Control of Airborne Noise Emissions from Ships

International Conference on Advances and Challenges in Marine Noise and Vibration
MARNAV 2012, Glasgow, Scotland, UK, 5-7 September 2012
CONTROL OF AIRBORNE NOISE EMISSIONS FROM SHIPS
A. Badino1, D. Borelli1, T. Gaggero2, E. Rizzuto3, C. Schenone1
1
DIME,
University
of
Genova,
Italy,
[email protected],
[email protected]
2
DINET, University of Genova, Italy, [email protected]
3
DICCA, University of Genova, Italy, [email protected]
[email protected],
ABSTRACT
Noise pollution is an important part of the environmental impact of ships. The noise inside the vessel (affecting
crew and passengers) has been regulated since a few decades, while the impact of emissions in water on the
marine fauna has been under consideration at the International Maritime Organisation (IMO) only in the last few
years. No specific effort seems to be in place towards a regulatory framework for external airborne noise
emissions from sea going commercial ships. Only the particular cases of inland vessels and pleasure crafts are
partially covered by rules. The airborne noise pollution from commercial ships, on the other hand, does affect
people living near channels, in coastal areas with intense traffic or near ports, where ships enter and stay at
wharf for loading/unloading processes. The actual dimension of the problem is remarked by several cases of
complaints sent out by citizens living in the urban areas affected. Within the SILENV project (Ships oriented
Innovative soLutions to rEduce Noise & Vibrations, funded by the E.U.) assessment criteria for the airborne
noise emitted by sea-going ships have been proposed: the background information, the general criteria at the
basis of the formulation, the aims as well as the verification procedure of the proposed limits are discussed in
the paper.
Keywords: SILENV project, noise pollution, airborne noise, ship noise
1.
INTRODUCTION
An assessment of the environmental impact of
ships involves consideration of different kind of
emissions both in air and in water. Significant
efforts have been dedicated in the last decades by
international Normative Bodies to the control and
mitigation of the effects of such emissions. As
regards specifically the noise impact of ships, as
pointed out in (Badino et al., 2011), the situation of
the normative framework is pretty much different in
the various types of ambient involved. The noise
radiation towards the spaces internal to the vessel
has been covered since a long time by Norms
regarding the preservation of acceptable working
conditions for the crew on board (IMO, 1981).
Staring in the ‗90thies, requirements have been
issued by Classification Societies (generally termed
Comfort Classes) having the target of assessing the
comfort of passengers and crew as regards noise
levels on board (as well as vibration levels and
other aspects). The subject of noise radiated by
ships into water started to be considered only
recently at IMO, see (IMO, 2007, 2009b, 2009c,
2010) and gained momentum also at European
level with the launch of dedicated research
programs (SILENV 2009, AQUO 2012).
As mentioned in (Badino et al., 2011), the control of
airborne emissions from ships and the assessment
of their impact on the external environment has not
been faced so far at an international level, despite
the fact that such impact results to be significant on
the inhabitants of living areas near channels, near
coastal waters with intense traffic or near ports,
both during the approach of ships and the period
spent at wharf for loading/unloading processes. On
the other hand, the problem has been often tackled
at a local level and with different approaches by
local administrations, driven by complaints sent out
by citizens living in the areas affected.
As pointed out in the cited paper, the lack of
coverage of this aspect by international normative
bodies is probably due to the fact that the
assessment of the impact of the noise emitted by
ships during coastal operations depends not only
on the ship, but also on the characteristics of the
coastal area (orography, urbanistics). Accordingly,
the Bodies involved in the assessment and the
control include quite different institutions at an
international as well as local level: (e.g. IMO, Class
Societies, Port Authorities, Municipalities, etc.). This
makes difficult a co-ordinated normative approach.
Notwithstanding what above, a qualified objective of
the SILENV project was to set goals for limiting the
various types of noise impact by ships revising and,
when appropriate, defining limits to be fulfilled in the
design process of the vessel.
The present paper describes how this objective has
been sought and obtained, with specific reference
to the external airborne noise emissions by
seagoing ships.
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International Conference on Advances and Challenges in Marine Noise and Vibration
MARNAV 2012, Glasgow, Scotland, UK, 5-7 September 2012
2.
EXISTING REQUIREMENTS IN SIMILAR
CASES
Prior to formulate a requirement for ship emissions
it is worthwhile to recall the requirements already
enforced in different fields but covering similar
aspects of airborne noise pollution. They will be
briefly analysed in the following.
