Percussive Rock Drilling Research Articles

Review of theoretical, experimental, and numerical studies on rotary percussive rock drilling

ABSTRACT Rotary percussion rock drills have been widely used in mining and civil engineering fields, and their performance needs to be further improved. To deeply understand the mechanisms of the rock drills, this article reviews the previous studies from fundamental theories to state-of-the-art techniques on the rotary percussive rock drilling processes, including stress wave propagation, dynamic bit penetration, and factors affecting drilling efficiency and penetration rates. An enormous variety of research approaches and their findings are provided to related researchers and engineers. In addition, important future issues are presented for discussing the further progress of rotary percussive rock drilling.

Realization of shorter substitutes for a percussive rock drill hammer by periodic redistribution of its mass and compliance

In percussive drilling of rock, use is made of high-amplitude short-duration force pulses generated through impacts of a hammer on a drill bit or a drill rod assembly. The mass of the hammer is crucial for the impact velocity and the impact frequency, while its length and variation of characteristic impedance control the duration and shape of the force pulses. High efficiency of conversion of kinetic impact energy to work results if the force pulses have suitable shapes and durations. Commonly, hammers of hydraulic rock drills are nearly uniform and generate nearly rectangular force pulses admitting high efficiencies. When space is constrained, short hammers are advantageous as they allow short rock drills. Here, therefore, shorter substitutes for uniform original hammers are sought. They should have the same mass as the original hammer, be shorter than the original hammer, and generate approximately the same impact force history as the original hammer. A method based on 1D theory is developed for realization of substitute hammers with axially periodic characteristic impedance. Impact force histories generated by such hammers agree well with that generated by a uniform original hammer in a 1D context, and also in a 3D context if 3D effects are small enough. Also, for bit-rock interaction conditions from soft to hard, the efficiencies obtained by use of substitute hammers agree well with that obtained by use of the original hammer.

Measurement of Stress Waves Propagation in Percussive Drilling.

This work describes the results of a test campaign aimed to measure the propagation of longitudinal, torsional, and flexural stress waves on a drill bit during percussive rock drilling. Although the stress wave propagation during percussive drilling has been extensively modeled and studied in the literature, its experimental characterization is poorly documented and generally limited to the detection of the longitudinal stress waves. The activity was performed under continuous drilling while varying three parameters, the type of concrete, the operator feeding force, and the drilling hammer rotational speed. It was found that axial stress wave frequencies and spectral amplitudes depend on the investigated parameters. Moreover, a relevant coupling between axial and torsional vibrations was evidenced, while negligible contribution was found from the bending modes. A finite element model of the drill bit and percussive element was developed to simulate the impact and the coupling between axial and torsional vibrations. A strong correlation was found between computed and measured axial stress spectra, but additional studies are required to achieve a satisfactory agreement between the measured and the simulated torque vibrations.

Experimental and Numerical Investigation of Hard Rock Breakage by Indenter Impact

To investigate the effect of indenter shape, impact energy, and impact velocity on the rock breakage performance, a test device for rock fragmentation by indenter impact was developed to obtain the rock breakage volume, depth, and area under different impact conditions. By comparing the rock breakage volume, depth, area, and specific energy consumption, the results show that indenter shape has a greater influence on the rock breakage performance than that of the impact velocity with the same impact energy, and impact energy plays a decisive role in rock breakage performance with an identical indenter shape and impact velocity. For the lowest to highest specific energy consumption, the order of indenter shape is cusp-conical, warhead, hemispherical, spherical-arc, and flat-top under the same impact energy and velocity, but the cusp-conical indenter is damaged after several impacts. The rock breakage volume, depth, and area all increase with the increase in impact energy, but the effect of the impact velocity could be ignored under the same impact energy. In addition, the rock breakage features of the numerical simulation and experiments are similar, which show that the crushing zone close to the indenter impact point is mainly caused by the high compressive stress, and then radial cracks are caused by the accumulative energy release. The findings of this study will contribute to progress in the performance and efficiency for percussive rock drilling.

