Seabed in and around turbines undergoing change

Fri, 08/14/2020 - 11:15am
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How are the five turbines of the Block Island Wind Farm impacting their immediate environment?

Two scientists, Jennifer Amaral and Monique LaFrance Bartley, recently presented their research on these questions and found that the seabeds are undergoing some interesting changes in the marine life gravitating toward that location.

The online webinar, hosted in late July, was titled “Learning from the Block Island Wind Farm: Sediment and Sound.”

Jennifer McCann, the host and moderator for the event, said in her introduction, “In the U.S., we are positioning ourselves to become a world producer for offshore wind. It’s an opportunity to create clean energy and economic growth. This webinar is an opportunity to communicate some of the science and ask some of the questions [and] to better understand how we can minimize the effect on wildlife, and find new opportunities and innovations. Today we are going back to our Rhode Island example, which is the Block Island Wind Farm, to get a little update on what we have learned and what are some advice,” said McCann.

The BIWF, operated by Ørsted and developed by Deepwater Wind, is America’s first offshore wind farm: five turbines produce 30 megawatts of power. The wind farm is located three miles off Block Island, and began operation in December 2016.

The impact of sound

Jennifer Amaral, who earned her B.S. and M.S. degrees in ocean engineering from the University of Rhode Island, is currently working towards her Ph.D. in the same discipline. Her doctoral research focuses on the study of underwater sounds and acoustics during the installation of offshore wind turbines. She is also a lead scientist and engineer with Marine Acoustics, Inc. in Middletown, R.I.

In her presentation, “Underwater Sounds of Offshore Wind Farms,” Amaral looked at three things: activities that produce sound at an offshore wind farm, the types of sounds that are generated, and sound reduction methods.

Amaral also classified two sets of sounds: impulsive and non-impulsive, with impulsive sounds serving a short-duration and are explosive, such as impact pile driving, and non-impulsive sounds are continuous, such as the rotating blades.

During the construction of the BIWF, pile driving was used to secure the foundation to the seabed. “A hammer hits the top of a steel pile and sends sound into the air, water, and sediment.” This construction produces “high level impulsive noise that is detectable many kilometers away.”

Now that the wind farm is operational, Amaral said, “vibrations are generated when the turbine blades rotate. These vibrations travel down the tower into the foundation and radiate into the water and seabed,” which produces non-impulsive sounds that are low level.

Amaral said the sound of impact pile driving travelled 7.5 kilometers. The sounds made by blade rotations could be heard in a radius of 50 meters. For each hammer strike that hit the steel piles during the pile driving construction, the frequency (measured in hertz) reached up to 3,000 hertz.

Recordings collected during the wind turbine operation reached 70 hertz. She added fin whale calls were recorded during the same time of the turbine operation recordings reached around 20 hertz.

She concluded that the “sounds from the operation of the turbines were only detectable in calm conditions with minimal boat traffic”.

As more offshore wind farms are being constructed, Amaral said her recommendations would be to “consider foundations or installation methods that generate less sound, or use sound mitigation systems.”

Benthic and epifaunal monitoring

Monique LaFrance Bartley, who earned her M.S. and Ph.D. degrees in oceanography at the Graduate School of Oceanography at U.R.I., is currently a marine ecologist with the National Park Service in the Ocean and Coastal Resources Branch. Bartley’s primary research concentration is on what is called benthic habitat mapping, which serves to help protect fragile undersea landscapes.

She began her presentation by introducing the key terms in her research: benthos and epifaunas. Benthos is the community of organisms “associated with the seafloor” and epifaunas are “organisms attached to a surface” — such as the surface of the windfarm turbine structures.

“Why do we care” about this kind of study? Bartley asked. “The benthos plays an important role in the ecosystem.” The benthos habitat contains a wide range of species, including fish and shellfish, as well as sediment stabilization, nutrient re-cycling, and water quality regulation.

Her study focused on turbines 1, 3 and 5. The studies were conducted from 2016 to 2019, through vessel-based benthic surveys, diver-based benthic surveys, and epifauna surveys, collecting samples, videos and photographs within the designated study areas.

The results Bartley collected found a diverse ecosystem: rock gunnels, cunners, Atlantic striped bass, bluefish, scup, spiny dogfish, monkfish, black sea bass, dog fish, squid, skate, and sea robin. The results for benthic macrofauna organisms showed diverse species of worms, blue mussels and barnacles; epifauna organisms on the turbine structures included “blue mussels, algae, anemones, crabs, and sea stars”.

Bartley said the benthic “habitats within the turbine footprints have, or are undergoing, profound change.” Specific worm species were migrating to coarser sand sedimentation, and mussels were being found in finer sedimentation. There were also “substantial increases in the abundance of larger mobile predators and scavengers” around the turbines, Bartley said.

For epifauna communities, “the foundations support dense assemblages of mussels” and other species including sponges, anemones, and coral.

To view the online events and recorded webinars: seagrant.gso.uri.edu/special-programs/ baird/