2.1
EUROPEAN UNION DIRECTIVES
The Directive 2002/49/EC is aimed at quantifying
the noise impact in the short, medium and long term
of major sources (roads, rail and aircraft: vehicles
and infrastructures, industrial plants, equipments
and mobile machineries). The indicator used is the
LDEN defined as follows:
1 
 10lg 12 10 10  4 10
24 





(1)
where Lday, Levening, Lnight are the A-weighted longterm average sound levels determined over all the
day, evening, night periods of a year.
These limits are meant to be verified at the border
of the industrial area of interest in the closest
position that a common inhabitant of the
surrounding areas can access. The levels actually
present in the mentioned position, however, are
highly dependent on the local geography of the
area; in particular for a ship at harbour, relevant
aspects would be: position of the quay where the
ship is moored, of the surrounding buildings,
altimetry of the area (all affecting the transmission
losses from the ship to the receivers).
The LDEN values do not characterise a source in
itself, but the source in a specific ambient and is
therefore a quantity non particularly suitable for a
use in a design phase when specific environmental
conditions are not available.
The Directive 2006/87/EC, regarding inland
navigation vessels, provides, on the contrary, limits
expressed in dB(A) levels. In particular Article 8.10
‗Noise emitted by vessels‘ reads:
1. The noise produced by a vessel under way, and
in particular the engine air intake and exhaust
noises, shall be damped by using appropriate
means. The noise generated by a vessel under way
shall not exceed 75 dB(A) at a lateral distance of 25
m from the ship's side.
3. Apart from transhipment operations the noise
generated by a stationary vessel shall not exceed
65 dB(A) at a lateral distance of 25 m from the
ship's side.
As apparent, in this second directive requirements
are explicitly devoted to limit the noise generated by
vessels at source, in both the conditions of ship at
Lday
LDEN
22
Levening 5
10
Lnight 10
 8 10
10
Figure 1: Measurement surface for sources with
large dimensions (from ISO 3746:2010).
quay and ship sailing. It is to be noted that
―transhipment operations” (loading/unloading) are
excluded.
2.2
TECHNICAL STANDARDS
A significant reference for the characterisation of
large sources placed on a reflecting plane is given
in ISO 3746.2010 from which Figure 1 is taken.
The standard refers to the determination of sound
power levels by sound pressure surveys carried out
on a measurement surface in the proximity of a
static source, as a moored ship can be considered.
In the Figure a parallelepiped surface is shown.
The aim of the present investigation is not a
characterisation in terms of power radiation of the
ship, but the same criteria for the definition of a
representative measurement surface around the
source and of the grid of point to be placed on it can
very well be adapted to the case of the ship moored
at quay.
Other interesting measurement standards (referring
to vehicles in motion) are contained in the ISO
2922:2000 regarding the airborne sound emitted by
vessels for inland navigation, in motion and at
harbour.
The suggested characterisation in this case is
represented by a single measurement point at
255m from the vessel at an height of 3.5 m over
the water surface.
The same approach is contained in ISO 145091:2008, referring to pleasure boats in motion. The
procedure consists in a pass by test with the
configuration represented in Figure 2. The same
reference distance and vertical position as for
inland vessels apply.
Very similar arrangements are used also for other
vehicles, like trains, cars, motorcycles or trucks: see
for trains Figure 3, taken from ISO 3095:2001.
International Conference on Advances and Challenges in Marine Noise and Vibration
MARNAV 2012, Glasgow, Scotland, UK, 5-7 September 2012
microphone
craft
Figure 2: Pass by test measurement layout
modified from ISO 14509-1:2009 – vertical view.
The distance from the source adopted in the
standards depends on the vehicle typology, due to
the different dimensions and also to the different
situation in predicting and tracking the actual
trajectory of the vehicle during the test.
On the other hand, some common features can be
identified among the various procedures:
a) a
single measurement position is selected along the
path followed by the moving vehicle (controlling the
time duration of the survey a ‗scan‘ of the vehicle
emission along its longitudinal axis is obtained), b)
maximum sound pressure level during pass-by test
is adopted as emitted noise indicator and c) the
vertical position is chosen in order to cover the
presence of local sources in the upper part of the
unit (this is explicitly mentioned in the ISO 3095,
with 2 microphones foreseen in the test: Figure 3).
3.
SCOPE OF THE PRESENT ANALYSIS
The present analysis focuses on seagoing vessels
and aims at setting limits on the noise radiation at
source that can be effective in reducing the impact
of such radiation on the exposed population of
coastal areas.
3.1
STRATEGIES IN DEFINING LIMITS
In principle, to reach the goal of controlling the
impact of noise on close living areas, limits could be
set directly at the receiving position where the
control of noise effects is needed. This is the
strategy followed by the Directive 2002/49/EC
above recalled, which is however best suited in the
assessment of an existing plant, when details of the
areas surrounding the source are available. At a
design level, an approach based on the limitation of
the emission at source is more suitable, even
Figure 3: Arrangement for pass by tests on trains,
from ISO 3095.2001.
though the final impact of noise radiation will
depend also on the surrounding environment.
