Application of the New IADC Dull Grading System for Fixed Cutter Bits

Abstract

Abstract This paper describes a recently developed standardized system for dull grading natural diamond, PDC, and TSP (Thermally Stable Polycrystalline diamond) bits, otherwise known as fixed cutter bits. The new system is consistent with the recently revised dull grading system for roller bits. It describes the condition of the cutting structure, the primary (with location) and secondary dull characteristics, the gauge condition, and the reason the bit was pulled. Examples of all commonly seen dull characteristics of fixed cutter bits are given. In addition, several worn fixed cutter bits are dull graded using the system. Introduction A dull bit can tell a lot about what happened while it, was downhole. Were the operating conditions right? Was it the best bit for that application? The IADC recognized the value of dull grading 25 years ago when it established the roller bit dull grading system that is used world wide today. A counterpart dull grading system for fixed cutter bits was never established; further, the original "T-B-G" roller bit system does not work for fixed cutter bits. Such descriptions as "50% worn" or "burned up" are usually all that can be found on bit records. Many drillers, tool pushers, and company men carefully examine their dull bits and make significant observations, but they are stymied because the standard report forms have no place to record fixed cutter bit information. The need for a standardized dull grading system became more obvious over the past five years because of the increased use of PDC bits. Therefore, the IADC, recently adopted its first standardized dull grading system for fixed cutter bits and scheduled it to take effect in March, 1987. The term "diamond bits" has been commonly used to describe any fixed cutter bit. However, it is not descriptive enough, since natural diamonds are not necessarily used in all fixed cutter bits. The term "fixed cutter bit" was chosen by the Drill Bits Subcommittee from many possibilities (such as drag bits, rigid cutter bits, and solid body bits) as the most descriptive phrase for non-roller cone bits. In this paper, only examples of natural diamond, PDC, and TSP drilling bits are used. However, there is no reason not to use the dull grading system for core bits also. There is no means in the system of distinguishing between drilling and coring bits. That information must be gotten from the bit record. SYSTEM CONCEPT The IADC Drill Bits Subcommittee recently revised the original roller bit "T-B-G" dull grading system (Reference 1). At the same time, the fixed cutter dull grading system was designed to be compatible in form and function to the roller bit system. The objective of the system is to "paint a mental picture" of the worn bit's physical condition. This partly require that the user knows what the bit looked like in new condition. Thus, the use of the new dull grading system is related to the use of the new fixed cutter bit classification system (Reference 2). Further, the dull grading system is a means of recording the physical condition of the bit for future reference. It is not intended to replace visual inspection of the bit. While the dull grading system was being designed, the need for as much detail as possible about the dull bit was balanced against the need to keep the system simple and easy to use. This system is flexible enough to work well in manufacturers' bit records, in operators' mornings reports, in the IADC's daily drilling report, and in data bases. P. 851^