Roller cone bit design
Roller cone bit design
Wide varieties of roller cone bits are available. They provide optimum performance in Wide varieties of roller cone bits are available. They provide optimum performance in specific formations and/or particular drilling environments. Modern drill bits
specific formations and/or particular drilling environments. Modern drill bits incorporateincorporate significantly different cutting structures and use
significantly different cutting structures and use vastly improved materials, resulting invastly improved materials, resulting in improved bit efficiency. Manufacturers work closely with drilling companies to collect improved bit efficiency. Manufacturers work closely with drilling companies to collect information about their bits to
information about their bits to identify opportunitiidentify opportunities for design improvements.es for design improvements.
Contents
Contents
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1 Roller cone bit design goals
1 Roller cone bit design goals
2 Basic
2 Basic design principdesign principlesles
3 Design methods and tools
3 Design methods and tools
o
o 3.1 How teeth and inserts drill3.1 How teeth and inserts drill o
o 3.2 Bit design method3.2 Bit design method
3.2.1 Bit 3.2.1 Bit diametediameter/availr/available spaceable space
3.2.2 Jornal angle3.2.2 Jornal angle
3.2.3.2.3 3 !on!one e o""so""setet
3.2.# $eeth and 3.2.# $eeth and insertinsertss
# Design as
# Design as applied to ctting strctapplied to ctting strctrere
o
o #.1 %nserts/te#.1 %nserts/teeth and eth and the ctting the ctting strctstrctrere
&
& 'ateri'aterials designals design
o
o &.1 %nserts and wear(resistant hard("acing materials&.1 %nserts and wear(resistant hard("acing materials
&.1.1 )roperties o" tngsten carbide composites&.1.1 )roperties o" tngsten carbide composites
&.1.1.1 $ngsten carbide insert *$!%+ design&.1.1.1 $ngsten carbide insert *$!%+ design
&.1.1.3 Diamond(enhanced tngsten carbide inserts *$!%s+ &.1.1.# $ngsten carbide hard "acing
- pecial prpose roller cone bit designs
o -.1 'onocone bits o -.2 $wo(cone bits
Re"erences 0 ee also
oteworth papers in 4ne)etro 15 67ternal lin8s
Roller cone bit design goals
Roller-cone bit design goals epect the bit to do the following!
"unction at a low cost per foot drilled.
#ave a long downhole life that minimi$es re%uirements for tripping.
&rovide stable and vibration-free operation at the intended rotational speed and
weight on bit 'W()*.
+ut gauge accurately throughout the life of the bit.
To achieve these goals, bit designers consider several factors. mong these are!
The formation and drilling environment.
pected rotary speed.
pected weight on bit 'W()*.
#ydraulic arrangements.
esign focal points include!
The bit body
+one configurations
+utting structures
Metallurgical, tribological, and hydraulic considerations in engineering bit design
solutions. 'Tribology is a science that deals with the design, friction, wear, and lubrication of interacting surfaces in relative motion.*
)asic design principles
rill-bit performance is influenced by the environment in which it operates. The way that bits are designed and their operating performance takes into consideration many operating choices, such as!
pplied W()
Rotary speed
#ydraulic arrangements
lso of critical importance in bit performance and design are environmental factors, such as!
The nature of the formation to be drilled
#ole depth and direction
+haracteristics of drilling fluids
The way in which a drill rig is operated
ngineers consider these factors for all designs, and every design should begin with close cooperation between the designer and the drilling company to ensure that all applicable inputs contribute to the design.
esign activities are focused principally on four general areas!
Material selection for the bit body and cones
Mechanical operating re%uirements
#ydraulic re%uirements
The dimensions of a bit at the gauge 'outside diameter* and pin 'arrangement for
attachment to a drillstem* are fied, usually by industry standards, and resultant design dimensions always accommodate them 'Fig 1*.
Fig. 1—Roller-cone bit general nomenclature.
