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Basics of drilling 1

  1. 1. BASICS OF DRILLING-1 I. Satyanarayana, M.Tech,MBA,MCA Dy. Manager, Project Planning, SCCL,, visit at ISN
  2. 2. Mine operation Drilling Blasting Overburden Removal Loading Haulage Reclamation Water and Air Management ISN
  3. 3. DRILLING In virtually all forms of mining, rock is broken through drilling and blasting. Except in dimension stone quarrying, drilling and blasting are required in most surface mining. Only the weakest rock, if loosely consolidated or weathered, can be broken without explosives, using mechanical excavators (ripper, wheel excavators, shovels etc.) or occasionally a more novel device, such as a hydraulic jet. ISN
  4. 4. In the mining cycle, drilling performed for the placement of explosives is termed production drilling. Drilling is also used in surface mining for purposes other than providing blast-holes. There are minor applications of rock penetration in surface mining other than drilling. In quarrying, dimension stone is freed by cutting, channeling, or sawing. ISN
  5. 5. 2. Classification of methods A classification of drilling methods can be made on several bases. These include size of hole, method of mounting and type of power. The scheme that seems the most logical to employ is based on the form of rock attack or mode of energy application leading to penetration. ISN
  6. 6. (1) Mechanical attack The application of mechanical energy to rock can be performed basically in only one of two ways: by percussive or rotary action. Combining the two results in hybrid methods termed roller-bit rotary and rotary-percussion drilling. The mechanical category, of course, encompasses by far the majority (probably 98%) of rock penetration applications today. In surface mining, roller-bit rotaries and large percussion drills are the machines in widest current use, with rotaries heavily favored. ISN
  7. 7. (1) Mechanical attack Generally speaking, percussion drilling with top hammers is appropriate for drilling small holes in hard rock, DTHs for large holes in hard rock, and rotary drilling for large holes in soft to medium hard rock. When drilling in rock, the sharp end of the drilling tool, or bit, is driven into the rock by means of a dynamic (percussion, in which the bit rotates slightly in response to each stroke) or static force (rotation). The material excavated by the bit is stripped out of the hole by shearing as the bit advances. The bit generally has a knife-edge made of steel alloy or an inserted carbide tip, or is composed of multiple round, conical, ballistic, or parabolic button tips made of hard alloy that are inserted into the leading face. • In some cases, the rotary cutting method can be used without percussion in drilling very soft rock such as coal or limestone that contains little silica. In the rotary crushing method, a three-cone bit similar to that employed in oil well drilling is used. The bit is constructed of three cones covered in variously shaped teeth or buttons that rotate freely like planetary gears and crush the rock as the drilling rig rotates the bit. • Teeth made of steel alloy are used at shallow levels in soft rock, and carbide buttons are used for hard rock. • Air is blown through the drill-rod to remove cuttings and to cool the rotation ISN bearings and the bit.
  8. 8. (1) Mechanical attack It is important to note that rotary drills are capable of two methods of drilling. The majority of the units operate as pure rotary drills, driving tricone or fixed-type bits. The fixed-type bits, such as claw or drag bits, have no moving parts and cut through rock by shearing it. Thus, these bits are limited to the softest material. The other method utilized by rotary drill rigs is down-the hole (DTH) drilling. High- pressure air compressors are used to provide compressed air through the drill string to drive the DTH hammer. The primary difference between rotary drilling and other methods is the absence of percussion. In most rotary applications, the preferred bit is the tricone bit. Tricone bits rely on crushing and spalling the rock. This is accomplished through transferring downforce, known as pulldown, to the bit while rotating in order to drive the carbides into the rock as the three cones rotate around their respective axis. Rotation is provided by a hydraulic or electric motor-driven gearbox (called a rotary head) that moves up and down the tower derrick or mast) via a feed system. Feed systems utilize cables, chains or rack-and pinion mechanisms driven by hydraulic cylinders, hydraulic motors or electric motors. The preference is to use cables for pulldown, as they are light weight and inexpensive, and allow easier detection of wear to help avoid catastrophic failures. Pulldown is the force generated by the feed system. The actual weight on bit, or bit load, is the pulldown plus any dead weight such as the rotary head, drill rods and cables. ISN
  9. 9. Schematic drawings of three types of drill A) top hammer, B) DTH, C) simple rotary drill a) tip, b) bit, c) rod, d) sleeve, e) drill pipe, f) piston, g) cylinder, h) percussion mechanism, i) rotation mechanism, j) flushing ISN
  10. 10. Down-The-Hole method Rotary drilling method ISN
  11. 11. (1) Drill Adaptability The type of rock drill used in mining drilling is generally determined by the hole diameter required and the mechanical properties, principally hardness, of the rock. A top hammer is commonly employed for drilling of holes less than 125 mm in diameter in all but the hardest rock, although a rotary cutting method may instead be used for soft rock. For hole diameters greater than 125 mm, rotary crushing with a three-cone bit is used in rock weaker than an upper limit that depends on the diameter of the hole, and DTH drilling is used for harder rocks. If rotary drilling is feasible, drilling performance will generally be higher than obtainable with DTHs. In practice, small-diameter drilling is required in underground mining and tunneling, medium-diameter drilling in quarries, and large-diameter drilling in large-scale open pit mining. • For large-scale open pit mining, it is common to use rotary drilling rigs equipped with three-cone bits or DTHs to drill blastholes. These tools are mechanically compatible with each other, and make it possible to choose the most efficient method according to the particular rock hardness. • Most drilling functions are hydraulically driven. Powering these hydraulic systems, along with the air compressor, is a diesel engine or electric motor. • Most rotary drills are diesel powered for good mobility. Electric powered units offer some advantages such as lower power cost (in most areas), no diesel ISN emissions, no refueling requirement and less maintenance.
