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SUCCESSFUL START-UP AND OPERATION OF GE FRAME 9E GAS TURBINE REFRIGERANT STRINGS

Steven Judd LNG Machinery Group Lead

ExxonMobil Development Company, Houston, Texas steven.p.judd@exxonmobil.com

Roy Salisbury Machinery Engineer Lead

Qatargas Operating Limited, Doha, Qatar rsalisbury@qatargas.com.qa

Peter Rasmussen Chief Machinery Engineer

ExxonMobil Upstream Research Company, Houston, Texas peter.c.rasmussen@exxonmobil.com

Paolo Battagli Qatar Large LNG Engineering Project Leader

GE Oil and Gas, Florence, Italy paolo.battagli@ge.com

Darrell Mosier Machinery Engineer Lead

RasGas Company Limited, Doha, Qatar drmosier@rasgas.com.qa

Arthur Smith, III W.S. Nelson, New Orleans, Louisiana

Arthur.Smith@wsnelson.com

ABSTRACT

Qatar Liquefied Gas Company Limited (II) (Qatargas) and Ras Laffan Liquefied Natural Gas Company Limited (3) (RasGas) have successfully completed commissioning, start-up and operation of multiple Frame 9E gas turbine refrigerant strings. These 100+ megawatt compressor strings provide energy for the propane, mixed refrigerant and nitrogen cooling loops as well as electrical power to the four largest 7.8 MTA (million tons per annum) LNG Trains in the world at Ras Laffan Industrial City in Qatar.

This paper will share the commissioning and start-up experience of these gas turbines, which are included within the LNG liquefaction process and integral to the power generation system of the LNG plant. The paper specifically addresses the unique and complex operating modes associated with starting these large gas turbines, along with full and part load operating conditions. The rigor applied to both static and dynamic commissioning and the success achieved will be shared. Operating experience on how this unique application of Frame 9E gas turbines have successfully responded to a wide mixture of fuel compositions and load upsets while maintaining production and complying with stringent site emissions requirements, mitigated by dry low NOx (DLN) technology and refrigeration loop pressurized starting, will also be covered.

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SYNOPSIS

The commissioning and start-up of the first Frame 9E driven refrigeration compressor strings for the new Qatar LNG trains required a unique approach. The parallel execution of multiple mega-projects, installation of the world’s largest LNG equipment and validation of many new technologies represented unprecedented challenges to the project partners.

To take on these challenges, Qatargas and RasGas together with ExxonMobil, General Electric (GE) and their sub-vendors assembled a team of experienced people and applied structured processes to ensure the technology validation program effectively addressed remaining risks prior to hand over of the refrigeration compressor strings to operations. To minimize the risk of reliability problems post start-up, innovative strategies were adopted to ensure system cleanliness. The entire twelve strings were also thoroughly tested during commissioning including final validation of the DLN combustion systems and variable frequency drive (VFD) starter/motor/generators. Consistent with the strategy of using the first mega-train, Qatargas Train 4, as the leading train for complete refrigeration string design validation testing in Massa Italy, real-time site implementation lessons learned were also carefully captured and transferred immediately to the three remaining mega-trains during simultaneous site execution.

INTRODUCTION

Over the last year Qatargas and RasGas, with strong support from its shareholders Qatar Petroleum and ExxonMobil, have successfully started operation of the largest LNG trains in the world (7.8 MTA each). At the heart of these LNG trains are twelve of the largest refrigeration compressor strings manufactured by GE. The function of the three refrigerant strings (per LNG train) is illustrated in Figure 1. These are the first to be driven by Frame 9E dry low NOx gas turbines with a combined overall compression power of more than 1000 megawatts (one gigawatt) and incorporate advanced technologies not previously used in the LNG industry.

Figure 1. AP-X® LNG Process

The execution challenges associated with installing, commissioning and starting these twelve large machinery strings were unprecedented. The successful start-up can be primarily attributed to the strong working relationship among all of the major organizations involved in these projects. Most of the activities were executed in parallel by two separate project teams, in adjacent sites,

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within the different operating companies of Qatargas and RasGas. This created the need for close coordination of work and the introduction of processes that facilitated real time sharing of lessons within and across organizational boundaries. These processes were critical in capturing the full benefit from the execution learnings obtained on Qatargas Train 4.

The complex sequence of commissioning programs across both plants and the extent of critical activity overlap can be seen in the overall schedule depicted in Figure 2.

Figure 2. Train Start-up Timeline

Simultaneously, Ras Laffan Industrial City was experiencing a period of unprecedented construction activity with a peak daily workforce of approximately 50,000 workers on the sites of the four new Qatargas and RasGas LNG trains.

