COMPARATIVE VERSATILITY

This week we are lining out some of the advantages and disadvantages of DTH systems in comparison to other popular drilling systems, namely rotary drilling and top hammer drilling. At the end of the article, we’re offering a look into the differences in their respective performance levels under varying rock conditions.

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ADVANTAGES OF DTH HAMMERS OVER ROTARY DRILLING

Rotary Drilling relies upon high rotational speeds and thrust without percussion to achieve the desired effect and outputs. Heavy drill collars are sometimes needed to add extra thrust to the bit.

Rotary machines are usually large self-contained, hydraulically powered units with sufficient weight to provide the thrust on the drill bit to drill the hole. The harder the rock the greater the thrust required, meaning that a heavier the machine is needed. This in turn increases the initial capital outlay, and results in higher operating costs. The same factors apply when hole diameters are increased.

Rotary drilling can be cost effective in soft/medium low abrasive formations but not in medium hard abrasive conditions. Furthermore, tricone rotary bits are not suited to holes below 140 mm (5.1/2”) diameter due to the premature destruction of the bearings within the bit and drag bits are only successful in soft formations, excessive thrusts can lead to deviation of the bore hole, particularly when drilling at angles in bedded formations.

DTH hammers do not require the powerful down thrust and torque of the rotary rig and, therefore, can be used on lighter, less expensive, and more mobile machines, while larger diameter holes can be drilled with DTH usually with the same size of rig. The low torque and thrust required by the DTH hammer means, that rotation head vibration is appreciably less than that created by the rotary method. Because minimal thrust is applied to the DTH hammer, it deviates from its course very little and drills a straighter hole than the rotary bit.

What is more, DTH offers longer bit life and faster penetration rates in medium to hard rock at any depth, and the hammers can successfully drill mixed, hard, and medium formations more efficiently than rotary equipment.

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ADVANTAGES OF DTH HAMMERS OVER DRIFTER (Top Hammer, TH) EQUIPMENT

Drifter Drilling uses high-energy, drill mast mounted hammers to drive the drill string and bit into the rock. There are both air-powered and hydraulically powered versions. Hydraulic drifter hammers are faster and more economical than air powered ones. In each case the hole is flushed clean with air or water.

There are 2 main types of drifter systems - “Conventional Drifter” and “Tube Drifter”.

“Conventional Drifters” are designed for short hole drilling in medium to hard conditions with maximum hole diameter up to 115 mm (4.1/2”). They use slender drill rods for ease of handling, which have a relatively small flushing hole for air to pass through to the drill bit to clear debris from the hole. The diameter of these rods leads to flexing within the bore hole, which in turn leads to hole deviation. This deviation is further accentuated by depth.

“Tube drifters”, which use heavy wall tubes in an attempt to provide a more rigid drill string, are designed to drill deeper with larger diameters. Tube drifters are always hydraulically driven from self-contained automatic machines. Even with tube drifters, experience has shown that the high impact on the drill bit can still cause deviation of the bore hole and in broken ground the drill bit can easily jam. The high energy impact can also have the detrimental effect of destroying the drill tubes to give a low finite life for these expensive components. Additionally, the drilling rate within the bore hole will decrease as the hole deepens, due to energy loss in the rods and couplings as the drill bit gets further away from the energy source.

Drifters are more successful in solid rocks and less so in soft, variable and fractured rocks.

With DTH hammers, there is little or no reduction in penetration rate as the hammer drills deeper in solid rock. Whatever drilling speeds can be obtained at a depth of 10 m (30 ft) can generally, subject to compressor capacity, be obtained at a depth of 100 m (330 ft). This is due to the hammer piston always directly striking the drill bit, unlike the drifter system, whose rods and couplings absorb blow energy resulting in progressively slower drilling as more rods are added.

With DTH, the operator works in relative comfort because the sound of the hammer is muffled in the hole. With the drifter hammer, on the other hand, the operator may be continuously subjected to intense noise.

Because they are unable to clear cuttings efficiently at depth, drifters often have to do a certain amount of non-productive drilling per hole to ensure they achieve the full required depth. With DTH, clearance of cuttings is more effective, resulting in less recrushing of rock and therefore more efficient drilling. This is achieved in two ways:

1. By utilising a smaller annulus between the drill string and the bore hole.
2. By making a greater volume of air available at the rock face than can be passed through the narrow duct in a drifter rod.

Drill tube changing is easier and quicker with DTH because DTH tubes are lighter and do not absorb percussive energy as drifter rods and couplings do, becoming heated and are therefore far more subject to wear on the threads. Consequently, DTH drill tubes can last for years whereas drifter rods and couplings are regarded as major consumable items.

DTH hammers can make use of high-pressure air whilst drifters are unable to do so because of the limitations of the drill steel bore.

DTH hammers are also kinder to the drill rig because their energy is transmitted to the rock and most vibration is absorbed down the hole. The drifter’s initial transfer of energy takes place on the mast which can be subjected to heavy vibration.

A major benefit with DTH hammers is that the operator is immediately aware of any malfunction such as binding or insert breakage by the sound emitted from the bore hole. This is not possible with the drifter due to the intense noise from the drifter head. In the event of changing a DTH hammer, either for servicing or fitting a different size or type, the DTH conversion can be carried out in a short time without immobilizing the rig. This is not the case with the drifter hammer.

Generally speaking, DTH hammers give way to drifters at depths of less than 10 m where hole diameters below 100 mm (4”) are required, due to the faster penetration speeds of the drifter in these conditions.

Many drifter drill rigs can be converted to the advantages of DTH hammer by equipping the drill rig with a rotation gearbox, enabling the rig to drill larger diameter holes than previously possible with a drifter head. On blasting applications, the subsequent increase in burden and spacing provided by a larger diameter hole can result in more rock being produced at less cost and without the capital outlay of a new drill rig.

The ability of the DTH hammer however to generally drill a straighter hole, its quieter operation, lower air consumption than an air powered drifter and more efficient hole cleaning capability are points worthy of consideration, even for short holes of small diameter.

PERFORMANCE IN SOFT ROCK CONDITIONS

‘Rotary’ systems work well in soft conditions, the rate decreasing as hardness increases.

‘Drifter’ systems do not drill well in soft conditions but drill better as the rock hardens and becomes more solid.

‘DTH’ systems are less efficient at the lower end of the scale but work extremely well at a consistent level in all other conditions.

PERFORMANCE IN HARD AND SOLID ROCK CONDITIONS

‘Rotary’ systems are more affected as the rock hardens.

‘Drifter’ systems work well in solid conditions with high drilling rates on shallow holes.

‘DTH’ systems are at the peak of efficiency being less affected than other systems in these conditions.

PERFORMANCE IN HARD BUT FRACTURED ROCK CONDITIONS

‘Rotary’ systems slow down rapidly as hardness increases.

‘Drifter’ systems tend to deviate and jam in fractured conditions.

‘DTH’ systems consistently produce good results and are less affected even at depth.

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