aeroderivative gas turbine final

© 2008 Bechtel Corporation. All rights reserved. 1

INTRODUCTION

Aeroderivative engines fit the ConocoPhillips Optimized Cascade® Process1 because of the

two-trains-in-one design concept that facilitates the use of such engines. Further, the application of a range of larger aeroderivative engines that are now available allows for a flexible design fit for this 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. For instance, the Darwin liquefied natural gas (LNG) plant is able to operate at reduced rates of 50% to 70% in the event that one refrigeration compressor is down.

Several practical aspects of the application of aeroderivative gas turbines as refrigeration drivers along with design and implementation considerations are discussed below. The selection of aeroderivative engines and their configurations for various train sizes, and evaluation of emission considerations are also covered.

OVERVIEW OF THE DARWIN LNG PROJECT

On February 14, 2006, the Darwin LNG plant was successfully commissioned

and the first LNG cargo was supplied to the buyers, Tokyo Electric and Tokyo Gas. The plant represents an innovative benchmark in the LNG industry as the world’s first facility to use high-efficiency aeroderivative gas turbine drivers. This benchmark follows another landmark innovation by ConocoPhillips: the first application of gas turbine drivers at the Kenai LNG plant in Alaska built in 1969.

The Darwin plant is a nominal 3.7 million tonnes per annum (MTPA) capacity LNG plant at Wickham Point, located in Darwin Harbour, Northern Territory, Australia. The plant is connected via a 500 km, 26-inch-diameter subsea pipeline to the Bayu-Undan offshore facilities. The Bayu-Undan field was discovered in 1995 approximately 500 km northwest of Darwin in the Timor Sea (see Figure 1). Delineation drilling over the next 2 years determined the field to be of world-class quality with 3.4 trillion cubic feet (tcf) of gas and 400 million barrels (MMbbl) of recoverable condensate and liquefied petroleum gas (LPG). The Bayu-Undan offshore facility began operating in February 2004; current production averages 70,000 bbl of condensate and 40,000 bbl of LPG per day.

The Darwin project was developed through a lump-sum, turnkey (LSTK) contract with Bechtel

Abstract—Market pressures for new thermally efficient and environmentally friendly liquefied natural gas (LNG) plants, coupled with the need for high plant availability, have resulted in the world’s first application of high-performance PGT25+ aeroderivative gas turbines for the 3.7 MTPA Darwin LNG plant in Australia’s Northern Territory. The plant was operational several months ahead of contract schedule and exceeded its production target for 2006. This paper describes the philosophy leading to this first-of-a-kind aeroderivative gas turbine plant and future potential for the application of larger aeroderivative drivers, which are an excellent fit for the ConocoPhillips Optimized Cascade® Process.

Keywords—aeroderivative, gas turbine, greenhouse gas, LNG, LNG liquefaction, thermal efficiency

Originally Issued: April 2007Updated: December 2008

Cyrus B. Meher-Homji

[email protected]

Tim Hattenbach

[email protected]

Dave Messersmith

[email protected]

Hans P. Weyermann

[email protected]

Karl Masani

[email protected]

Satish Gandhi, PhD

[email protected]

WORLD’S FIRST APPLICATION OF AERODERIVATIVE GAS TURBINE DRIVERS FOR THE CONOCOPHILLIPS OPTIMIZED CASCADE® LNG PROCESS

1 ConocoPhillips Optimized Cascade Process services are provided by ConocoPhillips Company and Bechtel Corporation via a collaborative relationship with ConocoPhillips Company.

Bechtel Technology Journal 2

ABBREVIATIONS, ACRONYMS, AND TERMS

ASME American Society of Mechanical Engineers

bbl barrel

CC combined cycle

CIT compressor inlet temperature

CNG compressed natural gas

CO2 carbon dioxide

DBT dry bulb temperature

DLE dry low emissions

FEED front-end engineering design

FOB free on board

GENP General Electric Nuovo Pignone

GT gas turbine

HHV higher heating value

HPT high-pressure turbine

HSPT high-speed power turbine

ISO International Organization for Standardization

kg/sec kilograms per second

LM2500+G4 gas generator manufactured by GE Industrial Aeroderivative group

LNG liquefied natural gas

LPG liquefied petroleum gas

LSTK lump-sum, turnkey

MDEA methyldiethanolamine

MMbbl million barrels

MMBtu million British thermal units

MTPA million tonnes per annum

NGL natural gas liquid

NOx nitrogen oxide

NPV net present value

PGT25+ GENP designation of the LM2500 engine with HSPT

ppm parts per million

RH relative humidity

rpm revolutions per minute

SAC single annular combustor

SC simple cycle

SHP shaft horsepower

tcf trillion cubic feet

TMY2 typical meteorological year 2

VFD variable-frequency drive

DILL

SUAI

TIMOR LESTE

JPDA

SUNRISE

EKKN

ABADI

EVANS SHOAL

DARWIN

NORTHERN TERRITORY

PUTREL

TERN

BLACKTIP

BAYU-UNDANTIMOR SEA

CRUX

SCOTT REEFBREWSTER

BRECKNOCK

INDONESIA

AUSTRALIA

AUSTRALIA

0 50 100km

N

Figure 1. Bayu-Undan Field Location and the Darwin LNG Plant

December 2008 • Volume 1, Number 1 3

Corporation that was signed in April 2003 with notice to proceed for construction issued in June 2003. An aerial photo of the completed plant is shown in Figure 2. Details regarding the development of the Darwin LNG project have been provided by Yates. [1, 2]

Not only has the Darwin plant established a new benchmark in the LNG industry by being the first LNG plant to use aeroderivative gas turbines as refrigerant compressor drivers, it also is the first to use evaporative coolers. The GE PGT25+2 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 required fuel, which reduces greenhouse gas in two ways. First, CO2 emissions are reduced due to a lower quantum of fuel burned, and second, the total feed gas required for the same LNG production also is reduced. The feed gas coming to the Darwin LNG facility contains carbon dioxide, which is removed in an amine system before 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. They 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 contains ship vapor recovery equipment. Both of these features not only reduce emissions that would have been produced from fired equipment and flares, but they also lead to reduced plant fuel requirements, which reduce the carbon dioxide released to the atmosphere.

