John G. Greenslade
Phillips Petroleum Resources Ltd.
Calgary
Edward Wichert
Gascan Resources Ltd.
Calgary
Important new safety and operational features and some industry firsts are employed in a PC-based supervisory control and data acquisition (scada) system at Phillips Petroleum Resources Ltd.'s Ghost River sour-gas field and pipeline in a populated area near Calgary.
The scada system monitors and controls wells, line-heaters, pumps, and alarm and shutdown systems. Facilities are operated on a partially attended basis.
Operators carry cellular telephones and laptop computers equipped with internal modems to enable them to receive alarms and take appropriate action promptly.
Several safety features are incorporated into the alarm and shutdown system.
All aboveground facilities are equipped with atmospheric monitors for H2S.
Leak detection is inferred from continuous material-balance computation. Should a sour-gas leak be suspected, an automated resident-notification system provides early notice by telephone to potentially affected residents.
Noteworthy operational features include automated measurement, continuous meter-discrepancy calculation, and flow control of the wells. Observed pressure drops are compared with predictions for each pipe segment to monitor for obstructions to flow.
A pig-tracking routine displays both the location of a pig and its expected arrival time at the pig receiver.
GHOST RIVER FACILITIES
Since April 1990, Phillips has operated a four-well sour-gas gathering and pipeline system in the Ghost River area west of Cochrane, Alta. (Fig. 1). The gathering lines and pipeline are located in scenic foothills country and traverse Indian lands and ranchlands close to developed acreages.
Major crossings include one river, two highways, and a reservoir which has significant year-round recreational use. More than 130 residences are located in the emergency planning zone which encompasses the extent of the potential 100 ppm H2S isopleth.
The sour gas contains essentially no C3+ compounds. The acid-gas content varies slightly among the four wells; the H2S content is approximately 15-1/2% and the CO2 content is approximately 3-1/2%.
The hydrate temperature is about 26 C. at operating pressure. Normally, hydrates are prevented by the gas being heated. At low gas temperatures, methanol is automatically injected into the gas stream to inhibit hydrate formation.
The Ghost River field facilities include the following:
- Four wells with wellsite heaters, downhole hot-fluid circulation, inhibitor and methanol injection, flow control, and automated flow measurement
- 5 km of insulated 114.3 mm OD Gathering lines
- 28 km of insulated 168.3 mm OD pipeline
- Eight emergency shutdown (ESD) valve stations and two line heaters along the pipeline
- Methanol-injection facilities at each ESD valve station
- Fuel-gas line (2-in. OD) from the plant to the ESD valves, lineheaters, and wells.
The piping at each wellsite includes an ESD valve located near the wellhead and a flow-control valve after one pass through the wellsite heater. The wellsite sour-gas piping is designed for the initial wellhead shut-in pressure of 21.4 MPa (3,100 psi).
Downstream of the wellsite heaters, the sour-gas facilities are designed for a maximum operating pressure of 9,760 kPa (1,415 psi).
SCADA SYSTEM
The scada hardware (Fig. 2) includes a master terminal unit (MTU) and offline data-processing computer located in the field office and 11 remote terminal units (RTUs), one at each wellsite and ESD valve station.
The primary scada contractor was Galvanic Analytical Systems Ltd., Calgary. At the time of the project, Galvanic was a distributor for Bristol Babcock Inc. The RTUs, MTUs, modems, and software development were by Bristol Babcock.
ESD valve No. 1 (ESDV 1) is located at a wellsite and shares an RTU with the wellsite.
The communication link between the MTU and the RTUs is by radio, except for the RTU at ESDV 2 near the field office, which is hardwired. Repco Radio Canada Ltd. supplied the RTU radios. The base-station radio at the MTU was supplied by Midland International.
The hilly terrain necessitates the use of a radio repeater station.
Equipment at the field office includes the following:
- Two linked MTUs with Intel 186 processors and multiple communications cards
- One offline 386 processor-based computer with multiple communications cards
- An uninterruptible power supply (UPS)
- A 26-w UHF base radio
- A 1,200-baud external modem
- A telephone autodialer.
The equipment at each of the 11 RTUs includes the following:
- An Intel 186 processor-based RTU
- A 2-w UHF radio and 1,200-baud modem
- Analog and discrete input/output (I/O) devices
- A UPS.
