Greenhouse gas reduction strategy considering fleet compositions

In the shipping sector, the adoption of onboard carbon capture and storage (OCCS) systems and the transition to e-fuels are expected to play key roles in achieving net-zero emissions. However, because these measures have limitations in both greenhouse gas (GHG) reduction potential and cost, it is essential to design a vessel fleet that integrates these measures to meet emission-reduction targets in a cost-effective manner. This study quantitatively estimates the GHG fuel intensity (GFI) and total annualized expenditure (TAE) of vessels operating with different various marine fuels (MGO, LNG, e-MeOH, and e-NH 3 ) as well as OCCS system, under scenarios that consider both fossil and renewable electricity sources and projected green H 2 cost. The results identify the optimal composition of a fleet that minimizes TAE while meeting the emission-reduction target. In scenarios where the green H 2 cost exceeds 2.5 USD/kg H2 , fossil-fueled vessels with OCCS system dominate the fleet compositions, whereas e-fuel vessels dominate in scenario with a green H 2 cost of 1.45 USD/kg H2 . Across all scenarios, achieving net-zero emissions consistently requires e-MeOH vessels with OCCS system, as only these vessels achieve negative emissions sufficient to offset the positive emissions from the rest of the vessels. • This study presents the most cost-optimal fleet composition meeting emission targets. • When green hydrogen cost exceeds 2.5 $/kg, fossil-fueled vessels with OCCS dominate. • When green hydrogen cost is 1.45 $/kg, e-fueled vessels dominate. • E-MeOH vessels with OCCS are essential as they achieve negative GHG emissions.