Numerical Simulation Research on Cutting Rock with a PDC Cutter Assisted by an Impact Force

Extensive studies have been carried out on cutting rock with a PDC cutter, but cutting rock assisted by impact force is rarely studied. In this paper, cutting rock using conical and cylindrical PDC cutters assisted by impact force were researched with the explicit dynamic model. The laws of cutting rock using a cylindrical cutter assisted by impact force are the same as those of a conical cutter. There are thresholds of impact frequency and amplitude when they are single variables. When impact frequency is lower than the threshold frequency, the impact frequency is the dominant frequency in the frequency spectrum of weight on bit (WOB), and the amplitude of dominant frequency and removal volume decreases with the increase of impact frequency. When the impact frequency is higher than the threshold frequency, there is no dominant frequency in the frequency spectrum of WOB, and the removal volume behaves the same. When the impact force is lower than the threshold amplitude, there is no dominant frequency in the frequency spectrum of WOB, and it does not affect the removal volume but the removal volume is positively correlated with the impact amplitude. When the impact amplitude is higher than the threshold amplitude, the removal volume is also positively correlated with the impact amplitude, and the removal volume assisted by low-frequency (20 Hz and 40 Hz) impact force is higher. The frequency threshold and amplitude threshold of the conical cutter are smaller than those of the cylindrical cutter. Although the cutting depth and removal volume of the conical cutter are lower than those of the cylindrical cutter, the amplifications of cutting depth and removal volume of the conical cutter are higher than those of the cylindrical cutter when assisted by impact force.

Numerical Study on Fragmentation Characteristics of Granite Under a Single Polycrystalline Diamond Compact Cutter in Rotary-Percussive Drilling

Fragmentation characteristics of granite in rotary-percussive drilling are studied using the distinct element method. We developed a model to investigate the interaction between the rock and a Polycrystalline Diamond Compact cutter. The micro contact parameters in the model are calibrated by conducting a series of simulated mechanical tests of the rock. Sensitivity analyses are then conducted according the drilling performances which are quantified as the penetration displacement, the fragmentation volume and the specific energy, as well as the lateral force and the particle size distribution. Results show that the model can well represent the typical fracture system under indentation of the cutter, the torque fluctuation phenomenon in drilling and the formation of lateral chips, which verify the reliability of the model. The cutter with a back rake angle of 55°and impact frequency of 30Hz has the best penetration performance in evaluated parameters. Increasing the frequency has a great effect on the rock breaking speed under the coupling effect of impact and cutting in the low frequency range. Considering crushing efficiency, 50 Hz is the recommended impact frequency. This paper provides a useful tool to represent the fragmentation performance of rotary-percussive drilling and sensitivity analyses shed light on the potential ways to improve the performance.

Design of a New Type of Torsional Impactor and Analysis of Its Impact Performance

In view of the stick-slip phenomenon in deep and hard rock drilling, a new type of torsional impactor that can provide torsional impact vibration was designed. According to the working principle and structural characteristics of the designed torsional impactor, this paper theoretically analyzes the influences of different structural parameters and motion parameters on the impact frequency, impact force, and impact torque of the torsional impactor. The results show that the impact frequency f is directly proportional to the rotational speed VZ of the transmission shaft and the installed number n of torsional impact generating devices. Additionally, the impact force F is directly proportional to sine value of the impact angle α (i.e., sinα), impact hammer mass m, impact hammer rotation speed VZ (i.e., transmission shaft rotation speed), and impact hammer rotation radius r and is inversely proportional to action time Δt of the impact hammer and impact anvil. Furthermore, the impact torque M is directly proportional to the impact force F and rotary radius r of the impact hammer. This article lays a foundation for further theoretical and experimental research of torsional impactors and provides a reference for the design and testing of torsional impactors.

Theoretical and experimental studies of impact energy and rock-drilling efficiency in vibro-impact drilling

Vibro-impact drilling has been proven to be a viable technique for enhancing the Rate of Penetration (ROP) in deep and ultra-deep well drilling. It is essential to study the effects of impact parameters on impact energy and rock-drilling efficiency for impact tool design and operating parameter optimization. In this paper, the influences of impact parameters including impact frequency, dynamic loading amplitude and loading on impact energy were analyzed by theoretical method. Then a full-scale drilling experiment was conducted to study the rock-drilling efficiency. The results are as follows: the optimal frequency is higher than the resonance frequency of the rock. The impact energy increase with the dynamic loading amplitude. The penetration rate at dynamic loading amplitude of 4 KN (0.13137 mm/s) is 38.7% higher than that of 2 KN (0.09473mm/s). When the impact frequency is lower than150 Hz, the rock-drilling efficiency increases with the impact frequency and dynamic loading amplitude. The penetration rate is 0.1051 mm/s at impact frequency of 150 Hz, which is 29.8% higher than that of 10 Hz. The impact energy and penetration rate at square loading waveform are the largest. The impact energy per second at loading waveform of square, sine and triangular is 19.6 J, 12 J and 7.91 J respectively when the impact frequency is set to optimal frequency of impact energy. This study provides a theoretical guidance for the optimization design of vibro-impact drilling technology.

