Offshore piling noise attenuation: theory and new applications on monopiles and jacket foundations

Karl-Heinz ELMER; Benedikt BRUNS; Christian KUHN OffNoise-Solutions GmbH, Neustadt a. Rbge., Germany

ABSTRACT Since some years a noise prevention concept for the protection of marine animals exists in Germany. Based on that, the acoustic underwater noise from the pile driving at offshore wind farms is required to be less than 160 dB (SEL) at a distance of 750 m. This value, however, is often exceeded so that the use of a soundproofing system is necessary. Piling noise is radiated from the pile surface directly into the surrounding water and deeper into the soil as travelling waves. These waves can interact with the water and induce additional underwater noise far away from the piling site. Underwater noise in the far field is usually reduced by bubble curtains with diameters of 150 -250m. For the special near field conditions and radiation of underwater noise into the surrounding water a very effective noise mitigation method is developed. It uses small elastic elements of different sizes and materials in the surrounding water to produce reflections, scattering effects and material damping effects with resonant vibrations in the water. This leads to controllable effective underwater noise reductions in the near field of radiated piling noise. The effectiveness of the arranged elements were investigated in laboratory tests and in situ under offshore conditions while different offshore installations. In the meantime two offshore wind farms with monopile foundations were installed by realizing these near field damping elements in combination with a far field big bubble curtain. Also the noise mitigation was improved and was determined by an acoustic institute to up to 25 dB (SEL) at a distance of 750m. In current projects the system will be used also during pre-piling for jacket foundations and for large the installation of large monopoles with today`s biggest offshore pile drivers. This article describes the theory, flexibility and the implementation of the effective noise mitigation method during previous and current projects. Furthermore a summary will be given about the results to show the effectiveness of the whole noise mitigation system. Keywords: Underwater noise, noise control on offshore platform, dynamic absorbers I-INCE Classification of Subjects Number(s): 54.3, 54.2, 47.4

1. INTRODUCTION

1.1 Problem of underwater piling noise The founding of offshore wind turbines by means of pile driving induces considerable hydro sound

emissions. Peak sound pressure levels have been measured high above 200 dB re 1 μPa at a distance of 750m from the pile driver (1). This construction noise of offshore wind turbines is potentially harmful to marine life, in particular to marine mammals and it induces flee reactions in a large area, as water is a very efficient conductor of sound.

Different zones of underwater noise immissions can be defined in the surrounding of a source of acoustic noise. Within the zone of audibility with moderate exposure levels, marine animals like harbour porpoises, harbour seals, grey seals and also fish will show some kind of reaction or change in their behaviour .

At higher exposure levels, within the closer zone, underwater noise can induce temporary (TTS) or permanent (PTS) threshold shift. Important acoustic information might be masked caused by reductions in hearing sensitivity of an animal.

Close to a very loud source of noise like pile driving, extreme intensity levels of underwater noise can cause physical trauma or death.

Even the lowest level of damage, which is a temporary threshold shift (TTS), must be avoided.

INTER-NOISE 2016

7169

Due to larger piles requiring higher driving energies, even higher underwater noise levels are

expected in future offshore projects and this is also accompanied by an increasing number of erected offshore wind turbines.

The mitigation of underwater noise is necessary, getting sound levels below recommended acoustic emission thresholds that are no longer harmful and disturbing to marine mammals and other protected animals. Numerical simulations of offshore systems and measurements and investigations of underwater noise during construction of offshore foundations are used to develop effective noise mitigation systems.

1.2 Radiation and propagation of piling noise The piling, in particular using hydraulic hammers creates high frequency noise with considerable

underwater sound levels during the installation of offshore wind foundations such as monopiles, tripods, tripiles and jackets. Numerical simulations and measurements of underwater noise during construction of the first offshore research platform in the North Sea FINO1 were already used to study the generation, radiation and attenuation of underwater noise (1).

The aim was to determine the impact area of offshore wind farms, to allow the formulation of recommendations for acoustic emission thresholds for offshore wind farms in cooperation with biologists, to develop standard procedures for the determination and assessment of nois e emissions and to get base information about the underwater noise to develop forecasting hydro sound models and to develop noise mitigation systems.

Most of the impact energy of the hydraulic hammer is driven into the sea ground. Only a small amount of the whole ram energy is radiated directly from the wet surface of the pile into the surrounding water inducing very high underwater sound levels. Depending on the properties of the ground material one part of the impact energy is radiated indirectly from the sea ground into the water, resulting in additional underwater sound.

