American Oil and Gas Reporter - October 2016 - 78

SpecialReport: Natural Gas Update
FIGURE 5
Optimization Tool Results from Example Compressor
4

Load Step

3
2
1
0
6/1/2015

6/6/2015

6/11/2015

6/16/2015

6/21/2015

6/26/2015

6/6/2015

6/11/2015

6/16/2015

6/21/2015

6/26/2015

Recommended Load Step

7/1/2015

7/6/2015

7/11/2015

7/16/2015

7/1/2015

7/6/2015

7/11/2015

7/16/2015

Actual Load Step

150

140
130

Pressure (psig)

120
110
100

90
80
70

60
6/1/2015

Unit Suction Pressure

Utilization is calculated as the throughput
at the current configuration divided by
the optimal throughput, and a target of
95 percent or greater is desired.
Major supporting parameters are the
throughput gain from the current to optimal
configuration, the performance code, and
warnings of an unsafe operating condition.
Figure 4 shows a screen shot of the utilization screen. All parameters can be
trended and a playback feature was incorporated for monitoring fleet performance.
Implementation Results
In 2014, Anadarko was increasing production rapidly within the D-J Basin
complex, and compression was being installed to the Wattenberg gathering system
as fast as possible to keep up. Production
growth and compression additions slowed
significantly in 2015 with the drop in
commodity prices.
It became imperative at that time to
reduce capital and operating expenses
while increasing efficiency of the fleet to
improve profitability. "Energize the base"
became the directive, and the optimization
tool quickly moved from demonstration
into full implementation to help increase
production with the existing compression
infrastructure.
78 THE AMERICAN OIL & GAS REPORTER

Field Pressure

Immediately after implementing the
optimization tool in the Wattenberg system,
there was an issue with units tripping on
first-stage "differential pressure." The differential pressure shutdown is an alternative method for rod load protection, as
compared with direct calculation of the
forces. The allowable differential pressure
calculation is a linear equation that returns
a maximum discharge pressure for a given
suction pressure to keep the rod load at a
safe level.
During subsequent investigation of
the PLC logic, it was determined that the
three-stage compressors were not programmed with the correct coefficients.
The units had been relocated from another
field where coalbed methane process gas
was much lighter than process gas in the
Wattenberg Field. The heavier gas affected
flow across the valves and the internal
cylinder pressure, which in turn, impacted
rod load. New coefficients for the differential shutdown equations were derived
for the new gas composition and were
programmed into the PLCs.
A second issue with differential shutdown also was discovered during this investigation. Because the packager had
not installed suction transmitters in the
interstages on these units, the prior-stage
discharge pressure was being used as the

next-stage suction pressure in differential
pressure calculations. This reduced accuracy by not considering the pressure
drop across the cooler.
Accordingly, a static value for the
cooler pressure drop was added to the
code to account for the pressure drop,
and was based on manual measurements.
All 3606 packages were updated to the
proper differential shutdown limits and
the premature shutdown problem was resolved.
Another limitation was related to the
engine drivers. The engines' factory control
system outputs a load factor (percent
load), which was being used as the basis
to determine whether a unit was fully
loaded. A wide spread was seen initially
from unit to unit in the load factor versus
the calculated percent load from the compressor model.
Anadarko's maintenance staff found
some lingering mechanical issues, but
most of the spread was a consequence of
how the units were tuned.
Other problems such as fouled spark
plugs, plugged precombustion needle
valves, and other issues were found during
implementation. Minor repairs and tuning
were incorporated as part of implementation. One key was tuning with the actual
fuel gas heating value to the particular
engines at each location. Gains of 50100 useable horsepower were realized
on many units. Once this procedure became part of the implementation process,
more consistent performance was achieved
and operation at full load became reliable.
There was a concern that active management of compressor VVCPs would
allow engines to overload or walk into
differential shutdown (rod load) if the
field pressure was to rise and the suction
set point was not set correctly. The solution
was to switch suction control from set
point control to load control.
This involved establishing a maximum
suction pressure limit, or clamp, for each
compressor design. The suction control
valve then opened to the maximum suction
pressure for a given load step setting-
limited by the maximum clamp-to automatically hold the unit at maximum load
or throughput condition.

American Oil and Gas Reporter - October 2016

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