CA1146530A - Rolling cutter drill bit - Google Patents

Abstract

ROLLING CUTTER DRILL BIT

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

ROLLING C~TTER DRILL_BIT

BACKGROUND OF THE INVENTION

In the drilling of boreholes through underground for-mations for the purposes of locating and producing oil and gas, and for the purposes of mining and production of steam energy through thermal wells, the most common type of dri]ling appara-tus used today is the tri-cone rolling cutter drill bit. This bit generally comprises a central body section having three legs extending downwardly therefrom. Each leg has an inwardly projecting bearing journal upon which is rotatably mounted a frustoconical cutter. Generally, the most prevalent type of cutting structure utilized in the tri-cone bit is the tungsten carbide insert cutting structure. Tungsten carbide cutting elements are press-fit in holes drilled in the frustoconical cutters and protrude outwardly to pro~ide a digging, 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 30 p ~. Schumacher, Jr. (~. S. Patent No. 3,495,668) in which, ~or the first time, an insert bit successfully incorporated offset i530 axis cutters to achieve greater gouging and scraping action in the borehole. A subse~uent 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 assignee of the present invention, Reed Rock Bit Company, in 1970, and have become the most prcvalent type of drill bi-ts in the drilling industry over the past ten years.
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 formations. Theamount of offset utilized in these bits ranges 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 amount of offset introduced into thc 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.
25 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 abovc about 1/32 inch per inch of bit diameter would not add any significant efficiency 33 or increased drilling rate to the bit to justify the increased breakage that such increased offset would introduce. In fact, ~ r!
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drilliny tests conducted utilizing insert bits with offset some-what greater than 1/32 inch per inch of bit diameter have indi-cated 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 having an offset range of from zero to 1/32 inch per inch of bit diameter.

The present invention utilizes a uni~lue insert bit design having great amounts of offset in the cutting structure far exceeding those ranges utilized in conventional offset-axis LO 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 performance can be ob-tained. For some reason un~nown 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 ~0 inch of bit diameter a significant jwnp 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, ~5 dated August 23, 1921, in which a two-cone rolling cuttcr 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 1:~4~530 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 introduced to the use of insert type bits utilizing the offset already present in milled tooth bits. The reason that the high offset cutters were not thought practical was that in-creases in offset above the 1/32 inch limit previously mentioned would gain very little in cut-ting 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 interference 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 oftwo different types. The oldest type is the regular drilling fluid system where 'arge, 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 bo-ttom of the borehole.
By necessity, this is a low-volume, low-velocity flow since the fluid stream impinges directly upon thc 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 flui~ is gene-rated from the bit body directly against the formation face with-out impinging on any cutting elements or any portion 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 face to maximize hydraulic energy of the fluid stream impinging thc formation face. The conventional jet bits do not emit fluid agalnst any cutting elements because of the adverse cffect of erosion from the high-pressure drilling Eluid. The present invention differs from these two conventional types in that it uses a directed jet spray which impinges directly upon the cutter inserts.

The present invention discloses an insert type bit, as opposed to a milled tooth bit, which insert bit utilizes rol-ling cone cutting elements rotatably mounted on lugs having ro-tational axes ~ith large offset from the xotational axis of the drill bit. The amoun-t of offset ranges between 1/16 and 1/8 inch per inch of bit diameter. The resulting invention produces lS greatly increased rates of penetration and drilling efficiency when utilized in soft to medium-soft formations. It should be noted that the present invention, when embodied in a tri-cone oilwell drilliny bit, suffers a greater amount of erosion and breakage of the hard metal cutting inserts in the cones, but the total gain in drilling efficiency and rate of penetration far offsets the increased wear and breakage of the cutting ele-ments.

In addition to the aforementioncd uniquc drill bit construction, the present invention also embodics a new and unique nozzle jet-ting sys-tem for delivering drilling fluid to thc cut-ting elements and the face of the formation as it is being drillcd.

