According to Cornelius Wittig, a PhD student in fluid mechanics at KTH, the study provides insight into the way shear stress—the frictional force exerted by water flowing over a surface—interacts with biofilms over time to promote slime fouling on surfaces such as the hull of a vessel.
As explained, biofilm creates a costly drag on fuel efficiency for ocean-going cargo ships, opening the door to more emissions and, ultimately, higher consumer prices.
To put it into perspective, the preliminary results of a November 2021 study by the Global Industry Alliance (GIA) for Maine Biosafety, an initiative addressing biofouling management, suggested that removing slime from ships’ hulls can slash greenhouse gas emissions (GHG) emissions by up to 25%.
Within this context, the International Maritime Organisation (IMO) had also underscored that one of the greatest factors impacting the efficiency of all vessels in service is associated with the resistance generated by the underwater area.
The IMO has also acknowledged the importance of biofouling management in cutting down on emissions and achieving worldwide sustainability goals, considering this a ‘crucial’ optimization element for the Carbon Intensity Calculator (CII).
Concerning the fouling process itself, it is understood that it starts with microcolonies of bacteria attaching to a surface and forming base structures from which string-shaped streamers begin to grow. Together, the base structures and streamers combine to create surface imperfections that lead to friction between the hull and water.
As a result, streamers increase, and imperfections build up at varying rates depending on the level of stress induced by fluid flow against the surface.
Wittig elaborated that slime fouling adds ‘significantly’ to fluid friction at sea, necessitating an uptick in a ship’s shaft power by what is estimated to be as much as 18%. “A ship covered with a thin biofilm may experience up to an 80% increase in fuel consumption. In many cases, a ship does not get scheduled for cleaning until the operator sees a spike in fuel consumption. By then, it is too late and a lot of fuel has been wasted.”
In this sense, Wittig suggested that to reap the benefits of de-fouling a ship, timing plays a ‘critical’ role.
“The potential fuel savings gained by cleaning need to be great enough to offset the costs of the dry-dock and cleaning procedures. Accurate predictions would allow for efficient scheduling of cleaning for ship operators.”
He further added that current evidence hints that shear stress can be roughly estimated just by being aware of the speed of a vessel. Nonetheless, Wittig emphasized that more research is needed in more ‘realistic’ conditions before the KTH-developed model can be used in shipping.
“From a ship operator’s perspective, it is interesting to know if the ship should be cleaned in next month or next year, so an estimate is sufficient to increase the accuracy of their decision making.”
In June 2023, Norway-headquartered marine coatings manufacturer Jotun found that almost two-thirds (59%) of the shipping industry underestimated the negative environmental impacts of biofouling, with as much as 1 in 4 claiming to know little about the issue.
The study suggested that if vessels operated with a clean hull, free from biofouling, carbon dioxide (CO2) emissions could be cut by a fifth, while fuel spending could be reduced by 19%.
Recently, numerous maritime industry stakeholders have made efforts to discover and create new solutions to tackle the prevailing biofouling issue. At the end of October 2024, the UK-based classification society Lloyd’s Register (LR) awarded the industry’s ‘first’ enhanced antifouling type approval to Canada-headquartered coatings company Graphite Innovation & Technologies (GIT Coatings).
German container shipping line Hapag-Lloyd also revealed in December last year that it slashed harmful emissions by implementing Shipshave’s In Transit Cleaning of Hull (ITCH) solution which was documented and verified by Norway’s classification society DNV.
As disclosed, following the assessment, DNV confirmed that the two boxships that were part of the study, carrying an 8,749 TEU and 18,800 TEU capacity respectively, achieved ‘significant’ fuel savings and reduction in emissions based on ‘improved’ energy efficiency.
