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Experimental research on the influence of working parameters on the drilling efficiency
Tunnelling and Underground Space Technology 95 (2020) 103174
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Experimental research on the influence of working parameters on the drilling efficiency
T
Hongsheng Li, Songyong Liu , Huanhuan Chang ⁎
School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou 221116, China Jiangsu Collaborative Innovation Center of Intelligent Mining Equipment, China University of Mining and Technology, Xuzhou 221108, China
ARTICLE INFO
ABSTRACT
Keywords: Drilling bits Drilling velocity Specific energy Working parameters Regression analysis
Rock drills are usually applied in petroleum engineering applications, tunnel excavation, and hard rock mining. The rock drills directly impacted the rock through the hydraulic impact power mechanism and are controlled by propulsion force and torque. However, there still exists the most serious problem in impact drilling which is low efficiency during the process of impact drilling, and thus the orthogonal experiment using Taguchi method L25 (56) is conducted to analyse the influence of the working parameters, including impact power, propulsion force, rotation speed and bit type, on the rock drilling efficiency. Unconfined compressive strength tests and Brazilian splitting tests are conducted first to present the bluestone characteristics. The drilling efficiency including drilling velocity and specific energy is the evaluating indicators for orthogonal experiments under different drilling conditions. The results demonstrate that the factor order affecting drilling efficiency is impact power, rotation speed, propulsion force and bit type. In addition, the impact power has a highly appreciable influence on the drilling efficiency, whereas the influence of rotation speed, propulsion force and bit type on that in their level range is slight. However, the influencing parameters of impact power, rotation speed, propulsion force and bit type in terms of percentage contribution on drilling velocity and specific energy is 38.54%, 20%, 20%, 20% and 35.307%, 20%, 20%, 20%, respectively. Moreover, the optimum combination in view of drilling velocity and specific energy are 6.4 kW, 240 r/min, 3800 N and 7-button for impact power, rotation speed, propulsion force, and bit type respectively. Based on the regression analysis of the main working parameters which affect drilling velocity, the relationship between drilling velocity and working parameters is obtained and verified, thus it easily provides a basis about equipment selection, optimal working parameters, equipment improvement and progress prediction.
1. Introduction
to increase the drilling efficiency. The working principle of the hydraulic impact rock breaking device is shown in Fig. 1 and the work process is consist of four tasks, feed, impact, rotation and flushing. Impact is generated by the impact piston impacting the shank adapter to transfer energy into the rock causing it to become fragmented. Feed is applied to keep rock surface contacting with drilling bit and exiting from the broken hole under feed. The rotation work turns the bit into a new position after each impact is completed, and the cracked rock surface is partially stripped off in the process of rotation. Flushing (not shown in Fig. 1) removes the broken debris from the drilling hole via high pressure water or compressed air to avoid repeated drilling, which slows down drilling velocity and wears the drilling bit. To explore the complex impact mechanism, researchers have proposed many theoretical models, such as the Sikarskie shear
Rock drill is a common device for rock breaking in geotechnical engineering, especially in underground excavating, coalbed methane and shale gas mining, petroleum engineering applications as well as hard rock mining (Liu et al., 2019; Liu et al., 2019). According to statistics, only in China's state-owned key coal mines, the annual amount of rock roadway excavation works is above 2000 km. Since 2011, the average annual mileage of China's highway tunnels has increased by 1000 km, and the planned railway tunnels have exceeded 10000 km. Percussive drilling is most productive method of blasting a hole, and the efficiency has a close relationship with the working parameters other than rock hardness; however, there are some problems at present, such as frequent sticking, serious bit wear, and low efficiency. Thus, it is significant to research the optimum combination of working parameters
⁎
Corresponding author. E-mail addresses: [email protected] (H. Li), [email protected] (S.Y. Liu).
https://doi.org/10.1016/j.tust.2019.103174 Received 29 July 2018; Received in revised form 23 September 2019; Accepted 28 October 2019 0886-7798/ © 2019 Elsevier Ltd. All rights reserved.
Tunnelling and Underground Space Technology 95 (2020) 103174
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Drill rod
Failure bit
Drilling bit
Hydraulic impact power Rock Guideway
Hydraulic impact rock breaking device
Impact
Feed
Rotation
Wear button Cracks
Rock Piston
Shank adapter
Working principle
Fig. 1. Working principle of the hydraulic impact rock breaking device.
2008) with rotary pressure and drilling velocity, respectively. The rock breaking process was analysed for a single spherical tooth, double spherical teeth and a plurality of spherical teeth; the corresponding relationship between the length of the side splitting and the rock properties was obtained, and the effect of the arrangement of the spherical teeth on rock breaking was also explored (Liu et al., 2007; Franca, 2011; Kwon et al., 2014). The influences of the working parameters (pushing force, rotating torque) and the rock characteristics on the drilling efficiency and noise during drilling process was studied by replacing rock with cement (Yaşar et al., 2011; Kivade et al., 2013). The specific energy relationship was established with spherical teeth spacing and teeth number by theoretical analysis to determine the optimal ratio of spherical teeth spacing to depth and teeth number, and then minimum specific energy and maximum drilling velocity was obtained (Liu et al., 2008). The author also investigated the impact fragmentation and damage evolution of bluestone using multi-type bits based on the numerical simulation and the results were verified by experiments (Liu et al., 2017). However, the optimal parameters combination obtained for the impact device only considered the drilling velocity and only the propulsion force and bit type were verified with limited experiment because it is hard to independently adjust the impact frequency as well as impact energy in the actual test. The trajectory of the reamer and the failure of the drilling tools were analysed and a nonlinear dynamic finite element model was established to analyse the subsidence control capacity of the reaming-bottom hole assembly (Zhu and Yi, 2018). Navarro et al. (2019) investigated the quality of the drilling in underground blasting operations with a view to quantify the distance of the position assessed by the MWD system with respect to the actual end position of the blast hole logged. Moreover, although researchers have performed much research on the impact mechanism, rock properties, geographical factors and arrangement form of the drilling bit, the rock properties and geographical factors are not optional, and distribution of teeth on small diameter bit is impossible to implement in actual production. In addition, the factors considered by the existing research are not comprehensive, and few scholars studied the optimal combination of drilling performance using multi-type bits based on orthogonal experiment. In this paper, unconfined compressive strength tests and Brazilian splitting tests are conducted first to present the bluestone characteristic. Moreover, the high-efficiency impact rock fragmentation orthogonal experiment is conducted to analyse the influence of working parameters, including impact power, propulsion force, rotation speed and bit type, on the rock impact efficiency from the views of the drilling velocity and the specific energy, and to obtain the primary and secondary order and the optimal combination of the factors affecting the
fragmentation mechanism model (Benjumea and Sikarskie, 1969) and the Altiero tensile fragmentation mechanism model (Sikarskie and Altiero, 1973). The former is mainly based on the Mohr-Coulomb strength criterion, providing predictions that are in reasonable consistent with the test results, and the latter is based on the Griffith fracture criterion. It has been found in experiments that the bit shape has a great influence on the failure forms and the drilling depth, and fracture under impact is composed of three regions: broken pit, direct damage zone and crack propagation zone. The experiment using coring bits was conducted to break brittle material (Kumano and Goldsmith, 1982); the results indicated that the bit with 60°angle and six-teeth is the best for the Ultrasonic/sonic driller/corer coring experimental drill tests. A mechanical models and corresponding equations was established for calculating various construction-related parameters during the pulling operation in horizontal directional drilling (Cheng and Polak, 2007). The research of rotational torque prediction using horizontal directional drilling in rock strata was conducted to offer the engineers selecting appropriate size reamers in rock strata (Lan et al., 2011). Intergranular failure caused by tensile and shear forces, and rock grain size and grain boundary had great influence on the form and expansion cracks, which were caused tensile or shear stress exceed rock tensile or shear strength around impact of the contact zone (Zhang et al., 2005; Riedel et al., 2010; Saksala et al., 2014). Thus, the main rock failure mode under impact of wedged tool and button tool is shear failure and tensile failure (Shan et al., 1996). In addition, the craters, crack shape and size were determined by bit shape, rock properties and load character (Tan et al., 1998). Single-indenter impact experiment on rock impact fragmentation under low energy impact loadings was performed by (Qin et al., 2014). Moreover, simulation models were established (Saadati et al., 2014; Saadati et al., 2015; Saksala, 2010a,b, 2013) based on the Mohr Coulomb strength criterion to study the failure mode and crack propagation shape better: the visco plastic damage model combined with rock brittle and ductile properties and the KST-DFH material model. In addition, researchers explored how the rock properties and geographical factors influence impact drilling efficiency (drilling velocity and tool wear). The rock properties included tensile strength, Schmidt hammer value, hardness, point load strength, elastic modulus, uniaxial compressive strength, density, impact strength, and the geographical factors included rock joint, text direction, compactness, porosity, thermal decomposition degree, weathering and equivalent quartz content (Schormair et al., 2006). The finite element method is used to discretize the system considering the torsional vibration to identify the parameters of the bit-rock interaction model (Ritto, 2015). To obtain maximum drilling velocity and minimum deviation, propulsion force relationship was studied (Cavanough et al., 2
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Experiment system 16
15
14
13
Water supply system 12
11
10
8
9
7
Rock
17
6
4 3
M
2
5 1
1 Water tank; 2 Filter; 3 Relief valve; 4 Water pump; 5 Motor; 6 Pressure gage; 7 Reversing valve; 8 Throttle valve; 9 Cooler; 10 Propulsion cylinder; 11 Baseplate; 12 Hydraulic rock drill; 13 Impact rotary sealing device; 14 Lifting platform; 15 Drill rod; 16 Bit; 17 Lift cylinder Fig. 2. Impact rock breaking performance test device.
drilling velocity. In addition, the regression orthogonal test of drilling system is carried out and the regression equation is established, which provides a basis for equipment selection, optimal working parameters and equipment improvement. 2. Experiment procedures
Laptop
2.1. Experiment equipment Impact rock breaking experiment equipment, shown in Fig. 2, is mainly composed of a drill bit, a drill rod, a rotary sensor, a hydraulic rock drill, a propulsion cylinder, an oil pump system, a lifting guide rail platform, a hydraulic system, a displacement sensor, and a water supply system. The lifting guide rail device lifts hydraulic rock drill with range of 0.5–1.5 m, and provides a guide to the operation. The hydraulic rock drill is power generating device for impact rock breaking, and its main technical parameters are shown in Table 1. The propulsion cylinder, which causes the hydraulic rock drill to advance and recede, provides the propulsion force range of 1–20 kN, and propulsion speed range of 0.1–10 m/min. The speed sensor and the displacement sensor measure the rotation speed and propulsion displacement, respectively. The oil pump system provides power for the hydraulic rock drill, lifting guide rail platform, and propulsion cylinder; the system parameters are shown in Table 2. The water supply system is used for flushing the hole bottom debris and reducing the dust amount.
