Alton Meyer
Aquila Gas Pipeline Corp.
Giddings, Tex.
The supervisory control and data acquisition (scada) system along Aquila Gas Pipeline's more than 2,000-mile system evolved without a controlling plan over the last 14 years to become, nonetheless, workable and effective.
For 10 years until 1991, a network of dumb remote terminal devices transmitted data to the control center at Giddings via 173 mhz VHF radios. This system was strictly for monitoring the pipeline; there were no remote controls.
The old system operated at about 20 bits per see (bps), monitoring 210 points and taking a complete poll in 20 min. Although this system served the company well, in 1991 the company bought another pipeline and agreed to sell gas to a power plant.
This change involved adding two new highly automated metering stations that required a more sophisticated remote terminal unit (RTU).
To fulfill this need, two Fisher remote operating controllers (ROCs) were purchased. The vendor also provided three more to be tried as electronic flow meters.
Meanwhile, horizontal drilling arrived in the Central Texas Austin Chalk oil field, prompting rapid expansion. Sales of natural gas increased from 60 MMcfd in 1991 to more than 300 MMcfd in 1994.
The pipeline network increased from 1,200 miles in 1991 to more than 2,000 miles in 1994. Telemetering stations increased from 35 to 62, and requirements for speed and accuracy changed drastically.
Two new requirements were remote operation of block valves and control valves.
AUSTIN CHALK LOCALE
Aquila Gas Pipeline operates in Central Texas in an area approximately 105 miles x 30 miles in the Giddings oil field. This area extends from 30 miles east of Austin to 65 miles northwest of Houston (Fig. 1).(30955 bytes)
The company Is network of more than 2,000 miles of pipeline varies from 3-in. polyvinyl pipe to 20-in. steel. Operating this complex are 62 instrumented stations, including 2 cryogenic gas plants, 1 amine treating plant, 40 compressor stations, 3 pig launcher/receiver sites, and 14 meter stations.
Aquila buys gas from more than 1,000 wells, gathers and processes it, and sells the NGL and residue gas.
Aquila's gathering system is typical of any natural-gas gathering pipeline in that it is in constant evolution that follows the typical lifecycle of a gas field.
A new field has relatively high pressure and requires a minimal amount of compression. As the field ages, the field's pressure drops require more compression. Later, as the field is further depleted, there is less gas production and fewer compressor stations.
Closing old compressor stations and building new ones causes frequent movement of scada stations and modifications - such as instrumenting new functions - to existing ones.
Another scada problem occurs at the delivery stations where the company sells or delivers gas to another pipeline. Aquila has seven sales points and exchanges gas with other companies at many points.
Exchanging gas with other companies occurs if one company buys gas from a well near another company's pipeline, the gas will be put into that pipeline and later paid back at a payback point.
Many marketing decisions require accurate real-time metering and flow control because the market changes hourly. Scada provides this timely information.
There is keen competition among different companies' gas buyers to buy gas from oil and gas producers. Aquila's buyers woo producers with promises of how quickly the company can build pipelines to producers' wells and how efficiently these lines will be operated.
Once the deal is closed, the rest of the company must make good on these promises. One promise is for scada-equipped compressor stations to provide efficient operation. This in turn should provide constant low line pressure allowing the gas wells to flow at their optimum rate.
Construction of compressor stations proceeded so rapidly that it overloaded the communications section which is responsible for the scada system. Sometimes the scada portion of a station was put up in 1 day. At other times, the instrumentation was done piecemeal over a period of days as the station was being constructed.
Stations were often built so fast that there were no blueprints. After the fact, engineering drew "as-built" blueprints.
During construction, communications personnel worked busily to keep up with construction.
Aquila Gas Pipeline has a variety of scada stations with many functions. A small compressor station may be only one compressor with only compressor status being transmitted. A larger compressor station may be instrumented to send five pressure and temperature readings, a flow rate, and the status of compressors.
On a meter station, the flow rate and the standard gas meter readings are telemetered. Flow rate is controlled with a PID-operated control valve (PID = proportional integral derivative).