Characterising and limiting the emission source is,
in short, only a part of the whole problem of
controlling noise effects, but, on the other hand, is
the only one that can be developed at a design
phase of the system which represents the source.
It is also worthwhile noting that dealing with a
known source allows to seek the final goal also by
means of operative constraints (in the case of the
ship: speed limitations, distance from the shore,
manoeuvring speed, time limitations, location within
the harbour) that can be imposed depending on the
source strength. A proper characterisation and
control of the ship source levels is therefore to be
intended as a key point of the control of port noise.
With this in mind, within the SILENV project
experimental characterisation procedures and
reference levels for airborne emissions from ships
were set, which, together with analogous limits on
emissions in water and on the propagation towards
the internal spaces of the ship make up the socalled ‗green label‘ requirement with respect to
noise emissions.
3.2
CHARACTERISTICS OF THE SHIP AS A
NOISE SOURCE
The ship is a complex source of airborne radiated
noise. Many single sources can contribute to the
general noise emission, each one with different
characteristics. Structure-borne sources internal to
the ship (in particular the propulsion engines) can
excite vibrations in the hull (or in portions of it) and
in turn this can generate noise waves in air. This
radiation mechanism, however, is not very effective
at larger distances, while affecting significantly
people on board the vessel (in the internal spaces
or on the decks). More effective from the viewpoint
of the propagation outside the vessel are the
sources of airborne noise that, even though placed
inside the vessel, are however in communication
with the surrounding ambient through openings.
Examples of these sources are represented by the
funnel, where exhaust gases at high speed are sent
in the atmosphere from the exhaust system and
inlet/outlet ducts belonging to heating, ventilation
and air-conditioning (HVAC) systems.
The above mentioned single sources are distributed
along the entire ship both in the longitudinal and in
the vertical direction (see Figure 4), generating a
complex 3D noise field radiating from the ship.
Moreover, different operating modes for the ship
can be identified, each one characterised by
different sources on board and/or different relative
contributions to the total emission:
a) ship sailing along the coast
b) ship manoeuvring (entering/exiting the harbour)
c) ship at quay (no cargo processing)
d) ship loading or unloading (equipment for cargo
processing in function)
23
International Conference on Advances and Challenges in Marine Noise and Vibration
MARNAV 2012, Glasgow, Scotland, UK, 5-7 September 2012
Figure 1: Airborne noise source on ships.
In a) the most important source is represented by
the main propulsion engines and their related
exhaust discharges.
In b) the main engines are working in off-design
conditions. Other manoeuvring equipment are in
function (bow and stern thrusters, auxiliary
azimuthal propulsors, winches, windlasses, etc.).
In c) and d) the main engines are supposed not to
be operating, while many of the auxiliaries are
running. Case d) is characterised by specific noise
components due to the operation of cranes,
buckets, ramps and other charging/discharging
means (which may produce noise with an impulsive
nature or anyway peculiar frequency contents
and/or durations).
In the present context the focus is on operating
modes a) and c), whose noise emission are in
principle stationary and which are identifiable in a
similar way for all types of ship (the other two being
more dependent on the specific features of the
single ship).
3.3
FEATURES OF THE NOISE FIELD
RADIATED BY SHIPS
Experimental surveys and predictions on the sound
field radiated by ships at quay were carried out in
the SILENV project. Figure 5 presents prediction
results for a multipurpose ship see (Badino et al.,
2012), while Figure 6 and Table 1 report the
experimental results for the same ship obtained in a
campaign carried out in the framework of the
SILENV project by the High Technological Park –
Technical University of Varna (HTP-TUV). Aim of
the campaign was to validate mathematical models
Figure 6: Points surveyed during the experimental
campaign.
and to compare the existing vessels‘ emitted levels
with the preliminary target levels fixed at the
beginning of the project. Other analogous data are
available from literature see e.g. (Draganchev et al.,
2012).
It is quite clear that the near field sound radiation in
the proximity of the ship is complex and need to be
described carefully. The complexity arises from the
presence on board of different sources interacting
with several reflecting surfaces and a number
diffracting edges existing on board and, possibly, on
shore.
The influence of the ‗shadowing‘ effect of the sidedeck connection on the propagation of noise from a
source placed on the deck is visible both in Figure 5
and in Table 1.
In Table 1 the colour scale highlights (in red) the
presence in the two sections of strong sources on
the deck (fans). Shadow cones close to the side are
present at a lower height.