"or roller-cone bits, steels must have!
ppropriate yield strength
#ardenability
0mpact resistance
Machineability
#eat treatment properties
The ability to accept hard facing without damage
+utting structure designs provide efficient penetration of the formation's* to be drilled and accurately cut gauge. The importance of bearing reliability in roller-cone bits cannot be
as a unit, and their designs are closely interrelated. )earing systems must function normally when!
1nder high loads from W()
0n conditions of large impact loads
While immersed in abrasive- and chemical-laden drilling fluids
0n relatively high-temperature environments.
#ydraulic configurations are designed to efficiently remove cuttings from cutting structure and bottomhole and then evacuate cuttings to the surface.
esign methods and tools
How teeth and inserts drill
To understand design parameters for cone bits, it is important to understand how roller-cone bits drill. Two types of drilling action take place at the bit. crushing action takes place when weight applied to the bit forces inserts 'or teeth* into the formation being drilled 'W() inFig. 2*. 0n addition, a skidding, gouging type of action results partly because the designed ais of cone rotation is slightly angled to the ais of bit rotation 'rotation in Fig. 2*. 2kidding and gouging also take place because the rotary motion of a bit does not permit a penetrated insert to rotate out of a crushed $one it has created without causing it to eert a lateral force at the $one perimiter. )oth effects contribute to cutting action 'Fig. 2*.
Fig. 2—Cutting actions for roller-cone bits.
Bit design method
The bit geometry and cutting structure engineering method of )entson has since 3456 been the root from which most roller-cone bit design methods have been designed 738. lthough modern engineering techni%ues and tools have advanced dramatically from those used in 3456, )entson9s method is the heritage of modern design and continues to be useful for background eplanation.
Bit diameter/available sace
Well diameter and the bit diameter re%uired to achieve it influence every design feature incorporated into every efficient bit. The first consideration in the physical design of a roller-cone bit is the permissible bit diameter or, in the words of the designer, available space. very element of a roller-cone bit must fit within a circle representative of the re%uired well diameter. The &0 has issued specifications establishing permissible tolerances for standard bit diameters.: The si$es of ;ournals, bearings, cones, and hydraulic and lubrication features are collectively governed by the circular cross section of the well. 0ndividually, the si$ing of the various elements can, to an etent, be varied. Repositioning or altering the si$e or shape of a single component nearly always re%uires subse%uent additional changes in one or more of the other components. 0n smaller bits, finding good compromises can be difficult because of a shortage of space.
!ournal angle
<=ournal angle> describes an angle formed by a line perpendicular to the ais of a bit and the ais of the bit9s leg ;ournal. =ournal angle is usually the first element in a roller-cone bit
design. 0t optimi$es bit insert 'or tooth* penetration into the formation being drilled? generally, bits with relatively small ;ournal angles are best suited for drilling in softer formations, and those with larger angles perform best in harder formations.
Cone offset
To increase the skidding-gouging action, bit designers generate additional working force by offsetting the centerlines of the cones so that they do not intersect at a common point on the bit. This <cone offset> is defined as the hori$ontal distance between the ais of a bit and the vertical plane through the ais of its ;ournal. (ffset forces a cone to turn within the limits of the hole rather than on its own ais. (ffset is established by moving the centerline of a cone away from the centerline of the bit in such a way that a vertical plane through the cone
centerline is brllel to the vertical centerline of the bit. )asic cone geometry is directly affected by increases or decreases in either ;ournal or offset angles, and a change in one of the two re%uires a compensating change in the other. 2kidding-gouging improves penetration in soft and medium formations at the epense of increased insert or tooth wear. 0n abrasive
formations, offset can reduce cutting structure service life to an impractical level. )it designers thus limit the use of offset so that results ;ust meet re%uirements for formation penetration.
"eeth and inserts
Tooth and insert design is governed primarily by structural re%uirements for the insert or tooth and formation re%uirements, such as!
&enetration
0mpact
brasion
With borehole diameter and knowledge of formation re%uirements, the designer selects structurally satisfactory cutting elements 'steel teeth or Tungsten +arbide 0nserts 'T+0s** that provide an optimum insert/tooth pattern for efficient drilling of the formation.
