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Sea-trial verification of ultrasonic antifouling control

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An ultrasonic antifouling treatment was applied to a 96,000 m³ class drill-ship to verify its feasibility through a sea-trial. Soon after the hull cleaning had been performed, six ultrasonic projectors were evenly deployed around the starboard shell plate. Driven by a 23 kHz sinusoidal ultrasound in an intermittent manner, the projectors emitted a high-intensity sound reaching 214 dB at the source level causing cavitation around the adjacent water and eventually deterring the settlement of marine fouling organisms. Underwater photographs acquired after four months showed fairly clean slabs on the starboard side, but heavy fouling on the port side. This experiment revealed that ultrasound treatment is a promising method for inhibiting fouling accumulation, even for large-scale ship applications.
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Biofouling
The Journal of Bioadhesion and Biofilm Research
ISSN: 0892-7014 (Print) 1029-2454 (Online) Journal homepage: http://www.tandfonline.com/loi/gbif20
Sea-trial verification of ultrasonic antifouling
control
Ji-Soo Park & Jeung-Hoon Lee
To cite this article: Ji-Soo Park & Jeung-Hoon Lee (2018) Sea-trial verification of ultrasonic
antifouling control, Biofouling, 34:1, 98-110, DOI: 10.1080/08927014.2017.1409347
To link to this article: https://doi.org/10.1080/08927014.2017.1409347
Published online: 12 Dec 2017.
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BIOFOULING, 2018
VOL. 34, NO. 1, 98110
https://doi.org/10.1080/08927014.2017.1409347
Sea-trial verication of ultrasonic antifouling control
Ji-SooParka and Jeung-HoonLeeb
aHydrodynamics Research, Samsung Ship Model Basin, Samsung Heavy Industries, Daejeon, Republic of Korea; bSchool of Mechanical
Engineering, Changwon National University, Changwon, Gyeongnam, Republic of Korea
ABSTRACT
An ultrasonic antifouling treatment was applied to a 96,000m3 class drill-ship to verify its feasibility
through a sea-trial. Soon after the hull cleaning had been performed, six ultrasonic projectors
were evenly deployed around the starboard shell plate. Driven by a 23kHz sinusoidal ultrasound
in an intermittent manner, the projectors emitted a high-intensity sound reaching 214dB at the
source level causing cavitation around the adjacent water and eventually deterring the settlement
of marine fouling organisms. Underwater photographs acquired after four months showed fairly
clean slabs on the starboard side, but heavy fouling on the port side. This experiment revealed that
ultrasound treatment is a promising method for inhibiting fouling accumulation, even for large-
scale ship applications.
Introduction
Marine biofouling causes signicant adverse issues for
ships and boats by increasing the hydrodynamic frictional
resistance, and subsequently fuel consumption. Schultz et
al. (2011) analyzed the economic impact of biofouling on
a mid-sized naval surface ship; the costs including addi-
tional fuel, hull cleaning and associated repainting were
~$56 million per annum or $1 billion over 15years. is
estimate excluded the environmental pollution resulting
from the large quantities of organic waste produced dur-
ing hull cleaning.
Heavily fouled marine structures gain weight, lead-
ing to a loss of stability or hydrodynamic performance.
Furthermore, when fouling communities develop on
major underwater equipment (eg sonar and riser), they
interfere with the ecient operation of the host structure.
Hence, the issue of fouling is of great concern in oil and
gas oshore platforms, such as oating liqueed natural
gasication (FLNG) vessels, oating production storage
and ooading (FPSO) vessels, oating storage regasica-
tion unit (FSRU) vessels and drill-ships (Qian et al. 2000;
Whomersley and Picken 2003).
e most popular treatment is the application of
antifouling (AF) paints to prevent growth. AF coatings
containing tributyltin (TBT) compounds emerged in the
mid-1950s and eventually were used on ~70% of the world
eet (omas and Brooks 2010). In the 1970s, self-polish-
ing copolymers (SPCs) based on TBT with the addition
of copper and other metallic compounds achieved wide
recognition, replacing free-TBT as an eective method
for fouling prevention (Phang et al. 2007). Although TBT-
free SPC coatings have been developed and are regarded
as the most eective approach for fouling inhibition (Wu
et al. 1997; Billinghurst et al. 1998; Kem et al. 2003) there
are environmental concerns regarding copper and various
co-biocides. Hence, more eco-friendly alternative solu-
tions to control marine fouling are necessary.
e aeration method has been considered as a non-
toxic solution (Scardino et al. 2009; Bullard et al. 2010).
In this method, a copious ow of compressed air is con-
tinuously distributed around the wetted surface to prevent
biofouling. However, a spatial arrangement of several noz-
zles and related complex piping systems are required for
its implementation in the hull and this can be hindered by
the shipbuilding environment and/or maintenance issues.
Furthermore, a large air-producer capacity required to
supply plenty of air is a critical issue.