Prediction Model of Drilling Performance for Percussive Rock Drilling Tool

This study suggests a method for quantitatively estimating the drilling performance of the down‐the‐hole (DTH) hammer during percussive drilling of rock surfaces. A pneumatic dynamic model of the DTH hammer was developed that considers the mass flow rate relations representing the orifice opening areas of the air tube, the piston, and bit flushing channels. A drill bit motion model was developed to represent the dynamics of a drill bit impacted by a dropped piston and explain the impact stress propagation and rock‐crushing mechanism. The rock‐crushing effect of the drill button bit was measured through a piston drop test. The pneumatic hammer model and drill bit motion model were then combined in a prediction model to determine the impact efficiency according to different rock types (i.e., soft, medium‐hard, and very hard). The drilling efficiency was defined as the input value of the prediction model, which was used to simulate the drilling performance of DTH hammers considering the rock type and dynamic effect of the drill bit. Finally, the simulation results were compared with the results of in situ drilling tests for verification.

Experimental Assessment of Water Hammer-Induced Column Separation in Oil-Hydraulic Pipe Flow

Cavitation erosion through water hammer and column separation is a major concern in hydraulic applications such as percussive rock drilling. Cavitation aspects must be considered both in early and late design stages, which require deep knowledge and tools for prediction. In this study, an oil-hydraulic test equipment for water hammer assessment was designed using state-of-the-art simulation tools. Several tests were performed, with and without column separation, showing good repeatability on measured pressures. At higher flow rates, column separation was the dominating feature and several high-pressure peaks with subsequent time delay reduction could be observed. These patterns were affected by the oil temperature, with most substantial changes at lower temperature ranges (<32 °C). Standard transmission line simulations managed to predict the water hammer, but as expected not the column separation, which is the theme of future work using this setup.

Theoretical investigation of the energy transfer efficiency under percussive drilling loads

With the deepening of oil and gas exploration, high performance drilling is required in these hard rocks now being penetrated. It is demonstrated that percussive drilling could promote the ROP (rate of penetration) effectively, and several impact drilling techniques have been applied to enhance the drilling performance. However, due to the various designs of rotary drilling tools as well as of working conditions, different shapes of stress waves would be generated during impact, which corresponds to different rate of penetration. The essence of percussive drilling is transmitting the impact energy in the form of stress wave, the objective of this research was to compare the energy transfer efficiencies of four different incident wave shapes, thereby providing theoretical guidance for the down-hole percussion drilling tool design. In this paper, the governing general equation of force-displacement was established in percussion drilling, and then the energy transfer efficiencies of four different incident wave shapes (exponent, rectangle, triangle, and sine) are calculated. Results show that 81% and 80% of the energy for rectangular and sine incident stress waves are used to break the rock, while for the triangular wave, only utilizes 41% of its energy to break the rock. Therefore, the research could contribute to the design of systems for percussive drilling of rock and lower cost drilling in the exploration of hard units.

3D finite elements modelling of percussive rock drilling: Estimation of rate of penetration based on multiple impact simulations with a commercial drill bit

This paper deals with assessing the rate of penetration (ROP) in percussive drilling of rock based on finite element simulations. For this end, a method to simulate the dynamic indentation in percussive drilling is developed and validated. This method includes a recently developed constitutive model to describe the rock fracture and a bit-rock interaction model. The constitutive model has a viscoplasticity part to indicate the stress states leading to rock fracture and a damage model with separate damage variables for tension and compression to quantify the rock fracture. In the numerical examples, the present approach is validated by simulations of the dynamic Brazilian disc test and dynamic indentation simulations on Kuru granite. The multiple consecutive impacts simulations carried out show that the present continuum approach is able to predict the experimental ROP. However, a close match of the experimental ROP requires that the critical damage threshold values, beyond which a damaged element contributes to the volume of removed material, are selected by the experiments. In any case, the present modelling approach, along with the procedure to convert the simulation results into ROP, provides a tool for improving the performance of percussive drilling equipment.

