6.4. Construction Phase

6.4.1. Impact Piling

  1. The noise generated and radiated by a pile as it is driven into the ground is complex, due to the many components which constitute the generation and radiation mechanisms. Larger pile sizes can require a higher energy in order to drive them into the seabed, and different seabed and underlying substrate types can require use of different installation techniques including varying the hammer energies and the number of hammer strikes. In addition, the seabed characteristics can affect how noise propagates from the pile through the sub-surface geology, thus fundamentally affecting the acoustic field around the activity. The type of hammer method used (i.e. the force-impulse characteristics) can also affect the noise characteristics.

A useful measure of noise used in underwater acoustics is the SEL. This descriptor is used as a measure of the total noise energy of an event or a number of events (e.g. over the course of a day) and is normalised to one second. This allows the total acoustic energy contained in events lasting a different amount of time to be compared on a like for like basis. For impulsive noises it has become customary to utilise the T90 time period for calculating and reporting root mean squared (rms) sound pressure levels. This is the interval over which the cumulative energy curve rises from 5% to 95% of the total energy and therefore contains 90% of the noise energy.

  1. The estimation of source levels for noise propagation modelling of piling is an important aspect of the noise modelling methodology. Ideally, use can be made of noise data measurement for similar piles, installed using a similar hammer in similar conditions. However, since noise modelling for proposed wind farms often proposes the use of piles and hammers for which there is no currently available measured data, it is often necessary to utilise an alternative method to estimate the source level inputs to the model. One such method used in some previous noise modelling assessments is the use of energy conversion factors, which involves estimating the proportion of the hammer energy which is transmitted into the water column as noise. However, the subject of noise generation due to impact piling is an active area of research and the evidence base is constantly being updated by new measurements, research and published papers. It is therefore important to ensure that the methodology used for determining the source levels of piling take into account the most recent research.
  2. It is proposed to utilise scaling of measured data during pile driving for similar operations to the Array in order to determine source levels. The subject of noise generation due to impact piling is an active area of research and the evidence base is constantly being updated by new measurements, research and published papers. A recent peer-reviewed paper (von Pein et al., 2022) presents a methodology for the dependencies of the SEL on strike energy, diameter, ram weight, and water depth that can be used for scaling measured or computed SELs from one project to another. The method has been shown to be usable within practical ranges of accuracy, especially if the measurement uncertainties are taken into account. The paper suggests that scaling should be performed over either a small number of very similar piling situations or over a larger data set with according averaging. This is a recently published method for deriving the noise source level which provides a more scientifically robust method compared to using an energy conversion factor (the conversion factor method simply assumes that a percentage of the hammer energy is converted into noise irrespective of parameters such as pile size, water depth and hammer specifications). Since the von Pein et al. (2022) methodology takes into account several site-specific and pile-specific factors, in addition to hammer energy, and because it is based on a scientifically rigorous and peer reviewed study, it is therefore considered to be a significant improvement on the use of simple conversion factors alone. This methodology is further endorsed by the recent study undertaken by Jasco on behalf of Marine Scotland, which recommends use of scaling methods instead of those relying on conversion factors (Wood et al. 2023).
  3. Using the equation below (von Pein et al. 2022), a broadband source level value is calculates 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).
  2. To account for the pile penetration and use of submerged piling rigs, a correction is applied through the piling sequence based on Lippert et al. (2017) by considering the quotient of wetted pile length Lw and water depth hw using the following equation:

  1. This methodology therefore considers the following factors:
  • pile diameter;
  • pile length;
  • pile penetration;
  • water depth;
  • rated maximum hammer energy of the proposed hammer;
  • hammer energy being used;
  • ram mass for the hammer; and
  • acoustical parameters of the soil and water.
  1. The peak SPL 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 = .

  1. 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 noise 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 overestimates and should therefore be treated as highly conservative.
  2. The piling scenarios for the Array include the following phases:
  • initiation (including slow-start);
  • soft start;
  • ramp up; and
  • full power piling.
  1. These phases and the various associated parameters and durations are shown in Table 6.7   Open ▸ .
  2. The impact piling scenarios that have been modelled for the Array are as follows:
  • OSP foundations (piled jacket) using a maximum hammer energy of 4,400 kJ for a duration of up to eight hours, related to the MDS associated with the largest diameter OSP foundations.
  • Floating wind turbine foundations anchor piles using a maximum hammer energy of 3,000 kJ for a duration of up to eight hours, related to the MDS associated with the largest diameter wind turbine anchors.

 

Table 6.7:
Modelled Pile Installation Parameters (Pile Diameter 4.5 m for Both Pile Types)

Table 6.7: Modelled Pile Installation Parameters (Pile Diameter 4.5 m for Both Pile Types)

6.4.2. Drilled Pile Installation

  1. For drilled pile installation, source noise levels have been based on pile drilling for the Oyster 800 project (Kongsberg, 2011). The hydraulic rock breaking source noise levels are based on those measured by Lawrence (2016). The source levels used in the assessment are summarised in Table 6.8   Open ▸ .
  2. Rotary drilling is non-impulsive in character and therefore the non-impulsive injury and behavioural thresholds have been adopted for the assessment.

 

Table 6.8:
Drilled Pile Noise Source Levels Used in Assessment (Un-Weighted)

Table 6.8: Drilled Pile Noise Source Levels Used in Assessment (Un-Weighted)

  1. 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 source levels are presented in Table 6.9   Open ▸ .

 

Table 6.9:
Source Levels for Other Sources

Table 6.9: Source Levels for Other Sources

 

6.4.3. Vessels

  1. Use of vessels is addressed in section 6.7 for all phases of the Array.