Cost-effective Drilling Comes at a Price—But Pays Big Dividends
Recent conference highlights the information needed to plan, drill and shoot efficiently
By Russell A. Carter, Managing Editor
As described by an experienced appli-cations engineer for one of the major drill-equipment suppliers, D&B is "all about putting the right amount of energy in the right place at the right time at minimum cost to achieve maximum control over the shot rock volume and the resulting particle size distribution in the muck pile." The benefits of a well-designed blast—or the repercussions of a poorly executed D&B plan—reverberate far away from the actual blast site, as shown in the accompanying diagram that depicts how various elements of D&B practice can influence downstream operations
. Although the physics of sinking a sim-ple hole into the ground seem straightfor-ward, the path to consistently effective D&B strategy meanders through a thicket of thorny issues that demand attention, ranging from an understanding of local geo-logical conditions, proper drilling equip-ment selection and climate considerations, to the type of explosives required or locally available, for example. Accompanying those considerations are other factors such as volume of material to be excavated according to mine plan, hole diameter, optimum bench height, stemming material source, fragmentation requirements, and desired level of equipment utilization and availability, among others.
Adding to the technical difficulty is the quick pace of daily job duties, technologi-cal progress and product introductions that can make it hard for mine personnel to stay current on best practices for D&B success. In October, Sandvik Mining & Construction convened its first Mining Forum, aimed at bringing participants up to date by focusing on fundamentals as well as recent technological developments in surface-mine drilling and blasting oper-ations. The three-day event, which includ-ed 112 participants from 25 mineral pro-ducers and mining contractors, was held in Johannesburg, South Africa, against the backdrop of the African continent's vast mineral potential—and equally immense needs for drilling and blasting equipment, techniques and management strategies to effectively cope with its widely varying mine-site conditions.
Realizing Regional Potential
Although most forum presentations addressed specific aspects of D&B prac-tice, leadoff speaker Chris Brindley, presi-dent of Sandvik Mining & Construction Region Africa, began by highlighting Africa's strengths and weaknesses as they relate to the global mining industry. Noting that it's somewhat difficult to men-tally grasp the sheer size of the continent, Brindley displayed a slide showing how outline maps of the United States, Western Europe, China, India and Argentina could all be superimposed upon a map of Africa—with room to spare. Its 30.3-million km2 of land area contain mineralization currently representing about 90% of the world's known platinum resources, 80% of its chromite, 65% of diamonds and 40% of gold. Its 2010 esti-mated population of 1.013 billion people account for 14.8% of world population—a share that is expected to grow to 24% by 2050—and 60% of the current popula-tion is under the age of 24.
The positive or negative effects of a mine's drilling and blasting methods extend far beyond the blasthole
or blast pattern. (All figures courtesy of Sandvik Mining and Construction.)
However, the challenges facing African economic development are equally expan-sive, including the threat of nationalization of private assets, political corruption and instability, the prospect of increased taxa-tion on mining, a shortage of skilled work-ers and difficult logistics.
"Africa is probably the wealthiest conti-nent in the world when it comes to miner-als," said Brindley. "But as you can see, it faces a lot of challenges. Until [these chal-lenges] are resolved, it will be difficult to get major capital funding for projects in this part of the world."
On the other hand, he noted, "Africa is one of the best places in the world to find new orebodies."
Brindley said Sandvik currently has business operations in 12 African coun-tries, employing roughly 3,250 workers; and eight distributors throughout the continent.
In addition to the regional macro-eco-nomic trends and political issues, Africa-based mineral producers share mine-site operational challenges that are common throughout the global industry. Among these, the search for improvement in D&B economics may not be paramount in the list of cost-cutting concerns but is defi-nitely rising rapidly in importance. Several of the forum's presentations dwelt on fundamental aspects of identifying and selecting the most appropriate drilling equipment and methods for a given application.
Charles Deacon, Sandvik's vice presi-dent of marketing for the Africa region, explained that D&B activities may account for as much as 15% of total production costs, and are actually the most control-lable of these costs. Across an entire oper-ation, D&B can affect excavation rates, cost of loading, secondary breakage requirements, ore grade dilution, process-ing rates, slope stability concerns and mine site safety.
