在海上生产平台上使用滚动压缩技术回收储存罐内闪发气体[中文3850字]
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在海上生产平台上使用滚动压缩技术回收储存罐内闪发气体[中文3850字],海上,生产,出产,平台,使用,滚动,转动,压缩,紧缩,技术,回收,储存,贮存,罐内闪发,气体,中文
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Recover flash gas in storage tanks on offshore platforms by useing Rolling compression technology AbstractA major independent oil and gas producer (Producer) with operations located on the Outer Continental Shelf of the Gulf of Mexico had several facilities damaged by Hurricane Ike. As a part of restoring operations, one of the offshore platforms was refurbished.The refurbishment included upgrading the production train to handle additional oil and gas production from other nearby production platforms. The additional production to the platform required a vapor recovery system to recover facility flash gas.The project team chose the scroll compressor vapor recovery unit (VRU) to recover and recompress the flash gas. The project was the first application of scroll compression technology for vapor recovery in an offshore environment.The Producer installed the VRU allowing the facility to recover flash gas from the oil storage tanks and excess unused flash gas from the oil treater. The average volume recovered was approximately 58,000 standard cubic feet of natural gas per day during the initial phase of the project. The methane content of the recovered natural gas was approximately 69 percent by volume. The estimated methane recovered was 0.84 US tons per day and the estimated recovery of greenhouse gases were 17.6 US tons per day CO2e. Volatile organic compounds (VOC) recovered were 1.0 US tons per day. The scroll compressor VRU met the regulatory requirements of the U.S. Minerals Management Services flaring and venting regulations. The projected payout was 15 months(simple payout).The significance of this project includes:1. First use of scroll compression technology in an offshore application2. Small physical footprint of unit important to offshore operations with limited space3. Scroll technology requires less maintenance than typical mechanical compressors4. Lower initial costs and lower operating costs enhance economics of recovery5. Recovered flash gas that contained volatile organic compounds (VOCs) and methane, a greenhouse gasIntroductionMany oil and gas production platforms and pipelines operating in the Outer Continental Shelf of the Gulf of Mexico were damaged by Hurricane Ike in November of 2008. A major independent oil and gas producer (Producer) with operations located on the Gulf of Mexico had several facilities damaged by the storm. As a part of restoring operations, one the offshore platforms was refurbished. The refurbishment of the platform included upgrading and improving the production train to handle additional production from other nearby production platforms that could not send their production to the gathering pipelines due to the effects of Hurricane Ike. The additional production to the platform required the installation of a VRU to recover flash gas from the oil storage tanks. The Producers project team decided to utilize scroll compressors to recover and recompress the flash gas from the storage tanks and oil treater.The source of natural gas vapors from oil storage tanks include flashing losses, working losses and breathing losses. Flashing for a pressure vessel (e.g., separator, heater treater) or oil storage tank occurs when the crude oil or condensate with dissolved gases moves from a higher pressure to a lower pressure. As the pressure of the oil drops some of the lighter components dissolved in the oil are released or “flashed.” Working losses are due to displacement of the natural gas vapors within the storage tank vapor space as a tank is filled. Breathing losses are due to displacement of natural gas vapor within the storage tank vapor space due to changes in the tank temperature and pressure throughout the day. For this paper we refer to the vent gas from the oil storage tanks collectively as flash gas.Often flash gases from offshore production platforms are either vented directly to the atmosphere or burned by a flare. Historically VRUs have been used to recover flash gas when there is sufficient quantity to justify the investment and to meet air emission standards. The typical type of vapor recovery compressors used for vent flash gas has been natural gas driven rotary screw compressors and rotary vane compressors.The United States Minerals Management Service (MMS) is the regulatory agency with jurisdiction over venting of natural gas in the central and western areas of the Gulf of Mexico. MMS regulations require a facility to recover natural gas volumes over 50,000 standard cubic feet per day rather than venting directly to the atmosphere or burning in a flare. For offshore production platforms, deck space requirements are a significant consideration for vapor recovery units. To accommodate this limitation, the scroll compressor package has a footprint one-third the size of a traditional VRUs used. In addition, lower overall maintenance costs were a significant factor in the decision to utilize scroll compressor technology. The scroll compressor requires oil changes once per year compared to quarterly for the typical mechanical compressor. Equipment used in the offshore environment required capital upgrades to the typical onshore compression package due to the saltwater corrosive environment and additional safety controls required for operating offshore. For this project the standard onshore VRU was upgraded to meet specifications for the offshore conditions and regulations.Description and Application of Equipment and ProcessesScroll Compression Technology.Scroll compression technology is a positive displacement machine that uses two interleaved spiral-shaped scrolls to compress natural gas. With scroll compression technology, one of the scrolls is fixed, while the other orbits eccentrically, thereby trapping and pumping or compressing gas between through successively smaller scroll volume “pockets” until the gas reaches maximum pressure at the center. At the center, the gas is released through a discharge point in the fixed scroll. Compression is continuous since during orbit of the orbiting scroll, multiple gas pockets are compressed simultaneously.The driver for the compressor is an electric motor. The scroll compressor is a hermetic compressor designed for use with high-pressure refrigerants. It has a broad range of operation and is intrinsically leak free. Scroll compressor technology has been widely used in cooling system applications.The