi~hout impinging on the actual cutter surfaces. The spray cleanses any packed euttings which migllt be lodged between the various inserts and then moves outward and then downward to sweep the bottom of the borehole in ront of the eutters as they roll i.nto the formation surfaee. Fic3ure ~ is a partial sicic vicw o~ th. b.i.~ o1 li~1urc 7 snowinc3 a single eutter 16 and the multi-jet nozzle 56. In this figure, the nozzle 56 is shown in a eross-seeti~nal diagram and it ean be seen that the nozzle has a eentral passage 66 which eommunicates with the nozzle openings 60. Nozzle 56 is securely located in a bore 68 formed in bit body 12. Bit bocly 12 has a fluid cavity 70 ~ormed therein which communicatcs with threaded pin end 14 which also is tubular in naturc. Th~ls, it can ~e that drilling fluid pumped down the clril~ string passcs through threaded pin 14 into bit cavity 70, through no~.zle bore 66 and out the nozzle opening 60 into a jet or spray 64 whic}l imyinc3cs the major cutti.ng inserts on cone 16 and thell is dirccted eit!ler . agai.nst the face of the borehole or, as shown in 8, may ~e di-reeted ac3~ st t~le w~.ll o~ t}lc? bor~?l~ Wl~(~L~ 2.].~ vc~
down the wall and across the formatiorl face to ~ic,; ui~ ad-litio;lll loose cuttinc3s thereon.

1 16~8~5 Referrin~ nQW to l'i~ures ~ throu~ll 11, a second em-bodimcnt of the clirectcd nozzle systcm is disclosed in which the fluid jetting system is directed across the main cutting inserts and impincJes ~irectly upon the borehole face. In this embodi-ment, the projected nozzle arrangement is replaced by a slantedjet configur~tion formed through the wall of the bit body 12 and communicating with bit cavity 70. Figure 9 is a partial axial view showing part of two cutter concs 16, thc bit body 12 and a directed jet passage 74. The drilling fluid is emitted from jet passage 74 in a stream 76 which impinges the major cutting inserts in cones 16 and passes downward to impinge the bottom of the borehole. In this embodiment three of the jet passages 74 are formed in bit body 12 so that each conical cutter l6 has one jet passage associated therewith for swéeping cuttings from the inserts and impinging the bottom of the borehole. Figure 10 is a side view of one cutter looking from the central axis of the bit radially outward at the cutter. Jet passage 74 passes through bit body 12, communicating with the drilling fluid in the drill string by means of cavity 70 and pin 14. Figure 11 is a partial side schematic view of the cutter 16 of Figure 10 rotated approximately 90 degrees. In Figure 11 one of the three jet passages 74 is shown in comrnunication with cavity 70 and emitting a jet stream 60 of drilling fluid passing across the cutting inserts of cutter 16 and impinging the borehole bottom.

Referring to Figures 12 through 14, two additional embodiments of the present invention with the directed nozzle - system are indicate~. In Fi~ure 12 a drill bit is shown in the axial view loolcing up from the bottom of the borchole. The bit has three conical cutters 16 having a plurality of tungsten car-bide inserts 22 securely held in raised lands 24 on the cutters.

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1 1~485S
set of thrce peril~herally dil-~cted nozzles 80 are located around the outer periphery of l~it body 12, extending downward therefrom into the generally open areas between the outèr rows of inserts on the conical cutters. The embodiment of Figure 12 utilizes the three directed nozzles which are generally cylin-drical in nature, each havin~ a bevelled face 82 and a jet pas-sage 84 formed through face 82 and COn~lUnicatiny with a ccntral bore passage in nozzle 80. Jet passage 84 is formed such that a directed spray of fluid 86 is emitted therefrom which impinges across the main cutting inserts of the conical cutters which are located clockwise from each nozzle 80. Each jet passage 84 is aimed in a generally circumferential direction with re-spect to bit body 12 and in a tangential direction to cutter cones 16 such that the fluid spray emitted therefrom does not impinge squarely on the cone 16. Each nozzle 80 having thc single jet passage 84 is arranged to clean the inserts on the cutter located in a clockwise directlon from the nozzle. After the spray passes across the main cutting inserts, it is directed against the bottom of the borehole to further provide cleaning action during the arilling operation. In Figure 13, a slightly different embodiment of the peripheral nozzle system is dis-closed in which three double jet nozzles 90 are located around the periphery of the bit bottom extending downwardly therefrom between the outer edges of the cones 16. Each nozzle 90 has two ~ 25 jet passages formed therein passing through opposed bevelled ; faces 92 and 94. Thus, each nozzle 90 has a jet passage direct-ed at each cutter cone 16 located adjaccllt tl~ereto. Fi~ure 1~
is a diagramatic sketch showing the nozzle 90 from the side and illustratiny the two bevelled faces 92 and 94. The jet passages 96 pass through the two bevelled faces and communicate with an .