Remote equipment includes:
- An 18-w UHF radio repeater station with UPS
- Four 386 processor-based portable laptop microcomputers with internal modems
- Four portable cellular telephones
- An IBM PS/2 computer with internal modem
- An IBM PS/2 computer equipped with telephone boards, a voice chip, and an internal modem.
The laptop computers (Toshiba 3100SXs) and cellular telephones are carried by the operators and their supervisor to enable them to monitor and operate the facilities remotely.
The PS/2 computers are located in the regional office in Calgary.
One computer is intended to be used only for monitoring the system. The other is the host computer for the automated resident-notification system.
Fig. 3 illustrates the scada-controlled equipment at each wellsite. This equipment includes valves (wellhead ESD, fuel-gas ESD, and flow control) and pumps (downhole fluid circulation, methanol injection, corrosion-inhibitor injection).
The following variables are monitored at each wellsite by the scada system, as illustrated in Fig. 4:
- Pressures: wellhead, meter-run static and differential
- Temperatures: wellhead, meter run, heater bath
- Status: ESD valve position, heater fuel-gas ESD valve alarm
- Ambient air H2S concentrations: at seven locations.
Similarly, the following items at each ESD valve station on the pipeline are scada controlled or monitored: controlled-ESD valve and methanol injection pump; monitored-ESD valve position, sour-gas pressure and temperature, heater bath temperature, ambient air H2S concentration (Fig. 5).
Additionally, at ESDV 3 near the midpoint of the pipeline, air temperature and wind speed and direction are monitored. These data are valuable for responding to potential sour-gas releases.
At ESDV 1, the start of the pipeline, the outlet temperature from each gathering system lateral is monitored and used to control methanol injection into the gathering system.
The scada system was designed to enable the wells and pipeline to be handled by one operator. The. operator, two back-up operators at nearby facilities, and their supervisor from the Red Deer area office all carry portable cellular telephones and laptop computers.
Alarms and notification of automatic shutdowns are passed by the scada system to a telephone autodialer, which alerts one of the operating staff 24 hr/day of the need to log onto the system to monitor the situation.
The system provides remote access to the complete suite of online monitoring and control features plus the alarm summary and historical data. Multiple remote connections can be made to allow operators and their supervisor to consult, monitor, remotely control, and manually intervene as appropriate.
SAFETY FEATURES
Several features of the system ensure its reliability.
MONITORING AMBIENT AIR
It is critically important in sour-gas operations to monitor and limit the accidental release of sour gas. Protecting the public and operations personnel is the most important consideration in any incident.
Establishing and maintaining public confidence in the safety of a sour-gas operation is essential. As part of the public consultation process during the planning of the Ghost River project, Phillips committed to the installation of H2S monitors in strategic locations at all above-round facilities.
The total number of H2S monitors is 36. Monitors are located as follows:
- Wellsites:
Four corners of each lease
One near each wellhead
One in each heater building
One near each vent from each heater bath
- Pipeline:
One near each ESD valve
One near the vent from each lineheater bath.
Whenever a monitor detects 20 ppm H2S, an alarm is recorded. This generates a telephone call to an operator.
If the H2S concentration exceeds 95 ppm at any monitor on a wellsite, the well is automatically shut in. If a pipeline ESD valve station or lineheater records 95 ppm H2S, the entire system is automatically shut in.
LEAK DETECTION
Leak detection is an innovative safety feature which has been incorporated into the scada system.
Production into the pipeline system is measured at each wellsite, and the flow out of the pipeline system is measured at the plant. Pressure and temperature are monitored at each wellsite and each ESDV station on the pipeline.
The gas stored at any moment in the entire pipeline system (linepack) is calculated. A rolling hourly material balance is used to record an alarm whenever the material balance reaches a specified discrepancy.
The entire system is automatically shut in if the material balance reaches a negative discrepancy of 6% of linepack within 1 hr or less. This important safety feature prevents wells from feeding sour gas into a leaking pipeline system until low-pressure shutdown limits are reached.
The feature also isolates leaks long before the pipeline depressures to the set point of low-pressure switches at adjacent ESD valves. This dramatically reduces the volume of sour gas which is likely to be released.
Phillips believes this use of material balance-inferred leak detection automatically to shut in a raw sour-gas pipeline system and wells is an industry first which is a valuable enhancement of Phillips' ability safely to transport sour-gas through populated areas.
