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REFRIGERATION COMPRESSOR DRIVER SELECTION

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REFRIGERATION COMPRESSOR DRIVER SELECTION
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    1 REFRIGERATION COMPRESSOR DRIVER SELECTION AND TECHNOLOGY QUALIFICATION ENHANCES VALUE FOR THE WHEATSTONE PROJECT Pankaj Shah , Wheatstone Project, Chevron Mark Weatherwax, Wheatstone Project, Chevron Meredith Chapeaux, Wheatstone Project, Chevron Karl Masani, ConocoPhillips Company Cyrus Meher-Homji, Bechtel Corporation ABSTRACT Chevron Australia, as part of the Wheatstone Project, is constructing a two train liquefied natural gas (LNG) facility and domestic gas plant at the Ashburton North Strategic Industrial Area, 12 kilometres west of Onslow on the Pilbara coast of Western Australia. The initial two-train facility design capacity is 8.9 million tonnes per annum (MTPA). Eventually, the facility could expand to produce up to 25 MTPA of LNG. During the early stages of project development, a driver selection study was performed based on the ConocoPhillips Optimized Cascade ®  natural gas liquefaction process. This driver study evaluated a variety of project-specific parameters and resulted in the selection of a General Electric LM6000 PF aeroderivative gas turbine based on overall value to the shareholders considering production rates, production efficiency, fuel costs, greenhouse gas emissions, installed cost, operational and maintenance cost. The selection of the LM6000 gas turbine results in the first commercial use of the LM6000 engine in a mechanical drive application. The final decision to use the LM6000 engine was based on a detailed technology qualification. A collaborative qualification was performed with representatives from the EPC contractor, process licensor, end user/operator, and the equipment manufacturer. The technology qualification followed a systematic approach to identify the risks and novelties within the application. Risk items identified were analyzed with a combination of detailed studies, computer-based modelling and full-scale engine testing. Following the completion of the technology qualification, a detailed risk mitigation plan was developed. The plan was incorporated into the purchase order of the equipment and subsequently incorporated into the equipment manufacturer’s Failure Mode Effects Analysis (FMEA) process. Finally, detailed analysis and testing requirements were selected to address all risks that were highlighted in the risk mitigation plan. INTRODUCTION The Wheatstone Project is one of Australia’s largest resource projects – providing both LNG export and greater security of supply for domestic gas production. The project will provide significant benefits to the people of Australia, including significant employment opportunities during the construction and operational phases, government revenue and local business opportunities. The project includes offshore development wells and associated facilities as well as the onshore LNG and domestic gas plants located within the  Ashburton North Strategic Industrial Area (ANSIA), 12 kilometres west of Onslow in Western Australia’s Pilbara region. The Wheatstone Project is a joint venture between Australian subsidiaries of Chevron (64.14%), Apache (13%), Kuwait Foreign Petroleum Exploration Company (KUFPEC, 7%), Shell (6.4%), and Kyushu Electric Power Company (1.46%), together with PE Wheatstone Pty. Ltd. (partly owned by TEPCO, 8%). The Wheatstone Foundation Project will process gas from various fields located 145 km offshore in the West Carnarvon Basin. Eighty percent of the Wheatstone Foundation Project capacity will be fed with natural gas from the Wheatstone and Iago Field operations, which are operated by Chevron in joint venture with  Australian subsidiaries of Shell and Kyushu Electric Power Company, together with PE Wheatstone Pty. Ltd. The remaining 20 percent of gas will be supplied from the Apache and KUFPEC’s Julimar and Brunello fields. The Foundation Project will consist of two LNG processing trains, each with a capacity of approximately 4.45 (MTPA), as well as a Domgas plant with a nominal capacity of 190 MMSCFD. The overall project is expected to consist of at least five LNG trains and additional Domgas facilities with a nominal LNG production capacity of up to 25 MTPA.    2  A final investment decision to proceed with the Wheatstone Project was made on September 26, 2011, with construction beginning soon after in December 2011. LIQUEFACTION TECHNOLOGY The Wheatstone LNG Plant will utilize the ConocoPhillips Optimized Cascade ®  process. This technology was first used in the Kenai LNG plant in Alaska in 1969, and since then ConocoPhillips has licensed 22 LNG liquefaction trains with over 90 MTPA of liquefaction capacity in the world. A simplified process flow diagram of the Optimized Cascade ®  process for Wheatstone LNG shown in Figure 1. Figure 1: Simplified Process Flow Diagram of the Optimized Cascade  ®   Process The Optimized Cascade ®  technology offers the following benefits for the Wheatstone Project: ã   Commercially proven technology that can process natural gas of varying composition which is well suited to the development of a number of separate gas fields and the Wheatstone Project’s ‘hub’ concept; the technology has been used around the world, starting with Kenai in1969. ã   Operational flexibility enabling plant throughput to be tuned to market demand and available gas supply. ã   Benchmarks favorably with alternative processes and existing LNG plants in terms of its process efficiency and reliability. ã   Uses multiple, parallel compressor circuits within each liquefaction train that allow continued operation (at reduced rates) during periods of planned and unplanned gas turbine maintenance, reducing the number of full plant shutdowns and startups. ã   Use of parallel compressors allows the opportunity to use smaller compressor process drivers for a given LNG throughput, which complements the use of high-efficiency aeroderivative gas turbines. The ConocoPhillips-Bechtel LNG collaboration has provided constant innovation, specifically in the design and implementation of LNG driver configurations. The world’s first application of gas turbines in LNG service was implemented in 1969 at Kenai. The plant was built with six GE Frame 5 gas turbines and has operated for 43 years, never missing an LNG shipment. In 2006, ConocoPhillips and Bechtel designed and constructed the Darwin plant in Australia, which is still the only operating LNG facility to utilize high efficiency aeroderivative gas turbines (LM2500+G4) in LNG refrigeration service. Since then, eleven LNG liquefaction trains have implemented the LM2500+ for the primary refrigeration driver. This constant innovation and thirst for efficiency improvement has led to the