Biconvex versus bilinear force-penetration relationship in percussive drilling of rock

Because of variability of the force vs. penetration relationship (FPR) from one blow to another in percussive drilling, and difficulty to predict FPRs under such conditions, use is commonly made of simple FPR models, such as the bilinear one defined by its loading/unloading slopes. Here a biconvex model with an added parameter representing convexity is considered. One aim is to study the effect of convexity on maximal penetration, maximal force and efficiency. Another is to assess, with the biconvex FPR as an example, how well a bilinear FPR can be used to approximate one that is nonlinear. A simple percussive top-hammer drill model is considered, comprising a hammer, a drill rod and a bit with the same characteristic impedance. The maximal penetration is found to decrease and the maximal force to increase with increasing convexity. The efficiency has a maximum for a finite hammer length (incident wave duration), and the highest maximal efficiency is obtained for a linear FPR. With increasing convexity, the maximal efficiency decreases and occurs for shorter hammers (incident waves). The bilinear approximation of a biconvex FPR accurately predicts the position of the maximum in efficiency, even for large convexity, but somewhat overestimates its height and width.

Numerical study of the influence of hydrostatic and confining pressure on percussive drilling of hard rock

This paper presents a numerical investigation on the effect of hydrostatic pressure (lithostatic stress) and confining pressure (horizontal tectonic stress) on percussive drilling of hard rocks. A finite element based modeling approach was employed in the simulation of the dynamic bit-rock interaction process. The rock fracture in this approach is modeled with a damage-viscoplasticity model accounting for the main fracture types in the bit-rock interaction as well as the strain-rate effects. The model was calibrated for Kuru grey granite in the unconfined case. In the simulations of single impact with a multiple-button bit both hydrostatic and confining pressures were applied at different levels up to 100MPa and the results were compared to the case with no pressure. The simulations show that at this pressure level the volume of the failed elements was 38% in case of hydrostatic pressure and 56% in case of confining pressure of the volume in the pressure free case. The same figures apply to corresponding rates of penetration. The cause of this reduction in the material removal or the rate of penetration is the compressive stress state, of either pressure type tested, which effectively suppresses tensile and shear fractures.

Force-penetration curves of a button bit generated during impact penetration into rock

To improve the performance and efficiency of percussion rock drills, it is essential to understand the impact penetration behavior of a bit drilling into rock. However, little knowledge to date has been published about the penetration behavior of the button bit, which is commonly used for hard rock. In this study, the impact penetration behavior of the button bit was investigated in laboratory tests. An impact penetration tester built from the same components used in an actual rock drill was developed and used in single-blow impact penetration tests. Unnatural fluctuations were observed in the force-penetration curves calculated using a two-point strain measurement method, which were probably due to not only the difference in rod stress as measured at two points on the rod, but also to the mismatch between the actual bit and the calculation model. A data correction method, in which the bit force calculated in a free end test is subtracted from that in the impact penetration test, was proposed using a numerical simulation. The method was applied to the measured rod stress, and force-penetration curves with a button bit were obtained from more than 40 impact penetration tests. Graph curves showed that peak force, initial slope and elastic penetration increased, while the maximum and final penetrations decreased with an increase in the secant slope. The crushing and elastic strain energies changed, but the sum of the energies was constant with an increase in the secant slope. The variations in the force-penetration curves are caused by the contact conditions between the bit and rock, the rock properties, and damage to the rock caused with each blow, not by the test conditions. The findings of this study will contribute to progress in the performance and efficiency of percussive rock drilling.

A Self-Optimizing Control System for Hard Rock Percussive Drilling

The derivation and application of a self-optimizing control system for percussive drilling of hard rock is presented. The control system superimposes an oscillating force signal onto the drill feed force and demodulates the signal from the drill rotational torque. The resultant demodulated signal is then used to control the drill, allowing real-time maximization of penetration rate, minimum hole deviation, and measurement of rock characteristics regardless of drilling conditions. The control system allows the drill to adapt to changing rock conditions. Testing has proven the control to be reliable and to increase drilling efficiency.