Gas turbine nitrogen oxide (NOx) control is derived by water injection, which allows the plant to control NOx emissions while maintaining the flexibility to accommodate fuel gas compositions needed for various plant operating conditions. At the same time, there is no need for costly fuel treatment facilities for dry low NOx combustors.

The Darwin plant uses a single LNG storage tank with a working capacity of 188,000 m3, one of the largest aboveground LNG tanks. 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 also uses vacuum jacketed piping in the storage and loading system to improve thermal efficiency and reduce insulation costs. Methyldiethanolamine

The Darwin plant

established a

new benchmark in

the LNG industry

by being the first

LNG plant to use

aeroderivative

gas turbines

as refrigerant

compressor drivers.

2 This engine uses a LM2500+ gas generator, coupled with a two-stage high-speed power turbine developed by GE Oil & Gas.

Figure 2. Aerial View of 3.7 MTPA Darwin LNG Plant — 188,000 m3 Storage Tank, 1,350 m Jetty, and Loading Dock


(MDEA) with a proprietary activator is used for acid gas removal. This amine selection lowers the required regeneration heat load, and for an inlet gas stream containing more than 6% carbon dioxide, this lower heat load leads to reduced equipment size and a corresponding reduction in equipment cost.

Plant Design The Darwin LNG Plant uses the ConocoPhillips Optimized Cascade Process, which was first used in the Kenai LNG plant in Alaska and more recently in Trinidad (four trains), Egypt (two trains), and a train in Equatorial Guinea. A simplified process flow diagram of the Optimized Cascade Process is shown in Figure 3.

Thermal Efficiency ConsiderationsSeveral fundamental conditions in today’s marketplace make aeroderivative engines an excellent solution:

• Sizes of available aeroderivative engines ideally fit the two-trains-in-one concept of the ConocoPhillips LNG Process.

• Aeroderivative engines are variable-speed drivers, which facilitate the flexibility of the process and allow startup without the use of large variable-frequency drive (VFD) starter motors commonly used on single-shaft gas turbines. Aeroderivative engines also allow startup under settle-out pressure conditions, with no need to depressurize the compressor as is common for single-shaft drivers.

• High efficiency results in a greener train with a significant reduction in greenhouse gas emissions.

• Several LNG projects are gas constrained due to a lack of available supplies. This situation occurs both on potential new projects and at existing LNG facilities. Under such constraints, any fuel reduction resulting from higher gas-turbine thermal efficiency means it can be converted to LNG.

• Gas supplies are also constrained due to greater national oil company control of the sources. Gas supplies are no longer available at low cost to LNG plants and the notion that fuel is free is now a thing of the past. Several current projects and front-end engineering design (FEED) studies

Sizes of

available

aeroderivative

engines ideally

fit the

two-trains-in-one

concept of the

ConocoPhillips

LNG process.

RAW GASFEED

PRETREATMENT

ETHYLENECOLD BOX

METHANECOLD BOX

AIR FIN HEATEXCHANGER


COMPRESSORS

TURBINES

PLANT FUEL

NGLTANK VAPOR

BLOWERSHIP VAPOR

BLOWER

STORAGE TANKSAND

LOADING PUMPS

VAPOR FROM SHIP WHEN LOADING

TO SHIP LOADINGFACILITIES

THYLCOLD BOX

LNG

HEAVIESREMOVAL

COMPRESSORS

TURBINES

COMPRESSORSTURBINES


PROPANEHEAT EXCHANGE ETHYLENE

METHANE

ETHYLENE

PROPANE

METHANE METHANE

Figure 3. Simplified Process Flow Diagram of the Optimized Cascade Process


have encountered fuel valued much higher than a decade ago. Host governments also are requiring more gas for domestic use, increasing the shortfalls for LNG plants.

Given this situation and the fact that fuel not consumed can be converted to LNG, use of high-efficiency aeroderivative engines delivers significant benefits with a net present value (NPV) of hundreds of millions of dollars. Because NPV is a strong function of feed gas costs and LNG sales price, it is highly affected by a plant’s thermal efficiency, especially when the free on board (FOB) LNG costs are high, as in the current market.

The present value of converting fuel into LNG for a nominal 5.0 MTPA plant is shown in Figure 4 for a range of driver efficiencies between 33% and 50%, as compared to a base case of 30%. Results are provided for FOB LNG prices ranging from $1 to $5 per million British thermal units (MMBtu). The present value of the gross margin (defined as LNG revenue—feed gas cost) is calculated over a 20-year life and a discount rate of 12%. The graph shows the strong influence of driver efficiency.

The thermal efficiency of an LNG facility depends on numerous factors such as gas composition, inlet pressure and temperature, and even more obscure factors such as the location of the loading dock relative to the site of the liquefaction process. Higher thermal efficiency is typically

a tradeoff between capital and life cycle costs. Gas turbine selection, the use of waste heat recovery and ship vapor recovery, and self-generation versus purchased power all have a significant effect on the overall thermal efficiency of the process. Process flexibility and stability of operation are of paramount importance and must be incorporated into the considerations regarding thermal efficiency because the value of a highly efficient process is diminished if plant reliability and availability are sacrificed.

