Mounts, coatings, blankets, silencers reduce noise and vibration aboard shipsJan 27, 2015 01:22 PM
In the engine room of a tugboat, all piping is equipped with rubber anti-vibration fittings.
Noise rules adopted last summer from the International Convention for the Safety of Life at Sea (SOLAS) are spurring shipowners to reduce threats to crewmembers’ hearing, concentration and sleep.
Ships’ spaces are being engineered to lower mechanical, aerodynamic and hydro-acoustic noise. Control measures include vibration-isolation mounts, structural damping, acoustical absorption and insulation, and new propeller designs.
Various U.S. companies are involved in controlling noise on vessels. Among their products are design and diagnostic services, isolation mounts, hullboards, Microlite blankets, damping sheets, acoustical panels, barriers and duct silencers.
On July 1, SOLAS was amended with the “Code on Noise Levels on Board Ships,” limiting sound on new ships from machinery spaces, workshops, control rooms, accommodations and other areas. These new standards are meant to make the maritime workplace healthier.
“Benefits to ship operators and crews from reducing noise include less fatigue and annoyance, improved communication and less risk of hearing damage,” said Jesse Spence, director of technology at Noise Control Engineering LLC (NCE). “This leads to higher productivity and crew retention, fewer adverse incidents and lower long-term operational costs.”
NCE, based in Billerica, Miss., consults with shipyards, naval architects, government agencies and private owners about noise and vibration. The company performs airborne, structural and underwater analyses and measurements to assess potential noise problems.
“We assist throughout the design cycle, from preliminary design through construction and trials, and on diagnostic work after a ship is built,” Spence said. “We recommend different treatments depending on specific situations. Each ship is different, and there is no one-size-fits-all solution.”
NCE developed an acoustic software program called Designer-NOISE for predicting noise levels on ships and offshore platforms. The product, which dates to 1998, has been improved since then. “It can be used to assess noise from diesel engines, turbines, propellers, thrusters, pumps, compressors and HVAC systems,” Spence said. Noise impacts at any location within the ship can be analyzed.
The software’s 3-D graphical user interface allows for rapid model creation and entry of parameters. A model can be made in days using basic physical design information, including material types, plate thicknesses and frame spacing. The software uses a hybrid statistical energy analysis algorithm with architectural acoustics to predict how vibrations and noise spread through a vessel. It simultaneously calculates octave-band and A-weighted noise levels.
On the same vessel, engine isolation mounts help sequester bothersome noise and vibration generated by the machinery. The goal is to make the work environment more comfortable and improve mariners’ quality of life.
“Results from Designer-NOISE can be used to identify sources and paths and treatment options,” Spence said. “Studies can be performed to find optimal treatments with limited cost, space and weight impacts.”
Isolation mounting of engines, turbines and other machinery reduces vibration transfers.
“Vibrating structures radiate noise,” Spence said. “Machinery vibrations travel throughout a vessel and cause noise in all compartments.” The extent of the noise depends on the magnitude of the vibrations. “One of the best ways to reduce these vibrations is through resilient mounting,” he said. “For common vessel designs, isolation mounting can lower overall noise by 10-plus decibels (dB) as long as the machinery is the dominant vibration source.”
Isolation mounting systems must be properly designed, however. “The machinery and mounting system will have natural frequencies of vibration,” Spence said. “If these natural frequencies are excited by the machinery, then excessive vibrations can occur.”
Poorly designed systems can inhibit mount performance and lead to increased noise. “A number of factors must be considered by capable personnel when designing an isolation system,” he said. “They include mount stiffness, mount loading, design of supporting structures above and below the mount, design of flexible connections to the mounted system, and the mass and mechanical properties of the machinery being mounted.”
NCE has provided noise assistance on hundreds of commercial and government vessels. In 2008, the company was the first in North America to be certified as an ambient environmental testing specialist by the American Bureau of Shipping. It’s one of a handful of companies in the world to be similarly rated. NCE doesn’t sell soundproofing materials or perform installations.