This jetting system utilizes directed nozzles which create a spray of pressurized drilling fluid and directs this spray across the protruding tungsten carbide inserts and against the formation face. The new jetting system provides a dual function of clean-ing material from the inserts and also sweeping the cuttings ~ .

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from the borehole face. This system is particularly 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 directed against the forma-tion face to further flush and clean the cuttings as they are gouged and scraped out of the formation.

The invention, in its broadest aspect, embodies a rolling cutter drill bit for drilling through underground formations, which comprises a main body having an upper end adapted for interconnection in a drilling string. A plurality of legs extend downwardly from the body with each leg having an inwardly projecting bearing journal formed thereon. A
generally frustoconical cutter is rotatably mounted on each bearing journal and has a rotational axis generally coinciding with the central axis of the journal. A plurality of hard metal cutting elements is inserted therein and protrudes from the surface thereof. A bearing means is located between each cutter and bearing journal. The drill bit has a cutting diameter defined by the radially outermost-located of the cutting elements on each cutter and has a bit rotational axis passing longitudinally therethrough. The central axes of ...... .

the journals are offset from the bit axis by at least one-sixteenth inch per inch of bit cutting diameter.

A variation of the drill bit described above has the journal axes offset from the bit axis by an amount ranging from about one-sixteenth inch to about one-eighth inch per inch of bit cutting diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure l is a side view of one embodiment of the pre-sent invention comprising a three-cone bit. Figure 2 is an axial bottom view of the three-cone bit of Figure l. Figure 3 is a schematic representation of the three cutter cones of the bit of Figures l and 2, showing the concept of offset cutter axes.
Figure 4 is a diagram of the cutter configuration in one embodi-ment 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 il]us-tration of one embodiment of this invention indicating the direct-ed nozzle system and its interaction with the cutter and theformation. Figures 7 and 8 are illustrations of a particular embodiment of the directed nozzle system shown schema-B

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tically in Figure 6; Figure 7 is all axial cnd-view of a central nozzle system, and Figure ~ is a partial cross-sectional side view of the nozzle of Fiyure 7. Figurc?s 9 through 11 are different views of a second embodimcllt of thc dircctcd nozzlc J 5 system utilizing an intermediatc jct. Figuresl2 throu(311 14 illustrate axial bottom views of a third embodiment of the pre-sent invention which utilizes a peripheral directed noz~le sys-tem.

DESCRIPTIC)N OF TllE PRE:FE~RRED E~BO[~I'II~NTS

Referring to Figure 1, a first embodiment of the in-vention shown in isometric view, this embodiment 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-ing the female end of a section of drill stem. The body sectio 12 has three downwardly extending legs 18 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, which is an axial 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 projectinc3 from raised lancls 24 formcd on the surfaces of the cones. In a typical embodimcnt the in-serts generally would comprise three different categories, the gauge row inserts 26, intermediate row inserts 28, and nose in-serts 30. As is well }~nown in the industry, the inserts are se-cured in the cones by drilling a hole in the cone for eacll in-sert with the hole having a sliglltly smaller diametcr than the .

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insert cliameter, thus resulting in an interfcrenc.c ~it. Tilc in-serts are then pre~sed undcr relatively hiC3il prcssurc into thc holes and the press fit insures that the inscrts arc securely held in the cones.

Although no~ shown in thc drawings, each cu~tcr 16 is rotatably mounted on a cylindrical bearing journal machined on each leg 8, as is well known in the art. As is also well known in t'ne art, bearings such as roller bearings, 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 therebetween 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 Houston, Texas.

In Figure 3, the cutters 16 are illustrated schematic-ally as simple frustoconical figures. Each cutter cone 16 has an axis of rotation 32 passing substantially through the center of the frustoconical figure. The central rotational a~is of the bit 10 is illustrated as point 34 in Figure 3 since Figure 3 is taken from a view looking directlyalong the rotational axis of the bit. From Figure 3, it can bc sccn that becau~.e of the offset of axes 3Z, none of the axes intersect axis 34 of the bit. In this flat projection, the intersection of the axes 32 forms an equilateral triangle 36. The amount of offset measured in a linear distance for any particular bit can be determined from a full scale diagram similar to-~igure 3 for that bit by measuring the distance from axis 3~ to the mid-point of any side of triangle 36.

