American Oil and Gas Reporter - October 2016 - 73

SpecialReport: Natural Gas Update

Wattenberg Gathering System
The Wattenberg gathering system includes approximately 180,000 horsepower
of compression with a nominal processing
capacity of 1.1 billion cubic feet a day,
and more than 2,100 miles of pipelines.
Unprocessed gas is gathered and delivered
to four Anadarko-owned processing plants.
The majority of gas originates from
Anadarko's production, but a portion is
from third-party producers.
Compressor stations typically consist
of multiple compressors, with a few
outlier stations with one or two smaller
units. Of the 89 gathering compressors
in Wattenberg, 42 are of like-kind designs:
four-throw, three-stage compressors that
Anadarko refers to as "3606 packages."
The engine driver is a Caterpillar G3606A3™ with a nameplate rating of 1,775
horsepower and site-rated power of 1,680
hp when derated for elevation and maximum ambient temperature.
The compressor is an Ariel JGK™/43 with bores of 14.125 inches for the

two first-stage cylinders, 10.5 inches for
the second stage, and 6.75 inches for the
third stage.
The particular design features relating
to control of the 3606 packages include:
* Manual variable volume clearance
pockets (VVCPs);
* Manual bypass valves;
* Unit programmable logic controllers
to control engine speed for startup and
shutdown, and overrides for high-discharge, low-suction and high temperature;
* Unit PLC control of inlet suction
pressure using individual or bank control
valves; and
* Engine-driven cooler fans (two)
with pneumatic control of louvers for individual process temperature control.
Prior to developing the optimization
tool, the typical compressor operation
was to set the suction control set point to
a fixed pressure and adjust the VVCP
through trial and error to a position such
that the unit provided the "nominal"
throughput. Nominal throughput is a
round number that approximates the
throughput at some typical suction and
discharge condition. It is a useful number
for overall gathering system design to
determine how many compressors are
needed to match expected production
rates. The nominal throughput value also
was used as the basis to determine if a
unit was being fully utilized.
However, when detailed modeling and

analysis of the Wattenberg compression
fleet began in 2013-14, it quickly became
evident that compression throughput could
be significantly greater than the nominal
rate. Among the factors that can affect
compressor throughput are engine load
levels, suction-pressure set points, and
actual VVCP positioning. Suction-pressure
set points and VVCP positions typically
were set to provide a conservative load
on the engine while still achieving the
nominal throughput. This provided a consistent and safe operating condition with
some margin for variations.
The field pressure, or inlet pressure
to the compressor station, often was much
higher than the typical control set point
used. Data trending showed field pressures
to be highly variable, and only occasionally
dropped below the suction control set
point.
Figure 1A shows a typical field pressure
trend at one station equipped with six of
the 3606 packages, which was representative of all stations. With the lower
suction control set point, efficiency was
being lost by compressing the gas at a
higher-than-necessary ratio.
Maximizing Capacity
With this knowledge, the modeling
effort focused first on illustrating the inefficiencies, and then on developing a
procedure to safely adjust compressors
to maximize capacity. Figure 1B shows
the safe operating envelope for the 3606

FIGURE 1A
Field Pressure Trend
14

Increase Suction Pressure

13
12

2

Increase Load

11
Throughput (MMcf/d)

After extensive modeling showed the
throughput of the Wattenberg gathering
compression fleet could be increased significantly with more routine adjustments,
the optimization program was developed
to:
* Create an in-depth equipment database and build detailed models of all
compressors, including load step schedules;
* Create intuitive and easy-to-interpret
tables of load step schedules to distribute
to operations personnel;
* Internally develop an optimization
tool to automatically look up the optimal
settings for maximum throughput at existing conditions within safe operating
boundaries;
* Construct human-machine interface
screens to display current conditions and
compressor utilization, recommended settings, and predicted gains from adjustments;
* Work with operations to implement
the recommendations; and
* Document improvements and search
for other opportunities to further increase
companywide compression utilization
and efficiency.

10

Increased
Throughput

3

9

1

8
7

4

6
5

30

40

50

60
Safe Operating Envelope

70

80
Suction Pressure (psig)

Typical Operating Point

90

100

110

120

130

Maximum Capacity Point

OCTOBER 2016 73
American Oil and Gas Reporter - October 2016

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