Yates [3] has provided a detailed treatment of the design life cycle and environmental factors that affect plant thermal efficiency, such as feed gas characteristics, feed gas conditioning, and the LNG liquefaction cycle itself. Some of the key elements of this discussion are provided below, leading into the discussion of the selection of high-efficiency aeroderivative engines.

A common consideration in evaluating competing LNG technologies is the difference in thermal efficiency. The evaluation of thermal efficiency tends to be elusive and subjective in that each project introduces its own unique characteristics that determine its optimum thermal efficiency based on the project’s strongest economic and environmental merits. Different technologies or plant designs cannot be compared on thermal efficiency without understanding and compensating for such unique differences of each project.

Process flexibility

and stability

of operation are

of paramount

importance and

must be

incorporated into

the considerations

regarding thermal

efficiency.

Pres

ent V

alue

of G

ross

Mar

gin,

$ M

illio

n(D

elta B

etwe

en Ea

ch Ef

ficien

cy C

ase a

nd B

ase C

ase)

500

400

300

200

100

0

LNG Price, $/MMBtu

4.00 5.001.00 3.002.00

40%

37%

50%

33%

Value of Converting Fuel Savings into LNG for a 5.0 MTPA LNG Plant(Power Cycle Efficiency Increase vs. Power Cycle Efficiency of 30%)

Fixed feed gas flow: gas cost = $0.75/MMBtuPresent value calculated at discount rate = 12% and 20-year life

Availability is adjusted for aeroderivatives (+1%) and combined cycle (–2%)Capital cost adjusted for incremental capacity (SC: $150/tonne, CC: $300/tonne)

Figure 4. Present Value of Gross Margin as a Function of Driver Thermal Efficiency, for a Range of LNG FOB Prices


The definition of thermal efficiency also has proven to be subjective depending on whether an entire plant, an isolated system, or an item of equipment is being compared. Thermal efficiency, or train efficiency, has been expressed as the ratio of the total higher heating value (HHV) of the products to the total HHV of the feed. This definition fails to recognize the other forms of thermodynamic work or energy consumed by the process. For example, the definition would not account for the work of purchased power and electric motors if they are used for refrigeration and flashed gas compression. When evaluating the benefits of achieving high thermal efficiency with a specific LNG plant design, a true accounting of all of the energy being consumed in the process must be considered.

Turndown capabilities of an LNG process also need to be considered when thermal efficiency and life-cycle cost comparisons are being made. Thermal efficiency comparisons are typically based on the process operating at design conditions. In an actual plant environment, this design point is elusive, and an operator is always trying to attain a “sweet spot” where the plant will operate at its peak performance under prevailing conditions. As the temperature changes during the day, affecting the performance of the air coolers, turbines, or process fluid and equipment, the operator needs to continually adjust plant parameters to achieve optimal performance. Designing a plant to allow an operator to continually achieve this optimum performance will affect the plant’s overall thermal efficiency and life cycle costs.

The efficiency of an LNG process depends on many features. The two most significant ones are the efficiency of heat exchange and the turbomachinery efficiency. The heat exchange efficiency is a function of the process configuration and selection of the individual heat exchangers, which sets temperature approaches. The turbomachinery efficiency depends on the compressor and turbine efficiencies.

Cooling Curve PerformanceThe liquefaction cooling curve3 performance is another benchmark reviewed in LNG technology comparisons and is often misunderstood or incorrectly applied. Recent analyses by Ransbarger et al. [4] have comprehensively evaluated the issue of cooling curve performance with respect to overall thermal efficiency.

A liquefaction cooling curve plot depicts the temperature change of the heat sink and the heat source as a function of the heat transferred. Frequently, cooling curves are shown with only the feed gas as a heat source and then used as a means to compare different liquefaction processes. Cooling curves should include all duty that is transferred at a given temperature, which includes cooling and condensing of the refrigerants as well as the feed gas. The composite cooling curve analysis seeks to optimize the area or temperature difference between the heat source and the heat sink in a cost-effective manner. Each of the available liquefaction processes attempts to optimize this temperature difference in a different way.

Very often, process efficiencies of LNG technologies have been compared with the classical cascade process. It is important to note that the ConocoPhillips Optimized Cascade Process encompasses two major modifications:

• The addition and optimization of heat recovery schemes

• Where appropriate, the conversion of the traditional closed-loop methane refrigeration system to an open-loop system

The plate fin heat exchangers used in this process are also recognized for their ability to achieve an exceptionally close temperature approach. The use of pure refrigerants allows continually accurate prediction of refrigerant performance during plant operation without the need for on-line refrigerant monitoring. Therefore, for a given feed gas composition range, the cascade liquefaction technology provides the plant designer with flexibility in cooling stage locations, heat exchanger area, and operating pressure ranges during each stage, resulting in a process that can achieve high thermal efficiency under a wide range of feed conditions.

When using cooling curves, incorrect conclusions can be drawn if only the feed gas is used as a heat source. It is imperative that heat transfer associated with cooling and condensing refrigerants also be included4, so that a “complete cooling curve” can be derived. Complete cooling curves of the classical and Optimized Cascade Process are depicted in Figure 5. The average temperature approach of the classical cascade is 16 °F (8.89 °C), while the average approach temperature of the Optimized Cascade is

Turndown

capabilities

of an LNG process

also need to be

considered when

thermal efficiency

and life-cycle

cost comparisons

are being made.

3 Also known as a temperature-duty curve.

4 The Optimized Cascade Process would include heat transfer associated with the propane refrigerant loads necessary to cool and condense ethylene, as well as the ethylene refrigeration loads necessary to cool and condense methane flash vapors.


12 °F (6.67 °C), i.e., a reduction of 25%, which represents a 10% to 15% reduction in power.

The maturity of the liquefaction processes has approached a point at which changes in duty curve no longer represent the greatest impact. Two developments that have a significant impact on efficiency are the improvement in liquefaction compressor efficiency5 and the availability of high-efficiency gas turbine drivers.