Dynamic signal testing and analysis system
Uniaxial compression test
test are φ50 mm × 100 mm, and the cylinder specimens for Brazilian splitting test are φ50 mm × 25 mm. The mechanical properties of the rocks are shown in Table 3. 2.3. Experiment arrangement The research factors considered in this paper are as follows: impact power, rotation speed, propulsion force and bit type. According to specifications of the explosive in actual production, the diameter of drill bit is chosen as φ38 mm, and five types of drilling bit are considered (Fig. 4): cross bit, 3-button bit, 4-button bit, 7-button bit and 8-button
In this paper, bluestone, representing hard rock, is the study object. The rock is 800 mm high by 800 mm wide by 600 mm thick. To acquire the uniaxial compression strength and tensile strength, compression tests and brazilian splitting test on cylinder specimens are performed using the MTS 815 testing machine (shown in Fig. 3) in accordance with ISRM standards. The cylinder specimens for uniaxial compression
Table 3 Rock properties.
Table 1 Main technical parameters of hydraulic rock drill. ≥85 ≥280 8–15
Impact frequency (Hz) Impact pressure (MPa) Borehole diameter (mm)
Brazilian splitting test
Fig. 3. Rock parameter testing system.
2.2. Rock specimens
Impact energy (J) Rotational speed (r/min) Rotary pressure (MPa)
Rock
Rock specimen
1 2 3 Mean
≤52 12–15 φ32-50
Compressive strength σc (MPa)
Elastic modulus E (GPa)
Poisson's ratio μ
Tensile strength T (MPa)
Density ρ (g/cm3)
119.51 117.95 129.43 122.30
39.09 47.22 52.63 46.31
0.291 0.315 0.294 0.300
9.19 10.05 9.37 9.54
2.60 2.72 2.57 2.63
Table 2 Technical parameters of hydraulic pump station. Nominal pressure (MPa)
Flow (L/min)
Motor power (kW)
Motor speed (r/min)
Motor voltage (V)
Cooling mode
20
≥(75 + 40)
22 + 11
1470
380
Water-cooling
3
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(a) Cross
(b) 3-button
(c) 4-button
(d) 7-button
(e) 8-button
Fig. 4. Bits type.
drilling velocity and specific energy. The main test curve in the experiment is the displacement curve, which is used to get the drilling velocity. Taking the test number 19 (shown in Table 5) as an example, the displacement curve is shown in Fig. 5. As shown in Fig. 5, the amplitude in the original curve is large, this
Table 4 Factor levels. Factors
Impact power (kW) A
Rotation speed (r/ min) B
Propulsion force (N) C
Bit type D
Level Level Level Level Level
1.2 2.5 3.8 5.1 6.4
120 160 200 240 280
2000 2900 3800 4700 5600
cross 3-button 4-button 7-button 8-button
1 2 3 4 5
1200
Original data Smooth data Fitting data
800
Displacement(mm)
bit. To determine the changing tendency of rock breaking performance of impact equipment, the factor levels are set 5 according to the basic performance parameters of hydraulic rock drill and impact energy calibration test data, as shown in Table 4. The orthogonal test design method is adopted to conduct the experimental arrangement to reduce the number of tests and difficulty as well as to ensure the authenticity and comprehensiveness of the test results because the problem has multiple factors and multiple levels. L25 (56) is used for the orthogonal table considering the 4 research factors and the experimental arrangement is shown in Table 4.
Fast drilling
400
0
Hole-opening
Return stroke
-400
0
3. Results and discussion
10
20
30 40 Time(s)
50
60
70
Fig. 5. Displacement curve.
The evaluation index of this test is drilling efficiency including Table 5 Test results statistics. Test no.
Impact power (kW) A
Errors 1
Rotation speed (r/min) B
Propulsion force (N) C
Bit type D
Errors 2
Drilling velocity (m/min)
Specific energy (mJ/m3)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
1.2 1.2 1.2 1.2 1.2 2.5 2.5 2.5 2.5 2.5 3.8 3.8 3.8 3.8 3.8 5.1 5.1 5.1 5.1 5.1 6.4 6.4 6.4 6.4 6.4
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
120 160 200 240 280 160 200 240 280 120 200 240 280 120 160 240 280 120 160 200 280 120 160 200 240
2000 2900 3800 4700 5600 3800 4700 5600 2000 2900 5600 2000 2900 3800 4700 2900 3800 4700 5600 2000 4700 5600 2000 2900 3800
Cross 3 4 7 8 7 8 Cross 3 4 3 4 7 8 Cross 8 Cross 3 4 7 4 7 8 Cross 3
1 2 3 4 5 5 1 2 3 4 4 5 1 2 3 3 4 5 1 2 2 3 4 5 1
0.342 0.486 0.42 0.456 0.432 0.66 0.698 0.665 0.652 0.402 0.914 0.793 0.879 0.701 0.727 1.101 0.989 0.694 1.075 0.967 1.026 1.053 0.877 1.007 1.292
1232.13 949.89 1155.48 1071.94 1202.69 838.65 822.59 900.00 990.72 1327.85 743.76 903.38 843.03 892.93 887.94 711.47 813.80 1018.03 681.92 787.73 810.76 699.17 871.65 786.43 629.46
4
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(a) Orifice
(b) Bottom
(c) Wall
Fig. 6. Broken rock hole status.
is because the vibration of hydraulic rock drill is larger in the working process, and the rope displacement sensor fluctuates in a certain period. To solve this problem, the smooth data is used to get some rules easier. It can be seen that the displacement variation with time during the process of drilling can be divided into three stages: the hole-opening, the fast drilling and the return stroke. The displacement change is relatively slow in the hole-opening stage. It is worth noting that the drilling velocity in this stage should be small because the drill bit is easy to slide on the rock surface when drilling and causing the drill rod to bend. In the fast drilling stage, the rotary resistance increased with the increasing of drilling depth and the displacement reached the maximum when the drilling to the bottom. The drilling velocity is defined as the slope of the stable stage. In return stroke stage, after the hydraulic drilling machine reaches the predetermined depth, it touches the stroke switch and the oil cylinder reverse working to make hydraulic drilled out of the working state, and finally exits the drill hole. The experimental results analysed by range analysis and analysis of variance (ANOVA) were shown in Table 5.
drilling velocity. From Fig. 6(b), the shape on the bottom is similar to the bit shape, and rock between edge buttons and centre buttons is less broken, whereas rock among edge buttons is more broken; an annular bulge is not broken because the smaller impact energy does not cause cracks to run through edge buttons and the centre buttons. From Fig. 6(c), cracks mainly extend to rock text and crack direction, which indicates that the crack is extended along the original direction of rock. After the experiment, debris was collected and dried to obtain the debris shape, as shown in Fig. 7. It can be seen that the impact crushing rock debris is powdery, while other types of broken debris are mixed rock powder and rock blocks, indicating the rock is over broken. The largest debris is shown in red circle, and the order of rock debris size is as follows: three buttons > four buttons > seven buttons > eight buttons. The rock cracks among the buttons are easily intersected and form small rock mass because of the reduced distance among the buttons caused by the increased number of buttons. In general, the larger the block is, the smaller the energy consumption is, and the more effective to improve the efficiency of the impact equipment. The bits state after the test is shown in Fig. 8. It can be seen the bits are only missing paint, with no wear and breakage observed, except for the 7-button bit because the drilling direction deviation caused by bedding occurred in the test specimen fracture, causing the drill handle to bear uneven stress, resulting in the damage of the handle of the seven-button bit under the propulsion force.
3.1. Rock broken status The rock broken status after the test is illustrated in Fig. 6. Fig. 6(a), (b) and (c) represents orifice, bottom and wall surface broken state, respectively. From Fig. 6(a), the orifice is round and smooth, which indicates low
(a) Cross bit
0 1cm (b) 3-button bit
0 1cm
(c) 4-button bit
0 1cm (d) 7-button bit
0 1cm (e) 8-button bit
Fig. 7. Debris shapes.
