A specific energy and penetration interaction-based method for optimizing TBM operational parameters in hard rock

The energy method to predict disc cutter wear extent for hard rock TBMs

The paper by Ersoy and Atici (2007) aims at establishing correlations between seismic wave velocity and cutting specific energy (SCE) of some selected rocks, determined during circular sawing experiments. An indirect method of SCE estimation has been provided which could be useful in practice. In the stone processing industry, the SCE can be used to monitor the efficiency of a sawing process and for estimating the power requirements. However, SCE is not a fundamental intrinsic property of rock. It is mainly influenced by the operational parameters (cutting depth, sawblade peripheral speed, feed rate, cutting mode, blade specifications, etc.), and the rock properties (mechanical strength, mineralogical and petrographic properties, etc.). Consequently, it is often difficult to compare the SCE values reported by different researchers working in the field of stone processing. Although a precise comparison of the data provided by different researchers cannot be made, there is benefit in examining the magnitudes of SCE values obtained for different operating conditions and rocks. Ozcelik et al. (2001) carried out circular sawing tests in a stone processing plant to examine the relations between rock parameters and SCE in the processing of andesites. In their study, SCE values obtained from the sawing tests ranged from 1.66 to 2.00 J/mm. Buyuksagis and Goktan (2005) undertook an experimental study to investigate the sawability characteristics of seven different types of calcium carbonate marbles during circular sawing. Depending on the testing conditions and the marble tested, SCE values from 0.6 to 2.10 J/mm were obtained. Xipeng et al. (2001) conducted a literature survey on the specific energy of granites as determined in circular sawing with diamond segmented sawblades. They reported specific energy values ranging from 3.2 to 6.9 J/mm. As shown from the literature survey above, SCE values ranging from 0.6 to 6.9 J/ mm have been reported, which can be attributed to the differences in the adopted

Harder formations in geothermal and deeper oil and gas wells typically see lower rates of penetration (ROPs) and higher bit wear. Recent drilling experience in the Utah FORGE observed higher ROPs and longer runs using lighter drilling muds and new cutter technologies. Questions have arisen regarding reasons for the improved ROP and if the higher ROP is due to increased spurt loss or reduced hydrostatic pressure from lighter drilling fluids. A study was performed on an inhouse rig using two polycrystalline diamond compact (PDC) bits (4-bladed vs 5-bladed) to analyze the effect of confinement on ROP in Sierra White Granite (SWG). Tests were performed at confinements of 0, 1000, and 2000 psi and at constant rotational speeds of 80 and 150 RPM. Results show that when confinement is increased, ROPs are similar in the ineffective cutting Phase I. However, results in Phase II indicate that ROP is largely different between the unconfined and higher confinement testing. No significant effect on ROP is observed between 1000 and 2000 psi. At zero confinement, the 4-bladed bit outperformed the 5-bladed bit while at 1000 and 2000 psi confinement, the performance was similar. This paper presents the laboratory results with potential applications to hard rock geothermal drilling. INTRODUCTION Confining pressure in drilling terms is the hydrostatic pressure at the bottom of the well while drilling in impermeable formations. Many studies have outlined the effects of confining pressure on rock deformation but the effect of confinement on drilling efficiency and ROP is yet to be thoroughly investigated. In recent years, hard rock drilling especially in geothermal reservoirs has faced great challenges such as low penetration rates and high bit wear leading to high drilling costs. Advanced bit technology is required to maneuver the unique physical and mechanical properties of hard and abrasive rocks at high confining pressures. Although roller cone bits were predominantly used to drill hard and abrasive formations, PDC bits with enhanced cutter properties and ‘special’ cutter geometries are at the forefront of drilling research and technology. During the past years, new PDC cutter technology has improved cutter wear resistance and its capability to carry higher loads. Recent drilling experience in the Utah FORGE observed higher ROPs and longer bit runs using lighter drilling muds and new cutter technologies including shaped cutters (Dupriest and Noynaert, 2022). Glowka, (1985) discussed the potential for developing a PDC drill bit for hard rock applications such as geothermal drilling. Different cutter geometrical shapes and designs are currently being designed to address heavy impact damage in hard formations, prevent thermal damage and extend drilling runs while maximizing ROP. Zhu et al. (2022) analyzed the loading performance and rock cutting mechanism of different PDC cutters by using single cutter analysis to study geometry, aggressiveness, and stress distribution in granite. Xiong et al. (2022) studied the performance of a stinger PDC cutter in granite to evaluate cutting force and mechanical specific energy (MSE). Barnett et al. (2022) studied the effects of drillstring torsional vibration on ROP with PDC bits in hard rock. Wang et al. (2020) designed a PDC hybrid drill bit to navigate hard and abrasive formations. Akhtarmanesh et al. (2021) designed an ROP model for drilling hard and abrasive formations for PDC bits. It was seen that distinct phases of drilling exist where Phase I is the inefficient drilling phase due to insufficient WOB and Phase II is the efficient drilling phase due to higher WOB which results in a higher ROP. Figure 1 shows the different phases of drilling as described by Akhtarmanesh et al. (2021). Due to the complexity of drilling in hard formations, more testing is crucial to completely understand the effects of temperature, rock behavior, and confining pressure on ROP and bit cutting mechanism. An understanding of bit cutting action in hard rock is crucial for successful ROP optimization and improving drilling efficiency.

