Numerous offshore wind farms are planned, already granted, or currently under construction within Germany's exclusive economic zone (EEZ). Usually, the foundation of the wind turbines are planted by pile driving, e.g. for monopile, tripile, tripod constructions.
This technique induces underwater noise, which can be measured in distances of 20 km and more to the construction site. In the near surroundings of the pile, sound energy levels often exceed the present limit values prescribed by the German authorities to protect marine mammals, which are a single event sound pressure level (SEL) of 160 dB re 1 μPa and at a maximum peak level (Lpeak) of 190 dB re 1 μPa in a distance of 750 m to the pile. Therefore, various sound-damping techniques are currently in development or in field trial.
Major Targets of the BORA Project
The global target of BORA is to develop a profound calculation model to predict waterborne noise due to offshore pile driving. This includes especially models to predict:
- the sound development at the source due to pile deformation and vibration.
- the sound transmission into water and soil.
- the consideration of the sound attenuation due to the air-water mixture produced by bubble curtains or due to other sound-damping methods.
An important step when developing such models is the thorough validation of the approaches regarding their ability to accurately account for the sound origination at the pile, the transmission into water and soil, ant the sound attenuation due to possible migitation measures. For this purpose, three extensive offshore measurement campaigns will be performed in different wind farms in the German North Sea.
After validation, the detailed simulation model will be adapted to other offshore structures with different boundary conditions. Due to the complexity of the resulting simulation model and the necessary access to substantial computing capacities and specialist simulation software, this model can only be executed by experts in the field of numerical simulation. Therefore an additional stand-alone expert system will be developed, which will enable third party users to calculate basic noise predictions for offshore pile driving. Finally, standard datasets will be compiled to allow for a review of future computation models.
Tasks of the Project Partners
TUHH/IMB (see Partners) is in charge of the project leadership as well as the overall coordination, while TUHH/GBT holds primary responsibility for the coordination of the offshore measurement campaigns. The expert system will be developed by TUHH and LUH under the direction of LUH.
The project is organized in four work packages (AP 1 to 4), each of them dealing with a specific subdomain of the wave propagation problem, that will be solved by one of the project partners. Each of the work packages includes profound documentation work and the publication of the major results.
- TUHH/GBT will describe the pile deformation and thus the vibration on the pile surface which causes the hydroacoustic input into the water column. Furthermore, the near field of the structure-borne sound will be modelled.
- CAU/IfG will investigate the structure of the subsurface. Additionally, the far field of the structure-borne sound will be described.
- LUH/ISD will investigate the attenuating effect of different sound mitigation measures and set up corresponding models, which can be included into the overall sound propagation model. Thereby, an emphasis is on the complex bubble dynamics and thus the functional principle of bubble curtains.
- TUHH/IMB will develop numerical models for the propagation of the waterborne sound in the near field as well as in the far field and set up the overall model including source and mitigation measures.
The coupling of all subdomain models will allow for the creation of a complex integrated model, which enables for a detailed investigation of the resulting underwater sound due to pile driving.
AP 1 (TUHH/GBT)
The work package AP 1 aims at the investigation of the movement of the steel pipe and the oscillation of the seabed during the pile driving and therefore the capture of the sound emission at the pile and at the seabed on the basis of measurements and computer simulations. Measured and calculated vibrations of the boundary layers pile/air, pile/water and seabed/water will be handed over to the project partners.
The coordination, planning and implementation of the field measurements are conducted by the Institute of Geotechnical Engineering and Construction Management. The measuring concept plans to carry out in a total of three campaigns i) measurements of the oscillation of the pile and the seabed to the capture of the sound emissions and ii) sound level measurements to the capture of the sound immissions. As the comprehensive field measurements serve as validation basis for the computer models, the entire transmission path of the oscillations is acquired. Thereby, the influence quantities are to be considered, like e.g. the layering of the #building site, the structural data of the pile foundation, the ram energy, the swell condition, as well as the sound attenuation technique by itself as input dimensions.
During the whole project computer simulations of the pile driving are carried out on the basis of the finite element method. Dynamic analyses in the time domain are planned using implicit as well as explicit time integration. The numerical simulation concentrates in AP 1 on the pile driving sequence, the steel structure and the soil in the near field.
Up to the achievement of the first field test, the numerical simulation bases on a simplified model. In the second phase, the finite element model is improved for the following parameter studies. The main modifications concern the material behaviour of the two-phase soil, the oscillation behaviour of the pile, the coupling of the boundary layers, and the penetration process during the pile driving, which changes the state of the soil. With the improved simulation model, parameter studies and variation computations regarding the foundation structure will be carried out in the third phase.
Finally, the achieved findings from the three field tests result in the development of an expert system, which will be able to evaluate the expected sound input and the mitigation effect of the sound attenuation technique of the installation process. Thereby, the sound emission by the pile and the seabed has priority in AP 1. The expert system allows an extended user circle, for example licensing authorities, nature conservation agencies and biologists, to evaluate the sound immission due to pile driving at offshore wind farm foundations to a wide variety.
AP 2 (CAU/IfG)
The work within AP 2 aims at defining the geological subsurface structure prior to the pile driving process. Reflection seismics and Scholte-wave measurements will provide information about geological structure and shear modulus, which will then serve as boundary conditions for prediction models. During the pile driving process, ground movement will be recorded to quantify the hydroacoustic input of seismic waves in the far field. Numerical simulations of wave propagation will help to interpret the observed ground movement and predict dependencies between wave propagation and geological structure.
