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Putting rotary drilling into perspective

rotary drilling

Atlas Copco’s largest drill, the Pit Viper 351E, operates on a blast pattern at an open pit copper mine. Rotary blasthole drills are the predominant method of drilling 9 inch (229 mm) diameter holes or greater.

A complete range
With the acquisition of Ingersoll-Rand’s Drilling Solutions, Baker Hughes Mining Tools (BHMT) and Thiessen Team businesses, Atlas Copco has a complete range of products to offer to large quarries and open pit mines. Much of the world’s mining output begins through drilling of holes with rotary drills. Ingersoll-Rand built air-powered rotary drills for many years prior to the introduction of their first fully hydraulic unit, the T4, in 1968.
About rotary drills
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-thehole (DTH) drilling. High-pressure air compressors are used to provide compressed air through the drillstring to
drive the DTH hammer (see illustration page 20). 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 via a feed system. Feed systems utilize cables, chains or rack-andpinion mechanisms driven by hydraulic cylinders, hydraulic motors or electric motors. The preference at Atlas Copco is to use cables for pulldown, as they are lightweight and inexpensive, and
allow easier detection of wear to help avoid catastrophic failures.
rotary drilling
The tower supports the drill string during drilling as well as the rotation head and feed system

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.
More weight with rotary
It only takes one look to see that the biggest DTH and tophammer drill rigs are very different than the biggest rotary blasthole rigs. In fact, the Pit Viper 351 rotary drill rig weighs in excess of nine times that of Atlas Copco's largest DTH
hammer drill rig, the ROC L8. Yet the Pit Viper 351 is drilling a hole that is generally only twice the diameter. Take
a typical medium formation tricone bit with a recommended maximum loading of 900 kg/cm of bit diameter (5000 lb per inch of diameter). With a 200 mm (7-7/8 in) bit, you could run about 18,000 kg (40,000 lb) of weight on the bit. The laws of physics dictate that for every action, there is an equal and opposite reaction, meaning that if you push on the ground with 18,000 kg (40,000 lb), the same force will push back on the unit. There-fore, the weight of the machine must be over 18,000 kg (40,000 lb) at the location of the drill string to avoid the machine “lifting off” the jacks. To achieve a stable platform through proper placement of the tracks and levelling jacks, the distribution of weight results in an overall machine
weight that approaches or exceeds twice the bit load rating. This weight does add cost to the machine, but the size of the components also translates to long life. Even smaller rotary blasthole drills are built to run 30,000 hours of operation,
and some of the large blasthole drills have clocked over 100,000 hours of use.
rotary drilling
Rig design
With the exception of one model, the rubber-tire mounted T4BH, Atlas Copco’s rotary blasthole drills are mounted on excavator-style undercarriages. Powerful hydraulic-drive systems allow the machine to tram over a variety of ground
conditions, though rotary blasthole drills should always operate on firm, flat benches. The key component of a rotary blasthole drill is the tower, which is sometimes referred to as the derrick or mast. Atlas Copco towers are four main member, open front structures in which the rotary head slides up and down via a guide system. The length and weight of the tower ultimately dictates the size of the mainframe and undercarriage. 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 emissions, no refueling requirement and less maintenance. However, some operations are not setup with the proper electrical infrastructure or staffing to run electric units. Even when electric power is available, many customers avoid electric drills as the trailing cable used to provide power makes it harder to move the unit between holes
or patterns. Generally, electric power is preferred on large single-pass units used in major open pit metals mines where electric shovels are employed, though electric power is now available on smaller units such as the Atlas Copco Pit Viper 271, Pit Viper 275 and DML.
rotary drilling
The importance of air
A key parameter of rotary drilling is flushing the cuttings from the hole. In most rotary blasthole drills, cuttings are lifted between the wall of the hole and the drill rods by compressed air. Sufficient air volume is required to lift these cuttings. Many types of tricone bits have been developed to meet various drilling needs. Softer formation bits are built with long carbides with wide spacing on the face of the bit. This design yields large cuttings which increase drill speed and reduce dust. It is important to have sufficient clearance between the wall of the hole and the drill rods in order for such large cuttings to pass. If this clearance, known as annular area, is not sufficient, the cuttings will be ground between the wall of the hole and the rods or by the bit itself (called regrinding) until they are small enough to exit the hole. This results in excess dust and accelerated wear on the bit and drill rod.
Bailing velocity
A traditional rule-of-thumb is a minimum of 1,525 m3/min (5000 cfm) of uphole velocity, the speed at which air exits the hole. The actual amount of air required will vary widely based on the density of the material and the size of the cuttings. Dense cuttings as found in iron ore mines will settle much quicker than lightweight overburden in coal mines and thus need more air coming up the hole to lift them; 1,525 m3/min (5000 cfm) may not be enough. However, harder material is generally drilled with hard formation bits that utilize shorter cutting structures, thus yielding smaller chips. Conversely, some soft material can be drilled effectively with only 915 m3/min (3000 cfm) uphole velocity. Unfortunately, many operations have tried to increase uphole velocity by increasing the diameter of the drill rod. This is obviously much easier than getting a larger air compressor by retrofitting or purchasing a new machine. In some conditions, this strategy works, but more often, the reduced annular area results in increased wear and dust, and the drill rate may even drop. Whatever
the application, it is critical to have proper bailing air.
rotary drilling
Dust control