RasGas and Qatargas therefore relied on ExxonMobil, Chiyoda Technip Joint Venture (CTJV) and GE to draw on their worldwide resources and supply the large number of experienced and skilled people needed to take on such a set of challenges.

The scale and complexity of the hardware associated with these four large LNG trains intensified the challenge. For example, installing and aligning twenty four gas compressors each weighing up to 300 tonnes, including aligning and connecting suction piping up to 2.0m in diameter in ambient temperatures exceeding 40°C required well managed and skilled work teams using proven safe work practices.

The design and manufacturing challenges, along with the industry leading testing campaign associated with the Qatargas Train 4 main refrigeration compressor strings was presented in the LNG-15 paper [1] “Design, Manufacture, and Test Campaign of the World’s Largest LNG Refrigeration Compressor Strings”. This new paper completes the story by covering some of the execution and technical challenges associated with transitioning from a single fully tested prototype design for Qatargas Train 4 into twelve reliable operating refrigeration compressor strings in two of the world’s largest operating LNG plants.

Central to the successful start-up and operation of the Frame 9E refrigerant compressor strings was the integration of enabling technologies, rigorous processes and experience of the

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people working together as a team. The role played by these three key elements, as shown in Figure 3, will be illustrated throughout this paper.

Figure 3. Technology, People & Processes

PEOPLE

Successful projects involve highly skilled people working well together toward a common goal. The leadership of the project management teams of Qatargas (QG-2) and RasGas Expansion (RGX-2) mega projects set challenging objectives and the site personnel from all organizations worked together to ensure these were met. Teams at both plants included a well-balanced mix of young, enthusiastic engineers combined with very experienced engineers. The result was highly effective teams working in a positive environment.

The commissioning teams for the large compressor strings involved the three primary stakeholder organizations:

• Operating Companies (Qatargas/RasGas)

• Project Management Teams (QG-2/RGX-2/ExxonMobil)

• Machinery Vendor (GE Oil & Gas & subvendors)

The three primary organizations associated with each project worked together as shown in Figure 4 below. The site teams were continuously assisted by permanently assigned engineering and management teams at ExxonMobil Development Company (EMDC) and GE headquarters in the United States and Italy. CTJV provided much of the general execution support, and established their own “3LNG” Organization to ensure effective sharing and application of lessons learned between the Qatar mega-train LNG projects. Specialized technical support was provided by GE’s sub-vendors and other specialist organizations from around the globe.

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Figure 4. Site Machinery Commissioning Organizations

The operating companies, Qatargas and RasGas, allocated very experienced people to manage the new mega projects through the Project Management Teams (PMT) and also formed interface organizations responsible for transitioning the new LNG trains from the PMT to the operating companies. The technical and operating experience within these transition organizations proved pivotal in achieving flawless execution.

The QG-2 and RGX-2 PMTs included the largest seconded ExxonMobil Upstream engineering team ever mobilized. More than twenty five PMT machinery engineers and specialists were involved at the site supporting the installation and commissioning of the twelve strings. Engineers with significant depth of skills and years of experience were brought in from a variety of ExxonMobil’s global functional organizations - Upstream, Downstream and Chemicals.

Key project resources from concept, design development, and testing phases were also allocated to the site teams. These resources were critical for technical continuity and to ensure that all design validation objectives were achieved on the leading Qatargas Train 4 and subsequently shared with the project teams for the other LNG trains. By the time commissioning had commenced the support structure from all organizations had been in place for approximately six years.

GE Oil & Gas created an integrated team to manage both projects at the initiation of the conceptual study in 2003. The continuity and depth of this team structure also contributed significantly to the project’s success. Many of the roles of the key team members changed over the years as the execution phase moved from the factory, through the full load test in Massa Italy and then on to construction, installation, commissioning and start-up of the trains.

The integrated GE site teams had a scope of responsibility beyond that traditionally adopted to support machinery installation and commissioning. The extended scope included site management, engineering support, quality control, planning/scheduling and materials management

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in addition to the usual technical field support disciplines of Machinery/Mechanical, Electrical and Instrumentation & Controls.

TECHNOLOGY

The Air Products AP-X® liquefaction process requires three independent refrigeration compressor strings (Propane (C3), Mixed Refrigerant (MR) and Nitrogen (N2)). A schematic of the string layouts is depicted in Figure 5. Each string utilizes a GE Frame 9E single shaft gas turbine driving through an Ansaldo Sistemi Industriali (ASI) starter/motor/generator, powered by a Siemens Variable Frequency Drive (VFD).

