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a wind farm, efforts that can be taken to reduce sound levels, and how these sounds might be assessed for their potential environmental impact.
Construction of Offshore Wind Turbines
Once the development of a wind farm has been approved, the installation of the wind turbine foundations can begin. The type of foundation used will depend on parameters such as the water depth, seabed properties at the site, and turbine size. In water depths less than 50 meters, fixed foundations such as monopiles, gravity base, and jacket
foundations are used to secure the wind turbines to the seabed (Figure 1). A gravity base foundation is a type of reinforced concrete structure used in water depths less than 10 meters that sits on the seabed and is heavy enough to keep the wind turbine upright. A monopile foundation is a single steel tube with a typical diameter of 3-8 meters that is driven into the seafloor, whereas a jacket foundation is a steel structure composed of many smaller tubular members welded together that sits on top of the seafloor and is secured by multiple steel piles driven into the sediment (Wu et al., 2019). Monopiles can be driven to a depth of 20-45 meters below the sea- floor and the piles to secure jacket foundations can be driven to a depth of between 30 and 75 meters (JASCO and LGL, 2019).
Most installed wind turbines utilize bottom-fixed founda- tions, but these foundations become less feasible in water depths greater than 50 meters. In the United States, roughly 58% of the offshore wind potential is in water depths deeper than 60 meters (US Department of Energy, 2016). In these greater water depths, floating foundations that are tethered to the seabed using anchors are a more viable option.
Impact Pile Driving
Impact pile driving, where the top of the pile is pounded repeatedly by a heavy hammer, is a method used to install monopile and jacket foundations and generates sound in the air, water, and sediment. The installation of a jacket foundation requires multiple piles be driven into the seabed to secure the corners of the steel struc- ture, whereas installation of the monopile design requires one larger pile be driven (Norro et al., 2013). Impact pile driving is not used for the installation of most floating or gravity-based foundations and therefore is not an inher- ent part of wind farm construction if the water depths and sediment characteristics at the installation site are suitable for these alternate foundations.
The impact of the hammer on the top of the pile is the primary source of sound that is generated during impact pile driving (see High-amplitude sound pressure is generated that radiates away from the pile on an angle that is dependent on the material proper- ties of the pile and the sound speed in the surrounding water. This angle is typically between 15° and 19° relative to the pile axis (Figure 2; Dahl et al., 2015b). Characteris- tics of the sound generated from each hammer strike are strongly dependent on the pile configuration, hammer impact energy, and environmental properties (such as the water depth and seabed properties).
In addition to the sound pressure generated in the water, compressional, shear, and interface waves are generated in the seabed that propagate outward from the pile in all directions (Figure 2). Compressional waves are the fast- est traveling waves in the seafloor and are characterized by particle motion that is parallel to the direction of wave propagation, whereas shear waves, which arrive second, have particle motion that is perpendicular to the direction of the propagating wave (Miller et al., 2016). Interface (or Scholte) waves along the water-sediment interface occur as a result of interfering compressional and shear waves. The low-frequency and slow-moving interface waves propagate
  Figure 1. Schematic showing some types of offshore wind turbine foundation structures, with the wind turbine components labeled. Image courtesy of the Bureau of Safety and Environmental Enforcement (BSEE), Department of the Interior, See
  14 Acoustics Today • Summer 2020

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