The design of bit crown is an important part of polycrystalline diamond compact (PDC) bit design, although predecessors have done a lot of researches on the design principles of PDC bit crown, the study of the law about rock-breaking energy consumption according to different bit crown shape is not very systematic, and the mathematical model of design is over-simplified. In order to analyze the relation between rock-breaking energy consumption and bit crown shape quantificationally, the paper puts forward an idea to take “per revolution-specific rock-breaking work” as objective function, and analyzes the relationship between rock properties, inner cone angle, outer cone arc radius, and per revolution-specific rock-breaking work by means of explicit dynamic finite element method. Results show that the change law between per revolution-specific rock-breaking work and the radius of gyration is similar for rocks with different properties, it is beneficial to decrease rock-breaking energy consumption by decreasing inner cone angle or outer cone arc radius. Of course, we should also consider hydraulic structure and processing technology in the optimization design of PDC bit crown.

To expedite drilling operations in hard rock of coal mines, a new type of impact-shear drill bit was developed, and its mechanism of speed-up and efficiency increase was studied. The RHT constitutive model was used to describe the structural behavior of rock, and the rock-breaking simulation model of full-size bit was established. Compared with PDC bit and hammer bit, the rock-breaking force, bit torque, and rock stress characteristics of impact-shear bit were analyzed. The results show that, in comparison to PDC bit and hammer bit, the axial force of impact-shear bit was reduced by 68.25% and 71.40%, respectively, and the average torque was reduced by 91.79% and 83.36%, respectively. Notably, for the impact-shear bit, the fluctuation of drilling force was effectively mitigated, the stick–slip vibration of bit was weakened, the rock-breaking energy consumption was drastically reduced, and the rock-breaking efficiency and bit’s life were finally improved. In terms of rock stress characteristics, the pre-impact effect of the central hammer bit of the impact-shear bit can release the internal stress of the rock well, and the stress of the rock element on the hole wall was relatively reduced, thus making it easier for the external PDC bit to break the rock. Field test results show that, under the condition of the small drilling rig, the impact-shear bit can give full play to the pre-crushing function of the impact mechanism, thereby effectively protecting the PDC cutter of external PDC bit, and realizing the fast hole-forming in hard rock of coal mine.

During directional drilling, when drilling in hard and multi-interbedded formation, the ordinary flat-bottomed PDC bit wears seriously, in order to solve this problem, a new type of directional drilling PDC bit (straight angle drag PDC bit) is developed, and numerical simulation method is used to analyze the working characteristics of straight angle drag PDC bit. Simulation results show that, comparing with the ordinary flat-bottomed PDC bit, axial force needed by straight angle drag PDC bit reduces by up to 36%, and rock breaking efficiency increases nearly 50%. Also, Φ96 mm straight angle drag PDC bit is produced and field test is carried out; field test results show that: comparing with flat-bottomed PDC bit, bit life expectancy of straight angle drag PDC bit increases more than three times, and rock breaking efficiency increases about two times. So, when drilling in hard and multi-interbedded formation in the process of directional drilling, straight angle drag PDC bit helps to improve bit life and rock breaking efficiency and finally reduces drilling cost.

When drilling in hard rock formations, the wear of the complex curved PDC bit is extremely uneven. To solve this problem, a numerical simulation method was used to analyze the load distribution law of the cutters and the crown shape of the complex curved PDC bit was optimized. The simulation results showed that: 1) along the radial direction of the bit, the axial load on the cutters increased first and then decreased, while the radial load decreased first and then increased; 2) the axial load on the cutters was the largest at the nose and taper area of the bit, while the radial load on the cutters was the largest at the center and shoulder area of the bit; 3) the influence of the inner cone angle on the cutting load was mainly concentrated in the bit crown's inner cone area, while the influence of the outer arc radius on the cutting load was mainly concentrated in the bit crown nose and taper areas. Based on these conclusions, the crown shape of the complex curved PDC bit was optimized and a field test was carried out. The test results showed that, compared with the original complex curved PDC bit, the average bit life and drilling efficiency of the new designed bit was increased by 20% and 18%, respectively, especially in hard rock drilling. The average drilling efficiency of the new designed bit was increased by 50%.