6. Propagation Modelling

6.1. Propagation of Sound Underwater

104. As the distance from the sound source increases the level of sound recorded reduces, primarily due to the spreading of the sound energy with distance, in combination with attenuation due to absorption of sound energy by molecules in the water. This latter mechanism is more important for higher frequency sound than for lower frequencies.
105. The way that the sound 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 sound will propagate. However, in simple terms, the sound 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 sound energy may be somewhere between these two simplistic cases.
106. In acoustically shallow waters[6] in particular, the propagation mechanism is coloured by multiple interactions with the seabed and the water surface (Lurton, 2002; Etter, 2013; Urick, 1983; Brekhovskikh and Lysanov, 2014; Kinsler et al., 1999). Whereas in deeper waters, the sound will propagate further without encountering the surface or bottom of the sea, in shallower waters the sound may be reflected from either or both boundaries (potentially more than once).
107. At the sea surface, the majority of the sound is reflected into the water due to the difference in acoustic impedance (i.e. sound speed and density) between air and water. However, the scattering of sound at the surface of the sea can be an important factor in the propagation of sound. In an ideal case (i.e. for a perfectly smooth sea surface), the majority of sound energy will be reflected into the sea. However, for rough seas, much of the sound 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 life. 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.
108. Because surface scattering results in differences in reflected sound, its effect will be more important at longer ranges from the sound 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 sound, 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 sound will experience fewer reflections and so the effect of scattering should not be significant.
109. When sound waves encounter the bottom, the amount of sound reflected will depend on the geoacoustic properties of the bottom (e.g. grain size, porosity, density, sound speed, absorption coefficient and roughness) as well as the grazing angle and frequency of the sound (Cole, 1965; Hamilton, 1970; Mackenzie, 1960; McKinney and Anderson, 1964; Etter, 2013; Lurton, 2002; Urick, 1983). Thus, bottoms comprising primarily mud or other acoustically soft sediments will reflect less sound 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 bottom (Essen, 1994; Greaves and Stephen, 2003; McKinney and Anderson, 1964; Kuo, 1992), particularly on rough substrates (e.g. pebbles).
110. The waveguide effect should also be considered, which defines the shallow water columns do not allow the propagation of low frequency sound (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. Any sound below this frequency will not propagate far due to energy losses through multiple reflections.
111. Changes in the water temperature and the hydrostatic pressure with depth mean that the speed of sound varies throughout the water column. This can lead to significant variations in sound propagation and can also lead to sound channels, particularly for high-frequency sound. Sound can propagate in a duct-like manner within these channels, effectively focussing the sound, and conversely, they can also lead to shadow zones. The frequency at which this occurs depends on the characteristics of the sound channel but, for example, a 25 m thick layer would not act as a duct for frequencies below 1.5 kHz. The temperature gradient can vary throughout the year and thus there will be potential variation in sound propagation depending on the season.
112. Sound energy is also absorbed due to interactions at the molecular level converting the acoustic energy into heat. This is another frequency-dependent effect with higher frequencies experiencing much higher losses than lower frequencies.

6.2. Modelling approach

113. There are several methods available for modelling the propagation of sound between a source and receiver ranging from very simple models which simply assume spreading 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, whose complexity and accuracy are somewhere in between these two extremes.
114. 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”, NPL Guidance, (Dekeling et al., 2014) and in Farcas et al. (2016). Thus, in some situations (e.g. low risk due to underwater noise, where range dependent bathymetry is not an issue, i.e. for non-impulsive sound) a simple (N log R) model might be sufficient, particularly where other uncertainties outweigh the uncertainties due to modelling. On the other hand, some situations (e.g. very high source levels, impulsive sound, complex source and propagation path characteristics, highly sensitive receivers, and low uncertainties in assessment criteria) warrant a more complex modelling methodology.
115. 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.

