Norro, A.

Open Marine Archive

19
CHARACTERIZATION OF THE UNDERWATER 
SOUND EMITTED DURING THE INSTALLATION  
OF MONOPILE STEEL FOUNDATIONS  
AT THE NOBELWIND OFFSHORE WINDFARM  
AND CUMULATIVE EFFECTS
Royal Belgian Institute of Natural Sciences (RBINS), Operational Directorate Natural Environment 
(OD Nature), Aquatic and Terrestrial Ecology (ATECO), Marine Ecology and Management Section 
(MARECO), Gulledelle 100, 1200 Brussels, Belgium.
NORRO Alain
CHAPTER 2
Abstract
The construction works of the Nobelwind 
wind farm offshore on the Belgian coast was 
monitored for the emission of energy into the 
sea by means of underwater sound (pressure). 
Five complete piling events which cover the 
driving to full depth of 5 steel monopiles of 
5 m diameter using a hydraulic hammer with 
a maximum power of 1400 kJ are described. 
No direct mitigation of the produced sound 
pressure was used and at 750 m distance, 
measured maximum SELss ranged from 166 
to 174 dB re 1µ Pa2 s. Different metrics are 
proposed for the assessment of the cumula-
tive effect of the piling works. SELcum ranged 
from 201 to 209 dB re 1µ Pa2 s. Number of 
strokes used ranged from 2582 to 3696 while 
the energy used ranged respectively from 
1764080 to 4048143 kJ. During piling work 
foreseen for 2018 and 2019, the behavioural 
response zone for the harbour porpoise could 
reach some 2800 km2. 
1. Introduction
High intensity impulsive sound like the one 
produced by underwater explosions, pile 
driving or seismic surveys using air-guns 
are known to affect marine life adversely 
(Hawkins & Popper 2016). Most of the avail-
able studies characterize local exposition 
to impulsive underwater sound (de Jong & 
Ainslie 2008; Bailey et al. 2010; Norro 
et al. 2013; Haelters et al. 2015; Popper & 
Hawkins 2012; 2016). Today, the develop-
ment of marine renewable energy (MRE) in 
the North Sea shows numerous new projects. 
The risk exists that cumulative effects result-
ing from simultaneous construction or oper-
ation may affect marine life not anymore at 
the individual level but at a population lev-
el. Underwater sounds propagate at about 
1500 ms-1 over large distances and are not 
stopped by national boundaries making the 
assessment of cumulative sound pressure a 
regional matter. 
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Norro 
The purpose of this report was (1) to 
characterize the emitted underwater sound 
during piling events, (2) to evaluate the 
emitted sound during the piling of the foun-
dation of the Nobelwind offshore high vol-
tage station (OHVS) that is, currently the lar-
gest pile ever piled in the Belgian part of the 
North Sea (BPNS), and (3) to quantify cu-
mulative	effects	and	define	the	spatial	extent	
of behavioural response zone for the harbour 
porpoise. 
2. Material and methods
2.1. Study area
The Nobelwind wind farm is situated off-
shore the Belgian North Sea coast on the 
Bligh	Bank	(fig.	1).	The	wind	farm	actually	
represents the second phase of construction 
extending the Belwind wind farm operation-
al since January 2011. This second phase 
of the wind farm construction requires the 
installation of 50 steel monopiles of 5 m 
diameter (lower end) and of lengths ranging 
from 54 m to 76 m. One additional monopile 
for the offshore high voltage station (OHVS) 
was installed in the concession zone. This 
steel monopile of 6.8 m diameter (lower 
end) and a length of 72 m represents the larg-
est monopile ever driven into the Belgian 
seabed. 
The	 first	 steel	 monopile	 of	 the	
Nobelwind offshore wind farm was installed 
on 16 May 2016 (BBK01) and the last one 
was piled on 22 September 2016 (BBI04). 
During the piling construction works under-
water	 sound	 emitted	 during	 five	 complete	
pile driving events was recorded by means 
Figure 1. Implementation of the Nobelwind windfarm (NB on the map). In pink what is foreseen for 
construction in 2018 and in purple what is foreseen for 2019.
 Chapter 2. Installation of monopile steel foundations: underwater sounds and effects
21
of a moored station. A 1400 kJ hammer op-
erated from a jacking-up platform was used 
for the piling job. No directly emitted sound 
mitigation was in place but the construc-
tion permit (granted in 2008 and extended 
in 2015) required the use of an acoustic de-
terrent	(Lofitech	seal	scarer),	to	be	deployed	
one hour before piling starts and for a “ramp 
up” or “soft start” procedure to be used at the 
beginning of every piling event.
2.2.  Underwater sound measurement 
 equipment
Underwater sound was recorded from a 
moored station consisting of an instrument-
ed	 tripod	 (fig.	 2).	The	 tripod	was	 equipped	
with a complete measurement chain includ-
ing a recorder RTsys EA-SDA14 and one hy-
drophone B&K 8104. RTsys calibrated the 
complete measurement chain prior to ship-
ping from the factory. The calibration was 
verified	 using	 a	 calibrator	B&K	4229	 (pis-
ton-phone) prior to every deployment. 
The instrumented tripod was de-
ployed on 14 August 2016 using an 
acoustic release to lower it to the seabed in 
the vicinity of the planned piling location 
(WGS84 N 51° 39,875; E 002° 50,590 by 
38 m depth relative to mean sea level). As 
such, the distance between the measuring 
equipment and the piling locations ranged 
from 850 to 3600 m. No surface marker 
was left on site to reduce navigation risk 
inside the construction zone as well as to 
avoid any perturbing sound originating from 
a line linking a surface buoy to the tripod. 
Scientific	divers	serviced	 the	 instrument	on	
27 August 2016 and retrieved the recording 
instrument on 25 October 2016. 
2.3.  Underwater sound measurements 
and post-treatment
Sound pressure was recorded continuously 
at a sampling rate of 78125 Hz and stored on 
hard drive coded in WAVE format.
During the period of deployment, the 
following piling events (table 1) were fully 
recorded. 
Homemade routines in MATLAB were 
used for the post treatment of the records. 
Figure 2. Tripod with RTsys sound recorder and B&K 8104 hydrophone mounted, C-POD and acoustic 
release ready for deployment on the rear deck of the MTS Valour (© Alain Norro/RBINS).
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Norro 
Sound exposure level for a single strike and 
for a full piling event (SELss and SELcum) 
as well as the normalization of levels to the 
reference distance of 750 m were computed 
according to Norro et al. 2013.
Because the intensity of the sound emit-
ted depends on the size of the sound source, 
the intensity of the sound increases with the 
pile diameter. ITAP in Germany proposes an 
experimental model (Bellmann et al. 2017) 
that permits estimation of both SEL and Lz-p 
from the diameter of the monopile to be driv-
en into the sediment. The ITAP model will 
be used to estimate level generated by the 
piling of the OHVS.
2.4. Cumulative effects
Cumulative effects characterize the effects 
resulting from the total number of strokes 
required for the installation of a single mo-
nopile or for a complete wind farm or even 
for the construction of a cluster of wind 
farms forming a zone of energy produc-
tion. Metrics are elaborated to try to provi-
de such integrated information. The cumu-
lative sound exposure level (SELcum) is one 
of those metrics. In such context, the total 
energy spent during the complete operation 
of the piling of one monopile or wind farm 
or cluster of wind farms is another metric.
Worst-case scenarios for cumulative 
assessment occur here when construction 
in both Belgian and Netherland waters are 
simultaneously	 conducted	 (fig.	 1).	 In	 this	
case large zones around these works may 
impact the behaviour of harbour porpoises. 
De Jong et al. (2017) estimated that a SELss 
level of 140 dB re 1µPa2 s should not be ex-
ceeded to guarantee absence of behavioural 
response for the harbour porpoises. Based 
on such threshold and the propagation mod-
el proposed by Norro et al. (2013), impact 
area size are estimated by drawing circles 
centered on every new project foreseen for 
a given year.
3. Results
Not considering the OHVS, the measured 
Lz-p ranged from 190 to 198 dB re 1µPa 
while SELss ranged respectively from 166 to 
174 dB re 1 µPa2 s. Piling of the 6.8 m di-
ameter monopile used for the OHSV is mod-
elled to increase the maximum reported Lz-p 
by 5 dB and the SELss by 4 dB.
The installation of the OHVS required 
about twice the number of strokes compared 
to BBH03 and about three times the energy 
spent for piling BBH03. When consider-
ing the sound levels to be similar to the one 
measured for other monopiles (OHSVsimilar), 
a SELcum reaching 211 dB re 1 µPa
2 s is es-
timated for piling the OHVS pile. When ap-
plying the ITAP model (OHSVmod Itap), a Lz-p 
of 203 dB re 1 µPa and a maximum SELss of 
178 dB re 1 µPa2 s was estimated.
The propagation model proposed by 
Norro et al. (2013) estimated a 20 km radius 
impact zone centered on the sound emission 
point (OHVS not considered here).
Based on this and on the location of fu-
ture	piling	sites	presented	at	fig.	1,	one	can	
estimate a 2800 km2 behavioural response 
zone for harbour porpoise in the worst case 
scenario presented in this paper. 
4. Discussion
There is not only one adequate metric that 
could be used to translate the loudness of the 
Table 1. Data available for the Nobelwind 
windpark piling phase
	