There are also pig launcher/receiver stations designed to launch pigs and verify departure and arrival of pigs.
Valves are opened and closed, samplers are pulsed, and gas chromatographs are monitored through modbus.
(Modbus is a communications protocol that operates through the serial communications port (RS-232) of a host computer sending queries, consisting of an address of a register, to one or more remote devices. The remote device will reply with the information stored in the addressed register.)
The system also monitors the functions in three gas-processing plants. In one unattended plant, three programmable logic controllers are monitored through modbus, and there is an emergency shutdown feature allowing the plant to be shut down from the control center.
In 1981, Aquila (then Clajon Gas Co.) purchased a scada system that operated successfully for 10 years. This was a complex of dumb remotes with each point being polled separately via VHF radio.
This meant that a station with 10 points to be polled required 10 interrogations and 10 answers. The polling message and the reply each consisted of 20 bits of information.
Each one-way message took 1 sec; each reading, therefore, required about 2 sec. A complete poll of 210 points required about 20 min. Although slow, the system was adequate until the period of rapid expansion.
When the original system was installed, stations were instrumented to obtain required readings; the new network retains some of these instruments. The largest station was a gas plant from which 22 points, including six flow meters, were telemetered to the control center.
On a typical compressor station, six readings were sent, including field pressure, suction pressure, discharge meter pressure, temperature, flow rate, and status of the compressor engines. On a small compressor station, only the status of the compressor engines was monitored.
At its peak there were 42 stations on this old system. The central computer had no disk drives, and the program was all on PROM (programmable read-only memory). The central computer had two RS-232 ports with a serial printer on one and a serial video-display terminal on the other.
The company had no listing of the program; changes were made by the software vendor for $3,000 each time the program was changed.
This problem was solved by using a Radio Shack 6000 to replace the central computer, and the programming was written within the company. This computer was later replaced with an IBM model 25 PC.
The old system was completely replaced by the current system in March 1994.
SELECTING AN BTU
When in 1991 Aquila acquired a pipeline that was supplying gas to a power plant, two metering and control stations were planned at either end of this new line.
These new stations required more control functions than were available on the old system which could perform simple control functions but not remote flow calculations and PID control loops.
Two Fisher ROC's were purchased to perform the monitoring and control functions. These two stations were the most extensively instrumented stations ever built by Aquila company and represented the company's first experience with remote PID controls and smart remotes.
The first station had two control valves for pressure control that were hooked up in the split control mode. The station also had a turbine meter to measure flow rate. Another feature was an online gas chromatograph that was read by the ROC through modbus.
During construction of this station, company personnel underwent formal training on the Fisher ROC. Most of the training came from the experience gained working the bugs out of the system once it began operating.
Shortly after completion of the first station, a similar station was built at the other end of the pipeline. This station consisted of two orifice meters, a control valve to control flow, and an on-line gas chromatograph.
Communications to the stations were handled through dial-up telephone lines. The central computer at the control center was operating on GV110 software developed by Fisher for the ROC.
PROBLEMS, LESSONS
Many lessons were learned on these two stations.
The first had to do with programming the ROC for a turbine meter which required programming the discrete input module for the input pulses with the right timing. Numerous time settings were attempted before the correct one was found.
Another problem involved the modbus communications with the gas chromatograph.
Documentation for both pieces of equipment on modbus was very sketchy. Many hours were spent on the phone talking to people at both factories about the modbus protocol before the communications were properly working.
Many other small problems with the ROC programming and operation were solved by numerous phone calls and trial and error.
Meanwhile, the three ROCs given to the company to test as gas-well meters performed well as electronic flow meters. The only problem was that they had much more capability than was needed on a simple gas-well meter and were too costly.
Installation of a ROC on a meter requires installation of a ROC "tombstone" with a large solar cell. Additionally,
transmitters had to be installed on the meter and wired to the main unit.
No communications were used on these units. Periodically, an operator downloaded the readings to a laptop computer. Although the ROC was unsuitable as a gas meter, company technicians gained valuable experience.
The power plant being supplied gas was for peak demand, operating only during high demand for electrical power. Without that high demand, the plant would shut down.
