Influence of Weight-on-Bit on Percussive Drilling Performance
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ORIGINAL PAPER
Influence of Weight‑on‑Bit on Percussive Drilling Performance Xianfeng Song1 · Ole Morten Aamo1 · Pascal‑Alexandre Kane2 · Emmanuel Detournay3 Received: 8 April 2020 / Accepted: 18 August 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract A phenomenological model for percussive drilling systems is proposed in this paper to explain the experimentally demonstrated existence of an optimal weight-on-bit (WOB), for which the rate of penetration (ROP) is maximized. Several hypotheses have been previously proposed to explain this universal characteristic of percussive drilling, including increased wear of the bit, reduced indexing, and poor cleaning of debris under excessive WOB. Motivated by experimental evidence, we instead consider an increase of the pseudo-stiffness of the bit-rock interface (BRI) with increasing WOB, and investigate its effect on the impact energy transmitted to the rock. The 1D model approximates the dynamics underlying the drilling process by assuming that the impact of the hammer generates a longitudinal wave in the bit. It is shown that the BRI pseudo-stiffness influences the incident wave and associated energy transmitted from the bit to the rock. As a consequence, the drilling efficiency is affected by the dependence of the BRI stiffness on the WOB. The model indicates that there exist optimal conditions for the energy transfer from the bit to the rock in terms of the impedance ratio and the BRI stiffness/ WOB. Thus it confirms that there is a sweet spot as seen in practice, which suggests that the root cause of the existence of a sweet spot in the ROP-WOB relationship lies in the nature of the BRI laws, rather than with issues related to bit indexing, bit wear, and/or cleaning of the debris. Keywords Percussive drilling · Weight-on-bit · Rate of penetration · Impact transmission efficiency
1 Introduction A percussive drilling system, either a Top-Hammer (TH) or a Down-The-Hole (DTH) tool consists essentially of a pneumatically or hydraulically driven hammer and a bit assembly, see Fig. 1 for the schematic of a DTH tool. During the percussive drilling process, the hammering system converts the potential energy of the pressurized air or water into reciprocating movements of the hammer, which repeatedly impacts the bit assembly (Ghosh et al. 2017). An impact induces a compressive incident wave that travels through the bit assembly to eventually cause penetration by indentation of a collection of bit buttons into the rock (Fourmeau et al. 2015; Lundberg 1982).
* Emmanuel Detournay [email protected] 1
Norwegian University of Science and Technology, Trondheim, Norway
2
SINTEF Industry, Trondheim, Norway
3
University of Minnesota, Minneapolis, USA
Percussion tools are generally more efficient for drilling hard rock formations than conventional rotary equipment (Franca 2011; Han et al. 2009). Their advantages include lower weight-on-bit (WOB) requirement, reduced bit wear due to less contact time with the rock, less borehole deviation and larg
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