  12. 12. a) underground mining and tunneling, b) quarrying, c) strip mining ISN
  13. 13. Construction of Drilling Equipment The essential components of a drilling system are • the rock drill, • feed equipment, • drilling rods, • bit, • supports against the drilling reaction, • power source, and • cuttings disposal equipment. a) rock drill, b) feed equipment, c) drilling rod, d) bit, e) support, f) operating media, g) cuttings discharge system, h) carrier, i) accessories ISN
  14. 14. Thrust and Feed Equipment All drilling methods require a feeding system that pushes the drilling tool against the rock, maintains the advance of the bit as drilling progresses, and withdraws the tools once drilling is complete. A heavy weight or a pneumatic (pusher-) leg is used with hand-held rock drills. For larger mechanized rock drills, whether pneumatic or hydraulic, the feed equipment is designed so that the drill sits on a carriage that travels along a feeder called the guide shell. An alternative system used with light pneumatic drills comprises a screw that is inserted into a nut built into the drill body itself and rotated by a feed motor that drives the drill bit (screw feed). Most feed systems, however, are either chain or rope feeds. The chain feed incorporates a chain running along the guide shell that is driven by a pneumatic or hydraulic motor via a rotating sprocket. In a rope feed, the expansion and contraction of a hydraulic cylinder is transmitted to the drill via a rope passing through a sheave connected to the feed cylinder body. In hydraulically driven drilling systems, a hose reel is usually attached to the feed equipment. An anti-jamming feed control for controlling the thrust is indispensable in hydraulic drilling systems, in order to avoid uncontrolled jamming of the bit. ISN
  15. 15. Rotation System For percussion rock drills, a mechanism is necessary to rotate the bit between blows and monitor the torque on the bit to avoid jamming. Hydraulic or pneumatic rotation motors are most frequently used to rotate the rod, Drilling Rod The role of this element is the transmission of percussion or rotation power to the bit and the removal of cuttings from the bottom of the hole. Consequently, the drilling rod must be hollow in order to transmit fluid carrying the cuttings. When drilling long holes, it is usually necessary to connect a series of drilling rods together via threads; these threads need to be easily unscrewed to detach the drilling rods, but firmly secured during drilling operation. Rods have male threads on either end and are connected to each other by means of coupling sleeves, which are short pipes tapped with female threads. Long holes are drilled by connecting a series of these rods together. Drilling rod coupling sleeve ISN
  16. 16. Cuttings Removal (Flushing) Compressed air is generally used in surface mining to discharge cuttings from the bottom of the hole, because of its efficiency in long downward-sloping holes and versatility. Care must be taken to ensure that the annular gap between the rod and the hole wall is neither too narrow nor too wide: if the gap is too narrow, it will become clogged with coarse cuttings, whereas if it is too wide the air velocity will be too low to carry the cuttings. In the latter case, the cuttings will be reground at the bottom of the hole, wasting drilling energy. ISN
  17. 17. (2) Thermal attack The only thermal method having practical application today is flame attack with the jet pierce. It penetrates the rock by spalling,an action associated with hard rocks of high free-silica content. Because of its ready capability of forming various shapes of openings, oxygen or air jet burners are used not only to produce blast holes but to chamber them as well and to cut dimension stone. Jet piercing of blast holes, however, has decreased in popularity in recent years as mechanical drills have improved in versatility and penetrability. ISN
  18. 18. (3) Fluid attack While disintegration of rock by fluid injection is an attractive concept, the end result is more likely fragmentation than penetration. To produce a directed hole with pressurized fluid from an external source, jet action or erosion appears to be more feasible, but commercial application to date is limited. ISN
  19. 19. Hydraulic monitors have been used for over a century to mine placer deposits and to strip frozen overburden, and more recently, high- pressure hydraulic jets have been applied successfully to the mining of coal, and other consolidated materials of relatively low strength. Hydraulic and mechanical attack mechanisms assist and complement one another. For large holes, the hydraulic jet alone may be competitive with drilling. ISN
  20. 20. (4)Sonic attack Sometimes referred to as vibratory drilling, this method as presently conceived is a form of ultra- high-frequency percussion. Attractive but not presently commercial, actuation of sonic devices by hydraulic, electric, or pneumatic means is possible. ISN
  21. 21. (5) Chemical attack Chemical reaction, because of the time element, may be more attractive as an accessory rather than a primary means of penetration. The use of explosives is a distinct possibility, however, and several alternative systems are under investigation. Additives to the drilling fluid, termed softeners, have shown some improvement in penetration rate in conventional drilling. (6) Other methods of attack While some attempts to employ other forms of energy (electrical, light, or nuclear) have been made in experimental or hypothetical category at present. ISN