A comparison of LNG technologies at a single design condition does not address plant performance during variations in operating conditions. A two-shaft gas turbine such as the PGT25+ used at Darwin, with its ability to control compressor performance without the need for recycle, can deliver significant improvements in thermal efficiency during turndown operations. Due to significant production swings during the day as a result of changes in ambient temperature, described earlier, the performance of the gas turbine and compressor package needs to be considered in any comparison of plant thermal efficiency.

SELECTION OF AERODERIVATIVE ENGINES

The earlier discussion demonstrated that the selection of the gas turbine plays an important

role in efficiency, 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 life cycle cost of the project.

When high fuel costs are expected, the selection of a high-efficiency driver becomes a strong criterion in the life cycle cost evaluation. However, LNG projects are developed to monetize stranded gas reserves, while low-cost fuel has favored industrial gas turbines. This situation is changing and the value of gas is growing. Further, when the gas is pipeline or otherwise constrained, there is a clear benefit to consuming less fuel for a given amount of refrigeration power. In such cases, a high-efficiency gas turbine solution through which the saved fuel can be converted into LNG production can reap large benefits.

Figure 6 6 shows that aeroderivative gas turbines achieve significantly higher thermal efficiencies

than industrial gas turbines. The figure illustrates the engines’ thermal efficiency vs. specific work (kW per unit air mass flow). The higher efficiency of an aeroderivative gas turbine can result in a

Classical Cascade ProcessComplete Cooling Curve

–300

–250

–200

–150

–100

–50

0

50

100

150

0% 20% 40% 60% 80% 100%

Enthalpy Change

Tem

pera

ture

, °F

Average Approach = 16 °F

Optimized Cascade ProcessComplete Cooling Curve

–300

–250

–200

–150

–100

–50

0

50

100

150

0% 20% 40% 60% 80% 100%

Enthalpy Change

Tem

pera

ture

, °F

Average Approach = 12 °F

T sinkT sourceT avg

T sinkT sourceT avg

Figure 5. Comparison of Cooling Curves for Classical Cascade Process and ConocoPhillips Optimized Cascade Process

5 Compressor polytropic efficiencies now exceed 80% and high-efficiency gas turbines are available with simple-cycle thermal efficiencies of approximately 40%.

6 Based on Frame 5C, 5D, 7EA, and 9E frame type drivers and GE PGT25+, LM6000, RR 6761, and RR Trent aeroderivative units.

Ther

mal

Effi

cien

cy, %

50

45

40

35

30

25

GT-Specific Work, kW/kg/sec

Aeroderivative

Heavy-Duty Engines

350 400200 300250

Figure 6. Map of ISO Thermal Efficiency vs. Specific Work of Commonly Used Frame Drivers and Aeroderivative Engines

(Aeroderivative Engines Exhibit Higher Specific Work and Thermal Efficiency)


3% or greater increase in overall plant thermal efficiency. Further, plant availability significantly improves because a gas turbine generator (or even a complete turbine) can be completely changed out within 48 hours compared to the 14 or more days 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 Darwin. 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 in turn results in decreased CO2 emissions, as depicted in Figure 7, 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 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 is an approximately 9.3% reduction in total greenhouse gases.

Advantages of Aeroderivative Engines over Heavy Duty Gas TurbinesSeveral advantages of using aeroderivative engines, some of which have been discussed, include:

• Much higher efficiency that leads to reduced fuel consumption and greenhouse emissions

• Ability to rapidly swap engines and modules, thus improving maintenance flexibility

• High starting torque capacity; excellent torque-speed characteristics, allowing large trains to start up under settle-out pressure conditions

• Essentially zero-timed after 6 years; maintenance can also be done “on condition,” allowing additional flexibility

• Dry-low-emissions (DLE) technology, avail-able and proven on several engines

• Relatively easy installation due to low engine weight

Implementation of the PGT25+ in the Darwin Plant − Gas Turbine and Compressor ConfigurationsThe 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 + 3MCL1405Ethylene: 2 X PGT25+ + GB + 2MCL1006Methane: 2 X PGT25+ + MCL806 + MCL 806 + BCL608

Both the propane and ethylene trains 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 8, which facilitates a down-nozzle configuration for the compressors. A view of the six strings from the gas turbine inlet side is shown in Figure 9. The four once-through steam generators are on the four turbine exhausts to the left. The LM2500+ gas generator is shown in Figure 10.

AERODERIVATIVE ENGINE TECHNOLOGY FOR DARWIN LNG

The PGT25+ engine used at the Darwin plant has a long heritage, starting from the

TF39 GE aeroengine, as shown in Figure 11. 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 6,100 revolution-per-minute (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 history of the LM2500 gas turbine

Aeroderivative

gas turbines

achieve

significantly higher

thermal efficiencies

than industrial

gas turbines.

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Rolls Royce 6761

GE LM2500+

GE Frame

7EA

GE Frame

6B

GE Frame

5D

GE Frame

5C

Rela

tive C

O 2 Emiss

ions

Trent 60 DLE

GE LM6000PD

Figure 7. Relative CO2 Emissions from Different Classes of Gas Turbines


that was completed in December 1996. The LM2500+ was originally rated at 27.6 MW, with a nominal 37.5% ISO thermal efficiency. Since that time, its ratings have grown to the current 31.3 MW and 41% thermal efficiency.

The LM2500+ has a revised and upgraded compressor section with an added zero stage for a 23% increased airflow and pressure ratio, and revised materials and design in the high-pressure and power turbines. Details can be found in Wadia et al. [5]

Description of the PGT25+ Gas TurbineThe 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 stage7. As reported by Wadia et al. [5], the airflow rate is 84.5 kg/sec at a gas generator speed of 9,586 rpm. The axial compressor has a polytropic efficiency of 91%.

