Improving plant performance

and Doug Yates, P.E., Hans P. Weyermann, Karl Masani, Weldon Ransbarger, and Satish Gandhi, ConocoPhilllips Company, Houston, Texas

Market pressures for new thermally efficient and environmentally friendly LNG plants, coupled with the need for high plant availability, have resulted in the world’s first application of high-performance PGT2500+ aeroderivative gas turbines for the 3.7 million tons per annum (MTPA) Darwin LNG plant in Australia. The plant was operational several months ahead of contract schedule, and has exceeded its early production targets. There have been several milestones along the way, including the world’s first aeroderivative-based gas turbine plant, and future potential for the application of larger aeroderivative drivers. These facilities are an excellent fit for the ConocoPhillips Optimized CascadeSM LNG process.

Aeroderivative engines fit the Optimized Cascade process because of the “two-trains-in-one” design concept that facilitates the use of available aeroderivative engines. The plant is able to operate at reduced rates of 50% to 70% in the event that one refrigeration compressor is down. The application of a range of larger aeroderivative engines that are now available allow a flexible design fit for the Optimized Cascade process. Benefits of aeroderivative engines over large heavy-duty single and two-shaft engines include significantly higher thermal efficiency and lower greenhouse gas emissions, the ability to start up without the use of large helper motors, and improved production efficiency due to modular engine change outs.

The goal here is to cover several practical aspects of the application of aeroderivative gas turbines as refrigeration drivers, and discuss design and implementation considerations. The selection of aeroderivative engines and their configurations for various train sizes, and evaluation of emission considerations are covered as well.

Darwin LNG project

The Darwin LNG plant was successfully commissioned and the first LNG cargo was supplied to the buyers, Tokyo Electric and Tokyo Gas, on February 14, 2006. The Darwin LNG plant represents an innovative benchmark in the LNG industry as the first to use aeroderivative gas turbine drivers. This follows another landmark innovation by ConocoPhillips, which was the first company to apply gas turbine drivers at the Kenai LNG plant in Alaska built in 1969.

The Darwin plant is a nominal 3.7 MTPA capacity LNG plant at Wickham Point, located in Darwin Harbour, Northern Territory, Australia, and is connected via a 500-km, 26-in. subsea pipeline to the Bayu-Undan offshore facilities. The Bayu-Undan field was discovered in 1995 approximately 500 kilometers northwest of Darwin, Australia, in the Timor Sea. Delineation drilling over the next two years determined the Bayu-Undan field to be of world-class quality with 3.4 TCF gas and 400 MMbbls of recoverable condensate and LPG. In February 2004, the Bayu-Undan offshore facility commenced operation with current production averaging 70,000 bbls of condensate and 40,000 bbls of LPG per day.

The Darwin plant has established a new benchmark in the LNG industry by being the first LNG plant to use an aeroderivative gas turbine as refrigerant compressor drivers and also the first to use evaporative coolers. The GE PGT25+ is comparable in power output to the GE Frame 5D gas turbine but has an ISO thermal efficiency of 41% compared to 29% for the Frame 5D. This improvement in thermal efficiency results in a reduction of fuel required, which reduces greenhouse gas in two ways. First there is a reduction in CO2 emissions due to a lower quantum of fuel burned. The second greenhouse gas benefit results from a reduction in the total feed gas required for the same LNG production. The feed gas coming to the Darwin LNG facility contains carbon dioxide, which is removed in an amine system prior to LNG liquefaction and is released to the atmosphere. The reduction in the feed gas (due to the lower fuel gas requirements) results in a reduction of carbon dioxide or greenhouse gas emissions from the unit.

The Darwin plant incorporates several other design features to reduce greenhouse gas emissions. These include the use of waste heat recovery on the PGT25+ turbine exhaust that is used for a variety of heating requirements within the plant. The facility also includes the installation of ship vapor recovery equipment. The addition of waste heat and ship vapor recovery equipment not only reduces emissions that would have been produced from fired equipment and flares, but also result in a reduction in plant fuel requirements. This reduction in fuel gas results in a lowering of carbon dioxide released to the atmosphere.

The Darwin LNG plant has been designed to control nitrogen oxide emissions from the gas turbines by utilizing water injection into the combustor. Water injection allows the plant to control nitrogen oxide emissions while maintaining the flexibility to accommodate fuel gas compositions needed for various plant operating conditions, without costly fuel treatment facilities that may be needed for dry low NO_x combustors.

