Bit classification allows efficient selection and use of polycrystalline diamond compact (PDC) and diamond drill bit. The classification system currently in use was developed by the International Association of Drilling Contractors (IADC).IADC classification codes for each bit are generated by placing the bit style into the category that best describes it so that similar bit types are grouped within a single category. The system leaves a rather broad latitude for interpretation and is not as precise or useful as the IADC Classification System for Roller-Cone Bits.
Fixed-cutter bit classification system
The system is composed of four characters that designate:
- Body material.
- Cutter density.
- Cutter size or type.
- Bit profile.
It does not consider hydraulic features incorporated into a bit and does not attempt to give a detailed description of body style beyond basic classification of the overall length of the bit cutting face. Special designs incorporating unconventional use and densities of gauge cutters are not considered for classification.
Bit body material
The first digit in the IADC Fixed-Cutter Bit Classification describes the material from which the bit body is constructed: M or S for matrix- or steel-body construction, respectively.
The second IADC classification character is a digit that represents the density of cutting elements. Densities for PDC cutter and surface set diamond bits are described separately through use of numerals 1 through 4 for PDC bits and 6 through 8 for surface-set diamond bits. Numerals 0, 5, and 9 are not defined. Specifically, for PDC bits, density classification relates to cutter count; for surface-set bits, it relates to diamond size. Because heavier cutter densities generally correspond to tougher drilling applications, the density classification digit implies an applications aspect as it increases.
- PCD Bit Cutter Density. PDC bit cutter density represents total cutter count, usually including gauge cutter count. A designation of 1 represents a light cutter density; 4 represents a heavy density. Within the classification rules, a density of 1 refers to ≤ 30 cutters; a density of 2 refers to 30 to 40; density 3 indicates 40 to 50; and density 4 refers to ≥ 50 cutters.
Manufacturers classify their PDC bits within these four numeric categories, depending on a manufacturer’s internal criteria for cutter density. Bits that are “borderline” are placed into a higher or lower density category, depending on manufacturer preference.
- Surface-Set Diamond Bit Density. Surface-set diamond density, numerals 6 through 8, categorize variations in the size of the cutter material. The numeral 6 represents diamond sizes > 3 stones per carat; 7 represents diamond sizes from 3 to 7 stones per carat; and 8 represents diamond sizes < 7 stones per carat. Thus, diamond size becomes smaller as the density classification increases. This generally corresponds to what would be expected in surface-set bit designs intended for harder or more abrasive formations.
Cutter size or type
The third character in the IADC classification designates the “size” or “type” of cutter. This again differs for PDC and surface-set diamond bits. For PDC cutter bits, the third character is a digit that represents cutter size:
- 1 indicates PDC cutters > 24 mm in diameter.
- 2 represents cutters from 14 to 24 mm in diameter.
- 3 indicates PDC cutters < 14 but > 8 mm.
- 4 is used for cutters < 8 mm.
For surface-set bits, the third character represents diamond type:
- 1 indicates natural diamonds.
- 2 refers to TSP material.
- 3 represents combinations such as mixed diamond and TSP materials.
- 4 indicating impregnated diamond bits.
The final (fourth) character describes the basic appearance of the bit based on overall length of the cutting face. “Fishtail”-type PDC bits are an exception as bits; for this type of bit, the ability to clean in fast-drilling, soft formations is thought to be a more important body feature than profile. The numeral 1 represents fishtail PDC bits and “flat” TSP and natural diamond bits; 2, 3, and 4 indicate increasingly longer bit profiles of both types (a virtually flat PDC bit would be identified by 2, whereas a long-flanked “turbine style” bit would be categorized as 4). In lieu of developing a formula relating overall bit face length (depth) to bit diameter, each manufacturer classifies its own product profiles using these rules.
IADC bit dull grading
The IADC, in conjunction with SPE, has established a systematic method for communication of bit failures. The intent of the system is to facilitate and accelerate product and operational development based on accurate recording of bit experiences. This system is called dull grading. The IADC Dull Grading Protocol evaluates eight roller-cone or seven PDC bit areas, provides a mechanism for systematically evaluating the reasons for removal of a bit from service, and establishes a uniform method for reporting.