Hard rock mining is currently carried out by drill-and-blast. With the increasing demand for automated, selective and remote mining, the industry hopes to replace conventional drill-and-blast with mechanical excavation. Modern mining machines have sufficient power for cutting hard rocks. The ‘bottleneck’ which limits the use of mechanical excavation for hard rock mining is the cutting tool wear. Rock cutting tools usually use tungsten carbide (WC) tipped picks. The WC tipped picks are effective and adequate for cutting relatively soft rocks, but unsuitable for hard rocks. To address this issue, CSIRO has developed Super Material Abrasive Resistant Tool (SMART*CUT), aiming at providing an effective cutting tool for hard rock mining. The key feature of the SMART*CUT technology is replacing the conventional WC with thermally stable diamond composite (TSDC) as the cutting tip of the pick. The main advantages of the TSDC tipped picks include (a) good thermal stability, (b) high wear resistance and (c) the ability to mine relatively hard deposits in comparison with the WC tipped picks. The disadvantage of the TSDC tipped picks is their low fracture toughness. To optimise the application of the TSDC tipped picks, this work aimed to improve the understanding of the rock cutting process and mechanisms, in particular to gain a better insight into the interaction between tools and rocks through systematic experimental and numerical studies. The rock cutting processes with the TSDC and WC tipped point attack picks were comprehensively investigated in terms of cutting force, specific energy, tool tip temperature and cutting chips. A study of rock cutting was systematically carried out using the Taguchi method to determine the effects of the cutting parameters on the performance of the TSDC tipped picks. The signal-to-noise (S/N) ratios and the analysis of variance (ANOVA) were applied to investigate the effects of the depth of cut, attack angle, spacing and cutting speed on the mean cutting, normal and bending forces involved in the rock cutting process. The statistical significance of process factors was determined and the optimum parametric combinations were successfully identified. Furthermore, the multiple linear regression (MLR) and artificial neural network (ANN) techniques were adopted to develop the empirical models for predicting the mean cutting and normal forces with the TSDC tipped picks. The established empirical force models showed good predictive capabilities with acceptable accuracy. The ANN models offered better accuracy and less deviation than the MLR models. The pick cutting temperature involved in rock cutting was measured using thermal infrared imaging and embedded thermocouple techniques. The temperature distribution in the cutting area was acquired by using a thermal infrared camera with high speed imaging rates. The results showed that the TSDC tipped picks generated much lower thermal energy and much fewer sparks than the WC tipped picks. The temperature at the pick tip in the contact area with the rock was measured using a special thermocouple configuration. It was found that the pick tip temperature increased significantly with the increased depth of cut and cutting speed. The effect of spacing on the pick tip temperature was smaller than that of the depth of cut and cutting speed, particularly when the spacing was greater than that required to change from unrelieved to relieved cutting. A series of rock cutting tests was also conducted to investigate the effects of cutting parameters on the coarseness index for the TSDC tipped picks. It was found that depth of cut, cutting speed and spacing had relatively great contributions to the coarseness index. These key parameters were then chosen to examine their effects on the rock chip size distribution and specific energy. The results showed that the dust proportion decreased with the increased depth of cut and decreased cutting speed, while the fraction of large chips increased with the increased depth of cut and decreased cutting speed. The effect of the spacing on the proportion of dust and the formation of large chips was insignificant. The specific energy increased with the increased cutting speed and spacing, while it decreased and then approached a saturated value with increasing depth of cut. Three dimensional numerical models were developed to simulate the rock cutting process using an explicit finite element code LS-DYNA. Using these numerical models, the cutting forces and specific energies under different cutting conditions were obtained and then compared with the results obtained experimentally and analytically. The simulated forces and specific energies were in good agreement with the experimental data, which validated the numerical models. The simulated cutting forces were, in general, consistent with those predicted by the analytical models of Evans and Goktan. However, discrepancy existed because the Evans’s model is independent of attack angle and cutting speed, and Goktan’s model does not include the effect of cutting speed.

