SWITCH FROM SPHERES TO PIPELINE PIG IMPROVES GAS-GATHERING EFFICIENCY

Aug. 1, 1994
Roger E. Meadows GPM Gas Corp. Oklahoma City Removing accumulated line liquids with pigging rather than conventional sphere-launching methods has improved natural-gas flow efficiencies to more than 90% from 15% in 200 miles of a high-pressure gathering system in western Oklahoma. The improved efficiencies increased capacity on two legs of the system by more than 20 MMcfd and allowed operator GPM Gas Corp. to shift gas between three processing plants in the company's Oklahoma region -
Roger E. Meadows
GPM Gas Corp.
Oklahoma City

Removing accumulated line liquids with pigging rather than conventional sphere-launching methods has improved natural-gas flow efficiencies to more than 90% from 15% in 200 miles of a high-pressure gathering system in western Oklahoma.

The improved efficiencies increased capacity on two legs of the system by more than 20 MMcfd and allowed operator GPM Gas Corp. to shift gas between three processing plants in the company's Oklahoma region - Cimarron, Okarche, and Kingfisher.

This gas-flow shift improves plant efficiencies and reduces operating costs. It also provides gathering system flexibility and offers the potential for gathering-system growth with minimal investment through recovered gathering-system capacities.

GPM initiated this project through studies to optimize its gas gathering and plant operations and to reduce operating costs. The optimization studies focused primarily on staffing requirements, horsepower utilization, electrical expenses, chemical costs, and high-pressure line efficiencies.

OKLAHOMA REGION

GPM's Oklahoma region consists primarily of three gas-processing plants that are supported by more than 5,400 miles of low and high-pressure gathering systems. Figs. 1 and 2 show the locations of the project in the Oklahoma system.

Low-pressure gas is gathered from the wellhead and compressed by more than 80 remote compressor stations located primarily in west-central Oklahoma.

The gas from each compressor station is routed to high-pressure discharge lines that lead to the Kingfisher, Okarche, and Cimarron plants. Although the three plants utilize a typical cryogenic process, Kingfisher and Okarche are more efficient.

Pressures on the main discharge lines range from 550650 psig at the plant inlets to 950 psig on the outer limits of the gathering systems. The discharge lines vary in size but consist primarily of 10, 12, and 14-in. pipe.

Before this project, accumulated line liquids were re- moved from the discharge lines by conventional sphere-launching procedures.

PHASE I

The first phase of the project involved determining the flow efficiencies of more than 200 miles of high-pressure gathering lines (Fig. 2). The procedure consisted of determining the pressure drop along the gathering lines at intervals of approximately 5 miles.

Once pressure and flow data were determined, efficiencies were calculated based on the U.S. Bureau of Mines' Monograph 9 Equation. 1

Results of the pressure survey indicated efficiencies varied from 15 to 95% on the key high-pressure discharge lines leading to the three plants.

Analysis of the pressure data indicated that reduced line capacities from accumulated liquids was limiting the amount of gas that could be delivered to the more efficient Kingfisher and Okarche plants.

PHASE 2

The second phase of the project involved improving flow efficiencies on the 55-mile, 12-in. high-pressure discharge line leading to the Okarche plant (Fig. 2).

Accumulated liquids from this discharge line were historically removed by launching a sphere weekly from the Fonda booster and receiving it at the Okarche plant.

Initial pressure surveys indicated that efficiencies varied from a low of 25% at the midpoint of the 12-in. line to a high of 95% near the inlet of the Okarche plant.

Detailed pressure surveys revealed that the pipeline segment with the lowest efficiency was located in an area called Bucher Canyon. Fig, 3 shows that the elevation changes dramatically within the 3-mile section of the canyon.

GPM modified the sphering procedures to improve efficiencies in the canyon section. Sphere-sizing techniques were reviewed to make sure line liquids were not bypassing the sphere.

This was accomplished by use of a sizing ring to match the largest internal diameter of the 12-in. pipe through which the sphere would travel.

This step proved unsuccessful.

GPM then increased the sphere-launching frequency to daily and later launched two simultaneous spheres. These two efforts also failed to improve efficiencies.

Marking a sphere revealed that one of the two launched spheres was being left in the canyon section of the discharge line and that subsequent launched spheres would only displace the stationary sphere.

Efficiencies would improve to 60% immediately after the sphere would pass the canyon section but would return to 25% within 24 hr. GPM believes a combination of severe elevation changes and low gas velocities caused this situation.

After attempts with modified sphering failed to improve efficiencies, GPM modified the sphere launcher at the Fonda booster to accommodate a conventional polyurethane pig. No modifications were needed at the Okarche plant to receive the incoming pigs,

Subsequent launching of the 12-in. pig improved efficiencies to more than 95% along the 55-mile portion of this line. The Bucher Canyon area pressure drop was reduced to about 5-10 psig from 100 psig.

