Page 565 - Air and Gas Drilling Manual

Chapter 11: Specialized Drilling Equipment 11-3
The Foremost Model DR-24 drilling rig shown in Figure 11-1 has two rotary
drive heads, a top rotary drive that rotates the drill string clockwise inside the
casing, and a bottom rotary drive that rotates the casing counterclockwise ( the
sections of casing are welded together as the drilling advances). Figure 11-2 shows a
view (looking up) of the two rotary drive heads.

Figure 11-2: Dual rotary drive heads, at the top is the drill string rotary drive head, at the
bottom is the casing rotary drive head (courtesy of Foremost Industries Incorporated).
A casing shoe ring drag type bit is welded to the bottom of the casing to give
the casing rock destruction capability as it is advanced (see Figure 11-3).
These dual rotary drilling rigs are designed primarily for shallow drilling
operations and can utilize drilling mud or a variety of air (or gas) drilling fluids.
The rigs are used to drill monitoring wells, water wells, and mining and
geotechnical boreholes. These dual rotary drilling rigs have only been in use for a
little over a decade. The dual rotary drilling rig is characterized by two masts each
having a track. Figure 11-1 shows these two masts. The taller narrower track has
the head rotary drive for the drill sting and the lower wider track has the head rotary
drive for the casing. The heads are moved on their respective tracks by hydraulic
power.
These dual rotary drilling rigs are available in several rig sizes. The model
number gives the capability of the rigs. Model DR 24 (shown in Figure 11-1)
denotes a 24 ft downward stroke of the drill string rotary drive head (in its respective
track). This model also has a 7 ft downward stroke of the casing rotary drive head