Pile name Distance from 
recorder (m) 
Start End Total 
duration 
BBH01 1600 18/8/16 18/8/16 2h09 
BBH03 860 17/8/16 17/8/16 4h32 
BBH05 2100 30/8/16 30/8/16 4h24 
BBH06 2000 27/8/16 27/8/16 2h46 
BBH07 3400 31/8/16 31/8/16 2h10 
 Chapter 2. Installation of monopile steel foundations: underwater sounds and effects
23
produced sound to an effect on the marine 
biota. Hawkins and Popper (2016) showed 
that the cumulative sound exposure level 
(SELcum) introduced for marine mammals is 
not	the	appropriate	metric	to	be	used	for	fi-
shes and invertebrates. Hawkins and Popper 
(2016) propose characterizing the emitted 
sound using other metrics such as the sound 
exposure level of a single stroke (SELss) to-
gether with the total time of piling and the 
total number of strokes. 
The	 figures	 proposed	 here	 for	 a	 lim-
ited number of monopile installations can 
be extrapolated to the complete construc-
tion resulting in very high levels (above 
185 dB re 1 µPa Lz-p ) of underwater sound in 
the vicinity of the construction site.
Moreover, because of the concentration 
of the zones reserved for energy production 
(fig.	 1),	 a	 cross-border	 strategy	on	 cumula-
tive sound emissions needs to be encouraged 
should a reduction of excessive underwater 
sound be strived for in the near future.
The 20 km circle radius of be-
havioural disturbance for harbour porpoise 
(Phocoena phocoena)	confirms	the	radius	of	
16 km that was already proposed by Norro 
et al. (2013) for the major behavioural dis-
turbance zone based on a level peak uto peak 
(Lp-p) of 155 dB re 1 µPa. That radius was 
further	 confirmed	 by	Haelters	 et al. (2015) 
investigating harbour porpoise distribution 
changes during piling activities. There is a 
need for more research and standardization 
from the bio-acoustician in the development 
of behavioural response thresholds for ma-
rine mammals as well as for other animals 
like	fishes	and	invertebrates.	
In 2018, construction of three new wind 
farms in the zone is planned. Two of them 
will be installed in Dutch waters and one 
in Belgian waters. For 2019, the construc-
tion	 of	 another	 five	wind	 farms	 is	 planned	
for, with three inside Belgian and two inside 
Dutch waters. From the above it is clear that 
any construction inside the Belgian zone will 
impact Dutch waters and any construction 
inside most of the Dutch Borssele zone will 
impact Belgian waters. 
5. Conclusion
For a monopile of 5 m diameter and a hy-
dro-hammer of 1400 kJ without direct 
underwater sound mitigation, a Lz-p rang-
ing from 190 to 198 db re 1 µPa at 750 m 
Pile name Lz-p  
in  
dB re 1µ Pa 
 
