Bubbles: Can Dampen Offshore Energy Sound for Whales

Sound is energy in the form of a wave. In the dark depths of the sea, whales and other marine mammals use gentle sound pulses to communicate about feeding, mating, and to keep their groups together. But as humans increasingly plumb the ocean for their own forms of energy, the loud sounds of exploration, development, and construction send powerful waves that can confuse and harm even the mightiest denizens of the deep. Offshore oil and wind power companies are studying an unusual but promising means of lessening the impact of sound on marine mammals: bubble curtains. Adapting a technique that proved successful in underwater bridge building, energy firms are testing the benefits of surrounding their operations with walls of bubbles that actually alter the shape of the noise waves. (Related: "Offshore Energy Clash Over Undersea Sound") It is too early to say whether the method will be effective. But research into this technology and other means of dampening human sound will be crucial for marine mammals living in the Arctic and coastal habitats now being eyed for their vast fossil and renewable energy potential. A Bridge to Insight The retrofit of the San Francisco-Oakland Bay Bridge provided evidence that bubbles can be used to lessen the impact of undersea sound. In 2003 and 2004, the California Department of Transportation (Caltrans) needed to drive new pilings—8 feet (2.44 meters) in diameter and 300 feet (91.44 meters) long—into the seabed for the project. Caltrans was concerned that the short, powerful pressure waves created by the enormous hammers striking the pilings would affect the behavior of nearby seals. Bud Abbott of Oakland, California, a marine biologist consulting on the project, warned the waves might kill fish as well. Sure enough, during an early test, a number of dead fish appeared on the surface. Abbott collected the fish and conducted necropsies. The swim bladders of the fish had burst and their kidneys had suffered terrific damage. Abbott's verdict: The pressure wave compressed the air in the swim bladder, which then quickly expanded again, bursting the bladder and damaging the kidneys. The discovery, alarming as it was, pointed the way to potential methods for lessening the impact of sound waves. "When a pressure wave hits an air bubble, it will compress the bubble, then it will expand again, so energy is lost," Abbott explained. Although scientists disagree on the amount of energy lost in this process, Abbott said, there is no doubt that the air bubble actually changes the shape of the wave. "Sound travels faster through water than air," said Abbott. "It slows down as it hits the air bubble." This creates a much smoother wave, altering it from a brief percussive bang to a longer, weaker wave. For the Bay Bridge project, Caltrans placed a wall of air bubbles directly against each pile as it was being driven. This attenuated the waves caused by the pile driving. Overall fish deaths were reduced and delayed mortalities did not occur beyond 69 meters (226 feet) from the piling. Germany's Offshore Wind Discovery In Germany, where offshore wind farms are an important component of the nation's ambitious plans for expansion of renewable energy, the impact on the rich marine life in the North and Baltic seas has been a growing concern. Building a wind farm into the sea floor is a massive undertaking; turbines in Germany's first offshore wind project each stood about 150 meters (490 feet) high and weighed 1,000 tons. A government study found that porpoises avoided swimming within 20 kilometers (13 miles) of a wind farm, Alpha Ventus, while the complex of 12 wind turbines was being built in 2009 and 2010 some 45 kilometers (28 miles) north of the German island of Borkum. In the wake of this research, the German government has set a mandatory 160-decibel limit on the sound levels allowed in wind farm construction, measured at a distance of 750 meters (half a mile) from the pile or source of noise. The German Federal Agency for Nature Conservation (BfN) listed bubble curtains as an option to meet the new standard and mask the sound of underwater wind turbine pile driving. A new wind farm now under construction in the North Sea, Borkum West II, being developed by a consortium of German power companies, will be the first offshore application of bubble curtains. Scientists familiar with the effort say the developers expect to be able to meet the German standard of 160 decibels. The Whales of the Arctic The oil company Shell* also is researching the use of bubble curtains to protect marine life from the noise of its undersea operations. This issue is of special concern in its proposed Arctic exploration in U.S. waters, because the marine mammals of the Beaufort and Chukchi seas off the coast of Alaska, where Shell currently holds leases, are among the most diverse in the world, according to the U.S. Geological Survey. The 15 species common to the area include ice seals, walruses, baleen and toothed whales, and polar bears. For the endangered bowhead whale, almost an exclusively Arctic species, noise, oil pollution, and warming are all important concerns, USGS says. (Related: "Chilean Wind Farm Faces Turbulence Over Whales" and "Report: Whales Could Be Harmed by Oil Search Noises") The ocean is actually filled with sound from waves, rain, thunder, and marine life (bowhead whale songs and tones have been measured at ranges from 128 to 189 decibels). And ocean ambient sound levels have been increasing, due to human-generated sounds, including an increase in commercial shipping. For comparison, normal conversation is about 60 decibels, 130 decibels is painful to the human ear, and the sound of a jet engine is about 140 decibels. But sound in water is not the same as sound in air. Hal Dreyer, president of Gunderboom, an Anchorage, Alaska-based engineering company that designs and builds aquatic filters and bubble curtains, notes that a sound that is 100 decibels on the surface, just "on the threshold of being uncomfortable" to human ears, is roughly equal to 162 decibels underwater. The U.S. Marine Mammal Protection Act requires that permits be issued for any underwater human activity, like oil exploration and drilling, that could affect protected species. A continuous sound (like that of a drilling ship) measured underwater at greater than 120 decibels or an "impulsive" sound (like that of a seismic survey air gun) greater than 160 decibels could disturb the behavior of marine mammals and requires a permit, say scientists at the U.S. National Oceanic and Atmospheric Administration (NOAA). In Shell's current Arctic exploration plan, NOAA's National Marine Fisheries Service estimated that an area of 481 square miles (1,245 square kilometers) would be exposed to 120-decibel sound or greater for one of two drill ships Shell is considering using, and 30 square miles (78 square kilometers) for the other. NMFS estimated an area of 74 square miles (191 square kilometers) would be exposed to seismic survey sounds greater than 160 decibels. Shell's work on bubble curtains is still at an early stage, so the technology is not included in the current mitigation plans for the Chukchi and Beaufort seas that the company submitted to the U.S. government for proposed exploratory drilling in the 2012 season. But Mitch Winkler, manager of the Arctic Technology Program at Shell International Exploration and Production, said the company recognizes that "marine sound is important."