Acoustic methods for biofouling control emerged in
the 1980s. A signal generator, power amplier and piezo-
electric transducers (herein aer projectors) are the main
devices in this system. e amplied signals are fed into
the projectors, directly emitting acoustic waves into the
water that cause vibrations in the vessel hull. e methods
KEYWORDS
Ultrasound; antifouling; sea-
trial; fouling rating (FR)
ARTICLE HISTORY
Received 13 September 2017
Accepted17 November 2017
© 2017 Informa UK Limited, trading as Taylor & Francis Group
CONTACT Jeung-Hoon Lee jhoonlee@changwon.ac.kr
BIOFOULING 99
al. 1974) are available. However, this is not the case for
ultrasonic treatment.
e current investigation presents a sea-trial veri-
cation of ultrasonic biofouling control based on the
guidelines of Guo et al. (2011b). is work, developed
during a four-month-long test period, placed emphasis
on the size of the test vessel. As summarized in Table 1,
the ship had a length between perpendiculars of 228m, a
breadth of 42m, and an operating draught of 10.5m. e
large size of the target vessel oered the following advan-
tages. First, the distinction between the treatment and
control groups became clear by placing the ultrasonic
projectors only on one-side of the ship. Second, using
a conguration of multiple projectors, it was possible
to obtain enough data to evaluate the eectiveness of
the ultrasonic biofouling treatment over the large wet-
ted surface area. Finally, the test results provided direct
evidence to demonstrate its applicability to large-size
vessels or eets.
To implement the laboratory test based scheme of Guo
et al. (2011b) in the sea, an ultrasonic antifouling system
with six channels was developed by considering sound
attenuation over distance. As detailed in the subsequent
section, a 3dB correction was made on the pressure cri-
teria to account for the dierence between the anechoic
(sea) and reverberant (lab) acoustic elds. e projectors
were then tethered from the ships deck and were evenly
distributed only around the starboard side (STBD) of the
ship. roughout the trial period, each projector emit-
ting a high acoustic pressure was intermittently driven
by a 23-kHz bursting signal, ie ‘0.5 s on and 0.5 s o’ in
a cyclical operation mode. At the end of the trial, pho-
tographic data were obtained through an underwater
camera to qualitatively compare the fouling settlement
on the STBD with that of the port side. Further analysis
was performed in a quantitative manner using the fouling
rating (FR) scale (NSTM 2006).
Materials and methods
Description of the test ship and the experimental
environment
e test ship was a 96,000m3 class drill-ship (named Pacic
Zonda) built by Samsung Heavy Industries Co. Ltd (Geoje,
Korea) in 2016. e vessel was anchored near the builder’s
can be classied into two types according to the frequency
range of the signal: audible (20Hz–20 kHz) and ultra-
sonic (>20kHz) (Legg et al. 2015). In the former case, a
lack of consistency has been found among research results
(Latour and Murphy 1981; Branscomb and Rittschof
1984; Fischer et al. 1984; Donskoy et al. 1996; Choi et al.
2013). Based on laboratory tests, Branscomb and Rittschof
(1984) reported that a 30Hz sound wave was the most
eective for the inhibition of fouling. However, other
researchers (Fischer et al. 1984; Choi et al. 2013) could
not obtain reproducible outcomes. For example, Choi et
al. (2013) observed that the deterrence of barnacle set-
tlement increased with vibrating frequency. Moreover, it
was recently discovered that vessel noise with frequency
compositions mainly from 30Hz to 2kHz could stimulate
fouling accumulation rather than prevention (Wilkens et
al. 2012; McDonald et al. 2014; Stanley et al. 2014). us,
thorough investigations of the audible frequency ranges
are necessary before these can be practically utilized in
the eld.
Studies on ultrasonic biofouling control have been
reported in the literature (Kitamura et al. 1995; Mazue
et al. 2011; Guo et al. 2011a, 2011b; Choi et al. 2013).
In this approach, the cavitation phenomenon developed
by the high intensity ultrasound is likely to be responsi-
ble for biofouling control. Cavitation creates high liquid
shear forces that deter the settlement of organisms on
the underwater surface, followed by a violent implosion
(Guo et al. 2011a). In addition, it has been shown that
micro-steaming generated during cavitation is capable of
injuring cells or damaging organisms. Several researchers
(Hao et al. 2004; Ma et al. 2005; Kratochvíl and Morntein
2006) stated that cavitation appears at lower, rather than
higher frequencies as long as the exciting pressure level
is kept constant. In other words, the ecacy of ultrasonic
biofouling prevention is degraded with an increase in
frequency. is tendency was validated by the following
laboratory-scale tests. For three selected frequencies of
19.5, 28, and 50kHz, Kitamura et al. (1995) observed
that the lowest, 19.5kHz, was the most eective for the
inhibition of larval settlement. Similar results were found
by Guo et al. (2011b), who reported that 23kHz was
the optimum frequency among three dierent frequen-
cies (23, 63, and 102kHz). ey pointed out that such a
frequency dependence may be due to the lower cavita-
tion threshold at lower ultrasonic frequencies along with
stronger implosions.
Despite these conrmative results, the previous studies
regarding ultrasonic biofouling control (Kitamura et al.
1995; Guo et al. 2011b) would be much more valuable if
they had included real sea experiments for further veri-
cation. For the audible frequency studies, some sea-trial
records (Waldvogel and Pieczynski 1959; Sheherbakov et
Table 1.Specifications of the test ship (96,000 m3 class drilling
vessel).
Item Value
Length overall [m] 228.2
Breadth [m] 42.0
Design draught [m] 12.0
Test draught [m] 8.5
100 JS. PARK AND JH. LEE
shipyard (34°5850.4′′ N, 128°3321.9′′ E) aer its launch at
the end of 2015. Before delivery, the authors had an oppor-
tunity to conduct a trial during the entire summer period
from the beginning of May to the end of August of 2016.
