Investigating the influence of a bubble curtain on the underwater sound wave field generated by offshore pile driving

The offshore wind energy plays a crucial role for the future German energy supply. For the foundation of offshore wind turbines, typically, piles are driven into the seabed. During this process, high sound levels occur in the water column even at a great distance from the pile (Bailey , 2010). These noise emissions are potentially dangerous for the marine environment. For protection, the German authorities introduced a dual criterion which includes a limit value of 160 dB re $1 μPa2 s$ of unweighted sound exposure level SEL (⁠ $LE,p$⁠) and a limit value of zero-to-peak sound pressure level (⁠ $Lpeak$⁠) of 190 dB re $1 μPa$ at 750 m distance to the pile (Juretzek , 2021). To reduce the sound pressure level in the water column and comply with the regulations, noise abatement systems are used. In particular, the bubble curtain is applied frequently (Bellmann, 2014), due to its possibility to be combined to pile near systems. Since the actual pile driving noise emission greatly exceeds the limits, combined systems represent the state of the art in noise control.

In recent years, the generation and propagation of sound waves in the water column and sediment due to pile driving have been extensively studied. The hammer impact result in a radial expansion of the pile which travels along the pile (Reinhall and Dahl, 2011) forming sound waves in the sediment and the water column. The latter acts as a waveguide due to the shallow water conditions (Ainslie, 2010). The waves radiating from the pile into the sediment are partly refracted upwards due to the positive sound velocity gradient in the sediment and enter the water column at a greater distance from the pile. These superimpose with the waves travelling in the water column and form a complex sound wave field (Ruhnau , 2016).

Since this is well understood for the unmitigated case, the question arises as to how the insertion of a bubble curtain alters the sound wave field and how this affects the sound level reduction. This question forms the central focus of our research in this work. A set of measured and simulated data specific to a construction site located within the Global Tech I wind farm is analyzed. The measurement data set includes data from eight different measurement stations, positioned at varying water depths and distances from the pile. To the best of the authors knowledge, no previous work has addressed the question of how the insertion of a bubble curtain alters the sound wave field, on the basis of such a comprehensive data set, making this the first original contribution of this work.

Numerical studies including the bubble curtain have not been possible for a long time due to a lack of modeling approaches that properly represent the acoustic properties of the bubble curtain. In recent years, a detailed modelling approach of the bubble curtain has been developed and validated (Bohne , 2019, 2020). The approach incorporates inter alia submodels of the fluid dynamics and bubble formation process at the nozzle hose and allows one to model the most important processes in the bubble curtain determining its acoustic properties. Other authors have since adopted and developed this approach. Peng (2021b) implemented the whole model in their pile driving noise model (Peng , 2021a; Tsouvalas and Metrikine, 2014) and recently extended it (Peng , 2023) with a pneumatic model to account for the pressure variation along the nozzle hose. Beelen and Krug (2024) focussed on the bubble formation process at the nozzle hose and adapted the corresponding submodel to represent also the transition from the axisymmetric nozzle flow to the plane bubble plume. They compared their modeling results with data sets from extensive measurements carried out in two freshwater tanks of the Dutch Marine Research Institute (Beelen , 2023). In the present work, the original acoustic model is combined with a seismo-acoustic model (Bohne , 2024) to obtain a realistic representation of the acoustic pile driving scenario with bubble curtain, making this the second original contribution.

Results of a small study based on simulation data of the construction of the FINO3 research platform (Bohne , 2016) indicate that the bubble curtain deflects part of the sound wave passing through it, resulting in an increased sound wave transmission into the upper sediment layers. In order to determine the influence of the bubble curtain on the sound wave field in the water column on a quantitative basis, a metric based on the flow of sound energy through the water column and the concept of transmission loss is introduced. This metric allows one to distinguish between the losses related to the interaction between the sound wave and the bubbles and the losses related to the change of the interaction between the sound wave and the sediment. This is the third original contribution of this work.