(a) cross bit
(b) 3-button bit
(c) 4-button bit Fig. 8. Bit states after tests. 5
Damaged 7-button bit
(d) 7-button bit
(e) 8-button bit
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indicates that there exists an optimum rotation speed which makes drilling velocity maximum. This is because each impact process will form a dense core at the bottom of the drilling hole, which will disperse the energy and reduce the impact crushing effect. Therefore, it is necessary to turn to a new position to avoid the dense core. At the same time, micro-cracks can be generated by impact fragmentation, so that the next cycle of impact can break larger volume of rock. When the rotation speed is small, the bit does not cross the crushing pit. When the impact occurs in the next cycle, the dense core is repeatedly crushed and the penetration zone formed is small. The energy is not effectively utilized, which reduces the impact crushing effect. When the rotation speed is too high, the distance between the new crushing position of the drill bit and the initial crushing position is too far, and the cracks between the two cannot be penetrated and the broken rock volume is small, which reduces the impact crushing effect. Propulsion force mainly affects drilling velocity from energy transfer efficiency and the cutting action of drill bit two aspects. In order to break rocks, energy needs to be transmitted to rocks through drill bits. When the propulsion force is small, the drill bits cannot form good contact with rocks. When energy is transmitted to rocks through drill bits, more energy is reflected at the bit, resulting in the continuous transmission and dissipation of energy in the drill rod, which makes the efficiency of energy transmission low. Therefore, the energy enters the rock less, the volume of rock fragmentation is smaller, and the drilling velocity is smaller. With the increase of propulsion force, the bit can keep good contact with the rock in a relatively short time, and the energy transfer efficiency is high, which is conducive to rock fragmentation. When the thrust force is in level 3(3800N), the drilling velocity appears extreme value, which indicates that a higher energy transfer efficiency can be achieved at this level. If the propulsive force continues to increase, such as level 4 (4700N), it has little effect on improving energy transfer efficiency, but increase the rotary resistance. Due to the limitation of hydraulic rock drill's rotary power, its rotation speed will be reduced, and it is easy to deviate from the optimal rotation speed, resulting in the decline of drilling velocity. In another aspect, when the propulsion force is relatively small, the recoil force generated by the drill bit in the process of rotation is greater than the propulsion force, and the phenomenon of the drill bit retreating after cutting occurs. As a result, the drill bit cannot or rarely cut the rock, but turns to a new position to prepare for the next impact on the broken rock. When the propulsion force is larger than the recoil force generated by the drill bit in the process of rotation, the phenomenon of retraction is not easy to happen. In the process of rotation, the drill bit cuts and breaks the rock and can turn to a new position, thus increasing the drilling velocity. And this is the reason drilling velocity at level 5 (5600N) greater than that at level 4 (4700N). In order to verify the second hypothesis of the influence of propulsion force on drilling velocity, a pure cutting test of the bit is added (taking a 3-button bit as an example). In the test, the rotation speed and the propulsion force is set as 240 r/min, level 4 (4700N) and level 5 (5600N), respectively. And the bit is rotated for half a minute without water. The state of rock breaking after pure cutting is shown in Fig. 10. The cutting depth of the left hole is 1 mm and that of the right hole is 4 mm, which shows that increasing the propulsion force is beneficial to the rock cutting. And this verifies the hypothesis that the propulsion force affects the drilling velocity. However, it is found that the drill bit sticking is easy to occur when the propulsion force is in level 5 (5600N), resulting in shutdown in actual production and affecting work efficiency. Although the factors order affecting the drilling velocity can be obtained by range analysis, it is unable to determine whether the drilling velocity is caused by the corresponding level variation or the error, and the factors on drilling velocity was not obtained; thus, it is necessary to apply the orthogonal test ANOVA of results for further analysis. The drilling velocity variation with working parameters and the ANOVA of drilling velocity is shown in Fig. 11 and Table 7. From
Table 6 Range analysis of drilling velocity. Factors
Impact power (kW)
Errors 1
Rotation speed (r/ min)
Propulsion force (N)
Bit type
Errors 2
K1 K2 K3 K4 K5
2.136 3.077 4.014 4.826 5.255
4.043 4.019 3.535 3.891 3.820
3.192 3.825 4.006 4.307 3.978
3.631 3.875 4.062 3.601 4.139
3.730 4.038 3.716 4.015 3.809
4.286 3.845 3.953 3.638 3.586
k1 k2 k3 k4 k5
0.427 0.615 0.803 0.965 1.051
0.809 0.804 0.707 0.778 0.764
0.638 0.765 0.801 0.861 0.796
0.726 0.775 0.812 0.720 0.828
0.746 0.808 0.743 0.803 0.762
0.857 0.769 0.791 0.728 0.717
Rang R
0.624
0.102
0.223
0.108
0.065
0.140
3.2. Drilling velocity As is illustrated in Table 6, Ki and ki (i = 1, 2,…, 5) is the sum and average value of experiments with factor level number i. R means the degree for effects of factors on experimental results. The range values of impact power, rotation speed, propulsion force, and bit type are 5.517, 1.843, 0.892 and 0.537 times the mean value of the error range compared to the range value R of the parameters; therefore, the order of factors which affects drilling velocity is impact power, rotation speed, propulsion force and bit type. Moreover, the range values of propulsion force and bit type are less than the error range, indicating that they have little influence on the drilling velocity. It can be concluded that the optimal scheme is A5B4C5D2, in which the impact power, rotation speed, propulsion force and bit type are 6.4 kW, 240 r/min, 5600 N and 3-button bit, respectively, by comparing the average value ki of each level drilling velocity. According to the changing trend of drilling velocity shown in Fig. 9, the drilling velocity is found to increase sharply with impact power; however, with increasing rotation speed, it increases first and later decreases with the maxima level of 4 (240 r/min), which indicates the optimal speed occurs near the level, and the optimal speed has no relationship with bit type. The drilling velocity for the cross bit, 3-button bit, 7-button bit and 8-button bit was 1.004, 1.087, 1.080 and 1.025 times that of the 4-button bit, respectively. However, this does not mean that propulsion force and bit type have no influence on drilling velocity, but only that the influence of propulsion force and bit type on drilling velocity is inconspicuous relative to impact power. The peak value of the influence of rotation speed on drilling velocity 1.2 Level 1 Level 2 Level 3 Level 4
1.1
Drilling velocity/(m/min)
1.0
Level 5
0.9 0.8 0.7 0.6 0.5 0.4 0.3
Impact power
Errors 1 Rotation speed Propulsion force Bit type
Errors 2
Factors Fig. 9. The changing trend of drilling velocity with factors. 6
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H=
E × 103 V
(1) (2)
E = Ft v + Pe + Ph = Ft v + pe Qe + ph Qh V=
D2 4
v
(3)
where H – Crushing specific energy consumption, mJ/m ; É – Energy consumption per unit of time, J; V – Rock crushing volume per unit time, m3; Ft – Propulsion force, N; v – Drilling velocity of the hydraulic rock drill, m/s; Pe – Impact power consumption, W; Ph – Rotary power consumption, W; pe, ph – Impact pressure, rotary pressure, Pa; Qe, Qh – Impact flow, Rotary flow, m3/s. Range analysis of specific energy is shown in Table 8. By contrasting the range values R of the factors, each factor on the drilling velocity of primary and secondary order was obtained: impact power, rotation speed, bit type and propulsion force. It can be concluded that the optimal scheme is A5B4C5D4, in which the impact power, rotation speed, 3
Fig. 10. Rock crushing state after bit cutting.
the value of F in the table, it can be obtained that the order affecting drilling velocity is impact power, rotation speed, propulsion force and bit type, which was in agreement with range analysis. From the P-value and conspicuousness, it can be observed that impact power has appreciable influence on drilling velocity. Moreover, rotation speed has certain degree of influence, while propulsion force and bit type have little effect in this study. However, the influencing parameters of impact power, rotation speed, propulsion force and bit type in terms of percentage contribution on drilling velocity is 38.54%, 20%, 20%, 20%, respectively.
Table 8 Range analysis of specific energy.
3.3. Specific energy The specific energy is used as an index of energy consumption in rock breaking. The energy consumption in the process of hydraulic rock drill impact crushing mainly includes impact energy, rotary energy and propulsion energy. The calculation of specific energy (Liu et al., 2017; Liu et al., 2017) can be expressed by the following formulas,
Factors
Impact power (kW)
Errors 1
Rotation speed (r/ min)
Propulsion force (N)
Bit type
Errors 2
K1 K2 K3 K4 K5
5954.63 5138.39 4478.52 4167.07 3962.59
4547.52 4395.98 5036.23 4636.22 5085.25
5304.55 4529.85 4491.15 4433.58 4942.07
5029.64 4836.58 4558.73 4840.06 4436.19
4843.40 4547.21 5102.67 4471.43 4736.49
4391.32 4546.69 4693.67 5063.67 5005.85
k1 k2 k3 k4 k5
1190.93 1027.68 895.70 833.41 792.52
909.50 879.20 1007.25 927.24 1017.05
1060.91 905.97 898.23 886.72 988.41
1005.93 967.32 911.75 968.01 887.24
968.68 909.44 1020.53 894.29 947.30
878.26 909.34 938.73 1012.73 1001.17
Rang R
398.41
137.85
174.19
118.69
126.25
134.47
Fig. 11. Drilling velocity variations with (a) impact power and rotation speed; (b) propulsion force and bit type.
Table 7 ANOVA of drilling velocity. Factors
Sum of squares
Freedom f
Mean squares
F
P-value
Significant
Contribution
Impact power Rotational speed Propulsion force Bit type Errors Total
1.298 0.136 0.048 0.019 0.096 1.60
4 4 4 4 8 24
0.324 0.034 0.012 0.005 0.012 –
26.942 2.834 0.990 0.398 – –
0.000 0.098 0.465 0.805 – –
*** *
38.54% 20% 20% 20% – –
7
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value of F in the table, it can be obtained that the factor order affecting specific energy is impact power, rotation speed, bit type and propulsion force. Moreover, from the P-value and conspicuousness, impact power has a highly appreciable influence on specific energy. However, the influencing parameters of impact power, rotation speed, propulsion force and bit type in terms of percentage contribution on specific energy is 35.307%, 20%, 20%, 20%, respectively. From the drilling velocity point of view, the optimal combination is A5B4C5D2. From the view of specific energy, the optimal combination is A5B4C5D4. However, in the test process, it was observed the hydraulic rock drill often suffers from the sticking phenomenon and interruption when propulsion force with level 5 (5600 N) occurs in the test, which will cause the drill to grind to a halt in practical application; thus, the overall drilling efficiency will be reduced. From the range analysis of the drilling velocity and specific energy, it can be obtained the difference of the results between level 3 (3800 N) and the level 5 (5600 N) is slight, which indicates that it can achieve better rock breaking performance and achieve high drilling velocity and lower specific energy when the propulsion force is level 3 (3800 N). Although the difference of drilling velocity and specific energy between the 3-button bit and 7button bit is small, it is less prone to sticking under the same operating parameters because the number of button is higher and the bit force is uniform. In conclusion, when the rock is uniform and less crack, the best work parameters for the impact device is A5B4C3D2, in which the impact power, rotation speed, propulsion force, and bit type are 6.4 kW, 240 r/min, 3800 N and 3-button bit, respectively, for the homogeneous and less crack rock. For the uneven and rock with greater numbers of cracks, inclusions and beddings, the optimum combination is A5B4C5D4, in which the impact power, rotation speed, propulsion force, and bit type are 6.4 kW, 240 r/min, 3800 N and 7-button bit, respectively. The calculation of the specific energy and its structure is largely related, which makes the results lacking general applicability, thus, specific energy is no longer discussed here because the rotating torque in the work of hydraulic rock drill is difficult to measure.
1200
Level 1 Level 2 Level 3 Level 4 Level 5
Specific energy(mJ/m3)
1100
1000
900
800
700
Impact power
Errors 1
Rotation speed Propulsion force Bit type
Factors
Errors 2
Fig. 12. Specific energy variations with factors.
propulsion force, and bit type are 6.4 kW, 240 r/min, 5600 N and 7button bit, respectively, from comparison of the average value ki of each level drilling velocity. From the variation trend of specific energy shown in Fig. 12, it can be concluded the specific energy reduces substantially with the increasing of impact power; however, with increasing rotation speed and propulsion force, it first decreases and later increases with the maxima level of 4 (240 r/min). Moreover, it can be concluded from Fig. 12 that the 7-button bit has the lowest specific energy, whereas the four-button bit has the highest value. The variation trend of specific energy is found to be just the opposite of the drilling velocity by comparing Fig. 9 and Fig. 12. The specific energy variation with working parameters and the ANOVA of the specific energy is shown in Fig. 13 and Table 9. From the
Fig. 13. Specific energy variation with (a) impact power and rotation speed; (b) propulsion force and bit type. Table 9 ANOVA of specific energy. Factors
Sum of squares
Freedom f
Mean squares
F
P-value
Significant
Contribution
Impact power Rotation speed Propulsion force Bit type Errors Total
432,640 133,780 41,997 50,587 121,200 780,202
4 4 4 4 8 24
108,160 33,445 10,499 12,647 15,150 –
7.14 2.21 0.69 0.83 – –
0.009 0.158 0.617 0.540 – –
*** – – – – –
35.307% 20% 20% 20% – –
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3.4. Regression orthogonal analysis
Table 12 Model determination coefficients.