"actors that must be considered to design an efficient insert/tooth and establish an advantageous bottomhole pattern include!
)earing assembly arrangement
+one offset angle
=ournal angle
+one profile angles
0nsert/tooth material
0nsert/tooth count
0nsert/tooth spacing
When these re%uirements have been satisfied, remaining space is allocated between insert/tooth contour and cutting structure geometry to best suit the formation.
0n general, the physical appearance of cutting structures designed for soft, medium, and hard formations can readily be recogni$ed by the length and geometric arrangement of their cutting elements.
esign as applied to cutting structure
pplication of design factors produces diverse results 'Fig. #*. The cutting structure on the left is designed for the softest formation types? that on the right, for formations that are harder.
Fig. #—Cutting structure for soft $left% and hard $right% formations.
The action of bit cones on a formation is of prime importance in achieving a desirable penetration rate. 2oft-formation bits re%uire a gouging-scraping action. #ard-formation bits re%uire a chipping-crushing action. These actions are governed primarily by the degree to which the cones roll and skid. Maimum gouging-scraping 'soft-formation* actions re%uire a significant amount of skid. +onversely, a chipping-crushing 'hard-formation* action re%uires that cone roll approach a <true roll> condition with very little skidding. "or soft formations, a combination of small ;ournal angle, large offset angle, and significant variation in cone profile is re%uired to develop the cone action that skids more than it rolls. #ard formations re%uire a combination of large ;ournal angle, no offset, and minimum variation in cone profile. These will result in cone action closely approaching true roll with little skidding.
&nserts/teeth and the cutting structure
)ecause formations are not homogeneous, si$able variations eist in their drillability and have a large impact on cutting structure geometry. "or a given W(), wide spacing between inserts or teeth results in improved penetration and relatively higher lateral loading on the inserts or teeth. +losely spacing inserts or teeth reduces loading at the epense of reduced penetration. The design of inserts and teeth themselves depends largely on the hardness and drillability of the formation. &enetration of inserts and teeth, cuttings production rate, and hydraulic re%uirements are interrelated, as shown in "able 1.
"able 1-&nterrelationshi Between &nserts' "eeth' H(draulic Re)uirements' *nd "he Formation
"ormation and cuttings removal influence cutting structure design. 2oft, low-compressive-strength formations re%uire long, sharp, and widely spaced inserts/teeth. &enetration rate in this type of formation is partially a function of insert/tooth length, and maimum insert/tooth depth must be used. @imits for maimum insert/tooth length are dictated by minimum
re%uirements for cone-shell thickness and bearing-structure si$e. 0nsert/tooth spacing must be sufficiently large to ensure efficient fluid flows for cleaning and cuttings evacuation.
Re%uirements for hard, high-compressive-strength formation bits are usually the direct opposite of those for soft-formation types. 0nserts are shallow, heavy, and closely spaced. )ecause of the abrasiveness of most hard formations and the chipping action associated with drilling of hard formations, the teeth must be closely spaced 'Fig. +*. This close spacing distributes loading widely to minimi$e insert/tooth wear rates and to limit lateral loading on individual teeth. t the same time, inserts are stubby and milled tooth angles are large to withstand the heavy W() loadings re%uired to overcome the formation9s compressive strength. +lose spacing often limits the si$e of inserts/teeth.
Fig. +—Comarison of softer &*,C +2( $left% and harder # $right% cutting structures
0n softer and, to some etent, medium-hardness formations, formation characteristics are such that provisions for efficient cleaning re%uire careful attention from designers. 0f cutting structure geometry does not promote cuttings removal, bit penetration will be impeded and force the rate of penetration 'R(&* to decrease. +onversely, successful cutting structure engineering encourages both cone shell cleaning and cuttings removal.
Materials design
Materials properties are a crucial aspect of roller-cone bit performance. +omponents must be resistant to abrasive wear, erosion, and impact loading. The eventual performance and longevity results for a bit take into account several metallurgical characteristics, such as!