Houston-based Mascoat sells a coating that can reduce structural-borne noise throughout most marine vessels, said Will Conner, the company’s marketing director. “Mascot Sound Control-dB is a coating for marine uses that’s lighter-weight than most other sound-damping methods on the market. It gets similar or better results with less weight added. Our product is a one-part, water-based coating with sound-deadening particulates, allowing it to greatly reduce noise on any type of marine vessel.” This coating is used on commercial vessels, barges, workboats and heavy machinery.
Sound Control-dB can be applied to aluminum, stainless steel, brass or fiberglass. For applications on carbon steel substrates, a primer coat should be applied first, Conner said. The product is spray-applied in coats of up to 40 mils or 1 mm each. “It has a paintlike consistency that cures quickly so equipment doesn’t have to be shut down for a long time,” he said. “Paint crews can install it overnight, avoiding daytime work that might interfere with other activities.”
For an engine room, Mascoat usually recommends applying the coating in up to 120 to160 mils, or 3 mm to 4 mm, Conner said. “It can be applied to accommodations and crew spaces, in and around thruster compartments, on wave slap areas of the bow, stacks and generator and engine rooms,” he said. “The farther you get away from the source of vibrations and noise, the less product you need. Because it’s a coating, this method of damping is versatile and can be adjusted to fit many budgets and sound requirements.”
Most shipbuilders and large vessel operators have noise engineers who run sound surveys, Conner said. “These surveyors can add Sound Control-dB to their calculations, and they can figure out how much material is needed to keep areas of the vessel below required noise levels,” he said. “You would, for example, need less coating on a 97-foot pushboat than a 300-foot PSV.”
Two issues in particular make noise hard to control, Conner said. “First, it’s very difficult to apply conventional types of damping systems to vessel areas that experience the brunt of sound,” he said. “And secondly, thrusters put off high noise and frequency ranges. Many traditional sound-damping products don’t perform well against this extreme noise.”
A worker applies a Mascoat coating, a paint-like substance that can be used instead of tiles to reduce noise around engine rooms, thruster compartments and wave slap areas of a bow.
When offshore vessel provider Seacor Marine in Houma, La., was concerned about sound in one of its newbuilds in 2009, the company used Mascoat Sound Control-dB, Conner said. “The vessel, under construction at the Gulf Craft shipyard in Patterson, La., had unwanted noise traveling throughout the boat from its bow thrusters. Our Mascoat business development specialist Andrew Margarit met with Seacor and Gulf Craft, and he found the bow thrusters putting out a great amount of noise.”
Mascoat’s engineers calculated that applying Mascoat Sound Control-dB, with a thickness of between 80 and 120 mils, or 2 mm to 3 mm, to the vessel’s forward machinery spaces and bow thruster area dampened noise to acceptable levels. The vessel was completed at Gulf Craft in 2010 and released for sea trials.
“Seacor told us that Mascoat Sound Control-dB significantly lowered noise in the concerned areas, contributing to an all-around quieter vessel and a more comfortable crew,” Conner said. Seacor continues to use the product on its newbuilds. “One of their more recent vessels, the Aaron S. McCall, was launched with Mascoat Sound Control-dB, and the reports have been positive,” Conner said.
Mascoat Sound Control-dB is one of the lightest sound-damping solutions and requires less labor and fewer days to apply than other companies’ products, Conner said. It is certified by the U.S. Coast Guard, Norway-based classification group Det Norske Veritas, Lloyd’s Register, American Bureau of Shipping and the Royal Institution of Naval Architects.
Denver, Colo.-based Johns Manville (JM) manufactures two types of sound insulation products for marine use, the Incombustible Hullboard and the Incombustible Microlite blanket, said Jeff Reinsma, JM’s performance-material portfolio leader.
The Incombustible Hullboard, made from felted glass fibers, provides acoustical and thermal insulating for the hulls and deckheads of naval and merchant vessels and drilling rig platforms. “The semi-rigid hullboard has a smooth surface, allowing it to receive a facing for architectural appearance and adhesion to the structure,” Reinsma said.
This product was the first incombustible hullboard developed for the marine industry. “Our hullboard is the most preferred brand of fiberglass thermal and acoustical board insulation for marine usage,” Reinsma said. The board is easy to cut and fit and can be fabricated with minimal time and effort, he said.