The Darwin plant uses a single LNG storage tank, with a working capacity of 188,000-m3, which is one of the largest above ground LNG tanks constructed to date. A ground flare is used instead of a conventional stack to minimize visual effects from the facility and any intrusion on aviation traffic in the Darwin area. The plant uses vacuum jacketed piping in the storage and loading system to improve thermal efficiency and reduce insulation costs. MDEA with a proprietary activator is used for acid gas removal. This amine selection lowers the regeneration heat load required, and for an inlet gas stream containing over 6% carbon dioxide, this lower heat load results in a reduction in equipment size and a corresponding reduction in equipment cost.

The Darwin LNG project was developed through a lump sum turnkey contract with Bechtel Corporation that was signed in April 2003 with notice to proceed for construction issued in June 2003. The plant was completed on budget and two months ahead of schedule.

Plant design

As mentioned, the Darwin LNG plant utilizes the ConocoPhillips Optimized Cascade LNG process. This technology was first used in the Kenai LNG plant in Alaska and more recently at the Atlantic LNG in Trinidad (four trains), Egypt LNG (two trains), and a train in Equatorial Guinea. A simplified process flow diagram of the Optimized Cascade process is shown in Figure 1.

Selection of aeroderivative engines

The selection of the gas turbine plays an important role in the efficiency, level of greenhouse gas emissions, and flexibility under various operating conditions. The gas turbine selection for Darwin LNG was based on the economic merits that the turbine would deliver for the overall lifecycle cost of the project.

Where high fuel costs are expected, the selection of a high efficiency driver becomes a strong criterion in the lifecycle cost evaluation. However, LNG projects are developed to monetize stranded gas reserves, where the low-cost fuel has favored industrial gas turbines. Further, in situations where the gas is pipeline or otherwise constrained, there is a clear benefit in consuming less fuel for a given amount of refrigeration power. In such cases, a high efficiency gas turbine solution, where the saved fuel can be converted into LNG production, can result in large benefits.

Aeroderivative gas turbines achieve significantly higher thermal efficiencies than industrial gas turbines as shown in Figure 2. This figure shows the engines’ thermal efficiency versus specific work (kW per unit air mass flow). The higher efficiency of an aeroderivative can result in a 3% or greater increase in overall plant thermal efficiency. Further, there is a significant improvement in plant availability as a result of the ability to completely change out a gas turbine generator (or even a complete turbine) within 48 hours versus fourteen or more days that would be required for a major overhaul of a heavy duty gas turbine.

The GE PGT25+ aeroderivative gas turbine is used as the refrigerant compressor driver at the Darwin plant. The PGT25+ is comparable in power output to the GE Frame 5D but has a significantly higher thermal efficiency of 41.1%. This improvement in thermal efficiency results directly in a reduction of specific fuel required per unit of LNG production. This reduction in fuel consumption results in a reduction in CO2 emissions. This impact is depicted in Figure 3, which shows relative CO2 emissions for various drivers.

A similar beneficial greenhouse gas reduction comes from the use of waste heat recovery on the PGT25+ turbine exhaust that is used for various heating requirements within the plant. The use of this heat recovery eliminates greenhouse gas emissions that would have been released had gas fired equipment been used. The result of using waste heat recovery equipment is a reduction in greenhouse gases by approximately 9.3% of the total emissions without the installation of this equipment.

Advantages of aeroderivative engines

There are several advantages of using aeroderivative engines. These include: 

• Much higher efficiency with its advantages of reduced fuel consumption and reduced greenhouse emissions. 
• Ability to rapidly swap engines and modules thus improving maintenance flexibility. 
• Excellent starting torque capacity – aeroderivative engines have excellent torque-speed characteristics allowing large trains to start up under settle out pressure conditions. 
• The engine is essentially zero timed after six years. Maintenance can also be done “on condition,” allowing additional flexibility. 
• DLE technology available and proven on several engines. 
• Relatively easy installation due to low engine weight.

Compressor configurations

The Darwin LNG compressor configuration encompasses the hallmark two-in-one design of the Optimized Cascade process, with a total of six refrigeration compressors configured as shown below in a 2+2+2 configuration. All of the turbomachinery was supplied by GE Oil & Gas (Nuovo Pignone). 

• Propane: 2 X PGT25+ + GB + 3MCL1405 
• Ethylene: 2 X PGT25+ + GB + 2MCL1006 
• Methane: 2 X PGT25+ + MCL806 + MCL 806 + BCL608.

