Study on the Axial Vibration Dynamics of Drill Rod Systems in Long-Reach Directional Drilling in Underground Coal Mines

In response to issues such as axial vibration, attenuation of drilling pressure transmission and increased impact loads on components near the drill bit that frequently occur during long-distance drilling in near-horizontal directional boreholes in underground coal mines, a multi-degree-of-freedom axial vibration model for directional long-hole drill-string systems has been established based on structural and stress analysis of the drill-string system. The model accounts for borehole-wall friction, buoyancy correction, self-weight, Rayleigh damping, drilling pressure input, and the velocity interaction between the drill bit and the coal–rock formation, and employs numerical integration to solve for the displacement, velocity, and spectral response of the drill bit and the pulse probe. The results indicate that an increase in the friction coefficient widens the vibration envelope, whilst the principal frequencies remain concentrated at approximately 5 Hz and its harmonics. When the drilling pressure is increased from 80 kN to 110 kN, the response increases gradually, and under high-pressure (120 kN) and heavy-duty drilling tool combinations, the displacement and velocity are significantly amplified; when the drill-string length was increased from 400 m to 550 m, the system’s response shifted from a relatively regular periodic response to a low-frequency, multi-peak response with amplitude modulation. The velocity response of the pulse probe was generally higher than that of the drill bit, indicating that it is a component sensitive to axial impacts near the drill bit. The research findings provide a theoretical basis for optimising drilling pressure, controlling frictional resistance and designing vibration-damping drill-string configurations for long directional boreholes in coal mines.