One of the basic informational needs for determining the best drilling approach for the application, said Deacon, is knowl-edge of rock mass "drillability"—defined by three factors: drilling rate (penetration), bit wear rate (time elapsed between regrinds), and bit life (distance drilled before reaching end of economic bit life). The most well-known indicator of drillabil-ity is the Drilling Rate Index, a relative measure of penetration rates in a given rock type. DRI is determined by two com-mon tests that measure rock toughness and rock surface hardness. In general, the lower the DRI, the lower the drilling rate that can be expected, and vice versa.
Armed with knowledge of local rock characteristics, the customer still faces a long list of factors that must be considered when choosing the proper drilling method. These involve both technical and commer-cial issues, according to Deacon, and include:
Recommended effective material and hole-diameter ranges for each major drilling method.
• Hole diameter
• Hole depth/bench height
• Production rate
• Size of operation
• Special techniques required
• Legal requirements – dust, noise, etc.
• Rock hardness
• Hole angle
• Power availability
• Fleet size
• Economic life
• Technical support
• Parts supply
• Operating cost
Putting Together the Right Rotary String
For those operators considering rotary drilling methods, Mark Baker, Sandvik's global product line manager for rotary tools, highlighted the physical limits of the equip-ment and the importance of using the prop-er drill string and bit setup. He emphasized that effective control of the feed and rota-tion applied by a rotary drill rig are essen-tial to productive and cost effective opera-tion of the drill. Excessive loading by either parameter will significantly reduce con-sumable life and increase mining costs.
In addition, careful selection of every drill
string component is vital to achieve accurate
holes, optimal fragmentation and opera-tional efficiency. A complete rotary drill
string assembly can include the following:
• Shock sub (optional) – Recommended for use in applications with high levels of axial and lateral vibration (>10g) such as drilling in fractured formations. Benefits include increased drill availability, re-duced mast maintenance and less rotary drive head repairs, smoother on-bottom running and improved torque control.
• Top sub – The connection between the drill pipe and rotary motor or shock sub.
• Drill pipe – Based on the outer blasthole diameter, a proper drill string OD should be selected that will provide the neces-sary column support to reduce flexing, as well as sufficient annular area for cut-tings evacuation.
• Deck bushing – Guides the drill string, reduces risk of wobbling, prevents reduc-tion of rotary head torque and supports drill string configuration in producing straight holes.
• Bottom sub or stabilizer – Allows for con-nection of the bit to the pipe. Roller sta-bilizers are used for improved hole sta-bility in hard and broken formations, where hole caving is prevalent. Blade stabilizers are used in softer formation where the gauging and scraping of the hole wall improves hole quality.
• Rotary bit – Proper drill bit selection is vital for achieving desired results. Pay attention to factors such as ground condi-tions (rock hardness, abrasiveness, com-petency and ground water); study product specifications and local availability; deter-mine correct cutting structure, bearing configuration (sealed or air-cooled) and air-nozzle sizing for site conditions.
Baker cited several case studies in
which changeover to a properly configured
drill string produced significant results,
• A copper-gold mine at which average drill pipe life without a shock sub was 25,000–30,000 m with eventual break-downs usually due to thread failure from vibration. With shock subs in place, drill pipe life increased to 42,000 m, with end failure resulting from eventual sur-face erosion.
• In another application, drill pipe conver-sion from 40/20 ft to 33 ft (x2) resulted in less handling, improved ease of rota-tion and longer service life. Savings amounted to S270,000 per year, primarily from improved efficiency.
Turning Money into Air—and Vice Versa
Compressed air requirements differ among drilling methods. Rotary drilling requires low-pressure, high-volume air fed through the center of the drill pipe to the bit for hole cleaning (cuttings removal) and bearing cooling. Similarly, top hammer drill-rig com-pressor capacity is calculated according to hole-cleaning requirements, but with DTH drilling the rating of the hammer defines the required compressor capacity. Whether a customer chooses rotary or percussion drilling, it's important for them to under-stand and know how to determine the right compressed-air volume and pressure for the selected drilling application, explained Karl Ingmarsson, vice president of marketing for Sandvik Mining & Construction. At a mini-mum, the user should be familiar with the following concepts:
• The purposes of compressed air in drilling.
• How to make a quick and simple calcu-lation of correct up-hole velocity.
• Why sufficient volume is required for a DTH hammer to perform well.
• How to match rod or tube size to topham-mer bit sizes.
• How to estimate cutting settling velocity and target exit velocity.
• Why air nozzle selection is important for rotary tools.
• How to interpret in-cab pressure readings.
• How to compensate for high altitude.