Figure 6.1: The Indicative Location of the Proposed Piles (Yellow Circles) in the Proposed Development, General Bathymetry Depth Around the Survey Region (Darker Is Deeper Water), and the Different Transects Employed for the Study Radiating out from one of the Modelled Source Locations

116. For the sound field model, relevant survey parameters were chosen based on a combination of data provided by the Applicant combined with the information gathered from the publicly available literature. These parameters were fed into an appropriate propagation model routine, in this case the Weston Energy Flux model (for more information see volume 3, appendix 10.1, annex C; Weston, 1971; 1980a; 1980b), suited to the region and the frequencies of interest. The frequency-dependent loss of acoustic energy with distance (TL) values were then evaluated along different transects around the chosen source points. The frequencies of interest in the present study are from 20 Hz to 1,000 kHz (1 MHz), with different noise sources operating in different frequency bands. These frequencies overlap with the hearing sensitivities (as per Figure 4.1   Open ▸ ) of some of the marine mammals that are likely to be present in the survey area.
117. A more detailed justification for the choice of noise model is provided in a separate technical note in volume 3, appendix 10.1, annex A. A calibration of the Weston Flux Energy underwater noise model against other underwater noise models is provided in volume 3, appendix 10.1, annex C.

Table 6.1: Regions of Transmission Loss Derived by Weston (1971)

118. The propagation loss is calculated using one for the four formulae detailed in the table above, depending on the distance of the receiver location from the source, and related to the frequency and the seafloor conditions such as depth and its composition.
119. In Table 6.1   Open ▸ , is the depth at the source, is the depth at the receiver, is the minimum depth along the bathymetry profile (between the source and the receiver), is the critical grazing angle (related to the speed of sound in both seawater and the seafloor material), and are the wavelength and wavenumber as usual, and is the seabed reflection loss gradient, empirically derived to be 12.4 dB/rad in Weston (1971).
120. The spherical spreading region exists in the immediate vicinity of the source, which is followed by a region where the propagation follows a cylindrical spread out until the grazing angle is equal to the critical grazing angle . Above the critical grazing angle in the mode stripping region an additional loss factor is introduced which is due to seafloor reflection loss, where higher modes are attenuated faster due to their larger grazing angles. In the final region, the single-mode region, all modes but the lowest have been fully attenuated.
121. For estimation of propagation loss of acoustic energy at different distances away from the noise source location (in different directions), the following steps were considered:

• The bathymetry information around this chosen source point (north-east point as marked in Figure 6.1   Open ▸ was extracted from the GEBCO database up to 80 km (where possible) in 72 different transects.
• A geoacoustic model of the different seafloor layers in the survey region was calculated.
• A calibrated Weston Energy model was employed to estimate the TL matrices for different frequencies of interest (from 25 Hz to 80 kHz) along the 72 different transects.
• The source level values calculated were combined with the TL results to achieve a frequency and range dependant RL of acoustic energy around the chosen source position.
• The recommended marine mammal weightings (m-weightings) were employed for injury and the TTS and PTS potential impact ranges for different marine mammal groups were calculated using relevant metrics (from Southall et al., 2019) and by employing a fleeing marine mammal model where necessary.
  122. The propagation and sound exposure calculations were conducted over a range of water column depths to determine the likely range for injury and disturbance. For the results shown in tables in section 7 a representative pile location in the middle of the site was chosen (wind turbine 96 for the 179T layout). For sound level contours, an additional five points were modelled, chosen as the extremities of the field (north, south, east and west), with the fifth being an additional point near the Firth of Forth. These six points are seen in Figure 6.2   Open ▸ .

Figure 6.2: Six Representative Modelling Points within the Proposed Development. These Correspond to Indicative Foundation Locations 1, 40, 83, 135 and 179 from the 179T Layout. The Central Point (83) was Used for Potential Impact Ranges.

123. It should be borne in mind that noise levels (and associated range of effects) will vary depending on actual conditions at the time (day-to-day and season-to-season) and that the model predicts a typical worst-case scenario. Considering factors such as animal behaviour and habituation, any injury and disturbance ranges should be viewed as indicative and probabilistic ranges to assist in understanding potential impacts on marine life rather than lines either side of which a potential impact will or will not occur[7].
124. The Weston noise model used for this assessment has been calibrated against a range of other noise models showing good agreement (typically within +/- 1 dB to a range of 2.5 km). The results of this comparison are summarised in volume 3, appendix 10.1, annex C. The acoustical properties of different layers employed in the propagation modelling are presented in Table 6.2   Open ▸ . This data is evaluated using recommendations by Hamilton (1980) based on the geological layers present in the survey region and the acoustic properties of the water column. Due to the relatively shallow nature of the area, only a single speed of sound in the water column was considered.

