5. Source Noise Levels

5.1. General

55. Underwater noise sources are usually quantified in dB scale with values generally referenced to 1 μPa pressure amplitude as if measured at a hypothetical distance of 1 m from the source (called the Source Level). In practice, it is not usually possible to measure at 1 m from a source, but the metric allows comparison and reporting of different source levels on a like-for-like basis. In reality, for a large sound source this imagined point at 1 m from the acoustic centre does not exist. Furthermore, the energy is distributed across the source and does not all emanate from this imagined acoustic centre point. Therefore, the stated sound pressure level at 1 m does not occur for large sources. In the acoustic near field (i.e. close to the source), the sound pressure level will be significantly lower than the value predicted by the SL.
56. A wealth of experimental data and literature-based information is available for quantifying the noise emission from different construction operations. This information, which allows us to predict with a good degree of accuracy the sound generated by a noise source at discrete frequencies in one-third octave bands, will be employed to characterise their acoustic emission in the underwater environment. Sections 5.2 to 5.7 will detail the types of noise sources present during different parts of the construction activities, their potential signatures in different frequency bands, and acoustic levels.

5.2. Types of Noise Sources

57. The noise sources and activities which were investigated during the underwater noise assessment study are summarised in Table 5.1   Open ▸ .

Table 5.1: Summary of Noise Sources and Activities Included in the Underwater Noise Assessment

58. The above sources for each project phase are considered in more detail in the following sections.

5.3. Pre-Construction Phase

5.3.1.    Geophysical Surveys

59. It is understood that several sonar based survey types will potentially be used for the geophysical surveys. Sound source data for the types of equipment likely to be used has been provided by the Applicant.
60. During the survey a transmitter emits an acoustic signal directly toward the seabed (or alongside, at an angle to the seabed, in the case of side scan techniques). The equipment likely to be used can typically work at a range of signal frequencies, depending on the distance to the bottom and the required resolution. The signal is highly directional and acts as a beam, with the energy narrowly concentrated within a few degrees of the direction in which it is aimed. The signal is emitted in pulses, the length of which can be varied as per the survey requirements. The assumed pulse rate, pulse width and beam width used in the assessment are based on a review of typical units used in other similar surveys. It should be noted that sonar based survey sources are classed as non-impulsive sound because they generally compromise a single (or multiple discrete) frequency (e.g. a sine wave or swept sine wave) as opposed to a broadband signal with high kurtosis, high peak pressures and rapid rise times.
61. The characteristics assumed for each device modelled in this assessment are summarised in Table 5.2   Open ▸ . For the purpose of potential impacts, these sources are considered to be continuous (non-impulsive).

Table 5.2: Sonar Based Survey Equipment Parameters Used in Assessment

62. The assumed pulse rate has been used to calculate the SEL, which is normalised to one second, from the rms sound pressure level. Directivity corrections were calculated based on the transducer dimensions and ping frequency and taken from manufacturer’s datasheets. It is important to note that directivity will vary significantly with frequency, but that these directivity values have been used in line with the modelling assumptions stated above.
63. Unlike the sonar-based surveys, the UHRS is likely to utilise a sparker, which produces an impulsive, broadband source signal. The parameters used in the noise modelling are summarised in Table 5.3   Open ▸ .

Table 5.3: UHRS Survey Equipment Parameters Used in Assessment

5.3.2.    Geotechnical Surveys

64. Source noise data for the proposed CPTs was reported by Erbe and McPherson (2017). In this report, the SEL measurements at two different sites in Western Australia at a measured distance of 10 m were presented. The signature is generally broadband in nature with levels measured generally 20 dB above the acoustic ocean noise floor. The report also mentions other paths for acoustic energy including direct air to water transmission and other multipath directions, which implied that measured sound level is strongly dependant on depth and range from the source. The third octave band SEL levels from the CPT extracted are presented in Table 5.4   Open ▸ . For the purpose of potential impacts, these sources are considered as continuous (non-impulsive) sounds.

Table 5.4: CPT Source Levels in Different Octave Band Frequencies (SEL metric) Used for the Assessment (Erbe and McPherson, 2017)

65. CPT noise is classified as impulsive at source since it has a rapid rise time and a high peak level of 220 dB re 1 µPa (pk).
66. The seismic CPT test is typically conducted at various depths for each location every three to five minutes with between 10 and 20 strikes per depth.
67. Measurements of a vibro-core test (Reiser et al., 2011) show underwater source sound pressure levels of approximately 187 dB re 1 µPa re 1 m (rms). The SEL has been calculated based on a one hour sample time which, it is understood, is the typical maximum time required for each sample. The vessel would then move on to the next location and take the next sample with approximately one-hour break between each operation. The vibro-core sound is considered to be continuous (non-impulsive).