Figure 5. Refrigeration Strings

The string design met demanding operability, reliability and availability requirements, and in addition, the environmental objectives of reducing NOx and CO emissions and minimizing flaring. The large starter/motor/generator with VFDs, combined with the revised Frame 9E start logic, provided the capability to start the compressor strings with the refrigeration loops still pressurized. This eliminates the need to flare the refrigerant prior to start-up. This reduced the operational complexity and the start-up time for each LNG train by approximately 7.5 hours which increases annual LNG production by ~ 0.5% (assuming 6 starts per year). Starting while still pressurized also eliminates the flaring of more than 1200 tonnes of hydrocarbon gas each year from Qatargas 2 and RasGas (preventing ~3500 tonnes of CO2 from being discharged into the atmosphere each year). During the summer months, when ambient temperatures are high and gas turbine power is limited, the large starter/motor/generator operates in helper mode to add shaft power to the string enabling constant annual LNG production rates.

The ability to vary the string speed to optimize LNG production while utilizing excess gas turbine power to produce high quality, constant 50Hz electrical power from the starter/motor/generator was another design first for these projects.

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In summary the new technologies incorporated into the 12 compressor strings include:

• First mechanical drive variable speed Frame 9E gas turbine customized for LNG

o Dry Low NOx (DLN) combustion system and controls able to accommodate large variations in fuel gas composition, load demand and ambient temperatures

o New control software to match mechanical drive application, wide range of operating conditions and application specific start-up requirements

• Large centrifugal compressors and associated casings

o Largest single piece impellers

o Largest dry gas seals

• Largest starter/motor/generators and VFDs ever used in a gas turbine driven compressor string

o 45 MW (60 MW peak power at start-up) variable frequency electric motor

o 60 MW Siemens VFDs (4X15 MW) / VFD Redundancy / 4 -3 phase stator winding motor

o Active Front End (AFE), capable of generating electrical power at non-synchronous speed

• Common off-skid oil lubrication system for an entire string

• Integrated gas turbine, VFD, plant process control and power management system controls

To ensure the success of so many new technologies, complete validation of the final refrigerant compressor string design was required. Due to practicality limitations this validation effort was divided into two phases.

First, a prototype of each unique string was built as part of the Qatargas Train 4 project and full load tested in Italy during the Massa String tests completed in June of 2006. This test program successfully validated the majority of the new design aspects that would be used on all of the Qatar mega LNG trains including the operability of the complete strings, functionality of the redesigned DLN combustion control strategy, the performance and reliability of the VFD drive and starter/motor/generator system, the pressurized restart capability and the performance of the gas compressors and dry gas seals.

Second, the remaining technology areas that were not practical to test on the prototypes in Massa required validation at the site on the actual strings after mechanical acceptance was achieved. This included:

1. Dynamic full load operation on plant fuel gas with high nitrogen concentration

This validated the DLN combustion system and, in particular the operability and reliability during rapid changes in fuel gas composition

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2. High ambient gas turbine package performance

Ambient conditions significantly influence gas turbine combustion system performance and can also impact ancillary systems. Field testing proved the performance and reliability of the complete package at the Qatar conditions

3. Operation within Qatargas and RasGas plant power systems

The differences in power system designs could create sources of electrical system excitation frequencies or instabilities that could create adverse effects on the refrigeration compressor string, balance of plant motor driven equipment, as well as the other electrical power generation equipment (steam and gas turbine driven generators)

4. Pressurized starting validation with plant piping configuration

As the process loops at the Massa test site did not exactly match the final plant configuration the final settle out pressures were not previously validated. Final verification of the ability to complete a full pressure restart from settle out without the need for flaring was therefore completed at the site

5. Gas turbine enclosure ventilation performance

The Massa testing was completed with a temporary enclosure in a temperate climate. The site field validation verified final ventilation air flow patterns and the corresponding thermal effects on the turbine casing.

The challenge for the QG-2 and RGX-2 teams was to ensure that this latter stage site validation effort was completed, to the maximum extent possible, prior to first LNG production. This required extensive planning and the use of numerous rigorous work execution processes.

PROCESSES

Both RasGas and Qatargas have a proven history of successfully executing large LNG projects and safely starting-up, and reliably operating and maintaining these complex facilities. This has been achieved through the use of well proven work processes that govern day-to-day activities. ExxonMobil and GE are also organizations well known for such structured work processes, many of which were a valuable contribution.

The proprietary capital project management systems of ExxonMobil and the Operating Companies guided the development of a structured framework for planning the installation and commissioning activities associated with these multiple large machinery packages. Experience on previous Qatar LNG projects led to early design enhancements and also to procedural improvements integrated into all phases of the project execution.

