7.2.3.    Drilled Piling

  1. The potential impact ranges for drilled piling are small (or not exceeded) for all marine mammal species groups, due to the low broadband SEL levels expected from these operations, at 160 dB re 1 µPa2s (see Table 7.19   Open ▸ ). The behavioural threshold range for all marine mammal groups is also report.

 

Table 7.19:
Potential Impact Ranges (m) for Marine Mammal Exposed to Drilled Piling

Table 7.19: Potential Impact Ranges (m) for Marine Mammal Exposed to Drilled Piling

 

  1. The ranges for recoverable injury and TTS for Group 3 and 4 Fish are presented in Table 7.20   Open ▸ based on the thresholds contained in Popper et al. (2014). It should be noted that fish would need to be exposed within these potential impact ranges for a period of 48 hours continuously in the case of recoverable injury and 12 hours continuously in the case of TTS for the effect to occur. It is therefore considered that these ranges are highly precautionary, and injury is unlikely to occur in reality.

 

Table 7.20:
Median Potential Impact Ranges (m) for Group 3 and 4 Fish Exposed to Drilled Piling

Table 7.20: Median Potential Impact Ranges (m) for Group 3 and 4 Fish Exposed to Drilled Piling

 

7.2.4.    Other Operations

  1. The potential impact ranges from other construction related activities (such as cable trenching, cable laying and supporting jack-up rigs) on different marine mammal groups are presented in Table 7.21   Open ▸ . The potential impact ranges for fish are presented in Table 7.22   Open ▸ .

 

Table 7.21:
Potential Impact Ranges (m) for Marine Mammals During other Construction Related Operations

Table 7.21: Potential Impact Ranges (m) for Marine Mammals During other Construction Related Operations

 

Table 7.22:
Median Potential Impact Ranges (m) for Group 3 and 4 Fish Exposed to Other Construction Related Operations

Table 7.22: Median Potential Impact Ranges (m) for Group 3 and 4 Fish Exposed to Other Construction Related Operations

 

7.2.5.    Vessels

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

7.3. Operation and Maintenance Phase

  1. The primary sources of underwater sound during the operation and maintenance phase of an offshore wind farm are vibration of the wind turbine’s gear box and generator, and vessel noise associated with operation and maintenance activities.
  2. Vibration of the wind turbine’s gear box and generator is transmitted down the tower and radiated as sound from the tower wall. Sound radiation by surface waves is difficult to quantitatively predict, in particular for the boundary regions, and is highly dependent upon the conditions of both the wind turbine itself, including generator and tower condition, and on the seawater conditions. There have been few empirical investigations of operational offshore wind farms, and as such measurement data is also scarce.
  3. The distances and exposures of mammals and fish reported by studies that investigate the potential impact of operational offshore wind farms present a range of values, but the majority conclude that in the order of hundreds of metres distance from the wind turbines, sound levels would likely be audible but not at a level sufficient to cause injury or behavioural changes (Betke, 2006; Nedwell et al., 2007; Norro, et al., 2011; Ward, et al., 2006; Jansen, 2016). Norro et al. (2011) compared measurements of a range of different foundation types and wind turbine ratings in the Belgian part of the North Sea, as well as comparing those to other European waters. A summary of these studies is shown in Table 7.23   Open ▸ . The authors found a slight increase in SPL compared to the ambient noise measured before the construction of the wind farms. They concluded that even the highest increases found within the dataset (20 to 25 dB re 1µ Pa) are unlikely to cause a significant potential impact and are significantly lower than those during the construction phase. They do however caution that this noise is of a much longer duration over the operational lifespan of the wind farm, and that little is known of the potential long-term impacts to aquatic life.

 

Table 7.23:
Desktop Study of Operational Noise from Wind Turbines

Table 7.23: Desktop Study of Operational Noise from Wind Turbines

 

  1. The potential impact ranges for the maintenance noise source are reported in Table 7.24   Open ▸ and Table 7.25   Open ▸ .

 

Table 7.24:
Potential Impact Ranges (m) for Marine Mammal Groups from other Maintenance Operations

Table 7.24: Potential Impact Ranges (m) for Marine Mammal Groups from other Maintenance Operations

Table 7.25:
Median Potential Impact Ranges (m) for Groups 3 and 4 Fish

 

Table 7.25   Open ▸ : Median Potential Impact Ranges (m) for Groups 3 and 4 Fish

 

7.3.2.    Vessels

  1. Vessels employed during the operation and maintenance phase are likely to be similar in size and noise signature to those employed in the construction phase. This includes for operations such as jack-up vessels, cable installation (repair) vessels and Crew Transfer Vessels (CTVs). Jack-up vessels and cable repair vessels will be used to facilitate any component replacement works or cable repair/remediation works. CTVs are likely to be required on a day to day basis for routine inspection and maintenance activities. Vessel noise associated with operation and maintenance activities is likely to similar in nature to activities at other parts of the survey.
  2. The potential impact ranges for vessels are included in section 7.4, which summarises the vessel modelling results for all phases of the development.