Table 6.2: Acoustical Properties of the Water Layer and Sediment Used for Propagation Modelling

125. The level of detail presented in terms of noise modelling needs to be considered in relation to the level of uncertainty for animal injury and disturbance thresholds. Uncertainty in the sound level predictions will be higher over larger propagation distances (i.e. in relation to disturbance thresholds) and much lower over shorter ones (i.e. in relation to injury thresholds). Nevertheless, it is considered that the uncertainty in animal injury and disturbance thresholds is likely to be higher than uncertainty in sound predictions. This is further compounded by differences in individual animal response, sensitivity, and behaviour. It would therefore be wholly misleading to present any injury or disturbance ranges as a hard and fast line beyond which no effect can occur, and it would be equally misleading to present any noise modelling results in such a way.

6.3. Batch Processing

126. To improve the performance and reduce the time taken to process and evaluate multiple TL calculations required for this study, Seiche Ltd’s proprietary software was employed. This software iteratively evaluates the propagation modelling routine for the specified number of azimuthal bearings radiating from a source point, providing a fan of range-dependent TL curves departing from the noise source for each given frequency and receiver depth. In-house routines are then employed to interpolate the TL values across transects, to give an estimate of the sound field for the whole area around the source point. For more information on the methodology followed, see volume 3, appendix 10.1.Once the TL values were evaluated at the source points, in all azimuthal directions, and at all frequencies of interest for various sources, the results were then coupled with the corresponding SL values in third octave frequency bands. The combination of SL with TL data provided us with the third octave band RL at each point in the receiver grid (i.e. at each modelled range, depth, and azimuth of the receiver).
127. The received levels were evaluated for the SPLpk, SPLrms or SEL metric, for each source type, source location, and azimuthal transect to produce the associated 2-D maps. The broadband RL were then calculated for these metrics and from the third octave band results. The set of simulated RL transects were circularly interpolated to generate the broadband 2-D RL maps centred around each source point.

6.4. Exposure Calculations

128. As well as calculating the un-weighted sound levels at various distances from different source, it is also necessary to calculate the acoustic signal in the SEL metric (where necessary and possible) for a mammal using the relevant hearing weightings to which it is exposed. For operation of the different sources, the SEL sound data was numerically equal to the SPL rms value integrated over one second window as the sources are continuous and non-impulsive. These SEL values are employed for calculation of cumulative SEL (SELcum) metric for different marine mammal groups to assess potential impact ranges.
129. Simplified exposure modelling could assume that the mammal either being static and at a fixed distance away from the noise source, or that the mammal is swimming at a constant speed in a perpendicular direction away from a noise source. For fixed receiver calculations, it has generally been assumed (in literature) that an animal will stay at a known distance from the noise source for a period of 24 hours. As the animal does not move, the noise will be constant over the integration period of 24 hours (assuming the source does not change its operational characteristics over this time). This, however, would give an unrealistic level of exposure, as the animals are highly unlikely to remain stationary when exposed to loud noise, and is therefore expected to swim away from the source. The approximation used in these calculations, therefore, is that the animals flee directly away from the source.
130. It should be noted that the sound exposure calculations are based on the simplistic assumption that the noise source is active continuously (or intermittently based on shot-timings) over a 24 hour period. The real world situation is more complex. The SEL calculations presented in this study do not take any breaks in activity into account, such as repositioning of the piling vessel.
131. Furthermore, the sound criteria described in the Southall et al. (2019) guidelines assume that the animal does not recover hearing between periods of activity. It is likely that both the intervals between operations could allow some recovery from temporary hearing threshold shifts for animals exposed to the sound and, therefore, the assessment of sound exposure level is conservative.
132. In order to carry out the swimming mammal calculation, it has been assumed that a mammal will swim away from the noise source at the onset of activities. For impulsive sounds of piledriving the calculation considers each pulse to be established separately resulting in a series of discrete SEL values of decreasing magnitude (see Figure 6.3   Open ▸ ).