The orthogonal test can take advantage of reduced test times to obtain comprehensive test results. However, within the experimental range, this test cannot confirm the relationship between the test indices and the research factors as well as the corresponding regression equation according to test datum. To establish a highly accurate regression equation using few test times during the regression equation mathematical model set up, it needs to get rid of the traditional regression analysis which can only passively address the experimental data, and the test arrangement, data processing and the precision of the regression equation must be taken into consideration uniformly. Accordingly, it is necessary to combine the regression analysis method with the orthogonal test method to form a new one, the regression orthogonal design method. According to the research on the rock-breaking performance of impact equipment on bluestone, it can be observed that the influence factors of impact power and rotation speed affect the drilling velocity, among which, the impact power has the greatest influence. To enhance the accuracy of the regression model, the propulsion force was taken into consideration; thus, some research factors of the regression test model were chosen, including impact power, rotation speed, propulsion force and interaction of impact power and rotation speed, and the 3button bit was chosen as the drilling tool. On the basis of the one regression orthogonal design method, various factor levels are shown in Table 10, and the test arrangement is shown in Table 11. The test results analysed by statistics are shown in Table 11. Analysis results mainly include the model decision coefficient, the model significance test and the factor regression coefficient. The determination coefficient R2 showed in Table 12 is 0.946, which illustrates that four chosen research factors can explain 94.6% of the variation of the drilling velocity for the test on bluestone. The factor regression coefficients are shown in Table 13, in which x1, x2, x3, and x1x2 represent impact power, rotation speed, propulsion force, and the interaction of impact power and rotation speed, respectively. From the table, it is easy to obtain both the regression coefficient of the constant term and the various factors, and the P-value is used to measure the influence degree of the factors on indices. The table also indicates that the impact power and the rotation speed have more effect on the drilling velocity than the propulsion force and the interaction of the impact
Model
R
R2
Adj. R2
1
0.972
0.946
0.914
Table 13 Factor regression coefficients of bluestone. Model
Non-standardized coefficients
1 (Constant) x1 x2 x3 x1x2
b
Errors
0.829 0.305 0.082 0.017 0.043
0.024 0.029 0.029 0.029 0.029
F
P-value
35.035 10.537 2.842 0.574 1.471
0.000 0.000 0.025 0.584 0.185
power and the rotation speed; thus, the latter two factors can be ignored in rough calculation. The relationships between each of the four factors and the drilling velocity are positively related, which means that with the increase of each factor, the drilling velocity will also increase gradually. However, the trends of drilling velocity changing with rotation speed and propulsion force are different from the test without interaction. The reason of this situation is that there are only two level numbers of each factor in one regression orthogonal design method. Depending on the P-value of the different factors, the regression relationship between drilling velocity and impact power and the regression relationship between drilling velocity and rotation speed obviously have reached to a significant level; nevertheless, the regression relationships between drilling velocity and the other two factors were insignificant. The analysis results are consistent with the results of the test with interaction. Summarising the above, the multivariate regression equation is as follows:
y = 0.829 + 0.305x1 + 0.082x2 + 0.017x3 + 0.043x1 x2 where x1 = 2.6 , x2 = 80 , x3 = The final fitting formula is A
3.8
B
200
C
3800 . 1800
y = 0.07596A + 0.000239B + 0.0000094C + 0.0002067AB + 0.2994
Table 10 Factor levels for the regression orthogonal test. Parameters
Impact power (kW) A
Rotation speed (r/ min) B
Propulsion force (N) C
Lower level −1 Upper level 1 Zero level 0 Interval
1.2 6.4 3.8 2.6
120 280 200 80
2000 5600 3800 1800
(4)
(5)
where y is the drilling velocity (m/min); A is the impact power (kW); B is the rotation speed (r/min); C is the propulsion force (N). The loss of fit was tested to verify the fitting situation of intermediate value of factor interval:
t=
|0.829 0.916| 7 + 3 = 1.903 < t (0.05, 10) = 2.228 0.047 + 0.000847 1 12 + 1 4 (6)
Table 11 Test arrangement for the regression orthogonal test. Test no.
Impact power (kW)
Rotation Speed (r/min)
Impact power × Rotation speed
Propulsion force (N)
5
6
7
Drilling velocity (m/min)
1 2 3 4 5 6 7 8 9 10 11 12
6.4 6.4 6.4 6.4 1.2 1.2 1.2 1.2 3.8 3.8 3.8 3.8
280 280 120 120 280 280 120 120 200 200 200 200
1 1 −1 −1 −1 −1 1 1 0 0 0 0
5600 2000 5600 2000 5600 2000 5600 2000 3800 3800 3800 3800
1 −1 1 −1 −1 1 −1 1 0 0 0 0
1 −1 −1 1 1 −1 −1 1 0 0 0 0
1 −1 −1 1 −1 1 1 −1 0 0 0 0
1.251 1.181 0.981 0.951 0.525 0.515 0.452 0.429 0.910 0.895 0.925 0.933
9
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estimating according to site condition and construction progress, one regression orthogonal test for rock burst of impact equipment is carried out. When the bit diameter is 38 mm and the bit type is 3button bit, regression equation obtained for drilling velocity of bluestone is as follows and the errors are within 12% in verification.
Table 14 Bluestone drilling velocity verification. Test no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Impact power (kW)
6.4 6.4 6.4 6.4 1.2 1.2 1.2 1.2 3.8 3.8 3.8 3.8 1.2 2.5 3.8 6.4 5 5
Rotation speed (r/ min)
280 280 120 120 280 280 120 120 200 200 200 200 160 280 200 240 270 270
Propulsion force (N)
5600 2000 5600 2000 5600 2000 5600 2000 3800 3800 3800 3800 2900 2000 5600 3800 5600 3800
Calculation value of drilling velocity (m/min)
Test value of drilling velocity (m/min)
Errors (%)
1.276 1.242 1.026 0.992 0.580 0.546 0.502 0.468 0.829 0.829 0.829 0.829 0.496 0.720 0.846 1.196 1.075 1.058
1.251 1.181 0.981 0.951 0.525 0.515 0.452 0.429 0.910 0.895 0.925 0.933 0.486 0.652 0.914 1.298 1.030 1.150
1.96 5.14 4.55 4.29 10.39 5.97 10.98 9.04 8.93 7.41 10.42 11.20 2.00 10.38 7.53 7.85 4.41 7.96
y = 0.07596A + 0.000239B + 0.0000094C + 0.0002067AB + 0.2994 Declaration of Competing Interest The authors declare that there are no conflicts of interest regarding the publication of this paper. Acknowledgments This work is supported by the Future Scientists Program of “Double First Class” of China University of Mining and Technology (No. 2019WLKXJ032). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.tust.2019.103174.
The result shows that the established regression equation is in good agreement with the actual situation. The correctness of regression formula can be verified by test datum. The verification of the test on bluestone is shown in Table 14. The table shows that the errors are within 12%. Since the impact power has no effect for equipment measured by the impact energy calibration test in the regression analysis, the formula can be applied to the selection of such equipment or the initially identified working parameters. In addition, the existing working parameters of hydraulic rock drill can be used to estimate the optimal drilling velocity on bluestone, which can provide basis for equipment selection and activity duration estimation according to requirements, such as site condition and construction schedule. When the accuracy of the regression model is not satisfied, two or higher power regression orthogonal designs can be conducted.
References Benjumea, R., Sikarskie, D.L., 1969. A note on the penetration of a rigid wedge into a nonisotropic brittle material. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 6, 343–352. Cavanough, G.L., Kochanek, M., Cunningham, J.B., Gipps, I.D., 2008. A self-optimizing control system for hard rock percussive drilling. IEEE/ASME Trans. Mechatron. 13, 153–157. Cheng, E., Polak, M.A., 2007. Theoretical model for calculating pulling loads for pipes in horizontal directional drilling. Tunn. Undergr. SP Tech. 22, 633–643. Franca, L.F.P., 2011. A bit–rock interaction model for rotary–percussive drilling. Int. J. Rock Mech. Min. 48, 827–835. Kivade, S.B., Murthy, C.S.N., Vardhan, H., 2013. Laboratory investigations on percussive drilling. J. Inst. Eng. (India): D 94, 81–87. Kumano, A., Goldsmith, W., 1982. Analytical and experimental investigation of the effect of impact on coarse granular rocks. Rock Mech. 15, 67–97. Kwon, K., Song, C., Park, J., Oh, J., Lee, J., Cho, J., 2014. Evaluation of drilling efficiency by percussion testing of a drill bit with new button arrangement. Int. J. Precis. Eng. Man 15, 1063–1068. Lan, H., Ma, B., Shu, B., Wu, Z., 2011. Prediction of rotational torque and design of reaming program using horizontal directional drilling in rock strata. Tunn. Undergr. SP. Tech. 26, 415–421. Liu, H.Y., Kou, S.Q., Lindqvist, P.A., 2008. Numerical studies on bit-rock fragmentation mechanisms. Int. J. Geomech. 8, 45–67. Liu, S., Chang, H., Li, H., Cheng, G., 2017a. Numerical and experimental investigation of the impact fragmentation of bluestone using multi-type bits. Int. J. Rock Mech. Min. 91, 18–28. Liu, X., Nie, B., Wang, W., Wang, Z., Zhang, L., 2019a. The use of AFM in quantitative analysis of pore characteristics in coal and coal-bearing shale. Mar. Petrol. Geol. 105, 331–337. Liu, X., Song, D., He, X., Wang, Z., Zeng, M., Wang, L., 2019b. Quantitative analysis of coal nanopore characteristics using atomic force microscopy. Powder Technol. 346, 332–340. Liu, Y., Mavroidis, C., Bar-Cohen, Y., Chang, Z., 2007. Analytical and experimental study of determining the optimal number of wedge shape cutting teeth in coring bits used in percussive drilling. J. Manuf. Sci. Eng. 129, 760–769. Liu, Z.H., Du, C.L., Zheng, Y.L., Zhang, Q.B., Zhao, J., 2017b. Effects of nozzle position and waterjet pressure on rock-breaking performance of roadheader. Tunn. Undergr. SP Tech. 69, 18–27. Navarro, J., Segarra, P., Sanchidrián, J.A., Castedo, R., López, L.M., 2019. Assessment of drilling deviations in underground operations. Tunn. Undergr. SP Tech. 83, 254–261. Qin, Z., Baolin, L., Mingtao, J., Tong, L., Guoxin, W., Jun, Z., 2014. Experiment research on impact fragmentation mechanism of single-indenter under low power condition. Procedia Eng. 73, 186–193. Riedel, W., Hiermaier, S., Thoma, K., 2010. Transient stress and failure analysis of impact experiments with ceramics. Mater. Sci. Eng. B 173, 139–147. Ritto, T.G., 2015. Bayesian approach to identify the bit–rock interaction parameters of a drill-string dynamical model. J. Braz. Soc. Mech. Sci. 37, 1173–1182. Saadati, M., Forquin, P., Weddfelt, K., Larsson, P., Hild, F., 2015. A numerical study of the influence from pre-existing cracks on granite rock fragmentation at percussive drilling. Int. J. Numer. Anal. Met. 39, 558–570. Saadati, M., Forquin, P., Weddfelt, K., Larsson, P.L., Hild, F., 2014. Granite rock fragmentation at percussive drilling - experimental and numerical investigation. Int. J. Numer. Anal. Met. 38, 828–843.
