Alexandra Foley, COMSOL, www.comsol.com
Offshore wind farms come with a few more benefits than the onshore variety, such as the presence of more regular and higher speed winds that produce higher capacity factors. However, as the wind industry grows offshore, engineers are asked to investigate how turbine noise there might affect marine wildlife.
The turbine foundations
Three proposed foundations include a gravity base structure sitting on the seabed, a jacket with pin-pile connections to the seabed, and a monopile assembled on the seabed with a transition piece. Jacket and gravity bases are used in water 50m or deeper, while the monopile is generally not used at depths exceeding 30m.
Working for the Scottish Government, Brett Marmo, Iain Roberts, and Mark-Paul Buckingham used simulation software to determine the noise produced by different wind turbine foundations. Ian Davies and Kate Brookes, marine biologists working for Marine Scotland, identified the marine species most likely to be affected, based on water depth, turbine size, and foundation types of turbines submitted to the Scottish Government for licensing permits.
“We explored how different bases affect the noise produced, and whether or not this noise was loud enough to be heard by marine life,” Marmo explains. “We studied three different foundations – monopiles, jack foundations, and gravity bases – and examined the possible effects that the noise produced by each could have on local whales, porpoise, seals, dolphins, trout, and salmon.”
Determining noise production Sources of noise
Noise is produced in the drive train by rotational imbalances, blade pass, gear meshing in the gearbox, and by electromagnetic effects between the poles and stators in the generator. The geometry was modeled to determine the frequencies and strength of the noise.
Turbine vibrations travel from the tower into the foundation and are released as noise into the surrounding marine environment. “Because the noise is emitted at the interface between the foundation and seawater, it’s likely that the intensity and frequency of the noise will vary with the type of foundation,” describes Marmo. “Using finite-element analysis, we modeled three identical wind turbines, only altering the structure of the foundation.”
Noise entering the marine environment is mainly produced by machinery housed in the turbine’s gearbox and drivetrain by rotational imbalances, gear meshing, blade pass, and by electromagnetic effects between the poles and stators in the generator. Once vibrations enter the foundation, the amplitude of the noise emitted is affected by the excitation force, the frequency of structural resonance, and the amount of damping in the structure. Higher winds increase the rotor torque, likely causing higher noise.
Average sound pressure levels versus frequency
At frequencies lower than 180 Hz, the monopile produces the largest amount of noise. Of the three foundations, the monopile continues to produce larger SPL values up to 500 Hz. Around 600 Hz, all three foundations are comparable with the jacket foundation becoming the noisiest at frequencies greater than 700 Hz. The analysis was conducted in COMSOL Multiphysics.
“Understanding the effect of damping — the dissipation of vibration energy from a structure — was one of the key analyses we conducted,” says Marmo. “In general, steel structures, such as the jacket foundation, have less damping than those built from granular materials, such as the gravity base, which is made of concrete.” The amount of internal damping within a structure therefore affects the noise emitted. To determine how these factors affect noise production, the team turned to the simulation software COMSOL Multiphysics.
Underwater acoustics and sound propagation
At the interface between the turbine foundation and seawater, foundation vibrations oscillate water molecules to produce a pressure wave that radiates from the foundation as sound. Marmo and his team analyzed the foundations at wind speeds of 5, 10, and 15 m/s.
The jacket base demonstrated the lowest sound pressure level (SPL) of the three at low frequencies, below ~200 Hz. However, at high frequencies, it produced the highest SPL. The monopile and gravity bases exhibit comparable SPLs at lower frequencies, while at higher frequencies the gravity base produces the lowest SPL of the three bases.
Frequencies at which the foundations are loudest
Results from COMSOL Multiphysics show sound pressure levels around each of the foundations for the frequency at which the foundation produces the loudest noise.
“Simulation let us model the noise produced by each foundation under identical operating conditions — something we wouldn’t have been able to achieve by taking measurements of in-service wind turbines,” says Marmo. “Without simulation, the different environments and wind loads these turbines experience would have made it difficult to determine if it was truly the foundation that was affecting the noise produced and not another, unaccounted-for variable.”
Marmo and team also looked at the propagation of sound at greater distances from a wind farm. Beyond a certain distance, ambient noise masks the sound produced, making it nearly impossible to hear. “We found that each of the bases produced the loudest sound in the far-field at different frequencies,” says Marmo. “At wind speeds of 10 and 15 m/s, the monopile and gravity bases are audible at least 18 km away at most frequencies below 800 Hz, while the jacket is most audible only at 250 Hz 10 km away and at 630 Hz 18 km away.”
Noise detection by marine species
The team next determined the frequencies at which marine species could detect sound levels and over what distances. Cormac Booth and Stephanie King of SMRU Marine at St. Andrews University were the key marine biologists who analyzed the hearing thresholds of marine species and determined whether the noise produced could affect the animal’s behavior.
Hearing thresholds for marine animals
The hearing thresholds are for dolphins, minke whales, porpoises, and seals. Analysis was conducted in COMSOL Multiphysics.
Of the species examined, minke whales have the most sensitive hearing at low frequencies (less than 200 Hz) and were able to hear the turbine from the farthest distances. “We predict that minke whales will be able to detect wind farms constructed of either monopile or gravity foundations up to 18 km away at most frequencies below 800 Hz for all three wind speeds,” says Marmo. “On the other hand, bottlenose dolphins and porpoises are less sensitive to low frequencies. Dolphins can detect a wind farm on a gravity base 4 km away, but can only detect jackets and monopiles at close ranges of less than 1 km.”
Neither seal species nor bottlenose dolphins were predicted to exhibit a behavioral response to the sounds generated under any of the scenarios. However, from 4 to 13 km, 10% of minke whales encountering the noise field produced by the monopile foundation are expected to move away. Jacket foundations appear to generate the lowest marine mammal impact ranges when compared to gravity and monopile foundations.
The team found little to no detrimental effects from wind turbine noise on marine species. Future studies could look into using the presence of marine life as an indicator to determine the optimal type of foundation for a certain location, when water depth permits its use. For example, the jacket foundation (which produces noise at high frequencies) would be best used in locations where minke whales are known to be, because their hearing is most sensitive at low frequencies. Although the topic deserves further research, Marmo and team demonstrate that the future of offshore wind is looking positive for wind power proponents and marine species alike. WPE
Filed Under: News, Offshore wind