Figure 6.3: A Comparison of Discrete “Pulse” Based SEL and a Cumulative of SEL Values

133. As a marine mammal swims away from the sound source, the noise it experiences will become progressively more attenuated; the cumulative SEL is derived by logarithmically adding the SEL to which the mammal is exposed as it travels away from the source. This calculation was used to estimate the approximate minimum start distance for a marine mammal in order for it to be exposed to sufficient sound energy to result in the onset of potential injury. It should be noted that the sound exposure calculations are based on the simplistic assumption that the animal will continue to swim away at a fairly constant relative speed. The real-world situation is more complex, and the animal is likely to move in a more complex manner.
134. The swim speeds of marine mammals used in this assessment were presented during Marine Mammal Road Map Meeting 2 on 20 October 2021 (see volume 3, appendix 10.3) and no concerns were raised by NatureScot and MSS at the meeting or in subsequent correspondence. The swim speeds applied are summarised in Table 6.3   Open ▸ along with the source papers for the assumptions.

Table 6.3: Swim Speeds Assumed for Exposure Modelling

135. To perform this calculation, the first step is to parameterise the m-weighted sound exposure levels for single strikes of a given energy via a line of best fit. This function is then used to predict the exposure level for each strike in the planned hammer schedule (periods of slow start, ramp up and full power).
136. In addition to the single-source pile driving, simplified situations of simultaneous pile driving from two piling rigs have been considered. The flight response has been approximated as fleeing straight away from the nearest point on a line between the two sources. For simplicity, the sources are considered to be omnidirectional and the piling schedules (soft start, ramp up, etc) are synchronised, entering each stage of the schedule at the same time.

6.5. UXO Noise Modelling

6.5.1.    Detonation

137. Noise modelling for UXO clearance has been undertaken using the methodology described in Soloway and Dahl (2014). The equation provides a simple relationship between distance from an explosion and the weight of the charge (or equivalent TNT weight) but does not take into account bottom topography or sediment characteristics.

138. Where W is the equivalent TNT charge weight and R is the distance from source to receiver.
139. Since the charge is assumed to be freely standing in mid-water, unlike a UXO which would be resting on the seabed and could potentially be buried, degraded or subject to other significant attenuation, this estimation of the source level can be considered conservative.
140. According to Soloway and Dahl (2014), the SEL can be estimated by the following equation:

Figure 6.4: Assumed Explosive Spectrum Shape Used to Estimate Hearing Weighting Corrections to SEL

141. In order to compare to the marine mammal hearing weighted thresholds, it is necessary to apply the frequency dependent weighting functions at each distance from the source. This was accomplished by determining a transfer function between unweighted and weighted SEL values at various distances based on an assumed spectrum shape (see Figure 6.4   Open ▸ ) and taking into account molecular absorption at various ranges. Furthermore, because there is potential for more than one UXO clearance event per day (a maximum of two per day is assumed) then it is also necessary to take this into account in the exposure calculation.

6.5.2.    Low Order Techniques

142. According to Robinson et al. (2020), low order deflagration (a specific method of low order UXO clearance) results in a much lower amplitude of peak sound pressure than high order detonations. The study concluded that peak sound pressure during deflagration is due only to the size of the shaped charge used to initiate deflagration and, consequently, that the acoustic output can be predicted for deflagration as long as the size of the shaped charge is known.
143. Noise modelling for low order techniques (such as deflagration) has therefore been based on the methodology described in section 5.3 for detonations, using a smaller donor charge size.

7. Sound Modelling Results

7.1. Pre-construction Phase

144. The estimated ranges for injury to marine mammals due to various proposed activities invited in the pre-construction surveying phase of the operations are presented in this section. These include geophysical and geotechnical survey activities, UXO clearance and supported vessel activities.
145. The potential ranges presented for injury and disturbance are not a hard and fast ‘line’ where an impact will occur on one side and not on the other. Potential impact is more probabilistic than that; dose dependency in PTS onset, individual variations and uncertainties regarding behavioural response and swim speed/direction all mean that it is much more complex than drawing a contour around a location. These ranges are designed to provide an understandable way in which a wider audience can appreciate the potential spatial extent of the impact.

