Rotary drilling
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Soft rock Significant depths up to 5,000 metersCore drilling
All soil types Drilling for soil analysis
Rotary drilling: This type of drilling digs a hole by grinding the rock with a tool under the effect of weight and rotation. It is mainly used in oil production. The cuttings are brought to the surface under the pressure of a drilling fluid that is injected into the drill string. Drilling fluid also helps consolidate the walls of the borehole, so it is not necessary to case it as the operation progresses.
Down-the-hole drilling (rotary percussion drilling or rotary hammer drilling): this type of drilling uses drilling rigs that drill with a rotary cutter (drill head) and hammer the bottom of the hole with a pneumatic hammer. The excavated material rises to the surface under the pressure of the compressed air used for the hammer. These drilling rigs are powerful and have high drilling rates, especially in rock. For artesian wells (wells where water naturally rises to the surface under groundwater pressure) or geothermal drilling, an air injection at the end of the drill pipe pushes the water up the borehole, allowing the operator to immediately determine the water flow encountered. This type of drilling is used for hard and cracked ground up to 4,000 meters deep. When the drilling bit is eccentric, the drilling can be done with a casing advancement system (installation of tubes by gravity) which reinforces the hole as the drilling progresses.
Top hammer drilling: this type of drilling uses drilling rigs that also operate on a percussion system, but one that remains outside the borehole. They are mainly used for small diameter drilling, for example for mineral exploration or the installation of tie rods in rock. This system is used for shallow boreholes, as there can be significant deviation due to the structure of the ground drilled through. These drilling rigs can operate in any direction however.
Auger boring: This is a type of rotary drilling using a worm screw (the auger). This type of drilling is mainly used for unconsolidated clay or sand terrain. The excavated material is directly brought to the surface by the worm screw. For deep drilling, it may be necessary to regularly raise the auger to empty it of its excavated material.
Core drilling: Core drilling is similar to rotary drilling, but it uses a crown to extract a sample, called a core, inside the tool.
Percussion drilling: consists in breaking the rock by dropping a tool, the drill bit, by gravity. This type of drilling can only be used vertically. To prevent the borehole from collapsing, it is necessary to case it as it is drilled. The cuttings are reassembled as soon as they interfere with drilling by replacing the drill bit with a flap valve bailer. The flap valve bailer is a type of tube equipped with a tilting flap in the lower part. This tilting flap allows the tube to be filled with excavated material which is then transported to the surface.
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Pipe jacking: is mainly used to install horizontal pipes. The tube is pushed progressively by percussion, the excavated material being removed at the end of the drilling operation either by water injection or with an auger. This technique is used in particular to avoid transmitting vibrations to the surrounding buildings or to avoid digging a trench.
One of the requests most often voiced by drill operators is for automated and remote-controlled drilling machines capable of drilling an entire work session before being removed for the blasting operation. But while control systems might theoretically enable this, new solutions are necessary for consumables to practically allow it.
Flexible and fast to set up top hammer drills are ideal for narrow and short boreholes in hard rockFlexible and fast to set up, top hammer drills are ideal for narrow and short boreholes in hard rock
"The traditional drilling bits can't last long enough, but Robit developed special bits with diamond coated carbides that can perform in the desired way. That means one drill bit being able to drill typically about 3,000 - 3,300ft (900 - 1,000m), and it will be able to maintain the drilling performance well all the way towards the end of its life without sharpening," says Vuojolainen. "While the diamond bit unit cost is clearly higher than a usual TH bit, it does offer an extremely good total cost per metre payback."
Together with the general improvements in the durability of drilling consumables, operators' ever prevalent desire to improve efficiency and the consequent continuous increase in drilling power are also creating new demands to the drilling string, especially the threads. As a response, the market is seeing new thread designs, like for example shoulder supported male/female thread, that many operators have found an improvement to the traditional thread designs.
However, it is not one specific innovation or application which will decide the efficiency and profitability of top hammer drilling on a specific project. Rather it is, stresses Vuojolainen, the old skill of adding up all cost-determining factors and establishing the total drilling cost (TDC). "TDC is a sum of productivity cost of drilling in terms of time and cost per metre (drill string endurance)," he explains.
Energy saving is another key factor as sometimes drilling is set-up to drill with highest pressures, while not necessarily needed. In short, Vuojolainen concludes, what needs to be focussed on is what can be done to increase productivity by increasing drilling speed, avoiding downtime and managing the lifetime of wear products.
On top of this, top hammer drilling allows for a larger number of smaller holes enabling more accurate and controlled blasting, something which is usually highly critical in underground operations.
Lastly, top hammer drilling is also required for bolting operations to secure tunnels, and, although the drilling machines used for bolting and production drilling can be slightly different, they share the same drilling methodology and require the same skillset from operators.
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