Airborne sound propagation over sea during offshore wind farm piling

Abstract

Offshore piling for wind farm construction has attracted a lot of attention in recent years due to the extremely high noise emission levels associated with such operations. While underwater noise levels were shown to be harmful for the marine biology, the propagation of airborne piling noise over sea has not been studied in detail before. In this study, detailed numerical calculations have been performed with the Green's Function Parabolic Equation (GFPE) method to estimate noise levels up to a distance of 10 km. Measured noise emission levels during piling of pinpiles for a jacket-foundation wind turbine were assessed and used together with combinations of the sea surface state and idealized vertical sound speed profiles (downwind sound propagation). Effective impedances were found and used to represent non-flat sea surfaces at low-wind sea states 2, 3, and 4. Calculations show that scattering by a rough sea surface, which decreases sound pressure levels, exceeds refractive effects, which increase sound pressure levels under downwind conditions. This suggests that the presence of wind, even when blowing downwind to potential receivers, is beneficial to increase the attenuation of piling sound over the sea. A fully flat sea surface therefore represents a worst-case scenario.

Atmospheric Sound Propagation Over Rough Sea: Numerical Evaluation of Equivalent Acoustic Impedance of Varying Sea States

“…an equivalent impedance can be used [6,8]. Validation of this approach applied to water roughness is lacking in literature.…”

“…The impedance was used to inform PE predictions compared to long range acoustic measurements. A second approach by Van Renterghem et al [8] used Finite Difference Time Domain (FDTD) simulations over short distances (source and receiver separated by 5 m) to replicate a method previously reported by Boulanger and Attenborough [19] to find equivalent impedances of pseudorandom sea surfaces. The equivalent impedance determined with the FDTD method was implemented in PE predictions of atmospheric acoustic propagation above water over a 10 km range.…”

“…The spectrum depends only on U 19 , which can often be replaced by wind speed measured at 10 meter height with the approximation of U 19 ≈ 1.026 U 10 . This model has been applied to sound scattering predictions due to water roughness in both atmospheric and underwater problems [8,17,23]. Despite the broad use and a recent revisitation by Alves et al, the PM spectrum is unsuitable for describing high frequency gravity waves or capillary waves [24].…”

“…Pseudorandom sea surfaces can be generated using the inverse spatial Fourier transform (FFT) of power sea spectra as described in previous works [8,27] This approach requires a uniform distribution of spatial frequency, k. The resolution, ∆k, and the minimum spatial frequency, k min , of this distribution are both equal to 2π/L, where L is the horizontal length of the generated surfaces. As a consequence, the spectra are always unevenly sampled, because of the logarithmic nature of the wave energy distribution.…”

“…For many decades, atmospheric sound propagation studies have been conducted predominantly on land [1-4], with only a few studies conducted over sea and small bodies of water [5]. In recent years, however, research in sound propagation in coastal areas has garnered increased interest, driven by the growth of off-shore wind farms [6][7][8], oil and gas platforms, offshore airport construction [9], and other activities. The study of sound propagation over sea is also critical for detection of vessels at sea in cases of low visibility, as well as for echolocation over long ranges, on the order of kilometers.…”