During the test period, the seawater temperature steadily
rose from 14 to 25°C and the surface salinity was 35 ppt
(‰) on average. ese sea conditions were regarded as
appropriate for vigorous growth of fouling.
e SPC paint was applied on the outer shell plate
with a mean thickness of ~750 μm during the ship-
building stage. Six to seven months aer launching, a
light degree of fouling was evident around the ship. Guo
et al. (2011b) stated that ultrasonic irradiation could
deter incipient cyprid attachment, while also damaging
or removing barnacles. However, mortality was restricted
to the sub-millimeter sized organisms. In other words, the
ultrasound eect may diminish for mature fouling. Hence,
underwater hull cleaning was conducted beforehand to
initialize the hull plate and secure the accuracy of the
test. Cleaning was performed by divers using mechanical
brushes. Although damage on the coating was probable,
this was unimportant for the relative comparison between
the port- and STBD-sides.
Conguration of the ultrasonic AF system
Figure 1 represents a schematic diagram of the six-chan-
nel ultrasonic AF system. e driving signal from the
waveform generator (Agilent 33120A, Santa Clara, CA,
USA) was fed into each power amplier (Yamaha P5000S,
Hamamatsu, Japan). For the purpose of impedance
matching, a step-up transformer with a turning ratio of 1:4
(Jeil-trans, Busan, Korea) was employed between the 70 Ø
spherical piezoelectric projector (Neptune D26, Kelk, UK)
and the amplier. e projector, with an omni-directional
property up to 35kHz, was able to emit a maximum sound
power of 850W into the water at a resonance frequency of
26kHz. e system was equipped with emergency battery
packs in case of a blackout on the ship.
e multi-channel conguration shown in Figure 1 is
supported by the two following considerations. First, the
wave attenuation was considered as the sound pressure
level dropped to 6dB per doubling of distance due to the
spherical-spreading characteristic of the projector. at is,
a single channel only covered a nite area restricted by a
certain pressure threshold. Hence, the second consider-
ation was to employ the pressure amplitude to a specic
area that a single projector could accommodate. In this
regard, Guo et al. (2011b) stated that a dynamic pressure
amplitude of 5kPa would be the minimum requirement
for an inhibitory eect. However, it should be noted that
the proposed value, equivalent to 194dB relative to 1 μPa,1
was based on the measurements in a reverberant acrylic
water tank. If the tests were conducted in an anechoic
eld, ie the seawater environment, the minimum criterion
would be decreased to 3.5kPa by 3dB dierence between
the two acoustic elds (Kinsler et al. 2000). Within the
allowable operational range of the equipment, each chan-
nel in the system could produce up to 52.5kPa at the
source level (1m away from the projector). us, the esti-
mated coverage of a single channel was a radius of 15m by
considering the 3.5kPa criterion with a pressure decrease
of 6dB per distance doubling. is meant that one entire
side of the ship, except for both far ends, could be engaged
Figure 1.Six-channel array configuration for the ultrasonic AF system.
BIOFOULING 101
e determination of the submergence depth also
required careful consideration. Because sunlight, an
essential factor for the growth of algal fouling, is con-
centrated near the sea-surface, it may seem intuitive that
the projectors should be placed just below the waterline.
However, the low acoustic impedance of air causes the
water-to-air interface to behave as an acoustic mirror. If
the omni-directional projector, acting like a monopole
source, is located within a wavelength from the water sur-
face, it would immediately become a dipole whose radi-
ation eciency is very weak (Blackstock 2000). Hence,
the projectors needed to be positioned several (normally
10) wavelengths away from the water-line for a practical
application. At 23kHz, the corresponding wavelength λ
was estimated to be 65mm using the following dispersion
relationship:
where c (=1,500m s−1) denotes the speed of sound in sea-
water and f (=23kHz) is the excitation frequency. In addi-
tion to the minimum necessity of 650mm (=10 × λ), the
sea-margin factor originated from waves and currents was
reected in specifying the submergence depth of 1.5m.
Preliminary tests
e generation of cavities using the developed system
were conrmed prior to the sea-trial because the ultra-
sonic AF method depended highly on cavitation. A cavity
observation test was conducted in a small water basin. As
shown in Figure 4, the excitation with the signal pattern
shown in Figure 2 resulted in the generation of numerous
cavitation bubbles around the projector, traveling random
paths owing to the buoyancy and acoustic stimulation.
Kamiirisa (2001) found that the number of nuclei in
(1)
𝜆=cf
in the ultrasonic treatment once the six projectors were
evenly distributed with a spacing of 30m. Needless to say,
the correction on the criteria of Guo et al. (2011b) was
benecial as it reduced the number of projectors required
to protect the entire area. If the criterion of 5kPa was
adopted, the coverage radius would be reduced to 8.9m,
leading to an over-treatment of the ultrasonic AF system
with 20 channels.
Figure 2 shows the driving signal pattern for the pro-
jectors. It represents a repetition of a 23kHz sinusoidal
wave switched on and o each 0.5 s. Guo et al. (2011b)
originally suggested the repeating pattern of ‘5min on and
20min o’ to achieve the same eect as with a continuous
mode. Such an intermittent operation could reduce the
power consumption and extend the lifespan of ultrasonic
devices. However, the operational prole seemed to be
particular to their own devices. When the projector in this
study was subjected to the scheme of Guo et al. (2011b)
with a high amplier gain, limitations in the durability
did not permit it to last for 24h. erefore, trial and error
was used to determine the ‘0.5 s on and 0.5 s o’ driving
regime. Similar issues may arise if other types of projectors
are employed.