Numerical simulations and measurements of the impacts of a pile hammer show that the resulting traveling wave within the piles is reflected up to several times at stepped cross sections of a pile and at both ends of a pile after Figure 1 until all the kinetic energy is damped out and radiated into the ground. This traveling wave induces sound waves in the surrounding water, propagating with the speed of sound of water of about 1500m/s into all directions of the shallow water after Figure 1, reflected at the free water surface and at the sea ground.

Figure 1: Traveling impact wave within the pile inducing underwater sound waves and reflections of the

underwater sound waves at the free surface of the water and at the sea ground after (1).

INTER-NOISE 2016

7170

1.3 Measuring of piling noise Underwater piling noise is usually described by two sound levels. The first level is the peak Sound

Pressure Level (peak SPL) in decibels (dB) of the maximum instantaneous posit ive or negative sound pressure |ppeak| of the measured impact noise that is referred to the underwater sound pressure of p0 = 1 Pa.

0

log20p

pSPLpeak peak

in dB re:1 Pa. ´ (1) The second quantity for describing pile driving underwater noise is the Sound Exposure Level SEL

in decibels (e.g. dB re:1 Pa2s), which is an equivalent energy level of the noise of a single pile driving impulse, based on T0 = 1s.

2

120

2

0

)(1log10T

T

dtptp

TSEL

in dB re:1 Pa2s. (2) The SEL is the level of a continuous sound with 1s duration and the same sound energy as the pile

driving impulse. Measurements of the underwater piling noise show peak levels of more than 210 dB (SPL) re 1 Pa

and sound exposure levels of more than 180 dB (SEL) re 1 Pa2s at a distance of 750 m from pile driving sites, depending on ram energy and pile size.

Hence, effective noise reducing methods are in great demand to keep the German Federal Maritime and Hydrographic Agency (BSH) standard sound exposure level of 160 dB SEL and the peak level of 190 dB (SPL) at 750m distance from offshore pile driving sites.

Spectral information of pile strokes are given by third octave spectra of the sound exposure levels

SEL of three different hydraulic hammers in Figure 2, measured at a distance of about 750 m. The highest spectral levels of the measured underwater ram noise of the hammers are shown in the

low frequency range from 100 to 300 Hz, responsible to the high broadband level of piling noise. They have to be reduced by effective noise mitigation measures like hydro sound damper systems. Spectral information of pile strokes are given by third-octave spectra of the sound exposure levels

(SEL) of three different hydraulic hammers in Figure 2. The highest spectral levels of the measured underwater ram noise of the hammers are shown in the low frequency range from 100 to 300 Hz, responsible to the high broadband level of piling noise.

150

155

160

165

170

175

180

185

190

195

200

10 100 1000 10000

SEL

dB re

1μP

a

frequency [Hz]

MENCK (ESRa - 300 kJ)IHC (London Array - 300 kJ)

0 0.05 0.1 0.15 0.2 0.25-1.5

-1

-0.5

0

0.5

1x 105

Am

plitu

de [P

a]

Zeit [s]

0 0.05 0.1 0.15 0.2 0.25-1.5

-1

-0.5

0

0.5

1x 105

Zeit [s]

Am

plitu

de [P

a]

Referenzschlag MENCK (ESRa) 300 kJ

Referenzschlag IHC (London Array) 300 kJ

MENCK: SEL 194 dB, LPeak 222 dB

IHC: SEL 199 dB, LPeak 219 dB

Figure 2: Third octave spectra of measured underwater noise of offshore projects with different hydraulic

hammers, (2).

INTER-NOISE 2016

7171

2. MITIGATION METHOD OF HYDRO SOUND DAMPERS

Air is ideal to reduce hydro sound. The new patented noise mitigation method of hydro sound dampers (HSD) only uses small thin gas

filled latex balloons and robust PE-foam elements, fixed to a pile surrounding fishing net and tuned to resonant frequencies to get very effective underwater noise reductions.

Steel is only used as heavy weights at the sea ground to compensate buoyancy of HSD elements. At these volume rates vertical forces from buoyancy and horizontal forces from tide currents are

still small.

Figure 3: Staggered HSD-net, large fishing net Figure 4: HSD-net fixed to a ram or piling frame and telescopic frames after (3). or covering the ground after (3).

