Abstract
The objective of this report is to investigate methods for the estimation of both rock strength and the tooth wear of roller cone bits while drilling. Drilling tests were conducted using milled tooth bits of 98.43 mmdia with different tooth wear. Two types of andesite, tuff and sandstone were drilled in the tests. The uniaxial compressive strength of these rocks ranged from 14.0 MPa to 118 MPa.
We first obtained the equations for the primary performance lines of each bit from the plots of F/d vs. u/N and 8T/d2 vs. u/N for each rock, where F is the bit weight, d the bit diameter, u the penetration rate, N the rotary speed, and T the torque.
Second, we estimated information related to rock strength using multiple regression analysis, based on the equations obtained. From this estimation, the relation between Feu /Nd and (8Fe/d2) 2 was investigated, where Fe is the effective weight and Te the effective torque. From the investigation, it became clear that this relation is closely linked to rock strength, as shown in Fig. 8. Thus, the drilling strength of rock (Ds) was defined based on this relation.
Finally, the method to estimate tooth wear, based on Ds, was investigated. This investigation revealed that the condition of tooth wear can be estimated from the relations between Ds and Is, Ds and Fc/d, Ds and Se, Ds and 8Tc /d2, where Is is the penetration strength of rock, Fe the threshold weight, Se the specific energy calculated from the effective rotary energy, and Tc the threshold torque.
