Page 86 - Air and gas Drilling Field Guide 3rd Edition

4.3 Bottom Hole Assembly 77

STRING TYPE
STABILIZER

30 FT.
DRILL COLLAR

STRING TYPE
STABILIZER

SHORT
DRILL COLLAR

NEAR BIT
STABILIZER
BIT

FIGURE 4-16. Typical bottom hole assembly for direct circulation rotary drilling operations.

borehole. For more details regarding the design of stabilized BHA and how to
apply such assemblies, readers are referred elsewhere [1, 4] or to service com-
pany literature.

4.3.1 Drill Collars
Drill collars are thick-walled tubulars used at the bottom of the drill string (see
Figure 4-1). Their principal purpose in the drill string is to provide the axial force
needed to advance the drill bit (see Figure 2-2). When drilling a vertical borehole,
the axial force is the weight of the drill collars. Drill collars are available in the API
range lengths given in Table 2-1. Figure 4-16 shows a BHA with Range 2 ( 30 ft or
9m long) drill collars. Range 2 lengths are typical for double and triple land
rotary drilling rigs. Also shown in Figure 4-16 are short drill collar lengths used