Rock penetration into cemented carbide drill buttons during rock drilling

In percussive and rotary percussive rock drilling, the rock is crushed into small fragments by the repeated hard impact of the drill bit, and subsequently removed by flushing water or air. To avoid excessive wear, the steel drill bit is equipped with a set of cemented carbide buttons that protrude from the bit to take the actual impact. The severe contact against the rock results in some wear of the button, but also in formation of surface layers of rock material and penetration and impregnation of rock material into the cemented carbide structure. This situation, with serious implications for the wear and fracture of the buttons, have previously not been reported. The present findings represent a significantly new understanding of the wear of the rock button material. The deterioration mechanisms are described in detail, using examples from a range of real drilling applications in different rock types. During operation, material in the surface layer of the drill button shifts from that of the original cemented carbide into an uncontrolled composite. This composite is formed by the WC carbide hard phase and a binder consisting of a mixture of cobalt and rock. This new material should be expected to exhibit properties significantly different from the original cemented carbide.

Efficiency of a percussive rock drilling process with consideration of wave energy radiation into the rock

The efficiency of churn drilling was studied with consideration of wave energy radiation into the rock. In this percussive drilling process, a hammer with cutters on its front subjects the rock to direct impact. A 3D elastic finite element model was used for the hammer and the rock, with a nonlinear force versus penetration relationship representing the rock crushing process. Two rock models with the same crushing properties were employed: (i) one perfectly rigid and (ii) one linearly elastic, representing Westerly Granite. Under favourable drilling conditions, with efficiencies higher than 60%, the elastic response of the rock significantly influences the efficiency of the drilling process. However, the radiation of wave energy into the rock, less than 3% of the impact energy, does not play an important role. Under unfavourable drilling conditions, with efficiencies lower than 40%, the elastic response of the rock does not significantly influence the efficiency of the drilling process even though the energy radiated into the rock is greater than 5% of the impact energy.

Influence of 3D effects on the efficiency of percussive rock drilling

The aim is to investigate the influence of 3D effects on the efficiency of three processes for percussive rock drilling, viz., hammer drilling, down-the-hole drilling and churn drilling, each based on the use of tube-shaped members. A 3D axisymmetric finite element study is carried out for systems with idealised geometries. The efficiencies obtained are compared with formulae obtained from 1D analyses. It is found that the efficiencies based on 3D analyses are generally slightly lower than those based on 1D analyses. The difference is typically about 4% for hammer drilling, of the order of a percent or less for down-the-hole drilling, and practically non-existent for churn drilling. In the case of hammer drilling, the reduction of efficiency due to 3D effects is found to be slightly larger for tubes with relatively thin walls than for tubes with massive cross-sections. The lower efficiency in 3D is partially due to contributions to the kinetic and potential energies from components of velocity and stress which do not promote the performance of work on the rock. It is concluded that from a practical point of view, there is no need of 3D corrections of the 1D results for efficiency except possibly in the case of hammer drilling.

Identification of a percussive drill rod joint from its response to stress wave loading

In percussive drilling of rock, elastic stress waves are generated in a drill string through repeated axial impacts by the hammer of a rock drill. For holes deeper than a few meters, several drill rods are commonly joined by means of cylindrical coupling sleeves with internal threads which connect drill rods with external threads at their ends. Each coupling sleeve (CS) joint serves to transfer stress wave energy from one drill rod to the next with minimum loss of energy due to reflection and dissipation. This paper deals with the development of an identification procedure for the nonlinear dissipative spring mass (NDSM) model of a CS joint developed by Lundberg et al. Stiffness, friction and mass parameters were determined by minimizing the diffence between simulated and measured responses of the joint, in reflection or transmission, to the same incident stress wave loading. Similar results were obtained as with an existing mixed static and dynamic identification procedure, but with considerably less expenditure of equipment and time. The most reliable results and the smallest deviation between simulated and measured responses were achieved in transmission.