The product comes in standard sizes of 24-by-36-inches and 24-by-48-inches, with thicknesses of 1, 1.75, 2, 3 and 4 inches. Standard sizes save cutting and trimming time and reduce waste. Kerfing “V grooves” for beam insulation can be done cleanly on cutting tables with hand tools or mechanical devices.
Johns Manville’s second product, the Incombustible Microlite blanket, is made of fine glass fibers and bonded with a thermo-setting resin. “It’s lightweight and highly resilient so that it recovers to design thickness after compression,” Reinsma said. “It provides excellent sound absorption.” The blanket can be installed when design parameters prevent the use of a rigid product. In addition to reducing sound, the blanket’s low thermal conductivity facilitates shipboard heating and air conditioning efficiency. The blanket ranges from 1.5 to 3 inches thick and is 48 inches wide and up to 100 feet long.
“Equipment is sometimes mounted on dampeners to reduce noise and vibration,” Reinsma said. “Our fiberglass insulation is not used in those applications since equipment can be very heavy and would crush the fiberglass.”
The Incombustible Hullboard and Incombustible Microlite blanket help control cabin noise and temperatures and are fire resistant. Both are U.S. Coast Guard-approved and comply with U.S. Navy and Nuclear Regulatory Commission product standards, Reinsma said.
Acoustical Surfaces Inc., in Chaska, Minn., provides soundproofing and noise control solutions to the transportation and other industries, including ships. “As a sailboat owner and a former musician, concert mixer and recording engineer, I’m personally committed to lower noise levels,” said the company’s retail sales director, John Calder. “We strongly advocate reducing workplace sound levels.”
The company’s VBlok VB-2 vibration damping sheet prevents the conversion of sound energy into vibration energy and can be used in hull bulkhead applications. VB-2 is a high-performance, lightweight sheet, engineered to turn vibration energy into low-level heat through viscous friction. VB-2 vinyl damping materials have a silica-mica and ceramic mineral load, which has proved better at reducing sound than asphalt-based materials. VB-2 is moldable and has an acrylic adhesive, allowing complete adherence to a vibrating structure.
A sample of Johns Manville’s Incombustible Hullboard, a semi-rigid felted glass fiber product that provides acoustical and thermal insulation on ships.
Courtesy Johns Manville
“Our Silk Metal acoustical panel has a smooth fabric surface and an excellent NRC or noise reduction coefficient rating of 0.80, with a 4-inch airspace,” Calder said. NRCratings.com does laboratory assessments of a material’s sound absorption quality. “The Silk Metal panel is made of a patented, angled, micro-perforated aluminum material,” Calder said. It absorbs energy generated by sound and electromagnetic waves. These panels achieve acoustical value without acoustical liners. They can be fabricated into various forms and sizes, provide heat mitigation, are fireproof and resist mold.
Because its high-performance damping products work together, the company recommends consulting one of its acoustical surface specialists for solutions.
An Acoustical Surfaces product that can help ships control noise is the Duct V-Max vibration damping sheet, made of an advanced, non-curing, butylene rubber bonded to a thin layer of aluminum. VMAX uses sheer force to control vibrations. The product’s thin aluminum layer prevents a substrate from bending beyond a given point. VMAX damping sheets are lightweight and can be applied to sheet metal, wood, fiberglass and plastic surfaces.
Additional company products used on ships include Pipe Noise S.T.O.P., which does pipe lagging with a decoupler; the Class-A Mass Loaded Vinyl Barrier, an MLV noise barrier; the Barrier-Decoupler, a vinyl barrier with a foam decoupler; the Acousti-Board Ultra, a soundproofing backer, and Silent-Mod Duct Silencers for HVAC noise quieting.
The U.S. Navy and several federal agencies have rules for noise on board ships. Noise-induced hearing loss is the Navy’s top occupational health expense. The Navy chooses quiet equipment, systems and tools at the earliest acquisition stages to reduce source noise. It has a “Buy Quiet” policy for equipment aboard ships. Such equipment can be more expensive but it’s usually built better and offers efficiencies that reduce maintenance costs and, of course, health threats to the crew.