4. Conclusions The high-efficiency impact rock fragmentation orthogonal experiment is conducted to analyse the influence of working parameters, including impact power, propulsion force, rotation speed and bit type, on the rock impact efficiency from the views of the drilling velocity and the specific energy. The results are analysed to get the order of the factors on impact fragmentation and the relationship between the test indices and the research factors as well as the corresponding regression equation. The following conclusions can be concluded: (1) From range analysis of drilling velocity and specific energy, it can be obtained the difference of the results between level 3 (3800 N) and the level 5 (5600 N) is slight, which indicates that it can achieve better rock breaking performance and achieve high drilling velocity and lower specific energy when the propulsion force in level 3 (3800 N). (2) The results of the rock breaking test using drilling bit reveals that the order affecting drilling efficiency is impact power, rotation speed, propulsion force and bit type. Moreover, the impact power has a highly appreciable influence on the efficiency, whereas the influence of rotation speed, propulsion force and bit type on that in their level range is slight. And the optimum combination in view of drilling velocity and specific energy are 6.4 kW, 240 r/min, 3800 N and 7-button for impact power, rotation speed, propulsion force, and bit type respectively. (3) In order to provide basis for equipment selection and duration 10
Tunnelling and Underground Space Technology 95 (2020) 103174
H. Li, et al. Saksala, T., 2010a. Numerical modelling of bit-rock fracture mechanisms in percussive drilling with a continuum approach. Int. J. Numer. Anal. Met. 35, 1483–1505. Saksala, T., 2010b. Damage-viscoplastic consistency model with a parabolic cap for rocks with brittle and ductile behavior under low-velocity impact loading. Int. J. Numer. Anal. Met. 34, 1362–1386. Saksala, T., 2013. 3D numerical modelling of bit-rock fracture mechanisms in percussive drilling with a multiple-button bit. Int. J. Numer. Anal. Met. 37, 309–324. Saksala, T., Gomon, D., Hokka, M., Kuokkala, V., 2014. Numerical and experimental study of percussive drilling with a triple-button bit on Kuru granite. Int. J. Impact Eng. 72, 56–66. Schormair, N., Thuro, K., Plinninger, R., 2006. The influence of anisotropy on hard rock drilling and cutting. Int. Assoc. Eng. Geol. Environ. 491, 1–11. Shan, R., Wang, H.X., Yang, Y.Q., 1996. Some laws of impact rupture of spherical pressure
head on rock. Chinese J. Rock Mech. Eng. (China) 15, 564–567. Sikarskie, D.L., Altiero, N.J., 1973. the formation of chips in the penetration of elasticbrittle materials (rock). J. Appl. Mech. 40, 791–798. Tan, X.C., Kou, S.Q., Lindqvist, P.A., 1998. Application of the DDM and fracture mechanics model on the simulation of rock breakage by mechanical tools. Eng. Geol. 49, 277–284. Yaşar, E., Ranjith, P.G., Viete, D.R., 2011. An experimental investigation into the drilling and physico-mechanical properties of a rock-like brittle material. J Petrol. Sci. Eng. 76, 185–193. Zhang, M.H., Shim, V.P.W., Lu, G., Chew, C.W., 2005. Resistance of high-strength concrete to projectile impact. Int. J. Impact Eng. 31, 825–841. Zhu, X., Yi, Q., 2018. Research and application of reaming subsidence control in horizontal directional drilling. Tunn. Undergr. SP Tech. 75, 1–10.
11
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Experimental research on the influence of working parameters on the drilling efficiency
T
Hongsheng Li, Songyong Liu , Huanhuan Chang ⁎
School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou 221116, China Jiangsu Collaborative Innovation Center of Intelligent Mining Equipment, China University of Mining and Technology, Xuzhou 221108, China
ARTICLE INFO
ABSTRACT
Keywords: Drilling bits Drilling velocity Specific energy Working parameters Regression analysis
Rock drills are usually applied in petroleum engineering applications, tunnel excavation, and hard rock mining. The rock drills directly impacted the rock through the hydraulic impact power mechanism and are controlled by propulsion force and torque. However, there still exists the most serious problem in impact drilling which is low efficiency during the process of impact drilling, and thus the orthogonal experiment using Taguchi method L25 (56) is conducted to analyse the influence of the working parameters, including impact power, propulsion force, rotation speed and bit type, on the rock drilling efficiency. Unconfined compressive strength tests and Brazilian splitting tests are conducted first to present the bluestone characteristics. The drilling efficiency including drilling velocity and specific energy is the evaluating indicators for orthogonal experiments under different drilling conditions. The results demonstrate that the factor order affecting drilling efficiency is impact power, rotation speed, propulsion force and bit type. In addition, the impact power has a highly appreciable influence on the drilling efficiency, whereas the influence of rotation speed, propulsion force and bit type on that in their level range is slight. However, the influencing parameters of impact power, rotation speed, propulsion force and bit type in terms of percentage contribution on drilling velocity and specific energy is 38.54%, 20%, 20%, 20% and 35.307%, 20%, 20%, 20%, respectively. Moreover, the optimum combination in view of drilling velocity and specific energy are 6.4 kW, 240 r/min, 3800 N and 7-button for impact power, rotation speed, propulsion force, and bit type respectively. Based on the regression analysis of the main working parameters which affect drilling velocity, the relationship between drilling velocity and working parameters is obtained and verified, thus it easily provides a basis about equipment selection, optimal working parameters, equipment improvement and progress prediction.
1. Introduction
to increase the drilling efficiency. The working principle of the hydraulic impact rock breaking device is shown in Fig. 1 and the work process is consist of four tasks, feed, impact, rotation and flushing. Impact is generated by the impact piston impacting the shank adapter to transfer energy into the rock causing it to become fragmented. Feed is applied to keep rock surface contacting with drilling bit and exiting from the broken hole under feed. The rotation work turns the bit into a new position after each impact is completed, and the cracked rock surface is partially stripped off in the process of rotation. Flushing (not shown in Fig. 1) removes the broken debris from the drilling hole via high pressure water or compressed air to avoid repeated drilling, which slows down drilling velocity and wears the drilling bit. To explore the complex impact mechanism, researchers have proposed many theoretical models, such as the Sikarskie shear
Rock drill is a common device for rock breaking in geotechnical engineering, especially in underground excavating, coalbed methane and shale gas mining, petroleum engineering applications as well as hard rock mining (Liu et al., 2019; Liu et al., 2019). According to statistics, only in China's state-owned key coal mines, the annual amount of rock roadway excavation works is above 2000 km. Since 2011, the average annual mileage of China's highway tunnels has increased by 1000 km, and the planned railway tunnels have exceeded 10000 km. Percussive drilling is most productive method of blasting a hole, and the efficiency has a close relationship with the working parameters other than rock hardness; however, there are some problems at present, such as frequent sticking, serious bit wear, and low efficiency. Thus, it is significant to research the optimum combination of working parameters
⁎
Corresponding author. E-mail addresses: [email protected] (H. Li), [email protected] (S.Y. Liu).
https://doi.org/10.1016/j.tust.2019.103174 Received 29 July 2018; Received in revised form 23 September 2019; Accepted 28 October 2019 0886-7798/ © 2019 Elsevier Ltd. All rights reserved.
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Drill rod
Failure bit
Drilling bit
Hydraulic impact power Rock Guideway
Hydraulic impact rock breaking device
Impact
Feed
Rotation
Wear button Cracks
Rock Piston
Shank adapter
Working principle
Fig. 1. Working principle of the hydraulic impact rock breaking device.
2008) with rotary pressure and drilling velocity, respectively. The rock breaking process was analysed for a single spherical tooth, double spherical teeth and a plurality of spherical teeth; the corresponding relationship between the length of the side splitting and the rock properties was obtained, and the effect of the arrangement of the spherical teeth on rock breaking was also explored (Liu et al., 2007; Franca, 2011; Kwon et al., 2014). The influences of the working parameters (pushing force, rotating torque) and the rock characteristics on the drilling efficiency and noise during drilling process was studied by replacing rock with cement (Yaşar et al., 2011; Kivade et al., 2013). The specific energy relationship was established with spherical teeth spacing and teeth number by theoretical analysis to determine the optimal ratio of spherical teeth spacing to depth and teeth number, and then minimum specific energy and maximum drilling velocity was obtained (Liu et al., 2008). The author also investigated the impact fragmentation and damage evolution of bluestone using multi-type bits based on the numerical simulation and the results were verified by experiments (Liu et al., 2017). However, the optimal parameters combination obtained for the impact device only considered the drilling velocity and only the propulsion force and bit type were verified with limited experiment because it is hard to independently adjust the impact frequency as well as impact energy in the actual test. The trajectory of the reamer and the failure of the drilling tools were analysed and a nonlinear dynamic finite element model was established to analyse the subsidence control capacity of the reaming-bottom hole assembly (Zhu and Yi, 2018). Navarro et al. (2019) investigated the quality of the drilling in underground blasting operations with a view to quantify the distance of the position assessed by the MWD system with respect to the actual end position of the blast hole logged. Moreover, although researchers have performed much research on the impact mechanism, rock properties, geographical factors and arrangement form of the drilling bit, the rock properties and geographical factors are not optional, and distribution of teeth on small diameter bit is impossible to implement in actual production. In addition, the factors considered by the existing research are not comprehensive, and few scholars studied the optimal combination of drilling performance using multi-type bits based on orthogonal experiment. In this paper, unconfined compressive strength tests and Brazilian splitting tests are conducted first to present the bluestone characteristic. Moreover, the high-efficiency impact rock fragmentation orthogonal experiment is conducted to analyse the influence of working parameters, including impact power, propulsion force, rotation speed and bit type, on the rock impact efficiency from the views of the drilling velocity and the specific energy, and to obtain the primary and secondary order and the optimal combination of the factors affecting the
fragmentation mechanism model (Benjumea and Sikarskie, 1969) and the Altiero tensile fragmentation mechanism model (Sikarskie and Altiero, 1973). The former is mainly based on the Mohr-Coulomb strength criterion, providing predictions that are in reasonable consistent with the test results, and the latter is based on the Griffith fracture criterion. It has been found in experiments that the bit shape has a great influence on the failure forms and the drilling depth, and fracture under impact is composed of three regions: broken pit, direct damage zone and crack propagation zone. The experiment using coring bits was conducted to break brittle material (Kumano and Goldsmith, 1982); the results indicated that the bit with 60°angle and six-teeth is the best for the Ultrasonic/sonic driller/corer coring experimental drill tests. A mechanical models and corresponding equations was established for calculating various construction-related parameters during the pulling operation in horizontal directional drilling (Cheng and Polak, 2007). The research of rotational torque prediction using horizontal directional drilling in rock strata was conducted to offer the engineers selecting appropriate size reamers in rock strata (Lan et al., 2011). Intergranular failure caused by tensile and shear forces, and rock grain size and grain boundary had great influence on the form and expansion cracks, which were caused tensile or shear stress exceed rock tensile or shear strength around impact of the contact zone (Zhang et al., 2005; Riedel et al., 2010; Saksala et al., 2014). Thus, the main rock failure mode under impact of wedged tool and button tool is shear failure and tensile failure (Shan et al., 1996). In addition, the craters, crack shape and size were determined by bit shape, rock properties and load character (Tan et al., 1998). Single-indenter impact experiment on rock impact fragmentation under low energy impact loadings was performed by (Qin et al., 2014). Moreover, simulation models were established (Saadati et al., 2014; Saadati et al., 2015; Saksala, 2010a,b, 2013) based on the Mohr Coulomb strength criterion to study the failure mode and crack propagation shape better: the visco plastic damage model combined with rock brittle and ductile properties and the KST-DFH material model. In addition, researchers explored how the rock properties and geographical factors influence impact drilling efficiency (drilling velocity and tool wear). The rock properties included tensile strength, Schmidt hammer value, hardness, point load strength, elastic modulus, uniaxial compressive strength, density, impact strength, and the geographical factors included rock joint, text direction, compactness, porosity, thermal decomposition degree, weathering and equivalent quartz content (Schormair et al., 2006). The finite element method is used to discretize the system considering the torsional vibration to identify the parameters of the bit-rock interaction model (Ritto, 2015). To obtain maximum drilling velocity and minimum deviation, propulsion force relationship was studied (Cavanough et al., 2
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Experiment system 16
15
14
13
Water supply system 12
11
10
8
9
7
Rock
17
6
4 3
M
2
5 1
1 Water tank; 2 Filter; 3 Relief valve; 4 Water pump; 5 Motor; 6 Pressure gage; 7 Reversing valve; 8 Throttle valve; 9 Cooler; 10 Propulsion cylinder; 11 Baseplate; 12 Hydraulic rock drill; 13 Impact rotary sealing device; 14 Lifting platform; 15 Drill rod; 16 Bit; 17 Lift cylinder Fig. 2. Impact rock breaking performance test device.
