On Sweet Spot in Percussive Drilling: Single Impact Experiments

Abstract

ABSTRACT: This work presents results of research on down-the-hole percussive drilling and investigates the origin of the observed non-monotonic dependency of the rate of penetration (ROP) on the applied weight-on-bit (WOB). Field experiments were carried out at a mining site with a drilling rig that was instrumented with a tailored monitoring system. The field data clearly shows in several instances that the ROP is maximized for specific values of WOB; this phenomenon is commonly referred to as Sweet Spot. In order to investigate whether the Sweet Spot is intrinsic to the bit-rock interface or stems from the system dynamics, laboratory experiments were conducted with a novel apparatus that drills shallow holes with discrete events (one impact at a time); prior to each impact of the piston on the drill bit, the initial conditions are guaranteed to be re-established. The effect of drilling parameters can be studied independently with the experimental setup, such as WOB, piston impact energy, and angular displacement of the bit between successive impacts. Preliminary results show no intrinsic dependency of the rate of penetration with the applied WOB. 1 INTRODUCTION Down-the-hole (DTH) percussive drilling is a widely employed technique in mining and civil engineering. In DTH drilling, a pneumatically or hydraulically activated piston (or hammer) strikes the drill bit, which fragments the rock surface and propagates the borehole. Concurrently, the drill bit rotates, and either compressed air or water is used to flush out rock debris and maintain bit cooling (Hustrulid and Fairhurst, 1971; Amjad, 1996; Zhao et al., 2014; Depouhon, 2014). In the context of DTH percussive drilling, several laboratory and field experimental studies have reported the existence of optimal operating conditions that result in the highest drilling efficiency and effectiveness (Cheetham and Inett, 1953; Lundquist, 1968; Hustrulid and Fairhurst, 1972; Unger and Fumanti, 1972; Amjad, 1996; Schunnesson, 1998; Kivade et al., 2015; Ghosh et al., 2017; Shen et al., 2022). The rate of penetration (ROP) increases with the increase of weight-on-bit (WOB) up to a point and then starts to decrease with a further increase in WOB (refer to Figure 1). This phenomenon is commonly referred to in the literature as the Sweet Spot (Depouhon, 2014); in this paper, however, it will be referred to as Optimum Drilling State (ODS). Despite numerous studies investigating potential causes of the Optimum Drilling State (ODS), there remains a lack of consensus or definitive explanation for its occurrence. This absence of a precise understanding regarding the correlation between drilling parameters and the manifestation of the ODS impedes the development of optimization and control algorithms aimed at maximizing the Rate of Penetration (ROP) amidst changing borehole conditions (Song et al., 2019).