7.1.2.    Geophysical Surveys

146. Geophysical surveying includes many sonar based operations and the resulting injury and disturbance ranges for marine mammals are presented in Table 7.1   Open ▸ , based on a comparison to the non-impulsive thresholds set out in Southall et al. (2019). Table 7.2   Open ▸ presents the results for geotechnical investigations. CPT injury ranges are based on a comparison to the Southall et al. (2019) thresholds for impulsive sound (with the peak injury range presented in brackets if exceeded) whereas borehole drilling and vibro-core results are compared against the non-impulsive thresholds. Borehole drilling source levels were reported as 142 dB to 145 dB re 1 µPa rms at 1 m, indicating little to no disturbance.
147. The potential impact distances from these operations vary based on their frequencies of operation and source levels and are rounded to the nearest 5 m. It should be noted that, for the sonar-based surveys, many of the injury ranges are limited to approximately 65 m as this is the approximate water depth in the area. Sonar based systems have very strong directivity which effectively means that there is only potential for injury when a marine mammal is directly underneath the sound source. Once the animal moves outside of the main beam, there is no potential for injury. The same is true in many cases for TTS where an animal is only exposed to enough energy to cause TTS when inside the direct beam of the sonar. For this reason, many of the TTS and PTS ranges are similar (i.e. limited by the depth of the water).

Table 7.1: Potential Impact Ranges (m) for Marine Mammals During the Various Geophysical Investigation Activities Based on Comparison to Southall et al. (2019) SEL Thresholds

Table 7.2: Potential Impact Ranges for Geotechnical Site Investigation Activites Based on Comparison to Southall et al. (2019) SEL Thresholds (Comparison to Ranges for Peak SPL Where Threshold was Exceeded Shown in Brackets)

7.1.3.    Vessels

148. The potential impact ranges for vessels are included in section 7.4, which summarises the vessel modelling results for all phases of the development.

7.1.4.    UXO Clearance

Low order disposal

149. The Applicant has committed to using low order techniques for UXO clearance. The predicted injury ranges for low order disposal are presented in Table 7.3   Open ▸ and Table 7.4   Open ▸ whereas the predicted ranges for the clearance shot to remove any residual explosive material from the seabed are shown in Table 7.5   Open ▸ and Table 7.6   Open ▸ .
150. All UXO injury and disturbance ranges are based on a comparison to the relevant impulsive sound thresholds as set out in section 4.

Table 7.3: Injury Ranges for Marine Mammals due to Detonation of 0.08 kg Donor Charge (Low Order Disposal)

Table 7.4: Injury Ranges for Fish due to Detonation of 0.08 kg Donor Charge (Low Order Disposal)

Table 7.5: Injury Ranges for Marine Mammals due to Detonation of 0.5 kg Clearance Shot

Table 7.6:  Injury Ranges for Fish due to Detonation of 0.5 kg Clearance Shot

Detonation – high order disposal

151. There is a small chance that the use of low order techniques (such as deflagration) could end up resulting in a high order detonation event. The predicted injury ranges for marine mammals and fish are shown in Table 7.7   Open ▸ and Table 7.8   Open ▸ for a realistic worst case 300 kg UXO detonation. It should be noted that, due to a combination of dispersion (i.e. where the waveform elongates), multiple reflections from the sea surface and bottom and molecular absorption of high frequency energy, the sound is unlikely to still be impulsive in character once it has propagated more than a few kilometres. Consequently, great caution should be used when interpreting any results with predicted injury ranges in the order of tens of kilometres. Furthermore, the modelling assumes that the UXO acts like a charge suspended in open water whereas in reality it is likely to be partially buried in the sediment. In addition, it is possible that the explosive material will have deteriorated over time meaning that the predicted noise levels are likely to be over-estimated. In combination, these factors mean that the results should be treated as precautionary potential impact ranges which are likely to be significantly lower than predicted.

Table 7.7: Potential Injury Ranges for Marine Mammals due to High Order Detonation of 300 kg UXO

Table 7.8: Potential Injury Ranges for Fish due to High Order Detonation of 300 kg UXO

7.2. Construction Phase

152. The construction phase of operations contains some of the loudest possible types of noise sources (including impact piling operations) and a range of vessels to support these activities.
153. Results are provided for piling of wind turbine foundations including maximum energy piling, a realistic maximum energy piling and piling of OSP/Offshore convertor station platform foundations, simultaneous pile installation by two vessels of wind turbine foundations at maximum energy piling and realistic maximum energy, and construction vessel noise (see section 5.7).

