Abstract
Maritime decarbonization is increasingly shaped by overlapping regulations affecting fuel choice, operating cost, and carbon performance. This study evaluates the combined effects of the European Union Emissions Trading System (EU ETS), FuelEU Maritime, and the Carbon Intensity Indicator (CII) using 2025 operational data from two sister container ships. Baseline outcomes were compared, and biofuel blend scenarios from B10 to B100 were developed for Vessel A together with allowance price sensitivity, pooling, and CII projection through 2030. The results show that technically similar ships can produce different compliance outcomes because of differences in fuel mix, voyage coverage, and operational practice. Increasing the biofuel share improved FuelEU performance and reduced EU ETS exposure, while CII projection indicated persistent rating risk under unchanged operations. Overall, the findings support integrated assessment of fuel strategy, compliance costs, and carbon-intensity risk.