Production Technology

Electric generator that uses gas flow for energy source proves robust

 Chevron tried out the turbo-electric generator offshore and onshore Cabinda, Angola. Wet and corrosive gas were not problems. 

Neil C. Saucier, Natural Gas Turbine Technologies, Prior Lake, Minnesota

Solar panels are not always reliable, given the vagaries of weather, and, offshore, they are sometimes stolen. Chevron had been searching for years for a more reliable remote power source for its gas-lift control valves and SCADA equipment at its offshore and onshore operations in Cabinda, Angola.

In 2003, Chevron contacted Natural Gas Turbine Technologies, Inc., to discuss the feasibility of using its technology to generate power in remote locations using wet and potentially sour natural gas. The company has been building power-generating turbines since 1991, primarily in the US domestic gas distribution market. These turbines, which use only the flow of gas to generate electricity, have been running in some locations for 14 years.

Unlike a vane turbine, these turbines exploit the energy from gas flowing around a “knurled” central rotor, minimizing pressure drop and gas-expansion cooling. The sealed ceramic ball bearings are non-corrosive and will run for years even if they run completely dry.

For Chevron, the question was whether this technology could stand up to very harsh environments and operate successfully using wet, contaminated lift gas. In 2004, a 150-watt prototype turbo-electric generator was delivered for testing on one of Chevron’s offshore well jackets in its Takula oil field in Cabinda, Angola. The unit was identical to a standard, proven GT-150 unit in every respect, except the housing was electroless nickel plated inside and out. In addition, it was rated for an operating pressure of 1,800 psi, rather than a standard 600 psi, and the internal electrical components were epoxy encapsulated to stand up to wet/sour gas.

The prototype was installed in late 2004. With the exception of a few problems with the electronic battery charger, which were unrelated to the operation of the turbo-electric generator, the prototype ran for nearly one full year before being taken out of service for disassembly and inspection. Upon inspection, the prototype unit showed almost no signs of wear. It had held up well against the wet gas going through it and had demonstrated to Chevron’s satisfaction that this technology could operate successfully in harsh environments using wet lift gas.

On the strength of the prototype’s results, Chevron and NGTT settled on a new specification for a production unit, the GT-400. As the name implies, the unit was to deliver 400 watts rather than 150 watts of power, using substantially the same amount of gas flow as the prototype. It was to be rated at 1,800 psi operating pressure and be FM Approved for Class 1, Div. II locations. The electronic control was specified to run 24-VDC equipment directly and charge backup batteries at the same time. In addition, the electronic control would have reporting capabilities using Modbus protocol and be capable of remote shut down. In 2005, Chevron ordered 10 units contingent upon meeting the specifications.

A 400-watt turbine being tested offshore Cabinda, Angola.

The turbines performed to Chevron’s specifications, and the 10 units were delivered in July 2006. The first 400-watt unit was installed on Chevron’s Wamba-Bravo offshore well jacket in November 2006. The unit takes lift gas from a 910-psi line. The gas enters the unit through a ¾-in. line. After turning the turbo-rotor flywheel and generating the required electricity, the gas is returned to a 180 psi oil production line (see figure). It is estimated that 57,600 cfd are passing through the generator, providing enough DC power to run 14 Flowserve Valtek StarPac control valves and a PLC, while keeping 16, 12V-backup batteries charged—about 12 amps at 24 VDC.

It is Chevron’s intention to install two GT-400s at each of its locations where lift gas is available. One unit will be primary and the other a backup in the event the primary unit fails for any reason. The electronic control package that comes with the GT-400 makes it possible to switch to the backup unit by using its onboard 24V relay to open and close valves in front of each unit, using a built-in RS485 port.   

THE AUTHOR
	Neil C. Saucier JD is president and founder of Natural Gas Turbine Technologies, Inc. He earned a doctorate in law and is an accomplished machinist and tool maker. Saucier has been involved in new technology development companies most of his 25-year career. He can be contacted at: neilsaucier@integraonline.com.