Application of the 1987 Roller Bit Dull Grading System

Abstract

SPE and IADC Members Abstract The IADC has adopted an improved rock bit dull grading system modeled after the previously used "T, B, g Remarks" approach. This paper discusses the improved system as it applies to roller cone bits (application of the new system for fixed cutter bits is presented in another paper). A complete definition of the system is given. Descriptions and possible causes of many dulling characteristics of roller cone bits are presented and briefly discussed. Some example dull bits are graded using the improved system. An application of the dull grading system as used in well planning is demonstrated. Introduction The use of dull bit evaluation methods represents a key step in the advance of rock bit technology. Historically, a driller would learn through experience how to examine a "dull" to determine what type of bit to run next, and how it should be run (WOB, RPM etc.). This was part of the art that separated the best drillers from the rest. An industry wide effort began about 30 years ago to teach the art of bit wear analysis to a broader range of personnel so that dull bit evaluation would become an integral part of daily drilling practice. General guidelines were established in the mid-1950's for relating typical bit wear patterns to the possible causes and remedies. This approach was helpful but limited by the lack of a common vocabulary for describing bit wear and documenting the dull condition in drilling reports. Meanwhile, basic bit performance studies produced a greater appreciation for the economic impact of bit wear and its close relationship to bit selection and operating practices. An industry standard for gauging bit wear was clearly needed. The Weight/Speed/Penetration Sub-Committee of the American Association of Drilling Contractors (AAODC) established the first dull grading standard in 1961. This is the origin of the familiar "tooth-bearing-gauge" format for reporting bit wear. The original method called for grading teeth and bearings on a 1 - 4 scale where a 4 represented teeth totally worn flat or broken and bearings that were either locked or missing. Reaction to the AAODC bit grading method was favorable as the drilling industry learned to use it. The system was expanded in 1963 to a 1 - 8scale and numerous abbreviated codes were defined for describing dulls in greater detail. This system remained essentially unchanged through 1985.Bit manufacturers contributed substantially to the success of the technique by teaching its use to customers via schools and numerous publications. In March of 1985 the IADC Drill Bits Sub-Committee of the IADC Drilling Technology Committee (the modern counterpart to the AAODC Weight/Speed/Penetration Sub-Committee) met and discussed the role of dull bit grading in the planning of wells, and reviewed the then-current dull grading system. It was agreed that dull bit gradings are an important part of bit records, and essential to well planners and field people charged with drilling wells in an efficient and cost-effective manner. P. 859^