Red Flag Reviews

GE’s Red Flag Reviews are based on project-specific dedicated checklists that are verified and approved by the site team at critical commissioning stages and become part of the official project documentation. All sub-systems such as, the lube oil and fuel gas systems are physically checked at the site for correct assembly, integrity, and adherence to design requirements. A set of detailed functional test procedures, first developed at the Massa string tests, were validated

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and updated accordingly for site conditions. This process has now become the standard approach for GE.

Lessons Learned Process – Real Time Sharing

Due to the large scope of parallel work execution at two adjacent sites, ExxonMobil and GE assigned key leadership positions responsible for ensuring real-time capturing and sharing of lessons across the site boundaries. Initiatives adopted included regular meetings between the ExxonMobil, Qatargas and RasGas machinery site teams at which general execution learnings were shared. Dedicated joint meetings were also held to review specific topics such as piping alignment strategies and duty run procedures. A Lessons Learned Register tracked closure of key GE learnings from QG-2 that were applicable to RasGas. These were reviewed on a weekly basis with the RasGas site teams.

Real-time integration of lessons learned across projects brought immediate results with a reduction in the commissioning time (motor solo-run to first LNG) of many months for QG-2 Train 5 compared to QG-2 Train 4. These lessons learned also enabled a reduction in commissioning schedule on both RasGas trains.

FMEA – Unique Application of an Industry Standard Tool

The traditional Failure Mode & Effect Analysis (FMEA) is commonly used in many industries to identify and address design deficiencies in the early stages of a product development. ExxonMobil and GE have also successfully utilized FMEA to identify and prioritize remaining integrity, schedule and reliability risks that still remained in the latter phases of our most complex projects. The types of failures in focus included those that could be attributed to incomplete technology validation during the first phase in Massa and potential execution process problems during the second phase at site.

The unique use of FMEA for this application was triggered by a comprehensive strategy initiated by the QG-2 project team to assess and manage the risks of successfully introducing a large number of new technologies (not limited to the compression strings) on the world’s first 7.8MTA Trains.

Large multi-disciplinary teams from Qatargas, RasGas, ExxonMobil, GE, Siemens and the other sub-vendors, conducted three independent reviews of the gas turbine package, compressor packages and Siemens VFD systems to address all aspects of the final as-tested design, the commissioning and validation plans, and first start-up/operating procedures.

A ranked list of potential risks and mitigating action plans (including hardware and software changes or specific validation tests) was produced for evaluation and implementation by GE/Siemens and the two project teams, ensuring all high priority risks were addressed prior to start-up. The role of the FMEA Reviews is illustrated in Figure 6.

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Figure 6. FMEA Reviews

The FMEA reviews identified more than 50 significant reliability or schedule risks. These

ranged from simple issues that were easily addressed to more complex problems requiring considerable effort and resource deployment to ensure adequate mitigation prior to start-up. For example:

• Revised installation procedures for the hydraulic fit couplings to address possible shaft damage during installation

• Tooling improvements and procedural changes for the installation of the dry gas seals

• Validating total string alignment in the field by installing targets at critical reference points along the string and using optical surveying tools to measure the final installed shaft offset in the idle and operating conditions

• Field validation of the enclosure ventilation system CFD analysis by installing thermocouples at critical locations on the turbine casing to measure temperatures against predicted values. This eliminated the potential risk of casing distortion due to inadequate or uneven cooling.

One of the more important and complex issues identified during the FMEA of the Siemens VFD system was the potential for arc flash related damage: a thermal and/or mechanical effect created by the sudden, even if relatively small, pressure increase inside the cabinets where the power equipment is installed.

To minimize this risk a thorough study of the effectiveness of an arc flash detection system concluded that a well designed system could significantly abate the amount of energy involved in an arc flash event. Such a system detects the light of the initiating arc flash and cuts the power to the equipment within a few milliseconds, thus stopping escalation and reducing the extent of damage.

Qatargas and RasGas installed arc flash detection systems and reinforced the cabinets to reduce the damage consequence. Siemens calculated the potential pressure rise within the cabinet which determined the cabinet reinforcement requirements. Pressure release vents were also built into the cabinet roof to safely release any potential rapid rise in pressure, improving

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safety for any nearby operators and maintenance personnel. Finally, tests were performed in a laboratory to prove the effectiveness of the adopted measures.

The installation of the arc flash detection system was completed prior to start-up and provided immediate benefits during site commissioning when the system activated and prevented severe damage following a 33kV VFD input transformer fault.

In summary, the unique application of the traditional FMEA during project execution was a critical step in identifying and mitigating risks that would have impacted the success of the start-up and subsequent reliable operation of the new Qatargas and RasGas LNG trains.

