6.5. Operation and Maintenance Phase

6.5.1. Operational Noise From wind Turbines

  1. Operational noise due to the floating wind turbines and mooring lines is dealt with qualitatively in section 8.3.2.

6.5.2. Geophysical Surveys

  1. Routine geophysical surveys will be similar to the geophysical surveys already discussed for the pre-construction phase (refer to section 6.3).

6.5.3. Routine Operation and Maintenance

  1. There are very few activities during the operations and maintenance phase that generate significant amounts of underwater noise. These noise generating activities are anticipated at this stage to be characterised by vessel movements.

6.5.4. Vessels

  1. The potential for vessel use to create underwater noise is presented in section 6.7 for all phases of the Array.

6.6. Decommissioning Phase

6.6.1. Vessels

  1. Only the potential impact of noise from vessel activity has been included in the underwater noise assessment for the decommissioning phase of the Array. It should be noted that cavitation from the vessels themselves is likely to dominate the noisescape for other decommissioning activities (e.g. removal of subsea structures). The potential impact of vessels noise emissions is addressed in section 6.7 for all phases of the Array.

6.7. Vessels (All Phases)

  1. The noise emissions from the types of vessels that may be used for the Array are quantified in Table 6.10   Open ▸ , based on a review of publicly available data. Noise from the vessels themselves (e.g. propeller, thrusters and sonar (if used)) primarily dominates the emission level, hence noise from activities such as seabed preparation, trenching and rock placement (if required) have not been included separately.
  2. In Table 6.10   Open ▸ , SELs have been estimated for each source based on 24 hours continuous operation, although it is important to note that it is highly unlikely that any marine mammal or fish would stay at a stationary location or within a fixed radius of a vessel (or any other noise source) for 24 hours. Consequently, the acoustic modelling has been undertaken based on an animal swimming away from the source (or the source moving away from an animal).
  3. Source noise levels for vessels depend on the vessel size and speed as well as propeller design and other factors. There can be considerable variation in noise magnitude and character between vessels even within the same class. Therefore, source data for the Array has been based on MDS assumptions (i.e. using noise data toward the higher end of the scale for the relevant class of ship as a proxy). In the case of the cable laying vessel, no publicly available information was available for a similar vessel and therefore measurements on a suction dredger using Dynamic Positioning (DP) thrusters were used as a proxy. This is considered an appropriate proxy because it is a similar size of vessel using dynamic positioning and therefore likely to have a similar acoustic footprint.

 

Table 6.10:
Source Noise Data for Site Preparation, Construction, Operation and Maintenance and Decommissioning Vessels

Table 6.10: Source Noise Data for Site Preparation, Construction, Operation and Maintenance and Decommissioning Vessels

 

