Dynamics study of hot dry rock percussive drilling tool based on the drill string axial vibration

Drill string vibration is the main factors affecting the drilling efficiency, and the control of drill string vibration is concerned in deep hot dry rock (HDR) drilling, it is believed that percussive drilling is one of the methods to improve the rate of penetration (ROP) of HDR. If the drill string vibration energy could be converted into impact energy to increase the ROP of the HDR, at the same time, it can also reduce the vibration of the drill string and protect the drill bit. Therefore, a novel HDR drilling tool is proposed in this paper, the basic principle is that the axial vibration of drill string is converted to impact load to improve the drilling efficiency. Therefore, firstly the mathematical model of the drill string axial vibration, drill string axial vibration dynamics model with the percussive drilling tool, drill bit and formation interaction model are established. Thereafter, the influence of flow rate, spring stiffness, WOB and outlet size are investigated, aiming to analyze the impact load magnitude of the impact drilling tool and vibration displacement of drill string and the impact drilling tool. The results demonstrate that the drill string vibration reduction ratio reaches 82.8%, and the HDR drilling tool could converter the random varied weight on bit to sinusoidal impact load, only with increasing of the WOB, the impact load keeps unchanged when the WOB is constant, the variation of spring stiffness and outlet size have no influence on the impact load. However, the force acted on cross-section of the load transfer head F-2 and the spring force F-1 varies with the variation of WOB and etc.; with increasing of the spring stiffness k(2), the drill string vibration decreases, and the drill string vibration decreases with the increasing of the flow rate, WOB and outlet size; validates that the impact load of the HDR drilling tool shows sinusoidal shape. The research provides a theoretical guidance to drill string vibration control and the percussive drilling tool design.