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Computer Simulations Uncover How Barnacles Slow Down Ships

Published Aug 02, 2022
Barnacles are a problem for the shipping industry and maritime institutions. Here, a man pressure-washes a boat hull, where barnacles tend to gather.
Barnacles are a problem for the shipping industry and maritime institutions. Here, a man pressure-washes a boat hull, where barnacles tend to gather.

Every year, over a billion tons of stuff gets shipped around the world. Cars, clothes, electronics, food, and fuel glide across oceans on cargo ships, on their way to stores or front porches. This hefty load comes at a price: The global shipping industry, which relies heavily on fossil fuels, is responsible for an estimated 3% of global greenhouse gas emissions each year, more than is produced by the entire country of Germany.

Worse still, a tiny menace is forcing ships to burn even more fuel. Barnacles, mussels, and other marine organisms can gather in such numbers on the hulls of ships that they can mess with the vessels’ efficiency and performance, a phenomenon known as “biofouling.”

Now, new research, published July 16 in Physical Review Fluids, seeks to understand the specifics of barnacle biofouling using computer simulations, which could inform new tools to better beat back the pesky creatures.

“[Biofouling] is a wide and important problem, and one that has an emissions impact and an economic impact,” says Angela Busse of the University of Glasgow and a co-author of the paper. When a ship moves through the sea, a layer of fluid forms at the ship’s surface as water flows over it. This “turbulent boundary layer” creates drag, preventing the ship from cutting cleanly through the water.

With effort, ships can cope with this drag—but any roughness on the ship’s hull can slow it even more, and a lot of small barnacles can have a huge impact. A 2007 study led by Michael Schultz of the United States Naval Academy estimated that a ship with heavy biofouling might have to overcome up to 80% more resistance than a clean ship.

As a result, more fuel is required to propel a ship forward, Busse explains. A 2011 study, also by Schultz, estimated that a Navy destroyer with fouling might require 20.4% more fuel. And a preliminary report from November 2021 found that barnacle or tubeworm biofouling could increase emissions by up to 55%, depending on the ship.

“Not only is it bad for greenhouse gas emissions, but also the bottom line,” Busse says.

Since the 1960s, toxic chemicals used to strip barnacles from ship hulls have been phased out or banned, and today, shipping companies and maritime institutions like the US Navy implement extensive cleaning and painting regimens to remove or prevent barnacle growth on their ships. It’s an essential step to keep boats in ship-shape, but it’s costly.

For the new paper, Busse and Sotirios Sarakinos, a co-author and fellow University of Glasgow researcher, developed a simulation that models how different levels of barnacle fouling affect how water moves over a surface. Past studies relied on simulations that didn’t realistically replicate barnacle colonization, or required scientists to observe real barnacles—a time-consuming task.

In their new study, Busse and Sarakinos created an algorithm that mimics barnacles’ natural behavior, including their tendency to settle near one another—what’s known as “gregarious” behavior.

Researchers created a simulation that models how different numbers of barnacles affect how water flows over a surface. Here, small cones represent barnacles as they gather in increasing numbers.
Credit: Sotirios Sarakinos and Angela Busse/Phys. Rev. Fluids 7 (2022)

“The idea was to place ‘barnacles’ on a surface, and then try to create small groups that would eventually grow and meet other groups,” Sarakinos explains. Using this algorithm, they created seven different virtual surfaces dotted with an ascending quantity of barnacles, from 10% to 85% coverage. The researchers then used a computational approach called a direct numerical simulation (DNS) to create virtual turbulence flows and passed them over each surface type.

The duo found that, with low barnacle coverage, the “water” moving over the surface retained some behavior seen over smooth walls, a result not typically seen in previous studies. And when they characterized the topographical properties of the layer of fluid blanketing the surfaces in the same way they’d characterize the surfaces themselves, they found a simple, linear relationship between the roughness’s effect on the flow and the barnacles’ “effective slope”—a measure based on the height and density of the rough layer.

In other words, the greater this particular measure of barnacle roughness, the greater the effect of that roughness on the water’s flow—“an unexpected result,” Busse says.

“The thing that is exciting about this work to me is that it opens up the door” to develop real-life tools, says Schultz, who was not involved with the study. For example, the Navy performs periodic underwater inspections to document how much fouling has built up on ship hulls. “But they don’t have a great way at present of taking that knowledge and saying, ‘What is the hydrodynamic penalty of that?’” Schultz says.

If this were possible, inspectors for the Navy or commercial shipping operations could make informed decisions on whether cleaning the hull at a given time would be economically “worth it” to save fuel, he explains. Studies like Sotirios’s and Busse’s will help researchers develop the models needed to make those decisions, Schultz says. “We’re getting better at doing that, given data like these.”

Tess Joosse

Tess Joosse is a science writer based in the Midwest.

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