Page 145 - Drilling Technology in Nontechnical Language

136 Drilling Technology in Nontechnical Language Second Edition

electrically or hydraulically powered. If a rig is fitted with a top drive, it
will still have a rotary table because sometimes rotation is needed from the

drill floor to manipulate tools directly when the top drive cannot be used.
Also if the top drive should fail, the rotary table and kelly can be used
while the top drive undergoes repair.

Drillpipe

The drillpipe represents a large investment for the drilling contractor.
Drillpipe comes in various sizes; probably the most common size
worldwide is 5" outside diameter (OD) pipe that has an inside diameter
of 4.276" and comes in joints of about 31 ft long. The ends of a joint of
drillpipe have welded onto them a piece of thick-walled pipe on which
threaded connections are machined. Joints of pipe are screwed together
using these connections. Two of the basic characteristics of drillpipe that
identify the pipe are the OD and the type of connection.

Drillpipe is not only specified by OD and connection but also by the

type of steel used to make the pipe. In the oil field, steel used in making
downhole tubulars comes in various grades, which are defined by

characteristics such as carbon content of the steel, amounts of impurities,
and heat treatment. For drillers, one of the most important properties of
the steel is the strength—how much force can be applied to the pipe before
it fails.
There are several grades in common use (though other grades are
available), such as E75, X95, G105, and S135.
The number part of the grade refers to the minimum yield strength of the
steel, in thousands of pounds per square inch. The minimum yield strength
is arrived at by testing a sample of the steel in tension and measuring at
what force the material increases in length by a set percentage. For normal
strength steel, E75, the minimum yield is found at the point when the
sample has increased in length by 0.5% and the minimum yield stress is
75,000 psi. For higher strength steels, it is more; grade G105 is 0.6%, and
grade S135 is 0.7% stretch, to give 105,000 and 135,000 psi, respectively.
If the minimum yield strength of the material and the cross-sectional
area of the pipe are known, the strength can be calculated. For instance,
for 5" grade E drillpipe, when new, the cross-sectional area at an inside
diameter of 4.276" is 5.274 in . If this is multiplied by the minimum
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