• 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 SAC features a through-flow, venturi swirler to provide a uniform exit temperature profile

Figure 8. Compressor Trains at Darwin LNG Plant

Figure 9. Compressor Block Viewed from Gas Turbine Filter House End (Note Four Once-Through

Steam Generators on Gas Turbines)

C-5

DC-10LM2500+G4/PGT25+G4

LM2500+/PGT25+

LM2500/PGT25

34.3/46,00039%–41%

31.3/42,00039%–41%

23/32,00038%

Power OutputMW/SHP

Thermal Efficiency

LM2500/PGT25/

LM2500+/PGT25+TF39/CF6-6

(Source: GE Energy)

Figure 11. LM2500 Engine Evolution

Figure 10. Installation of LM2500+ Gas Generator 7 The zero stage operates at a stage pressure ratio of

1.43:1 and an inlet tip relative mach number of 1.19.


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 NOx 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 HSPT with 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 6,100 rpm with an operating speed range of 3,050 to 6,400 rpm. The high-speed two-stage power turbine can be operated over a cubic load curve for mechanical drive applications.

• Engine-mounted accessory gearbox driven by a radial drive shaft — The PGT25+ 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, starter, and variable-geometry control. An overview of the engine, including the HSPT, is shown in Figure 12.

Maintenance A critical factor in any LNG operation is the life- cycle cost influenced in part by the maintenance cycle and engine availability. Aeroderivative engines have several features that facilitate “on condition” maintenance, rather than strict time-based 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.

The PGT25+ is designed to allow for rapid on-site exchange of major modules within the gas turbine. Component removal and replacement can be accomplished in less than 100 hours, and the complete gas generator unit can be replaced and be back online within 48 hours. The hot-section repair interval for the aeroderivative is 25,000 hours on natural gas; however, waterinjection for NOx control shortens this interval to between 16,000 hours and 20,000 hours, depending on the NOx target level8.

Performance Deterioration and RecoveryGas turbine performance deterioration is of great concern to any LNG operation (see [6, 7, and 8]). Total performance loss is attributable to a combination of recoverable (by washing) and non-recoverable (recoverable only by component replacement or repair) losses. Recoverable performance loss is caused by airborne contami-nant fouling of airfoil surfaces. The magnitude of recoverable performance loss and the frequency of washing are determined by site environment and operational profile. Generally, compressor fouling is the predominant cause of this type of loss. Periodic washing of the gas turbine, using online and crank-soak wash procedures, will recover 98% to 100% of these losses. The objective of online washing is to increase the time interval between crank washes. The best approach is to couple online and offline washing.

The cooldown time for an aeroderivative engine is much less than that for a heavy-duty frame machine due to the lower casing mass. Crank washes can therefore be done with less downtime.

(Source: GE Energy)

Figure 12. PGT25+ Gas Turbine A critical factor

in any LNG

operation is the

life-cycle cost

influenced

in part by the

maintenance cycle

and

engine availability.

8 The level of water injection is a function of the NOx target level.


Upgrades of the PGT25+Another advantage of using aeroderivative engines for LNG service is that they can be uprated to newer variants, generally within the same space constraints—a useful feature for future debottlenecking. The Darwin LNG plant is implementing this upgrade.

The LM2500+G4 is the newest member of GE’s LM2500 family of aeroderivative engines. The engine, shown in Figure 13, retains the basic design of the LM2500+ but increases the power capability by approximately 10% without sacrificing hot-section life. The modification increases the engine’s power capability by increasing the airflow, improving the materials, and increasing the internal cooling. The number of compressor and turbine stages and the majority of the airfoils and the combustor designs remain unchanged from the LM2500. Details on the LM2500+G4 can be found in [9].

The increased power of this variant compared to the base engine is shown in Figure 14.

POWER AUGMENTATION BY EVAPORATIVE COOLING

LNG production is highly dependent on the power capability of the gas turbine drivers of

the propane, ethylene, and methane compressors. Industrial gas turbines lose approximately 0.7% of their power for every 1 °C rise in ambient temperature. This effect is more pronounced in aeroderivative gas turbines due to their higher specific work for which the sensitivity can increase to much greater than 1% per °C. The impact of ambient temperature on the PGT25+ power and air flow is depicted in Figure 15.

As aeroderivative machines are more sensitive to ambient temperature, they benefit significantly from inlet air cooling. Darwin LNG uses media-type evaporative coolers—another first for LNG refrigeration drivers. Details on

(Source: GE Energy)

Figure 13. Uprated LM2500+G4 Engine — DLE Variant

Shaf

t Pow

er O

utpu

t, kW

Ambient Temperature, °C

40,000

36,000

32,000

28,000

24,000

20,000–30.0 –15.0 0.0 15.0 30.0

LM2500+G4 SAC PowerLM2500+ SAC PowerLM2500 SAC Power

(Source: GE Energy)

Figure 14. Comparative Power Output of LM2500+G4 Variant

Powe

r, kW

Air M

ass F

low

Rate

, kg/

sec

Ambient Temperature, °C

40,000

35,000

30,000

25,000

20,000

10,000

15,000

95

90

85

80

75

65

60

70

25201510 30 355 40

kWAirflow, kg/sec

Figure 15. Variations in Power Output and Airflow Rate for PGT25+ Gas Turbine


media-based evaporative cooling can be found in Johnson. [10]

Among the key advantages of power augmen-tation are that it leads to:

• Greater LNG production due to reduced gas turbine compressor inlet air temperature, increasing the air mass flow rate and power

• Improved thermal efficiency of the gas turbine, resulting in lower CO2 emissions

There is considerable evaporative cooling potential available in Darwin especially during the periods of high ambient temperatures, because the relative humidity tends to drop as the temperature increases. The average daily temperature profile at Darwin is shown in Figure 16, and the relationship of relative humidity and dry bulb temperature for the month of September is shown in Figure 17 9. Details regarding the climatic analysis of evaporative cooling potential can be found in [11].