drilling velocity. In addition, the regression orthogonal test of drilling system is carried out and the regression equation is established, which provides a basis for equipment selection, optimal working parameters and equipment improvement. 2. Experiment procedures
Laptop
2.1. Experiment equipment Impact rock breaking experiment equipment, shown in Fig. 2, is mainly composed of a drill bit, a drill rod, a rotary sensor, a hydraulic rock drill, a propulsion cylinder, an oil pump system, a lifting guide rail platform, a hydraulic system, a displacement sensor, and a water supply system. The lifting guide rail device lifts hydraulic rock drill with range of 0.5–1.5 m, and provides a guide to the operation. The hydraulic rock drill is power generating device for impact rock breaking, and its main technical parameters are shown in Table 1. The propulsion cylinder, which causes the hydraulic rock drill to advance and recede, provides the propulsion force range of 1–20 kN, and propulsion speed range of 0.1–10 m/min. The speed sensor and the displacement sensor measure the rotation speed and propulsion displacement, respectively. The oil pump system provides power for the hydraulic rock drill, lifting guide rail platform, and propulsion cylinder; the system parameters are shown in Table 2. The water supply system is used for flushing the hole bottom debris and reducing the dust amount.
Dynamic signal testing and analysis system
Uniaxial compression test
test are φ50 mm × 100 mm, and the cylinder specimens for Brazilian splitting test are φ50 mm × 25 mm. The mechanical properties of the rocks are shown in Table 3. 2.3. Experiment arrangement The research factors considered in this paper are as follows: impact power, rotation speed, propulsion force and bit type. According to specifications of the explosive in actual production, the diameter of drill bit is chosen as φ38 mm, and five types of drilling bit are considered (Fig. 4): cross bit, 3-button bit, 4-button bit, 7-button bit and 8-button
In this paper, bluestone, representing hard rock, is the study object. The rock is 800 mm high by 800 mm wide by 600 mm thick. To acquire the uniaxial compression strength and tensile strength, compression tests and brazilian splitting test on cylinder specimens are performed using the MTS 815 testing machine (shown in Fig. 3) in accordance with ISRM standards. The cylinder specimens for uniaxial compression
Table 3 Rock properties.
Table 1 Main technical parameters of hydraulic rock drill. ≥85 ≥280 8–15
Impact frequency (Hz) Impact pressure (MPa) Borehole diameter (mm)
Brazilian splitting test
Fig. 3. Rock parameter testing system.
2.2. Rock specimens
Impact energy (J) Rotational speed (r/min) Rotary pressure (MPa)
Rock
Rock specimen
1 2 3 Mean
≤52 12–15 φ32-50
Compressive strength σc (MPa)
Elastic modulus E (GPa)
Poisson's ratio μ
Tensile strength T (MPa)
Density ρ (g/cm3)
119.51 117.95 129.43 122.30
39.09 47.22 52.63 46.31
0.291 0.315 0.294 0.300
9.19 10.05 9.37 9.54
2.60 2.72 2.57 2.63
Table 2 Technical parameters of hydraulic pump station. Nominal pressure (MPa)
Flow (L/min)
Motor power (kW)
Motor speed (r/min)
Motor voltage (V)
Cooling mode
20
≥(75 + 40)
22 + 11
1470
380
Water-cooling
3
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(a) Cross
(b) 3-button
(c) 4-button
(d) 7-button
(e) 8-button
Fig. 4. Bits type.
drilling velocity and specific energy. The main test curve in the experiment is the displacement curve, which is used to get the drilling velocity. Taking the test number 19 (shown in Table 5) as an example, the displacement curve is shown in Fig. 5. As shown in Fig. 5, the amplitude in the original curve is large, this
Table 4 Factor levels. Factors
Impact power (kW) A
Rotation speed (r/ min) B
Propulsion force (N) C
Bit type D
Level Level Level Level Level
1.2 2.5 3.8 5.1 6.4
120 160 200 240 280
2000 2900 3800 4700 5600
cross 3-button 4-button 7-button 8-button
1 2 3 4 5
1200
Original data Smooth data Fitting data
800
Displacement(mm)
bit. To determine the changing tendency of rock breaking performance of impact equipment, the factor levels are set 5 according to the basic performance parameters of hydraulic rock drill and impact energy calibration test data, as shown in Table 4. The orthogonal test design method is adopted to conduct the experimental arrangement to reduce the number of tests and difficulty as well as to ensure the authenticity and comprehensiveness of the test results because the problem has multiple factors and multiple levels. L25 (56) is used for the orthogonal table considering the 4 research factors and the experimental arrangement is shown in Table 4.
Fast drilling
400
0
Hole-opening
Return stroke
-400
0
3. Results and discussion
10
20
30 40 Time(s)
50
60
70
Fig. 5. Displacement curve.
The evaluation index of this test is drilling efficiency including Table 5 Test results statistics. Test no.
Impact power (kW) A
Errors 1
Rotation speed (r/min) B
Propulsion force (N) C
Bit type D
Errors 2
Drilling velocity (m/min)
Specific energy (mJ/m3)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
1.2 1.2 1.2 1.2 1.2 2.5 2.5 2.5 2.5 2.5 3.8 3.8 3.8 3.8 3.8 5.1 5.1 5.1 5.1 5.1 6.4 6.4 6.4 6.4 6.4
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
120 160 200 240 280 160 200 240 280 120 200 240 280 120 160 240 280 120 160 200 280 120 160 200 240
2000 2900 3800 4700 5600 3800 4700 5600 2000 2900 5600 2000 2900 3800 4700 2900 3800 4700 5600 2000 4700 5600 2000 2900 3800
Cross 3 4 7 8 7 8 Cross 3 4 3 4 7 8 Cross 8 Cross 3 4 7 4 7 8 Cross 3
1 2 3 4 5 5 1 2 3 4 4 5 1 2 3 3 4 5 1 2 2 3 4 5 1
0.342 0.486 0.42 0.456 0.432 0.66 0.698 0.665 0.652 0.402 0.914 0.793 0.879 0.701 0.727 1.101 0.989 0.694 1.075 0.967 1.026 1.053 0.877 1.007 1.292
1232.13 949.89 1155.48 1071.94 1202.69 838.65 822.59 900.00 990.72 1327.85 743.76 903.38 843.03 892.93 887.94 711.47 813.80 1018.03 681.92 787.73 810.76 699.17 871.65 786.43 629.46
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(a) Orifice
(b) Bottom
(c) Wall
Fig. 6. Broken rock hole status.
is because the vibration of hydraulic rock drill is larger in the working process, and the rope displacement sensor fluctuates in a certain period. To solve this problem, the smooth data is used to get some rules easier. It can be seen that the displacement variation with time during the process of drilling can be divided into three stages: the hole-opening, the fast drilling and the return stroke. The displacement change is relatively slow in the hole-opening stage. It is worth noting that the drilling velocity in this stage should be small because the drill bit is easy to slide on the rock surface when drilling and causing the drill rod to bend. In the fast drilling stage, the rotary resistance increased with the increasing of drilling depth and the displacement reached the maximum when the drilling to the bottom. The drilling velocity is defined as the slope of the stable stage. In return stroke stage, after the hydraulic drilling machine reaches the predetermined depth, it touches the stroke switch and the oil cylinder reverse working to make hydraulic drilled out of the working state, and finally exits the drill hole. The experimental results analysed by range analysis and analysis of variance (ANOVA) were shown in Table 5.
drilling velocity. From Fig. 6(b), the shape on the bottom is similar to the bit shape, and rock between edge buttons and centre buttons is less broken, whereas rock among edge buttons is more broken; an annular bulge is not broken because the smaller impact energy does not cause cracks to run through edge buttons and the centre buttons. From Fig. 6(c), cracks mainly extend to rock text and crack direction, which indicates that the crack is extended along the original direction of rock. After the experiment, debris was collected and dried to obtain the debris shape, as shown in Fig. 7. It can be seen that the impact crushing rock debris is powdery, while other types of broken debris are mixed rock powder and rock blocks, indicating the rock is over broken. The largest debris is shown in red circle, and the order of rock debris size is as follows: three buttons > four buttons > seven buttons > eight buttons. The rock cracks among the buttons are easily intersected and form small rock mass because of the reduced distance among the buttons caused by the increased number of buttons. In general, the larger the block is, the smaller the energy consumption is, and the more effective to improve the efficiency of the impact equipment. The bits state after the test is shown in Fig. 8. It can be seen the bits are only missing paint, with no wear and breakage observed, except for the 7-button bit because the drilling direction deviation caused by bedding occurred in the test specimen fracture, causing the drill handle to bear uneven stress, resulting in the damage of the handle of the seven-button bit under the propulsion force.
3.1. Rock broken status The rock broken status after the test is illustrated in Fig. 6. Fig. 6(a), (b) and (c) represents orifice, bottom and wall surface broken state, respectively. From Fig. 6(a), the orifice is round and smooth, which indicates low
(a) Cross bit
0 1cm (b) 3-button bit
0 1cm
(c) 4-button bit
0 1cm (d) 7-button bit
0 1cm (e) 8-button bit
Fig. 7. Debris shapes.
