Higher Grade Pipe In The Inclined Section Of The Well
The previous discussion was restricted to the simple case when the higher
grade pipe is totally in the vertical portion of the well. If the higher grade
pipe is used through a build-up section, the calculation becomes more
difficult. A rough approximation could be obtained by treating each stand
length as a straight section of hole and using the average inclination of that
course length. The weight this exerts along the borehole is found from:
Weight acting along borehole = weight of stand x cos (ave. inc.)
This, however, ignores drag which may be significant.
Similarly, for an inclined section of the well where the inclination is
constant, the weight acting along hole will be the air-weight of the pipe
multiplied by the cosine of the average inclination. Notice again that in this
particular calculation we do not use a buoyancy factor. This is because
although the entire drillstring is subject to a buoyancy force, that force is
acting on the lower portion of the string and affects the weight pulling
down on the top joint of lower grade pipe from below, but not the weight of
the joints of higher grade pipe at the top of the string.
It must be emphasized that if a higher grade pipe extends below the vertical
part of the well, then an accurate analysis of the axial stresses requires the
use of “Torque and Drag” programs.
The previous discussion was restricted to the simple case when the higher
grade pipe is totally in the vertical portion of the well. If the higher grade
pipe is used through a build-up section, the calculation becomes more
difficult. A rough approximation could be obtained by treating each stand
length as a straight section of hole and using the average inclination of that
course length. The weight this exerts along the borehole is found from:
Weight acting along borehole = weight of stand x cos (ave. inc.)
This, however, ignores drag which may be significant.
Similarly, for an inclined section of the well where the inclination is
constant, the weight acting along hole will be the air-weight of the pipe
multiplied by the cosine of the average inclination. Notice again that in this
particular calculation we do not use a buoyancy factor. This is because
although the entire drillstring is subject to a buoyancy force, that force is
acting on the lower portion of the string and affects the weight pulling
down on the top joint of lower grade pipe from below, but not the weight of
the joints of higher grade pipe at the top of the string.
It must be emphasized that if a higher grade pipe extends below the vertical
part of the well, then an accurate analysis of the axial stresses requires the
use of “Torque and Drag” programs.