As shown in Figure 3, the six projectors were placed
on the STBD side of the ship, denoted as the ‘treatment
group’ hereinaer. e opposite side (port side) repre-
sented the control group. Despite the signicant sepa-
ration between the projectors, the eect of destructive
interference needed to be addressed. Destructive inter-
ference may cause anti-nodes, where out-of-phase acous-
tic pressures from the two neighboring projectors cancel
each other out, potentially reducing the AF performance.
Accordingly, projectors with additional weights were sus-
pended from the deck, allowing them to move freely in a
nearly horizontal plane.
Figure 2.Driving signal pattern for ultrasonic projectors: 23kHz sinusoidal ultrasound waveform, switched on for 0.5 s and off for 0.5 s,
was repeated throughout the sea-trial.
102 JS. PARK AND JH. LEE
nature of the sound source. Hence, representative results
among the equal distance levels were selected for the sake
of simplicity as shown in Figure 5. e background noise
obtained by turning o the projector included the con-
tribution from the vessel hull (mainly due to the power
generators), with predominant components in the low
frequency range below 2kHz.
When the projector was activated, the exciting fre-
quency component at 23 kHz and its harmonics were
clearly identied irrespective of the measurement loca-
tions (Figure 5A–C). Even if there were slight deviations
between the measurements, the source level data in
Figure5A conrmed that the sound pressure amplitude
at the fundamental frequency was around 214dB (equiv-
alent to 52.5kPa), as expected. e sound pressure dier-
ence between the source level and the halfway point was
~21dB at the rst harmonic, due to the spherical spread-
ing, described previously, or simply the transmission loss
(TL) in the following equation:
where r is the distance from the source to the measurement
location. e readings at key frequencies for all measure-
ments are summarized in Table 2 and it was veried that
the transmission loss relationship held for other locations.
Figure 5C shows the measurements at the far ends, ena
-
bling the observation of the eects of wave superposition.
Because the outermost region was mostly dominated by
a single projector alone, the measurement at this location
indicated whether the destructive interference concerned
(2)
TL
=20 log
10
(r)
[
dB
]
seawater is ~10 times higher than that in the freshwater.
at is, under the same exciting condition, seawater is
more prone to cavitate than freshwater. erefore, it could
be anticipated that there would be a greater number of
cavitating bubbles in the seawater than those shown in
Figure 4.
Unfortunately, the authors could not obtain a high-qual-
ity photograph in the sea environment. Instead, the sound
pressure was measured as a second preliminary test aer
the installation. Several locations for measurement are
depicted in Figure 3. Six of them were near the projec-
tors (ie the source level marked by the red ×), ve were
at the halfway point between the two projectors (marked
by the blue ×), and the remaining two were at far ends of
the coverage area (marked by the green ×). In a similar
way to the projector deployment, the hydrophone (B&K
8103, Nærum, Denmark) with weights was tethered from
the deck and immersed at the same depth as the projec-
tors. During the ultrasonic emission, the signals from the
hydrophone passed through a signal conditioner (B&K
2690) and were logged into a computer-based dynamic
signal analyzer (B&K 3052) with a suciently high sam-
pling rate of 256kHz. A fast Fourier transform was then
used to obtain the amplitude spectrum in the frequency
domain, estimated by applying a rectangular window, with
an ensemble average of 3,000 times with 75% overlapping
and a frequency resolution of 64Hz (Bendat and Piersol
2010).
Provided that the distances from the projector were
identical, the measured spectra for the STBD side exhibited
no noteworthy dierences because of the monopole-like
Figure 3.Arrangement of the six projectors and the locations for acoustic measurements.
BIOFOULING 103
Practically, higher order components are unavoidable
even in the pure tone generation, which was also the
case in this study. However, their amplitudes were insig
-
nicant compared to those of the fundamental order,
as the pressure dierences between the rst- and sec-
ond-order were more than 35dB on average. Given that
the cavitation threshold increases with frequency, the
existence of harmonics are ineective in the ultrasonic
AF treatment.
Finally, a measurement was made at the middle of the
port side (marked by the black × in Figure 3) for a compar-
ison with the results for the STBD side. Figure5D shows
almost no change in the spectrum regardless of whether
the projector was in operation or not. Neither the tonal
components at the high frequencies nor the broadband
noise at the low frequencies was noticed, ensuring that
the ultrasound did not aect the portside. erefore,
the visual inspection could be replaced by the acoustic
measurement.
was substantial. As the pressure amplitudes at the halfway
point were 1–2dB higher than those at the far ends, the
area between the projectors was assumed to be assisted by
the constructive interference rather than hindered by the
destructive interference.
Further, it is interesting to note that the broadband
noise was superimposed on the tonal spectra. When the
projector was turned on, the spectrum for the frequency
range below 20kHz showed a distinct increase from the
background noise. Moreover, the sound pressure level in
these frequencies decreased in accordance with the dis-
tance from the sound source. As found in several studies
(Guo et al. 2011b; Lee and Seo 2013; Lee et al. 2014), the
most likely cause is attributed to the shock wave radiation
during the collapse of the cavitation bubble. As a result,
the emergence of broadband noise indicates the occur-
rence of cavitation around the projector.
e appearance of higher harmonics and its inu-
ence on the ultrasonic treatment can also be argued.