1 16~8~5 inncr ~orc in no..-.lcs 90. ~rceisuri~.cd drillillc3 ~luid ~)~sscs through the drill bit and into the nozzlcs 90 in a manncr simi-lar to that of the embodiment shown in Figurel2.

The nozzles utilized in the embodiments illustrated in Figures 6 through 14 are preferably forme~ by casting, for-ging, and/or machillincl from a hard mcltcr;al sucll ~s stc~cl or ono of the hard metal alloys such as tungstcn carbide in a cobalt matri~. The tungsten carbide-cobalt alloy can be of the type using sintered tungsten carbide, cast tungsten carbide, or a combination of both. Alternatively, the nozzles could be formed of any material which successfully resists erosion.

Th~s, the present invention cleEincs scveral ulli~lue features, one of wllich is the utilization o~ an cxtrcmc amount of offset in the cutter axes of an insert type bit. Another feature is the novel fluid jetting system which provides a high -ly efficient cleaning of the protruding inserts as well as a cleaning of the formation face as it is being drilled.

This system directs the high-~rcssure [luid jcL at or near a tangent to the cutter cones in a position to swccp the main cutting inserts, thereby cleaning the balled up material therefrom, and the fluid stream thercafter passes from the in-sert region to the formation face directly, or from the insert region to the borehole wall and thcn down thc wall and across the formation face.

~lthough certain prefcrrcd embodimcnts o~ the prcscnt invention have been herein described in order to provide an 1 16~8S~

underst.Illdillc~ oc thc ~3cneral prilI~ e; o[ th(' inVC.`n~iOIl, it. wi be ai)~reciate~d tlIat various chal~gcs and innovations can be ef-fected in thc described cIrill bit structurc Wit}IO~It cleparture frorn thesc ~rinciples. For e~ample, whereas a tri-cone bit havin~3 thrce conical cutters is disclosed, it is clear that the ~it structùrc could be of thc four-conc type, and still embocly the principlcs of the present inventiolI. Like~ise, the number ancl location oE the directcd nozzles coulcl be variecI from those shown and still obtain equivalent operation, function, and re-sults. Thus, al1 modificatiolIs and Cllall(3CS oE this ~pc al-c deemed to embraced by the spirit and scope of the invention e~-cept as the same may be necessarily limitecl by the appended claims or reasonable equivalents thereof.

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A rotary drill bit for drilling a well bore, the bit comprising:
a bit body having a threaded pin at its upper end adapted to be detachably secured to drill pipe for rotating the bit, a chamber therein adapted to receive drilling fluid under pressure from the drill pipe, a plurality of depending legs at its lower end, each leg being spaced from the other legs and having an inwardly and downwardly extending, generally cylindrical bearing journal at its lower end, and a plurality of nozzles in flow communication with the chamber for exit of the drilling fluid from the bit body; and a plurality of roller cutters, one for each leg, each roller cutter comprising a generally conical cutter body rotatably mounted on the bearing journal of the respective leg and a plurality of cutting elements on the body;
each of said nozzles having a nozzle orifice above the central axis of the bearing journal of an adjacent roller cutter at its inner end with respect to the bit body, each nozzle directing the drilling fluid to flow downwardly and in the direction opposite to the direction of rotation of the bit, with the fluid flowing in a stream generally tangent to the cutter body of the adjacent roller cutter and thereafter im-pinging portions of the bottom of the well bore closely adjacent to, but spaced apart from the points of engagement of the cut-ting elements of the adjacent roller cutter with the bottom of the bore, whereby the drilling fluid engages and cleans at least some of the cutting elements and well bore bottom immediate-ly prior to the engagement of said portions of the well bore bottom by the cutting elements for enhanced drill bit cutting action.

2. A drill bit as set forth in claim 1 wherein said cutting elements for each roller cutter are generally elongate members of tungsten carbide material and are mounted on the roller cutter body with a portion thereof projecting outwardly beyond the generally conical surface of the roller cutter body

3. A drill bit as set forth in claim 1 wherein the cutting elements of each roller cutter are arranged in annular rows around the cutter body, the stream of drilling fluid from each nozzle impinging cutting elements of at least one of the outer rows of cutting elements of the respective roller cutter

4. A drill bit as set forth in claim 1 wherein the nozzle orifice is above the central axis of the respective bearing journal at its outer end with respect to the bit body.

5. A drill bit as set forth in claim 1 wherein each nozzle comprises a tubular member depending from the underside of the bit body.

6. A drill bit as set forth in claim 5 wherein said legs are spaced at equal intervals around the periphery of the bit body, and one of said nozzles extends down between each pair of adjacent legs.