7.2.2.    Impact Piling

154. The impact piling scenarios modelled were as follows:

• single piling rig – Wind turbine foundations - Maximum design scenario (4,000 kJ);
• single piling rig – Wind turbine foundations - Realistic design scenario (3,000 kJ);
• single piling rig - OSP/Offshore convertor station platform foundations - Maximum design scenario (4,000 kJ);
• two rigs concurrent piling – Wind turbine foundations - Maximum design scenario (4,000 kJ); and
• two rigs concurrent piling – Wind turbine foundations - Realistic design scenario (3,000 kJ).
  155. As described in section 5.4.1, all source sound levels have been calculated based on a conversion factor of 4% reducing to 0.5%.
  156. There is a possibility that during the piling operations it will be necessary for two pile installation vessels to operate concurrently. For the concurrent piling scenarios, two separate maximum adverse case assumptions were identified, as follows:
• separation distance of 1 km (the minimum distance between foundations) as a maximum adverse scenario for injury; and
• separation distance of 42.9 km as a maximum adverse scenario for disturbance.
  157. The reason the maximum adverse scenario assumptions for injury and disturbance differ is that the scenario which results in the greatest potential for injury is when two rigs are operating in close proximity, meaning that the animal is exposed to sound from both rigs at relatively high levels. Conversely, the maximum area of disturbance occurs when both rigs are operating at a further distance apart in the AfL and their disturbance ranges are just overlapping. For the latter case, the maximum adverse scenario is not necessarily the greatest possible separation distance and piles wind turbine 1 and wind turbine 179 and piles wind turbine 40 and wind turbine 135 were chosen as representative as the combined maximum adverse scenario in terms of separation distance and bathymetry.
  158. All impact piling injury and disturbance ranges are based on a comparison to the relevant impulsive sound thresholds as set out in section 4.
  159. The injury ranges for peak sound pressure are based on the first strike the animal experiences at the closest point during each phase of the pile installation. Consequently, the peak pressure ranges for simultaneous piling do not differ from the peak injury ranges identified for single rigs.
  160. Injury ranges for marine mammals due to impact pile driving for the “realistic” and “maximum” pile driving schedule for the 24 MW option, and for the piling of the OSP/Offshore convertor station platform are summarised in Table 7.9   Open ▸ .
  161. During impact piling the interaction with the seafloor and the water column is complex. In these cases, a combination of dispersion (i.e. where the waveform shape elongates), and multiple reflections from the sea surface and bottom and molecular absorption of high frequency energy, the sound will lose its impulsive shape after some distance (generally in order of several kilometres).
  162. A recent article by Southall (2021) discusses this aspect in detail, and notes that “…when onset criteria levels were applied to relatively high-intensity impulsive sources (e.g. pile driving), TTS onset was predicted in some instances at ranges of tens of kilometers from the sources. In reality, acoustic propagation over such ranges transforms impulsive characteristics in time and frequency (see Hastie et al., 2019; Amaral et al., 2020; Martin et al., 2020). Changes to received signals include less rapid signal onset, longer total duration, reduced crest factor, reduced kurtosis, and narrower bandwidth (reduced high-frequency content). A better means of accounting for these changes can avoid overly precautionary conclusions, although how to do so is proving vexing”. The point is reenforced later in the discussion which points out that “…it should be recognized that the use of impulsive exposure criteria for receivers at greater ranges (tens of kilometers) is almost certainly an overly precautionary interpretation of existing criteria”.
  163. Consequently, great caution should be used when interpreting any results with predicted injury ranges in the order of tens of kilometres. Further discussion on this topic is provided in the technical note in volume 3, appendix 10.1, annex D.

