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Session: System Design and Optimization I
Session starts: Monday 07 October, 11:10
Presentation starts: 11:30
Room: Ruys & Rijckenvorsel Zaal

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Leonardo Pierobon (Technical Unversity of Denmark)
Ulrik Larsen (Technical Unversity of Denmark)
Fredrik Haglind (Technical Unversity of Denmark)

Abstract:
In off-shore platforms gas turbines are employed to provide the load required in processes such as crude oil separation, gas compression, seawater injection and oil and gas export. Off-shore gas turbines are designed to ensure high system performance, fuel flexibility and compactness. Furthermore, one or more redundant gas turbines are installed to improve the reliability during maintenance periods. In order to enhance the performance of the power generation system, an organic Rankine cycle can be utilized to recuperate the gas turbine exhaust heat. Benefits of this technology are high thermal efficiency in full and part-load, low weight, high compactness and little complexity. Furthermore, the organic Rankine cycle eliminates the problem of turbine blade erosion due to liquid droplet formation by utilizing a “dry” fluid as working fluid. On the Draugen off-shore oil and gas platform (North Sea, Norway) three SGT-500 gas turbines are installed. The normal load is normally shared the two engines while the third is on stand-by for maintenance. Hence, the analysis of off-shore combined gas turbine – organic Rankine cycle requires an accurate part-load model. The present paper aims at deriving the off-design performance of the combined SGT-500 gas turbine - organic Rankine cycle. First, we model the part-load performance of the gas turbine by means of the stage stacking approach. Subsequently, we evaluate the organic Rankine cycle at off-design by introducing specific equations for the pump, heat exchangers and expander. The results indicate that the mass flow and the exhaust temperature of the SGT-500 gas turbine are assessed with an error lower than 2.5% at 50% load and 9.0% at 10% load. The simulations indicate that the thermal efficiency of combined SGT-500 - organic Rankine cycle drops down from 43.3% to 20.7% when the load is decreased from 100% to 15%. The performance of the SGT-500 engine declines substantially, i.e. from 31.3% to 9.8%, while the thermal efficiency of the organic Rankine cycle lies within a smaller range (between 27.4% and 23.3%). As a practical consequence, the methodology can be applied to simulate the real time operation of the combined gas turbine - organic Rankine cycle on the Draugen platform and to evaluate the fuel savings and the reduction of CO2 and pollutants emission during the year.