Basics of drilling 2


Published on

Published in: Technology, Business
No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Basics of drilling 2

  1. 1. BASICS OF DRILLING-2 I. Satyanarayana, M.Tech,MBA,MCADy. Manager, Project Planning, SCCL,, visit ISN
  2. 2. Rotary drillsIn rotary drilling, the drill bit attacks the rock withenergy supplied to it by a rotating drill stem.The drill stem is rotated while a thrust is applied toit by a pull-down mechanism using up to 65% of theweight of the machine, forcing the bit into the rock.The drill bit breaks and removes the rock by either aploughing-scraping action in soft rock, or acrushing-chipping action in hard rock, or by acombination of the two. ISN
  3. 3. Compressed air is supplied to the bit via the drill stem. The air both cools the bit and provides a medium for flushing the cuttings from the hole.Water may be used in addition to the compressed air to suppress the effects of dust, however, this is normally found to have a detrimental effects on bit wear.Blast hole sizes produced by rotary machines vary in the range of 100 to 445 mm diameter with the most common sizes being 200, 250, 311 and 381mm.These drills usually operate in the vertical position , although many types can drill up to 25 or 300off the vertical.Drills are manufactured that can drill horizontal holes used in overburden stripping where hard bands of material are located low in the high-wall face. ISN
  4. 4. PRINCIPLES OF ROTARY DRILLINGRotary drilling can be subdivided into rotary cutting and rotary crushing.Rotary cutting creates the hole by shear forces, breaking the rockstensile strength. The drill bit is furnished with cutter inserts of hard metalalloys, and the energy for breaking rock is provided by rotation torque inthe drill rod. This technique is limited to rock with low tensile strengthsuch as salt, silt and soft limestone not containing abrasive quartzminerals.Rotary crushing breaks the rock by high point load, accomplished by atoothed drill bit, which is pushed downwards with high force. The bit,being of tricone roller type fitted with tungsten carbide buttons, issimultaneously rotated, and drill cuttings are removed from the holebottom by blowing compressed air through the bit. Drill rigs used forrotary drilling are large and heavy. The downwards thrust is achieved byutilising the weight of the drill rig itself, and the rotation, via a hydraulicor electric motor, applied at the end of the drill pipe. Common holediameters range from 8 to 17½ in (200-440 mm) and, because adding theheavy drill pipes is cumbersome, most blast hole drill rigs use long mastsand pipes to accommodate single-pass drilling of maximum 20 m (65 ft). ISN
  5. 5. Electric power is usually chosen for the large rigs, whereas smaller rigsare often powered by diesel engines. Rotation rates vary from 50 to 120rpm, and the weight applied to the bit varies from 0.5t/in of bit diameterin soft rock, to as much as 4 t/in of bit diameter in hard rock.Recent technical advances include:Improved operator cab comfort; automatic control and adjustment ofoptimum feed force and rotation speed to prevailing geology and bit typeand diameter; and incorporation of latest technology in electric andhydraulic drive systems. Rotary drilling, which is still the dominantmethod in large open pits, has limitations in that the rigs cannot drillholes off the vertical line. Many rotary rig masts have pinningcapabilities permitting drilling at angles as much as 30 degrees out of thevertical. However, the inclined hole drilling capabilities in rotary drillingare limited by the heavy feed force required, since part of this force isdirected backwards. This causes rig stability problems, reducedpenetration, and shorter life of drilling consumables. Consequently, mostblast hole drilling using rotary drill rigs is in vertical holes. ISN
  6. 6. (1) Rotary drill penetration rateOne of the most important factors in drilling is howfast can drill hole be produced while the machine isactually drilling.This factor almost entirely influences productivityand has a strong influence on unit costs of the hole.The penetration rate of the rotary drill is about15m/hr for the size of 250mm hole. ISN
  7. 7. (2) Rotary drill rotation speedThe rotary drive motor turns the drill tool stringthus turning the drill bit at the bottom of the hole.As the rotary speed increases, so does the numberof contacts and the penetration rate.The limit to rotary speed is hot bearings in the bit。Current rotary speeds ranges from 60 to 90 rpm forhard materials with greater speeds for softer rocks ISN
  8. 8. (3)Rotary drill pull-down weight A portion of the machine weight is applied through the pull-down motor via the pull-down chain or chains, rotary head and drill stems to the drill bit.As the bit diameter increases, the bearing size increases thus allowing an increase in the tolerable load. Overloading the bit results in severe loss of bit life ISN
  9. 9. (4) Rotary drill bits Tri-cone rotary drill bit has evolved from the drag bit and two cone bits.Mill, or steel tooth bits are used for soft rock with the cutoff being a medium-strength limestone.The average footage per bit is about 800 for rock with compressive strength of 80MPa.The larger bit sizes, incorporating larger bearings, etc. can be expected to give higher footages per bit. This has not always proved the case. These footages are averages for the highest-grade tungsten carbide bit of different manufacture, the ore having variable compressive strengths approximately 204Mpa. ISN
  10. 10. (5) Rotary drill air requirements Air is used to bail the drill cutting from the hole as well as cool the bit bearings and, when used, roller stabilizer bearings. The air volume is the primary requirement for bailing cuttings from the hole. Air velocity up the hole is dependent on the air volume per minute as well as the hole annulus. ISN
  11. 11. The velocity of the drill cuttings in this air isdependent on the chip size, density, and shapeAt air velocity above this balancing value, the chipbegins to move.A bailing velocity of 1800mpm is usually adequateto bail13mm chips. ISN
  12. 12. Choosing the higher velocity has the following advantages:1) Will bail larger chips;2) Tend to give higher bit life3) Will help cater for hole cavities, etc.4) May give higher penetration rates and possibly lower cost per ft5) Reduce the volume of cuttings in the hole for a given penetration rate.But they will give increased stabilizer and pipe wear. ISN
  13. 13. (6) Cold weather operation Drills to be used in cold climatic conditions should be purchased with a cold weather package.(7) Single pass drilling Many new mining ventures selected bench height and drill equipment to utilize single pass drilling. ISN
  14. 14. Advantages of single pass are: 1) Eliminates adding stems 2) Reduces associated thread damage 3) Reduces machine downtime for rod changing equipment and tool racks 4) Facilitates the cleaning of boreholes 5) Permits a continuous air flow through the bit at all times. This is especially important in wet holes as it eliminates the possibility of siphoning water and cuttings up into the bit. ISN
  15. 15. Disadvantages:1) High masts make the drill more unstable, back brace become almost mandatory,2) Extra care needs to be taken when moving the drill over medium or long moves,3) Pull-down chains become long and may require special attention. ISN
  16. 16. For operations choosing to reduce bench height inorder to attain single pass drilling, the followingother benefits may also be experienced:1) Multi-row blasts become easier to blast,2) Drill cuttings volume is reduced (especially for the larger holes),3) Contour areas are easier to drill and blast,4) Ore grading is easier. ISN
  17. 17. (8) Rotary drill productivity and costsThe larger the sizes of the hole, the greater the productivity of the rotary drill.(9) Drilling trends Rotary drills have increased their dominance of blast-hole production in open pit mining. The trend has been to larger sturdier drills, to yield higher mechanical availability and operating performance. This increased availability has been achieved through improvement of crawler track frames, masts, propel chain, pull-down mechanisms, rotary head drives complete with automatic lubrication and greasing, on the new models of machine. ISN
  18. 18. ISN