STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drill String: Composition and Design 731
(text continued from page 722)
Example
Suppose you want to select the best connections for 9 4 x 2 2-in. ID drill collars.
For average conditions, you should select in this order of preference (see
Figure 4-131):
1. Best = N.C. 70 (shaded area and nearest reference line.)
2. Second best = 7Q in. REG. (low torque). (Light area to left and nearest to
reference line.)
3. Third best = 79 in. H-90. (Light area to right and nearest to reference line.)
But in extremely abrasive and/or corrosive conditions, you might want to select
in this order of preference:
1. Best = 79 in. REG. (Low torque) = strongest box.*
2. Second best = N.C. 70 = second strongest box.
3. Third best = 79 in. H-90 = weakest box.
Recommended Makeup Torque for Drill Collars
The rotary shoulder connections must be made up with such torque that the
shoulders will not separate under downhole conditions. This is of critical
importance because the shoulder is the only area of seal in a rorary shoulder
connection. Threads are designed to provide a clearance between crest and root
that acts as a channel for lubricant and also accommodates the small solid particles.
To keep the shoulders together, the shoulder load must be high enough to
create a compressive stress at the shoulder face capable of offsetting the bending
that occurs due to drill collar buckling. This backup load is generated by a
makeup torque. Field observations indicate that an average stress of 62,500 psi
in pin or box, whichever is weaker (cross-sectional area), should be created by
the makeup torque to prevent shoulder separation in most drilling conditions.
It should be pointed out that the makeup torque creates the tensile stress in
the pin and, consequently, the number of cycles for fatigue failure of the pin
is decreased. Therefore, too high a makeup torque has a detrimental effect on
the drill collar service life.
The recommended makeup torque for drill collars is given in Tables 4-75
and 4-76.
Drill Collars Buckling
In a vertical straight hole with no weight on the bit, a string of drill collars
remains straight. As the weight for which the straight form of the string is not
stable is reached, the drill string buckles and contacts the wall. If weight on
the bit is further increased, the string buckles a second time and contacts the
borehole wall at two points. With still further increased weight on the bit, the
third and higher order of buckling occurs. The problem of drill collars buckling
(text continued on page 7?4)
*The connection furthest to the left on the chart has the strongest box. This connection should he
considered as possible first choice for very abrasive formations or corrosive conditions.