(a) cross bit
(b) 3-button bit
(c) 4-button bit Fig. 8. Bit states after tests. 5
Damaged 7-button bit
(d) 7-button bit
(e) 8-button bit
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indicates that there exists an optimum rotation speed which makes drilling velocity maximum. This is because each impact process will form a dense core at the bottom of the drilling hole, which will disperse the energy and reduce the impact crushing effect. Therefore, it is necessary to turn to a new position to avoid the dense core. At the same time, micro-cracks can be generated by impact fragmentation, so that the next cycle of impact can break larger volume of rock. When the rotation speed is small, the bit does not cross the crushing pit. When the impact occurs in the next cycle, the dense core is repeatedly crushed and the penetration zone formed is small. The energy is not effectively utilized, which reduces the impact crushing effect. When the rotation speed is too high, the distance between the new crushing position of the drill bit and the initial crushing position is too far, and the cracks between the two cannot be penetrated and the broken rock volume is small, which reduces the impact crushing effect. Propulsion force mainly affects drilling velocity from energy transfer efficiency and the cutting action of drill bit two aspects. In order to break rocks, energy needs to be transmitted to rocks through drill bits. When the propulsion force is small, the drill bits cannot form good contact with rocks. When energy is transmitted to rocks through drill bits, more energy is reflected at the bit, resulting in the continuous transmission and dissipation of energy in the drill rod, which makes the efficiency of energy transmission low. Therefore, the energy enters the rock less, the volume of rock fragmentation is smaller, and the drilling velocity is smaller. With the increase of propulsion force, the bit can keep good contact with the rock in a relatively short time, and the energy transfer efficiency is high, which is conducive to rock fragmentation. When the thrust force is in level 3(3800N), the drilling velocity appears extreme value, which indicates that a higher energy transfer efficiency can be achieved at this level. If the propulsive force continues to increase, such as level 4 (4700N), it has little effect on improving energy transfer efficiency, but increase the rotary resistance. Due to the limitation of hydraulic rock drill's rotary power, its rotation speed will be reduced, and it is easy to deviate from the optimal rotation speed, resulting in the decline of drilling velocity. In another aspect, when the propulsion force is relatively small, the recoil force generated by the drill bit in the process of rotation is greater than the propulsion force, and the phenomenon of the drill bit retreating after cutting occurs. As a result, the drill bit cannot or rarely cut the rock, but turns to a new position to prepare for the next impact on the broken rock. When the propulsion force is larger than the recoil force generated by the drill bit in the process of rotation, the phenomenon of retraction is not easy to happen. In the process of rotation, the drill bit cuts and breaks the rock and can turn to a new position, thus increasing the drilling velocity. And this is the reason drilling velocity at level 5 (5600N) greater than that at level 4 (4700N). In order to verify the second hypothesis of the influence of propulsion force on drilling velocity, a pure cutting test of the bit is added (taking a 3-button bit as an example). In the test, the rotation speed and the propulsion force is set as 240 r/min, level 4 (4700N) and level 5 (5600N), respectively. And the bit is rotated for half a minute without water. The state of rock breaking after pure cutting is shown in Fig. 10. The cutting depth of the left hole is 1 mm and that of the right hole is 4 mm, which shows that increasing the propulsion force is beneficial to the rock cutting. And this verifies the hypothesis that the propulsion force affects the drilling velocity. However, it is found that the drill bit sticking is easy to occur when the propulsion force is in level 5 (5600N), resulting in shutdown in actual production and affecting work efficiency. Although the factors order affecting the drilling velocity can be obtained by range analysis, it is unable to determine whether the drilling velocity is caused by the corresponding level variation or the error, and the factors on drilling velocity was not obtained; thus, it is necessary to apply the orthogonal test ANOVA of results for further analysis. The drilling velocity variation with working parameters and the ANOVA of drilling velocity is shown in Fig. 11 and Table 7. From
Table 6 Range analysis of drilling velocity. Factors
Impact power (kW)
Errors 1
Rotation speed (r/ min)
Propulsion force (N)
Bit type
Errors 2
K1 K2 K3 K4 K5
2.136 3.077 4.014 4.826 5.255
4.043 4.019 3.535 3.891 3.820
3.192 3.825 4.006 4.307 3.978
3.631 3.875 4.062 3.601 4.139
3.730 4.038 3.716 4.015 3.809
4.286 3.845 3.953 3.638 3.586
k1 k2 k3 k4 k5
0.427 0.615 0.803 0.965 1.051
0.809 0.804 0.707 0.778 0.764
0.638 0.765 0.801 0.861 0.796
0.726 0.775 0.812 0.720 0.828
0.746 0.808 0.743 0.803 0.762
0.857 0.769 0.791 0.728 0.717
Rang R
0.624
0.102
0.223
0.108
0.065
0.140
3.2. Drilling velocity As is illustrated in Table 6, Ki and ki (i = 1, 2,…, 5) is the sum and average value of experiments with factor level number i. R means the degree for effects of factors on experimental results. The range values of impact power, rotation speed, propulsion force, and bit type are 5.517, 1.843, 0.892 and 0.537 times the mean value of the error range compared to the range value R of the parameters; therefore, the order of factors which affects drilling velocity is impact power, rotation speed, propulsion force and bit type. Moreover, the range values of propulsion force and bit type are less than the error range, indicating that they have little influence on the drilling velocity. It can be concluded that the optimal scheme is A5B4C5D2, in which the impact power, rotation speed, propulsion force and bit type are 6.4 kW, 240 r/min, 5600 N and 3-button bit, respectively, by comparing the average value ki of each level drilling velocity. According to the changing trend of drilling velocity shown in Fig. 9, the drilling velocity is found to increase sharply with impact power; however, with increasing rotation speed, it increases first and later decreases with the maxima level of 4 (240 r/min), which indicates the optimal speed occurs near the level, and the optimal speed has no relationship with bit type. The drilling velocity for the cross bit, 3-button bit, 7-button bit and 8-button bit was 1.004, 1.087, 1.080 and 1.025 times that of the 4-button bit, respectively. However, this does not mean that propulsion force and bit type have no influence on drilling velocity, but only that the influence of propulsion force and bit type on drilling velocity is inconspicuous relative to impact power. The peak value of the influence of rotation speed on drilling velocity 1.2 Level 1 Level 2 Level 3 Level 4
1.1
Drilling velocity/(m/min)
1.0
Level 5
0.9 0.8 0.7 0.6 0.5 0.4 0.3
Impact power
Errors 1 Rotation speed Propulsion force Bit type
Errors 2
Factors Fig. 9. The changing trend of drilling velocity with factors. 6
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H=
E × 103 V
(1) (2)
E = Ft v + Pe + Ph = Ft v + pe Qe + ph Qh V=
D2 4
v
(3)
where H – Crushing specific energy consumption, mJ/m ; É – Energy consumption per unit of time, J; V – Rock crushing volume per unit time, m3; Ft – Propulsion force, N; v – Drilling velocity of the hydraulic rock drill, m/s; Pe – Impact power consumption, W; Ph – Rotary power consumption, W; pe, ph – Impact pressure, rotary pressure, Pa; Qe, Qh – Impact flow, Rotary flow, m3/s. Range analysis of specific energy is shown in Table 8. By contrasting the range values R of the factors, each factor on the drilling velocity of primary and secondary order was obtained: impact power, rotation speed, bit type and propulsion force. It can be concluded that the optimal scheme is A5B4C5D4, in which the impact power, rotation speed, 3
Fig. 10. Rock crushing state after bit cutting.
the value of F in the table, it can be obtained that the order affecting drilling velocity is impact power, rotation speed, propulsion force and bit type, which was in agreement with range analysis. From the P-value and conspicuousness, it can be observed that impact power has appreciable influence on drilling velocity. Moreover, rotation speed has certain degree of influence, while propulsion force and bit type have little effect in this study. However, the influencing parameters of impact power, rotation speed, propulsion force and bit type in terms of percentage contribution on drilling velocity is 38.54%, 20%, 20%, 20%, respectively.
Table 8 Range analysis of specific energy.
3.3. Specific energy The specific energy is used as an index of energy consumption in rock breaking. The energy consumption in the process of hydraulic rock drill impact crushing mainly includes impact energy, rotary energy and propulsion energy. The calculation of specific energy (Liu et al., 2017; Liu et al., 2017) can be expressed by the following formulas,
Factors
Impact power (kW)
Errors 1
Rotation speed (r/ min)
Propulsion force (N)
Bit type
Errors 2
K1 K2 K3 K4 K5
5954.63 5138.39 4478.52 4167.07 3962.59
4547.52 4395.98 5036.23 4636.22 5085.25
5304.55 4529.85 4491.15 4433.58 4942.07
5029.64 4836.58 4558.73 4840.06 4436.19
4843.40 4547.21 5102.67 4471.43 4736.49
4391.32 4546.69 4693.67 5063.67 5005.85
k1 k2 k3 k4 k5
1190.93 1027.68 895.70 833.41 792.52
909.50 879.20 1007.25 927.24 1017.05
1060.91 905.97 898.23 886.72 988.41
1005.93 967.32 911.75 968.01 887.24
968.68 909.44 1020.53 894.29 947.30
878.26 909.34 938.73 1012.73 1001.17
Rang R
398.41
137.85
174.19
118.69
126.25
134.47
Fig. 11. Drilling velocity variations with (a) impact power and rotation speed; (b) propulsion force and bit type.
Table 7 ANOVA of drilling velocity. Factors
Sum of squares
Freedom f
Mean squares
F
P-value
Significant
Contribution
Impact power Rotational speed Propulsion force Bit type Errors Total
1.298 0.136 0.048 0.019 0.096 1.60
4 4 4 4 8 24
0.324 0.034 0.012 0.005 0.012 –
26.942 2.834 0.990 0.398 – –
0.000 0.098 0.465 0.805 – –
*** *
38.54% 20% 20% 20% – –
7
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value of F in the table, it can be obtained that the factor order affecting specific energy is impact power, rotation speed, bit type and propulsion force. Moreover, from the P-value and conspicuousness, impact power has a highly appreciable influence on specific energy. However, the influencing parameters of impact power, rotation speed, propulsion force and bit type in terms of percentage contribution on specific energy is 35.307%, 20%, 20%, 20%, respectively. From the drilling velocity point of view, the optimal combination is A5B4C5D2. From the view of specific energy, the optimal combination is A5B4C5D4. However, in the test process, it was observed the hydraulic rock drill often suffers from the sticking phenomenon and interruption when propulsion force with level 5 (5600 N) occurs in the test, which will cause the drill to grind to a halt in practical application; thus, the overall drilling efficiency will be reduced. From the range analysis of the drilling velocity and specific energy, it can be obtained the difference of the results between level 3 (3800 N) and the level 5 (5600 N) is slight, which indicates that it can achieve better rock breaking performance and achieve high drilling velocity and lower specific energy when the propulsion force is level 3 (3800 N). Although the difference of drilling velocity and specific energy between the 3-button bit and 7button bit is small, it is less prone to sticking under the same operating parameters because the number of button is higher and the bit force is uniform. In conclusion, when the rock is uniform and less crack, the best work parameters for the impact device is A5B4C3D2, in which the impact power, rotation speed, propulsion force, and bit type are 6.4 kW, 240 r/min, 3800 N and 3-button bit, respectively, for the homogeneous and less crack rock. For the uneven and rock with greater numbers of cracks, inclusions and beddings, the optimum combination is A5B4C5D4, in which the impact power, rotation speed, propulsion force, and bit type are 6.4 kW, 240 r/min, 3800 N and 7-button bit, respectively. The calculation of the specific energy and its structure is largely related, which makes the results lacking general applicability, thus, specific energy is no longer discussed here because the rotating torque in the work of hydraulic rock drill is difficult to measure.
