5.2. Subsea Noise

5.2.1. Introduction

  1. This section of this Scoping Report presents the relevant subsea noise aspects to the Array and considers the scope of assessment of subsea noise producing activities associated with the construction, operation and maintenance, and decommissioning of the Array.
  2. Subsea noise assessment (primarily modelling) will form a technical appendix to the offshore EIA Report and which will be used to inform the impact assessment for the following receptor groups:
  • fish and shellfish ecology (section 6.2); and
  • marine mammals (section 6.3).
  1. Subsea noise and vibration sources during construction may include piling, hammering or drilling for the wind turbine moorings and anchoring systems and Offshore Substation Platform (OSP) foundations, and will include the use of barges and vessels, heavy machinery and generators on the vessels. Other mooring and anchoring systems considered within the Project Description (section 2), such as drag embedment or suction caisson anchors would not use percussive piling and are therefore not considered further within the subsea noise section. Subsea noise during operation could include noise transmitted into the water from aerodynamic noise, from wind turbine blades passing through the air via the air to water interface, and structure borne mechanical noise from the gearbox and generators of the turbines.
  2. A subsea noise modelling study will be undertaken to provide an assessment of the level of subsea noise generated from the Array area and will be provided as a technical appendix to support the relevant chapters of the Array EIA Report.

5.2.2. Study Area

  1. No separate study area has been outlined for subsea noise as this is defined by the receptors and discussed within the relevant topics listed in section 5.2.1.

5.2.3. Baseline Environment

  1. Background or “ambient” subsea sound is created by several natural sources, such as rain, breaking waves, wind at the surface, seismic sound, biological sound and thermal sound. Biological sources include marine mammals (using sound to communicate, build up an image of their environment and detect prey and predators) as well as certain fish and shrimp. Anthropogenic sources of sound in the marine environment include fishing boats, ships (non-impulsive), marine construction noise, seismic surveys and leisure activities (all could be either impulsive or non-impulsive), all of which add to ambient background sound. The sound can be either impulsive (pulsed) such as impact piling, or non-impulsive (continuous) such as ship engines, and the magnitude of the impact on marine life will depend heavily on these characteristics. Anthropogenic sound within the vicinity of the Array will arise primarily from shipping, the offshore oil and gas industry, subsea geophysical and geotechnical surveys and the offshore renewables industry.
  2. Historically, research relating to both physiological effects and behavioural disturbance of noise on marine receptors has typically been based on determining the absolute noise level for the onset of that effect (whether presented as a single onset threshold or a dose response/probabilistic function). Consequently, the available numerical criteria for assessing the effects of noise on marine mammals, fish and shellfish, tend to be based on the absolute noise criteria, rather than the difference between the baseline noise level and the noise being assessed (Southall et al., 2019). This is the approach that is proposed for the Array.
  3. Baseline noise levels vary significantly depending on multiple factors, such as seasonal variations and different sea states. Although there are some limited studies in relation to baseline ambient sound in UK waters (e.g. Merchant et al., 2016), lack of long term measurements/sound data is a widely recognised gap in knowledge in relation to general soundscape and potential effects of human activities on marine life. There is lack of available evidence-based studies on the effects of noise relative to background on marine receptors feeding into established thresholds for impact.
  4. A quantitative assessment of the magnitude or significance of the impact of underwater noise on marine mammals is based on noise modelling which utilises various data to predict sound propagation. It is a general approach to underwater noise modelling used to inform marine mammal impact assessments for offshore wind farm EIAs to obtain historic data from publicly available data sources. For example, given that marine mammals are highly mobile and wide-ranging species, utilising historic bathymetry data that cover a much larger area is likely to be more representative of the soundscape individuals are experiencing than using site-specific data that covers only a small portion of the animal’s range.
  5. Based on the information presented in paragraphs 251 to 253, existing and published noise data (where available) will be used to characterise the baseline environment and site-specific subsea noise data collection is not proposed.

5.2.4. Potential Subsea Noise Producing Activities

  1. A range of potential subsea noise-producing activities have been identified which may occur during the pre-construction, construction, operation and maintenance, and decommissioning phases of the Array in the absence of designed in measures ( Table 5.4   Open ▸ ).