Media-based evaporative coolers use corrugated media over which water is passed. The media material is placed in the gas turbine airflow path within the air filter house and is wetted via water distribution headers. The construction of the media allows water to penetrate through it, and any non-evaporated water returns to a catch basin. The material also provides sufficient airflow channels for efficient heat transfer and minimal pressure drop. As the gas turbine airflow passes over the media, the airstream absorbs moisture (evaporated water). Heat content in the airstream is given up to the wetted media, resulting in a lower compressor inlet temperature. A typical evaporative cooler effectiveness range is 85% to 90%, and is defined as follows:

Because

aeroderivative

machines are

more sensitive

to ambient

temperature,

they benefit

significantly from

inlet air cooling.

9 Data is for Darwin Airport, from the typical meteorological year (TMY2) database.

Tem

pera

ture

, °C

Time of Day

35

33

31

29

27

25

23

21

19

17

15

22:3

0

20:3

0

18:3

0

16:3

0

14:3

0

12:3

0

10:3

0

8:30

6:30

4:30

2:30

Mea

n

0:30

Dec

Nov

Oct

Sep

Aug

Jul

Jun

May

Apr

Mar

Feb

Jan

Figure 16. Darwin Temperature Profile Based on Time of Day over 12-Month Period

0

10

20

30

40

50

60

70

80

90

100

10 12 14 16 18 20 22 24 26 28 30 32 34 36

DBT, °C

RH, %

Figure 17. RH vs. DBT at Darwin Airport for the Month of September (Considerable Evaporative Cooling Potential is Available During Hot Daytime Hours)


Where:

T1DB = entering-air dry bulb temperature

T2DB = leaving-air dry bulb temperature

T2WB = leaving-air wet bulb temperature

Effectiveness is the measure of how capable the evaporative cooler is in lowering the inlet-air dry bulb temperature to the coincident wet bulb temperature. Drift eliminators are used to protect the downstream inlet system components from water damage, caused by carryover of large water droplets.

The presence of a media-type evaporative cooler inherently creates a pressure drop, which reduces turbine output. For most gas turbines, media thickness of 12 inches will result in a pressure drop of approximately 0.5 in. to 1 in. of water. Increases in inlet duct differential pressure will cause a reduction of compressor mass flow and engine operating pressure. The large inlet temperature drop derived from evaporative cooling more than compensates for the small drop in per-formance due to the additional pressure drop.

Inlet temperature drops of approximately 10 °C have been achieved at Darwin LNG, which results in a power boost of approxi-mately 8% to 10%. Figure 18 shows calculated compressor inlet temperatures (CITs) with the evaporative cooler for a typical summer month of January.

FUTURE POTENTIAL OF AERODERIVATIVE ENGINES USING THE OPTIMIZED CASCADE PROCESS

Several factors must be considered in choosing an optimal train size, including:

• Gas availability from the field• Market demand and LNG growth profile

(which would also dictate the buildup and timing between subsequent trains)

• Overall optimization of production, storage, and shipping logistics

• Operational flexibility, reliability, and maintenance of the refrigeration block (Flexibility is of extreme importance in today’s operational market environment, which has seen some departure from long-term LNG supply contracts.)

As the Optimized Cascade Process uses a two-train-in-one concept, in which two parallel compressor strings are used for each refrigeration service, the application of larger aeroderivative engines is an ideal fit. Using the Optimized Cascade Process, the loss of any refrigeration string does not shut down the train but only necessitates a reduction in plant feed, with overall LNG production remaining between 60% and 70% of full capacity10.

The significant benefits of aeroderivative engines as opposed to large single-shaft gas turbines make large aeroderivative units an attractive proposition for high-efficiency, high-output LNG plants. Larger LNG plant sizes can be derived

DBT, CIT Media Evaporative Efficiency = 90% (TMY2 Database Data for Month of January )

10

15

20

25

30

35

40DB

T, CI

T, °C

1 33 65 97 129

161

193

225

257

289

321

353

385

417

449

481

513

545

577

609

641

673

705

Hours per Month

5

DBT

CIT with Evaporative Cooler

DBT, °CCIT, °C; Efficiency = 90%

Figure 18. Calculated CITs due to Evaporative Cooling During the Summer Month of January

Because the

Optimized Cascade

Process uses a

two-train-in-one

concept, in which

two parallel

compressor strings

are used for

each refrigeration

service, the

application

of larger

aeroderivative

engines is

an ideal fit.

10 Obtained by shifting refrigerant loads to the other drivers.

(T1DB – T2DB)(T1DB – T2WB)

Effectiveness =


by adding gas turbines, as shown in Table 1. While the output with one driver down in a 2+2+2 configuration is approximately 60% to 70%, the output would be even higher with a larger number of drivers.

As split-shaft industrial gas turbines are not available in the power class of large aeroderivative gas turbines, the application of aeroderivative engines offers the significant advantage of not requiring costly and complex large starter motors and their associated power generation costs.

For example, the LM6000 depicted in Figure 19is a 44 MW driver11, with a thermal efficiency of 42%, operating at a pressure ratio of 30:1 and with an exhaust mass flow rate of 124 kg/sec. This engine is a two-spool gas turbine with the load driven by the low-speed spool, which is mounted inside the high-speed spool, enabling the two spools to turn at different speeds. The output speed of this machine is 3,400 rpm.