SITE COMMISSIONING / VALIDATION PROGRAM

Safely starting the machinery on schedule while minimizing future unplanned shutdowns was the team’s priority

With twelve strings to commission, early planning commenced in Qatar in mid-2007 attended by technical experts and lead commissioning engineers from Qatargas, RasGas, ExxonMobil, GE and their key sub-vendors. The detailed commissioning program scope and execution plans were then further refined by the individual site teams considering the results of the Massa testing program, the FMEA review output and the many schedule, hardware and resource constraints.

Test protocols were needed to validate applied technologies and reveal any new issues so that they could be addressed prior to first LNG production. The teams quickly concluded that this could be best achieved utilizing a comprehensive testing program that included operation of the complete machinery strings under load (duty runs) prior to first LNG.

Prior to Mechanical Acceptance Certification (MAC) all hardware was confirmed to be installed correctly using the PMT’s proprietary capital project management system and GE’s Red Flag Review checklists.

The first phase of the commissioning program, Static Commissioning, commenced prior to reaching the Mechanical Acceptance milestone and was therefore executed by the main contractor CTJV. This first phase of commissioning involved energizing equipment and validating correct functionality of the many individual subsystems involved. It included the instrument loop checks, switch gear function tests, motor solo-runs and cause and effect testing needed to ensure safe operation of these large and complex machinery packages.

The second phase of commissioning starts when process fluids are introduced, activities become more complex and multi-discipline teams are involved in working to rigorous step by step procedures. These activities were primarily led by the operating company interface organizations.

Figure 7 below depicts the relative schedule of key activities in both the Static and Dynamic Commissioning phases.

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Figure 7. Commissioning Program Schedule

Due to the abundance of new technologies and associated rigorous validation required, prerequisite steps were incorporated to confirm closure of FMEA and Red Flag Review items prior to starting each test run activity. The complexity of the systems and addition of the prerequisite steps increased the scope and resource requirements to execute the starter/helper motor solo runs and gas turbine full speed no-load (FSNL) tests.

One such prerequisite step involved confirmation of the cleanliness of the Inlet Bleed Heating (IBH) system prior to commencing the gas turbine FSNL test. Construction debris remaining in the system resulted in IBH control valve damage on the earlier generation RasGas Train 5 project and during the Massa String test despite the use of industry standard pipe cleaning processes. This issue was raised during the FMEA reviews and possible mitigation options were proposed.

As a result, GE developed a new procedure that involved temporarily removing the IBH control valve and installing a temporary strainer in the line upstream of the air inlet ducting (refer to Figure 8). The gas turbine was then started prior to the gas turbine solo-run and the Frame 9 axial compressor discharge air was directed at high velocity through the IBH line for a period of time to ensure removal of all debris. The new procedure was successful and there were no subsequent IBH control valve reliability issues on any of the twelve Frame 9E gas turbines.

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Figure 8. Inlet Bleed Heating Cleaning

The successful completion of the motor solo run and gas turbine FSNL tests were pivotal milestones for the project execution and were required prior to Mechanical Acceptance.

Another system cleanliness lesson from previous Qatar LNG projects was the potential risk of construction debris remaining in the main process piping and compressor discharge coolers leading to potential damage of compressor anti-surge valve seats and other sensitive equipment. To address this production risk dynamic air blows were identified as a key commissioning strategy to validate line cleanliness.

An extensive dynamic air blow procedure, utilizing the Frame 9E as the driver, was developed. The use of “tee” type suction strainers eliminated the need to disturb the main compressor piping and lowered shaft alignment risks. The strainer cover was modified to provide a second source of suction air (refer D-Plate cover in Figure 9) to the compressor leaving the original design strainer mesh intact and protecting the compressor from foreign material ingestion during the air blow. This combined with minor modification to the dry gas seal buffer system also mitigated possible dry gas seal damage due to reverse pressurization should the compressor suction pressure drop to sub-atmospheric levels.

The dynamic air blow run time was limited due to the difference in the gas properties (mole weight and ratio of specific heats) of air compared to the normal process gas. This difference increased the compressor discharge temperature which approached the trip level established by both the compressor and process piping. Although the dynamic air blow ρv2 (fluid momentum) could not match the final operating values, analysis of the achievable momentum confirmed the majority of any harmful debris left in the refrigeration loops would be removed by this cleaning process. This cleaning effort would significantly reduce the probability of additional train shutdowns during plant start-up due to strainer blockage or valve damage. Figure 9 shows the piping set-up for a typical dynamic air blow and highlights the two air suction sources and final discharge point.

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Figure 9. Typical Air Blow Piping Configuration

The Dynamic Commissioning phase also included validation tests of two key technologies not able to be fully validated during the Massa String tests. The Frame 9E Mechanical Drive DLN system needed to be operated using the final plant fuel gas system and the starter/motor/generator/VFD needed to be proved reliable when integrated into the total power system of each of the plants.