Table 5.5: Vibro-Core Source Levels Used in the Assessment

68. The frequency spectral shape for vibro-coring is presented in Figure 5.1   Open ▸

Figure 5.1: Frequency Spectral Shape Used for Vibro-Coring

69. Source levels for borehole drilling ahead of standard penetration testing was reported in Erbe and McPherson (2017), with source levels of 142 dB to 145 dB re 1 µPa re 1 m (rms). A set of one third octave band levels, calculated from the spectrum presented in the paper are shown in Figure 5.2   Open ▸ .

Figure 5.2: Borehole Drilling Source Level Spectrum Shape Used in the Assessment

70. As for other non-impulsive sources, impact assessment criteria is the SEL metric applied to a fleeing target.

5.3.3.    Vessels

71. Vessels are dealt with in section 5.7 for all phases of the project.

5.3.4.    UXO Clearance

72. The precise details and locations of potential UXOs is unknown at this time. For the purposes of this assessment, it has been assumed that the maximum realistic worst case will be 300 kg.
73. The Applicant has committed to the use of low order techniques (subsonic combustion) as the intended methodology for clearance of UXO. For example, one such technique (deflagration) uses a single charge of 30 g to 80 g Net Explosive Quantity (NEQ) which is placed in close proximity to the UXO to target a specific entry point. When detonated, a shaped charge penetrates the casing of the UXO to introduce a small, clinical plasma jet into the main explosive filling. The intention is to excite the explosive molecules within the main filling to generate enough pressure to burst the UXO casing, producing a deflagration of the main filling and neutralising the UXO.
74. Recent controlled experiments showed low order deflagration to result in a substantial reduction in acoustic output over traditional high order methods, with SPLpk and SEL being typically significantly lower for the deflagration of the same size munition, and with the acoustic output being proportional to the size of the shaped charge, rather than the size of the UXO itself (Robinson et al., 2020). Using this low order deflagration method, the probability of a low order outcome is high; however, there is a small inherent risk with these clearance methods that the UXO will detonate or deflagrate violently.
75. It is possible that there will be residual explosive material remaining on the seabed following the use of low order techniques for unexploded ordnance disposal. In this case, recovery will be performed as outlined in paragraph 73, including the potential need of a small (500 g) ‘clearing shot’.
76. The noise modelling has been undertaken for a range of donor charge configurations as set out in Table 5.6   Open ▸ . In addition, the noise modelling investigated the potential range of effects for an accidental high order detonation based on a realistic maximum scenario UXO size of 300 kg.

Table 5.6: Details of UXO and their Relevant Charge Sizes Employed for Modelling

77. The source levels for UXO are included within the terms for propagation modelling and are described in section 6.5.

5.4. Construction Phase

5.4.1.    Impact Piling

78. The sound generated and radiated by a pile as it is driven into the ground is complex, due to the many components which make up the generation and radiation mechanisms. However, a wealth of experimental data is available which allow us to predict with a good degree of accuracy the sound generated by a pile at discrete frequencies. Third octave band noise spectra have been presented in literature for various piling activities (e.g. Matuschek and Betke, 2009; De Jong and Ainslie, 2008; Wyatt, 2008; Nedwell et al., 2003; Nedwell and Edwards, 2004; Nedwell et al., 2007; CDoT, 2001; Nehls et al., 2007; Thomsen et al., 2006; Robinson et al., 2020; Lepper et al., 2009).
79. For the Proposed Development, the assessments have been carried out for the wind turbine installation of up to 5.5 m diameter piles with an average maximum hammer energy typically at 3,000 kJ (maximum spatial scenario), but also considering an absolute maximum hammer energy of 4,000 kJ. Assessments have been carried out for the offshore substation platform installation of up to 4 m diameter piles, with a maximum hammer energy up to 4,000 kJ.
80. The assessment has been carried out at two locations on opposite sides of the Proposed Development array area, chosen to represent extremes of location. The bathymetry of the site is relatively flat, therefore the two locations were selected to represent the points closest and furthest away from the shoreline. In this case, these are represented by the indicative wind turbine foundation locations (e.g. wind turbine 1 and wind turbine 179, or wind turbine 40 and wind turbine 135 ( Figure 6.2   Open ▸ ). Results are therefore presented as a range from smallest to largest ranges of potential impact.

