Development Of Large Diameter Eccentric Pneumatic Hammer Drill Bit With Reverse Circulation Used For Casing-while-drilling

Air Down-The-Hole(DTH)hammer reverse circulation drilling technology of Casing-while-Drilling(Cw D)is widely used in the construction of anti-slip pile for geological disaster control with its stable and efficient advantages.However,for large landslides with deep sliding surface and complicated geological environment,various problems often occur during construction,including the failure of retractable mechanism,the difficulty in forming reverse circulation,low drilling efficiency and the difficulty in lowering casing,etc.Especially for large-diameter drill bits of Cw D,not only the impact energy is high during drilling,but also the amount of cuttings per footage is large,so the problem is more prominent.The key to solve the above problems is to improve and optimize the bit structure to ensure the reliable operation of the retractable mechanism of drill bit and the strong reverse-circulation of the gas passage.Based on the national key research and development plan 'Research and Development of Emergency Disposal and Rapid Treatment Technology for Extra Large Landslide' topic3 'Research on Large Diameter Drilling Tools and Construction Machinery Miniaturization for Extra Large Landslide Prevention and Control',a novel retractable drill bit of Cw D of large diameter DTH hammer was designed,which was composed of upper drill bit,lower drill bit body and suspension pin.The upper and lower drill bit are connected by the suspension pin,and the diameter is reduced by the arc-shaped circular shaft and the arc groove.In order to analyze the reliability of the retractable mechanism,the kinematic characteristics of the mechanism were analyzed and studied by means of mechanical simulation and test.The results show that the drill bit can be opened and retracted through positive and negative rotation,and the retractable mechanism is flexible.The maximum opening diameter is 600 mm,and the diameter after retracted is 530 mm,which meets the design requirements.In order to further improve the reliability of drill bit retraction,an inclined plane is arranged on the bit body.When the drill bit is lifted,it will retract under the pressure of the casing boot.In addition,the interaction area between the upper and lower drill bit is large,which can efficiently transfer impact stress and ensure good rock crushing effect.Based on the working principle of ejection reverse circulation drill bit,the reverse circulation structure of the drill bit was innovatively designed,and the reverse circulation effect and cuttings carrying capacity of the drill bit were improved by setting up suction channel and spiral slots on the bottom lip.Experimental and multiphase flow numerical simulation results show that the suction channel increases the passage of cuttings and air into the center channel of the drill bit,which contributes to the improvement of the reverse circulation cuttings removal effect.The spiral slots at the bottom of the drill bit has relatively little influence on the reverse circulation effect,but is more conducive to carrying cuttings.The cuttings carrying ratio ? can reach 99% and the suction ratio ? can reach 20% by optimizing the design of the inner jet nozzles,the flushing nozzles,the suction channel and the spiral slots in the drill bit.Combined with the rock-crushing effect of carbide inserts,the arrangement method of carbide inserts on the bottom of the drill bit was analyzed.The total number of carbide insert on the bottom of the drill bit was determined to be 102,and the carbide insert on the bottom of the drill bit was arranged in a circular shape around the central channel of the bottom of the drill bit.The total number of carbide insert on the bottom of the drill bit was 95,with a diameter of 20 mm.The number of side carbide insert is 6,with a diameter of 25mm;A carbide insert with a diameter of22 mm is arranged at the entrance of the central passage at the bottom of the drill bit.Ansys Workbench software and Abaqus software were used to simulate the strength of drill bit under static load and impact load respectively.Under the static load of WOB less than 50 k N and torque less than 1500N·m,and under the impact load of 2500 J,all parts of the drill bit keep good strength and stiffness.Based on the above research results,a large-diameter DTH reverse circulation drill bit of Cw D was designed and machined,and field test was carried out.The test formation is mainly argillous sandstone.During the operation,the retractable mechanism of drill bit can open and close smoothly as required.The drill bit has strong reverse circulation ability,good cuttings carrying effect,high rock crushing efficiency,firmly inlaid of carbide inserts and no obvious wear,which meets the needs of reverse circulation of large-diameter DTH hammer of Cw D and provides important support for the construction of large-diameter anti-slide pile for landslide disaster control.