Proposed practical methods to estimate rock strength and tooth wear while drilling with roller-cone bits

The authors have previously developed and proposed methods to estimate the in situ rock strength and tooth wear while drilling with roller-cone bits [Karasawa et al., 2002, ASME J. Energy Resour Technol. 124, pp. 125-132 (Pt. 1) 133-140 (Pt. 2)]. The purpose of this paper is to provide a follow-up to these two reports and to propose, for both previous methods, alternate techniques that can be more readily implemented in the field than those originally presented. The data presented in Part 1 of the previously mentioned work [1] were reanalyzed in order to find a new and simple parameter that can be used to estimate rock strength. This parameter uses only one set of data that consists 6 bit weight, torque, penetration rate, rotary speed, and bit diameter. It was also demonstrated that the effect of tooth wear on this new parameter is small. In addition, more practical methods, which employ two parameters, are derived and proposed to evaluate the tooth wear of roller cone bits.