81. Using the equation below (De Jong and Ainslie, 2008), a broadband source level value is evaluated for the noise emitted during impact pile driving operation in each operation window.

SEL = .

82. In this equation, β is the energy transmitted from the pile into the water column, E is the hammer energy employed in joules, C0 is the speed of sound in the water column, and ρ is the density of the water. From the SEL result calculated using the equation above, source level spectra can also be calculated for different third octave frequency bands.
83. The assumption used for the modelling is that the amount of sound radiated into the water column depends on both the hammer energy and the length of pile exposed above the seabed in the water column. During the Marine Mammal Road Map pre-Application consultation process, the validity of different conversion factors relating to how energy is converted between hammer energy (kinetic) into sound energy was discussed with key stakeholders (NatureScot, Marine Scotland Licensing and Operations Team (MS-LOT) and Marine Scotland Science (MSS), and it was agreed that a range of values should be investigated with a robust justification for the conversion factor that would be taken forward to the full Marine Mammal Impact Assessment. The underwater noise modelling study therefore investigated the following:

• a constant conversion factor of 1% (a conservative value that was evaluated for Seagreen alongside the 0.5% conversion factor applied to the assessment);
• a reducing conversion factor commencing at 10% reducing to 1% (with the starting value derived from a study by Thompson et al., 2020); and
• a reducing conversion factor commencing at 4% reducing to 0.5% (with the starting value based on studies by Lippert et al., 2017).
  84. A comprehensive study evaluating the evidence and justification for different conversion factors was undertaken following advice received during the Marine Mammal Road Map pre-Application consultation process (volume 3, appendix 10.1, annex A). Consequently, a variable conversion factor (β) has been used ranging from β = 4% at the start of piling to β = 0.5% at the end of piling when the pile is almost fully embedded in the seabed. This scenario has been chosen as it was considered to represent the best balance of realism and precaution in conversion factor, particularly compared to a conversion factor of 10% reducing to 1% which was considered over-precautionary and therefore misrepresentative of the potential kinetic energy converted to sound energy. A 1% constant conversion factor was considered less representative compared to 4% reducing to 0.5% for a partially submersible hammer as would be used for the Proposed Development. Note, however, to adopt a precautionary assessment and to mitigate for uncertainties in the true value of the conversion factor the marine mammal EIA took forward the predicted ranges from either the 4% reducing conversion factor or 1% constant conversion factor, whichever led to the greatest ranges using the relevant noise thresholds for injury and disturbance.
  85. The justification for this assumption is provided in full in volume 3, appendix 10.1, annex A, and is summarised as follows:
• Measurements on piles using above water impact hammers show approximately linear SEL to hammer energy relationship (e.g. Bailey et al., 2010; Robinson et al., 2007; Robinson et al., 2009; Lepper et al., 2012; Robinson et al., 2013).
• Peer reviewed literature which considers theoretical concepts, concluded that a representative energy conversion factor is likely to be in the range β ≈ 0.3% to 1.5% (Zampolli et al., 2013), whilst Dahl et al. (2015) concluded that β ≈ 0.5% based on a review of both theoretical considerations and measurement data by others.
• The theoretical upper limit of the energy conversion factor is therefore approximately 1.5%, although this is only likely to apply when the hammer is operating at the lower end of its power rating, with lower conversion factors being more likely throughout the remainder of the piling period (that are subject to higher hammer energies). An average hammer energy conversion factor of β ≈ 1% is therefore concluded to be representative and precautionary across the range of hammer energies used during a pile installation using an above water hammer.
• Peer reviewed studies based on measurements on above water piling hammers determined real world energy conversion factors of β = 0.8% (De Jong and Ainslie, 2008) and β ≈ 1% (Dahl and Reinhall, 2013). However, use of a submersible hammer can result in the conversion factor varying depending on pile penetration depth.
• Both measurement data and detailed source modelling presented for a partially submersible hammer in Lippert et al. (2017) supports a varying conversion factor of between β ≈ 2% and 0.5% depending on penetration depth and the length of pile above water.
• Thompson et al. (2020), whilst ostensibly indicating conversion factors ranging between β ≈ 10% and 1% for a fully submersible hammer, is considered to be a considerable overestimate of the true energy radiated into the water column caused by discrepancies between the noise modelling and real world propagation. True conversion factors are thought likely to be in the order of half these values, or less, as demonstrated by differences in the conversion factors derived at different ranges from each pile (a full analysis and explanation for this is provided in volume 3, appendix 10.1, annex A).