This work is structured as follows. First, the modeling approach is presented (Sec. II). Then, the site conditions, the measurement concept, and the data processing are described (Sec. III). Next, the measured and simulated data are analyzed and compared (Sec. IV). Then the simulated sound wave field altered by the bubble curtain is examined in detail (Sec. V). Finally, the results are discussed (Sec. VI) and a conclusion is drawn (Sec. VII).

In order to investigate in detail the acoustic scenario of a monopile driven into the seabed with a bubble curtain deployed at several dozen meters from the pile, a model has been developed. Therefore, a recently developed seismo-acoustic model (Bohne , 2024), hereafter abbreviated as SAMPD, was extended by an acoustic model of the bubble curtain (Bohne , 2019, 2020).

The SAMPD consists of a hammer model (Fricke and Rolfes, 2015), a close-range model (CRM), and a long-range model (LRM). The CRM is set up in a finite element solver that treats the soil as an elastic medium. In the LRM, the higher-order elastic parabolic equation (Collins, 1989) is solved using the Crank-Nicolson integration method for frequencies $f≤56 Hz$⁠. For $f>56 Hz$⁠, the acoustic parabolic equation is solved using the split-step Padé approach (Collins, 1993). This frequency-dependent approach ensures that shear and interfacial waves, which predominate at lower frequencies (Jensen , 2011), are properly accounted for in the model and that the computational effort does not become too great at higher frequencies. The limit frequency of 56 Hz was found by Bohne (2024) through a parameter study investigating the influence of these seismic wave types at greater distances to the pile and therefore in the LRM.

The acoustic model of the bubble curtain includes a model of the plane bubble flow to determine the local gas fraction in the bubble curtain, and a model representing the bubble formation process above the nozzle to obtain the bubble size distribution. The resulting model is hereafter referred to as SAMPD-BC.

To extend the SAMPD by a bubble curtain, two scenarios must be considered. First, the coupling radius $rc$ between CRM and LRM is larger than the radial position of the bubble curtain (⁠ $rc>rBC$⁠). Then, the bubble curtain can be considered as a local acoustic equivalent fluid within the CRM, cf. Bohne (2019). Second, the coupling radius is smaller (⁠ $rc<rBC$⁠). Then, the bubble curtain must be considered in the LRM. Due to its definition and restriction on range-independent elastic wave guides, the bubble curtain must be introduced differently. Virtually, the LRM is applied twice. First, the LRM is integrated from $rc$ to $rBC$⁠. Then, the resulting fields at the radial position of the bubble curtain $rBC$ are corrected by applying a transfer function mapping from the inner onto the outer side of the bubble curtain, cf. Sec. II B. The resulting field is then used as the starting field for the second run of the LRM from the bubble curtain position $rBC$ to the maximum radius considered $rmax$⁠. Since the described process ensures that the coupling between CRM and LRM only occurs at the distance calculated according to Eq. (1), even with a bubble curtain, no increased coupling error is expected.

This approach introduces a potential inaccuracy when accounting for the reflected wave field at the bubble curtain. By directly integrating the bubble curtain into the computational domain of the CRM, the solution incorporates the influence of the wave field reflected at the curtain. Conversely, in the LRM, where only the forward-propagating wave field is modified, the reflected wave field at the bubble curtain is not considered. To quantify the error resulting from this assumption, a simplified scenario of a bubble curtain under a homogeneous incident wave field is examined. The model parameters of the nozzle hose are given in Table II and the water depth is given in Fig. 2.