The size of the bodies, the effective frequency range, the damping rate, the number and distribution

of the hydro sound dampers (HSD) and the influence from hydrostatic water pressure can be fully con -trolled, if the envelope bodies are fixed to pile surrounding fishing nets or to frames in Figure 4. Covering the sea ground by HSD nets after Figure 4 allows to reduce the additional underwater sound that is indirectly radiated from the sea ground into the water.

The HSD-system of Figure 5 is a donut-like container, enclosing the HSD-net. It can be fixed below

a piling frame, or below the hydraulic ram, or it is swimming round the pile as shown in Figure 5.

Figure 5. HSD-net enclosed in a donut-like container after (2).

INTER-NOISE 2016

7172

In contrast to conventional air bubbles, hydro sound dampers of both kinds, gas filled balloons and PE-foam elements allow to use three different physical effects for underwater noise attenuation:

Resonant effects of air filled small latex balloons in water increase the scattering and

extinction cross sections of the vibrating balloons by a factor of 1000 and more compared to the physical cross section, thus reducing underwater sound up to 35 dB and more as it is known from small air bubbles in water. The resonance frequency of an air filled latex balloon in water is adjustable, even to low frequency ranges, in contrast to free natural air bubbles, and it is inversely proportional to the diameter of a balloon. It is also depending on the gas pressure inside, the water depth and the stiffness of the encapsulating material. The resonant effect is of course frequency dependent and acts as a very effective noise mitigation method near the resonance frequency of a balloon.

Dissipation and material damping effects according to the material damping potential of the

envelope material and the filling material inside the vibrating balloons and foam elements. Maximum damping is obtained near the resonant frequency of a damped HSD-element, achieving noise reductions between 10 and 30 dB (SEL).

Reflections of sound waves at impedance steps, as air filled balloons in water increase the

compressibility of the mixed water-body, decrease the bulk modulus of the mixture and decrease the sound speed and the specific impedance of the mixture very much. There are sound reflections at the transition from original water to the water volume, filled with HSD-balloons of about 1 – 3 % volume rates, resulting in noise reductions between 5 and 15 dB (SEL). This effect of changing the material property of the surrounding water is not frequency dependent.

The important resonant effect with high scattering, multiple reflections and effective absorption of

sound waves in the water is to be seen in Figure 6. The very strong interaction of a vibrating HSD-element with the surrounding water is to be seen at the water surface in Figure 6. This interaction also takes place under water, but it is not visible there.

Figure 6: Scattering, radiation and strong interaction of a vibrating HSD-element after (4).

Hydro sound dampers are used in the whole frequency range of pile driving noise from 30 Hz up to

several kHz. It is possible to control the damping rate, the size, the number and the distribution of the HSD around the pile.

The efficacy of hydro sound dampers in reducing underwater noise depends on the natural frequencies of the HSD-elements and on the volume rate. Rates of about 1-2% of the HSD are already sufficient to obtain good results. At these volume rates vertical forces from buoyancy and horizontal forces from tide currents are still small.

Finally, hydro sound dampers are not expensive and they don’t need compressed air supply.

INTER-NOISE 2016

7173

SEL

[dB

re 1

P

a]

150

160

170

180

190

200

f [Hz]10 100 1000 10000

Referenz NMS HSD

23 dB 23 dB 23 dB

m

3. APPLICATIONS OF HYDRO SOUND DAMPERS

3.1 HSD offshore test “ESRa”, Germany, 2012 Offshore test results in the Baltic Sea confirm the high underwater sound attenuation of both, gas

filled balloons and PE-foam elements. The first tested HSD-System is a self-swimming construction of 10t weight of the offshore test ESRa in Germany, 2012, in Figure 7. All elements are tuned to the resonance frequency of 120 Hz. The radiated noise was measured at 4 m above the ground at a distance of 6 m from the pile to get most of the directly radiated sound and to avoid influences of reflections from the sea bed. Figure 8 shows the SEL spectrum of the nearfield noise mitigation. There is a very broad noise reduction up to 23 dB (SEL) within the most important range of 100 - 600 Hz after [5]. That means, 99.5% of the sound energy is damped out although the HSD-net is only covered by less than 10% of its surface. Higher frequencies and smaller elements are not tested.

