Carbon intensity compliance of a crude oil tanker: A five-year voyage-level analysis

The Carbon Intensity Indicator (CII) of ships is evaluated annually; however, this aggregated calculation may conceal substantial variation in carbon intensity across different voyage types. To investigate how voyage parameters affect CII outcomes, this study presents a five-year (2020–2024) voyage-level empirical analysis of a Suezmax crude oil tanker. The findings indicate that laden voyages with high waiting time (>40% of voyage days) exhibit the poorest CII performance, averaging 4.84 gCO₂/dwt·nm, well above the 2024 required threshold of 3.30 gCO₂/dwt · nm. Speed reduction in both laden and ballast conditions produces meaningful but bounded fuel savings well below cubic-law expectations, and the vessel's rating trajectory was shaped less by operational change than by the tightening of the regulatory threshold itself. The findings underline the importance of voyage-level analysis as a complement to annual reporting and indicate that ship operators should consider voyage characteristics and idle time alongside speed optimization, particularly as reduction factors continue to escalate.