Weight and resistance reduction mechanism of drilling rods for drilling in soft coal seam and its application

Gas drainage boreholes during construction is a necessary means to control gas hazards. In response to the issues such as high drilling resistance, low drilling efficiency, and high labor intensity caused by the significant self-weight of the drilling rod in drilling into the soft coal seams, a research approach of reducing the self-weight and lowering the drilling resistance by optimizing the drilling rod structure was proposed. Thus, a four-winged concave grooved drilling rod was designed to address both of its advancing resistance and rotating torque losses. Meanwhile, a drilling resistance model for the drilling rod was established. On this basis, the strength verification analysis and parameter optimization for the four-winged concave grooved drilling rod was conducted with the ANSYS numerical simulation method. Subsequently, the drilling rod was developed, and industrial-scale field experiments were carried out. The study shows that: (1) , the four-wing concave grooved drilling rod reduces the weight by 21.88% in every 100-meter drilling depth, compared to a circular grooved drilling rod. During normal drilling, the rotating torque loss of the four-wing concave grooved drilling rod is reduced by 25.10% compared to a circular rod and by 20.90% compared to a circular grooved drilling rod. In case of a collapse hole of 10 meters, the drilling resistance of the four-wing concave grooved drilling rod is reduced by 51.44% compared to a circular rod, and by 43.65% compared to a circular grooved drilling rod. (2) Strength analysis and parameter optimization of the four-wing concave grooved drilling rod show that under the action of a normal tunnel drilling machine, the maximum stress of the drilling rod is positively correlated with the concave depth and negatively correlated with the wall thickness. The influence of wall thickness on stress is greater than that of concave depth. Besides, the safety factor is 1.59 when the concave width is 33 mm, the depth is 4 mm, and the wall thickness is 11.5 mm. (3) On-site industrial experiments demonstrate that the four-wing concave grooved drilling rod works for smooth slag discharge, with reliable strength, having the drilling depth increased by 18.18%, and drilling efficiency improved by 17.80%. By effectively reducing the weight of drilling rods through innovative structural design while ensuring their strength, significant improvements can be achieved in reducing labor intensity, increasing drilling efficiency, and achieving a higher hole formation rate. Generally, this research provides a new method for promoting the development of drilling technology in complex geological formations.