#eat treatment properties
Weldability
The capacity to accept hard facing without damage
Machineability
&hysical properties for bit components are contingent on the raw material from which a component is constructed, the way the material has been processed, and the type of heat treatment that has been applied. 2teels used in roller-cone bit components are all melted to eacting chemistries, cleanliness, and interior properties. ll are wrought because of grain structure refinements obtained by the rolling process. Most manufacturers begin with forged
blanks for both cones and legs, because of further refinement and orientation of microstructure that result from the forging process.
2tructural re%uirements and the need for abrasion and erosion resistance are different for roller-cone bit legs and cones. &redictably, the materials from which these components are constructed are normally matched to the special needs of the component. "urthermore, different sections of a component often re%uire different physical properties. @eg ;ournal sections, for eample, re%uire high hardenabilities that resist wear from bearing loads, whereas the upper portion of legs are configured to provide high tensile strengths that can support large structural loads.
Roller-cone bit legs and cones are manufactured from low-alloy steels. @egs are made of a material that is easily machinable before heat treatment, is weldable, has high tensile
strength, and can be hardened to a relatively high degree. +ones are made from materials that can be easily machined when soft, are weldable when soft, and can be case hardened to provide higher resistance to abrasion and erosion.
&nserts and wear-resistant hard-facing materials
Tungsten carbide is one of the hardest materials known. 0ts hardness makes it etremely useful as a cutting and abrasion-resisting material for roller-cone bits. The compressive strength of tungsten carbide is much greater than its tensile strength. 0t is thus a material whose usefulness is fully gained only when a design maimi$es compressive loading while minimi$ing shear and tension. Tungsten carbide is the most popular material for drill-bit cutting elements. #ard-facing materials containing tungsten carbide grains are the standard for protection against abrasive wear on bit surfaces.
When most people say <tungsten carbide,> they do not refer to the chemical compound 'W+* but rather to a sintered composite of tungsten carbide grains embedded in, and
metallurgically bonded to, a ductile matri or binder phase. 2uch materials are included in a family of materials called ceramic metal, or <cermets.> )inders support tungsten carbide grains and provide tensile strength. )ecause of binders, cutters can be formed into useful shapes that orient tungsten carbide grains so they will be loaded under compression. Tungsten carbide cermets can also be polished to very smooth finishes that reduce sliding friction. Through the controlled grain si$e and binder content, hardness and strength
properties of tungsten carbide cermets are tailored for specific cutting or abrasion resistances.
The most common binder metals used with tungsten carbide are iron, nickel, and cobalt. These materials are related on the periodic table of elements and have an affinity for
tungsten carbide 'cobalt has the greatest affinity*. Tungsten carbide cermets normally have binder contents in the 6A to 36A 'by weight* range. )ecause tungsten carbide grains are
metallurgically bonded with binder, there is no porosity at boundaries between the binder and grains of tungsten carbide, and the cermets are less susceptible to damage by shear and shock.
0roerties of tungsten carbide comosites
The process of <designing> cermet properties makes it possible to eactly match a material to the re%uirements for a given drilling application. +omposite material hardness, toughness, and strength are affected by!
Tungsten carbide particle si$e 'normally : to 6 Bm*
&article shape
&article distribution
)inder content 'as a weight percent*
s a generali$ation, increasing binder content for a given tungsten carbide grain si$e will cause hardness to decrease and fracture toughness to increase. +onversely, increasing tungsten carbide grain si$e affects both hardness and toughness. 2maller tungsten carbide particle si$e and less binder content produce higher hardness, higher compressive strength, and better wear resistance. 0n general, cermet grades are developed in a range in which hardness and toughness vary oppositely with changes in either particle si$e or binder content. 0n any case, subtle variations in tungsten carbide content, si$e distribution, and porosity can markedly affect material performance 'Fig. *.
Fig. —Hardness' toughness' and wear resistance of cemented tungsten carbide.
