STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drill String: Composition and Design 735
A. Lubinski also found [171] that to drill a vertical hole in homogeneous formations,
it is best to carry less weight on the bit than the critical value of the first
order at which the drill string buckles. However, if such weight is not sufficient,
it is advisable to avoid the weight that falls between the first and second buckling
order and to carry a weight close to the critical value of the third order.
For practical purposes, in many instances, the above statement holds trues if
formations being drilled are horizontal. When drilling in dipping formations, a
proper drill collar stabilization is required for vertical or nearly vertical hole
drilling. In an inclined hole, a critical value of weight on the bit that produces
buckling may be calculated from the formula given by R. Dawson and
P. R. Paslay [40]:
-I
EIpsina "
wm, = 2( (4-53)
where a = hole inclination measured from vertical in degrees (")
r = radial clearance between drill collar and borehole wall in ft
E,I,p = as for Equations 4-51 and 4-52
Few straightforward computations can reveal that, in regular drilling conditions,
the critical weight is very high. The reason why drill collars in an inclined
hole are very resistant to buckling is that the hole is supporting the drill collar
along its contact with the borehole wall.
This explains why heavy-weight drill pipe is successfully used for creating
weight on the bit in highly deviated holes. However, in drilling a vertical or
nearly vertical hole, a drill pipe must never be run in effective compression or,
in other words, the neutral point must always reside in the drill collar string.
Rig Maintenance of Drill Collars [38]
It is recommended practice to break a different joint on each trip, giving the
crew an opportunity to look at each pin and box every third trip. Inspect the
shoulders for signs of loose connections, galls, and possible washouts.
Thread protectors should be used on pin and box when picking up or laying
down the drill collars.
Periodically, based on drilling conditions and experience, a magnetic particle
inspection should be performed using a wet fluorescent and black light method.
Before storing, the drill collars should be cleaned. If necessary, reface the
shoulders with a shoulder refacing tool, and remove the fins on the shoulders
by beveling. A good rust preventative or drill collar compound should be applied
to the connections liberally, and thread protectors should be installed.
Drill Pipe
The major portion of drill string is composed of drill pipe. The drill pipe is
manufactured by the seamless process. According to API Specification 5A
(Thirty-fifth Edition, March 1981), seamless pipe is defined as a wrought steel
tabular product made without a welded seam. It is manufactured by hot working
steel or, if necessary, by subsequently cold finishing the hot worked tabular
product to produce the desired shape, dimensions and properties.