To acquire Johnson–Cook (J-C) constitutive parameters that accurately depict the mechanical behavior of powder liner under conditions of high pressure, elevated temperature, and large deformation, as well as Holmquist–Johnson–Cook (HJC) constitutive parameters that precisely describe the dynamic damage of hard rock and make them suitable for numerical simulations for hard rock perforation, the present study introduces a constitutive parameter inversion method based on finite element simulation. Firstly, based on the experiments of perforating steel targets and underground perforating hard rock targets, a dynamic simulation of the perforating process of a shaped charge perforating target was carried out using ANSYS/LS-DYNA, and the influence law of each constitutive parameter on perforating depth and perforating aperture was systematically analyzed. Subsequently, the key parameters of the J-C constitutive model for powder liner and the HJC constitutive model for hard rock were optimized and determined using a response surface method, multi-genetic algorithm, and experimental data. A numerical simulation of the perforating process was finally conducted using the retrieved constitutive parameters of powder liner and hard rock, which were then compared with the experimental results. The results demonstrated that the discrepancy between the experimental and simulated data was within 5%, indicating that the constitutive parameters obtained through this inversion method could more reliably reflect the mechanical behavior of the powder mold and hard rock used in this study during perforation.

Drilling through hard crystalline rocks like granites is challenging and taxing on the overall performance due to reduced rate of penetration (ROP). While efforts have been made in improving polycrystalline diamond compact (PDC) bits to increase ROP in hard rocks, the evidence of their field performance is currently restricted to only a few sites. There exists scope for alternative drilling technology where a significant fraction of hard rocks are present, such as in deep geothermal wells for electricity generation. In the ORCHYD project, two mature technologies – high pressure water jetting (HPWJ) and percussion drilling - were combined. A combination of bit profile and peripherical groove (slotted by HPWJ) creates a stress-relief effect releasing the rock from surrounding geological stresses, requiring lower energy to break the rock using a mud hammer. Furthermore, pressure waves due to percussion are reflected by free surfaces at the groove aiding in rock breakage. In this project, an experimental study on the influence of operating conditions such as HPWJ pressure, bottom hole pressure and surrounding geological stresses on the drilling performance was conducted. Several tests were performed at a dedicated drilling laboratory where the operational parameters can be varied to emulate drilling conditions for depths up to 4 km. As compared to tricone roller bits, ORCHYD technology guaranteed at least 4 times increase in the drilling performance. The performance of HPWJ in slotting a peripheral groove and mud hammer in rock breakage were strongly influenced by the operational conditions, e.g., for a given jet pressure the groove depth decreased significantly with increased bottom hole pressure. In this work, effects of such operating conditions on drilling performance were tested for different types of rocks such as Sidobre, Kuru Grey and Red Bohus. A sensitivity analysis of the influencing parameters on drilling performance of this technology is presented in this work. With increasing geological stresses, the proposed drilling technique is more effective in increasing ROP due to the stress relief effect. A novel technique combining HPWJ and percussion drilling using a mud hammer prototype was developed to show improved drilling performance in deep, hard rocks as compared to conventional drilling technique. Through this work, the performance of this method under different realistic drilling conditions was studied to optimize ROP, especially when drilling hard abrasive formations in deep oil and gas or geothermal wells.

A successful excavation of roadheaders depends on the cutting performance and the tool life of conical picks. Tool life is important in terms of wear rate which is affected by different rock parameters such as equivalent quartz content, mineral grain size, as well as cutting parameters on the cutterhead. In this study, analyses among wear rate, specific energy, advance rate, and cutter consumption were carried out. The wear mechanisms of two different models of conical picks were examined from different aspects depending on rock and machine parameters. Their relation with the mechanical and abrasivity properties of rocks and petrographic analyses were investigated. In addition, the metallurgic content and Rockwell hardness of conical picks were determined to describe the metal alloys and their effects on the wear of cutting tool. The results showed that the metallurgic content, pick positions, and other environmental conditions influence the wear mechanism. Finally, two different models were proposed to estimate the pick consumption in sandstone and siltstone rocks based on actual data obtained from coalfield.

Investigations on thermal spallation drilling performance using the specific energy method

Stress conditions are critical in deep hard rock mining and significantly influence hard rock cuttability. The peak cutting force (PCF), cutting work (CW), and specific energy (SE) can reflect rock cuttability and determine the feasibility and saving of mechanized mining to some extent. In this paper, the influence of uniaxial lateral stress on rock cuttability was investigated by an indentation experiment on cuboid rock using a conical pick, and a theoretical model was proposed to analyze the PCF and associated factors. The PCF, CW, and SE were used as indices to measure hard rock cuttability. The regression analyses show that rock cuttability presents as decreasing followed by increasing as uniaxial lateral stresses increases. The theoretical model was established by simplifying rock fragments into three-dimensional ellipse cones, and a formula was derived based on the elastic fracture mechanics theory. The error between the calculated and experimental values is 3.8%, which confirms the accuracy of the prediction formula. Finally, rock fragmentation by using conical picks was successfully applied on the field mining stope by inducing high geostresses to promote adjustments in stress and improve ore-rock cuttability.

