281 The combination of PDC bits and inserted roller cones utilises the compressive rockbreaking action of the roller cones to pre-fracture hard rock, thereby enhancing the effectiveness of the cutting tools and improving bit stability. Research on PDC cutters has seen significant development in recent years(Huang et al., 2020;Niu et al., 2018). However, research on the rock-breaking efficiency of inserted roller cones is relatively scarce, especially regarding the indentation mechanism under deep high-stress environments and the performance differences between inserts of different shapes (such as spherical and conical), which still require further investigation. The key operational parameters affecting the rock-breaking effectiveness of cutters mainly include indentation speed, penetration depth, and insert spacing. Indentation speed is related to the bit rotary speed and the self-rotation speed of the inserted roller cone, penetration depth reflects the weight on bit (WOB) during rock breaking, and insert spacing is an important parameter for the layout of inserts on the roller cone. Regarding the influence of indentation speed, Wang et al.(Wang et al., 2023) conducted impact rock-breaking tests using SHPB, finding that an increase in impact speed linearly increases the peak impact force and penetration depth, with high-strength rock specimens showing a greater increase in penetration depth than low-strength ones. While studying conical insert indentation into water-saturated sandstone, found that when the indentation speed increased from 0.3 mm/s to 20 mm/s, the specific energy for rock breaking decreased by 32%. Zou et al.(Zou et al., 2020) conducted numerical simulations of conical insert rock-breaking using displacement-controlled and force-controlled loading methods, respectively. They found that the peak rock-breaking force initially decreased with increasing loading rate but started to rise when the loading rate exceeded 0.8 kN/s. When the penetration speed increased from 0.25 mm/min to 2.0 mm/min, the inserts experienced significant lateral forces, causing crack propagation to deviate from the specimen axis. At this point, shear failure during crack propagation became more pronounced than splitting failure. This indicates that at lower indentation speeds, there exists an optimal range for its effect on specific energy, while once the indentation speed exceeds a certain threshold, the rockbreaking force increases rapidly. Regarding factors such as penetration depth and insert spacing, Cho et al.(Cho et al., 2013)suggested that when the ratio of roller cutter spacing to penetration reaches 15, the interaction between adjacent grooves is no longer significant. Park et al.(Park et al., 2018), while investigating the influence of cutting depth on pick cutter rock breaking, found that within a certain range, rock-breaking efficiency is highest when the ratio of cutter spacing to cutting depth is between 2 and 3. To address the research gaps mentioned above, this study is dedicated to investigating rock-breaking laws under conditions closer to actual drilling operations. The main innovations are as follows:
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