Partly because of dull analyses, bit design processes and product operating efficiencies evolve rapidly. Engineers identify successful design features that can be reapplied and unsuccessful features that must be corrected or abandoned; manufacturing units receive feedback on product quality; sales personnel migrate performance gains and avoid duplication of mistakes between similar applications, and so forth. All bit manufacturers require collection of dull information for every bit run.
IADC dull grading is closely associated with its bit classification systems, and the general formats for fixed-cutter bit and roller-cone bit dull grading are similar. There are important differences that must be taken into account and the two approaches are not interchangeable. The following explains IADC Dull Grading and points out the differences between diamond/PDC and roller-cone bit rules.
IADC dull grading system
IADC dull grading reviews four general bit wear categories:
- Cutting structure (T).
- Bearings and seals (B).
- Gauge (G).
These and their subcategories are outlined in Fig. 1.
Fig. 1—IADC dull grading categories.
Cutting structure wear grading (T)
For dull grading purposes, cutting structures are subdivided into four subcategories: inner rows, outer rows, major dull characteristic of the cutting structure, and location on bit face where the major dull characteristic occurs. Fig. 2 illustrates the dull grading system.
ORIGINAL IMAGE HERE pdc2.png
Fig. 2—IADC Dull Grading System.
- Roller Cone Cutting Structure Evaluation. Dull grading begins with evaluation of wear on the inner rows of inserts/teeth (i.e., with the cutting elements not touching the wall of the hole bore). Grading involves measurement of combined inner row structure reduction caused by loss, wear, and/or breakage with the measurement method described above. Outer rows of inserts/teeth are those that touch the wall of the hole bore. Grading involves measurement of combined outer row teeth/insert structure reduction caused by loss, wear, and/or breakage with the measurement method described above.
- Roller-Cone Cutter or Insert/Tooth Wear Measurement. Measurement of roller-cone cutting structure condition requires evaluation of bit tooth/insert wear status. Wear is reported by use of an eight-increment wear scale in which no wear is represented by “0” and completely worn (100%) is represented by “8” (Fig. 3).
Fig. 3—Tooth height measurement.
- PDC Bit Cutter Wear Evaluation. Cutter wear is graded with a 0 to 8 scale in which 0 represents no wear and 8 indicates that no usable cutting surface remains (Fig. 4). PDC cutter wear is measured across the diamond table, regardless of the cutter shape, size, type, or exposure. The location of cutter wear is categorized as either the inner two-thirds or outer third of the bit radius (Fig. 5).
. 4—Cutter wear convention (zero is no wear).
Fig. 5—Inner/outer body designation for PDC and impregnated bits.
- PDC Bit Inner and Outer Row Cutter Wear Measurement. For both PDC and surface-set diamond bits, a value is given to cutter wear with the method described above. To obtain average wear for the inner rows of cutters depicted in Fig. 5, the six included cutters must be individually graded, summed as a group, and averaged to obtain the inner row wear grade, (a + b + c + d + e + f) / 6 . This analysis is repeated for each blade, and blade results are summed and averaged for the final result. A similar analysis is made for the seven cutters used in the outer bit rows, and the two results are recorded in the first two spaces of the dull grading form.
- Dull Characteristic (D). The cutting structure dull characteristic is the observed characteristic most likely to limit further use of the bit in the intended application. A two-letter code is used to indicate the major dull characteristics of the cutting structure.
The primary cutter dull characteristic, the third cutting structure subcategory, is recorded in the third space on the dull grading record. (Note that noncutting structure or “other” dull characteristics that a bit might exhibit are noted in the seventh grading category.) Category 3 defines only primary cutter wear, whereas Category 7 can be used to describe either secondary cutting structure wear or wear characteristics that relate to the bit as a whole and are unrelated to cutting structure. Grading codes for the other dull characteristics category are the same as those listed above.