scroll compressor VRU installed had a horizontal design that has a low profile, low noise, low vibration, and uses variable speed control motors. Depending on the application, the range of inlet pressures of gas to the scroll compressor VRUs may vary from -10.4 to 101.3 pounds per square inch gage and the discharge pressures can range from 43.5 to 363 pounds per square inch gage. The compression ratio ranges from 3 to 15.Scroll compression technology has been used in oil and gas vapor recovery applications since 2004.Application of Scroll Technology.In May of 2009, COMM and the Producer began working together to modify a typical onshore scroll compressor VRU for the platform that was damaged and being refurbished.The scroll compressor VRU consisted of two stacked modules each 8-foot long by 4-foot wide by 4-foot high steel skids each with an inlet gas scrubber. Each module contained two 15-horsepower scroll compressors and an aftercooler. Each module also included a control panel with Programmable Logic Control (PLC) and variable frequency drive (VFD). The design recovery capacity of this twin module package used was 200,000 standard cubic feet per day.A suction line connected to the oil storage tanks common vent and to the oil treater (i.e., heater treater) vent was installed to the inlet scrubber of the scroll compressor VRU. The suction line to the oil treater was used to collect excess gas from the oil treater that was not used as platform fuel gas. A flow meter was placed on the suction line prior to the inlet of the scroll compressor VRU to measure the amount of natural gas recovered. The discharge of the scroll compressor package was piped to the suction separator/scrubber of the onsite main compressor. This main compressor compresses natural gas for ultimate injection into the sales pipeline.A purge gas system was installed and used to recycle gas through the scroll compressor VRU when there is insufficient pressure from flash gas in the storage tanks. The purpose of the purge gas system is to keep VRU operating to maintain the scroll compressors oil temperature at a minimum of 235 degrees Fahrenheit. By maintaining the oil temperature at or above 235 degrees F, the flash gas will remain in a gas phase.As a safety measure, a blanket gas system was installed on the storage tanks to maintain approximately 0.5 ounce per square inch of pressure on the tanks to keep oxygen from entering the tanks.Figure 1 contains a simplified process flow for the VRU.The control panels with VFDs were located in the motor control center (MCC) and wiring was run to the scroll compressor VRU which was located on a lower deck of the platform.Functionally, the scroll compressor operates normally in the recycle mode at 2400 revolutions per minute (rpm). When the pressure builds in the oil storage tanks, a pressure transmitter sends a signal enabling the speed of the compressor to increase to 4800 rpms and the flash gas is recovered and compressed. Once the flash gas from the storage tanks is recovered and the pressure drops in the storage tanks, the VFD ramps the compressor speed down to 2400 rpms. Then the VRU is in recycle mode again.Any liquids recovered by the gas scrubber are pumped back to the oil storage tanks.Modifications to VRU Package.To meet offshore specification, the structural components of the scroll compressor package were already hot dipped galvanized and suitable for offshore installation but other components required refinishing to withstand the corrosive saltwater environment. The compressors and several other components were removed from the modules and specially coated with a three part epoxy coating to withstand the corrosive environment.In addition to the special coatings needed for offshore, there was a number of safety system modifications needed to make the scroll compressor VRU compliant with the United States Minerals Management Service (MMS) regulations. Offshore operators are required to abide by the American Petroleum Institute (API) Recommended Practices 14C (RP 14C). API RP 14C contains the criteria for designing, installing and testing a safety system on an offshore platform. It identifies each undesirable event that could affect a process component and discusses safety device selection criteria for each component type.Failure to meet RP 14C requirements can result in fines to the operators and in some cases, require an interruption of production which could result in losses of income to the operator until compliance is restored.Specifically, the modifications in response to RP-14C were:1. Installation of test circuit for monthly testing of high level alarm/shutdown on the gas scrubber2. Installation of test circuit for monthly testing of high discharge pressure alarm/shutdown on compressor discharge line3. Installation of test circuit for monthly testing of low pressure alarm/shutdown on oil storage tanks4. Addition on redundant oil storage tank pressure transmitter. Installation of test circuit for monthly testing of high pressure alarm/shutdown on oil storage tanks.Additionally, the Producers offshore specifications required the replacement ofseveral valves to steel construction rather than brass.The scroll compressor VRU was shipped to the platform in July 2009. The interconnecting piping to and from the scroll compressor VRU was completed in August 2009. Once the installation was completed and the platform was placed into operation, the scroll compressor VRU was brought into operation.Presentation of Data and ResultsFor this installation, the scroll compressor VRU had an average recovery of tank flash gas over the initial operating period of 58,000 standard cubic feet per day. The peak flowrate documented was 215,000 standard cubic feet of flash gas per day. A sample of the recovered flash gas that was chemically analyzed had a molecular weight of 26.6 and contained approximately 69 percent by volume of methane. Volatile organic compounds (nonmethane, nonethane hydrocarbons) amounted to approximately 29 percent by volume. The higher heating value was approximately 1540 British Thermal Units (BTU) per standard cubic feet.The hydrogen sulfide content of the flash gas was considered de minimus based on the facility processing sweet natural gas.The calculated simple payout of this scroll compressor VRU based on the average recovery and gas price of USD 5/MMBTU is 15 months.The estimated methane emissions recovered were 0.84 US tons per day and the estimated recovery of greenhouse gases were 17.6 US tons per day CO2e. Volatile or
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