Pipeline & Gas Journal
October issue 1999:

BC Gas Field Benefits

PLC-Based Control System Holds 100% Engine Power Output

Talisman Energy Inc. does not buy engine horsepower to have it sit idle. In compression packages on its natural gas fields, the company expects continuous engine operation at 100 percent load, with greater than 96 percent availability.

Talisman is achieving those objectives on two sour gas fields in the Rocky Mountain foothills of British Columbia, Canada. The Sukunka and West Sukunka gas fields near Chetwynd, about 60 miles (100 km) southwest of Dawson Creek, produce a combined 102 million cfd. At both fields, Caterpillar G3600 Series gas engines drive Dresser Rand HOS reciprocating compressors, operating at 1,000 rpm.

At West Sukunka, a PLC-based control system automatically adjusts compressor swept volume to keep a 12-cylinder G3612 gas engine fully loaded at 3,550 hp. At Sukunka, a 6-cylinder G3606 unit delivers a continuous full-load at 1,715 hp. In both cases, output exceeds the nameplate rating by 3 percent—the manufacturer granted uprates based on Talisman’s advanced monitoring and maintenance practices and high-quality fuel supply.

Strong Track Record
Aggressive operating practices have helped propel Talisman to five consecutive years of record production, revenues and cash flow. The company, based in Calgary, posted total 1997 revenues of $1.4 billion and reported a record 851 successful oil and gas wells during the year, with a 91 percent success rate.

Talisman, an independent upstream oil and gas company, focuses on exploration, development, production and marketing of crude oil, natural gas and natural gas liquids. Production averages 700 MMcf/d of natural gas and 145,000 bpd of liquids, mainly from Canada, the North Sea and Indonesia. The company also has interests in oil fields and exploration in Algeria, Trinidad and Peru.

The two Chetwynd-area fields are among Talisman’s largest gas producers. Sukunka, established in 1978, now produces 30 MMcf/d. West Sukunka, established in 1991, delivers 72 MMcf/d. Both produce sour gas containing 8-9 percent H2S and 6-7 percent CO2 and saturated at suction pressure.
Talisman compresses the gas, dehydrates it using molecular sieve technology, and delivers it to the Grizzly Valley Pipeline System, owned by Westcoast Energy Inc. of Vancouver, BC. The gas travels to the WEI Pine River Gas Plant at Chetwynd for H2S and CO2 removal, then enters the Mainline "T South" line for transmission through British Columbia to the state of Washington.

The WEI Pine River plant supplies the two fields, via pipeline, with sweetened fuel gas approaching 99 percent methane.

Extracting Full Power
The Sukunka site, staffed around the clock, serves as the remote monitoring station for West Sukunka and a third Talisman gas field at Bull Moose, B.C. West Sukunka is staffed 8-10 hours per day. Reliable operation is critical, especially in winter, when the rugged terrain and typical snow depths of 9 to 10 feet make access to the compressor stations difficult.

At Sukunka, the G3606 engine and four-throw compressor take gas from suction pressure to 1,200 psi for entry to the gathering system. That package has operated since September 1996. At West Sukunka, the G3612 with a six-throw compressor, in service since October 1997, acts as a booster station, raising gas from suction pressure to 400 psi. Another compression package then elevates pressure to the final 1,200 psi.
The Caterpillar G3600 series engines are turbocharged and aftercooled units with a 300 mm bore and 9.2:1 compression ratio. High-energy ignition, an enriched prechamber design, a deep-cup oil gallery piston and electronic controls combine to deliver efficient combustion for low fuel consumption and low emissions (1 gram/hp-hr NOx at 1,000 rpm).

Talisman’s maintenance and operating practices are geared toward maximizing engine load and uptime. "Our operating philosophy is to assume our engines will deliver 100 percent power continuously during the first five years of operation," says Joe Bichel, senior facilities engineer for Talisman.
To that end, the G3612-driven compressor package at West Sukunka is controlled by an Allen-Bradley PLC programmed to monitor compressor suction and discharge pressure and automatically adjust compressor cylinder volume or engine speed to keep the engine fully loaded at all times.
The compressor’s six cylinders are equipped with:

• Head end suction valve unloaders (SVUs) which, when open, allow gas to bypass the suction valve, reducing compressor efficiency.
• Three pairs of different-sized head end fixed volume clearance pockets (FVCPs) which, when closed, decrease compressor swept volume.

Different combinations of open and closed SVUs and FVPCs create a total of 12 compressor load steps. During compressor operation, the PLC reads suction pressure and selects a load step, then checks engine load. If engine load is less than 100 percent, the PLC adjusts the compressor to the next highest load step. The process continues until the engine is fully loaded.

If the engine remains below 100 percent output at maximum compressor swept volume, the PLC orders an increase in engine speed.
To check the accuracy of the PLC control system and the power output reading on the engine control panel, Talisman performs pressure-volume analysis on the compressor. A transducer measures pressure in the cylinder during the stroke and feeds the result to a processor, which instantly calculates actual engine horsepower delivered.

Automatic Operation
Since installation, the Sukunka and West Sukunka engines have achieved 96.5 to 97 availability, a record Bichel credits to diligent maintenance and around-the-clock electronic engine monitoring designed to provide early warning against malfunctions and eliminate operator error.
Engines at both sites are equipped with the microprocessor-based Caterpillar engine supervisory system (ESS), which continuously monitors all important engine functions and controls for variations in fuel and load. The system continuously displays engine operating status, system fault indications and caution warnings and controls safety shutdown functions.

Sensors in each cylinder read engine speed, combustion timing and air temperature and feed the information to the microprocessor. If the values differ from preset levels, the microprocessor signals actuators that adjust air and fuel delivery to the combustion chambers.
In addition, an ionization sensor in each cylinder helps minimize exhaust emissions. It measures how long the flame front takes to pass across the combustion chamber and, if necessary, adjusts ignition timing accordingly.

A Customer Communication Module feeds data from the ESS to a Human-Machine Interface (HMI) in the central control facility at Sukunka and to Talisman’s custom-designed, Windows-based performance monitoring system.

"We use the system as a constant check on engine functions and as a database for maintenance and troubleshooting," says Bichel. "The system maintains a full day’s worth of historic engine operating data in memory. By looking at fault codes, we can detect minor, recurring problems and correct them before a costly failure occurs.

"In the event of an alarm or a shutdown, the system records the time, date and engine run hours and permanently saves all operating data up to one hour before the event. That enables us to go back and determine the cause. Quick diagnosis helps our technicians attack the problem immediately and get the unit back on line quickly."

Proactive Maintenance
Offsets Downtime
Talisman’s maintenance practices help keep downtime to a minimum. The Sukunka and West Sukunka engines are maintained in strict adherence to the manufacturer’s recommendations. Talisman staff members perform scheduled maintenance, including regular lube oil analysis. The Grand Prairie, Alta., office of Finning Power Systems, the local Caterpillar dealer, supplies major service, including a complete annual engine checkout that includes examination for valve recession and bore-scoping to monitor cylinder liner wear. Inspections to date indicate both engines are likely to operate for 30,000 to 35,000 hours before their first top-end overhaul.

For now, Bichel reports activity in the Sukunka and West Sukunka fields remains strong, in part, because of the high availability of service parts and the engines’ ability to supply the required horsepower. P&GJ