A new study shows a way to calculate when a hull needs to be defouled, so that drydock time can be scheduled optimally, reducing costs and emissions.
Researchers at Sweden’s KTH Royal Institute of Technology developed a model for estimating the speed and extent of biofilm growth, which could enable ship operators to schedule hull cleaning at optimal intervals to save on fuel consumption.
Cornelius Wittig, a PhD student in fluid mechanics at KTH, said the study offers a first-ever explanation of how shear stress interacts with biofilms over time to foster slime fouling on surfaces such as the hull of a ship.
The fouling process begins with micro-colonies of bacteria attaching to a surface. These bacteria form base structures from which string-shaped streamers begin to grow (see image below). Together, the base structures and streamers combine to form surface imperfections that lead to friction between the hull and water. Streamers grow and imperfections build up at different rates depending on the level of stress induced by fluid flow against the surface.
Slime fouling adds significantly to fluid friction at sea – requiring an increase in a vessel’s shaft power by as much as 18%, the study shows. A ship covered with a thin biofilm may experience up to an 80% increase in fuel consumption.
In many cases a ship doesn’t get scheduled for cleaning until the operator sees a spike in fuel consumption. By then it’s too late and a lot of fuel has been wasted.
“The potential fuel savings gained by cleaning need to be great enough to offset the costs of the dry-dock and cleaning procedures,” Wittig said. “Accurate predictions would allow for efficient scheduling of cleaning for ship operators.”
Pictured below the formation of biofilm over three days.