"ungsten carbide insert $"C&% design
T+0 design takes the properties of tungsten carbide materials and the geometric efficiency for drilling of a particular rock formation into account. s noted, softer materials re%uire geometries that are long and sharp to encourage rapid penetration. 0mpact loads are low, but abrasive wear can be high. #ard formations are drilled more by a crushing and grinding action than by penetration. 0mpact loads and abrasion can be very high. Tough materials, such as carbonates, are drilled by a gouging action and can sustain high impact loads and
high operating temperatures. Cariations in the way that drilling is accomplished and rock formation properties govern the shape and grade of the correct T+0s to be selected. The shape and grade of T+0s are influenced by their respective location on a cone. 0nner rows of inserts function differently from outer rows. 0nner rows have relatively lower rotational velocities about both the cone and bit aes. s a result, they have a natural tendency to gouge and scrape rather than roll. 0nner insert rows generally use softer, tougher insert grades that best withstand crushing, gouging, and scraping actions. auge inserts are commonly constructed of harder, more wear-resistant tungsten carbide grades that best withstand severe abrasive wear. 0t is thus seen that re%uirements at different bit locations dictate different insert solutions. large variety of insert geometries, si$es, and grades through which bit performance can be optimi$ed are available to the designer ' Fig. * 7:8.
Fig. —"(ical insert t(es $height 34 in. but varies with bit si5e%.
6auge cutting structure
The most critical cutting structure feature is the gauge row. auge cutting structures must cut both the hole bottom and its outside diameter. )ecause of the severity of gauge
demands on a bit, both milled tooth and insert type bits can use either tungsten carbide or diamond-enhanced inserts on the gauge. 1nder abrasive conditions, severe wear or gauge rounding is common, and, at high rotary speeds, the gauge row can eperience
temperatures that lead to heat checking, chipping, and breakage. ,iamond-enhanced tungsten carbide inserts $"C&s%
iamond-enhanced inserts are used to prevent wear in the highly loaded, highly abraded gauge area of bits and in all insert positions for difficult drilling conditions. They are made up of polycrystalline diamond compact '&+*, which is chemically bonded, synthetic diamond grit supported in a matri of tungsten carbide cermet. &+ has higher compressive strength and higher hardness than tungsten carbide. 0n addition, diamond materials are largely
unaffected by chemical interactions and are less sensitive to heat than tungsten carbides. These properties make it possible for diamond-enhanced materials to function normally in drilling environments in which tungsten carbide grades deliver disappointing or
"able 2-Comarison 7f ,iamond' 0,C' *nd "ungsten Carbide 8aterials
When diamond-enhanced inserts are designed, higher diamond densities increase impact resistance and ability to economically penetrate abrasive formations. 0ncreased diamond density increases insert cost, however. 0n the past, diamond-enhanced inserts have been available only in symmetrical shapes. The first of these was the semiround top insert. Today, some manufacturers have developed processes that make it possible to produce comple diamond-enhanced insert shapes.
"ungsten carbide hard facing
#ard-facing materials are designed to provide wear resistance 'abrasion, erosion, and impact* for the bit 'Fig. *. To be effective, hard facing must be resistant to loss of material by flaking, chipping, and bond failure with the bit. #ard facing provides wear protection on the lower 'shirttail* area of all roller-cone bit legs and as a cutting structure material on milled-tooth bits 'Fig. *.
Fig. —"(ical hard-facing alications on a milled-tooth bit.
Fig. —9:loded view of seal and bearing comonents.
#ard facing is commonly installed manually by welding. hollow steel tube containing appropriately si$ed grains of tungsten carbide is held in a flame until it melts. The resulting
molten steel bonds, through surface melting, with the bit feature being hard faced. 0n the process, tungsten carbide grains flow as a solid, with molten steel from t he rod, onto the bit. The steel then solidifies around the tungsten carbide particles, firmly attaching them to the bit.