Figure 4.Cavities which developed around the projector (left: signal off, right: signal on). Cavitating bubbles appear as white spots.
104 JS. PARK AND JH. LEE
Figure 5.Comparison of sound power spectrum at different locations. Processing parameters: rectangular window, 3,000 ensemble
average, 75% overlapping, frequency resolution of 64Hz. (A) Measurement at S3; (B) measurement at H34; (C) measurement at F1; (D)
measurement at the middle of the port side.
BIOFOULING 105
As shown in the third column in Figure 6, a small
amount of slime was found at Guo’s criteria locations
where the sound pressure fell to ~5kPa. However, this
was not the case for animal fouling, eg juvenile barna-
cles. It was concluded that ultrasound with a low-pressure
amplitude did not control slime but was eective enough
to inhibit settlement of calcareous fouling organisms.
Additionally, the quantity of slime showed a random dis-
tribution within this area, reecting the ever-changing
current direction around the test vessel.
At the halfway region where the acoustic pressure was
further decreased to 3.5kPa, while still satisfying the min-
imum pressure criteria, slime and colonies of Ciona were
observed on the shell surface, as shown in the last column
in Figure 6. Clearly, the 3.5kPa pressure amplitude was
not powerful enough to generate sucient cavitating bub-
bles. Nonetheless, the comparison with the control group
proved the eectiveness of the ultrasound in AF even in
an area of low exciting pressure.
It is noteworthy that the snapshots taken at both far
ends (labeled F1 and F6 in Figure 6) showed heavier foul-
ing accumulation than those at the halfway locations. As
mentioned in the previous section, this might be attrib-
uted to a superposition eect of sound waves. While the
sound eld in the middle of the projectors had a greater
possibility of receiving large pressure amplitudes owing
to the constructive interference, the outermost region
had almost no chance of pressure increments. e fouling
development around the bow and a areas of the starboard
side can then be explained because the eectiveness of the
ultrasonic treatment was proportional to sound pressure.
Quantitative comparison using the fouling rating
(FR) scale
e previous analysis demonstrated the qualitative validity
of the ultrasonic method. However, an assessment using an
objective measure is necessary to explain the eect quan-
titatively. For this purpose, the authors adopted the US
Navy’s fouling rating (FR) scale (NSTM 2006) as a standard
for underwater hull cleaning. Ranging from 0 (the lightest)
to 100 (the heaviest), FR describes the 10 most frequently
encountered fouling patterns. As summarized in Table 3,
a zero rating describes a fouling-free surface, while ratings
of 30 or less indicate the existence of so (vegetable-type)
fouling only. At a rating of 30, the visibility of the underlying
paint color is lower and the fouling accumulation is not easily
wiped o by hand. A FR of 40 is equivalent to the hull surface
with an incipient animal-type fouling. Growth of calcare-
ous fouling raises the rating, with a maximum value of 100
corresponding to heavy colonization by composite fouling.
However, the FR scale alone is not adequate for quan-
tifying the severity of a fouled state as it only represents
Results and discussion
Underwater camera inspection: qualitative
comparisons
e application of six projectors for four months produced
distinct levels of fouling accumulation in the treatment
(STBD) and control (port) groups. In the rst column in
Figure 6, photographs taken at the opposite sides of the
projector revealed that the control group was extremely
fouled by various macroorganisms. As well as algae, ani-
mal fouling was also present. Colonies of algae and Ciona
were randomly formed, exhibiting no location depend-
ence. is pattern is expected in for fouling under uncon
-
trolled circumstances.
Conversely, the treatment group had a relatively clean
shell plate. e abundance of fouling was highly dependent
upon location, as the sound pressure diminished with dis-
tance from the source. at is, the higher acoustic pressure
led to larger cavitation bubbles, which in turn kept the
treatment group from being uncontaminated in the areas
closer to projectors. us, photographs for the STBD shell
plate in Figure 6 were documented for three distances from
the projectors, specically, 1, 8.9 and 15 m, denoted as
source-, Guo’s criteria-, and halfway-locations, respectively.
By comparing the two groups, it was possible to qualify
the denite eectiveness of the ultrasonic AF method. In
the treatment group, the areas near the sources showed
an almost identical condition to that four months before.
A detailed inspection conrmed that even slime was not
observable in this region. According to Guo et al. (2011a),
high-intensity ultrasound (>20 kPa) directly generates
cyprid mortality, as well as providing a hostile environ-
ment for other marine fouling organisms. us, it can be
concluded that the clean surface near the projectors was
the result of a strong sound pressure exceeding 50kPa.
Table 2.Readings for sound pressure levels at the first three fun-
damental frequencies.
Meas-
urement
location
Value [dB] (dB ref. 1Pa)
Remark
1st
harmonic
(23kHz)
2nd
harmonic
(46kHz)
3rd
harmonic
(69kHz)
S1 213.8 176.9 168.3 Source level
S2 213.4 179.3 170.3
S3 214.3 178.1 171.2
S4 213.9 176.3 169.3
S5 214.5 177.5 170.5
S6 213.8 178.6 170.3
Average 213.9 177.8 169.9
H12 191.5 152.7 140.9 Halfway
levelH23 190.4 154.8 142.5
H34 193.0 153.3 143.1
H45 194.0 151.7 141.6
H56 190.4 153.6 143.0
Average 191.9 153.2 142.2
F1 189.3 152.1 139.9 Far-end
levelF2 188.5 149.2 137.5
Average 189.9 150.6 138.7
106 JS. PARK AND JH. LEE
product of the two measures was used in this study to
characterize the fouling severity (FS) as follows:
with a scale of 0 (no severity) to 100 (extreme severity).