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Referring now to Figure 4, in which a cutter layout is illustrated, the profiles or cross-sections of each of the cutters on the tri-cone bit of the preferred embodiment are layed out in relation to each othcr to show the intermes11 o~ thc cut-ting elements or inserts 22. Generally, each cuttcr in a ~ri-cone bit is of a slightly different profile in order to allow optimum spacing of the inserts for the entire bit. In ~igure 4, the three cutters are labeled A, B and C. The C cutter has been divided to illustrate its intermesh with both cutters A and B. It should be noted that the projections have been flattened out, and because 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 A, B and C, has a rotational axis 32 which is offset b~ a distance Y from 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 coverage.Each insert in the profile of Figure 5 is labeled according to the particular cutter cone in which the insert is located. The angle X is indicated to show tne journal anyle 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 embodiment it was found that the preferred range of insert protrusion above the cutter surface --lt)--6534:~

should be greater than or equal to about one-half the diameter of the insert. Any protrusion significantly less -than one-half the diameter would make the gouging and scraping action result-ing from the large amount of offset in~ffcc~ive. ll~e ~I-c~e~-lc~
range of insert protrusion is Erom onc-llalf to onc ti Incs th-' insert diameter. The preferred shape of the protruding portion of the insert is conical or chisel. ~cceptable alternate shapes are the hemispherical and the sharpened hemispherical inserts.

Whereas the insert can be made of any hard metal alloy such as titanium carbide, tantalum carbide, or chromium carbide, in a suitable matrix, one particular range of embodirnents uti-lizes tungsten carbide in a cobalt matrix. The cobalt content ranges from about 5% to about 20~ by weight of the insert materi-al, with the remainder of the metal being either sintered or cast tungsten carbide, or both. The hardness of the inserts is con-trolled by varying the cobalt content and by other well-known methods. The hardness ranges frorn about 85 Rockwell A to about 90 Rockwell A. In one particular ernbodiment, conical inserts having a protrusion greater than one-half of their diameter were used, with the inserts being made of tunysten carbide-cobalt alloy, having a cobalt content of around 12~ and a hardness of about 86.5 Rockwell A.

Referring now to Figure 6, a schematic sketch oE the directed nozzle fluid system of the invention is illus-trated.
In Figure 6, a generally cylindrical jet nozzle 40 is shown connected to bit body 12 and communicating with a high pressure drilling fluid passage 42 passing therethrough. Nozzle 40 has an exit jet 44 from which high pressure drilling fluid 46 is ~ ~19L6536:~

emitted in a tight directcd spray. sit lcg 18 is illustratcd having conical cutter 16 locatcd tncrcon. A dircction arrow 48 is drawn on leg 18 to indicate thc direction of movcment of the bit leg in thc boreholc as the drill ~it is rot;lte~l. ri~c~ ,c, a second rotation arrow 50 is drawn on cuttcr 16 ~o 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 exactly 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 preferred embodiment is to have stream 46 either tangential to or slightly displaced from cu-tter shell 16, a slight impinge-ment o 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 22 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 goLIged from the formation by inserts 22. The drilling fluid thcn passcs below thc cutter 16 and moves back upward outside the drill bit and up through the borehole in the conventional manner.

