Implementing IMO indices in warships: An in-depth case study in the Spanish Navy

Abstract

Contemporary navies are increasingly committed to environmental stewardship, yet no study has quantified how warships measure up to civilian green-ship standards. This article is pioneering in its use of International Maritime Organization (IMO) indicators to quantify the energy efficiency of a fleet of warships. The IMO’s Energy Efficiency Existing Ship Index (EEXI) and Carbon Intensity Index (CII) have been adapted to 25 major Spanish Navy ships over a two-year period (Dec. 2019–Dec. 2021). These indices were originally designed for merchant vessels. A combination of fuel consumption data with displacement and speed-dependent consumption curves was utilized to reconstruct each vessel’s EEXI and annual CII under both typical and full-load scenarios. It is evident that none of the warships in question satisfied the IMO’s requisite EEXI. Indeed, the most antiquated gas-turbine frigates exceeded the EEXI limit by a factor of four and the CII threshold by up to ten. In contrast, naval tankers and transport ships exhibited the closest proximity to the prescribed values, with deviations within the range of 20%–30%. The findings (25 ships; 24 months; 4–10 overrun) indicate a significant efficiency disparity and establish a robust framework — combining IMO indices with customized consumption models — for benchmarking military fleets globally.