Norro, A.

Open Marine Archive

ON THE EFFECTIVENESS OF A SINGLE BIG 
BUBBLE CURTAIN AS MITIGATION MEASURE 
 FOR OFFSHORE WIND FARM PILING SOUND  
IN BELGIAN WATERS
Royal Belgian Institute of Natural Sciences (RBINS), Operational Directorate Natural Environment 
(OD Nature), Aquatic and Terrestrial Ecology (ATECO), Marine Ecology and Management (MARECO), 
Vautierstraat 29, 1000 Brussels, Belgium
Corresponding author: alain.norro@naturalsciences.be
NORRO Alain
CHAPTER 2
Abstract
The construction works of the Rentel wind 
farm off the Belgian coast was monitored 
for the emission of energy into the sea by 
means of underwater sound (pressure). 
Thirteen complete piling events were mon-
itored, covering the driving to full depth of 
13 steel monopiles of 7,8 m diameter using 
a hydraulic hammer with a maximum power 
of 4000 kJ. Sound mitigation in the form of 
a single big bubble curtain (BBC) was used. 
Measured zero to peak level (L
z-p
) normalized 
to 750 m distance from the source showed 
values ranging from 185 to 194 dB re 1µ Pa 
at the end of the piling event when maximal 
hammer energy is used (2100-4000 kJ). The 
efficiency of the BBC is in the lower range 
proposed by the literature with a reduction 
of a maximum of 11-13 dB re 1 µPa (L
z-p
). 
More than one mitigation measure should be 
used simultaneously in order to comply with 
the Belgian Marine Strategy Framework 
Directive requirements for such project. 
 
1. Introduction
The size of commercially available wind 
turbines has increased in the last decades. 
Whereas in 1991, the first offshore wind 
farm used 450 kW turbines currently pro-
jects typically use 8 MW or larger turbines. 
The offshore wind energy sector has had to 
adapt turbine foundation design in order to 
keep up with this increase in size. Taking 
into account the cost and construction time, 
the followed option has been by increa-
sing the size of the monopile foundations. 
However, more powerful hammers are re-
quired to drive such XL or XXL steel mo-
nopiles into the seafloor. As a result, higher 
levels of impulsive sound are introduced into 
the marine environment raising concerns 
about possible negative impacts on marine 
life (i.e, Popper & Hawkins 2012; 2016). In 
absence of mitigation measures, pile driving 
of an 8 m diameter monopile would emit im-
pulsive underwater sound zero to peak levels 
(L
z-p
) of about 204 dB re 1µ Pa at 750 m dis-
tance from the source (ITAP model, see be-
low). The reduction of the generated sound 
by sound mitigation measures hence no lon-
ger is an option but compulsory; this given 
19
the need to respect maximum admissible 
levels of sound defined at national level for 
the Marine Strategy Framework Directive 
(MSFD). For Belgium, that limit is set at a 
maximum L
z-p 
of 185 dB re 1µ Pa at 750 m 
from the source. In Germany, maximum L
z-p 
must stay below 190 dB re 1 µ Pa at 750 m 
and sound exposure level (SEL) must be be-
low 160 dB re 1 µ Pa2 s at 750 m from the 
sound source. 
In 2017, the company Rentel built a 
new wind farm off the Belgian coast locat-
ed between the C-Power and the Northwind 
wind farms. This new wind farm consists of 
42 monopiles with a diameter ranging from 
7.5 m (10 piles) over 7.8 m (26 monopiles) 
to 8 m (6 monopiles). For this project, the 
noise mitigation measure proposed and used 
was a single big bubble curtain (BBC).
The purpose of this report is (1) to quan-
tify the emitted underwater sound during pil-
ing events and (2) to assess and evaluate the 
efficiency of the noise mitigation measure 
(insertion loss).
2. Material and methods
2.1. Research strategy
Underwater sound generated by driving an 
8 m diameter XXL steel monopile into the 
seabed while applying a big bubble curtain 
as sound mitigation measure was measured in 
situ during construction. The L
z-p
, the sound 
exposure levels of a single stroke (SEL
ss
) 
and the cumulative sound exposure levels 
(SEL
cum
) were computed. The effectiveness 
of the sound mitigation measure was assessed 
comparing the measured value on site with the 
theoretical figures obtained for such monopile 
diameter using the ITAP model.
2.2. Construction activities
The first steel monopile of the Rentel 
offshore wind farm was installed on 
21 July 2017 (RC03) and the last one was 
piled on 23 September 2017 (RD05). During 
construction, underwater sound was record-
ed by means of a moored station during 
13 complete pile driving events. A Hydro 
hammer S-4000 from IHC IQIP (4000 kJ) 
was deployed from the jacking-up platform 
Innovation. 
Sound mitigation was in place in the 
form of a single big bubble curtain (BBC) 
of 700 m long (table 1). The flow of air was 
provided by eight oil-free compressors (AC 
PTS 916) of 40.3 m3 min-1 each at a maximum 
pressure of 10 bar.
BBCs are expected to reduce the sound 
levels by 14 dB L
z-p
 (range 11-17 dB) or 
11 dB SEL (range 9-13 dB) (OSPAR 2014). 
The best sound reduction is achieved with 
an optimal air supply and BBC design, e.g., 
distance between holes and dimension of the 
holes (OSPAR 2014; Nehls et al. 2015).
2.3.  Underwater sound measurement 
equipment
Underwater sound was recorded from a 
moored station (figs 1 & 2). The mooring was 
equipped with a measuring chain consisting of 
	