Prior to the pile driving process, high resolution 2.5-D reflection seismics will be carried out in order to define the geological structure in the vicinity of the pile up to the planned foundation depth. Therefore, about 25 parallel and cross seismic lines will be recorded in an area of 500m x 500m around the future position of the pile. The sediment structure will then serve as boundary condition for later modelling.
Additionally, boundary surface waves (Scholte waves) will be recorded along a seismic transect using ocean bottom seismometers (OBS). These waves provide seismic parameters of the sediment which are essential for viscoelastic modeling and help to detect lateral variations of the shear modulus and shearwave velocity via inversion.
During the pile driving process, OBS will be placed at three different distances from the pile and record vertical and radial ground movement. Data recorded by seismometers and hydrophones will be used to identify seismic waves and to validate numerical models. Furthermore, hydrophone data allow to draw conclusions about the compressional part of seismic waves. It will be possible to directly identify hydroacoustic waves in the water column that are triggered by certain seismic waves in the subsurface.
Next, reflection seismic data will be digitally processed and interpreted using commercialized software. This includes digital signal processing such as filtering and deconvolution, as well as determination of seismic velocities. The analysis of dispersion curves using Scholte waves will result in a depth distribution model of shearwave velocity and dynamic shear modulus. In the end, an overall subsurface model will give information about geological boundaries as well as elastomechanical parameters of the geological layers.
In the last step, two-dimensional viscoelastic FD-modeling of seismic wave propagation in the far field will help to understand and quantify the relationship between wave propagation and geological structure.
AP 3 (LUH/ISD)
Within AP 3, reliable parametric numerical models for the prognosis of the mitigation effect of bubble curtains and further sound attenuation concepts will be developed. To reach this goal, the programming of suitable finite elements is intended. Both the development and the optimization of the computational models will be performed using a global approach and an implicit code. When generating mathematical models for mitigation concepts based on the principle of bubble curtains, interactions between single bubbles have to be taken into account.
In a first step, a mathematical model for the mitigation concept of the bubble curtain, developed within a recently completed research project, will be upgraded, taking into account effects of sound scattering and measured bubble size distribution.
As a next step, the simulation models for bubble curtains and typical further concepts, like noise mitigation screens, IHC-sleeve, cofferdam, different pile sleeves etc. will be developed. Combinations of the aforementioned will be considered, too. Regarding the bubble curtain, results of numerical studies simulating local interaction effects of single bubbles will be taken into account. The upgraded model of the previous work package serves for verification.
After having accomplished modelling, the simulation codes will be validated by data gained from different offshore experiments, in which underwater sound pressures for several foundation types and distances will have been detected. In the following and as a result of the comparison with measurements, last model modifications might be necessary.
The validated, sophisticated computational models will be implemented into the overall acoustic propagation model of TUHH/IMB. During a next step, last model optimizations can be performed, if required. Finally, sensitivity analyses for different boundary conditions and system properties of the single mitigation concepts will be carried out.
Within the last phase of the project, LUH/ISD will coordinate the compilation of an expert system for estimating the pile driving noise during OWT construction. All results of the above mentioned sub work packages will be integrated into the underlying knowledge base.
AP 4 (TUHH/IMB)
The work within AP 4 aims at the development of a computational model for simulating the propagation of pile driving noise in the water. The hydroacoustic waves, emitted by the movement of the pile and the seabed that will be determined within AP 1 and AP 2, respectively, serve as starting conditions. Furthermore, the sound attenuating effect of bubble curtains, for which corresponding models will be created within AP 3, as well as other mitigation measures are taken into account.
As a first step, the near field of the water column is modeled in a radius of about 50 m around the pile. Therefore, a suitable modeling technique, e.g. the finite element method, will be applied. The near field model will be coupled both to the pile structure and the seabed. To assure an undisturbed propagation of the hydroacoustic waves at the borders of the discretised water domain, free field conditions at the element free faces will be realized by using, e.g., special boundary conditions, perfectly matched layer approaches, or infinite elements.
In a second step, the far field of the water column up to a distance of several kilometers to the pile will be computed. Due to the huge dimension of the domain of interest, a more global approach, e.g. based on normal modes or ray tracing techniques, will be applied. The far field model will exhibit a coupling to the seabed as well as to the near field model to consider their corresponding contributions to underwater noise.
In the course of the project, the overall model will continuously be validated both with laboratory experiments as well as with three comprehensive offshore tests, in which the pile and soil dynamics and the resulting hydroacoustic signals are measured with a high degree of detail (for more information, see despriction of AP 1). To enable a broad validation of the simulation model, different foundation types, e.g. tripiles, tripods, and monopiles, as well as different sound attenuation techniques should be investigated within the offshore measurement campaigns. A detailed comparison of calculated and measured data will be achieved by setting up a corresponding simulation model for each of the three offshore tests, which considers the particular construction and the special conditions at the offshore location.
The detailed and sophisticated computational model described above allows for a comprehensive prediction of the resulting underwater sound field. However, a profound knowledge of numerical modeling techniques, the access to substantial computer resources, and the availability of special simulation software are basic preconditions to set up and execute such a model. Therefore, an additional expert system for estimating the pile driving noise during OWT construction will be developed, which can also be handled by users without an experienced background in numerical simulation and acoustics. The expert system will be based on a knowledge base, which is generated by executing the detailed computational model for a broad range of different parameter combinations regarding foundation type, pile dimensions, soil conditions, attenuation measures, etc.