Table 5.4:
Potential Subsea Noise Producing Activities in the Absence of Designed in Measures

Table 5.4: Potential Subsea Noise Producing Activities in the Absence of Designed in Measures


5.2.5. Designed in Measures

  1. Measures adopted as part of the Array are discussed within each of the relevant sections of this Scoping Report for which subsea noise is considered relevant due to the potential impacts from subsea noise on certain receptors (section 6.2 and section 6.3). Each of the proposed measures adopted as part of the Array relating to reducing potential impacts on receptors from subsea noise will be modelled to assess their efficacy in a quantitative way, and for piling will likely include a combination of slow start, soft start, ramp up and/or Acoustic Deterrent Devices (ADDs). The requirement and feasibility of any further measures will be dependent on the significance of the effects of subsea noise on the receptors associated with each topic and will be consulted upon with statutory consultees through the EIA process. Any approach to noise mitigation will be informed by best available evidence and latest guidance, including any outputs from work undertaken during assessment and construction of the nearby operational offshore wind farms and lessons learnt within the industry.

5.2.6. Potential Subsea Noise Producing Activities After the Implementation of Designed in Measures

  1. The subsea noise producing activities that have been scoped into the Array assessment are outlined in Table 5.5   Open ▸ together with supporting analyses (e.g. modelling) that will be required to enable a full assessment of the impacts in the relevant Array EIA Report chapters.
Table 5.5:
Subsea Noise Producing Activities Proposed to be Scoped into the Array Assessment for Subsea Noise. Project Phase Refers to Construction (C), Operation and Maintenance (O) and Decommissioning (D) Phase of the Array Area

Table 5.5: Subsea Noise Producing Activities Proposed to be Scoped into the Array Assessment for Subsea Noise. Project Phase Refers to Construction (C), Operation and Maintenance (O) and Decommissioning (D) Phase of the Array Area


5.2.7. Proposed Approach to Subsea Noise

  1. The following guidance documents will be considered when preparing the Subsea Noise Technical Report:
  • Good practice guide to underwater noise measurement (National Physical Laboratory (NPL), 2014);
  • Review of underwater acoustic propagation models (NPL) (Wang et al., 2014);
  • National Oceanic and Atmospheric Administration (NOAA) technical guidance for assessing the effects of anthropogenic sound on marine mammal hearing (National Marine Fisheries Service (NMFS), 2016);
  • Underwater acoustic thresholds for onset of permanent and temporary threshold shifts (NMFS, 2018);
  • Marine mammal noise exposure criteria: Updated scientific recommendations for residual hearing effects (Southall et al., 2019);
  • Sound exposure guidelines for Fishes and Sea Turtles (Popper et al., 2014);
  • Statutory nature conservation agency protocol for minimising the risk of injury to marine mammals from piling noise (JNCC, 2010);
  • JNCC guidelines for minimising the risk of injury to marine mammals from geophysical surveys (JNCC, 2017);
  • Guidance on noise management in harbour porpoise Phocoena phocoena Special Areas of Conservation (SACs) (JNCC, 2020a);
  • The European Union (EU) Marine Strategy Framework Directive (Directive 2008/56/EC). This seeks to achieve good environmental status (GES) in Europe’s seas by 2020. The qualitative descriptors for determining GES include "Introduction of energy, including underwater noise, is at levels that do not adversely affect the marine environment" This Directive was transposed into UK law by the Marine Strategy Regulations 2010 and remains applicable after EU Exit; and
  • Department for Business, Energy and Industrial Strategy (BEIS) Policy Statement - Marine environment: unexploded ordnance clearance joint interim position statement (BEIS, 2022a).
  1. The impact criteria will be based on the most recent and up-to-date scientific research and guidance, such as that included in Southall et al. (2019) and NMFS (2018), while utilising a precautionary approach. Potential impacts arising from subsea noise on marine mammals and fish will be assessed with respect to the potential for injury and behavioural disturbance. Where possible, noise source data will be based on measured data from similar wind turbine devices. Source noise levels will be based on a combination of theoretical and empirical predictions, and detailed source level modelling where appropriate. The associated source levels of other types of subsea noise associated with the Array area will be based on published data and established prediction methodologies.
  2. Source sound levels for piling will be determined using the method established by von Pein et al. (2022), using the equation below. A broadband source level value is evaluated for the noise emitted during impact pile driving operation in each operation window.