Inner diameter hose 102 mm 
 
FAD (Free Air Delivery) 8x40 m³/min/compressor 
  320 m³/min 
FAD per meter 0,44 m³/m/min 
Diameter holes 2 mm every 100-300 mm 
Table 1. Specificities of the single big bubble curtain, nozzle hose (as provided by the concessioner)
20
Norro 
an acoustic release (Benthos 866 A/P), one un-
derwater sound recorder (RTsys EA-SDA14), 
one hydrophone (B&K 8104 or HTG) and a 
flotation device used to maintain the systems 
upright and tied. One additional acoustically 
commanded pop-up buoy (Benthos 875-PUB) 
was used for recovery of the mooring block 
afterwards. A wood block (fig. 2) was used to 
assure a silent behavior of the pop-up buoy in 
case of strong tidal currents and wave action. 
The manufacturer RTsys calibrated the com-
plete measurement chain prior to shipping 
from the factory. The calibration was verified 
using a calibrator B&K 4229 (piston-phone) 
prior to every deployment.
The mooring was deployed on 
14 July 2017 from RV Belgica at the posi-
tion WGS84 N 51° 35,129; E 002° 56,037. 
The mooring was retrieved on the 
25 August 2017 when another one was 
placed at the position WGS84 51° 35,114 N, 
002° 56,04 E. As such, the distance between 
the measuring equipment and the piling 
locations ranged from 808 to 4691 m. No 
surface marker was left on site to reduce nav-
igation risk inside the construction zone as 
well as to avoid any perturbing sound origi-
nating from a line linking a surface buoy to 
the mooring.
2.4. Underwater sound measurements  
and post-treatment
Sound pressure was recorded continuously 
at a sampling rate of 78125 Hz and stored on 
a hard drive coded on WAV format. During 
the period of deployment, 13 piling events 
occurred (table 2).
MATLAB was used for the post treat-
ment of the records. SEL
ss
, SEL
cum
 as well as 
the normalization of the sound levels to the 
reference distance of 750 m were computed 
following the material and methods section 
of Norro et al. (2013). Because the intensity 
of the sound depends on the size of the sound 
Figure 1. Mooring design of the underwater 
sound measurement equipment. 1 pop-up buoy 
(acoustic command), 2 concrete blocks 300 kg, 
3 acoustic release, 4 underwater noise recorder 
& hydrophone, 5 rigid flotation (total 650 N 
buoyancy), 6 attachment for deployment, to-
tal height 3 m, all links in stainless steel cables 
8 mm.
Figure 2. Underwater sound measuring chain 
prior to deployment from RV Belgica. A C-POD 
(acoustic porpoise detector) is added to the 
mooring (next to the acoustic release). A pop-
up buoy (far end) is used for recovery of the 
concrete block. Photograph by A. Norro.
 Chapter 2. Mitigation measure for offshore wind farm piling sound: a single big bubble curtain
21
source and associated hammer, the intensity 
of the sound increases with the pile diameter. 
For assessing the efficiency of the sound 
mitigation measure, the ITAP model was 
used to estimate the sound levels generated 
by the piling without mitigation measures.
ITAP proposes a model based on past 
observations (Bellmann et al. 2017) that al-
lows estimating both SEL and L
z-p
 from the 
diameter of the monopile to be driven into 
the seabed. 
3. Results
L
z-p
 normalized at 750 m ranged, at the start 
of the piling when a maximum of 500 kJ was 
used, from 176 to 187 dB re 1µPa, while at 
the end of the piling when maximum energy 
was used, between 186 and 193 dB re 1µPa 
(table 3).
SEL
ss
 ranged from 165 to 
173 dB re 1 µPa2 s while SEL
cum 
ranged from 
200 to 208 dB re 1 µPa2 s (table 4). The dif-
ference between SEL
5 
and SEL
50 
(the SEL 
percentile
 
