This article focuses on various research efforts that are being undertaken to address underwater noise. One of the U.S. National Oceanographic and Atmospheric Administration (NOAA)’s findings is that underwater sound has been doubling every 10 years. Most of this sound is man-made, from the ever expanding fleet of ships that ride our oceans. Researchers believe that intrusive sound is harming sea life. Many organizations around the US shipbuilding industry have seen the need to address underwater noise. Standards organizations such as International Organization for Standardization (ISO), American National Standards Institute (ANSI), and the Acoustical Society of America have been working overtime to develop standards for the measurement of underwater noise from ships, oil and gas exploration, pile driving, and other sources. The ship classification societies are adding underwater noise to their library of regulations. In the United States, the Society of Naval Architects and Marine Engineers are planning to add their own regulations or guidelines in the near future.
Scientists from the U.S. National Oceanographic and Atmospheric Administration for more than a dozen years have been studying something that we humans normally don’t sense: increasing underwater noise and its effect on marine mammals and other sea life.
One of NOAA’s findings is that underwater sound has been doubling every 10 years. Most of this sound is man-made, from the ever-expanding fleet of ships that ride our oceans. Researchers believe that intrusive sound is harming sea life. Whales in particular, are greatly affected by increasing undersea sounds, as they depend on hearing to find food and each other.
In response, the International Maritime Organization, part of the United Nations, has been deliberating on draft guidelines for the reduction of underwater noise from commercial ships.
Technologies for quiet ships have been in use for military vessels, especially submarines, since World War II. The submarine, after all, has been called “the original stealth fighter” in a defense contractor’s advertisement.
Designers of commercial ships began paying attention to noise about 20 years ago, with the design of quiet marine research vessels. The quieter the vessel, the less likely it is to disturb the populations of creatures that it is trying to study. This led the marine science community to push engineers, naval architects, and noise-control engineers to build quiet vessels for their work.
Most of the world’s research vessels, or R/Vs, are configured in the same way. They typically range in length from 100 to 300 feet. Most have the same deck configuration with roughly one-third of the ship length dedicated to an open “working deck” in the aft part of the ship. Here stand multiple cranes, winches, and A-frames used to deposit and retrieve objects in the water. Inside the main deck of the ship are usually a collection of labs. Research vessels often distinguish these rooms as a “wet lab” for doing work on marine life or a “dry lab” for all other work. The rest of the ship may be like any other. The lower decks are dedicated to machinery, and the upper decks include mess, lounges, and staterooms for crew and scientists.
Building an R/V that is acoustically quiet requires concerted effort by the owner, operator, naval architect, shipyard, and most certainly the noise-control engineer.
The first concern is the propeller, which must be designed to avoid cavitation—the formation and implosion of cavities in a liquid. It occurs on both sides of the blades of a rapidly rotating propeller.
Low-noise propellers are cavitation-free, at least up to certain speeds. They have large- diameter blades with a high skew, and they spin relatively slowly.
After the propeller, the noisiest problem is the diesel engine. Direct-drive configurations that link diesel engines to the propeller are noisy. The solution is using a diesel generator to power high-quality electric motors that turn the propellers.
Building an R/V that is acoustically quiet requires concerted effort by the owner, operator, naval architect, shipyard, and most certainly the noise- control engineer.
It is easier to isolate the vibration of a generator than that of an engine driving a propeller. The use of high-quality electric motors (dc or ac) will also minimize hull vibration.
Lesser vibration problems include refrigeration plants, air compressors, seawater cooling systems, large pumps, large ventilation fans, and hydraulic power units. All can be vibration isolated.
High-transmission-loss bulkhead insulation can provide additional noise reduction. The insulation can consist of multiple layers of fiberglass or a single layer of heavier mineral wool.
Another noise-control treatment, used extensively on submarines, is hull damping. This uses a spray-on material or a plastic tile that absorbs vibratory energy in the hull.
The Quiet R/V Fleet
As far as acoustically quieted research vessels go, Europe led the way. The first vessel constructed to be acoustically quieted was the Fisheries Research Vessel (FRV) Corystes, put into service in 1988 for the United Kingdom. Over the last twenty years Europe has put at least seven more quiet R/Vs into service.
Only in 2004 did the U.S. launch its first quiet R/V with the delivery of the FRV Oscar Dyson. The Dyson and four later ships of the same class are owned and operated by NOAA.
All NOAA fisheries research vessels have a 40-day endurance (the ability to stay at sea for up to 40 days). Hence this class of ships is referred to as the FRV-40 Class. All these ships have an overall length of 210 feet (64 m) and have a diesel-electric plant. For the sake of quiet, the diesel generators are installed on a two-stage isolation mounting system.
The FRV-40 Class is propelled by a single 14-foot (4.3 m) diameter propeller turned by a 2,250 kilowatt dc motor. The U.S. Navy designed the propeller and the hull form. Nearly every piece of rotating machinery and all major electrical transformers are mounted on marinegrade vibration isolators.