Referring now to Figures 7 and ~, one embodiment of the directed jetting systcm is disclosed. This embodiment 53~

utilizes a multi-orifice jet nozzlc wi~ich protrudcs clownwardly from thc central area of the bit body towards the ccntral arc~
between the three conical cutters. E;igurc 7 is a partial axial end-view of the bit 10 part:ially illustratinc~ two cuttcrs 16 and the location of the multi-orifice jet 5~. Jet 56 is gcne-rally cylindrical in nature having a bevelled edge 58 at the downward projecting end thereof and having three nozzle openings 60 formed through the bevelled surface 58. A flat closed end 62 is located at the bottom of the nozzle. A fluid spray 64 is shown emanating from one of the openings 60. This spray passes across the inserts in the cu~ters 16 without impinging on the actual cutter surfaces. The spray cleanses any packed cuttings which might be lodged between the various inserts and then moves outward and then downward to sweep the bottom of the borehole in front of the cutters as they roll into the formation surface. Figure ~ is a partial side view of the bit of Figure 7 showincJ a sinyle cutter 16 and the multi-jet nozzle 56. In this figure, the nozzle 56 is shown in a cross-scctiollal diagl-am and it can be seen that the nozzle has a central passage 66 which cor~nunicates with the nozzle openings 60. Nozzle 56 is securely located in a bore 68 formed in bit body 12. Bit body 12 has a fluid cavity 70 formed therein which communicates with threaded pin end 14 which also is tubular in nature. Thus, it can be seen that drilling fluid pumpcd down the drill strinc3 passcs through threadcd pin 14 into bit cavi~y 70, throu(3ll nozzle ~OI-C 66 arld out the nozzle openillg 60 into a jct or spray 64 wl~ich impirlcJcs the major cuttinq inserts on cone 16 and then is directed either ayainst the face of thc borehole or, as shown in 8, may be di-rected against the wall of thc bore~lole wilcrcupon the fluid moves down the wall and across the formation facc to pick up additional loosc cuttincJs thcrcon.

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..
Re~erring now to ricJurcs '~ throuc3h 11, a seconcl em-bodimcnt of the directcd nozzlo system is disclosed in which the fluid jetting system is directed across thc main cutting inserts an~ impingcs directly upon thc borehole face. In this embodi-ment, the orojecte~ nozzle arrangement is replaced by a slantedjet configuration 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 cones 16, the 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 16 has one jet passage associated therewith for sweeping 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 b~ 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 communication with cavity 70 and emitting a jet stream 60 of drilling fluid passing across the cutting inserts oE cutter 16 and impinging the borehole bottom.

Referring to Figures 12 throug}l 14, two addltional embodiments of the present invention with the directed nozzle ; system are indicate~. In Figure 12 a drill bit is shown in the axial view looking up from the bottom of the borehole. The bit has three conical cutters 16 havir~g a plurality oE tungsten car-bide inserts 22 securely held in raised lancls 24 on tl~e cutters.

;i530 A set of three periL)h~rally clircctc~ noz7.1~s ao ar~ locatcd around the outer periphcry of bit l~ody 12, cxtcndlng downwar~
therefrom into the generally open arcas betwcen the outer rows of inserts on the conic~l cutters. Thc emboclimcnt of Figure 12 utilizes the threc directed nozzles which are generally cylin-drical in nature, each having a bevelled face 82 and a jet pas-sage 84 formed through face 82 and communicating with a central 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 clirection t~ith 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 the single jet passage 84 is arranged to clean the inserts on the cutter located in a clockwise di~ection 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 drilling 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 downwarclly therefrom between the outer edges of the cones 16. Each nozzle 90 has two jet passages formed therein passing through opposed bcvellcd faces 92 and 94. Thus, each nozzle 90 has a jet passage direct-ed at each cutter cone 16 located adjacent thereto. Figure 14 is a diagramatic sketch showing the nozzle 90 from the side and illustrating the two bevelled faces 92 and 94. The jet passages 96 pass through the two bevelle~ faces and communicate with an 3~

inner ~ore in no~ lcs 90. ~rcssurizcd drilling ~luid E~asscs throuqh the drill bit and into the nozzlcs ~0 in a manncr simi-lar to that of the embodiment shown in Figurcl2~

The nozzles utilized in the embodlments illustrated in Figures 6 through 14 are preferably formed by casting, for-ging, and/or machining from a hard material such as steel or one of the hard metal alloys such as tungsten carbide in a cobalt matri.Y. The tungsten carbide-cobalt alloy can be of the type using sintered tungsten carbide, cast tungsten carbide, or a combination of both. A~ternatively, the nozzles could be formed of any material which successfully resists erosion.

