Roller cone bit design

Wide varieties of roller cone bits are available. They provide optimum performance in specific formations and/or particular drilling environments. Modern drill bits incorporate significantly different cutting structures and use vastly improved materials, resulting in improved bit efficiency. Manufacturers work closely with drilling companies to collect information about their bits to identify opportunities for design improvements.

Roller cone bit design goals

Roller-cone bit design goals expect the bit to do the following:

• Function at a low cost per foot drilled.
• Have a long downhole life that minimizes requirements for tripping.
• Provide stable and vibration-free operation at the intended rotational speed and weight on bit (WOB).
• Cut gauge accurately throughout the life of the bit.

To achieve these goals, bit designers consider several factors. Among these are:

• The formation and drilling environment.
• Expected rotary speed.
• Expected weight on bit (WOB).
• Hydraulic arrangements.
• Anticipated wear rates from abrasion and impact.

Design focal points include:

• The bit body
• Cone configurations
• Cutting 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.)

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Design methods and tools

How teeth and inserts drill

To understand design parameters for roller-cone bits, it is important to understand how roller-cone bits drill. Two types of drilling action take place at the bit. A crushing action takes place when weight applied to the bit forces inserts (or teeth) into the formation being drilled (WOB in Fig. 2). In addition, a skidding, gouging type of action results partly because the designed axis of cone rotation is slightly angled to the axis of bit rotation (rotation in Fig. 2). Skidding and gouging also take place because the rotary motion of a bit does not permit a penetrated insert to rotate out of a crushed zone it has created without causing it to exert a lateral force at the zone perimiter. Both effects contribute to cutting action (Fig. 2).

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  Fig. 2—Cutting actions for roller-cone bits.

Bit design method

The bit geometry and cutting structure engineering method of Bentson has since 1956 been the root from which most roller-cone bit design methods have been designed . Although modern engineering techniques and tools have advanced dramatically from those used in 1956, Bentson’s method is the heritage of modern design and continues to be useful for background explanation.

Bit diameter/available space

Well diameter and the bit diameter required 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. Every element of a roller-cone bit must fit within a circle representative of the required well diameter. The API has issued specifications establishing permissible tolerances for standard bit diameters.2 The sizes of journals, bearings, cones, and hydraulic and lubrication features are collectively governed by the circular cross section of the well. Individually, the sizing of the various elements can, to an extent, be varied. Repositioning or altering the size or shape of a single component nearly always requires subsequent additional changes in one or more of the other components. In smaller bits, finding good compromises can be difficult because of a shortage of space.

Journal angle

“Journal angle” describes an angle formed by a line perpendicular to the axis of a bit and the axis of the bit’s leg journal. Journal angle is usually the first element in a roller-cone bit design. It optimizes bit insert (or tooth) penetration into the formation being drilled; generally, bits with relatively small journal 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 horizontal distance between the axis of a bit and the vertical plane through the axis of its journal. Offset forces a cone to turn within the limits of the hole rather than on its own axis. Offset 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. Basic cone geometry is directly affected by increases or decreases in either journal or offset angles, and a change in one of the two requires a compensating change in the other. Skidding-gouging improves penetration in soft and medium formations at the expense of increased insert or tooth wear. In abrasive formations, offset can reduce cutting structure service life to an impractical level. Bit designers thus limit the use of offset so that results just meet requirements for formation penetration.

Teeth and inserts

Tooth and insert design is governed primarily by structural requirements for the insert or tooth and formation requirements, such as:

• Penetration
• Impact
• Abrasion

With borehole diameter and knowledge of formation requirements, the designer selects structurally satisfactory cutting elements (steel teeth or Tungsten Carbide Inserts (TCIs)) that provide an optimum insert/tooth pattern for efficient drilling of the formation.

Factors that must be considered to design an efficient insert/tooth and establish an advantageous bottomhole pattern include:

• Bearing assembly arrangement
• Cone offset angle
• Journal angle
• Cone profile angles
• Insert/tooth material
• Insert/tooth count
• Insert/tooth spacing

When these requirements have been satisfied, remaining space is allocated between insert/tooth contour and cutting structure geometry to best suit the formation.

In general, the physical appearance of cutting structures designed for soft, medium, and hard formations can readily be recognized by the length and geometric arrangement of their cutting elements.

Design as applied to cutting structure

Application of design factors produces diverse results (Fig. 3). The cutting structure on the left is designed for the softest formation types; that on the right, for formations that are harder.