Figure 1 shows the reflection, transmission, and absorption coefficients of the bubble curtain over frequency. The incident energy flux on the inner side of the bubble curtain is compared with the reflected and transmitted energy flux. Note that the incident energy flux corresponds to the wavefield radiated from the pile and does not account for the reflections of the reflected wave on the pile. It is observed that in the frequency range of 0 to 100 Hz, approximately half of the energy is reflected by the bubble curtain. Assuming constructive interference, the resulting wave field deviates by about 1.7 dB from the incident field without reflection. For frequencies above 100 Hz, the reflection coefficient decreases rapidly. At 200 Hz, the coefficient falls below 0.3. The resulting wave field deviates by only about 1.14 dB from the incident field without reflection.

One-dimensional cylindrical wave propagation in the positive radial direction is assumed. Therefore, the wave field is produced by an infinite, homogenous line source which is located at r = 0 and the Helmholtz equation reduces to the Bessel equation (Jensen , 2011). At the radius $r=ris$⁠, the wave field reaches the inner side of the bubble curtain which is modelled as a column of n layers of constant acoustic properties. The layer interfaces are located at r = r_j. At the outer side of the bubble curtain (⁠ $r=rn+1=ros$⁠), the column is bounded by an infinite space.

As part of the BORA project funded by the Federal Ministry for Economic Affairs and Energy, an extensive measurement campaign was carried out at a construction site in the OWF Global Tech I (GTI) during the installation process of three piles for the foundation of a tripod structure. A Menck MHU 1200S hammer was used for the installation. To reduce the noise emissions, a bubble curtain was deployed, which enclosed the entire construction site. The hydroacoustic measurements were conducted by the Institute for Technical and Applied Physics (ITAP). Measurement stations were deployed in the southeast and northwest directions seen from the construction site. A more detailed description of the construction site and the measurement campaign can be found in the project report (von Estorff , 2015), the work of von Pein (2021), and the measurement report (Bellmann , 2015).

In this work, the data from each pile and the measuring stations MP8, MP10, MP11, and MP12, which are located south-east of the construction site, are used. In addition, the lowest hydrophone of a hydrophone array (HA) near the piles is used to characterize the pile sound radiation. For MP8, MP11, and MP12 data from two depths, namely, 1 to 2 m and 10 m above the seafloor, have been available. For MP10, only data from 1 to 2 m above the seafloor has been available. The arrangement of the measurement stations in relation to each pile is shown in Fig. 2. The positions of the measurement stations are based on the measurement report (Bellmann , 2015). It should be noted that the positions of the measurement stations are not known exactly, due to the prevailing tidal flow. Therefore, the given values should be considered as approximations. The identifiers of the measurement stations correspond to the measurement report (Bellmann , 2015). The founding process proceeded as follows. First, the tripod was set up. Subsequently, the piles were guided through the pile sleeves. Each of these piles was vibrated in first and then driven to the final depth. The piles were driven in sequence, with the third pile being driven with the bubble curtain switched off as a reference. The identifiers of the piles and their arrangement with respect to the bubble curtain can be found in Bohne (2019).

The recorded sound signal from a measurement station has been analysed as follows. For each pile, a segment comprising 40 blows is identified, corresponding to an embedded pile length of 14.5 m. The sound signal of each blow is isolated. Subsequently, the sound exposure level is obtained as single value and in one-third octave bands. Assuming the same boundary conditions for each blow within a segment, the arithmetic mean and the standard deviation are calculated for each segment. Thus, each segment represents a measurement case which is used in the following for investigation and comparison with the simulated data. Table I summarizes the information for each segment.

In the first step, the fluid dynamics of the bubble curtain has been obtained by the SAMPD-BC. The model parameters of the nozzle hose are listed in Table II. Figure 3 shows the resulting spatial distribution of the gas fraction in the bubble curtain. The bubble curtain spreads almost linearly with decreasing depth resulting in a decrease in gas fraction. Due to the decreasing static pressure, the gas fraction reaches a minimum at $z=−15 m$ and increases approaching the sea surface. In combination with the bubble size distribution, given in Fig. 4, the local bubble number density distribution and the local sound velocity field can be obtained. A more detailed explanation of the acoustic model of the bubble curtain, including the related fluid dynamic and acoustic parameters, can be found in Bohne (2019, 2020).