(3)
Fouling severity (FS)
=
Fouling Rating
(FR)×
Percentage Coverage
(PC
)
specic types of occurrences, not the amount of fouling.
Indeed, another measure is necessary along with the foul-
ing rating. For this purpose, NSTM (2006) supplements
the evaluation reference with the percentage coverage
(PC), quantifying the fouled area for a given plate. For
example, a FR of 10–30% corresponds to a fouling pattern
of FR 10 covering 30% of the observed area. us, the
Figure 6.Underwater inspection of the hull-plate after the sea-trial. Following the outer shell plates of the ship, the divers continuously
recorded a series of videos. Thus, this figure displays representative images captured from the acquired movie file. In general, photographs
in the first column show that the control group (port side) was extremely fouled by complex macroorganisms. However, the treatment
group (STBD side) remained with a relatively clean shell plate, as shown in the last three columns.
BIOFOULING 107
side of projector no. 1 where constructive interference was
not formed. is was the reason why FS in the bow zone
(depicted as ‘F1’) scored a relatively high value of up to 12.
e area adjacent to the location of Guo’s criteria (8.9m
from the projector) consisted mostly of heavy slime with
areas of light slime as seen in the third column in Figure 6.
Although the discrimination of the plate surface beneath
the slime was obscure, it was clear that no animal fouling
was present. As a result, the FR value in this area was
mostly estimated to be 20–30, and consequently FS was
4–15 by considering the broad range of PC (10–50%).
Together with the so non-calcareous fouling, some
incipient animal-type fouling, such as Ciona, tubeworms
and small-sized colonies, were observed around the
halfway locations (refer to the last column in Figure 6).
rough approximate measurements of their size were
made, these occurrences were found to be sucient to
obtain a FR value of 30. e fouled area far exceeded
half of the unit plate, resulting in a PC>50%. Hence, the
resultant FS values outweighed those in other areas of the
STBD side.
In the case of the control group, FR was estimated to be
over 40 for most regions, indicating that heavily colonized
animal fouling was formed over a wide area. However, the
PC of the control group showed a random distribution for
the entire plate. Accordingly, the spread of FS had no spe-
cic pattern on the port-side plates. e calculated average
was ~19.3, much higher than the value of the treatment
group of 5.2. For a brief comparison, a summary of FS for
several locations is presented in Table 4.
e ratio of the FS average for both sides implied
that the AF performance of the ultrasound was ~73%.
Although Guo et al. (2011b) used a dierent measuring
index, they reported that the ultrasound prevented foul-
ing growth by 70%, which is similar to the results in the
present study. us, by comparing all the photographic
Figure 7 represents distributions of FR and PC on
the STBD side and FS on both sides, drawn by dividing
the entire hull surface into several grid elements with
sizes 2.5 m (width) × 1.0 m (height). All distributions
were nearly symmetrical with respect to the amidships
section; hence only half of the ship (from bow to amid-
ships) is shown. For clarication, the region below the
water depth of 4m remained clean. Normal practices,
including this work, oen reveal relatively light fouling
for a deeply immersed object, possibly due to decreased
sunlight as it propagates through the water. erefore, in
Figure 7 descriptions for the region below 4m depth were
excluded.
e size of the grid element is seemingly large, and
was chosen as the authors were interested in the overall
distribution of fouling along the hull surface rather than
the characteristics within a local area. Despite this, the
grid system composed of 152 (= 38 × 4) elements had suf-
cient spatial resolution to evaluate the AF performance
of the ultrasonic method. However, several dierent com-
binations of FRPC were obviously possible in each grid,
because a wide range of biofouling, from so to heavy,
existed. In this work, averaging of both FR and PC was
employed. For example, if the composition in a grid was
FR20–10, FR30–30, and FR40–15%, then the representa-
tive assigned FRPC for that grid was FR30–18.3%.
For the treatment group (STBD side), both the FR and
PC (Figure 7A and B) appeared to generally have radial
symmetric distributions around the projector owing to
its omni-directional characteristic. us, the product of
FR and PC, ie FS (Figure 7C), resembled the sound radi-
ation pattern of the projector. In more detail, no sign of
fouling growth was found within the radial distance of
about 5.0m from each projector, and the correspond-
ing FS values remained<1 (indicated by the lightest gray
color). However, a fouling-free area shrank at the right
Table 3.Detailed descriptions of fouling rating (Naval Ships Technical Manual (NSTM) 2006).
Fouling rating (FR) Description
Vegetable fouling(eg slime,
algae, grass)
0 Clean, fouling-free surface
10 Red and green shadow
Possible to identify surface through fouling
20 Bottle-green slime (brown region)
Difficult to identify surface through fouling
30 Fibered grass > 76mm size
Fouling thickness ≈ 6.4mm
Difficult to clean fouling by hand
Animal fouling(eg barnacles,
tubeworms, Ciona)
40 Fouling tubeworm ≈ 6.4mm size
50 Fouling barnacle ≈ 6.4mm size
60 Tubeworm and barnacle composite < 6.4mm size
70 Tubeworm and barnacle composite > 6.4mm size
80 Colonized settlement of animal fouling
Tubeworm and barnacle composite < 6.4mm size
Formation of white-region calcareous layer
90 Dense colonization of animal fouling
Tubeworm and barnacle composite > 6.4mm size
Formation of brown-region calcareous layer
Composite 100 All animal fouling composite
108 JS. PARK AND JH. LEE
study could not cover a wide range of fouling organisms,
including the various species that ourish during spawn-
ing periods. In addition, the trial period of four months
did not allow a full investigation of the eects of ultra-
sound on the ship’s structure or on coating disbondment.
us, future research should be designed to concentrate
on these unresolved issues. For this purpose, a long-term
trial is being carried out using small panels placed on a
shipyard quay.
data and quantitative measures, it was inferred that the
ultrasonic AF system showed good performance in con-
trolling fouling even for a large-scale ship.