Frame 9E DLN Combustion Validation Tests

The low-emission technology adopted for the Qatar large LNG projects was a step beyond GE’s experience increasing the priority on completing full validation testing. The gas turbines needed to accommodate the inherently large range of fuel gas compositions created within the LNG process, as well as the rate of change in conditions associated with plant equipment upsets.

The majority of the Qatargas and RasGas normal operating fuel gas is derived from nitrogen rich (~ 35 – 42%) End Flash Gas (EFG), a by-product of LNG production, and not available until the plant is online. The only fuel gas available prior to LNG production is the start-up and back-up fuel gas derived from the plant feed gas, Fuel From Feed (FFF), which has a very low nitrogen content (~ 2 - 4%). The variation in nitrogen content of these fuels results in a large variation in the fuel gas Wobbe Index, which is the primary parameter that defines the similarity of different fuels.

The project teams successfully mitigated the unplanned downtime risks associated with implementing the new emissions reduction technology on these challenging applications through a number of activities during the execution of the project. Many were completed a considerable time prior to shipping the machinery to site. Pre-shipping mitigation steps included:

Laboratory Combustion Tests. Combustion tests completed in 2005 in GE’s combustion laboratories in Greenville, South Carolina initially sized and then validated the recommended nozzle and dilution holes using actual site hardware and expected gas compositions. The tests validated the design and highlighted potential operating modes where final control system tuning

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might be required to avoid operability/reliability risks. A project specific risk matrix similar to that provided in Figure 10 provided the focus areas for the later site DLN testing program.

Figure 10. Greenville DLN Combustion Test Matrix (QG-2)

Combustion Control Logic Redesign. The Frame 9E control logic was redesigned to suit the unique LNG compressor mechanical drive application as opposed to GE’s previous applications for power generation. The redesigned logic incorporated:

• Modified light-off sequence for reliable start-up with variable fuel gas

• String acceleration sequence with coordinated control of Frame 9E, helper motor and process compressor

• New control logic for the DLN combustion modes during steady state and transient conditions: process upsets; start-up; shutdown

• Improved exhaust temperature control logic increasing control accuracy of combustion parameters while improving overall gas turbine performance

• Software to calculate remaining gas turbine power available to base load considering turbine degradation and a wide range of fuel gas compositions

Massa String Testing. The prototype string testing program provided initial validation of the new control logic while also validating the new combustion hardware performance in an operating gas turbine.

Fuel Gas System Design Optimization. The RGX-2 and QG-2 fuel systems, designed by CTJV, utilized the Greenville test results to establish the maximum allowable Wobbe Index rate of change. The size of the fuel gas mixing drum and the final fuel gas system control strategy were critical in preventing flame-out as the fuel changed from EFG to FFF and vice versa. Dynamic simulations of the entire Qatargas and RasGas fuel systems were run to ensure that all scenarios were covered. The highest risk scenario was a trip of the fuel gas compressor which would result in a sudden stop of the EFG fuel gas supply as can be seen in Figure 11. This scenario was tested at the site with the Frame 9s near full load, with no resulting shutdown of the refrigeration strings.

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Figure 11. Fuel System

Once the machinery was shipped and installed at the site in Qatar further steps were taken to mitigate any other potential DLN-related outages during the Dynamic Commissioning phase.

On the previous LNG projects there had been several outages after start-up to validate on-skid fuel gas line cleanliness. The QG-2 and RGX-2 teams realized that a modified approach was needed to avoid similar outages.

Adequate cleaning of the fuel gas supply lines is a prerequisite for reliable operation of the gas turbine since the fuel nozzles have relatively small holes that may clog or prematurely wear if debris is carried into the combustor. During commissioning, three temporary strainers were fitted immediately before the fuel gas manifolds on the gas turbine as shown in Figure 12. These strainers were only removed once the supply lines had been proven to meet the cleanliness criteria set by GE. This required operation at fuel flows (momentum ρv2) equivalent to the final operating conditions. For LNG applications the ρv2 for EFG is significantly higher than the ρv2 for FFF due to the different nitrogen content. Since EFG is not available until after start-up the temporary strainer cleaning usually takes place after first LNG production resulting in start-up and shutdown cycles to complete the cleanliness inspections. The elimination of production losses associated with multiple LNG train restarts became the incentive to develop the new procedure that facilitated early verification of adequate fuel line cleanliness prior to first LNG production.

