Oil and gas platforms are energy-intensive systems, which operate under changing boundary conditions over time. In this paper, the life performance of an offshore platform is analysed by comparing three representative stages of an oil field (early-life, plateau and end-life productions). The energy requirements are assessed by a process integration study, and the system inefficiencies are pinpointed by performing an exergy accounting. The heating and cooling requirements vary significantly over time, and most inefficiencies take place in processes where chemical exergy is consumed (similar or equal to 50-55%), thermal exergy is transferred (similar or equal to 15-20%), or mechanical exergy is varied (similar or equal to 0-15%). These findings are valid for all production periods: this suggests that more attention should be paid on a proper integration of the processing and utility plants, by, for instance, recovering heat from the turbine exhausts and from the exported gas. Multi-objective optimisations are conducted for evaluating the integration of steam and organic Rankine cycles, considering thermodynamic, economic and environmental performance indicators. They indicate that the profitability of a given improvement measure mainly depends on (i) the field properties, (ii) the platform operating strategy, and (iii) the production stage of the oil field. The implementation of steam networks appears promising, as it results in a better performance of the offshore platform and in larger economic profits. (C) 2014 Elsevier Ltd. All rights reserved.