2pecial purpose roller cone bit designs
8onocone bits
Monocone bits were first used in the 34DFs. The design has several theoretical advantages but has not been widely used. )it researchers, encouraged by advances in cutting structure materials, continue to keep this concept in mind, because it has the room for etremely large bearings and has very low cone rotation velocities, which suggest a potential for long bit life. While of a certain general interest, monocone bits are potentially particularly advantageous for use in small-diameter bits in which bearing si$ing presents significant engineering
problems.
Monocone bits drill differently from three-cone bits. rilling properties can be similar to both the beneficial crushing properties of roller-cone bits and the shearing action of &+ bits. +utting structure research thus focuses partly on eploitation of both mechanisms
encouraged by the promise of efficient shoe drillouts and drilling in formations with hard stingers interrupting otherwise <soft> formations. Modern ultrahard cutter materials properties can almost certainly etend insert life and epand the range of applications in which this design could be profitable. The design also provides ample space for no$$le placements for efficient bottomhole and cutting structure cleaning.
"wo-cone bits
The origin of two-cone bit designs lies in the distant past of rotary drilling. The first roller-cone patent, issued in ugust 34F4, covered a roller-cone bit. s with monoroller-cone bits, two-cone bits have available space for larger bearings and rotate at lower speeds than three-cone bits. )earing life and seal life for a particular bit diameter are greater than for
comparable three-cone bits. Two-cone bits, although not common, are available and perform well in special applications 'Fig ;*. Their advantages cause this design to persist, and
designers have never completely lost interest in them.
Fig. ;—"wo-cone bit.
The cutting action of two-cone bits is similar to that of three-cone bits, but fewer inserts simultaneously contact the hole bottom. &enetration per insert is enhanced, providing particularly beneficial results in applications in which capabilities to place W() are limited. The additional space available in two-cone designs has several advantages. 0t is possible to have large cone offset angles that produce increased scraping action at t he gauge. 2pace also enables ecellent hydraulic characteristics through room for placement of no$$les very close to bottom. 0t also allows the use of large inserts that can etend bit life and efficiency. Two-cone bits have a tendency to bounce and vibrate. This characteristic is a concern for directional drilling. )ecause of this concern and advances in three-cone bearing life and cutting structures, two-cone bits do not currently have many clear advantages. s with many roller-cone bit designs, however, modern materials and engineering capabilities may resolve problems and again underscore their recogni$ed advantages.
References
3. G )entson, #.., and 2mith 0ntl. 0nc. 3456. Roller-+one )it esign. @os ngeles, +alifornia! &0 ivision of &roduction, &acific +oast istrict.
:. G &ortwood, ., )oktor, )., Munger, R. et al. :FF3. evelopment of 0mproved &erformance Roller +one )its for Middle astern +arbonate rilling pplications. &resented at the 2&/0+ Middle ast rilling Technology +onference, )ahrain, ::-:E (ctober. 2&-H::4I-M2. http!//d.doi.org/3F.:33I/H::4I-M2 .
D. G Jeshavan, M.J., 2iracki, M.., and Russell, M.. 344D. iamond-nhanced 0nsert! Kew +ompositions and 2hapes for rilling 2oft-to-#ard "ormations. &resented at the 2&/0+ rilling +onference, msterdam, Ketherlands, ::-:5 "ebruary. 2&-:5HDH-M2.http!//d.doi.org/3F.:33I/:5HDH-M2 .
E. G 2alesky, W.=. and &ayne, ).R. 34IH. &reliminary "ield Test Results of iamond-nhanced 0nserts for Three-+one Rock )its. &resented at the 2&/0+ rilling +onference, Kew (rleans, @ouisiana, 35-3I March.
2&-36335-M2.http!//d.doi.org/3F.:33I/36335-M2 .
5. G 2alesky, W.=., 2winson, =.R., and Watson, .(. 34II. (ffshore Tests of iamond-nhanced Rock )its. &resented at the 2& nnual Technical +onference and hibition, #ouston, Teas, :-5 (ctober.
2ee also
Rotary drill bits
Roller cone bit components Roller cone bit classification
&#!0ntroduction to Roller-+one and &olycrystalline iamond rill )its
Koteworthy papers in (ne&etro
ternal links
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