Page 146 - Drilling Technology in Nontechnical Language

Chapter 5 – RIG SELECTION AND RIG EQUIPMENT 137

yield of 75,000 lb, the maximum force that can be applied to the drillpipe
is 395,595 lb. Higher strength pipe has a correspondingly higher
maximum force.

When used for drilling, drillpipe is subjected to wear. As it is rotated,
parts of the pipe will touch the wall of the hole and the inside of the casing.
As drillpipe wears, the thickness of the pipe wall (and hence the cross-
sectional area) also decrease, and so the strength of the drillpipe decreases.

To allow for this, drillpipe is given a classification that relates to the degree
of wear. New pipe is just that— it is within the original manufacturer’s
tolerances. Next comes premium pipe, which has up to 20% uniform

wear on the thickness at the OD. Two other classifications are class II and
class III, but these are rarely used except in drilling very shallow wells or
water wells.
Apart from transmitting torque to the drilling assembly and physically
supporting the weight of the entire string of pipe, the drillpipe also has to
withstand very high pressures from the inside. While drilling deep wells,
the pressure at the surface could exceed 3,000 psi, which the drillstring has
to be able to withstand.
To specify a particular pipe to use, a drilling engineer has to state the
size (OD), grade (of steel), connection type, and classification.

Drillpipe handling equipment

There are three particular items of equipment that are used on a rig to
work with drillpipe: the slips, elevators, and tongs.

The slips are wedge-shaped pieces of steel that fit inside the insert

bushing. The angle of the outside matches the inner profile of the bushings.
The slips have steel teeth on the inside face that grip the drillpipe. The
more weight is applied to the teeth of the slips, the more the slips are forced
down the tapered profile, and the tighter they grip the drillpipe (fig. 5–22).

In this way, the drill crew can easily hang the entire weight of the drillstring
(hundreds of thousands of pounds of weight in a deeper well) in the rotary
table using the slips. This is necessary when tripping into or out of the hole
to allow connections to be screwed together or unscrewed.

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