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  Fig. 3—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. Soft-formation bits require a gouging-scraping action. Hard-formation bits require a chipping-crushing action. These actions are governed primarily by the degree to which the cones roll and skid. Maximum gouging-scraping (soft-formation) actions require a significant amount of skid. Conversely, a chipping-crushing (hard-formation) action requires that cone roll approach a “true roll” condition with very little skidding. For soft formations, a combination of small journal angle, large offset angle, and significant variation in cone profile is required to develop the cone action that skids more than it rolls. Hard formations require a combination of large journal angle, no offset, and minimum variation in cone profile. These will result in cone action closely approaching true roll with little skidding.

Inserts/teeth and the cutting structure

Because formations are not homogeneous, sizable variations exist in their drillability and have a large impact on cutting structure geometry. For a given WOB, wide spacing between inserts or teeth results in improved penetration and relatively higher lateral loading on the inserts or teeth. Closely spacing inserts or teeth reduces loading at the expense of reduced penetration. The design of inserts and teeth themselves depends largely on the hardness and drillability of the formation. Penetration of inserts and teeth, cuttings production rate, and hydraulic requirements are interrelated, as shown in Table 1.

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  Table 1-Interrelationship Between Inserts, Teeth, Hydraulic Requirements, And The Formation

Formation and cuttings removal influence cutting structure design. Soft, low-compressive-strength formations require long, sharp, and widely spaced inserts/teeth. Penetration rate in this type of formation is partially a function of insert/tooth length, and maximum insert/tooth depth must be used. Limits for maximum insert/tooth length are dictated by minimum requirements for cone-shell thickness and bearing-structure size. Insert/tooth spacing must be sufficiently large to ensure efficient fluid flows for cleaning and cuttings evacuation.

Requirements for hard, high-compressive-strength formation bits are usually the direct opposite of those for soft-formation types. Inserts are shallow, heavy, and closely spaced. Because of the abrasiveness of most hard formations and the chipping action associated with drilling of hard formations, the teeth must be closely spaced (Fig. 4). This close spacing distributes loading widely to minimize insert/tooth wear rates and to limit lateral loading on individual teeth. At the same time, inserts are stubby and milled tooth angles are large to withstand the heavy WOB loadings required to overcome the formation’s compressive strength. Close spacing often limits the size of inserts/teeth.

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  Fig. 4—Comparison of softer IADC 427y (left) and harder 837Y (right) cutting structures

In softer and, to some extent, medium-hardness formations, formation characteristics are such that provisions for efficient cleaning require careful attention from designers. If cutting structure geometry does not promote cuttings removal, bit penetration will be impeded and force the rate of penetration (ROP) to decrease. Conversely, 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. Components 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:

• Heat treatment properties
• Weldability
• The capacity to accept hard facing without damage
• Machineability

Physical 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. Steels used in roller-cone bit components are all melted to exacting chemistries, cleanliness, and interior properties. All 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.

Structural requirements and the need for abrasion and erosion resistance are different for roller-cone bit legs and cones. Predictably, the materials from which these components are constructed are normally matched to the special needs of the component. Furthermore, different sections of a component often require different physical properties. Leg journal sections, for example, require 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. Legs 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. Cones 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.

Inserts and wear-resistant hard-facing materials

Tungsten carbide is one of the hardest materials known. Its hardness makes it extremely 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. It is thus a material whose usefulness is fully gained only when a design maximizes compressive loading while minimizing shear and tension. Tungsten carbide is the most popular material for drill-bit cutting elements. Hard-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 (WC) but rather to a sintered composite of tungsten carbide grains embedded in, and metallurgically bonded to, a ductile matrix or binder phase. Such materials are included in a family of materials called ceramic metal, or “cermets.” Binders support tungsten carbide grains and provide tensile strength. Because 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 size 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 6% to 16% (by weight) range. Because 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.

Properties of tungsten carbide composites

The process of “designing” cermet properties makes it possible to exactly match a material to the requirements for a given drilling application. Composite material hardness, toughness, and strength are affected by:

• Tungsten carbide particle size (normally 2 to 6 μm)
• Particle shape
• Particle distribution
• Binder content (as a weight percent)

As a generalization, increasing binder content for a given tungsten carbide grain size will cause hardness to decrease and fracture toughness to increase. Conversely, increasing tungsten carbide grain size affects both hardness and toughness. Smaller tungsten carbide particle size and less binder content produce higher hardness, higher compressive strength, and better wear resistance. In general, cermet grades are developed in a range in which hardness and toughness vary oppositely with changes in either particle size or binder content. In any case, subtle variations in tungsten carbide content, size distribution, and porosity can markedly affect material performance (Fig. 5).

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  Fig. 5—Hardness, toughness, and wear resistance of cemented tungsten carbide.