The LM6000 gas turbine makes extensive use of variable geometry to achieve a large operating envelope. The variable geometry includes the variable inlet guide vanes, variable bypass valves, and the variable stator vanes in the engine

compressor with each system independently controlled. The gas turbine consists of five major components: a 5-stage low-pressure compressor, a 14-stage high-pressure compressor, an annular combustor, a 2-stage high-pressure turbine, and a 5-stage low-pressure turbine. The low-pressure turbine drives the low-pressure compressor and the load. The engine is available in both a water-injected and DLE configuration, with a DLE capability of 15 parts per million (ppm) NOx.

The importance of high thermal efficiency and the details on the implementation and operating experience of aeroderivatives at Darwin LNG have been presented by Meher-Homji et al. [12]

CONCLUSIONS

In 1969, the ConocoPhillips-designed Kenai LNG plant in Alaska was the first LNG plant

to use gas turbines as refrigeration drivers. This plant has operated without a single missed shipment. Another groundbreaking step was made 38 years later with the world’s first successful application of high-efficiency aeroderivative gas turbines at the Darwin LNG plant. This efficient plant has shown how technology can be integrated into a reliable LNG process to minimize greenhouse gases and provide the high flexibility, availability, and efficiency of the Optimized Cascade Process. The plant, engineered and constructed by Bechtel, was started several months ahead of schedule and has exceeded its production targets. It has been successfully operated for close to 3 years and will shortly be upgraded by implementing PGT25+G4 engines as part of a debottlenecking effort.

The new generation of highly efficient and high-power aeroderivative engines in the 40 MW to 50 MW range available today is ideally suited to the Optimized Cascade Process due to its two-trains-in-one concept. The ConocoPhillips-Bechtel LNG collaboration will offer the engine for future LNG projects. In the meantime, the ConocoPhillips-Bechtel LNG Product Development Center continues to design and develop new and highly efficient plant designs that can be used for 5.0–8.0 MTPA train sizes.

11 To compare the power/wt ratio, the LM6000 core engine weighs 7.2 tons compared to 67 tons for a 32 MW Frame 5D engine (core engine only).

(Source: GE Energy)

Figure 19. LM6000 Gas Turbine

Table 1. Configuration/Size of LNG Plants Using Aeroderivative Engines

Aeroderivative Engine Confi guration (Propane/Ethylene/Methane) Approximate Train Size, MTPA

6 x LM2500+ 2/2/2 3.5

8 x LM2500+G4 DLE 3/3/2 5

6 x LM6000 DLE 2/2/2 5

9 x LM6000 DLE 3/3/3 7.5

The new generation

of highly efficient

and high-power

aeroderivative

engines in

the 40 MW to

50 MW range

available today

is ideally suited

to the Optimized

Cascade Process

due to its

two-trains-in-one

concept.


TRADEMARKS

ConocoPhillips Optimized Cascade is a registered trademark of ConocoPhillips.

REFERENCES

[1] D. Yates and C. Schuppert, “The Darwin LNG Project,” 14th International Conference and Exhibition on Liquefied Natural Gas (LNG 14), Doha, Qatar, March 21–24, 2004 <http://www.lng14.com.qa/lng14.nsf/attach/$file/PS6-1.ppt>.

[2] D. Yates and D. Lundeen, “The Darwin LNG Project,” LNG Journal, 2005.

[3] D. Yates, “Thermal Efficiency – Design, Lifecycle, and Environmental Considerations in LNG Plant Design,” GASTECH, 2002 <http://lnglicensing.conocophillips.com/NR/rdonlyres/8467A499-F292-48F8-9745-1F7AC1C57CAB/0/thermal.pdf>.

[4] W. Ransbarger et al., “The Impact of Process and Equipment Selection on LNG Plant Efficiency,” LNG Journal, April 2007.

[5] A.R. Wadia, D.P. Wolf, and F.G. Haaser, “Aerodynamic Design and Testing of an Axial Flow Compressor with Pressure Ratio of 23.3:1 for the LM2500+ Engine,” ASME Transactions, Journal of Turbomachinery, Vol. 124, Issue 3, July 2002, pp. 331−340, access via <http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JOTUEI000124000003000331000001&idtype=cvips&gifs=yes>.

[6] C.B. Meher-Homji, M. Chaker, and H. Motiwalla, “Gas Turbine Performance Deterioration,” Proceedings of the 30th Turbomachinery Symposium, Houston, Texas, September 17−20, 2001.

[7] C.B. Meher-Homji and A. Bromley, “Gas Turbine Axial Compressor Fouling and Washing,” Proceedings of the 33rd Turbomachinery Symposium, Houston, Texas, September 20−23, 2004, pp. 163-192 <http://turbolab.tamu.edu/pubs/Turbo33/T33pg163.pdf>.

[8] G.H. Badeer, “GE Aeroderivative Gas Turbines − Design and Operating Features,” GE Power Systems Reference Document GER-3695E, October 2000 <http://gepower.com/prod_serv/products/tech_docs/en/downloads/ger3695e.pdf>.

[9] G.H. Badeer, “GE’s LM2500+G4 Aeroderivative Gas Turbine for Marine and Industrial Applications,” GE Energy Reference Document GER-4250, September 2005 <http://gepower.com/prod_serv/products/tech_docs/en/downloads/ger4250.pdf>.

[10] R.S. Johnson, “The Theory and Operation of Evaporative Coolers for Industrial Gas Turbine Installations,” ASME International Gas Turbine and Aeroengine Congress and Exposition, Amsterdam, Netherlands, June 5−9, 1988, Paper No. 88-GT-41 <http://www.muellerenvironmental.com/documents/100-020-88-GT-41.pdf>.