In the second step, for each of the three cases in Table I, a simulation of the acoustic scenario has been performed, resulting in the transfer function. The axisymmetric model domain has been set up on basis of the pile parameters listed in Table III and the geoacoustic model of the construction site. The pile sleeve surrounding the lower part of the pile, as shown in Fig. 2, has been neglected. Furthermore, the tripod structure is not considered in the model domain as this breaks the axis symmetry. The geoacoustic model includes a water layer with the compression wave velocity $cc,w=1500 m/s$ and the density $ρw=1025 kg/m3$ and a sediment below. The sediment properties, shown in Fig. 5, have been derived on basis of Buckingham's model, cf. Buckingham (2005, 2020). The layer characteristics with sediment classification base on CPT-probing at the construction site, cf. von Estorff (2015).

In the third step, the hammer forcing function which represents the source spectrum has been obtained. The hammer model parameters are listed in Table IV. Figure 6 shows the Fourier transform of the forcing function.

The variation of the sound exposure levels over the radial distance from the pile is shown in Fig. 7 for the three cases (BC1, BC2, REF). The levels are provided for a height of 1 and 10 m above the seabed. Generally, all levels decrease with increasing distance. At short distances, this overall trend is overlaid by local peaks. For the cases BC1 and BC2, the calculated levels decrease significantly at the positions of the bubble curtain. From a distance of approximately 200 m, the levels become quite similar. Figure 7 also shows the measured values at the measurement stations. The measured and calculated levels agree well. A small deviation can be observed when looking at the BC cases. With increasing distance, the simulated levels for 10 m deviate from the measurements.

For a more detailed comparison, Fig. 8 shows the resulting sound exposure levels in one-third octave bands at the measurement stations HA, MP8, and MP11 at 1 m above the seafloor for the REF case. The three stations have been chosen to represent the sound generation and the full propagation path. This allows the accuracy of the model to be assessed in detail. The results for the BC cases are shown later in the context of the insertion loss. Similar trends can be observed at each measurement station. The sound exposure increases with frequency in the range from 20 to 100 Hz and peaks around 100 to 200 Hz. As frequency increases further, the sound exposure decreases. A quantitative agreement between the simulated and measured data can be observed over the whole frequency range and for each measurement station.

The radial variation of the single-valued insertion loss $IL=LE,p,REF−LE,p,BC$ at heights of 1 and 10 m above the seafloor is shown in Fig. 9. At the positions of the bubble curtains, the insertion loss increases to about 12 dB for each BC case, but then drops to about 2 dB within the next 100 m. An explanation for this drop can be found in Sec. V B. From a distance of 200 m, the insertion loss increases again to about 10 to 12 dB at a distance of about 300 m. As the distance increases further, the insertion loss decreases slightly and reaches 8 and 5 dB for 1 and 10 m above the seabed, respectively, at a distance of 5000 m. The measured and simulated data are in good agreement. In particular, for the height of 1 m above the seafloor only small deviations can be observed. For 10 m there is an increasing deviation with distance between simulated and measured data, similar to the trend seen in Fig. 7.

For a more detailed analysis, Fig. 10 shows the measured and simulated insertion loss in one-third octave bands for the closest (MP8) and the farthest (MP11) measurement station, for the two heights 1 and 10 m above the seafloor. The insertion loss increases from nearly 0 dB at 30 Hz to about 20 dB for MP8 and 15 dB for MP11 at 1000 Hz, respectively. In line with the observations made above, the insertion loss is lower at greater distances, especially for higher frequencies. This difference is evident in both the calculated and measured data. A good agreement between simulated and measured data can be seen for MP8 over the whole frequency range and for MP11 for the lower frequency range. A deviation can be seen at higher frequencies at MP11. In particular, at 10 m height and in the 400 to 800 Hz range, the insertion loss deviates by about 10 dB from the measurements. A potential explanation for the deviation between measured and simulated data is discussed in Sec. VI.

