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98 Part I Liquid Drilling Systems

drilling because of restricted flow paths and the limited number of casings
and liners. Accurately predicting surge and swab pressures is of great
importance in wells, where the pressure must be maintained within nar-
row limits to ensure trouble-free drilling and completion operations.
New drilling and completion technologies are challenging many
aspects of our operations. For example, running liners in a subsea casing
string with very tight tolerance can cause extremely high surge pressures.
Autofill float equipment and other new tools such as flow diverters (also
called circulation subs) have been developed to reduce surge pressure,
and they are effective. The questions are, what will the surge/swab pres-
sures be and what are the optimal tripping speeds?
To thoroughly analyze surge pressure, a comprehensive surge and
swab hydraulics computer model, SurgeMOD, has been developed to
assist in the analysis and design of tripping operations, especially for deep-
water wells or wells using new tools such as autofill float equipment, cir-
culation subs, and so on. The program simulates fairly complex wellbore
configurations, including multiple pipe sizes, wellbore intervals, and
annular sections, with very tight tolerance.

4.3.4 Analyzing Trip Operations
This section discusses the engineering analyses behind trip operations for
different pipe end conditions: closed, open, open with autofill or a bit, and
with a flow diverter. These four conditions are illustrated in Figure 4.8.
We will discuss the controlling parameters that affect surge pressure using
SurgeMOD. The surge and swab pressure analysis has two components: to
predict surge and swab pressure for a given running speed (analysis mode)
and to calculate optimal trip speeds at different string depths without
breaking down formations or causing a kick at weak zones (design mode).
As pipe is moved downward into a well, the original mud is displaced by
the new volume of the extending pipe, and the mud must move upward.
When the pipe is closed or contains a float sub, all displaced fluid passes up
the annulus. The flow rate in the annulus is equal to the pipe displacement
rate. It is therefore easy to calculate the frictional pressure drop in the annu-
lus. Surge pressure is calculated using standard hydraulics equations, but the
equations must be modified to account for movement of the pipe wall.

Fully Open Pipe
If the pipe is open-ended, the problem becomes more complicated, since
the distribution of flow between the inside pipe and the annulus cannot