The assessment of the exergy spent for production of oil and gas on offshore platforms is of primordial importance for evaluation of the environmental impact associated with petroleum derived substances. In this work the exergy cost for oil and gas produced on a Floating Production Storage and Offload (FPSO) ship is evaluated along the lifespan of the well taking off-design operation conditions of process plant and cogeneration plant into account. The impact of 3 different cogeneration plants and 2 different process plant operating modes was assessed. Distribution of exergy costs for the oil and gas was obtained using thermoeconomy to reduce the arbitrariness of cost partition criteria. Results reveal that the exergy cost of oil varies from 1.0 kJ/kJ to 3.2 kJ/kJ along the well lifespan depending on process plant operating mode and cogeneration plant configuration. The exergy cost of gas varies from 1.0 kJ/kJ to 2.4 kJ/kJ along the well lifespan depending on cogeneration plant configuration. The average emission of CO2 for the natural gas ranges from 19.0 gCO2/MJ to 19.8 gCO2/MJ depending on the cogeneration plant configuration while for the oil it ranges from 19.4 gCO2/MJ to 26.8 gCO2/MJ and it also depends on process plant operating mode. (C) 2018 Elsevier Ltd. All rights reserved.