STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

748 Drilling and Well Completions
The corrected collapse pressure resistance according to Equation 4-65 is
Hydrostatic pressure of the drilling fluid behind the drill string at the packer
level is
P, = (0.052)(12)(8,500) = 5,304 psi
Obtained safety factor 5,493/5,304 = 1.0356.
Since the obtained magnitude of safety factor (1.03) is less than desired (l.l),
the drill pipe must not be run empty inside.
Tool Joints
The heart of any drill pipe string is the threaded rotary shoulder connection
(Figure 4-133), known as the tool joint. Today, the only API standard tool joint
is the weld-on joint shown at the bottom of Figure 4-133.
Tool joint dimensions for drill pipe grades E, X, G and S (recommended by
API) are given in Table 4-84. Selection of tool joints should be discussed with
the manufacturer. This is due to the fact that, up to the present time, there are
no fully reliable formulas for calculating load capacity of tool joints. It is
recommended that a tool joint be selected in such a manner that the torsional
load capacity of the tool joint and the drill pipe would be comparable. The
decision can be based on data specified in Tables 4-85 through 4-88.
Makeup Torque of Tool Joints
The tool joint holds drill pipe together, and the shoulders (similar to drill
collars) form a metal-to-metal seal to avoid leakage. The tool joint threads are
designed to be made up with drilling fluid containing solids. Clearance must
be provided at the crest and root of threads in order to accommodate these
solids. Therefore, the shoulder is the only seal. To keep the shoulders together,
proper makeup torque is required.
However, makeup torque applied to the tool joint produces as axial preloading
in the pin and the box as well as a torsional stress.
In particular, makeup torque induces a tensile state of stress within the pin
and compression stress in the box. Thus, when the tool joint is exposed to the
additional axial load due to the weight of the drill string suspended below the
joint, the load capacity of the tool joint is determined by the tensile strength
of the pin.
The magnitude of the makeup torque corresponding to the maximum load
capacity of the tool joint is called the recommended makeup torque.
Therefore, the actual torque applied to the drill string should not exceed
the recommended makeup torque; otherwise, the load capacity of the tool joint
is reduced.
The API recommended makeup torque for different types of tool joints and
classes of drill pipe is given in Table 4-89.