Max SELss 
in  
dB re 1µ Pa2 s 
SEL cum 
in  
dB re 1µ Pa2 s 
Total 
energy 
(kJ) 
Number of 
strokes 
BBH01 197 174 209 2977919 3297 
BBH03 198 174 205 1764080 2582 
BBH05 196 171 206 2892379 3123 
BBH06 190 166 201 2229876 2753 
BBH07 191 169 205 4048143 3696 
OHVSsimilar ≈ ≈ ≈ 211 
5180744 5157 
OHVSmod Itap 203 178 215 
	
Table 2. Nobelwind construction phase 
Measured parameters Lz-p , Max SELss and SELCUM normalized at 750m distance from the pile. Total 
energy & number of strokes provide from the hammer log (Nobelwind data). Offshore High Voltage 
Station (OHVS) not monitored in situ but estimation proposed here in two options (see text).
24
Norro 
References
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underwater noise levels during pile-driving at an offshore windfarm and its potential effects on 
marine mammals. Marine Pollution Bulletin 60: 888-897.
Bellmann, M.A., Schuckenbrock, J., Gündert, S., Müller, M. & Holst Hand Remmers, P. 2017. 
Is there a state-of-the-art to reduce pile-driving noise? Presentations from the CWW2015 
Conferences. Berlin: Springer Verlag.
de Jong, C.A.F. & Ainslie, M.A. 2008. Underwater radiated noise due to the piling for the Q7 
Offshore Wind Park. Paris: Acoustics.
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marine	fishes	and	invertebrate.	ICES Journal of Marine Science 74 (3): 635-651. DOI:10.1093/
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Norro, A., Rumes, B.  & Degraer, S. 2013 . Differentiating between underwater construction noise 
of monopile and jacket foundations for offshore windmills: A case study from the Belgian part 
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from the source was detected, while at the 
same distance the SELss ranged from 166 
to 174 dB re 1 µPa2 s. An estimation of the 
emitted underwater sound resulting from the 
installation of a 6.8 m monopile used for the 
OHVS gave a Lz-p of about 200 dB re 1µPa 
at 750 m and a SELss of about 180 dB re 
1 µPa2 s.
A	zone	of	20	km	radius	was	confirmed	
as a behavioural response zone for harbour 
porpoise and concerns are highlighted for 
the coming year 2018 and 2019 when con-
struction of seven new windfarm projects is 
scheduled in the Belgian and adjacent Dutch 
(Borssele) offshore energy zones. During the 
year 2018 and 2019 the zone of behavioural 
response for harbour porpoises may reach 
some 2800 km2. 


Characterization of the underwater sound emitted during the installation of monopile steel foundations at the Nobelwind offshore windfarm and cumulative effects

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Characterization of the underwater sound emitted during the installation of monopile steel foundations at the Nobelwind offshore windfarm and cumulative effects

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