RESPONSE
When the operator receives an alarm that indicates a potential sour-gas leak, he logs on to the system, using his laptop computer if he is away from the field office. He calls up a graphical display of pressure trends in all pipeline segments. Should there be an abnormal pressure decline, the operator initiates an automated early notification of nearby residents.
Data bases are maintained of residents within the hazard zone defined by the 100-ppm isopleth around each wellsite, pipeline ESD valve, and pipeline segment.
A prerecorded voice message is transmitted by telephone simultaneously by multiple voice cards to potentially affected nearby residents, prioritized by distance from the facilities. The message advises them of the potential H2S danger and reminds them of Phillips' recommended initial response to a potential nearby sour-gas release.
Subsequent actions of the operator and emergency response team conform to normal industry practices.
The early warning provided by the automated resident-notification system enhances the safety of nearby residents in the event of a sour-gas leak.
BACKUP CONTROL SYSTEM
A high level of integrity and safety is maintained in the case of problems with the scada system.
Smart RTUs retain local control and store data upon loss of communication with the MTU. ln the event of an RTU failure, a watchdog feature integral to each RTU imposes a programmable "fail" status on each scada-controlled item.
RTU failures at wellsites shut in wells because of the loss of flow measurement. But RTU failures at pipeline ESD valve stations preserve the last status, which allows production to be maintained.
Conventional pneumatic controls provide redundant emergency shutdowns of wells and pipeline valves, totally independent of the scada system.
CONTROLS; MONITORING
The wellsites and pipeline ESD valve stations are controlled pneumatically. The scada instrumentation interfaces with the pneumatic instrumentation for well flow control and for opening and closing ESD valves.
Wells can be flow controlled manually, or by scada. Flow control by scada incorporates remote well start-up and shutdown, fixed flow rate (operator specified), or variable flow rate (scada controlled to produce a specified total flow rate or to maintain a desired pressure at the plant inlet).
The scada system has been programmed to control well flow rates to maintain a specified pressure at the plant inlet. In this mode of operation, the scada system requires that one or more wells be placed on variable flow.
The plant operators control the gas rate into the plant, and the scada system reacts to changes in flow and pressure at the plant to adjust the production from the wells on variable flow to maintain a specified pressure at the plant.
A spin-off from the material-balance calculation is a continuous evaluation of the metering difference between the sum of the wellsite's meters and the sum of the plant's inlet meters.
If there are no leaks in the system, which should always be the case, discrepancies in the material balance are due to metering error. A calculation has been incorporated in the software which determines the metering discrepancy between measured production and plant receipts.
When a large metering discrepancy occurs, wells can be shut in alternately for a short while to identify the faulty meter.
PRESSURE DROPS; PIGS
A routine has been built into the scada software which, upon demand, calculates the downstream pressure for each section of the gathering system and pipeline by the general flow equations. The calculated pressures are displayed along with the measured pressures.
This routine is used to detect and locate inaccurate pressure transmitters, liquid holdup, and restrictions in the pipelines such as hydrates.
The gathering lines and pipeline are pigged occasionally. Because it is always of interest to know the location of the pig during such operations, a pig tracking routine has been included in the scada software.
The routine displays the distance of the pig from the launch point and the next valve station, and the estimated time of arrival at the pig receiver. This information is particularly useful for determining when to open or close the block valves on the main line that divert the gas flow through the line heaters, which must be bypassed by the pig.
Also displayed are the current and average velocities of the pig. Knowledge of travel speed is handy for controlling the speed of internal inspection tools, and distance information would be useful in locating a stuck pig or tool.
ALARM NOTICE
The scada system recognizes many alarm conditions, as mentioned earlier which are all logged on a printer in the field office.
Operators are alerted to selected alarms by telephone through a callout feature incorporated in the scada system. The operator can remotely monitor and control the system by connecting a portable computer to the system by telephone.
The scada system has been designed so that the MTU and network monitoring computer (Compaq 386) are simultaneously accessible by two remote computers. Any compatible computer with a security key and modem can be connected by telephone to the scada system to access all real time and historical data and control the system.
This makes it possible to operate the facilities on a partially attended basis by one operator, without sacrificing response time when alarm conditions occur, and enables supervisors and technical staff to support the operation on short notice from remote locations.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
