New Study Examines Noise Reduction from Retrofitted Shipping Vessels

Researchers at UC San Diego’s Scripps Institution of Oceanography have been eavesdropping on the marine soundscape for decades to study the ecology of marine animals and the threats they face. This effort is made possible with instruments known as High-frequency Acoustic Recording Packages, or HARPs, that use an underwater microphone, or hydrophone, to record everything from whale songs and dolphin clicks to passing ships and military sonar.

For more than 10 years, one such HARP instrument has been continuously recording sounds in the Santa Barbara Channel, a heavily trafficked shipping route off Southern California that intersects the Channel Islands National Marine Sanctuary and is an important foraging region for the endangered Northeast Pacific Blue Whale and other marine life. Sitting on the seafloor at a depth of 580 meters (1,903 feet), the instrument opportunistically measured noise from container ships operated by Maersk—one of the largest container shipping companies in the world—before and after a major retrofitting initiative.

This long-term dataset has informed a new study led by researchers in the Scripps Whale Acoustics Laboratory in collaboration with Maersk, with funding by the National Resources Defense Council. The Scripps and Maersk teams worked together to identify vessel designs that would reduce radiated noise and source levels, with implications for quieting shipping on an international scale. Their findings were published March 16 in the journal PLOS One.

“Human-made underwater noise in the ocean can negatively impact marine organisms that rely on sound for daily life functions like communication, navigation, and foraging,” said Scripps PhD student Vanessa ZoBell, lead author of the study. “This paper was a first step in assessing whether retrofitting had an impact on noise levels, and it was an interdisciplinary effort involving marine scientists, industry, naval architects, and policy experts. We need all hands on deck to establish solutions for the modern ocean.”

The research collaboration emerged following Maersk’s completion of a $1 billion, five-year “Radical Retrofit” initiative focused on improving energy efficiency and fuel consumption to reduce emissions. During this effort, 12 of Maersk’s G-Class container ships were retrofitted from 2015 through 2018. Changes made to the sister ships included a redesign of the bulbous bow to reduce drag, reducing the engine’s output to improve vessel efficiency at slower speeds, reducing the number and size of propeller blades, and more.

While the primary goal of the retrofitting effort undertaken by Maersk was to increase fuel efficiency, both Scripps and Maersk were interested in exploring noise reduction as a potential co-benefit of the effort.

“Underwater noise is increasingly recognized as an important environmental factor, and is starting to be addressed at the International Maritime Organization level,” said Lee Kindberg, head of Environment and Sustainability for Maersk North America and a study co-author who was instrumental in the collaboration. “The team at Scripps has unique expertise in this area; it is clear that we need additional info to design most effectively to both reduce underwater noise and greenhouse gas emissions. The best solution must address both of these issues.”

The study identified retrofit-induced changes in the ships’ radiated noise levels and monopole source levels. Radiated noise measurements account for the distance between the recording device (such as the hydrophone) and the sound it is recording (the ship noise) by correcting for the distance in-between the ship and the recorder. Monopole source levels account for what you would hear if you were positioned one meter away from the source, which in commercial ships is predominantly the sound from propeller cavitation, or pressure-related water cavities due to propeller motion.

The researchers focused on 111 transits from the 12 sister ships, pre- and post-retrofitting, as they made their way to and from the ports of Los Angeles and Long Beach. Marine animals including humpback whales, blue whales, fin whales, and dolphins were also recorded at the study site.

Of the 177 total transits recorded between 2008 and 2018, 66 were excluded because of the presence of singing whales, as well as acoustic interference from other vessels and hydrophone cable strumming.

Looking at the data, the team identified a significant decrease of monopole source levels in the low-frequency band by over five decibels post-retrofitting. This noise reduction was likely due to the changes in the propeller and bow design, wrote the authors.

Post-retrofit, they also measured a slight increase in the radiated noise level from the ships, likely due to the increased number of containers stacked on top of the retrofitted vessels. The additional weight likely caused the vessels to sit deeper in the water, changing their draft. The authors note that from one perspective, the increased cargo capacity will lead to fewer trips and thereby reduce ocean noise, however, this will only be true if global demand stays constant.

“One of the goals of the vessel retrofit was to add more containers, allowing the ship to transport more cargo,” said John Hildebrand, principal investigator of the Scripps Whale Acoustics Lab and a co-author on the study. “This changed the draft of the ship which had an impact on the noise. It forced us to think about the noise relative to the number of containers transported, which was a new perspective.”

The interaction between retrofit and speed in this study was also noteworthy, highlighting that the effect of retrofitting on monopole source level was greatest at slower speeds.

The International Maritime Organization has recognized vessel design as a potential method to reduce underwater noise from commercial ships on an international level, and recommends investigating specific propeller and hull design modifications to identify the most effective designs for noise reduction. Because multiple design changes occurred at once in the retrofitted ships highlighted in this study, it is difficult for the authors to disentangle which changes were most effective in reducing sound levels, so further research in this area is needed.

“I am hoping that this paper can be used to start discussions within committees focused on noise reduction efforts in the International Maritime Organization, International Whaling Commission, and International Union for Conservation of Nature, whether that be to focus on reducing noise per transit, reducing transits, or reducing noise per container,” said ZoBell.

The authors also recommend that future studies further test the noise reductions found in this study with larger sample sizes, different ship types, and different design approaches to identify the most efficient methods for reducing underwater noise on an international level.

“Understanding the details of the vessel retrofit and ship operations, information that could only come from Maersk, was what made the study possible,” said Hildebrand. “We look forward to similar collaborations as commercial shipping is adapted to have a lower carbon footprint and hopefully less underwater noise.”

Additional co-authors of the study are Sean Wiggins and Kaitlin Frasier of Scripps Oceanography, and Martin Gassman, a former postdoctoral researcher at Scripps Oceanography who helped launch the research effort.