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Mud Hydraulics Optimization 103

pipe end conditions at various depths. As we can see from this graph,
engineers must pay close attention before the pipe reaches 10,000 ft. At
10,000 ft, the narrowest annular section is the longest, thus producing
the maximum surge pressure. When the liner enters the larger open hole
section, trip speeds can be increased. The most dangerous string depth is
not necessarily at the bottom of the well. Note also that the curves for
a pipe with autofill float equipment and a pipe with a circulation sub
coincide above the string depth of 5,555 ft. This is because above that
depth, the circulation sub, which is located at a depth of 9,445 ft, is not
in the well.
Figure 4.13 shows the sensitivities of surge pressures to tripping speeds
for pipe ending conditions that involve pipes that are closed, have autofill
float equipment, and have a flow diverter. The SurgeMOD program is
equipped with pipe tripping animation so the user can view the positions
of the pipe and pressure variation simultaneously. Figure 4.13 is for the
sensitivity of surge pressures at a depth of 15,000 ft. If we increase the
tripping speed, the surge pressure for the closed pipe will increase sharply
and the surge pressure for the pipe with a circulation sub will gradually
increase. The curve for the pipe with the autofill float equipment lies
between them.

Figure 4.13 Sensitivities of surge pressures to tripping speeds. Surge EMW @
15,000 (ft) when string reaches 15,000 (ft).