Evaluation of a North Sea oil platform using exergy analysis
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The motivation is to explore the applicability of exergy analysis as an evaluation and monitoring toolfor an offshore platform. The focus should be turned from energy efficiency to exergy efficiency,because to use an efficiency that does not account for the quality of the energy may be misleadingfor the improvement potential. The exergy efficiency contains the improvement potential, whilecurrent commonly used performance parameters only focus on power consumption. An exergyanalysis is a good method to detect, locate and quantify the theoretical potential for savings and it ismore useful the more complex and advanced the system to be analyzed is. Exergy efficiency can beused together with industries? own standard measures, such as specific CO2-emissions. An exergyanalysis is seldom systematically used in the industry yet, but the more details we have on the use ofexergy, the more opportunities we have to foster environmental friendly technologies.This thesis contains an exergy analysis of the oil and gas processing plant and the power generationsystem and distribution systems at a particular North Sea offshore platform. In the oil and gasprocessing a mix of reservoir petroleum and water is separated into oil, gas and water. The oil isexported through a 212 km long pipeline to an onshore terminal, gas and water are reinjected intothe reservoir and some of the gas is used for gaslift. Gas can also be imported through a 50 km longpipeline from a nearby gas center to cover the need for gas injection. A fraction of the gas iscombusted in power turbines and in pilot flames in the flare system. The oil and gas processing canbe divided into six sub-processes; the production manifolds, the separation train, the recompressiontrain, the reinjection train, the export system and the fuel gas system. The power generation systemconsists of three gas turbines, one mechanical drive and two generator drive. They cover the powerdemand at the platform. A seawater distribution system and a hot water distribution system suppliesconsumers with cold and hot water, respectively.A process flowsheet of the oil and gas processing plant, power generation system and distributionsystems is simulated in the chemical process simulator HYSYS. The exergy loss in the whole oil andgas processing process, the power generation system and the distribution system, in each subprocessand in each process unit, is calculated. This was done for two dates, with two years inbetween. In addition, the possibility for installing a combined cycle is studied. The exergy analysis ofthe oil and gas processing platform is also compared to another exergy analysis of a North Sea oilplatform performed by Voldsund et al. (Voldsund M. , Ertesvåg, He, & Kjelstrup, 2013).The specific power consumption was 31 kWh/Sm3/ 40 kWh/Sm3 and the exergetic efficiency was 36.6%/ 38.9 % for the whole oil and gas processing process at the platform. The highest losses wererelated to compression and cooling of gas in the recompression train and the reinjection train andthrottling in the production manifolds. Also heating and cooling in the separation train and exportsystem contributed a lot, respectively. The exhaust gas from the mechanical drive turbine does nothave enough heat to cover the electricity demand via a steam cycle. However, if one utilizes the heatfrom one of the generator drive gas compressors at part load in addition, the electricity demand iscovered. The total exergy destruction is reduced with 5.8 MW/ 10.0 MW.We see the advantage of analyzing exergy destruction and exergetic efficiency of the process. Theseparameters show other features of the processes, in addition to the industry?s own measures ofperformance.