Table 7.9: Injury and Disturbance Ranges Based on the Cumulative SEL Metric for Marine Mammals due to Impact Pile Driving for the “Realistic” and “Maximum” Pile Driving for Wind Turbine Jacket Foundations, and for the Piling of the OSP/Offshore Convertor Station Platform Jackets

164. All ranges presented are the 95th percentile across the bearings modelled. These results are identical to the given accuracy for the “maximum” and the “realistic” pile driving schemes. The schemes only differ by the final full power section, which is one hour longer in the “maximum” scenario. This identical result is due to the flight model, with the pile driving period being six hours after the start of piling.
165. The injury ranges for marine mammals based on peak pressure are summarised in Table 7.10   Open ▸ . These ranges represent the potential zone for instantaneous injury. The injury ranges are based on the first strike the animal experiences, and is therefore highly dependent upon the hammer energy, but independent of piling duration. As such, the ranges are presented for both PTS and TTS by each stage of the piling, but as piling energies are consistent across all three scenarios (wind turbine maximum energy, realistic energy and OSP/Offshore convertor station platform) the peak pressure ranges will also be the same across all three scenarios. It is therefore assumed that, although the piling and full power piling phases have larger injury ranges, the animal would have moved out of the ranges at the time those hammer energies are used. It should be noted that the peak SPLs were calculated on a 20 m grid spacing meaning that the results are presented to the nearest 20 m. Since the reported distance is the first “bin” where the peak SPL is below the threshold, any ranges of 20 m are in reality likely to be lower than this and possibly not exceeded. In this respect it is important to understand that a pile is a large and distributed source and therefore reporting injury ranges that are smaller than the physical size of the pile based on a point source sound level assumption (i.e. assumption of an infinitesimally small source size) could result in an overestimation of injury range.

Table 7.10: Summary of Peak Pressure Injury Ranges for Marine Mammals due to the Phase of Impact Piling Resulting in the Maximum Peak Sound Pressure Level, for Wind Turbine Foundations (“Maximum” and “Realistic” Scenarios) and OSP/Offshore Convertor Station Platform Foundations

166. The results of the noise modelling for fish and turtles are shown in Table 7.11   Open ▸ based on the cumulative sound exposure level thresholds, and in Table 7.12   Open ▸ based on the peak sound pressure thresholds. The tables show two results for Group 1 Fish, one based on the 0.5 m/s and another (in square brackets) showing the range for basking sharks using a higher swim speed of 1 m/s. Similarly, sea turtles have been assumed to swim at a speed of 0.5 m/s whereas fish eggs and larvae have been assumed to be static, resulting in a different potential impact range to reach the same numerical SEL criteria.

Table 7.11: Injury Ranges for Fish Based on the Cumulative SEL Metric due to Impact Pile Driving for the “Realistic” and “Maximum” Pile Driving for Wind Turbine Jacket Foundations, and for the Piling of the OSP/Offshore Convertor Station Platform Jackets based on the Cumulative SEL Metric

Table 7.12: Summary of Peak Pressure Injury Ranges for Fish due to the Phase of Impact Piling Resulting in the Maximum Peak Sound Pressure Level, for Both Wind Turbine Foundations and OSP/Offshore Convertor Station Platform Foundations Based on the Peak Pressure Metric

167. The disturbance range for fish, given by the 150 dB re 1 µPa SPLrms contour is 17 km for single pile driving.
168. The maximum design scenario was also modelled with the use of and ADD for 30 minutes prior to commencement of piling, the results of which are provided in Table 7.13   Open ▸ for cumulative SEL, and in Table 7.14   Open ▸ for peak sound level.

Table 7.13: Injury Ranges Based on the Cumulative SEL Metric for Marine Mammals due to Impact Pile Driving for the “Maximum” Pile Driving for Wind Turbine Jacket Foundations with and without 30 Minutes of ADD

169. As can be seen from Table 7.13   Open ▸ , the use of an ADD is effective in reducing all PTS injury ranges to a level not exceeding the thresholds, but has less beneficial benefit in reducing potential TTS ranges. This is because for the longer ranges associated with TTS, the distance the animal can swim during the 30 minutes of ADD activation is small compared to the overall potential range of TTS from the piling location.
170. The potential injury ranges due to the peak sound level metric will remain the same regardless of whether an ADD is used. However, if the animal is able to swim outside of the peak injury range during the period of ADD activation, the peak thresholds will not be exceeded.