Tungsten carbide insert (TCI) design

TCI design takes the properties of tungsten carbide materials and the geometric efficiency for drilling of a particular rock formation into account. As noted, softer materials require geometries that are long and sharp to encourage rapid penetration. Impact loads are low, but abrasive wear can be high. Hard formations are drilled more by a crushing and grinding action than by penetration. Impact 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. Variations in the way that drilling is accomplished and rock formation properties govern the shape and grade of the correct TCIs to be selected.

The shape and grade of TCIs are influenced by their respective location on a cone. Inner rows of inserts function differently from outer rows. Inner rows have relatively lower rotational velocities about both the cone and bit axes. As a result, they have a natural tendency to gouge and scrape rather than roll. Inner insert rows generally use softer, tougher insert grades that best withstand crushing, gouging, and scraping actions. Gauge inserts are commonly constructed of harder, more wear-resistant tungsten carbide grades that best withstand severe abrasive wear. It is thus seen that requirements at different bit locations dictate different insert solutions. A large variety of insert geometries, sizes, and grades through which bit performance can be optimized are available to the designer (Fig. 6) .

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  Fig. 6—Typical insert types (height ≈¾ in. but varies with bit size).

Gauge cutting structure

The most critical cutting structure feature is the gauge row. Gauge cutting structures must cut both the hole bottom and its outside diameter. Because 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. Under abrasive conditions, severe wear or gauge rounding is common, and, at high rotary speeds, the gauge row can experience temperatures that lead to heat checking, chipping, and breakage.

Diamond-enhanced tungsten carbide inserts (TCIs)

Diamond-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 (PDC), which is chemically bonded, synthetic diamond grit supported in a matrix of tungsten carbide cermet. PDC has higher compressive strength and higher hardness than tungsten carbide. In 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 unsatisfactory results (Table 2) ,,

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  Table 2-Comparison Of Diamond, PDC, And Tungsten Carbide Materials

When diamond-enhanced inserts are designed, higher diamond densities increase impact resistance and ability to economically penetrate abrasive formations. Increased diamond density increases insert cost, however. In 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 complex diamond-enhanced insert shapes.

Tungsten carbide hard facing

Hard-facing materials are designed to provide wear resistance (abrasion, erosion, and impact) for the bit (Fig. 7). To be effective, hard facing must be resistant to loss of material by flaking, chipping, and bond failure with the bit. Hard 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. 8).

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  Fig. 7—Typical hard-facing applications on a milled-tooth bit.

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  Fig. 8—Exploded view of seal and bearing components.

Hard facing is commonly installed manually by welding. A hollow steel tube containing appropriately sized 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. In the process, tungsten carbide grains flow as a solid, with molten steel from the rod, onto the bit. The steel then solidifies around the tungsten carbide particles, firmly attaching them to the bit.

Special purpose roller cone bit designs

Monocone bits

Monocone bits were first used in the 1930s. The design has several theoretical advantages but has not been widely used. Bit researchers, encouraged by advances in cutting structure materials, continue to keep this concept in mind, because it has the room for extremely 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 sizing presents significant engineering problems.

Monocone bits drill differently from three-cone bits. Drilling properties can be similar to both the beneficial crushing properties of roller-cone bits and the shearing action of PDC bits. Cutting structure research thus focuses partly on exploitation 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 extend insert life and expand the range of applications in which this design could be profitable. The design also provides ample space for nozzle placements for efficient bottomhole and cutting structure cleaning.

Two-cone bits

The origin of two-cone bit designs lies in the distant past of rotary drilling. The first roller-cone patent, issued in August 1909, covered a two-cone bit. As with monocone bits, two-cone bits have available space for larger bearings and rotate at lower speeds than three-cone bits. Bearing 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 9). Their advantages cause this design to persist, and designers have never completely lost interest in them.

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  Fig. 9—Two-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. Penetration per insert is enhanced, providing particularly beneficial results in applications in which capabilities to place WOB are limited.

The additional space available in two-cone designs has several advantages. It is possible to have large cone offset angles that produce increased scraping action at the gauge. Space also enables excellent hydraulic characteristics through room for placement of nozzles very close to bottom. It also allows the use of large inserts that can extend bit life and efficiency.

Two-cone bits have a tendency to bounce and vibrate. This characteristic is a concern for directional drilling. Because of this concern and advances in three-cone bearing life and cutting structures, two-cone bits do not currently have many clear advantages. As with many roller-cone bit designs, however, modern materials and engineering capabilities may resolve problems and again underscore their recognized advantages.

See also

Rotary drill bits

Roller cone bit components

Roller cone bit classification

PEH:Introduction_to_Roller-Cone_and_Polycrystalline_Diamond_Drill_Bits

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