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7 DRILL STRING COMPONENTS

Drill Collars

The early-day drill collar was a heavy-wall double-box sub
used to connect the bit to the drill pipe. When Hughes simplex bit
with a lubricating system arrived it was necessary to employ an
overshot drill collar at least 17 ft long to provide space within its
inner bore to accommodate a reservoir for the lubricant. Soon
afterwards it was common practice to include at least one
additional drill collar which, with one joint of drill pipe, made up a
full stand to set back.
Use of long drill collar assemblies began in 1934 and has
been expanded, in most hard rock areas, to 20 or more 30-ft
lengths. Rotary bits, in order to make hole, must be forced
downward against the bottom of the hole. This thrust is provided
by that portion of the drill string weight not supported by the rig.
At some point in the drill string there is neither tension nor
compression. This is referred to as the neutral point which will
move up or down the string depending on amount of weight
applied to the bit. Below the neutral point the drill string is in
compression and subject to buckling forces.
Drill pipe, if rotated while buckled, will not last long due to
the severe transverse stresses that are induced. Heavy-wall drill
collars, on the other hand, resist buckling much better. It is
considered good practice therefore to have more than enough
weight in the drill collars to provide the desired bit weight. As long
as the neutral point is in the drill collar section all drill pipes will
be in tension.
Drill collars are made of alloy steel, heat-treated after
machining, and are quite rugged. Even so, they buckle enough to
induce heavy stresses in connections and cause wear on the outer
surfaces. The drill collar connection, being the weakest part, has

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