5 and 50 respectively) is less than 
3 dB. The total energy used for the complete 
piling event and the total number of strokes 
are provided as additional information often 
requested by bio-acousticians (Hawkins & 
Popper 2016) in order to better evaluate cu-
mulative effects. 
Based on the ITAP model (Bellmann 
et al. 2017), piling of a 7.8 m steel monopile 
produce an average L
z-p 
of 204 dB re 1µPa at 
750 m distance (ranging from 199 to 209 dB 
re 1µPa) and a SEL
ss
 of 179 dB re 1 µPa2 s 
(range: 174-184 dB re 1 µPa2 s). With a re-
duction of sound of about 11-13 dB re 1 µPa 
(table 5), the efficiency of the BBC seems 
to be less than the predicted 14 dB re 1 µPa 
(ranging 11 to 17 dB re 1µPa) and hence 
would be closer to the lower limit of sound 
reduction cited for BBCs (OSPAR 2014). 
The efficiency of the BBC could probably 
have been enhanced by an optimal setup of 
the device (OSPAR 2014). Hole of 1.5 mm 
diameter (OSPAR 2014) in the hose instead 
of 2 mm (table 1) may have improved the 
quality of the mitigation.
Stated flow of 0.44 m3 m-1 min-1seems 
to be sufficient but one should remember 
that the bubble curtain is not placed at the 
surface but below 20 m of sea water. At that 
depth hydrostatic pressure is three time the 
Table 2. Sound data available for the Rentel wind farm piling phase. Position of the monopile and 
instrument, monopile diameter, distance from the monopile to the measuring equipment (instrument), 
RTsys1 position for the first measuring chain deployment, RTsys2 position for the second deployment
	
	
	
	
	
	
	
	
	
	
	
	
Location Center Point Position (Coordinates)  
as built (WGS84) 
Distance to instrument (m) Monopile diameter (m) 
  Latitude Longitude   
B4 51°34.934' N 2°55.412' E 808 7.8 
B5 51°34.670' N 2°55.874' E 872 7.8 
B6 51°34.411' N 2°56.346' E 1378 7.8 
B7 51°34.164' N 2°56.813' E 1974 7.5 
C4 51°34.729' N 2°56.895' E 1237 7.8 
C5 51°34.485' N 2°57.305' E 1869 7.8 
D2 51°35.915' N 2°56.020' E 1458 7.8 
D6 51°34.819' N 2°57.882' E 2196 7.8 
E2 51°35.910' N 2°57.089' E 1909 8 
E4 51°35.117' N 2°58.518' E 2861 7.5 
F3 51°35.857' N 2°58.411' E 3064 7.8 
G7 51°35.721' N 2°59.667' E 4337 7.5 
G8 51°35.485' N 3°0.058' E 4691 7.5 
22
Norro 
atmospheric pressure and therefore it a re-
duced pressure that is present in the SSB 
when immerged.
In the case presented here, we observe 
levels that exceed MSFD value as permit-
ted in Germany and the Netherlands (Rumes 
et al. 2016). 
	
Location Distance to 
instrument (m) 
Measured level Lzp 
START  
(dB re 1µ Pa) 
Nomalized level Lzp 
@ 750m START 
(dB re 1µ Pa) 
Measured level Lzp 
END  
(dB re 1µ Pa) 
Nomalized level Lzp 
@ 750m END  
(dB re 1µ Pa) 
B4 808 176 176 192 192 
B5 872 182 183 190 191 
B6 1378 181 185 189 193 
B7 1974 176 182 184 190 
C4 1237 180 183 188 191 
C5 1869 179 186 185 191 
D2 1458 181 185 188 192 
D6 2196 180 187 184 191 
E2 1909 179 185 185 191 
E4 2861 176 184 183 191 
F3 3064 175 185 182 186 
G7 4337 174 184 175 186 
G8 4691 174 184 176 188 
	
