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Mud Hydraulics Optimization 87

for different liner sizes. The liner size to use to drill a specific hole
section specified by the bit size should provide an adequate flow rate
for carrying up drill cuttings and sufficient pressure to drill the section
completely.
In field drilling operations, there is often confusion about the maxi-
mum pump pressure and the maximum circulation rates. Pump manufac-
turers publish what are called “maximum liner ratings” and “maximum
circulation rates” for specific pumps. These maximums, however, are sel-
dom used in drilling operations. The manufacturer’s published maximum
pressure is based on the maximum permissible force on the power end
bearings.
In actual drilling operations, the maximum pump pressure is rarely
achieved. Many arbitrary standards are used. One common standard uti-
lizes a fixed percentage of the maximum liner rating pressure. Most
operators would not exceed 90% of the liner rating, which is equivalent
to a safety factor of 1.11. Some rig operators ignore the safety factor
completely and simply specify a maximum surface pump pressure.
Because the combination of flow rate and pressure is limited by pump
horsepower, the high flow rate is available from a pump with a scarifying
(reduced) available pump pressure. On the other hand, high pressure is
available from a pump with a scarifying (reduced) available flow rate. The
procedure for selection of liner size is as follows:
1. For the given borehole geometry and fluid properties, calculate the
minimum flow rate required to transport drilling cuttings to the
surface (see Chapter 2).
2. Design a flow rate based on the minimum required flow rate with a
sufficient safety factor.
3. Look up the manufacturer’s pump specifications table to select a liner
size that meets the designed flow rate from two pumps.
4. Based on the designed flow rate, calculate the expected total parasitic
pressure loss at the total depth of the section.
5. Calculate the required pump pressure based on the total parasitic
pressure loss and the selected criterion for hydraulics optimization.
6. Look up the manufacturer’s pump specifications table to check the
maximum allowable pressure against the calculated pump pressure.
7. If the maximum allowable pressure is higher than the calculated pump
pressure, this liner size can be selected. Otherwise, go back to step 3
and consider a smaller liner size.