development of the LM6000 PF gas turbine in LNG service. Wheatstone LNG will be the first LNG facility utilizing the LM6000 to drive its refrigeration compressors. DRIVER ALTERNATIVES    3 The selection of the driver for the refrigeration compressors within an LNG plant has a key impact on the overall LNG plant efficiency and capacity of the plant. Natural gas liquefaction plants are generally designed to the limits of the available refrigeration compressor drivers to maximize train capacities. It is therefore critical that an adequate amount of effort is put into the evaluation of driver options. Figure 2 shows the evolution of the LNG process technology since the early years from steam turbine to gas turbines as the driver of choice for the refrigeration compressors. Figure 2: Evolution of Refrigeration Compressor Drivers in the LNG Industry There are three major types of drivers used in the baseload LNG industry as described below. Steam Turbine Drivers Most of the earlier LNG baseload plants used steam driven refrigeration compressors. However, their use has become less common over the past 25 years. The steam turbine driver can be customized to the precise desired power requirement for the compression system, has high equipment reliability and provides the ability to vary operating speed over a wide range. However, the use of the steam turbine driver requires an elaborate steam, water treatment and cooling system, making it relatively complex to operate and high in initial capital cost. While the equipment reliability of the steam turbine is high, this benefit has to be balanced against the reliability as well as thermal efficiency of the total system. Gas Turbine Drivers The gas turbines come in a variety of discrete sizes with ISO ratings varying from approximately 27 MW (for a Frame 5 gas turbine) to 130 MW (for a Frame 9E gas turbine). The first use of gas turbines in an LNG plant was at the ConocoPhillips LNG plant in Kenai, which started up in 1969. This plant, with an LNG production capacity of 1.5 MTPA, used Frame 5 gas turbines to drive the refrigeration compressors. Initially, smaller gas turbines were used resulting in plant sizes ranging from 1 to 3 MTPA. As the LNG industry matured and focused on efficiency and cost reduction, the single train capacities grew to 5 MTPA or more. The higher capacity trains offered the opportunity to use large-size gas turbines. Electric Drivers    4 In recent years, there has been increasing interest in using electric motors as drivers for the refrigeration compressors. Some in the industry have proposed that high availability of electric motors can increase the overall plant production efficiency (availability). While the extent of increase in production efficiency is debatable, this option typically requires significant additional investment for power plants and systems. The Snohvit project in Norway is the only LNG plant to use motors as primary refrigeration drivers. To improve the overall thermal efficiency of electric driver arrangements, combined cycle power generation would be required with similar pros and cons of the steam systems mentioned previously.  AERODERIVATIVE VS. INDUSTRIAL GAS TURBINES There are two broad categories of gas turbine drivers used as mechanical drivers in the LNG facilities: Industrial Gas Turbines Industrial gas turbines such as Frame 5, 6, 7 and 9 have been used in the LNG liquefaction industry as mechanical drivers for refrigeration compressors since the late 1960s. The first application was at the Kenai plant in 1969. Since the mid 1980s, most LNG plants have used industrial gas turbines as mechanical drivers for refrigeration systems. The thermal efficiencies, at ISO conditions, of these gas turbines range from 29% to 34% (based on lower heating value). Industrial gas turbines can be either a single shaft or dual shaft design. Most industrial gas turbines used in power generation application, such as Frame 6, 7 and 9, are single shaft machines and have a limited range of speed variation. Typical speed ranges for single-shaft gas turbines are on the order of 95% to 101%. These machines, when used in mechanical drive application, also require a variable speed startup motor for starting up the compression strings. Unlike the large industrial gas turbines, the Frame 5 gas turbines have a two-shaft design that allows high startup torques, making it possible to start the compression strings under refrigeration compressor settle out pressure conditions and without a large starter motor.  Aeroderivative Gas Turbines The use of aeroderivative gas turbines for mechanical drive dates back to the 1950s. They are attractive as mechanical drives because of their small size, fuel efficiency and ease of maintenance. Aeroderivative gas turbines, such as LM2500 and LM6000, have been developed from aircraft engines. They are relatively compact, light weight and have thermal efficiencies, at ISO conditions, ranging from 39% to 43% based on LHV.  Aeroderivative and industrial gas turbines have inherent technical differences, some of which may impact which type is selected for a particular LNG project. Both turbine types are available in a wide range of sizes that provide flexibility in the selection of the appropriate size for a given application. The sizes are finite and are not adjusted to fit the application, as are motors, steam turbines and the compressors they drive.  Aeroderivative units are “multi-shaft” designs, whereas most large industrial units are single-shaft design. This feature gives the aeroderivatives advantages in part load efficiency and speed variation over a wider range of operation. The aeroderivatives also have higher full load efficiency due to a higher compression ratio and firing temperature. Aeroderivative engines are physically smaller than industrials. They are also modular in design, which allows maintenance and major overhauls to be done “off site.” Industrial units are typically maintained and overhauled in place. A major item impacting the decision on the use of industrial or aeroderivative gas turbines is their ability to handle large variations in fuel composition and the rate of change between these fuels. Given the larger combustion system and longer residence times in the industrial gas turbines, they are more tolerant of fuel variation compared to aeroderivative gas turbines. There are a number of additional features that impact the comparison between aeroderivatives and industrials, including emission characteristics, engine degradation, performance at site conditions (site rating), and fuel pressure requirements. Both aeroderivatives and industrial gas turbines are well proven selections for use in LNG refrigeration service. POWER AUGMENTATION FOR GAS TURBINES

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Jul 23, 2017

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