Location SELss @750m 
(dB re 1 µ Pa2 s) 
SEL CUM  
(dB re 1 µ Pa2 s) 
Total E (kJ) Strokes (n) 
B5 166 201 6765005 3332 
B6 168 203 6046718 3071 
B4 170 205 7912667 3547 
C4 169 204 4819735 3247 
C5 173 208 5064200 3174 
E2 167 202 4987828 3035 
D6 170 205 5247363 3171 
B7 167 202 5573269 2986 
E4 169 204 5039575 3072 
F3 166 200 4480189 2784 
G8 165 199 3948502 2714 
G7 166 200 4517827 2562 
Table 3. Sound zero to peak levels normalized at 750 m distance from the piling location, measured du-
ring 13 piling events at the Rentel site. Start is at the start of the piling event within the first 10 minutes; 
end is for the end of the piling event when the maximum energy was used. Those results include the in-
sertion loss from a single big bubble curtain (BBC)
Table 4. Computed SEL
ss
 and SEL cumulative SEL
cum
 normalized at 750 m distance from the 13 piling lo-
cations as well as total energy provided by the hammer and the number of strokes needed for complete pe-
netration of the monopile. Those results include the insertion loss from a single big bubble curtain (BBC)
 Chapter 2. Mitigation measure for offshore wind farm piling sound: a single big bubble curtain
23
4. Discussion
During the first few minutes (< 10 min) of the 
piling, when the energy provided by the ham-
mer is less than 500 kJ and the BBC in place, 
L
z-p
 is generally below 185 dB re 1µPa which 
is the MSFD limit in Belgium. In the follow-
ing stages of a piling event, when the energy 
provided by the hammer is more than 500 kJ 
(500-4000 kJ), L
z-p
 is well above the Belgian 
MSFD limit.
An optimal tuning of the BBC is neces-
sary to obtain the full efficiency of the sys-
tem and to reach a reduction of about 17 dB 
re 1µPa for L
z-p 
(OSPAR 2014). The numbers 
presented in table 5 suggest that the BBC was 
not optimally configured or that the flow of air 
inside the BBC was insufficient for optimal 
noise mitigation (OSPAR 2014). Moreover, 
even with an optimized BBC, the maximum 
reduction is 17 dB re 1µPa and remains in-
sufficient to reduce L
z-p 
below 185 dB re 1µPa 
at 750 m distance (204 - 17 = 187). For such 
a project more than one mitigation measures 
must have been used.
Nevertheless, the apparent better effi-
ciency of the BBC observed for F3, G7 and 
G8 with 18 to 16 dB reduction is due to anoth-
er effect. It is an under estimation of the L
z-p
 
resulting from the computation of the normal-
ized value presented at table 5. 
Norro et al. 2013 presented in the results 
section a validated propagation model better 
suited for the Belgian part of the North Sea 
and that could have been used for the nor-
malization computation. One should remem-
ber that it is generally accepted that for such 
a comparison of normalized value at 750 m 
one uses a sound propagation law on ‘15 log’ 
as introduced and used by Muller & Zerbs 
(2011). As a result, we surmise that, when 
the difference between the normalization dis-
tance (750 m) and the actual distance between 
sound source and measurement increases, the 
underestimation of L
z-p
 increases as well.
Our results demonstrate that, when it 
is required to install XL or XXL monopiles 
by pile driving, it will be necessary to use a 
combination of at least two sound mitigation 
measures in order to comply with national 
MSFD regulations, as had been predicted by 
Rumes et al. (2017). 
Table 5. Efficiency estimate of the single big bubble curtain mitigation measure based on the diffe-
rence between the theoretically produced zero to peak level of sound (L
z-p
) at 750 m (ITAP model) of 
204 dB re 1µ Pa (on average) and the observed L
z-p
 normalized to 750 m
	
Location Nomalized level Lzp @ 750m E 
(dB re 1µ Pa) 
Observed efficiency of the BBC 
(dB re 1µ Pa) 
Distance to instrument (m) 
B4 192 12 808 
B5 191 13 872 
B6 193 11 1378 
B7 190 14 1974 
C4 191 13 1237 
C5 191 13 1869 
D2 192 12 1458 
D6 191 13 2196 
E2 191 13 1909 
E4 191 13 2861 
F3 186 18 3064 
G7 186 18 4337 
G8 188 16 4691 
24
Norro 
5. Conclusion
With L
z-p
 in excess of 185 dB re 1µPa at 
750 m even with BBC sound mitigation 
measure in place, the BBC proved to be less 
effective than predicted. For future construc-
tion activities involving such XL or XXL 
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monopiles, it will be required to combine 
two or more sound mitigation measures as to 
comply with the Belgian MSFD thresholds 
for impulsive underwater sound.
 Chapter 2. Mitigation measure for offshore wind farm piling sound: a single big bubble curtain
25


On the effectiveness of a single big bubble curtain as mitigation measure for offshore wind farm piling sound in Belgian waters

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On the effectiveness of a single big bubble curtain as mitigation measure for offshore wind farm piling sound in Belgian waters

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