Experimental Study on Axial Impact Mitigating Stick-Slip Vibration with a PDC Bit

Stick-slip vibration reduces the drilling rate of penetration, causes early wear of bits, and threatens the safety of downhole tools. Therefore, it is necessary to study suppression methods of stick-slip vibration to achieve efficient and safe drilling. Field tests show that the use of downhole axial impactors is helpful to mitigate stick-slip vibration and improve rock-breaking efficiency. However, there are many deficiencies in the study of how axial impact load affects stick-slip vibration of a PDC bit. In this paper, based on the two-degrees-of-freedom spring-mass-damper model and similarity theory, a laboratory experiment device for suppressing stick-slip vibration of a PDC bit under axial impact load has been developed, and systematic experimental research has been carried out. The results show that the axial impact force can suppress the stick-slip vibration by reducing the amplitude of weight on bit and torque fluctuations and by increasing the main frequency of torque. The amplitude of impact force affects the choice of the optimal back-rake angle. The impact frequency is negatively correlated with the fluctuation amplitude of the rotary speed. When the impact frequency is greater than 100 Hz, the fluctuation amplitude of the rotary speed will not decrease.

Parameters Optimization and Energy Absorption Evaluation of the Steel Ball Friction Energy Absorber

The energy absorber is used to simulate the reaction of a working piece subjected to a vibration stimulus, by which the consistent and repeatable reactions to the tool’s vibration inputs could be achieved. According to the proposed coupling simulation model by using commercial software RecurDyn and EDEM, the energy dissipated by the energy absorber and the contact force between the drill rod and the piston are evaluated under different load conditions such as the impact frequency and impact stroke. Moreover, the effects of the ball diameter, ball column height, and diameter on the energy absorption characteristics are also studied. The results show that the impact frequency and stroke influence the energy absorber by changing the impact force; the energy absorption is more obvious under higher impact frequency and long impact stroke. The filling ball diameter influences the energy reflectivity by changing the porosity, which is negatively correlated to the energy reflectivity, and a 6 mm filling ball diameter is suggested. The energy reflectivity is inversely proportional to the ball column height and diameter, and the suggested ball column diameter and height are 160 mm and 600 mm, respectively, with energy reflectivity of 0.045. Even when the increase in impact frequency and stroke will increase the contact force, the dynamic load factor decreases. The contact force and dynamic load factor are inversely proportional to the ball column height, but they are not influenced by the ball diameter and the ball column diameter.

SELECTION OF THE DRIVE OF THE PULSE GENERATOR FOR WAVE DEFORMATION HARDENING

The article is devoted to the analysis of known structures of impact devices used in industry in order to obtain recommendations for their adaptation or when creating new structures for wave strain hardening by surface plastic deformation. The analysis was carried out on the used drive and on the main parameters of impact devices: impact energy, impact frequency, relative metal consumption and efficiency. The options are the best combinations of parameters for electric, pneumatic and hydraulic drives. Recommendations are given on the use of such devices, with appropriate adaptation, as pulse generators for wave strain hardening.

Study on Working Performance of Multi-impact Drum

The current depletion of high-quality coal seams, hard coal mining had become the norm, while the traditional mechanical cutting methods were inefficient. A multi-impact cutting technology was proposed, which was to design a hydraulic system inside the drum. A hydraulic control system was formed by the hydraulic impact pick driver set by the ranging arm and the cutting motor to realize the reciprocating impact movement of multiple picks. The impact of picks was used to make the coal synchronously pre-crack, thereby reducing the difficulty of hard seam mining. In this paper, by analyzing the working process of the multi-impact drum, the corresponding mechanical model was established and the overall research plan was determined. A simplified drum model was established using CATIA, a coal model was established in EDEM based on the physical and mechanical properties of hard coal, and a simulation experiment composed of the cutting drum and coal model was based on orthogonality. The test method explored the working performance of the multi-impact drum under the action of multiple factors in cutting different media, using different impact frequencies and different drum speeds. The results showed that the coal breaking rate was used as the evaluation index, the order of the influencing factors was: A>C>B (coal hardness> drum speed> impact frequency), and the optimal plan combination was A2B1C3 (coal wall hardness was f5, impact frequency was 4Hz, drum speed was 40r·min -1 ); taking cutting specific energy consumption as the evaluation index, the order of influencing factors was: C>A>B (drum speed>coal hardness>impact frequency), the optimal plan combination was A2B1C3 (coal wall hardness was f5, impact frequency was 4Hz, drum speed was 40r·min -1 ). The matrix analysis method was further introduced to calculate that the order of the influence of each factor on the index value of the orthogonal test was C>A>B (drum speed>coal hardness>impact frequency), when the coal hardness was f5, the impact frequency was 4Hz, and the drum speed was 40r·min -1 , the working performance of the multiimpact cutting drum was the best. Under the same working conditions (coal hardness was f5, drum speed was 40r·min -1 ), a comparative simulation experiment of the traditional drum was carried out. Compared with the simulation results, the coal falling amount of the multi-impact drum was about 24.86% higher than that of the traditional drum, and the cutting specific energy consumption of the multi-impact drum was 0.7423kW·h/m 3 , which was about 21.67% lower than that of the traditional drum. Finally, a simplified multi-impact drum industrial cutting test was carried out. The test results showed that the cutting resistance of the multi-impact drum was about 17.22% lower than when there was no impact. Considering that the simplified multi-impact cutting drum had a reduced impact pre-cracking effect on the coal, it can be considered that the results of the industrial test and the discrete element simulation test were still relatively consistent. The multi-impact cutting drum had good working performance under hard coal conditions.