7.4. Vessel Noise

  1. Estimated ranges for injury to marine mammals due to the continuous noise sources (vessels) during different phases of the construction operations are presented below.
  2. It should be borne in mind that there is a considerable degree of uncertainty and variability in the onset of disturbance and therefore any disturbance ranges should be treated as potentially over precautionary. Another important consideration is that vessels and construction noise will be temporary and transitory, as opposed to permanent and fixed. In this respect, construction noise is unlikely to differ significantly from vessel traffic already in the area.
  3. The estimated median ranges for onset of TTS or PTS for different marine mammal groups exposure to different noise characteristics of different vessel traffic are shown in Table 7.26   Open ▸ . The exposure metrics for different marine mammal and flee speeds (as detailed in section 6.4) were employed.

 

Table 7.26:
Estimated PTS and TTS Ranges from Different Vessels for Marine Mammals

Table 7.26: Estimated PTS and TTS Ranges from Different Vessels for Marine Mammals

 

  1. The ranges for recoverable injury and TTS for Groups 3 and 4 Fish are presented in Table 7.27   Open ▸ based on the thresholds contained in Popper et al. (2014). It should be noted that fish would need to be exposed within these potential impact ranges for a period of 48 hours continuously in the case of recoverable injury and 12 hours continuously in the case of TTS for the effect to occur. It is therefore considered that these ranges are highly precautionary, and injury is unlikely to occur in reality.

 

Table 7.27:
Estimated Recoverable Injury and TTS Ranges from Vessels for Groups 3 and 4 Fish

Table 7.27: Estimated Recoverable Injury and TTS Ranges from Vessels for Groups 3 and 4 Fish

 

8. Summary

  1. Noise modelling has been undertaken to determine the range of potential effects on marine mammals, fish, and sea turtles due to noise from piling activities associated with construction of the Proposed Development. The results are summarised in Table 8.1   Open ▸ which shows the maximum injury range for each group of mammals, fish, and sea turtles, for individual and simultaneous piling (the worst-case scenario of cumulative SEL or peak). The potential PTS impact range is typically dominated by nearest pile, so these ranges don’t change for single or simultaneous pile driving (except for LF cetaceans).

 

Table 8.1:
Summary of Maximum PTS Injury Ranges for Marine Mammals, and Mortality for Fish, and Turtles due to Impact Piling of a Single Pile Based on Highest Range of Peak Pressure or SEL (N/E = Threshold Not Exceeded)

Table 8.1: Summary of Maximum PTS Injury Ranges for Marine Mammals, and Mortality for Fish, and Turtles due to Impact Piling of a Single Pile Based on Highest Range of Peak Pressure or SEL (N/E = Threshold Not Exceeded)

  1. Underwater noise emissions from the wind turbines, other relevant operational noises, and vessels during the operation and maintenance phase are unlikely to be at a level sufficient to cause injury or behavioural changes to marine mammals, fish, or sea turtles.
  2. The use of ADD means that no PTS injury thresholds are exceeded for marine mammals.

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[1] Historically, use was primarily made of rms and peak SPL metrics for assessing the potential effects of sound on marine life. However, the SEL is increasingly being used as it allows exposure duration and the effect of exposure to multiple events to be considered. 

[2] It is worth noting that hearing thresholds are sometimes shown as audiograms with sound level on the y axis rather than sensitivity, resulting in the graph shape being the inverse of the graph shown

[3] Based on an analysis of the time history graph in Lucke et al. (2007), the T90 period is estimated to be approximately 8 ms, resulting in a correction of 21 dB applied to the SEL to derive the rmsT90 sound pressure level. However, the T90 was not directly reported in the paper.

[4] Guideline exposure criteria for seismic surveys, continuous sound and naval sonar are also presented though are not applicable to the Proposed Development.

[5] It should be noted that the presence of a swim bladder does not necessarily mean that the fish can detect pressure. Some fish have swim bladders that are not involved in the hearing mechanism and can only detect particle motion.

[6] Acoustically, shallow water conditions exist whenever the propagation is characterised by multiple reflections with both the sea surface and bottom (Etter, 2013).Consequently, the depth at which water can be classified as acoustically deep or shallow depends upon numerous factors including the sound speed gradient, water depth, frequency of the sound and distance between the source and receiver.

[7] This is a similar approach to that adopted for airborne noise where a typical worst case is taken, though it is known that day to day levels may vary to those calculated by 5 to 10 dB depending on wind direction etc.