1200
Level 1 Level 2 Level 3 Level 4 Level 5
Specific energy(mJ/m3)
1100
1000
900
800
700
Impact power
Errors 1
Rotation speed Propulsion force Bit type
Factors
Errors 2
Fig. 12. Specific energy variations with factors.
propulsion force, and bit type are 6.4 kW, 240 r/min, 5600 N and 7button bit, respectively, from comparison of the average value ki of each level drilling velocity. From the variation trend of specific energy shown in Fig. 12, it can be concluded the specific energy reduces substantially with the increasing of impact power; however, with increasing rotation speed and propulsion force, it first decreases and later increases with the maxima level of 4 (240 r/min). Moreover, it can be concluded from Fig. 12 that the 7-button bit has the lowest specific energy, whereas the four-button bit has the highest value. The variation trend of specific energy is found to be just the opposite of the drilling velocity by comparing Fig. 9 and Fig. 12. The specific energy variation with working parameters and the ANOVA of the specific energy is shown in Fig. 13 and Table 9. From the
Fig. 13. Specific energy variation with (a) impact power and rotation speed; (b) propulsion force and bit type. Table 9 ANOVA of specific energy. Factors
Sum of squares
Freedom f
Mean squares
F
P-value
Significant
Contribution
Impact power Rotation speed Propulsion force Bit type Errors Total
432,640 133,780 41,997 50,587 121,200 780,202
4 4 4 4 8 24
108,160 33,445 10,499 12,647 15,150 –
7.14 2.21 0.69 0.83 – –
0.009 0.158 0.617 0.540 – –
*** – – – – –
35.307% 20% 20% 20% – –
8
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3.4. Regression orthogonal analysis
Table 12 Model determination coefficients.
The orthogonal test can take advantage of reduced test times to obtain comprehensive test results. However, within the experimental range, this test cannot confirm the relationship between the test indices and the research factors as well as the corresponding regression equation according to test datum. To establish a highly accurate regression equation using few test times during the regression equation mathematical model set up, it needs to get rid of the traditional regression analysis which can only passively address the experimental data, and the test arrangement, data processing and the precision of the regression equation must be taken into consideration uniformly. Accordingly, it is necessary to combine the regression analysis method with the orthogonal test method to form a new one, the regression orthogonal design method. According to the research on the rock-breaking performance of impact equipment on bluestone, it can be observed that the influence factors of impact power and rotation speed affect the drilling velocity, among which, the impact power has the greatest influence. To enhance the accuracy of the regression model, the propulsion force was taken into consideration; thus, some research factors of the regression test model were chosen, including impact power, rotation speed, propulsion force and interaction of impact power and rotation speed, and the 3button bit was chosen as the drilling tool. On the basis of the one regression orthogonal design method, various factor levels are shown in Table 10, and the test arrangement is shown in Table 11. The test results analysed by statistics are shown in Table 11. Analysis results mainly include the model decision coefficient, the model significance test and the factor regression coefficient. The determination coefficient R2 showed in Table 12 is 0.946, which illustrates that four chosen research factors can explain 94.6% of the variation of the drilling velocity for the test on bluestone. The factor regression coefficients are shown in Table 13, in which x1, x2, x3, and x1x2 represent impact power, rotation speed, propulsion force, and the interaction of impact power and rotation speed, respectively. From the table, it is easy to obtain both the regression coefficient of the constant term and the various factors, and the P-value is used to measure the influence degree of the factors on indices. The table also indicates that the impact power and the rotation speed have more effect on the drilling velocity than the propulsion force and the interaction of the impact
Model
R
R2
Adj. R2
1
0.972
0.946
0.914
Table 13 Factor regression coefficients of bluestone. Model
Non-standardized coefficients
1 (Constant) x1 x2 x3 x1x2
b
Errors
0.829 0.305 0.082 0.017 0.043
0.024 0.029 0.029 0.029 0.029
F
P-value
35.035 10.537 2.842 0.574 1.471
0.000 0.000 0.025 0.584 0.185
power and the rotation speed; thus, the latter two factors can be ignored in rough calculation. The relationships between each of the four factors and the drilling velocity are positively related, which means that with the increase of each factor, the drilling velocity will also increase gradually. However, the trends of drilling velocity changing with rotation speed and propulsion force are different from the test without interaction. The reason of this situation is that there are only two level numbers of each factor in one regression orthogonal design method. Depending on the P-value of the different factors, the regression relationship between drilling velocity and impact power and the regression relationship between drilling velocity and rotation speed obviously have reached to a significant level; nevertheless, the regression relationships between drilling velocity and the other two factors were insignificant. The analysis results are consistent with the results of the test with interaction. Summarising the above, the multivariate regression equation is as follows:
y = 0.829 + 0.305x1 + 0.082x2 + 0.017x3 + 0.043x1 x2 where x1 = 2.6 , x2 = 80 , x3 = The final fitting formula is A
3.8
B
200
C
3800 . 1800
y = 0.07596A + 0.000239B + 0.0000094C + 0.0002067AB + 0.2994
Table 10 Factor levels for the regression orthogonal test. Parameters
Impact power (kW) A
Rotation speed (r/ min) B
Propulsion force (N) C
Lower level −1 Upper level 1 Zero level 0 Interval
1.2 6.4 3.8 2.6
120 280 200 80
2000 5600 3800 1800
(4)
(5)
where y is the drilling velocity (m/min); A is the impact power (kW); B is the rotation speed (r/min); C is the propulsion force (N). The loss of fit was tested to verify the fitting situation of intermediate value of factor interval:
t=
|0.829 0.916| 7 + 3 = 1.903 < t (0.05, 10) = 2.228 0.047 + 0.000847 1 12 + 1 4 (6)
Table 11 Test arrangement for the regression orthogonal test. Test no.
Impact power (kW)
Rotation Speed (r/min)
Impact power × Rotation speed
Propulsion force (N)
5
6
7
Drilling velocity (m/min)
1 2 3 4 5 6 7 8 9 10 11 12
6.4 6.4 6.4 6.4 1.2 1.2 1.2 1.2 3.8 3.8 3.8 3.8
280 280 120 120 280 280 120 120 200 200 200 200
1 1 −1 −1 −1 −1 1 1 0 0 0 0
5600 2000 5600 2000 5600 2000 5600 2000 3800 3800 3800 3800
1 −1 1 −1 −1 1 −1 1 0 0 0 0
1 −1 −1 1 1 −1 −1 1 0 0 0 0
1 −1 −1 1 −1 1 1 −1 0 0 0 0
1.251 1.181 0.981 0.951 0.525 0.515 0.452 0.429 0.910 0.895 0.925 0.933
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estimating according to site condition and construction progress, one regression orthogonal test for rock burst of impact equipment is carried out. When the bit diameter is 38 mm and the bit type is 3button bit, regression equation obtained for drilling velocity of bluestone is as follows and the errors are within 12% in verification.
Table 14 Bluestone drilling velocity verification. Test no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Impact power (kW)
6.4 6.4 6.4 6.4 1.2 1.2 1.2 1.2 3.8 3.8 3.8 3.8 1.2 2.5 3.8 6.4 5 5
Rotation speed (r/ min)
280 280 120 120 280 280 120 120 200 200 200 200 160 280 200 240 270 270
Propulsion force (N)
5600 2000 5600 2000 5600 2000 5600 2000 3800 3800 3800 3800 2900 2000 5600 3800 5600 3800
Calculation value of drilling velocity (m/min)
Test value of drilling velocity (m/min)
Errors (%)
1.276 1.242 1.026 0.992 0.580 0.546 0.502 0.468 0.829 0.829 0.829 0.829 0.496 0.720 0.846 1.196 1.075 1.058
1.251 1.181 0.981 0.951 0.525 0.515 0.452 0.429 0.910 0.895 0.925 0.933 0.486 0.652 0.914 1.298 1.030 1.150
1.96 5.14 4.55 4.29 10.39 5.97 10.98 9.04 8.93 7.41 10.42 11.20 2.00 10.38 7.53 7.85 4.41 7.96
y = 0.07596A + 0.000239B + 0.0000094C + 0.0002067AB + 0.2994 Declaration of Competing Interest The authors declare that there are no conflicts of interest regarding the publication of this paper. Acknowledgments This work is supported by the Future Scientists Program of “Double First Class” of China University of Mining and Technology (No. 2019WLKXJ032). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.tust.2019.103174.
The result shows that the established regression equation is in good agreement with the actual situation. The correctness of regression formula can be verified by test datum. The verification of the test on bluestone is shown in Table 14. The table shows that the errors are within 12%. Since the impact power has no effect for equipment measured by the impact energy calibration test in the regression analysis, the formula can be applied to the selection of such equipment or the initially identified working parameters. In addition, the existing working parameters of hydraulic rock drill can be used to estimate the optimal drilling velocity on bluestone, which can provide basis for equipment selection and activity duration estimation according to requirements, such as site condition and construction schedule. When the accuracy of the regression model is not satisfied, two or higher power regression orthogonal designs can be conducted.
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4. Conclusions The high-efficiency impact rock fragmentation orthogonal experiment is conducted to analyse the influence of working parameters, including impact power, propulsion force, rotation speed and bit type, on the rock impact efficiency from the views of the drilling velocity and the specific energy. The results are analysed to get the order of the factors on impact fragmentation and the relationship between the test indices and the research factors as well as the corresponding regression equation. The following conclusions can be concluded: (1) From range analysis of drilling velocity and specific energy, it can be obtained the difference of the results between level 3 (3800 N) and the level 5 (5600 N) is slight, which indicates that it can achieve better rock breaking performance and achieve high drilling velocity and lower specific energy when the propulsion force in level 3 (3800 N). (2) The results of the rock breaking test using drilling bit reveals that the order affecting drilling efficiency is impact power, rotation speed, propulsion force and bit type. Moreover, the impact power has a highly appreciable influence on the efficiency, whereas the influence of rotation speed, propulsion force and bit type on that in their level range is slight. And the optimum combination in view of drilling velocity and specific energy are 6.4 kW, 240 r/min, 3800 N and 7-button for impact power, rotation speed, propulsion force, and bit type respectively. (3) In order to provide basis for equipment selection and duration 10
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