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Silence: Underwater noise from offshore pile driving

Public summary

To meet today’s increasing energy demand, a large number of offshore wind farms are planned to be constructed in the near future. Despite the plethora of the available foundation concepts in order to support the tower and the nacelle of offshore wind power generators, the most common of those is a steel monopile. Steel monopiles are driven into the sediment offshore with the help of large impact or vibratory hammers. During the piling process, the generated underwater noise levels are very high. Measurements indicate that the noise levels close to the pile due to impact piling can be in the order of 105 Pascals and that largely depends on the pile dimensions and input energy of the hydraulic hammer. Such high noise levels have naturally drawn the attention of regulatory bodies in various nations. The Dutch government permits pile driving only from the first of July till the end of December of each year in order to avoid disturbance of the breeding season of the harbour porpoise. In addition, the simultaneous construction of more than one offshore wind farms is not permitted. In the United Kingdom, an evaluation per project takes place. Measures like seal scarers are used together with a few low energy blows of the impact hammer mainly aiming at intimidating the mammals in the neighbourhood of the construction site. The German Federal government, on the contrary, adopts certain sound level criteria. These have been set to 160 dB re 1 μPa for the sound exposure level (SEL) and to 190 dB re 1 μPa for the sound peak pressure level (SPL), both at a distance of 750m from the sound source. Even though there is yet no overall consensus upon the most appropriate way of quantifying the level of noise which can be potentially harmful for marine species, all the involved parties recognise that certain measures have to be undertaken in order to protect the marine ecosystem.

In the scientific literature, there exist several studies which investigate the impact of pile driving operations in the marine ecosystem. However, the available knowledge regarding the physics of noise generation caused by marine piling is very limited. Without a proper understanding of the noise generation mechanisms, any attempt to mitigate the noise will fall short of expectation. This research project aims to fill this knowledge gap. The primary goal is to shed new light on the underlying physics of the underwater noise generated by marine piling in order to help the practitioners to develop more efficient noise mitigation equipment. This aim is reached by setting the following objectives:

1. development of a vibroacoustic model that can reproduce the physical mechanisms of the underwater sound generation and propagation;
2. analysis of data collected during an experimental campaign in order to identify the main sources that contribute to the underwater noise pollution and to validate the model;
3. theoretical investigation of the effectiveness of a chosen noise mitigation technique and, if possible, generalisation of the conclusions to several other noise mitigation concepts.

Regarding the first objective, considerable effort has been placed in the development of a computationally efficient model that can be used for underwater noise predictions caused by marine piling. The model consists generally of three subsystems, namely: the pile, the water and the soil. The impact hammer is substituted by a distributed force exerted at the pile head. The solution approach adopted to describe the coupled vibroacoustic behaviour of the pile-water-soil system is semianalytical which is considered to be advantageous for several reasons: (i) the computational time is considerably reduced when compared, for example, to the finite element or the boundary element methods; (ii) different stages of the installation process can be investigated with minimum computational effort; and (iii) considerable insight is gained into the physics of noise generation and into the contribution of the various modes to the total acoustic field.

The predictions of the model show that the acoustic field in the seawater consists of two noise paths. The primary noise path is characterised by the pressure conical waves radiated into the water region directly by the wave packet propagating along the pile after the hammer impact. As this wave packet enters the soil region, both shear and compressional waves are radiated, with the former being much stronger than the latter. At later moments in time, Scholte waves are observed along the seabedwater interface. These waves constitute the so-called secondary noise path and carry acoustic energy at low frequencies. A parametric study was also conducted in order to determine the critical parameters of the system and the way they influence the radiated sound. Among the various parameters examined, the pile diameter and the soil properties were found to be the most influential ones in the determination of the underwater sound field. The former defines largely the frequency spectrum of the radiated sound, whereas the latter the energy distribution among the various
subsystems. In particular, the shear rigidity of the soil was found to be crucial for the correct estimation of the noise levels close to the seabed surface due to the energy transferred into the interface waves. The higher the shear rigidity of the seabed, the larger the penetration depth of the Scholte wave into the fluid and, consequently, the higher the noise levels close to the seabed-water interface.

With regard to the second objective, time series data collected during a measurement campaign were analysed in the time, the frequency, and the time-frequency domains. The collected data were also  used for validation of the model described previously. The analysis has shown that the dynamic response of the system is blow-invariant provided that the input energy and penetration depth remain almost constant. The vibration modes with frequencies below the ring frequency of the shell structure in vacuo were shown to be best coupled to the surrounding fluid and therefore able to radiate considerable energy into the exterior fluid domain. This observation was, in fact, verified by the model predictions. In addition, the sound levels in the near-field region showed a strong depthdependence. Finally, the analysis of the geophone signals verified the existence of low-frequency oscillations close to the seabed level which are attributed to the Scholte waves that were predicted by
the prediction models.

Regarding the third objective, the theme of noise mitigation is treated. A state-of-the-art review of the available mitigation concepts is included and a final model is developed which includes an airbubble curtain. A parametric study is conducted in order to reveal the principal mechanism of noise reduction and the optimum system configuration for a specific case. The influence of a number of parameters, i.e. the volume of the air content, the thickness of the bubble curtain and the distance from the pile surface, on the predicted sound levels, were investigated. It is found that for piles of large diameter, the main mechanism responsible for the noise reduction is the impedance contrast between the seawater and the air-bubble medium. The dissipation effects due to resonance of the individual bubbles seem not to be important for the relatively low-frequencies associated with the sound radiation of large piles. Finally, the efficiency of the air-bubble curtain increases, the higher the
air-volume content, and the larger the horizontal distance it is placed from the pile. In addition to the above, collected data were analysed with and without the use of the Noise Mitigation Screen developed by Royal IHC (IHC-NMS) in order to reveal the potential reduction of the noise levels. The IHC-NMS is a mitigation technique that can withstand stronger sea currents when compared to the air-bubble curtain and therefore it is of interest to investigate its noise reduction potential. The analysis has shown that the IHC-NMS can reduce the noise levels but the noise reduction is strongly frequency-dependent, i.e. noise at low frequencies is less efficiently mitigated.

Apart from the scientific contributions mentioned above, there exists also a clear contribution to the original FLOW targets in economic terms. It is important to mention that the current legislation in The Netherlands permits the installation of piles with impact hammers only from first of July till the end of December of each year. In addition the simultaneous installation of foundation piles in more than one offshore wind parks by means of impact hammers is not permitted. This inevitably leads to considerable deceleration in the deployment of the offshore wind industry. The understanding of the underwater noise generation mechanisms (main contribution of the current project) helps the industry to develop more efficient noise mitigation equipment. This yields the following contribution to the overall FLOW targets:

1. Acceleration of the deployment of the offshore wind industry: The use of the (validated) tools for the underwater noise prediction during the installation of foundations piles, together with the better understanding of the noise generation mechanisms, will help the industry to develop new noise mitigation techniques and/or to optimise the ones currently in use. This could lead to a removal of the restrictions mentioned above regarding the limitation in the installation period and the non-allowance of the simultaneous construction of more than one offshore wind parks.
2. Cost reduction: The elimination of the restrictions currently imposed by current legislation will, in turn, allow the installation of piles by impact piling throughout the year. The industry could therefore make use of the best-possible weather window for the installation. This can speed up the installation process and reduce the risk for the marine contractor. Calculations with the FLOW-TKI model pointed out a reduction of the Levelised Cost of Energy (LCoE) in order of 1%.

 

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