Types of Drilling Equipment

Once a drilling method has been selected, you must decide on the type of drilling equipment or rig that best suits your situation. This section discusses the various types of rig available and their suitability and also provides an overview of drilling rig parts.

Choosing a drilling rig:

The type of rig chosen may be determined on the basis of the site geology, the anticipated depths of the boreholes, and their expected diameters. Access is an important consideration. All drilling machines, except the smallest units capable of being dismantled and reassembled on site, require transportation: a road may have to be cut through bush to reach a location. For the largest truck or trailer-mounted rigs this can be a significant problem during rainy seasons in remote areas. Heavy rigs are notorious for becoming stuck in mud, and in such difficult conditions they should be used only if rain is not expected, or if there are means of pulling the rig out of trouble.

Drilling rig components

a) Drill bit
No single type of drill bit can cope with all possible drilling conditions and formations. Some typical examples are shown in Figure 1.4: Drag bits consist of three or four serrated blades that shear the formation when the bit is rotated; they can penetrate softer formations such as poorly consolidated or stiff clays and mudstones rapidly. Roller cone bits (or tri-cone rock bits), which can be used with air or liquid flushing, are popular with the oil industry. They can be used to penetrate both soft and relatively hard formations.

b) Hammer
In air-circulation drilling, if a formation is too hard for penetration by a drag bit, a DTH hammer is generally employed. This tool was developed for the mining and quarrying industry. The ‘business’ end – the button bit – is studded with hemispherical tungsten carbide ‘buttons,’ and with channels built in to allow the passage of compressed air. When the hammer is pressed against the ground, the bit is forced into a pneumatic hammer action (like a road drill) by compressed air fed down the drill pipes. Then, as the tool is rotated in the hole, the buttons act across the entire base of the borehole.

Most hammers rotate at speeds of 20 to 30 revolutions per minute, and blows can be struck at rates of up to 4000 per minute. Debris is normally flushed (blown) out of the hole at the end of each drill pipe. DTH hammers are most effective in hard rock formations such as limestones or basalts; soft, fine-grained formations tend to clog the air ducts or jam the piston slides. Nonetheless, DTH hammers are extremely cost-effective and hence very popular with commercial drillers.