Both the propane and ethylene strings have speed reduction gearboxes. All compressors are horizontally split except for the last casing of the methane string which is a barrel design. The gas turbines and compressors are mezzanine mounted as shown in Figure 4, which facilitates a down nozzle configuration for the compressors.

Engine technology for Darwin project

The PGT25+ engine used at the Darwin plant has a long heritage starting from the TF-39 GE aeroengine as shown in Figure 5. This highly successful aeroengine resulted in the industrial LM2500 engine which was then upgraded to the LM2500+. The PGT25+ is essentially the LM2500+ gas generator coupled to a 6100 RPM high speed power turbine (HSPT). The latest variant of this engine is the G4, rated at 34 MW.

The first LM2500+, design was based on the successful heritage of the LM2500 gas turbine that was completed in December 1996. The LM2500+ was originally rated at 27.6 MW, and a nominal 37.5% ISO thermal efficiency. Since that time, its ratings have grown to its current level of 31.3 MW and a thermal efficiency of 41%.

The LM2500+ has a revised and upgraded compressor section with an added zero stage for increased air flow and pressure ratio by 23%, and revised materials and design in the high pressure and power turbines.

Description of gas turbine

The PGT25+ consists of the following components.

Axial flow compressor. The compressor is a 17-stage axial flow design with variable-geometry compressor inlet guide vanes that direct air at the optimum flow angle, and variable stator vanes to ensure ease of starting and smooth, efficient operation over the entire engine operating range. The axial flow compressor operates at a pressure ratio of 23:1 and has a transonic blisk as the zero stage. The airflow rate is 84.5 kg/sec at a gas generator speed of 9586 RPM. The axial compressor has a polytropic efficiency of 91%. The gas generator being installed at Darwin LNG is shown in Figure 6.

Annular combustor. The engine is provided with a single annular combustor (SAC) with coated combustor dome and liner similar to those used in flight applications. The single annular combustor features a through-flow, venturi swirler to provide a uniform exit temperature profile and distribution. This combustor configuration features individually replaceable fuel nozzles, a full-machined-ring liner for long life, and an yttrium stabilized zirconium thermal barrier coating to improve hot corrosive resistance. The engine is equipped with water injection for NO_x control.

High-pressure turbine (HPT). The PGT25+ HPT is a high efficiency air-cooled, two-stage design. The HPT section consists of the rotor and the first and second stage HPT nozzle assemblies. The HPT nozzles direct the hot gas from the combustor onto the turbine blades at the optimum angle and velocity. The high pressure turbine extracts energy from the gas stream to drive the axial flow compressor to which it is mechanically coupled.

High-speed power turbine. The PGT25+ gas generator is aerodynamically coupled to a high efficiency high speed power turbine. The high speed power turbine (HSPT) is a cantilever-supported, two-stage rotor design. The power turbine is attached to the gas generator by a transition duct that also serves to direct the exhaust gases from the gas generator into the stage one turbine nozzles. Output power is transmitted to the load by means of a coupling adapter on the aft end of the power turbine rotor shaft. The HSPT operates at a speed of 6100 RPM with an operating speed range of 3050 to 6400 rpm. The high-speed, two-stage power turbine can be operated over a cubic load curve for mechanical drive applications.

Engine-mounted gearbox. The PGT25+ gas generator has an engine-mounted accessory drive gearbox for starting the unit and supplying power for critical accessories. Power is extracted through a radial drive shaft at the forward end of the compressor. Drive pads are provided for accessories, including the lube and scavenge pump, the starter, and the variable-geometry control.

Maintenance

A critical factor in any LNG operation is the life cycle cost that is impacted in part by the maintenance cycle and engine availability. Aeroderivative engines have several features that facilitate “on condition” maintenance. Numerous boroscope ports allow on-station, internal inspections to determine the condition of internal components, thereby increasing the interval between scheduled, periodic removal of engines. When the condition of the internal components of the affected module has deteriorated to such an extent that continued operation is not practical, the maintenance program calls for exchange of that module. This allows “on-condition maintenance,” rather than strict time based maintenance.

The PGT25+ is designed to allow for on-site, rapid exchange of major modules within the gas turbine. On-site component removal and replacement can be accomplished in less than 100 man hours. The complete gas generator unit can be replaced and be back on-line within 48 hours. The hot-section repair interval for the aeroderivative is 25,000 hours on natural gas; however maintenance intervals are dependant on the extent of water injection used for NO_x control.

Acknowledgment

Based on a paper presented at the LNG 15 Conference held in Barcelona, Spain, April 24-27, 2007.