Figure 7: Hydraulic hammer, test pile and HSD mitigation system of ESRa test round the pile, [4].

Figure. 8: HSD during ESRa-Test (left), 1/3 octave SEL spectra of underwater with and without HSD,[4].

3.2 HSD offshore test “London Array”, GB, 2012 Another offshore test was done at the London Array (LA) wind farm in August 2012 in the North

Sea nearby the coast of the United Kingdom. The designed HSD is a self-expanding system with a total weight of only 17t and a diameter of 9 m after Figures 9. There are three parts: the buoyancy ring at the

INTER-NOISE 2016

7174

water surface, the HSD net and the ballast box. The compressed HSD has a height of 1,8m and is applicable in variable water depth of up to 28m as described in [5].

In addition to that, smaller and larger elements are applied to get a better noise reduction in the frequency range below 100 Hz and higher than 1000 Hz. The underwater sound mitigation in Figure 10 was measured at 1 m above the seabed at a distance of 15 m. Figure 10 shows the 1/3 octave SEL spectrum of the original piling noise and reduced noise with HSD in use.

The impact of the additional applied HSD-elements causes increased reductions of the nearfield noise up to 21dB between 20-00 Hz and above 1kHz. There is a broad noise reduction of 23 dB (SEL).

Figure. 9: HSD during London Array-Test, [5].

SEL

[dB

re 1

P

a²s]

150

160

170

180

190

200

f [Hz]10 100 1000 10000

Referenzpfahl F04, SEL: 203 dB NMS HSD Pfahl F05, SEL: 187 dB

20 dB

23 dB21 dB

m

18 dB

Figure 10: Third octave SEL spectra of under water piling noise with and without HSD, [5].

3.3 HSD offshore application “OWF Amrumbank West”, 2014 The first serial application was done within the monopile installation of the windfarm Amrumbank

West in the German North Sea in 2014. As hydraulic piling hammer the MENCK MHU 1999S was used.

For the noise reduction of the 6.0 m in diameter monopiles in up to 25m water depth a double bubble curtain and the HSD-system was used. The implemented HSD system was connected with a winch frame to the main crane hook. Figure 11 (left) shows the rigging with eight winch wires to control the system. The other pictures in this figure presenting the net box hanging above the monopile and the fully installed net in the water during piling.

The noise reduction of HSD and big bubble curtain within this project was determined up to 22 dB SEL at a distance of 750 m to the monopile.

Almost in the near field measurement figure 12 shows noise reductions up to 23 dB in certain frequency ranges.

INTER-NOISE 2016

7175

Figure. 11: HSD rigging (left), HSD-System above monopile (middle), HSD during piling (right)

SEL

[dB

re 1

μ

Pa²s

]

150

160

170

180

190

200

f [Hz]10 100 1000 10000

reference HSD

18 dB 23 dB 23 dB

Figure. 12: HSD nearfield noise mitigation in OWF Amrumbank West [5].

3.4 HSD offshore application “OWF Sandbank”, 2015 In 2015 the HSD and a double bubble curtain were used for noise mitigation of the foundations for

OWF Sandbank in the German North Sea. The monopiles are 6.4m and 6.8m in diameter. The water depth is between 25 m and 34 m. The HSD system was located on a separate winch frame below the pile gripper as visualized in Figure 13. With this solution the net box was controlled in parallel to other works of the main hook which caused no time delay. The picture on the right shows the site during piling. The blue colored HSD is shown on starboard side of the aft of the installation vessel.

Piling was done with a MENCK MHU 3500. The challenge was to reduce the predicted noise

immission of up to 180 dB. In total there occurred no problems regarding the noise limits. In [6] an overall noise reduction of the Sandbank piling using a HSD-system combined with a double bubble curtain of between 19 and 25 dB is published.

INTER-NOISE 2016

7176

Kran

Installationsschiff

Pfahl-führung

Hammer

HSD-Netz

Netzbox

Pfah

l

Winden-rahmen

© Bruns, 2015 Figure. 13: HSD-system implementation at OWF Sandbank, HSD-System above monopole

Foto by Bilfinger

Beeg, Philipp, 2015

Figure. 14: OWF Sandbank noise mitigation: combination of HSD-System and a double bubble curtain [6].