Thus, the present invention defines several unique features, one of which is the utilization of an extreme amount of offset in the cutter axes of an insert type bit. Another feature is the novel fluid jetting system whic'n provides a high-]y 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-pressure fluid jet at or near a tangent to the cutter cones in a position to s~eep the main cutting inserts, thereby cleaning tl-e balled up material therefrom, and the flu;d stream thereafter passes from the in-sert region to the formation face directly, or from the insert region to the borehole wall and thcn dowrl the wall alld across the formation face.

2~ ~lthou~h certaill preferred embodiments of the preserlt invention have been herein described in order to provide a -]6-~653~

understanding of the general principles of the invention, it will be appreciated that various changes and innovations can be ef~
fected in the described drill bit structure without departure from these principles. For example, whereas a tri-conc bit having three conical cutters is disclosed, it is clear that the bit structure could be of the four-cone type, and still embody the principles of the present invention. Likewise, the number and location of the directed nozzles could be varied from those shown and still obtain equivalent operation, function, and re-sults. Thus, all modifications and changes of this type aredeemed to embraced by the spirit and scope of the invention ex-cept as the same may be necessarily limited by the appended claims or reasonable equivalents thereof.

Claims (7)

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

1. A rolling cutter drill bit for drilling through underground formations, said bit comprising:
a main body having an upper end adapted for interconnection in a drilling string;
a plurality of legs extending downwardly from said body, each having an inwardly pro-jecting bearing journal formed thereon;
a generally frustoconical cutter rotatably mounted on each said bearing journal, having a rotational axis generally coin-ciding with the central axis of said jour-nal, and having a plurality of hard metal cutting elements inserted therein and protruding from the surface thereof;
bearing means between each said cutter and bearing journal;
said drill bit having a cutting diameter de-fined by the radially outermost located of said cutting elements on each said cutter and having a bit rotational axis passing longitudinally therethrough; and, the central axes of said journals being offset from said bit axis by at least one-sixteenth inch per inch of bit cutting diameter.

2. The drill bit of claim l wherein said journal axes are offset from said bit axis by an amount ranging from about one-sixteenth inch to about one-eighth inch per inch of bit cutting diameter.

3. The rolling cutter drill bit of claim 1 or claim 2 wherein said legs and cutters each number three and said cutting elements protrude at least one-half of the cutting element insert diameter from the cutter surface; said inserts being formed of a tungsten carbide-cobalt material wherein the cobalt content by weight ranges from about five percent to about twenty percent and the hardness of said insert is from about 85 Rockwell A to about 90 Rockwell A.

4. A tri-cone rolling cutter drill bit comprising:
a bit body having an upper threaded pin end for engaging a section of drill string, and further having an axis of rotation;
three downwardly extending, generally equispaced legs on said bit body, each having a cylin-drical bearing journal extending radially inward and downwardly from the lower end thereof, and each journal having a central longitudinal axis;
a generally frustoconical cutter rotatably mounted on each said bearing journal, and having a rotational axis generally coinciding with the central longitudinal axis of said journal;
bearing means between each said cutter and bearing journal;

a plurality of hard metal cutting elements in-serted in each said cutter and protruding outwardly therefrom; and, wherein said cutter rotational axes are offset from said bit rotational axis by a distance of from about one-sixteenth inch per inch to about one-eighth inch per inch of the diameter of said bit.

5. The tri-cone drilling bit of claim 4 in which said cutting elements comprise tungsten carbide grains in a cobalt matrix and the shape of the cutting elements protruding beyond the cutter surface is sub-stantially conical with a rounded top.

6. The tri-cone drill bit of claim 5 wherein the shape of the cutting element protruding beyond the cutter surface is conical with a rounded tip.

7. The tri-cone drill bit of claim 4 wherein at least one of said cutting elements comprises a tungsten carbide-cobalt insert having a chisel-shaped pro-truding portion.