- Roller-Cone Bit Dull Location (L). A two-letter code is used to indicate the location of the wear or failure that necessitated removal of the bit from service. These codes are listed in Fig. 2
- PDC Bit Cutting Structure Drill Location. The last of the cutting structure-related wear grades, dull location, indicates the location of the primary dull characteristic. Possible locations include the cone (C), nose (N), taper (T), shoulder (S), gauge (G), all areas (A), middle row (M), and heel row (H). Location grades are reported in the fourth space on the dull grading form.
Bearing and seal (B) criteria (not used for PDC bits)
IADC provides sebrte protocols for estimation of bearing and seal wear in nonsealed and sealed bearing assemblies. Seal and bearing grading applies only to roller-cone bits. It is always marked “X” for PDC bits.
- Estimating Wear on Nonsealed Bearings. For nonsealed bearings, wear is estimated on a linear scale of 0 to 8: 0 is new, 8 is 100% expended.
- Estimating Wear on Sealed Bearings. A checklist for the seal and bearing system condition is provided in Table 1. The grading protocol is as follows:
Table 1-Seal/Bearing Evaluation Checklist
- If no seal problems are encountered, use the grading code E.
- If any component in the assembly has failed, use the grading code F.
- If any portion of the bearing is exposed or missing, it is considered an ineffective assembly; again, use the grading code F.
- Use the grading code N if it is not possible to determine the condition of both the seal and the bearing.
- Grade each seal and bearing assembly sebrtely by cone number. If grading all assemblies as one, report the worst case.
Gauge grading (G)
The gauge category of the Dull Bit Grading System is used to report an undergauge condition for cutting elements intended to touch the wall of the hole bore. For diamond and PDC bits only, gauge is measured with an API-specified ring gauge. (API specifications for ring gauges for roller-cone bits have not been issued.)
- Roller-Cone Bit Gauge Grading. For three-cone bits, the “two-thirds rule” is applied to measuring the gauge condition. The amount out of gauge, as measured by the ring gauge, is multiplied by two-thirds to give the true gauge condition.
For two-cone bits, gauge is the measured distance from either the gauge or heel elements, whichever is closer to gauge.
Measurements are taken at either the gauge or heel cutting elements, whichever is closer to gauge (Fig. 6). Undergauge increments of 1/16 in. are reported. If a bit is 1/16 in. undergauge, the gauge report is 1. If a bit is 1/8 in. (2/16 in.) undergauge, the gauge report is 2. If, a bit is 3 ∕16 in. undergauge, the gauge report is 3, and so forth. Round to the nearest 1 ∕16 in. Gauge rules apply to cutting structure elements only.
Fig. 6—Measuring out of gauge.
- PDC Bit Gauge Grading. For diamond and PDC bits, gauge is measured with a nominal ring gauge. Use of an “IN” code indicates that the bit remains in gauge. Undergauge increments of ∕161 in. are reported. If a bit is 1 ∕16 in. undergauge, the gauge report is 1. If a bit is 1 ∕8 in. (2∕16 in.) undergauge, the gauge report is 2, and so forth. Round to nearest 1 ∕16 in. Gauge rules apply to cutting structure elements only. Measurements are taken at the gauge cutting elements.
Roller cone and PDC bit remarks
The “remarks” category allows explanation of dull characteristics that do not correctly fit into other categories and is the category in which the reason a bit was removed from service is recorded.
- Roller-Cone Bit Other Dull Characteristics (O). Dull characteristics can be used to report dull characteristics other than those reported under cutting structure dull characteristics (D). Evidence of secondary bit wear is reported in the seventh grading category. Such evidence could relate to cutting structure wear, as recorded in the third space, or may report identifiable wear, such as erosion, for the bit as a whole. The secondary dull characteristic often identifies the cause of the dull characteristic noted in the third space.
- Roller-Cone and PDC Bit Reason Pulled (R). The eighth dull grading category reports the reason why a bit was pulled.