ローラコーンビットによるビット荷重とトルクの振動特性に関する基礎的研究

Abstract

Vibrations of bit weight and torque during drilling by roller cone bits are converted into power spectra, and prominent frequencies are estimated. Three kinds of new bits and two kinds of worn bits with diameters from 43/4 to 45/8 in. are tested with a bit test equipment at room conditions. Vibrations of bit weight and torque are measured by load cells under the rock specimen drilled by the bit.Power spectra are calculated from normalized data with zero average value and unit standard deviation. The frequencies are normalized to eliminate the effect of bit rotational speed. The dimensionless spectra of experimental results comprise normalized spectra and frequencies.Theoretical dimensionless frequencies of bit motion are calculated under an assumption of no-slipped rotation of cones, and apparent power spectra of bit motion are made from the calculated values. A displacement factor is introduced as a measure of shift of the experimental spectra from the apparent ones. The displacement factor as well as the shape of the ower spectra are usefull to evaluate bit characteristics and status of bit. If bit cones slip, the displacement factor has a lower value than 1. Without slip, the factor may be equal to 1. As a result of experiments, the soft formation bit has the lower displacement factor than the hard formation bit. The shapes of dimensionless spectra obtained by worn bits differ irregularly from the shape of apparent spectra. The shape of spectra offers a means to detect a worn-out bit qualitatively.