With exception of the depth-averaging operation the given definition of the basic quantities follows ISO 18405:2017 (ISO, 2017).

At 300 Hz, the sound wave passes through the bubble curtain, particularly at mid-water depth, which results mainly from the typical $λ/2$ transmission (Bohne , 2019). This wave is then refracted upwards, reflected at the sea surface and then incident on the ground. At this point, the sound wave field penetrates more than 20 m into the ground, indicating significant energy transmission into the seabed. In comparison, the wave field in the REF case reaches only 10 m into the ground.

For 900 Hz, the bubble curtain is almost closed, allowing only waves close to the seabed to pass through. In the REF case, the up and down propagation of the sound wave can be clearly observed, and the propagation angle is retained over distance. In contrast, the wave field behind the bubble curtain is scattered resulting in a complex interference pattern. A steep ray leaves the bubble curtain. It is then reflected at the water surface. After that, it is transmitted into the seabed. This corresponds to the observation made for 300 Hz and the drop seen in Sec. IV. In conclusion, Fig. 11 visually demonstrates that the bubble curtain not only changes the amplitude of the sound wave passing through but also alters the wave field behind it.

In the second step, the metric introduced in Sec. V A is applied. Figure 12(a) shows the variation of the insertion loss due to the altered bottom loss over the radial distance from the pile for relevant mid-band frequencies (150, 300, and 900 Hz), as well as the single value. At around 200 m, the single valued insertion loss $ILE¯,sed$ increases rapidly from zero to around 3 dB. With further increasing distance the insertion loss increases but only slightly, reaching its maximum of 4 dB at around 800 m. From here, the insertion loss decreases again reaching nearly zero at 5000 m. The same applies for 300 Hz. For 900 Hz, the decrease is more pronounced, reaching −4 dB at 5000 m. For 150 Hz, the insertion loss remains at around 1 dB over the entire range considered.

For comparison, the single valued total insertion loss $ILE¯$ is also shown in Fig. 12(a). The variation of the total insertion loss is identical to that of the insertion loss due to the altered bottom loss, but with an offset of about 5 dB, which represents the insertion loss due to the interaction between the sound wave and the bubbles $ILE¯,bub$⁠. In conclusion, the insertion loss due to the altered bottom loss varies with range and, at a distance of 750 m from the pile—which is a typical measurement distance—accounts for almost half of the total insertion loss, thus influencing the efficiency of the bubble curtain.

A reason for the distance dependence can be found by considering the variation of the transmission losses, reduced by the cylindrical spreading loss, of the reference case (REF) and the bubble curtain case (BC1) over the radial distance in Fig. 12(b). For the REF case, there is a constant increase in transmission loss with distance. Conversely, the BC1 case shows a pronounced increase in transmission loss of about 3 to 4 dB in close proximity to the bubble curtain. As the distance increases, the curve of transmission loss becomes flatter. Consequently, at a certain distance, the transmission loss of the REF case will intersect that of the BC1 case. For higher frequencies, this intersection occurs at shorter distances, as seen for 900 Hz at 3000 m.

A potential explanation for the difference in transmission loss between the reference and bubble curtain cases is that the bubble curtain scatters the sound wave passing through it, as seen in Fig. 11. Thus, the uniform wave field radiated by the pile is split into different rays with different grazing angles. This results in rays that are absorbed more quickly and rays that are absorbed less quickly by the sediment than the reference wave field. Finally, this also explains the drop in insertion loss close behind the bubble curtain, as seen in Fig. 9. This can be interpreted as a ray with a steep grazing angle leaving the bubble curtain, passing the observation point and then being absorbed by the bottom.