Conclusions
To the best of the authors’ knowledge, the present study
is the rst sea-trial of the ultrasonic AF technique in a
large vessel. Because of the limited time schedule, this
Figure 7.Fouling quantification: (A) fouling rating (FR); (B) percentage coverage (PC); (C) fouling severity (FS) on the STBD side; (D)
fouling severity (FS) on the port side. Because both FR and PC for the STBD side generally resembles the sound radiation pattern of the
projector, FS (=FR × PC) in the treatment group appears to have a radial symmetric distribution around the projector. Conversely, not only
FR but also PC for the port side remained with a random distribution over the hull surface. As their plots do not make much difference to
what is shown in the plot of FS, FR and PC for the control group is not provided.
Table 4.Summary of FS for several locations.
*FS values at the port side were read at the exact opposite of the STBD side.
Locations F1 S1 H12 S2 H23 S3 H34 S4 H45 S5 H56 S6 F6
STBD 3.5 0.6 7.5 0.5 11.5 0.7 6.0 0.4 10.0 0.6 8.5 0.5 4.0
Port*28.3 27.5 19.1 18.0 17.7 21.2 14.9 24.1 15.7 17.6 21.4 22.2 30.1
BIOFOULING 109
eciency. If this factor is taken into consideration, the
return of investment (ROI) will be signicantly decreased.
Note
1. All decibel (dB) calculations in this study are referenced
to 1 μPa.
Disclosure statement
No potential conict of interest was reported by the authors.
Funding
is work was supported by the National Research Foundation
of Korea (NRF) [grant number 2017R1C1B5016831] funded
by the Korean government (Ministry of Science, ICT & Future
Planning).
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produce a negative eect on marine life. Noise may cause
stress to animals and interfere with sound-based orien-
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... Examples include flushing where high pressure water is forced through a pipeline, creating a strong shear stress to remove biofilms, however, flushing does not completely remove bacteria from pipe walls and can lead to the formation of compact biofilms (Douterelo et al., 2013). Ultrasonic waves have also been used to control biofouling by creating a physical, high stress environment, which can even cause programmed cell death (Broekman et al., 2010;Park and Lee, 2018). Pigging is another technique whereby the pipes are physically scrubbed clean my passing through a hard sponge bullet or ice, which also rotates to back-pressure build up (Liu et al., 2016a). ...
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To cope simultaneously with marine biological pollution and seawater corrosion, in this study, zinc acrylate resin was used as the base material to prepare an antifouling coating, and either zinc powder or aluminum paste was added to modify it. The addition of zinc powder or aluminum paste is 3%, 6% and 9% of resin content in the antifouling coating. A series of tests on the antifouling coating and modified coatings were carried out, including a contact angle test, laser confocal observation, marine hanging test, and salt spray test. The experiment results showed that the salt spray test time of the antifouling coating is only 240 h, while there was almost no corrosion appeared on the modified coatings with 6% and 9% zinc powder content after 408 hours of salt spray test. The initial electrochemical impedance values of the two modified coatings are 6.39×10 ⁷ Ω·cm ² and 2.18×10 ⁷ Ω·cm ² , respectively, both of which were greatly improved compared with the initial electrochemical impedance value of 1.41×10 ⁴ Ω·cm ² of the antifouling coating. After immersion in seawater for 50 days, there were no cracks on the surface of the two modified coatings, and their contact angles were 96° and 94°, respectively. After 120 days of the marine hanging test, there was no biofouling on the film surface of any coating.
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Ship fouling is one of the essential factors that affect the economic benefits of the operational ships and the marine ecological environment. This will produce adverse effects such as increased energy consumption, increased carbon emissions, biological invasion, and hull damage. In this paper, a novel ultrasonic-enhanced submerged cavitation jet ship fouling cleaning method is proposed. The objective of this paper is to verify the feasibility of this method. A submerged cavitation jet experimental set-up was built. A mass loss method was used to evaluate the experimental results. The experimental results showed that the mass loss of samples increases greatly when the submerged cavitation jet combined with ultrasonic. This is preliminarily proved that the method of ultrasonic-enhanced submerged cavitation jet for cleaning the ship fouling is feasible. Under the pump pressures of 10 MPa and 20 MPa, the maximum mass loss of the samples were increased by 12.9% and 9.5%, respectively.
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Marine biofouling remains one of the key challenges for maritime industries, both for seafaring and stationary structures. Currently used biocide-based approaches suffer from significant drawbacks, coming at a significant cost to the environment into which the biocides are released, whereas novel environmentally friendly approaches are often difficult to translate from lab bench to commercial scale. In this article, current biocide-based strategies and their adverse environmental effects are briefly outlined, showing significant gaps that could be addressed through advanced materials engineering. Current research towards the use of natural antifouling products and strategies based on physio-chemical properties is then reviewed, focusing on the recent progress and promising novel developments in the field of environmentally benign marine antifouling technologies based on advanced nanocomposites, synergistic effects and biomimetic approaches are discussed and their benefits and potential drawbacks are compared to existing techniques.