• Of the above two studies, the Lippert et al. (2017) study is considered scientifically robust because of the very strong correlation between the detailed finite element modelling and measured data.
• It is recognised that for the Lippert et al. (2017) study a significant proportion of the pile was above water at the start of the piling sequence which could have reduced the apparent conversion factor compared to a situation where the pile starts just above the water line. Assuming that the energy radiated into the water is approximately proportional to the length of pile which is exposed to the water then the conversion factor at the start of piling in the Lippert et al. (2017) study can be estimated to be approximately 3.5%.
• For the Proposed Development, although no detailed piling methodology is available at the point of Application, it is considered likely that in the deepest waters, piling will commence just above (or just below) the water line and will finalise with pile penetration just above the seafloor, in water depths of up to 70 m. Consequently, and in light of MSS and NatureScot’s request, a conversion factor of β ≈ 4% has been used for the Proposed Development at the start of the piling sequence. This 4% conversion factor is higher than that derived in the Lippert et al. (2017) study and is therefore considered conservative.
• Furthermore, it should be noted that any piles installed in shallower waters within the Proposed Development array area, are likely to result in lower source levels than derived for the start of piling in deep waters as a significant proportion of the pile could penetrate above the water line, meaning much of the energy would radiate into the air rather than water therefore not affecting marine mammals under water.
• In Lippert et al. (2017) study, the piles remained approximately 17 m above the seabed floor at the end of the piling sequence which means that the β ≈ 1% conversion factor at the end of the piling sequence is likely to be an overestimate compared to the Proposed Development case where a greater proportion of the pile will penetrate the seabed. Since the final pile position in the Lippert et al. (2017) study was a little below mid-water depth (and since, when the pile is subsea, the fall-off in acoustic energy cited by Lippert et al. (2017) is ~2.5 dB per halving of exposed pile above the seabed) this infers a final conversion factor of 0.5% or less at the end of piling.
• Consequently, based on this review, the assumption that piling is likely to use a submersible hammer for the majority of the piling operation, best available scientific evidence, professional judgement, and taking into account the advice of MSS and NatureScot, it is proposed to utilise a varying energy conversion factor of β = 4% at the start of piling to 0.5% at the end of piling for underwater noise modelling at the Proposed Development.
  86. A review of hammer energy conversion factors and further justification of the energy conversion factor assumptions is provided in volume 3, appendix 10.1, annex A. In addition, a sensitivity analysis using the three different hammer energy conversion factors (i.e. 4% reducing to 0.5%, 1% constant throughout the piling period, and 10% reducing to 1%) is provided in volume 3, appendix 10.1, annex B. Furthermore, the various inputs and assumptions used in the modelling, including the conversion factors used to derive the source levels, has been subjected to an independent peer review which is provided in volume 3, appendix 10.1, annex H.
  87. Figure 5.3   Open ▸ shows that due to the use of a reducing conversion factor, the highest sound exposure level no longer occurs during the period of maximum hammer energy (full power piling), but occurs during the period of piling that the maximum conversion factor is applied to (e.g. at the start of the piling sequence). This is consistent with the measurements taken by Lippert et al. (2017). Figure 5.4   Open ▸ shows the hammer energy over the same time period, for comparison, and illustrates the impact of conversion factor on the resultant noise level.

Figure 5.3: Representation of the Relationship Between the Varying Conversion Factor (4% to 0.5%) and SEL for the 4,000 kJ Scenario, Over the Piling Sequence as Indicated

Figure 5.4: Hammer Energy Throughout the Piling Sequence, Maximum Design Scenario

88. The spectral distribution of the source SELs for impact piling were derived from the reference spectrum provided in De Jong and Ainslie (2008), reproduced in Figure 5.5   Open ▸ .

Figure 5.5: Impact Piling Source Frequency Distribution Used in the Assessment

89. The impact piling scenarios that have been modelled for the Proposed Development are:

• wind turbine foundations (Piled Jacket) Maximum design scenario–o - 24 MW wind turbines (largest wind turbine) using an absolute maximum hammer energy of 4,000 kJ for the longest possible duration (up to ten hours) (see Table 5.7   Open ▸ );
• wind turbine foundations (Piled Jacket) realistic design scenario–o - 24 MW (see Table 5.8   Open ▸ ). Table 5.8   Open ▸ is based on the realistic average maximum hammer energy of 3,000 kJ for a realistic maximum duration (up to nine hours) and is included to provide context to the maximum hammer energy scenario; and
• OSP/Offshore convertor station platform foundations (jacket) maximum design scenario – using a maximum hammer energy of 4,000 kJ for a duration of up to eight hours (see Table 5.9   Open ▸ ).

