The Relationship Between Slime On Ship Hulls And The Increase In Greenhouse Gas Emissions

On November 4, 2021, the Global Industry Alliance (GIA) for Marine Biosafety, operating under the framework of the GEF-UNDP-IMO GloFouling Partnerships project ( released its preliminary results of the study on the Impact of Ship’s Biofouling on Greenhouse Gas Emissions (the “Study”) during the United Nations Climate Change Conference (COP 26). 1   The final Study is expected to be released in 2022.

The International Maritime Organization (“IMO”) summarized the substance of the preliminary results of the Study in the headline of its Latest News with the admonition: “Remove slime from ships to cut emissions.”   The Latest News announcement from the IMO stated:

Keeping ship’s hulls free from just a think layer of slime can reduce a ship’s GHG emissions by up to 25 percent, according to the preliminary finds of a new study, launched at COP 26 (4 November). 
The preliminary findings of the study . . . reveals that a layer of slime as thin as 0.55mm covering up to 50% of a hull surface can trigger an increase of GHG emissions in the range of 20 to 25%”2


According to the IMO, biofouling management is critical to reduce GHG emissions.  In an effort to reduce GHG emissions, the IMO has adopted certain legally binding requirements for (1) ship design, and (2) operational performance to reduce GHG emissions.  3

Slime—A Biofilm Coating—Leads to Biofouling

Slime—a green slick film—is a colony of microbes that over time become the breeding ground for algae, tubules, and barnacles. 4 

Slime is a complex, varied and dynamic organism predominantly made up of bacteria and diatoms.  There are thousands of individual species in the oceans that can result in varying slime film thicknesses, textures, compositions, surface roughness on ships’ hulls.   Slime begins to cultivate as soon as the vessel enters the water as bacteria populate the substrate and excrete Extracellular Polymeric Substances (EPS). 

5   In fact, slime has been a prevalent problem for seafarers for thousands of years. Biofouling of a ship’s hull is the accumulation of plants, algae, animals, and micro-organisms that increase the hydrodynamic drag. 6

The Increase of GHG Emissions Corresponds to the Severity of Hull Biofouling

The report of the Study presented at the COP 26 disclosed the relationship of various spectrums of biofouling on a ship’s hull with the resulting GHG emissions.  The Study first considered the impact of “Deteriorated coating or light slime” on a bulk carrier and the ratio of the increased GHG emissions.  The Study next analyzed the drag and resistance created by the presence of “Heavy slime.”   The report of the Study revealed that a 230 meter containership with 2.5 millimeters of barnacles covering 10% of the hull surface would increase GHG emissions by 34%.  Finally, the Study concluded that GHG emissions would increase 55% for a 320 meter tanker with barnacles 5 millimeters thick and 1% coverage of the hull. The Study concluded that the impact of biofouling had been “historically underestimated by the shipping community.”7

Biofouling Control Strategies

The Study further considered industry practices to manage and control biofouling of a ship’s hull.   Some of the measures referenced included hull cleaning, hull coatings, ultrasonic antifouling systems and propeller policing.   The preliminary findings of the Study concluded that prevention of biofouling should become a priority in the shipping industry.


2 Id.

3 Id.





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