7. Propagation Modelling

7.1. Propagation of Noise Underwater

  1. As the distance from the noise source increases the level of received or recorded noise reduces, primarily due to the spreading of the noise energy with distance, in combination with attenuation due to absorption of noise energy by molecules in the water. This latter mechanism results in higher attenuation at higher frequency noise than for lower frequencies.
  2. The way that the noise spreads (geometrical divergence) will depend upon several factors such as water column depth, pressure, temperature gradients, salinity as well as water surface and bottom (i.e. seabed) conditions. Thus, even for a given locality, there are temporal variations to the way that noise will propagate. However, in simple terms, the noise energy may spread out in a spherical pattern (close to the source) or a cylindrical pattern (much further from the source), although other factors mean that decay in noise energy may be somewhere between these two simplistic cases. The distance at which cylindrical spreading dominates is highly dependent on water depth. Noise propagation in shallow water depths will be dominated by cylindrical spreading as opposed to spherical spreading.
  3. In acoustically shallow waters[11] in particular, the propagation mechanism is influenced by multiple interactions with the seabed and the water surface (Lurton, 2002; Etter, 2013; Urick, 1983; Brekhovskikh et al, 2003; Kinsler et al., 1999). Whereas in deeper waters, the noise will propagate further without encountering the surface or bottom of the sea (seabed).
  4. At the sea surface, the majority of the noise is reflected into the water due to the difference in acoustic impedance (i.e. product of noise speed and density) between air and water. However, the scattering of noise at the surface of the sea can be an important factor in the propagation of noise. In an ideal case (i.e. for a perfectly smooth sea surface), the majority of noise energy will be reflected into the sea. However, for rough seas, much of the noise energy is scattered (e.g. Eckart, 1953; Fortuin, 1970; Marsh, Schulkin, and Kneale, 1961; Urick and Hoover, 1956). Scattering can also occur due to bubbles near the surface such as those generated by wind or fish or due to suspended solids in the water such as particulates and marine species. Scattering is more pronounced for higher frequencies than for low frequencies and is dependent on the sea state (i.e. wave height). However, the various factors affecting this mechanism are complex.
  5. As surface scattering results in differences in reflected noise, its effect will be more apparent at longer ranges from the noise source and in acoustically shallow water (i.e. where there are multiple reflections between the source and receiver). The degree of scattering will depend upon the sea state/wind speed, water depth, frequency of the noise, temperature gradient, grazing angle and range from source. It should be noted that variations in propagation due to scattering will vary temporally within an area primarily due to different sea-states/wind speeds at different times. However, over shorter ranges (e.g. several hundred meters or less) the noise will experience fewer reflections and so the effect of scattering should not be significant.
  6. When noise waves encounter the seabed, the amount of noise reflected will depend on the geoacoustic properties of the bottom (e.g. grain size, porosity, density, noise speed, absorption coefficient and roughness) as well as the grazing angle and frequency of the noise (Cole, 1965; Hamilton, 1970; Mackenzie, 1960; McKinney and Anderson, 1964; Etter, 2013; Lurton, 2002; Urick, 1983). Thus, seabeds comprising primarily mud or other acoustically soft sediments will reflect less noise than acoustically harder bottoms such as rock or sand. This will also depend on the profile of the bottom (e.g. the depth of the sediment layer and how the geoacoustic properties vary with depth below the seafloor). The effect is less pronounced at low frequencies (a few kHz and below). A scattering effect (similar to that which occurs at the surface) also occurs at the seabed (Essen, 1994; Greaves and Stephen, 2003; McKinney and Anderson, 1964; Kuo, 1992), particularly on rough substrates (e.g. pebbles).
  7. The waveguide effect should also be considered, which defines the shallow water columns that do not allow the propagation of low frequency noise (Urick, 1983; Etter, 2013). The cut-off frequency of the lowest mode in a channel can be calculated based on the water depth and knowledge of the sediment geoacoustic properties but, for example, the cut-off frequency as a function of water depth (based on the equations set out in Urick, 1983) is shown in Figure 7.1   Open ▸ for a range of seabed types. Any noise below this frequency will not propagate far due to energy losses through multiple reflections.

Figure 7.1:
Lower Cut-Off Frequency as a Function of Depth for a Range of Seabed Types

Figure 7.1: Lower Cut-Off Frequency as a Function of Depth for a Range of Seabed Types

 

  1. Changes in the water temperature and the hydrostatic pressure with depth mean that the speed of noise varies throughout the water column. This can lead to significant variations in noise propagation and can also lead to noise channels, particularly for high-frequency noise (Lurton 2002). Noise can propagate in a duct-like manner within these channels, effectively focussing the noise, and conversely, they can also lead to shadow zones. The frequency at which this occurs depends on the characteristics of the noise channel and since the temperature gradient can vary throughout the year there will be potential variation in noise propagation depending on the season.
  2. Noise energy is also absorbed due to interactions at the molecular level converting the acoustic energy into heat (Urick 1983). This is another frequency-dependent effect with higher frequencies experiencing much higher losses than lower frequencies.

7.2. Modelling Approach

  1. There are several methods available for modelling the propagation of noise between a source and receiver ranging from very simple models which simply assume spreading effects according to a 10 log (R) or 20 log (R) relationship (as discussed above, and where R is the range from source) to full acoustic models (e.g. ray tracing, normal mode, parabolic equation, wavenumber integration and energy flux models). In addition, semi-empirical models are available, in which complexity and accuracy are somewhere in between these two extremes.
  2. In choosing the correct propagation model to employ, it is important to ensure that it is fit for purpose and produces results with a suitable degree of accuracy for the application in question, taking into account the context, as detailed in “Monitoring Guidance for Underwater Noise in European Seas Part III”, National Physical Laboratory Guidance (Dekeling et al., 2014) and in Farcas et al. (2016). Thus, in some situations (e.g. low risk of auditory injury due to underwater noise, where range dependent bathymetry is not an issue, i.e. for non-impulsive noise) a simple (N log R) model might be sufficient, particularly where other uncertainties (such as uncertainties in source level or the impact thresholds) outweigh the uncertainties due to modelling. On the other hand, some situations (e.g. very high source levels, impulsive noise, complex source and propagation path characteristics, highly sensitive receivers, and low uncertainties in assessment criteria) warrant a more complex modelling methodology.
  3. The first step in choosing a propagation model is therefore to examine these various factors, such as:
  • balancing of errors/uncertainties;
  • range dependant bathymetry;
  • frequency dependence; and
  • source characteristics.