[11] M. Chaker and C.B. Meher-Homji, “Inlet Fogging of Gas Turbine Engines: Climatic Analysis of Gas Turbine Evaporative Cooling Potential of International Locations,” Journal of Engineering

for Gas Turbines and Power, Vol. 128, No. 4, October 2006, pp. 815–825, see <http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JETPEZ000128000004000815000001&idtype=cvips&gifs=yes> (see also Proceedings of ASME Turbo Expo 2002, Amsterdam, the Netherlands, June 3–6, 2002, Paper 2002-GT-30559 <http://www.meefog.com/downloads/30559_International_Cooling.pdf>).

[12] C.B. Meher-Homji, D. Messersmith, T. Hattenbach, J. Rockwell, H. Weyermann,and K. Masani, “Aeroderivative Gas Turbines for LNG Liquefaction Plants − Part 1: The Importance of Thermal Efficiency” and “Part 2: World’s First Application and Operating Experience,” Proceedings of ASME International Gas Turbine and Aeroengine Conference, Turbo Expo 2008, Paper Nos. GT2008-50839 and GT2008-50840, Berlin, Germany, June 9−13, 2008 (see http://www.asmeconferences.org/TE08//pdfs/TE08_FinalProgram.pdf, p. 86).

BIOGRAPHIESCyrus B. Meher-Homji is a Bechtel Fellow and senior principal engineer assigned to the Houston, Texas-based Bechtel-ConocoPhillips LNG Product Development Center as a turbomachinery specialist. His 29 years of industry experience covers gas turbine and compressor design, engine

development, and troubleshooting. Cyrus works on the selection, testing, and application of gas turbines and compressors for LNG plants. His areas of interest are turbine and compressor aerothermal analysis, gas turbine deterioration, and condition monitoring.

Cyrus is a Fellow of ASME and past chair of the Industrial & Cogeneration Committee of ASME’s International Gas Turbine Institute. He also is a life member of the American Institute of Aeronautics and Astronautics (AIAA) and is on the Advisory Committee of the Turbomachinery Symposium. Cyrus has more than 80 publications in the area of turbomachinery engineering.

Cyrus has an MBA from the University of Houston, Texas, an ME from Texas A&M University, College Station, and a BS in Mechanical Engineering from Shivaji University, Maharashtra, India. He is a registered professional engineer in the state of Texas.

Tim Hattenbach has worked in the oil and gas industry for 36 years, 30 of which have been with Bechtel. He is the team leader of the Compressor group in Bechtel’s Houston office and has worked on many LNG projects and proposals as well as a variety of gas plant and refinery projects.

A modification of the original version of this paper was presented at LNG 15, held April 24–27, 2007,

in Barcelona, Spain.


Tim is Bechtel’s voting representative on the American Petroleum Institute (API) Subcommittee on Mechanical Equipment and is a member of its Steering Committee. He is Taskforce Chairman of two API standards (API 616 – Gas Turbines and API 670 – Machinery Protection Systems).

Tim has an MS and a BS in Mechanical Engineeringfrom the University of Houston, Texas.

Dave Messersmith is deputy manager of the LNG and Gas Center of Excellence, responsible for LNG Technology Group and Services, for Bechtel’s Oil, Gas & ChemicalsGlobal Business Unit, located in Houston, Texas. He has served in various lead roles on LNG projects for 14 of the past

17 years, including work on the Atlantic LNG project conceptual design through startup as well as many other LNG studies, FEED studies, and projects. Dave’s experience includes various LNG and ethylene assignments over 17 years with Bechteland, previously, 10 years with M.W. Kellogg, Inc.

Dave holds a BS degree in Chemical Engineering from Carnegie Mellon University, Pittsburgh, Pennsylvania, and is a registered professional engineer in the state of Texas.

Hans P. Weyermann is a principal rotating equipment engineer in the Drilling and Production Department of the ConocoPhillips Upstream Technology Group. He supports all aspects of turbo-machinery for business units and grassroots capital projectsand is also responsible for

overseeing corporate rotating machinery technology development initiatives within the ConocoPhillips Upstream Technology Group.

Before joining ConocoPhillips, Hans was the supervisor of rotating equipment at Stone &Webster, Inc., in Houston, Texas. Earlier, he was an application/design engineer in the TurboCompressor Department at Sulzer Escher Wyss Turbomachinery in Zurich, Switzerland.

Hans is a member of ASME, the Texas A&M University Turbomachinery Advisory Committee, and the API SOME, and, in addition, serves on several API task forces.

Hans has a BS degree in Mechanical Engineering from the College of Engineering in Brugg-Windisch, Switzerland.

Karl Masani is a director for LNG Licensing & Technology in the Global Gas division of ConocoPhillips, where he is responsible for LNG project business development and project supervision. Previously, he held various managerial positions at General Electric Company, Duke Energy Corporation, and Enron Corporation.

Karl holds an MBA in Finance from Rice University, Houston, Texas, and a BS degree in Aerospace Engineering from the University of Texas at Austin.

Satish Gandhi is LNG Product Development Center (PDC) director and manages the center for the ConocoPhillips-Bechtel Corporation LNGCollaboration. He is responsiblefor establishing the work direction for the PDC to implement strategies and priorities set by the LNG

Collaboration Advisory Group.

Dr. Gandhi has more than 34 years of experience in technical computing and process design, as well as troubleshooting of process plants in general and LNG plants in particular. He was previously process director in the Process Technology & Engineering Department at Fluor Daniel with responsibilities for using state-of-the-art simulation software for the process design of gas processing, CNG, LNG, and refinery facilities. He also was manager of the dynamic simulation group at M.W. Kellogg, Ltd., responsible for technology development and management and implementation of dynamic simulation projects in support of LNG and other process engineering disciplines.

Dr. Gandhi received a PhD from the University of Houston, Texas; an MS from the Indian Institute of Technology, Kanpur, India; and aBS from Laxminarayan Institute of Technology,Nagpur, India, all in Chemical Engineering.

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