Stating that "air is money," Ingmarsson provided examples of how much fuel a typical, small DTH drill rig would burn in its lifetime (at 70 l/hr and average load factor of 74%, roughly 2.8 million l or 743,000 gal), or a large rotary blasthole drill (at 140 l/hr with same load factor, about 5.6 million l or 1.48 million gal)—of which about 2.3 million l and 4.5 million l, respectively, would be con-sumed to run the rig's compressor alone.
And with so much fuel being burned to provide compressed air, is that air being used economically?
Not usually, explained Ingmarsson. In recent years, as diesel engine OEMs built better monitoring systems into their prod-ucts, a rig's nondrilling fuel-burn rate has become much more noticeable. Traditional rigs, when in drilling mode, provide maximum air volume regardless of actual drilling conditions; when not drilling, they maintain maximum pres-sure, thus loading the engine for no par-ticular benefit.
After an extended effort to find ways to alleviate this problem, Sandvik recent-ly introduced its Compressor Manage-ment System (CMS), designed to reduce fuel consumption, extend engine life and reduce associated drilling costs by elec-tronically managing compressor operation to provide the necessary amount of air required at all times, ensuring the com-pressor runs at full volume only when needed (See E&MJ, May 2011, "New System Manages Main Compressor on Rotary Drills," pp.30-34). It also provides continuous feedback to the operator on downhole conditions and indicates how CMS is responding to current drill demands.
According to Ingmarsson, CMS is cur-rently available as a retrofit for Sandvik's rotary drill rig models, and will be avail-able for DTH rigs in 2012.
Common sources of drilling error.
Playing it Straight
No matter what drilling method is select-ed, overall D&B performance will suffer unless holes are drilled straight and according to plan, from collar to bottom. When an operation "drills holes that look like spaghetti," according to Arne Lislerud, surface applications manager for Sandvik, it can expect:
• Floor humps, hindering efficient loading due to uneven pit floors;
• Unstable pit walls and difficult first-row drilling;
• Safety concerns from flyrock;
• Stemming material blowouts that gener-ate safety, excessive dust and "bad toe" concerns;
• Poor blast direction, affecting quality of floors and walls;
• Shothole deflagration and/or misfires that produce safety hazards and poor muckpile diggability.
The keys to achieving consistent straight hole drilling, said Lislerud, are simple: Be aware of the numerous issues that lead to drillhole deviation; operate with a technically sound drill rig, drill string and instrumentation; and motivate drillers to strive for best results. Good practice dictates only 2%-3% maximum drillhole deviation in regular production drilling operations.
For collar position error control, Lislerud recommends:
• Using tape, optical squares or alignment
lasers for measuring-in collar positions; or
• Using GPS or total stations to measure collar positions;
• Marking collar positions using painted lines, not movable objects such as rocks, etc.;
• Protecting completed drillholes with shothole plugs to prevent holes from cav-ing in (and filling up); • Using GPS guided collar positioning devices, such as Sandvik's TIM3D drill rig navigation system. Similarly, to control drill-hole deflection:
• Select bits less influenced by rock-mass discontinuities;
• Reduce drill string deflection by using guide tubes, etc.;
• Reduce drill string bending by using less feed force;
• Reduce feed foot slippage since this will cause a misalignment of the feed and lead to excessive drill string bending;
• Avoid gravitational effects that lead to drill string sag when drilling inclined shotholes (>15°);
• Avoid excessive bench heights.
Choosing the proper bit face design can enhance drill-hole straightness, he also noted. When a percussion bit first starts to penetrate through a rock-joint surface at the hole bottom, for example, the gauge buttons tend to skid off this sur-face and thus deflect the bit. More aggres-sively shaped gauge inserts (ballistic / chisel inserts) and bit face gauge profiles (drop center) reduce this skidding effect by enabling the gauge buttons to "cut" through the joint surface quickly, thus resulting in less overall bit deflection.
The right bit-skirt design also helps: As the bit cuts through a joint surface, an uneven bit face loading condition arises; resulting in bit and drill string axial rotation that is proportional to bit impact force imbalance. A rear bit skirt support (retrac type bits) reduces bit and string axial rota-tion by "centralizing" the bit.
Other deviation countermeasures include using a longer bit body, adding a pilot tube behind the bit, using lower impact energy, or employing a drilling control system that can rapidly react to varying torque, feed and percussion or pulldown demands based on hole conditions.