To reveal the mechanism of the impactor-bit-rock interaction in geophysical prospecting percussion drilling, considering the coupling effect of the static pressure, impact force and rotary cutting, constructing the physical model of the impactor-bit-rock interaction, and using the finite element methods (FEM), three-dimensional (3D) model of the impactor-bit-rock interaction is established. Using the finite element analysis software (ANSYS/LS-DYNA), the 3D FEM analysis of the impactor-bit-rock interaction is carried out when compressed air pressure is 0.8 MPa, 0.9 MPa, 1.0 MPa, 1.1 MPa and 1.2 MPa respectively. The results show that: the energy transmission efficiency when piston impacts bit under different air pressure is not high and it should be improved further, bit can not fragment rock until it is impacted by piston, it is found that the best air pressure is 1.0 MPa when the impactor and bit are used to drill granite according to the volume of the fragmented rock and the depth of the crater, the speed and displacement on the radial direction of the piston which should be reduce even eliminate are very harmful. The results are further useful to extend the applications of the geophysical prospecting impactor and hammer bit.
Percussion drilling has been widely used in oil and gas industry, yet it still has some shortcomings, such as severe damages to drilling tools, low energy transferring efficiency and low rock-fragmenting efficiency. Thus it is necessary to reveal the mechanism of interactions between the hammer bit and rock in geophysical prospecting percussion drilling. Taking account of the coupling effect of the Weight on Bit (WOB), impact force and rotary torque, this paper constructed a Finite Element Method (FEM) model using the finite element analysis software (ANSYS/LS-DYNA) and conducted a computer simulation of bit-rock interaction under rotating and simple impact effect, which showed the rock-fragmenting process of hammer bit and the curves of volume-time and depth-time of craters as well as the effective stress-time curves of the centre tooth, second-row tooth and peripheral tooth. The results showed that: the percussion drilling process under rotating impact effect is characterized as four fundamental processes; the crater depth mainly depends on impact force rather than rotary torque; the crater created under rotating impact effect is twice the volume of that under impact effect; the effective stress of each tooth changes severely: the stress of second-row tooth is the largest, centre tooth the second, and peripheral tooth the smallest. This study provided a guide for the structural optimization of hammer bit and general applications of percussion drilling.
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