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EFM units monitor gas flow.

Several years ago, Transcontinental Gas Pipe Line Corp. decided to equip all its natural gas purchasing metering facilities with electronic flow measurement computers to monitor the gas flow. Data transmission radios were chosen to collect the flow data wherever geography was favorable and frequencies were available.

TGPL used an existing microwave backbone route across its Gulf of Mexico service area, along with 900-mHz short-range radio to link the backbone system with the individual flow computer remote terminal sites along the route.

A new data transmission scheme, 900-mHz multiple address system radio was chosen as the short-range transmission medium, A MAS consists of one or more master radio transmitter/receiver units and multiple remote radio transmitter/receiver units.

This type of system allowed reach remote RTU to be discretely addressed and interrogated for the gas flow being metered by each facility in real time.

The current microwave and MAS radio network, anchored by a computer-based supervisory control and data acquisition system located at TGPL's Houston headquarters, has grown to more than 530 land-based and offshore metering sites and 45 MAS master locations throughout the Gulf Coast gas production area and downstream delivery points.

Each RTU is polled sequentially at regular intervals by the SCADA system. Gas flow data from the entire network are gathered and reported on a continual basis for billing and productivity analyses.

Upgraded Equipment

In 1993, TGPL decided to upgrade the equipment at several 900-mHz MAS radio sites, most of them offshore. Production of the original equipment had been discontinued by the manufacturer and it was becoming cost-prohibitive to repair the units.

New radios offered frequency-synthesized operation with remote diagnostic capabilities. Due to the number of offshore locations, reducing helicopter trips by diagnosing defective radios from a land-based personal computer would help justify the cost of the replacement program.

The radio changeout program was coordinated with TGPL measurement technicians. The remote radios were installed during normal monthly testing of the flow computers to eliminate special trips to the platforms for set up.

Advanced Features

One type of MAS radio selected to replace the original equipment proved to be ideal for TGPL's applications. The radio, manufactured by Alligator Communications Inc., Sunnyvale, Calif., incorporates many advanced technological features that are especially valuable for communications with remote, unmanned locations.

The radio allow complete remote diagnostics and control from a master station. Alligator's "over-the-air" capabilities eliminate the need to spend valuable time and money traveling to remote radio sites to make adjustments, run diagnossic tests or change alarm limits.

"We found that the most useful function of the Alligator radio was the ability to change frequency and fine-tune the remote radios from the master station," said John Kryvanick, telecom measurement engineer TGPL's offshore base for the eastern Gulf of Mexico at Schriever, La.

Cell Changeout

The goal of the MAS cell changeout was to install the new remote radios tuned to the old frequency while the other remote equipment was still in place. The existing antennas and power supplies were not changed in the process.

The cutover involved setting the new Alligator Master A transmitter and receiver to the old radios' wideband frequency pair. The new Alligator Master B transmitter and receiver were preset for the new radios narrowband frequency pair.

The A equipment was put on the air until all remote units were in place. Then each remote was addressed in turn through the Alligator Master Station by a laptop computer running Alligator software. The remote radio frequencies and deviation were changed to the B settings and the Master's B transmitter while the A transmitter was re-turned to the new frequency. The remotes were then fine-tuned using the

Alligator frequency adjust option

"Alligator radios enabled us to change out the equipment in the entire MAS cell with virtually no downtime and few helicopter trips," said Kryvanick. "In a replacement procedures using a non-synthesized radio, a trip to each of 19 production platforms was necessary to change frequencies of the remote radios. This took a full day of helicopter time.

"The cutover with Alligator radios was accomplished with only one trip to the master site."

TGLP plans to continue the MAS radio replacement program throughout the entire network.

Software Driven

Remote frequency change and fine tuning is possible because the Alligator radios incorporate frequency-agile synthesizers that are tunable over a range of frequencies.

This minimizes the need for multiple spare radios as well as providing easy adjustment. Otherwise separate replacement units would be required for each set of frequencies in use with the MAS.

The entire network of remote radios can be monitored over- the-air thanks to advanced microprocessor control. Alligator software monitors and displays RSSI, frequency offset and deviation. It can also monitor forward and reflected power, received signal strength, power supply voltage, phase lock loop voltage and internal radio temperature.
COPYRIGHT 1994 Oildom Publishing Company of Texas, Inc.
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Copyright 1994 Gale, Cengage Learning. All rights reserved.

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Title Annotation:electronic flow measurement
Publication:Pipeline & Gas Journal
Date:Feb 1, 1994
Words:800
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