3.5 HSD offshore application “OWF Veja Mate”, 2016 A comparable system in Figure 15and 16 is currently in use for the foundation installation of OWF

Veja Mate also in the German North Sea. The HSD net box is connected to the lower gripper frame and controlled by winches. Due to this there is no delay in time. The monopiles of a diameter of 7.8m are installed in water depth of 40 m. For piling an IHC S-4000 is used. Results of the noise measurements are not published yet but both system are working well.

http://koopvaardij.blogspot.de Figure 15: Pile gripper with HSD net box during sea fastening position.

INTER-NOISE 2016

7177

http://koopvaardij.blogspot.de Figure 16: Pile gripper with HSD net box during piling.

3.6 HSD offshore application “OWF Wikinger”, 2016 At OWF Wikinger in the Baltic Sea jacket foundations with pre-piling are currently installed. The

pin piles have a diameter of 2.7m, the water depth is 40m. The noise mitigation concept includes a double big bubble curtain and the HSD as a backup for exceeding the limits. The system is also located on a separate frame and controlled by winches. Results are not published yet but both systems are working well.

3.7 Summary of HSD offshore applications systems The following table 1 gives an overview about the different applications of the HSD with the

project specific conditions.

Table 1 – Developing process of Hydro Sound Damper systems

OWF London Array

Amrumbank West Sandbank Veja Mate Wikinger

year 2012 2014-2015 2015-2016 2016 2016

foundation type Monopile monopile monopile monopile Jacket (prepiling)

pile diameter [m] 5.7m 6.0m 6.4m; 6.8m 7.8m 2.7m

water depth 25m 25m 34m 40m 40m

hammer IHC S-1400 MHU 1900S MHU 3500 IHC S-4000 MHU 1200

HSD implementation

single system prototype

connected to hammer separate frame separate frame separate frame

Noise reduction of

HSD and DBBC

(SEL@750m)

14 – 22 dB 19 – 25 dB not published not published

INTER-NOISE 2016

7178

4. CONCLUSIONS The innovative Hydro Sound Dampers (HSD) have been demonstrated to be a cost -effective

method of attenuating underwater piling noise in offshore marine piling projects. HSD is also applicable as an effective general method for reducing underwater piling no ise in ports and harbors. HSD may achieve reductions of more than 10 dB in underwater piling noise, even in the presence of strong tidal currents. HSD systems are small, not expensive and easy to handle by piling contractors for port and harbor applications.

The use of HSD will significantly reduce the observation area required during piling operations and provide improved protection of the marine environment from potential adverse impacts upon marine life from impulsive and cumulative underwater piling noise exposure.

Offshore measurements of using Hydro Sound Dampers in combination with an air bubble curtain result in noise reductions between 19 and 25 dB (SEL) after [6]. HSD-systems are already patented in Europe, USA, Germany and Australia, including all kind of air filled enveloped bodies, encapsulated bubbles and foam elements. Further international PCT patents are pending.

REFERENCES 1. Elmer, K.-H., Betke, K., & Neumann, T. (2007): “Standard Procedures for the Determination and As-

sessment of Noise Impact on Sea Life by OWF”,(German) BMU report 0329947, 2007. 2. Bruns, B., Kuhn, C., Stein, P., Gattermann, J., Elmer, K.-H.: The new noise mitigation system ‘Hydro

Sound Dampers’: History of development with several hydro sound and vibration measurements, inter-noise 2014 Conference, Melbourne, Australia, 2014.

3. Elmer, K.-H. (2010) “Pile driving noise red. using new HSD”, BSH-workshop on pile driving, ESC2010,Stralsund.

4. Elmer, K.-H., Gattermann,J., Kuhn, C., Bruns, B. & Stahlmann, J. (2012) “Mitigation of offshore piling noise using balloons and PE-foam elements as hydro sound dampers (HSD)”, Proceed. DEWEK2012 Conference, Bremen, G, Nov. 7/8, 2012.

5. Bruns, B., Kuhn, C., Stein, P., Gattermann, J., Elmer, K.-H. & Imhorst, F. (2012) “Underwater noise reduction from measurements and offshore tests using systems of hydro sound dam-pers (HSD)”, Proceed. DEWEK2012 Conference, Bremen, G, Nov. 7/8, 2012.

6. Beeg, O., Philipp, E.: Developer Case Study Dan Tysk & Sandbank, Windpower Monthly Foundations meeting Bremen 04.11.2015, Bremen, November 2015

INTER-NOISE 2016

7179