James Miller, URI professor of ocean engineering, is leading a team of scientists and students conducting acoustic monitoring of the pile-driving phase of the wind farm’s construction. He said that driving long metal poles 60-meters into the seabed to provide a foundation for the wind turbines is the noisiest part of the project.
“When a pile is struck, it’s like ringing a bell or like a hammer blow,” he explained. “The pressure wave travels to the bottom of the pile and bounces back up and down again. That radiates loudly in the water, sediment and air. We’re most interested in how that sound travels through the water and sediment and how it will affect whales, fish and diving birds.”
The construction of the first offshore wind turbines in the United States is providing an opportunity to learn about their effects on the environment and how best to assess them. The Bureau of Ocean Energy Management, an agency of the U.S. Department of Interior, has funded the study to capitalize on this opportunity. Pile driving noise has been investigated extensively in the Gulf of Mexico, where offshore oil and gas platforms are abundant. But Miller said the marine life, water conditions and sediments are very different there, so its effects may be very different in the North Atlantic.
“We’re particularly interested in the sediments because the sediment affects the propagation of sound in the water,” Miller said. “Some of our sediments are stiff and sandy, which would allow for a longer range impact.”
So what about that “most interesting scientific question?”
Miller said the question deals with where the sound transitions from high intensity, short duration signals to low intensity, long duration signals. It’s an important distinction that will help determine how to protect the hearing of sensitive whales and other marine life.
“Think of it like a lightning strike,” said Miller. “If you’re close to a lightning strike, you don’t hear any thunder, just the crack of the strike. But if you’re a couple miles away, the sound spreads out and you hear a rumble of thunder. The same thing happens under water. If you’re close to the pile driver, you hear the crack, but if you’re far away you hear the rumble.
“We know that high intensity, short duration sounds – like a gunshot or lightning strike – can affect our hearing,” he continued. “But at some point the sound transitions from high-intensity, short duration to low intensity rumbles. At what distance from the impact does that happen and how is it related to the environment?”
Miller and his colleagues, working as a subcontractor for HDR Environmental, Operations and Construction Inc. of Denver, are placing hydrophones in the water at various distances from the construction site to collect data about the noise levels generated by the pile driver. During three days when the pile driving is taking place, the researchers – including URI Professor Gopu Potty and Kathy Vigness-Raposa and Geoff Sisson of Middletown-based Marine Acoustics Inc. – will also tow a 200-foot array of hydrophones from a URI boat on a 30-kilometer route away from the construction site.
In addition, Tim Mason with Subacoustech Environmental Ltd. in England is taking underwater acoustic measurements at a fixed location 750 meters from the pile driver and on transects away from the pile to 20 kilometers. And HDR project leader Randy Gallien is also using student interns from URI’s Department of Ocean Engineering to measure airborne noise and the visual impacts of the construction.
Analyzing the data will be complicated, because the water temperature, salinity, and sea surface roughness affect how sound travels. The amount of sun and clouds in the sky during the pile driving will also affect their results.
Photo caption: URI Ocean Engineering Professor Gopu Potty (left) helps deploy an array of hydrophones to monitor the noise from construction of the Deepwater Wind turbines. (Photo courtesy of James Miller).
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