  22. 22. 3.1.2 Drilling application 1. Percussion drills Percussion drills generally plays a minor role as compared with rotary machines in surface mining operations. Their application is limited to production drilling for small mines, secondary drilling, development work and controlled blasting There are two main types of drill mounting. The smaller machines utilize drifter-type drills placed on self-propelled mountings designed to tow the required air compressor. Typical hole sizes are in the 63 to 150mm range ISN
  23. 23. The larger machines are crawler-mounted and self-contained Drill towers permit single pass drilling from 7.6 to 15.2 m with hole sizes in the range of 120 to 229mm in diameter. These larger machines are almost exclusively operated using down- the-hole hammers Percussive drilling breaks the rock by hammering impacts transferred from the rock drill to the drill bit at the hole bottom. The energy required to break the rock is generated by a pneumatic or hydraulic rock drill. A pressure is built up, which, when released, drives the piston forwards. The piston strikes on the shank adapter, and the kinetic energy of the piston is converted into a stress wave travelling through the drill string to the hole bottom. In order to obtain the best drilling economy, the entire system, rock drill to drill steel to rock, must harmonise. ISN
  24. 24. For many years, these machines were exclusively operated using pneumatic hammers. ( pneumatic leg drill ) Recently hydraulic machines have been used in the smaller size range. The higher capital cost of these hydraulic drills is offset by lower operating costs and increased productivity compared with pneumatic machines. Another aspect that is becoming increasingly more important is the reduced noise produced by the hydraulic drills. With percussion drilling, the bit is driven into the rock by either a top hammer, in which case the drill rods transmit the impact of a blow at the surface, or a “down the hole drill” in which the hammer itself is in the hole and impacts the bit directly. ISN
  25. 25. (1) Percussion drill productivity It will be observed that penetration rate decreases with both increasing hole size and increasing rock strength. As the piston area and stroke length are fairly inflexible for a given hole size, one main thrust to help improve percussion drill productivity has been to improve the drill penetration rate by increasing the hammer operating pressure. A typical increase in penetration rate experienced when changing from 0.7Mpa to 1.7Mpa would be of the order of 200% with an approximate doubling in the actual hole production rate. ISN
  26. 26. Some attempts have also been made to improve drill penetration rate using high frequency blow. However it has been found difficult, especially at the higher air pressures. The other option is to use hydraulic machines to improve the energy available at the drill bit. The penetration rate for the hydraulic drill is shown to be from 20 to 100% higher than the pneumatic machines. • Recently, many percussion rock drills have been converted from pneumatic operation to hydraulic operation, because of associated gains in efficiency and performance. • The rig weight of percussion rock drills can be made quite light because of the small thrust required, whereas the rig weight of rotary drills strongly influences their ultimate drilling performance. ISN
  27. 27. (1) Percussion drill costs the costs are dependent on both the blast-hole size and the strength of the rock. The high cost for hammer drilling is partly a result of lower penetration rates obtained as compared with rotary machines. The penetration rate of rotary machines is about 15m/hr, while the rate of hammer drilling is about 10m/hr. ISN
  28. 28. While an increase in operating air pressure has greatly improved blast-hole penetration rates, the drilling cost improvements have been less impressive because of higher maintenance costs, lower machine availabilities, etc. The results is that high pressure 178mm hammer drilling cost is approximately 100% higher than 250mm rotary drill per unit volume of material blasted. ISN
  29. 29. A comparison between pneumatic and hydraulic surface mounted drill costs follows: Pneumatic hydraulic Investment cost 1.0 1.27 Energy cost 1.0 0.24 Drill steel cost 1.0 0.86 Overall operating cost 1.0 0.78 ISN
  30. 30. 2. Jet-piercing drills The jet-piercing process relies upon a characteristic of rock known as spallability. The rock is broken down, or spalled, as a result of differential expansion of the rock crystals by thermally induced stresses. The jet-piercing drill essentially consists of a burner fixed to a blowpipe that produces a high temperature flame (of the order of 43000F) by burning fuel oil in oxygen. ISN
  31. 31. The drills have a typical drilling depth capacity of 15.2m. In addition to fuel oil and oxygen, water is also used to cool the burner and, in the form of steam, it helps eject the spalled rock cuttings from the blasthole. Hole diameters range from a minimum of approximately 229 mm up to 457 mm. At present very few jet-piercer drills are in operation. One of the main problems with the system has been the high cost of oxygen and fuel oil which has helped to make the drill uneconomic. ISN
  32. 32. ISN