Contents lists available at ScienceDirect International Journal of Rock Mechanics and Mining Sciences journal homepage: www.elsevier.com/locate/ijrmms The percussive process and energy transfer efficiency of percussive drilling with consideration of rock damage Hengyu Song, Huaizhong Shi * , Zhaosheng Ji, Xiaoguang Wu, Gensheng Li, Heqian Zhao, Gaosheng Wang, Yong Liu, Xinxu Hou State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing), Beijing, 102249, China ARTICLE INFO Keywords: Percussive drilling Rock damage Input energy Output energy Energy transfer efficiency ABSTRACT Considering the strong nonlinearity of the percussive process, this paper used the finite element method to establish a three-dimensional percussive system model to study the energy transfer efficiency of percussive drilling. The model consists of 12-button bit model, damage-plasticity model for the rock, bit-rock interaction model, etc. The explicit dynamics solver in ABAQUS was adopted to solve the above model. In order to study different impactors, the percussive system was loaded by the prescribed pulse load that can be generated by ultrasonic impactor or electro-magnetic impactor, and by impact hammer with initial velocity that can be driven by high-pressure fluid or compressed gas, respectively. The simulation results are as follows. Increasing load duration and dynamic amplitude of the pulse load can improve the input energy, output energy and energy transfer efficiency. With the increase of the load duration or dynamic amplitude, the energy transfer efficiency first increases rapidly in the low input energy phase and then increases slowly in the high input energy phase. For the three pulse shapes–sine, triangle, square in this paper, the input energy, output energy and energy transfer efficiency under the condition of square pulse are the largest when the load duration and dynamic amplitude of the pulse force are fixed. While, the energy transfer efficiency under the condition of square pulse are the smallest and that of sinusoidal pulse are the largest when the input energy of the pulse force is fixed. Both the output energy and the energy transfer efficiency under dynamic loading increase first and then decrease with the static load. For the impact hammer, the energy transfer efficiency increases with the impact velocity of impact hammer. 1. Introduction Achieving high-efficiency drilling is the goal pursued by oil and gas drilling industry. 1,2 However, huge challenges and high costs are en- countered when drilling hard formations in recent years. Therefore, it is essential to develop new technologies to improve the drilling efficiency. Percussive drilling has proven to be a feasible drilling technology capable to significantly increase the penetration rate when drilling hard rocks. 3–5 It utilizes the impact energy converted from the downhole energy to impact on and crush the rock. 3,5 The downhole energy here refers to the energy used to power the impactors. For different im- pactors (such as gas/hydraulic hammer impactor, ultrasonic impactor and electro-magnetic impactor), the downhole energy can be derived from compressed gas, high pressure liquid or electrical energy. To make this technique more efficient for drilling, the percussive process, rock- breaking mechanism, stress wave propagation and energy transfer ef- ficiency, etc. need to be studied. Predecessors have done a lot of related research. Simon 6 proposed a method of computing the conversion efficiency of the energy in the stress wave in the drill steel produced by the striker impact into work performed by the bit on the rock in percussive drilling and optimized the impact parameters to improve the energy transfer efficiency. Hus- trulid et al. 7–10 studied the percussive process, the force-penetration relationship of the bit and the specific energy of crushing rock by theoretical analysis and experiment, and then proposed a theoretical model to predict the penetration rate. Franca 5 proposed a bit–rock in- teraction model to evaluate the performance of rotary-percussive dril- ling and studied the impact system efficiency (energy transfer effi- ciency) based on the model. Krivtsov et al. 11 proposed a dry friction model of percussive drilling to study the influence of the static force and the dynamic amplitude on the material removal rate. Lundberg et al. 12,13 studied the efficiency of churn drilling with consideration of wave energy radiation into the rock and investigated the influence of 3D effects on the efficiency of percussive rock drilling. Pavlovskaia https://doi.org/10.1016/j.ijrmms.2019.04.012 Received 1 July 2018; Received in revised form 22 April 2019; Accepted 23 April 2019 * Corresponding author. E-mail address: shz@cup.edu.cn (H. Shi). International Journal of Rock Mechanics and Mining Sciences 119 (2019) 1–12 1365-1609/ © 2019 Published by Elsevier Ltd. T