A time scale regard on percussion drilling

This paper examines the performance of three families of percussive drilling methods, Churn (CD), Down-the-Hole (DTH), and Top-Hammer (TH), through the prism of time scales. These time scales characterize different aspects of the dynamics of the drilling process. One time scale represents the travel time of an elastic perturbation in the hammer, while another one corresponds to the travel time in the drillstring and/or the bit assembly. The duration of the response of the bit/rock interface to an impulse load, and the duration of the pulse generated by the impact of the hammer in DTH and TH tools are two other time scales. Within the simplified modeling framework considered in this study, the dynamics of the percussion tools is at most controlled by three numbers, which are ratios of time scales. However, some of these numbers could be irrelevant depending on the design and class of the percussion drilling system, because they are either too small or too large and thus do not affect the dynamical response. For example, the energy transfer efficiency—the fraction of the impact energy effectively delivered to the rock—depends on one number when drilling with a TH tool, but on three numbers for a hydraulically powered DTH tool. This approach enables the identification of the point of maximum efficiency in the parametric space of the time scale ratios. The so-called sweet spot can thus be understood as representing an optimum match of the different timescales characterizing the drilling system. For instance, maximum performance is achieved with a TH tool if the two time scales controlling its dynamics are equal.