Table 5.7: Impact Piling Schedule Used in Assessment - Wind Turbine Foundations (Maximum Design Scenario)

Table 5.8: Impact Piling Schedule Used in Assessment – Wind Turbine Foundations (Realistic Design Scenario)

Table 5.9: Impact Piling Schedule Used in Assessment - OSP/Offshore Convertor Station Platform Foundations

90. The peak sound pressure level can be calculated from SEL values via the empirical fitting between pile driving SEL and peak SPL data, given in Lippert et al. (2015), as:

SPLpk =

91. Root mean square (rms) sound pressure levels were calculated assuming a typical T90 pulse duration (i.e. the period that contains 90% of the total cumulative sound energy) of 100 ms. It should be noted that in reality the rms T90 period will increase significantly with distance which means that any ranges based on rms sound pressure levels at ranges of more than a few kilometres are likely to be significant over estimates and should therefore be treated as highly conservative.
92. The piling of wind turbine foundations described in Table 5.8   Open ▸ was also modelled with the inclusion of an acoustic deterrant device (ADD) before commencement of piling. Use of an ADD was modelled for a duration of 30 minutes prior to commencement ofpiling, all other stages of piling remained the same, and the ADD itself was assumed to not contribute towards any animal injury.

5.4.2.    Drilled Piles

93. For drilled piling, source sound levels have been based on pile drilling for the Oyster 800 project (Kongsberg, 2011). The hydraulic rock breaking source sound levels are based on those measured by Lawrence (2016). The source levels used in the assessment are summarised in Table 5.10   Open ▸ .
94. Rotary drilling is non-impulsive in character and therefore the non-impulsive injury and behavioural thresholds have been adopted for the assessment.

Table 5.10: Drilled Pile Noise Source Levels Used in Assessment (Un-Weighted)

95. The other noise source potentially active during the construction phase are related to cable installation (i.e. trenching and cable laying activities), and their related operations such as the jack-up rigs. The SEL based source levels are presented in Table 5.11   Open ▸ .

Table 5.11: SEL Based Source Levels for Other Noise Sources

5.4.3.    Vessels

96. Use of vessels is addressed in section 5.7 for all phases of the Proposed Development.

5.5. Operation and Maintenance Phase

5.5.1.    Operation Noise from Wind Turbines

97. A review of publicly available information on the potential for operation wind turbines to produce noise has been undertaken and is presented in section 7.3.

5.5.2.    Geophysical Surveys

98. Routine geophysical surveys will be similar to the geophysical surveys already discussed for the pre-construction phase (see section 5.3).

5.5.3.    Routine Operation and Maintenance

99. There are very few activities during the operation and maintenance phase that generate significant amounts of underwater noise. The source level for the general operations carried out in the operation and maintenance phase such as the jet cutting operation, which is considered to be the activity with the highest sound level, is presented in Table 5.12   Open ▸ .

Table 5.12: SEL Based Octave Band Levels Used for Different Operations in this Phase

5.5.4.    Vessels

100. The potential for vessels use to create underwater noise is presented in section 5.7 for all phases of the Proposed Development.

5.6. Decommissioning Phase

5.6.1.    Vessels

101. As agreed with stakeholders during the pre-Application consultation phase, only the potential impact of noise from vessel activity has been scoped into the underwater noise assessment for the decommissioning phase of the Proposed Development. It should be noted that noise cavitation from the vessels themselves is likely to dominate the soundscape for other decommissioning activities (e.g. removal of subsea structures). The potential impact of vessels noise is addressed in section 5.7 for all phases of the Proposed Development.

5.7. Vessels (All Phases)

102. The noise emissions from the types of vessels that may be used for the Proposed Development are quantified in Table 5.13   Open ▸ , based on a review of publicly available data. Sound 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.
103. In Table 5.13   Open ▸ , a correction of +3 dB has been applied to the rms sound pressure level to estimate the likely peak sound pressure level. 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). 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 Proposed Development has been based on worst-case 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 was 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 5.13: Source Noise Data for Construction and Installation Vessels