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Biofouling is a serious threat to marine renewable energy structures and marine aquaculture operations alike. As an alternative to toxic surface coatings, ultrasonic antifouling control has been proposed as an environmentally friendly means to reduce biofouling. However, the impact of ultrasound on fish farmed in offshore structures or in marine multi-purpose platforms, combining renewable energy production and aquaculture, has not yet been assessed. Here we study the impact of ultrasound on the growth and microbiota of farmed European sea bass (Dicentrarchus labrax) under laboratory conditions. Whereas growth and survival were not reduced by ultrasound exposure, microbiological analysis using plate counts and 16S rRNA gene based metataxonomics showed a perturbation of the gill and skin microbiota, including an increase in putative pathogenic bacteria. This warrants further research into the long-term effects of ultrasonic antifouling control on the health and wellbeing of farmed fish.
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Biological fouling is a significant problem to the shipping industry causing significant increases in fuel, maintenance, and downtime costs. Environmental concerns associated with toxic antifouling coatings have led to studies on alternative methods of biofouling control. This paper provides a literature review on laboratory and sea trial studies, which have used acoustic techniques for biofouling control. To the best of the authors׳ knowledge, this is the first in depth literature review on this topic. Applications of the reviewed studies have included the inhibition of biofouling on vessel hulls and pipes and also treatment of ballast water. The studies have used transducers operating in the audio and ultrasonic frequency range and sparkers. Variations were found in these acoustic parameters, which were reported to provide inhibition. Some have reported that low ultrasonic frequencies (about 20 kHz) may be optimal. The potential effect of marine life is considered. The use of ultrasonic frequencies for biofouling control appear to be more desirable than audio frequencies since they are outside the hearing range of most marine life. More studies are needed on this topic, which are well documented in terms of the parameters used and efficiency of the trials.
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Underwater noise is increasing globally, largely due to increased vessel numbers and international ocean trade. Vessels are also a major vector for translocation of non-indigenous marine species which can have serious implications for biosecurity. The possibility that underwater noise from fishing vessels may promote settlement of biofouling on hulls was investigated for the ascidian Ciona intestinalis. Spatial differences in biofouling appear to be correlated with spatial differences in the intensity and frequency of the noise emitted by the vessel's generator. This correlation was confirmed in laboratory experiments where C. intestinalis larvae showed significantly faster settlement and metamorphosis when exposed to the underwater noise produced by the vessel generator. Larval survival rates were also significantly higher in treatments exposed to vessel generator noise. Enhanced settlement attributable to vessel generator noise may indicate that vessels not only provide a suitable fouling substratum, but vessels running generators may be attracting larvae and enhancing their survival and growth.
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In marine propeller, the tip vortex cavitation and its relevant noise has been the subject of extensive researches up to now. Among the most cases of experimental approaches, accurate and objective detection of cavitation inception is primary, which is the main topic of this paper. While radiating an explosive-bursting noise into the surrounding medium, the cavitation occurs whenever the propeller blade passes through so-called wake-peak zone. Likewise the faulty bearing cases, hence, periodic occurrence of bursting noise induced from tip vortex cavitation provides a confirmative diagnostic proof, whereby the DEMON analysis(DEtection of envelope MOdulation on Noise or sometimes referred as Envelope analysis) can be effectively employed. Recently, the concept of spectral kurtosis significantly resolved an enhancement of such impulsiveness in a signal with a strong additive noise, which is a prerequisite for the DEMON processing. By taking advantage of the benefit, this paper deals with its applicability to the detection of tip vortex cavitation in propeller. For an acoustic measurement in water cavitation tunnel, the optimum filtration band from 65 kHz to 130 kHz could be obtainable by the spectral kurtosis. Also subsequent DEMON analysis showed a satisfactory result in detecting the cavitation inception.
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The effects of ultrasonic waves for survival and settlement of the barnacle, Balanus amphitrite, were investigated under laboratory condition. Among the three frequencies (19.5, 28, and 50kHz) of ultrasonic waves tested, 19.5kHz was found to be the most effective in decreasing the survival rate of the barnacle nauplii. A 50% reduction in cyprid settlement was achieved by a total irradiation of about 140kPa·sec, defined as the product of acoustic pressure (kPa) and irradiation time (sec), whereas some 4, 300kPa·s was necessary to reduce to 50% the survival rate of nauplii or cyprids.
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Globally billions of dollars are spent each year on attempting to reduce marine biofouling on commercial vessels, largely because it results in higher fuel costs due to increased hydrodynamic drag. Biofouling has been long assumed to be primarily due to the availability of vacant space on the surface of the hull. Here, it is shown that the addition of the noise emitted through a vessel's hull in port increases the settlement and growth of biofouling organisms within four weeks of clean surfaces being placed in the sea. More than twice as many bryozoans, oysters, calcareous tube worms and barnacles settled and established on surfaces with vessel noise compared to those without. Likewise, individuals from three species grew significantly larger in size in the presence of vessel noise. The results demonstrate that vessel noise in port is promoting biofouling on hulls and that underwater sound plays a much wider ecological role in the marine environment than was previously considered possible.