The FRV-40 Class of quiet research vessels was just the start for the U.S. Since 2004, one other quiet research vessel has been built and delivered, and six more ships are at various stages of procurement. The second quiet R/V in the United States belongs to the University of Delaware’s College of Marine Sciences, which took delivery of the R/V Hugh R. Sharp in 2006.
Quiet is needed on fisheries research vessels so they can sneak up on fish for the purpose of counting them.
The next quiet R/V to hit the water will operate in the Arctic. The Alaska Region Research Vessel, the R/V Sikuliaq, will be the only ship in the U.S. academic fleet rated for year-round icebreaking operations in first-year ice. The ship will be operated by the University of Alaska at Fairbanks.
The U.S. Navy is getting two quiet research vessels built at the Anacortes Washington shipyard by Dakota Creek Industries. The first ship of the Ocean Class AGOR Class (T-AGOR-27) will be the R/V Neil Armstrong, named after the first man to walk on the moon. This ship will be operated by the Woods Hole Oceanographic Institution in Massachusetts; a second ship will be operated by the Scripps Institution of Oceanography in San Diego.
Noise at Issue
Quiet is needed on these FRVs so they can sneak up on fish for the purpose of stock assessment (i.e. counting), which is done using sonar.
But underwater noise generated by ships may have a more serious effect on marine life than simply making fish hard to count, and various organizations are addressing the issue.
Measuring Underwater Noise
Underwater noise is measured with an underwater microphone known as a hydrophone. The unit of measurement for underwater noise, or sound pressure level, is the same as for airborne SPL, the Pascal. As with airborne SPL, underwater noise is customarily reported in decibels. Airborne SPL uses 20 μPa as a reference pressure; underwater SPL uses 1 μPa.
The measurement of the source level noise from any ship requires a very careful process first standardized in ANSI/ASA S12.64-2009, Quantities and Procedures for the Description and Measurement of Underwater Sound from Ships. This standard has three grades of measurement depending on the user’s needs and accuracy. Either one or three hydrophones (depending on the grade) are lowered to specific water depths.
The ship being measured will pass by the hydrophone at controlled conditions and speed at least 100 meters from the hydrophone array. The measured sound at the hydrophone is then adjusted based on the actual distance between the ship and hydrophone. This adjustment assumes the ship is a spherical point source with all sound coming from one point.
The horizontal distance between the hydrophone and the ship is measured at the surface using GPS, radar, or laser range finder. The vertical distance from the water surface to the hydrophone is measured in situ when the hydrophone is deployed.
The Pythagorean Theorem is used to determine the total distance. This distance is then used to adjust upwardly the measured SPL as follows SPL (source) = SPL (measured) + 20 × Log (D) where D is the diagonal distance.
NOAA has hosted meetings to address the rise in underwater sound. In 2004, it sponsored “Shipping Noise and Marine Mammals: A Forum for Science, Management, and Technology.” In 2007, it hosted a second meeting focused on vessel-quieting technologies for large commercial vessels.
In 2008 a private non-profit organization, Okeanos - Stiftung für das Meer (Foundation for the Sea), held a conference on shipping noise and marine mammals in Hamburg, Germany. It was attended by a wide crosssection of marine mammal experts, underwater noise experts, maritime industry executives, and shipbuilders from all parts of the world. The final report from the conference included a “Statement From Participants,” which called for global action to reduce underwater sound by 3 dB in the next 10 years and 10 dB in the next 30 years. The conference recognized that the International Maritime Organization was the only organization with the international authority for such a large task.
Many organizations around the U.S. shipbuilding industry have seen the need to address underwater noise. Standards organizations such as ISO, ANSI, and the Acoustical Society of America have been working overtime to develop standards for the measurement of underwater noise from ships, oil and gas exploration, pile driving, and other sources.
Does the future include regutations—not just guidetines—for underwater noise limits? It’s hard to say.
The ship classification societies are adding underwater noise to their library of regulations. Europe’s Det Norske Veritas was the first with its Silent Class notation. It includes a series of underwater noise limits for various types of shipping operations.
In the U.S., the Society of Naval Architects and Marine Engineers will be adding their own regulations or guidelines in the near future.
This March, as the Okeanos foundation had hoped, the International Maritime Organization’s Design and Equipment Committee met in London where an international group of experts reviewed and updated draft guidelines. The draft will be distributed for final review and may be approved at next year’s meeting of the IMO Marine Environment Protection Committee.
The voluntary guideline was proposed by the United States delegation in collaboration with a dozen other member nations and non-governmental organizations. This guide will list actions a ship designer can take to design a new ship to be quiet. It will also list things a ship operator can do to keep an existing vessel running less noisily.
Does the future include regulations—not just guidelines—for underwater noise limits? It’s hard to say, because if regulations come, they will be many years away.
For today, this much is safe to say: underwater noise is not just the military’s problem.