A necessary evil created by the air compressor in drilling operations is the generation of dust. To control the dust, the area surrounding the hole is enclosed by a dust hood. Dust hoods are sealed on the sides by dust curtains, and where the rod comes through the deck by a rod wiper or dust seal. A dust control system must be used in conjunction with the dust hood and curtains. The two most popular types of dust control are dry dust collectors and water injection. Dust collectors are essentially large vacuum cleaners that pull the dust away from the dust hood and run it through a collection of filter elements. Water injection systems inject a fine amount of water into the air stream. Water injection is the more effective solution for ensuring dust is minimized, but the introduction of water into the hole can slow down the drilling process by increasing the density of the cuttings at the bottom of the hole that the air compressor must move. Water injection systems require frequent refilling of the water tanks, and in freezing conditions, elaborate heating systems must be used. Dust collectors offer a productivity advantage, but they can become plugged if not turned off when wet material is encountered. This is particularly a problem if the wet material freezes in the system.
When rotary is better
Every drilling application is different, so we cannot say that there are particular breakpoints where you should transition between drilling methods. Generally, drilling below 152 mm (6 in) is best accomplished with tophammer units. Above this diameter, it is typically done with a rotary rig, although tophammer units are doing some of this work effectively with the introduction of larger platforms and more powerful rock drills. For harder material, say above 100 MPa (15,000 psi), unconfined compressive strength (UCS), DTH is often faster than pure rotary drilling if provided there is enough air pressure on board. Simply looking at our product range (see above) gives an indication of which methods are commonly used for the different diameters found in construction and mining. There are certain limitations imposed on each method of drilling. With tophammer percussive drills, the power of the rock drill itself limits the ability to transmit adequate force to larger diameter bits, especially at deeper depths when percussive energy is successively reduced with each new rod connection. Down-the-hole (DTH) tools solve this energy loss problem, but their maximum hole diameter is limited by the
volume of air. To build the air pressure that translates directly to impact energy, a certain volume of air is required. Take for example a Secoroc QL80 203 mm (8 in) DTH hammer that is designed to operate at 25 bar (350 psi). Even with our largest high pressure compressor 686 41 m3/min (1,450 cfm), the pressure will only build to 23 bar (325 psi), thus  providing less impact energy. In real terms, each blow of the piston is about 45 kg (100 lb) less than it is designed
for. In some cases, this method will still outperform rotary drilling. For most large diameter blasthole drilling, there is simply not enough air on-board for a DTH to be as cost effective as rotary drilling with a tricone bit. Rotary drilling is still the predominant method of drilling 230 mm (9 in) diameter or greater. This is driven primarily by the current limitations of tophammer units and rig air systems. Tricone bits also become more cost effective as the larger bits are equipped with larger bearings which in turn can handle higher loads. These higher loads translate to improved drill rates. Another advantage of rotary rigs is the length of the drill rods that can be carried on board. Longer rods mean fewer connections. Smaller rotary blasthole machines utilize 9.1 meter (30 ft) length rods, while larger units are capable of running 10.7 meter (35 ft) or 12.2 meter (40 ft) rods. By comparison, tophammer or DTH crawler drills use drill steel that is generally 6.1 meters (20 ft) or less in length. Further, some rotary rigs are large enough to handle a long tower that enables drilling of the entire bench height in a single-pass. At the largest open pit mines, rotary units are drilling 20 m (65 ft) deep holes in a single-pass to match the bench heights dictated by the large electric shovels that can dig a 17 m (55 ft) bench.
Productivity versus cost
Studies have shown that pure penetration rate will increase linearly with increased pulldown. The same has also been said of rotation speed. So why doesn’t every operation use more of each? Unfortunately, higher pulldown and rpm usually results in increased vibration and lower bit life. The vibration causes increased wear-and-tear on the rig, but more importantly, it creates a very unpleasant environment for the operator. What invariably happens is that the operator reduces the weight or rpm until the vibration returns to a comfortable level. Some operations limit bit load and rpm even if there is no vibration in order to improve bit life. This is often the wrong strategy as the overall drilling cost per unit, also known as Total Drilling Cost (TDC), should be considered. TDC is calculated using the bit cost per meter/foot and the total rig cost per hour. The unit cost per hour includes labor, maintenance and power, and possibly capital cost. The drilling speed really doesn’t impact this cost-per-hour figure. What it does impact though is the cost per unit produced cost/meter/foot, cost/ton, etc…). You generally want to push the rig harder to reduce the cost/foot, but there will be a point where the rig overloads the bits (see diagram).
rotary drilling

rotary drilling

Large versus small
There are some drawbacks to rotary rigs. Smaller crawler rigs are more flexible with many advantages such as articulating and extendable booms and guides that allow drilling at many different angles. Unlike crawler rigs, the components on rotary rigs are often not enclosed. They are mounted onto the frame in an open layout that makes them extremely easy to service. Large electric units normally have a machinery house to protect the electrical drive components, and newer midrange sized blasthole units such as the Pit Viper 235 have the option of a machine enclosure. The general trend for 165 mm (6-1/2 in) or less is towards the smaller, more flexible units. However, many large scale quarries and small mines still favor the durability, life and simplicity of the larger rotary rigs for these small diameters. For the large scale open pit operations that yield a high percentage of the total worldwide mineral production, it is anticipated that rotary drilling will remain the primary method for years to come.

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