This created a problem for the control center because a new market had to be found for the gas that had been going to the plant.
During the summer this was daily. There would be a peak demand for electrical power for air conditioning until 10 p.m. when the plant would be shutdown and then restarted the next morning. Two people had to be called out on overtime to switch the gas to another sales point.
The solution: Two control stations with control and block valves remotely operated through the ROC. This saved the company 8 Jar of overtime each evening.
It allowed monitoring and control of the sales point and provided a faster changeover from one sales point to another. The ability to control flow rate and sell a predetermined amount of gas at a sales point was an added bonus.
PRODUCTION INCREASES
Coincidentally, horizontal drilling arrived in the Giddings Austin Chalk oil field. Previously all the wells in this field were vertical holes drilled into the oil and gas-bearing formation.
The Austin Chalk is a nonporous formation with most of the petroleum in faults or cracks in the rock. A vertical well drilled into one fault would drain the oil and gas from that crack but not from adjacent ones sometimes only a few feet away.
In horizontal drilling, some welts are drilled with horizontal bore holes 6,000 ft in each direction from the vertical hole. This allows oil and gas production from an area 12,000 If long.
The fault lines generally run east and west, and the bore holes run north and south. This allowed producers to extract additional oil and gas from areas that had been previously drilled.
All this activity created a boom in gas production that increased Aquila's production five-fold in less than 3 years. During this boom, the company constructed many metering and compressor stations.
These new stations required new scada units, and the old units were no longer being manufactured. Also, replacement parts for the old system were no longer available.
The company started a formal study to find a new system. During the search, construction of new stations needing instrumentation continued at full speed.
As these stations were built, ROCs were bought and installed as the station RTUs. Communication with these new units was through a leased trunked radio network. (A more detailed discussion of the communications medium occurs later.)
Before completion of the study, 15 stations were equipped with ROCs. When the time came for a decision on buying a new RTU, the company already had a large investment in ROCs, and company technicians had developed considerable expertise on the ROC.
Aquila decided to approach the vendor for a bid on replacing the remaining old RTUs. The understanding was that if the bid price were unsatisfactory, Aquila would go to other vendors for bids on a new system.
The bid was satisfactory, and enough ROCs were purchased to convert the entire system. Aquila will never know if a better or more economical RTU was available. The important thing is that the system works and was available at an affordable price.
The natural-gas gathering business is very competitive. Speed in building lines and efficiency of operation is sometimes more important than the time expended finding the very best deal.
This does not mean price is not important. Aquila Gas Pipeline is a profit-making organization and operates in a volatile commodities market. This is an unconventional way to purchase a system, but the company has a very economical and workable system.
Until 1992, Aquila used GV110 software as the operating program in the central computer in gas control, but it was not versatile enough for the requirements of the system.
The solution was Factorylink, a software package from USData that interfaced with the ROC. It was placed in operation in fall 1992.
This program runs on an OS/2 operating system in a 486 Compaq computer and is adaptable to the changing requirements of the pipeline. It is almost a full-time job maintaining the software and making necessary changes.
SELECTING COMMUNICATIONS
The first ROCs communicated with the central computer by dial-up telephone lines. Although the telephone communications were reliable, it was a slow process requiring dial-up time and a data rate of 1200 baud.
It was also expensive, about $250-300/month for one station. And phone service is unavailable in some remote locations. Although telephones are still used at the most important stations as a backup to the radio system, this was unsatisfactory for more general use.
The next choice was the 173-mhz VHF radio frequency for which the company was already licensed. This frequency had some interference and the narrow band authorized for it prevented any data transmission faster than 1200 baud.
Even at this speed, only 50% of the data were successively transmitted. A new means of communication was needed quickly.
The next solution was buying service on a trunked system that used a series of tones to communicate data at 1200 baud. This system operated successfully for 2 years.
Its biggest drawbacks were cost and lack of service: The cost for 22 radios was about $1,200/month. Whenever the repeaters went down at night or holidays, the owners didn't want to repair the system until normal business hours.
In one instance, threats of a lawsuit were required to get them to come out in the middle of the night.