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Figure 12. Fuel Gas Strainer Graphic

The final procedure adopted by the teams at both Qatargas and RasGas included enhanced initial line cleaning methods such as high velocity air blows, hydro-milling and hand cleaning followed by operation at high Frame 9E power levels, at different fuel gas split rates, with the available fuel gas. The use of methane in the C3 / MR refrigeration loops, in conjunction with the VFD in generation mode achieved the desired Frame 9 loads. These extended runs maximized the velocities in each of the three DLN supply lines and adequately validated system cleanliness enabling early removal of the temporary strainers.

At this stage the final DLN performance validation tests began. The first of these final tests was the DLN tuning runs in premix mode on FFF gas with nitrogen in the nitrogen loop and methane in the C3 / MR loops. The use of the VFD in generation mode drastically simplified the power adjustments required to shift between DLN operating modes on FFF gas.

However the final DLN tests could only be completed after EFG fuel gas was available post start-up and this higher risk portion of the program was completed without issue due to the other comprehensive validation steps completed previously. The ability of the VFD to generate power at non-synchronous speed minimized LNG production losses associated with DLN tuning.

In summary, the Qatar Frame 9E mechanical drive DLN testing program (as shown in Figure 13) highlights were:

• Emissions performance targets can be met under operating conditions

• Combustion dynamics proven to be within expected range

• LNG production not significantly impacted during DLN test program

• No significant production losses associated with verifying system cleanliness

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Figure 13. Frame 9E Mechanical Drive DLN Validation

VFD Performance & Reliability Validation

During Massa string testing it was observed that a change in VFD input transformer tap settings created an approximate 8Hz active electrical power (MW) instability on both the line and load side of the variable frequency drives. This instability was not predicted or experienced during previous testing but created a concern considering it was close to a compression string torsional natural frequency. This meant that an electrical instability and a system mechanical resonance were very close to one another. Since the magnitude and frequency of the instability appeared to be affected by electrical system changes, a potential risk developed that the site electrical system configuration at either RasGas or Qatargas plants might negatively impact VFD stability creating string reliability concerns.

Although the GE string design was robust there were still unknowns associated with the primary cause of the VFD 8Hz instability experienced during string testing. The main concern was that the drive instability magnitude and frequency experienced during the Massa testing may change due to different site electrical systems and generation capacity. It was therefore decided to monitor the drive stability along with the response at the coupling during commissioning and start-up using high speed recording instrumentation installed on the string coupling, VFD drives, and plant electrical system.

The results from the site testing validated the string design and the final frequency of any electrical system instability. The site commissioning also confirmed that as each additional compression string was put on-line the associated increase in system short circuit current provided by the drives, along with increases in system generation capacity to support the increased plant load, caused the instability frequency to decrease. With one string on-line (i.e. 4 VFD threads) the frequency was 7.2Hz (as can be seen in Figure 14) and when all three strings were on-line (i.e. 12 threads) the frequency dropped to 6.6Hz. The site results were quite favorable because the decrease in VFD instability frequency increased the distance from the

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refrigeration string torsional resonance, as well as any possible excitation of balance of plant rotating equipment. This assured that the original design required no external suppression of the plant electrical system.

Figure 14. Electrical Instability - Massa vs. Site

Each Compression String VFD utilizes active front end (AFE) technology capable of supplying real power whenever the turbine power available exceeds the compression requirements as well as reactive power whenever the drives are online.

Since the VFDs isolate the motor/generator from the plant grid by converting plant power from 50 cycle AC to DC and then to a variable AC supplying the compression string motor/generator, power flows between the plant grid and the motor/generator independent of turbine driven motor/generator shaft speed.

Both real and reactive power produced by the compression string VFDs is controlled by a power management system (PMS) in both plants designed to respond to load changes and share power with all electrical power producers within the facilities. One unique feature of the power management system is its ability to scale the “size” of the VFD generation power available (0 to 45MW) based on compression string excess torque as calculated by the GE Frame 9E turbine controls. This unique feature allows proportionate load sharing between the compression string VFDs in generation mode with other power generation throughout the facility.

VFD generation testing under PMS system control validated the VFDs not only functioned as “typical” synchronous generators but also that they shared load with the remaining electrical power producers within the facility. This was accomplished by successful operation of the VFDs supplying real and reactive power in a plant environment and confirming that they respond to compression and or plant load fluctuations while supporting facility electrical power generation and proportional load sharing with other power producers. A schematic of how the multiple systems are integrated within each of the LNG plants is shown in Figure 15.

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Figure 15. Multiple Systems Integration

START-UP & OPERATION

The comprehensive operational and technology validation tests completed within the dynamic commissioning program prior to start-up, ensured that all major reliability risks were addressed prior to the introduction of feed gas into the LNG trains.

The remaining validation steps came once the refrigeration compressor strings were put into operation and LNG production commenced.

