Keywords:Offshore Bottoming cycle CO 2 power cycle Off-design Modelling and simulation a b s t r a c t Improved energy efficiency is an issue of increasing importance in offshore oil and gas installations. The power on offshore installations is generated by gas turbines operating in a simple cycle. There is an obvious possibility for heat recovery for further power generation from the exhaust heat. However, the limited space and weight available makes the inclusion of bottoming cycles challenging. Due to its high working pressure and thereby compact components CO 2 (carbon dioxide) could be a viable solution, combining compactness and efficiency. An in-house simulation tool is used to evaluate the performance of CO 2 bottoming cycles at design and off-design conditions. Both a simple recuperated single stage cycle and a more advanced dual stage system are modelled. Results from simulations show a potential for 10 e11%-points increase in net plant efficiency at 100% gas turbine load. Also off-design simulations taking the variation in heat exchanger performance into account are performed showing that the bottoming cycle improves the off-design performance compared to the standard gas turbine solution. Even at 60% GT (gas turbine) load, the combined cycle with CO 2 bottoming cycle can achieve up to 45% net plant efficiency, compared to 31% for only the gas turbine.

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Summary This paper will discuss novel technologies for increasing the energy efficiency of offshore oil and gas platforms. Three case studies are in progress that are based on actual oil-producing platforms--two on the Norwegian Continental Shelf (NCS) and one in the Brazilian basin. The current focus is on developing compact, novel bottoming cycles for recovery of waste heat from the gas turbine and heat recovery from the compressor train for gas export. The technologies under investigation use steam and alternative working fluids, such as carbon dioxide (CO2) and hydrocarbons. All the fluids investigated in this project are natural working fluids; hence, they will not cause any unexpected environmental issues in the future. A case study was performed that considered an 18-year period of operation on an actual platform and a scenario in which one gas turbine was removed and replaced with a CO2 bottoming cycle by use of the exhaust heat from a different gas turbine. The beauty of this scenario is that it would not increase the weight on the platform because the crate containing the gas turbine to be removed was of a weight similar to that of the crate containing the CO2 bottoming cycle. The substitution would not affect the ability to cover the heat demand on the platform because a waste-heat-recovery unit (WHRU) could be installed on the platform's other gas turbine. The case study indicates a significant reduction in CO2 emissions of 22% (63 000 t/a), and does not involve adding additional weight or volume to the platform. If operating on the NCS, the annual savings in reduced fuel costs and CO2 tax from implementing this scenario would be USD 17 million, although much lower in other territories.

Heat exchangers are important units in most industrial processes. They involve physical phenomena such as condensation and evaporation including several boiling regimes. Different types of heat exchangers constructed for different applications may differ much in geometrical design. This work explains and demonstrates a modelling framework which is capable of handling a multitude of geometries and relevant physical phenomena affecting the performance of the heat exchangers. The data structure and governing equations are explained, before the framework is demonstrated for a particular challenging test case with a heat exchanger operating similar to the main heat exchanger in a single mixed refrigerant cycle. In the test case, both evaporation and condensation may happen simultaneously along the length of the heat exchanger. 1000 cases with random changes within predefined intervals in inlet temperatures, mass flows and pressures were used to test the robustness of the model framework. The solution scheme converged in 98.7% of the cases, and in the non-converging cases, the operating conditions exceeded the physical limits of the heat exchanger. The framework demonstrated may thus be used to create flexible and robust heat exchanger models for use in process simulations, optimization, or as a stand-alone model.