A trunked system is satisfactory and convenient if owned by the company using it, allowing repairs to be made whenever necessary.
During first studies on a new scada system, a consultant told Aquila no multiple-address system (MAS) frequencies were available in the company's area. Some investigation, however, revealed that not to be the case.
Aquila applied for and received licenses for two MAS master stations. One was in Giddings at the control center and the other near College Station (Fig. 2).(47395 bytes)
The MAS station in Giddings is hard wired to the central computer in gas control. A point-to-point microwave is used to communicate from gas control to the MAS near College Station. This provides radio coverage for all Aquila Gas Pipeline's operational area.
The radios for this network were purchased from Microwave Data Systems and operate in the 928-952 mhz frequency bands and at a baud rate of 4,800.
Under most conditions, these radios operate well. Communications are degraded whenever a cold front with many thunderstorms is approaching. Also, communications are poor when high barometric pressure covers the area and temperatures exceed 100 F.
Ice on the antennas will detune them and cause communications failure. This seldom happens in Central Texas or, when it does, lasts for only a day or two.
A good signal path is much more important in the microwave band than in the VHF band. Many locations where VHF radios worked won't work with microwave radios. Some antennas had to be raised, and some relay stations are used to communicate with stations behind hills.
The company bought software with a terrain data base that will compute point-to-point signal paths and the terrain profile along the path. This is invaluable for planning new remote stations.
Also, a Magellan global positioning system navigator was purchased to pinpoint the latitude and longitude of new sites. This is an accurate device with an error of 100 m or less.
The combination of these two tools allows preconstruction analysis of new stations and prevents poor paths.
TERRAIN PROBLEMS
Aquila had two stations on low terrain by the power plant, one inside the plant and the other outside the fence. Analysis revealed that obtaining a good signal path required a 150-ft tower on each location. Space limitations at both spots required free standing towers. A freestanding tower that tall costs at least $25,000.
A station 2 miles away had a good signal path. The solution was spread-spectrum radios. These are radios that operate unlicensed under Part 15 of U.S. Federal Communications Commission rules.
There are, however, restrictions on how they operate. These include a limit of 1 w of power output. The radios must spread this power over a large frequency band or use frequency hopping.
The radios used by Aquila Gas Pipeline use frequency hopping.
They operate on 64 frequencies, hopping to them randomly and staying on one frequency for 250 ms.
A master radio will determine the hopping pattern for a network of slave radios which follow the master to these frequencies.
For Aquila, a spread-spectrum master radio was piggy-backed to the radio at the good site. It relayed signals to and from slave radios at the lower sites. Using these radios was very economical.
Spread-spectrum radios require no license and make excellent relay radios. The master spread-spectrum radio is piggy-backed to a MAS remote radio by a "Y" cable to connect the MAS radio to both the RTU and to the spread-spectrum master radio.
In this configuration, all messages received by the MAS radio are retransmitted by the spread-spectrum radio (Fig. 3).(49846 bytes) Alternatively all messages received by the spread-spectrum transceiver are retransmitted by the MAS radio. The slave spread-spectrum radios communicate with the master.
By September 1993, Aquila was still operating 27 stations with the old scada system in parallel with 18 stations operating on the ROCs. At this time a massive effort was started to finish the conversion to an all-ROC system.
A special crew was organized consisting of two tower service personnel and one company man to install MAS antennas on 45 different locations. They accomplished the task by traveling to all the sites with a 2,500-ft spool of heliax cable, numerous parts, and antennas.
The task was completed in 1 week.
Next a contractor assembled complete stations on 2 x 4 ft pieces of plywood. Each assembly consisted of a ROC, a radio, surge protectors, and all wiring. The wiring included a pigtail to be attached to the existing instrument wiring on the old stations.
Most of these units were installed in about 2 weeks. Some stations such as pig-signal stations were not installed until the ROC's were programmed to record pig-signal trip times.
The final conversion to ROC's was accomplished in March 1994, and the final radio changeover from trunked to microwave was accomplished in May 1994.
Copyright 1995 Oil & Gas Journal. All Rights Reserved.