One key operability feature validated after reaching normal operating conditions was the ability to complete a rapid pressurized restart following a trip of any of the machinery strings. The final settle-out-pressure of each refrigeration loop and the compressor anti-surge control (supplied by Compressor Controls Corporation - CCC) settings utilized at start-up were two critical factors.

The anti-surge valve position has a large effect on the compressor power consumption and hence is critical in controlling the string load during start-up from a pressurized condition. The conflicting need to keep the anti-surge valves open as far as possible to keep the compressor operating point at a safe margin from the surge line and yet as closed as possible to limit power consumption is the biggest challenge. The dynamic simulations completed during the design phase proved valuable in predicting the required valve openings and the corresponding turbine power requirements during this start-up mode. Figure 16 shows a full pressure restart of one of the propane compressors where the suction pressure can be seen dropping rapidly from settle-out and the starter motor torque requirement peaking at approx 70%. Similar tests completed on all strings validated the ability to meet the design objective of rapid re-start without flaring on all strings.

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Figure 16. Pressurized Restart Event – C3 Machine

Other operational modes validated during the dynamic commissioning and following start-up included:

Helper Mode: String operation with additional power added from the helper motor. Normally only required during high ambient temperature conditions but used in testing to unload the gas turbine without affecting LNG production.

Generator Mode: String operation when available gas turbine power is utilized to generate additional power for the plant power system. This was used extensively during DLN testing to add additional string power.

The LNG trains including the main refrigeration compressor strings completed performance verification testing following start-up and all results obtained during this testing verified that the design objectives of the mega train concept were met.

Since initial start-up the availability of all units has exceeded expectations with very few machinery related trips with the highest hour units achieving more than 6500 hours of reliable operation by the end of 2009.

CONCLUSION

Qatargas and RasGas with strong support from the technology and technical teams in ExxonMobil, and working closely with CTJV and GE, are proud to report the safe installation, commissioning and start-up of the twelve large refrigeration compression strings within the four new mega LNG trains in Qatar.

These unique compression strings incorporate many new features that would help reduce operating costs and the associated impact on the environment.

The economies of scale have been stretched once again, and the challenge to safely commission and start such a large number of complex facilities in parallel within an eighteen month period required significant support from all involved organizations. Meeting that challenge

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can be attributed to the quality of the people involved, teamwork and the structured processes used in bringing this state-of-the-art technology from concept to reality.

Success can be measured by the safe and efficient completion of the comprehensive commissioning and testing programs on all twelve refrigeration compressor strings and the minimization of significant post start-up production losses.

REFERENCE CITED

(1) Roy Salisbury et al, “Design, Manufacture, and Test Campaign of the World’s Largest LNG Refrigeration Compressor Strings”, LNG 15, April 2007.

BIBLIOGRAPHY

(1) Ching Thye Khoo et al, “Execution of LNG Mega Trains – The Qatargas 2 Experience”, 24tth World Gas Conference, October 2009

(2) Mark J. Roberts et al, “Large Capacity Single Train AP-X TM Hybrid LNG Process” , GasTech 2002, October 2002

DEFINITIONS

AFE Active Front End

CFD Computational Fluid Dynamics

Refrigeration Compressor String

complete machinery package including the Frame 9E Gas turbine driver, helper starter/motor/generator and process gas compressors

LNG Train single LNG processing plant that incorporates three refrigeration compressor strings (Propane (C3), Mixed Refrigerant (MR), and Nitrogen (N2)

LNG Plant total LNG processing plant that can incorporate numerous LNG trains

Duty Runs Operation of the gas turbine-driven compressor strings in recycle mode prior to LNG production commencing

Massa small town on the west coast of Italy where GE Oil & Gas has a testing site for completing full scale testing of complete machinery strings

VFD Variable Frequency Drive

FMEA Failure Mode & Effect Analysis

DLN Dry Low NOx

MTA Million Tons per Annum

CTJV Chiyoda Technip Joint Venture – Main Contractor

PMT Project Management Team

MAC Mechanical Acceptance Certification – Project milestone when installation by the main contractor was considered complete

FSNL Full Speed No Load – Early gas turbine test prior to connecting to the process compressors

Motor Solo Run Operation of the electric motor when disconnected from the driven equipment

EFG End Flash Gas – Fuel Gas with high N2 concentration

FFF Fuel From Feed – Fuel gas derived directly from plant feed gas

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BOG Boil Off Gas – Fuel gas derived from LNG boil off

IBH Inlet Bleed Heating: System used to re-circulate air from the gas turbine facilitating operation in DLN mode at low loads

ρv2 measure of gas flowing momentum that includes gas density and velocity

PMS Power Management System