One of the most labor-intensive operations in production of dimension stone is separation of a stone monolith from rock mass, cutting of the monolith into blocks and sawing of the blocks into slabs. This article reviews in brief the methods of cutting and sawing of high-strength rocks. The new method of percussion sawing is described. Based on the method, the structural layouts of machines for cutting and sawing of stone blocks are proposed. The percussi on sawing method is efficient in brittle hard and super-hard rocks such as granite and sometimes marble, with Protodyakonov-scale hardness from 6 to 20. The method provides high quality finishing of surfaces at average productivity and low cost of sawing. The authors describe various structural layouts and operation modes for different percussion sawing machines suitable for operation in stone quarries, stone working plants and workshops. The basic elements of percussion sawing machines are frame, percussion assembly, tooth saw, vertical and horizontal feed drives, and the cut blowing or washing facilities. The article describes the machine capacity calculation procedure confirmed by the experimental research data. The major machine parameters to affect its capacity are: blow energy of the percussion assembly, number of percussion assemblies, saw area at the contact with stone, seesaw velocity, and ultimate compression strength of stone. A new estimated figure is put forward as specific blow energy per kerf bottom surface. The change in the capacity of a quarry percussion sawing machine as a function of time of one saw pass is determined. The obtained relation is compared with performance of other two types of sawing machines.The comparative analysis of mechanical methods of hard rock sawing is performed. The efficiency of the proposed method for percussion sawing of hard and super hard rocks is proved.

To investigate the matching rules of cutters with different blade widths in hard rock and extremely hard rock environments, this study carries out a full-scale cutting test of 432-mm disc cutters with blade widths of 12 mm, 19 mm and 30 mm. A multifunctional cutter performance experimental system was used to test the cutting loads and rock-breaking quantity of rust stone granite (hereinafter referred to as hard rock) and granite (hereinafter referred to as extremely hard rock) as well as to analyse the specific energy consumption for rock breaking under different cutting parameters. The experimental results show that (1) the blade width has a greater influence on the normal force than on the rolling force. The rock-breaking resistance of the 12-mm-blade-width cutters under extremely hard rock conditions is 36.7% lower than that of conventional cutters. (2) Under the same conditions, the smaller the blade width is, the smaller the specific rock-breaking energy consumption is and the smaller the optimal cutter spacing is. Under extremely hard rock condition, the specific energy consumption of the 12-mm-blade-width cutters is 37.9% lower than that of conventional cutters. (3) With a certain gross thrust of a single cutter, the rock-breaking quantity of a single 12-mm-blade-width cutter is much larger than that of the 19-mm-blade-width cutter, regardless of the types of rock. Under extremely hard rock conditions, the S/P of the 12-mm-blade-width cutter with the highest rock-breaking efficiency is approximately 10–12.

Summary With the increasing demand for oil, gas, and geothermal resources worldwide, the efficient and economical construction of wells in deep and hard rocks has become very important, but conventional mechanical drilling technology cannot achieve this. In this paper, a new noncontact combined thermal spallation and melting technology by plasma jet is proposed. This technology can excavate rock materials by disintegrating brittle rocks into small fragments and melting plastic rock. Especially for hard granite, this method exhibits high rock removal efficiency with low specific energy. Furthermore, the plasma bit is not in contact with the rock, thus avoiding bit wear. A thermal spallation and melting experiment system is established, and laboratory tests are conducted. The influence of plasma current, plasma gas flow rate, confining pressure, and the types of rock-on-rock removal efficiency and specific energy are also researched. Results show that this novel technology can efficiently remove hard rocks, and hard granite is more likely to be removed under confining pressure than under the absence of confining pressure, indicating the feasibility of accessing geothermal, oil, and gas resources from deep and hard rock formations.

Full-scale linear cutting tests to study the influence of pre-groove depth on rock-cutting performance by TBM disc cutter

This paper is concerned with the application of artificial neural networks (ANNs) and regression analysis for the performance prediction of diamond sawblades in rock sawing. A particular hard rock (granitic) is sawn by diamond sawblades, and specific energy (SE) is considered as a performance criterion. Operating variables namely peripheral speed (V p), traverse speed (V c) and cutting depth (d) are varied at four levels for obtaining different results for the SE. Using the experimental results, the SE is modeled using ANN and regression analysis based on the operating variables. The developed models are then tested and compared using a test data set which is not utilized during construction of models. The regression model is also validated using various statistical approaches. The results reveal that both modeling approaches are capable of giving adequate prediction for the SE with an acceptable accuracy level. Additionally, the compared results show that the corresponding ANN model is more reliable than the regression model for the prediction of the SE.

Effect of confining pressure on rock breaking by high-pressure waterjet-assisted TBM