Table 7.14: Summary of Peak Pressure Injury Ranges for Marine Mammals due to Each Phase of Impact Piling for Wind Turbine Foundations “Maximum” Scenario: Showing Whether the Key Mammal Species can Flee the Injury Range During the Period of ADD

171. From Table 7.14   Open ▸ it can be seen that although the peak injury ranges do not change due to the use of an ADD, the use does give animals time to swim out with the potential injury range prior to the commencement of piling.

Noise contours

172. The potential noise contours (every 5 dB) for single strike SEL for a single piling event at all locations shown in Figure 6.2   Open ▸ are provided in volume 3, appendix 10.1, annex E.
173. The areas contained within the 140 and 160 dB re 1 µPa (rms) contours (equating to the NMFS mild and strong disturbance ranges) are shown in Table 7.15   Open ▸ .

Table 7.15: Disturbance Areas for Marine Mammals due to Impact Pile Driving at One Location

Two piling vessels operating concurrently

174. There is a possibility that during the piling operations it will be necessary for two pile installation vessels to operate concurrently. The potential cumulative SEL injury ranges for marine mammals due to impact pile driving for the “realistic” and “maximum” pile driving schedule are summarised in Table 7.16   Open ▸ , along with the mild and strong disturbance ranges based on rms sound pressure levels. Here the piles are modelled as following the same piling plans with all phases starting at the same time. For injury a worse case is considered to be that of two adjacent piles, separated by a distance of 2.2 km due to the maximal overlap of sound propagation contours leading to the maximum generated sound levels. Conversely, for disturbance the maximum separation between two piling locations would lead to the larger area ensonified at any one time and therefore the greatest disturbance.

Table 7.16: Injury and Disturbance Ranges Based on the Cumulative SEL Metric for Marine Mammals due to Impact Pile Driving at Two Locations Concurrently, for the “Realistic” and “Maximum” Pile Driving for Wind Turbine Jacket Foundations

175. The ranges for mortality and recoverable injury for the groups of fish and sea turtles do not change between single and double pile driving. This is because the injury range at distances close to the pile are dominated by energy from the closest pile, with little influence from the pile which is further away. For example, if the pulse SEL due to the nearest pile was 200 dB re 1 µPa2s and the SEL from the further pile was 190 dB re 1 µPa2s (i.e. 10 dB difference), then the cumulative SEL from both piles would only be 200.4 dB re 1 µPa2s. If the difference between SEL levels was 20 dB then the cumulative SEL of both pulses would only be 200.04 dB re 1 µPa2s. Consequently, it is only for injury ranges which are of a similar or greater magnitude to the separation distance between piles that a significant change in injury range will be derived for the simultaneous piling scenario. It should be noted that this assumes that the animal does not swim directly towards the furthest pile in order to end up at a close range to that pile having left the injury range of the original pile.
176. The TTS range for fish and basking sharks for simultaneous pile driving for close foundations is given in Table 7.17   Open ▸ . The disturbance range for fish, given by the 150 dB re 1 µPa SPL RMS contour is 23 km for simultaneous pile driving.

Table 7.17: TTS Injury Ranges based on the Cumulative SEL Metric for Fish due to Impact Pile Driving at Two Locations Concurrently, for the “Realistic” And “Maximum” Pile Driving for Wind Turbine Jacket Foundations Based on the Cumulative SEL Metric

177. Contours for single strike SEL for simultaneous piling at two piling locations (adjacent wind turbines 1 and 179 and wind turbines 40 and 135) are provided in volume 3, appendix 10.1, annex F.
178. The maximum design scenario was also modelled for concurrent piling in the presence of 30 minutes of ADD use, the results of which are shown in Table 7.18   Open ▸ for the potential impact of cumulative SEL. The potential impact of peak sound level on marine mammals will remain the same as the single piling case, as it did for the scenarios without ADD.

Table 7.18: Injury Ranges for Marine Mammals due to Impact Pile Driving at Two Locations Concurrently for the “Maximum” Pile Driving for Wind Turbine Jacket Foundations with and Without 30 Minutes of ADD

179. As can be seen from Table 7.13   Open ▸ , the use of ADD is useful for reducing all PTS injury ranges to a level not exceeding the thresholds, but has little impact on many of the TTS levels. This is because for these long distances the distance the mammal can swim during these 30 minutes is short compared to the overall distance from the piling.