Brandon, B.D.,* and Cerkovnik, Jerry,** Eastman Christensen; Koskie, Earl, DBS; Bayoud, B.B.,* Eastman Christensen; Colston, Fred, Smith Diamond; Clayton, R.I.,* Security; Anderson, M.E.,*Eastman Christensen; Hollister, K.T.,* Hycalog; Senger, Jim,* Security Div., Dresser Industries; and Niemi, Ralph,* Cliffs Drilling Co.
This paper is one of two describing changes to the IADC Classification and Dull Grading Systems for fixed cutter bits. Dull grading system revisions, described herein, were implemented to improve utilization and effectiveness of the dull grading system.
Classification system changes were required as a result of improvements in bit technology and applications, and are detailed in the companion paper SPE 23940.
The IADC Fixed Cutter Work Group this year audited the 1987 Fixed Cutter Dull Grading System and determined that some minor refinement was necessary. As was the case with introduction of the fixed cutter dull grading system in 1987, the objective of this revision was to facilitate creation of a "mental picture" of a worn bit's physical condition through a standardized evaluation of certain bit characteristics.
Because the system provides an industry-wide standard for recording the physical condition of the worn bit for future reference, the meaning of a dull grade should be subject to as little misinterpretation as possible. Therefore, committee discussions focused on two specific areas: improving the definition of "usable cutter height" as it relates to evaluation of PDC cutter wear, and making minor enhancements to the wear PDC cutter wear, and making minor enhancements to the wear characteristic codes.
The format of the dull grading chart, shown in Figure 1, has not changed under this revision. Eight factors are recorded: the first four spaces describe the extent and location of wear of the "Cutting Structure." The next two spaces address other criteria for bit evaluation, with the fifth space reserved for grading "Bearing" wear of roller cone bits. This space is always marked with an "X" when fixed cutter bits are graded.
The sixth space indicates "Gauge Measurement". The last two positions allow for "Remarks" which provide additional positions allow for "Remarks" which provide additional information concerning the dull bit, including "Other (or secondary) Dull Characteristics" and "Reason Pulled", respectively.
The revised system grades all PDC cutters based on condition of the visible diamond table of the cutter, regardless of cutter shape or exposure. This differs from the former practice of grading PDC cutters based on "usable cutter height" remaining. It was determined that the definition of "usable cutter height" for PDC bits was subject to misinterpretation, given the initial positioning of some PDC cutters "within" the bit blade on some designs.
Additional enhancements include addition of a dull characteristic code, "BF", to distinguish "bond failure" between the cutter and its support backing from "LT", loss of a cutter. In addition, the optional designations "RR" or "NR" were added to allow for indication of whether a bit is "rerunnable" or not.
Application of these minor revisions will further "standardize" the meaning of a dull grade. Examples of dull characteristics are shown in Figure 2.
Evaluating "Cutting Structure" Inner/Outer Rows: Spaces 1 and 2 Using a linear scale from 0 to 8, as before, a value is given to cutter wear in both the inner and outer rows of cutters. Grading numbers increase with amount of wear, with 0 representing no wear, and 8 meaning no usable cutters left. A grade of 4 indicates 50% wear.
For surface-set bits, the scale of cutter wear is determined by comparing the initial cutter height with the amount of usable cutter height remaining.