  1. In this equation, E is the hammer energy employed in Joules, d is the pile diameter, mr is the ram mass in kg, h is the water depth in m, is the reflection coefficient and is the propagation angle (approximately 17° for a Mach wave generated by impact piling). The equation allows measured pile noise data from one site (denoted by subscript 0) to be scaled to another site (denoted by subscript 1). This is a recently published method for deriving the sound source level which provides a more scientifically robust method compared to using an energy conversion factor, which simply assumes that a percentage of the hammer energy is converted into sound irrespective of parameters such as pile size, water depth and hammer specifications.
  2. If the proposed piling involves submersible hammers, a correction will be applied through the piling sequence to account for the pile penetration, based on Lippert et al. (2017) which demonstrates that when a pile is below the water surface, the fall-off in acoustic energy is ~2.5 dB per halving of exposed pile length above the seabed.
  3. Subsea noise modelling is planned to assess the potential impact of construction noise using a robust, peer reviewed model. Currently, there is a lack of quality operational data available to be able to carry out any operational noise modelling. Thus, it is proposed that operational modelling will be assessed qualitatively, and modelling will only be carried out if sufficient quality data of operational noise associated with wind turbines and mooring and anchoring systems is made available as a result of current research. In accordance with NPL guidance (NPL, 2014), the choice of model will depend upon many factors which will be determined during the post-scoping consultation and will depend on site-specific circumstances (such as bathymetry etc.). However, the chosen model will be appropriate and peer reviewed, such as the energy flux model (Weston, 1976). Such models have been successfully benchmarked against other sound propagation models (e.g. Etter, 2018; Toso et al., 2014; Schulkin and Mercer, 1985) and used in previous subsea noise assessments for offshore wind and tidal energy developments as well as for oil and gas and port developments. The noise model proposed for this assessment has been calibrated against a range of other noise models showing good agreement (typically within +/- 1 dB out to a range of 2.5 km).
  4. The exact scope, specification and methodology of the noise propagation modelling will be discussed and agreed with MSS, MS-LOT, and NatureScot. On the basis of previous subsea noise modelling completed for other recent offshore wind projects, the assessment will consider the bathymetry and other characteristics of the area, including the geo-acoustic properties of the seabed, as well as other factors such as the sound source characteristics and frequency range of interest. It is anticipated that the subsea noise assessment will include:
  1. The model will be used to estimate the unweighted and hearing group weighted SEL, Root Mean Square (rms) (T90) sound pressure level and peak (peak-to-peak) pressure level parameters, as recommended by Southall et al. (2019), NMFS (2018), Southall et al. (2007), Acoustic Society of America (ASA) Sound Exposure Guidelines for Fishes and Sea Turtles (Popper et al., 2014) and other guidance. The model will also incorporate swim speeds of marine mammals and fish to calculate cumulative SELs, which will be agreed with stakeholders during post-Scoping consultation.
  2. The combined effect of multiple events/operations will also be assessed/modelled and will consider the likely exposure times of species, allowing for safe distances and reaction ranges to be determined. Modelling scenarios will be undertaken for concurrent piling scenarios, including both typical (most likely) and maximum piling parameters within the PDE. Further, modelling will be undertaken with the consideration of mitigation, for example ADDs, comparing otherwise identical scenarios with and without ADDs.
  3. The results of the noise modelling will be presented in a Subsea Noise Technical Report.


5.2.8. Potential Cumulative Effects

  1. The Subsea Noise Technical Report will present the outcome of subsea noise modelling, which will be used to inform the impact assessment for the relevant receptor groups. The cumulative effects of subsea noise will therefore be assessed within technical chapters where appropriate

5.2.9. Potential Transboundary Impacts

  1. The Subsea Noise Technical Report will present the outcome of subsea noise modelling, which will be used to inform the impact assessment for the relevant receptor groups, therefore there are no potential transboundary impacts to be considered for subsea noise.

5.2.10. Scoping Questions to Consultees

  • Do you agree with the proposed modelling methodology?
  • Do you consider any particular sources or receptors should be included within the noise modelling assessment which have otherwise not been considered?

5.2.11. Next Steps

  1. The next steps for subsea noise will follow the strategy set out in section 4.3.4. The approach to subsea noise modelling will be discussed as part of the post-Scoping consultation process as described in the dSEP (Appendix 1).