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86 Part I Liquid Drilling Systems

maximum bottomhole jetting action. Therefore, the problem is one of
using the maximum jetting action that is economically feasible. Economic
feasibility depends on the maximum penetration rates possible, the hole
conditions, and other factors such as connection time and the mainte-
nance of support equipment.
When high-capacity pumps are available, it is possible to achieve a
higher level of bit hydraulics (horsepower, impact force, or nozzle velo-
city) than is needed to clean the hole bottom adequately. Using higher
than needed bit hydraulics not only is wasteful but also can be harmful.
This is because the high flow velocity in the system can result in bore-
hole and pipe erosion as well as premature failure of the pump’s parts.
It is important to know that pump maintenance costs go up as pump
pressures are increased. Showing a direct mathematical relationship
between pump pressures and maintenance costs is difficult because so
many other variables also have direct impacts on pump maintenance
expense. In fact, pump maintenance costs rise much faster than the
increase in pump pressures. For instance, the pump maintenance cost is
often more than doubled when the pump pressure increases from 2,500
to 3,000 psi (17,006 to 20,407 kPa). Precise numbers for specific rigs or
operations must be determined based on field operation conditions.
If the hole cleaning needs can be established from penetration rate data
taken in similar lithology under conditions of varying bit hydraulics, the
pump energy input should be reduced by decreasing the flow rate until
the desired level of bit hydraulics can be obtained if the pump is operated at
the maximum allowable pressure. This same logic could be applied using
either hydraulic horsepower or impact force as the hydraulic parameter.

4.3 THE OPTIMUM DESIGN OF A HYDRAULICS
PROGRAM
For a drilling operation with given mud pumps and mud programs,
a well hydraulics program is defined as a complete procedure of changing
the pump liner, the flow rate, and the nozzle size according to the depth
of drilling. Drilling performance can always be improved by optimizing
the hydraulics program.

4.3.1 Selecting the Liner Size
As shown in Table 3.6, pump manufacturers provide specifications of
pumps that indicate the maximum pressures and ranges of flow rates