Klaus Marco Gttsche, M.Eng Peter Mller Juhl, Ph.D Dr.-Ing. Ulrich - - PowerPoint PPT Presentation

Numerical prediction of underwater noise reduction during offshore pile driving by a Small Bubble Curtain

Klaus Marco Göttsche, M.Eng Peter Møller Juhl, Ph.D Dr.-Ing. Ulrich Steinhagen

Associated with the Joint Research Project between MENCK GmbH and BARD Engineering GmbH “Untersuchung und Erprobung eines Kleinen Blasenschleiers zur Minderung von Unterwasserschall bei Rammarbeiten für Gründungen von OWEA” funded by the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety due to an act of the German Parliament, Project 0325334G

Numerical prediction of underwater noise reduction during offshore pile driving by a Small Bubble Curtain

• 15. – 18. September 2013

Contents

• Motivation
• Concept of the Small Bubble Curtain
• Simulation of the unprotected case
• Attenuation due to Bubble Curtain
• Comparison to measurements
• Conclusion

Numerical prediction of underwater noise reduction during offshore pile driving by a Small Bubble Curtain

• 15. – 18. September 2013

Motivation

• Driven steel piles are the most common method for funding offshore wind

turbines and platforms.

• Piles are driven by hydraulic hammers up to 40 m deep.
• This leads to high noise emission under water.
• To protect the maritime fauna, the noise level has to be reduces (e.g. in

Germany: 160 dB SEL in 750 m).

• Small Bubble Curtains (SBC) are an effective method to reduce the noise.
• The goal is to develop a numerical method to predict the noise level due

to pile driving and to determine the attenuation due to a SBC.

Numerical prediction of underwater noise reduction during offshore pile driving by a Small Bubble Curtain

• 15. – 18. September 2013

Concept of the Small Bubble Curtain

Prototype of SBC by MENCK

• The pile is enclosed by a system of hoses or pipes.
• Air is injected through holes.
• Air bubbles surround the entire pile.
• Noise reduction by:
• Air bubbles reduce impedance of the water
• Reduced radiation due to pile vibration
• Reflection, Absorption and Scattering of

pressure wave

Numerical prediction of underwater noise reduction during offshore pile driving by a Small Bubble Curtain

• 15. – 18. September 2013

Simulation of the unprotected case

• The noise radiation in the near field is

solved by FEM.

• An axial symmetric model containing

solid and acoustic fluid elements is used.

• The transient pressure is exported for a

vertical array of points.

• The pressure spectrum is determined for

each point by FFT. Near field by FEM

Numerical prediction of underwater noise reduction during offshore pile driving by a Small Bubble Curtain

• 15. – 18. September 2013

Simulation of the unprotected case

• Transmission Loss is determined

for each source to a common receiver

• TL is applied to the spectrum from

the FEM

• Receiving spectrum is determined
• SEL is determined by integration of

the spectrum TL by e.g. Parabolic equation / Wavenumber integration Near field by FEM

Numerical prediction of underwater noise reduction during offshore pile driving by a Small Bubble Curtain

• 15. – 18. September 2013

Attenuation due to bubble curtain

• The bubble distribution is

determined by CFD-Simulation using Euler particle model.

• The volume fraction is exported

in a 2D grid.

• The depth division is the same

like the sources from the FEM.

• Within each cell, the Volume

Fraction is assumed to be constant.

Numerical prediction of underwater noise reduction during offshore pile driving by a Small Bubble Curtain

• 15. – 18. September 2013

Attenuation due to bubble curtain

• A uniform or Gaussian bubble

distribution is assumed for each cell.

• The transmission coefficient for

each cell is determined by Effective Medium Approach (EMA) as a function of frequency, range and depth.

• EMA considers bubble cloud as an
• scillating continuum.

Numerical prediction of underwater noise reduction during offshore pile driving by a Small Bubble Curtain

• 15. – 18. September 2013

Attenuation due to bubble curtain

• For each depth a total transmission

coefficient is determined.

• The transmission coefficient is

considered as reduction of the source strength. TL by e.g. Parabolic equation / Wavenumber integration Near field by FEM Attenuation due to EMA

Numerical prediction of underwater noise reduction during offshore pile driving by a Small Bubble Curtain

• 15. – 18. September 2013

Comparison to measurements

750 m Distance from the pile, 30 m below water level

SEL without SBC SEL with SBC Attenuation Measurement 173.6 dB 159.8 dB 13.8 dB Simulation 177.6 dB 159.3 dB 18.3 dB

Numerical prediction of underwater noise reduction during offshore pile driving by a Small Bubble Curtain

• 15. – 18. September 2013

Thank you for your attention !