Computer modeling and simulation of percussive drilling of rock : B. Lundberg, Comprehensive rock engineering. Vol. 4, ed J.A. Hudson, (Pergamon), 1993, pp 137–154

Computer modeling and simulation of percussive drilling of rock : B. Lundberg, Comprehensive rock engineering. Vol. 4, ed J.A. Hudson, (Pergamon), 1993, pp 137–154

Efficiency of percussive drilling of rock with a bent drill rod

Wave propagation and efficiency in percussive rock drilling with a bent rod is studied theoretically using a combined approach of FFT and matrix method. The bent drill rod is represented by straight uniform elements. The extensional and flexural stress waves in the drill rod are described by the one-dimensional wave equation and by Euler-Bernoulli theory, respectively. The conditions at the bit and rock during loading are represented by a penetration resistance, a lateral stiffness and a rotational stiffness. Numerical results are given for two cases; the first involves a drill rod with a constant radius of curvature through a certain bend angle, and the second a drill rod with a sharp bend defined by the same bend angle. It is found that for slightly bent rods the efficiency is practically the same as for straight rods. Thus, slight bends, which are always present in real drill rods, do not appear to be significant from the point of view of efficiency. Furthermore, it is found that the efficiency can remain high even if the rod is significantly bent. Thus, percussive rock drilling systems with special designs for drilling in curved paths are feasible from the point of view of efficiency.

Rattling Loose of Thread Connected Steels in Percussive Rock Drilling

A common practice to uncouple drill steels in percussive rock drilling is by rattling. Rattling is primarily a continuation of a drilling process without the rotation of the drill steels. By examining the forces which exist at the contacting threads between two drill steels and a coupling, it is shown that normal thread loads in tightly coupled steels and coupling systems always produce a loosening torque. The loosening torque is opposed by circumferential friction torques developed at the thread interface and at the drill steels’ interface. Using a theory developed by Goodier for loosening of threaded fasteners by vibration, it is shown how tensile forces generated by rattling can overcome the friction torques to provide a mechanism for uncoupling the drill steels. A sequence of events during rattling loose of multiple steels is postulated, and conditions which facilitate the loosening process are discussed. Repeated pushing and pulling tests of manually torqued-up steels and couplings in a tensile testing machine for four commonly used steels confirms that tensile forces generated during rattling are indeed responsible for rattling loose. Complete loosening was achieved for all four steels tested under various combinations of an initial tightening torque and the cyclic tensile load, and the total number of load cycles required for a complete loosening were determined. Test results also show that loosening due to compressive forces generated during rattling should be very small. The dependence of rattling loose on the helix angle and the static thread efficiency of the thread form is discussed.

Efficiency of percussive drilling of rock with dissipative joints

The nonlinear dissipative spring mass (NDSM) model for a percussive drill rod joint of the coupling sleeve (CS) type has been implemented into a Modula-2 program with the aid of which percussive drilling of rock is simulated. Transmission and dissipation of energy are first studied when a rectangular stress wave, generated through the impact by a uniform hammer, is transmitted through a single joint. The efficiency of energy transmission increases from 81 to 94% and the relative energy dissipation decreases from 8 to 1 or 2% when the length of the hammer varies from relatively short to relatively long. The effect of the joint preload is weak in the range from medium to relatively high preload. The efficiency of the percussive drilling process decreases with the number of joints but depends little on the joint preload. For soft rock, the efficiency increases with hammer length, whereas for medium and hard rock the dependence of efficiency on hammer length is not monotonic. This is because soft rock requires a long incident wave for efficient conversion of energy to work at the bit, whereas the reverse is true for hard rock. It is also found that the efficiency of the percussive drilling process may be considerably underestimated if the effects of each joint on the length and shape of the transmitted wave and of multiple reflections within the drill string are neglected.