The pig, because of its greater seating ability within the line, was able to dislodge the stuck spheres.

The increased efficiencies allowed GPM to shift 5 MMcfd of gas from the Cimarron plant to the more efficient Okarche plant. Currently, three pigs per week are required to maintain the 95% efficiency.

PHASE 3

The third phase of the project involved improving the flow efficiency on a 10-in. discharge line from the Elm Creek area to the Kingfisher plant. This 65-mile discharge line is depicted in Fig. 2,

Accumulated line liquids were previously removed by use of three separate sphere launchers and receivers located at approximately 20-mile intervals leading into the Kingfisher plant. Nine spheres per week were launched to remove the accumulated liquids.

Pressure surveys indicated efficiencies of less than 15% on approximately 16 miles of the northern section of this line near the Eh-r. Creek area.

The low efficiency on this section of line was due to gas velocities of less than 40 fps that made sphering the line extremely difficult.

GPM modified this 10-in., high-pressure line to allow a single pig to travel the entire 65 miles to the Kingfisher plant. This involved modifying the existing spheres launcher site at Elm Creek and removing the intermediate sphere-launching and receiving equipment.

No modifications were needed at the existing King-fisher plant receiver.

Subsequent pressure surveys revealed that efficiencies approached 95% along the entire 65 miles of the system with line pigging twice per week.

The increased efficiency allowed GPM to shift 7 MMcfd of gas from the Cimarron plant and route it to the more efficient Kingfisher plant.

PHASE 4

In the fourth phase of the project, efforts concentrated on a 45-mile, 8-in. discharge line (Fig. 2). Historically, gas on this line has been route,d to the Cimarron plant due to the reduced capacity on the 10-in. line feeding the Kingfisher plant, as described in Phase 3.

Pressure surveys indicated that efficiencies along the 8-in. line varied from 60% to less than 40%. Accumulated line liquids were being removed by numerous sphere launchers and receivers located along the 8-in. line.

GPM replaced the sphere launcher at the North Oakdale booster with a pig launcher, and a pig receiver was installed at the Elm Creek site. This improved efficiencies to more than 90% with line pigging twice per week. This improvement allowed GPM to shift 8 MMcfd of gas from the Cimarron plant to the 10-in. discharge line feeding the Kingfisher plant.

CIMARRON PLANT INLET

Historically, the Cimarron plant has been plagued with large slugs of line liquids from a 14-in., high-pressure inlet line (Fig. 2). The liquids were removed from this 35-mile line by conventional sphering techniques.

Over the years, gas volumes had dropped off significantly on this line, resulting in gas velocities of less than 3 fps. Pressure surveys indicated efficiencies were less than 25%.

Clearing the 14-in. line with spheres was difficult. The spheres coming into the Cimarron plant were very unpredictable, resulting in twice per month launchings.

When the plant did not receive a sphere, the volume of line liquids often exceeded the capacity of the 2,000-bbl inlet receiver. This overloaded the plant inlet liquid stabilization unit and compromised efficiencies.

GPM installed a pig launcher at the west end of the 14-in. line, and subsequent launchings increased efficiencies to more than 90%. Travel time of the 14-in. pig became very predictable, allowing the company to begin launching pigs twice per week. The increased frequency resulted in smaller incoming slugs of line liquids and provided for more efficient operation of the plant inlet receiver and liquid stabilization unit.

PIG USES

Improving and maintaining high-pressure line efficiencies are essential in optimizing a gas-gathering system. In systems where sphering procedures are ineffective, conventional pigs can be particularly important especially in locations of severe elevation changes and low gas velocities.

As a result of this project, GPM improved flow efficiencies to more than 90% in 200 miles of high-pressure, gas-gathering lines through the installation of pigging equipment, The project shifted 20 MMcfd of gas to the more efficient Okarche and Kingfisher plants,

The improved efficiencies increased line capacities and allowed GPM to shift gas between three of its plants in its Oklahoma region. This flexibility will allow for future growth of the system with minimal investment.

Although not quantified, fuel savings are expected from lower compressor discharge pressures. In some areas, discharge pressures were reduced by as much as 100 psig.

ACKNOWLEDGMENT

Efforts by GPM personnel in the Sooner and Cimarron gathering systems made this project successful.

REFERENCE

  1. Smith, R. V., et a]., "Flow of Natural Gas Through Experimental Pipe Lines and Transmission Lines," U.S. Bureau of Mines Monograph 9 (New York: American Gas Association, 1956).

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