In Sec. V, the scattering of the sound wave field due to the bubble curtain and the altered bottom loss has been identified as an additional insertion loss component of the bubble curtain. Close behind the bubble curtain, an increased transmission loss has been observed, which results from rays with steep grazing angles. As the simulated insertion loss agrees well with the measured one at a distance of 1088 m (MP8), this finding is considered confirmed. Moreover, the simulated data show a decrease in the insertion loss with increasing distance. This trend is also observed in the measured data, though to a lesser extent. Particularly at higher frequencies and at a distance of 5190 m (MP11), the simulated insertion loss is significantly lower than the measured one. There are two potential reasons for this discrepancy.

First, the sea surface is assumed to be flat in the simulation, which does not accurately reflect the environmental conditions during the measurement campaign, where a rough sea surface with a significant wave height of around 1.6 m (Bellmann , 2015) was observed. Consequently, extra scattering effects were neglected in the simulation.

Second, the scattering of the sound wave field at the bubble curtain may be partially inaccurately represented by the model. Due to the long propagation distances considered, it is plausible that even small amounts of false rays with relatively shallow grazing angles could lead to the observed discrepancies. The inaccuracy of the model can generally be attributed to modelling assumptions, such as the reduction of the complex flow field in the bubble curtain to a plane bubble flow or the neglect of cross-flow effects. A detailed analysis will be part of future work.

The aim of this work has been to investigate how a bubble curtain alters the sound wave field emitted by a pile during driving and how this affects the sound level reduction. For this purpose, a construction site at the Global Tech I offshore wind farm has been selected, where a bubble curtain was used as a noise abatement system and for which extensive acoustic measurement data has been available. In order to interpret the data set, a recently developed seismo-acoustic model for pile driving has been extended by an established acoustic model of the bubble curtain.

The simulated and measured data have been compared. The results reveal a dependency of the insertion loss with distance and depth, notably showing a strong variation close behind the bubble curtain and a decrease at greater distances. This decrease amounted to about 2 to 4 dB of the single value for 1 m above the seabed. Notably, both the measured and calculated datasets exhibit similar trends and quantitative agreement up to a distance of around 1000 m from the pile. For greater distances of around 5000 m, similar trends can be observed, namely the decrease in the insertion loss, but the model overpredicts this for higher frequencies.

Subsequently, a more detailed analysis of the spatially resolved simulation data has been conducted. To unambiguously determine the sound level reduction due to a bubble curtain at varying radial distances from the pile, the insertion loss has been formulated based on the transmission loss of the reference case and the mitigated case. The resulting metric includes the loss of sound energy due to the interaction between the sound wave and the bubbles, as well as the altered interaction between the sound wave that has passed through the bubble curtain and the sediment. The analysis indicates that, for instance, at a distance of 750 m from the pile—a distance specified by the German authorities for evaluating pile driving noise—the latter component accounts for almost half of the total insertion loss and, consequently, the efficiency of the bubble curtain. This component can be attributed to the scattering of the sound wave as it passes through the bubble curtain, producing sound rays with different grazing angles and consequently altered bottom loss characteristics. Close behind the bubble curtain, steep rays are absorbed by the sediment, increasing the insertion loss. Conversely, as the distance increases, the loss decreases because the rays with shallow angles remain in the water column for longer.

Finally, the results have been discussed with respect to the accuracy of the simulation. Discrepancies between the simulated and measured data at greater distances have been attributed to the neglect of the rough sea in the model and the sensitivity of the observed scattering effect at longer distances to model assumptions.

The Institute of Structural Analysis is part of the Center for Wind Energy Research For-Wind. Part of the research at Leibniz University Hannover was carried out in the frame of the BORA project in cooperation with project partners from the University of Kiel and the Hamburg University of Technology. The authors gratefully acknowledge the funding of the Federal Ministry for Economic Affairs and Climate Action due to an act of the German Parliament (project Ref. No. 0325421).

The authors state that they have no conflicts of interest.

The data that support the findings of this study are available within the article.