Decomposing ship carbon intensity under fuel-intensity regulation: Separating fuel decarbonization from operational energy efficiency

International shipping requires performance indicators that distinguish reductions in operational energy demand from reductions in the life-cycle greenhouse gas (GHG) intensity of marine fuels. The Carbon Intensity Indicator (CII) is emission-based but is often interpreted as a proxy for operational energy efficiency, an interpretation that becomes less reliable as alternative fuels, onshore power supply, and well-to-wake accounting gain importance. This study develops an integrated Data Collection System (DCS)–Greenhouse Gas Fuel Intensity (GFI) framework for comparing emission-based and energy-based ship performance indicators. An emission-based CII-type indicator is decomposed into transport energy intensity (TEI), defined as energy use per unit of transport work, and the energy-weighted average GHG intensity of the fuel mix. Annual operational data from five structured ship-operation cases are analysed. In controlled Bio30 and methanol comparisons, transport work, total energy use, and operating profile are held constant, fixing TEI while varying only fuel GHG intensity. Bio30 and methanol reduce the emission-based indicator by 12.8% and 20.4%, respectively, whereas the energy-based indicator remains unchanged. Thus, an apparent improvement in an emission-based carbon-intensity metric may reflect fuel decarbonization rather than lower energy use per unit of transport work. The findings support separate but complementary assessment of fuel decarbonization and operational energy efficiency. A dual structure in which the GFI addresses life-cycle fuel GHG intensity and an energy-based operational indicator monitors TEI could provide clearer regulatory signals for maritime decarbonization and energy productivity.