The shape of the shadow cones is different in the
two sections and the sound field appears not be
regular at 19 m from the side (in the fore section the
pressure levels keep on increasing with the
distance from the ship up to 19 m).
In Figure 7 a sketch is presented for the
quantification of the influence of the ‗shadowing‘
effect of the side-deck connection on the
propagation of noise from a source placed 1 m over
the deck.
Table 1: Recorded noise levels (dB(A)) in the
positions shown in Figure 6
Position
section in way of
engine room fans
(aft)
horizontal
19m
 vertical
Figure 5: Predicted noise field for a multipurpose
ship
24
section in way of
cargo hold fans
(fore)
11m
1m
19m
11m
1m
6m
64.6
68.3
75.6
64
65.5
71.8
3m
65.2
67.9
69.8
64.5
66.2
65.1
1.2m
62.8
67.2
68.9
65.9
65.6
62.2
International Conference on Advances and Challenges in Marine Noise and Vibration
MARNAV 2012, Glasgow, Scotland, UK, 5-7 September 2012
Figure 7: Sketch of the shadow zone generated by
the hull‘s side for a source on the deck.
The distance d (see Figure 5) that represents the
limit of the shadow zone can be easily evaluated by
the following expression:
B
(2)
d   (D  T )
2
T= draught, D= depth, B= beam
The situation may be different if measurements are
carried out in the far field: at larger distances from
the hull, the radiation features more regular patterns
(see Figure 5). On the other hand, the distance at
which a source can be considered as omnidirectional is by rule-of-thumb 7-8 times the
dimension of the source. This, in the case of a ship,
would bring the measurement location so far away
as to make quite difficult to get a high enough signal
to noise ratio.
This very preliminary analysis suggests that
measurements for a ship will most often be carried
out in the near field and therefore a suitable spatial
distribution (in particular in the vertical direction) of
the survey positions is to be selected to capture the
characteristic of the radiation field.
What above is noted with reference to the ship at
quay, but can be transferred also to the case of ship
sailing.
For the ship manoeuvring and loading/unloading
the situation may be further complicated by time
dependencies and random occurrences of the noise
radiation.
4.
REQUIREMENTS FOR SHIP AT QUAY
The strategy that has been adopted to characterise
the ship while floating still at quay is similar to the
one of the ISO 3746:2010 above presented. In the
present case, there is no need to adopt a closed
measurement surface (no evaluation of the power
emitted by the ship is sought). Accordingly, the top
horizontal plane of the parallelepiped is omitted, as
the noise radiated in the vertical direction
propagates in a direction where no receivers are
present.
4.1
POSITION OF THE MEASUREMENT
SURFACE
In defining a distance from the ship where the
measurements surface is placed, both technical
and practical aspects have been taken into account.
A short distance from the ship is more practical
because of easier accessibility. Moreover, the
influence of possible reflections from surfaces
external to the ship such as buildings, walls etc., is
likely to be less pronounced in this position.
Getting too close to the ship implies, however, to
increase the resolution in the grid of measurement
points, as the pressure field tends to become less
homogeneous and needs a more detailed
description.
On the other hand, at larger distances from the ship
side, such as the 25 m suggested in the European
directive 2006/87/EC and in ISO 2922:2000, other
drawbacks may arise: reflecting surfaces (e.g.
buildings, cranes, containers etc) may be present,
affecting the sound field, and background noise,
coming from external sources in the area, can
jeopardise the measurements.
In the light of what above, a distance of 10 m from
the ship sides (and from the bow and the stern) was
set for the measurement surface. Such distance is
suggested by practical as well as technical issues,
as it is likely that for the major part of shipyard and
port layouts this corridor along the ship side and the
adjacent area is free of obstacles. Further, in the
same space cranes are often located, which can be
used to move a microphone in the measurement
grid positions. From a technical point of view, the
distance is large enough to allow for a grid of points
not too dense (which would imply an increase in the
measurement effort).
The distance of 10 m is set with a small tolerance
(0.5 m) in order to avoid the use of corrections to
report the levels to the reference distance: the
adoption of this possibility would have implied the
definition of a transmission loss law which is not
easily definable in the near field of the ship.
The measurement surface, therefore, results to be
composed of four vertical planes (two parallel to the
ship‘s symmetry plane and two perpendicular to it
see Figure 8 below) at the reference distance of
100.5 m).
4.2
EXTENSION OF THE MEASUREMENT
SURFACE
The portion of the lateral area of the measurement
parallelepiped to be covered by the grid of points,
as suggested in ISO 3746:2010, should extend by
10 m (reference distance) beyond the maximum
dimensions of the ship in the three directions of
space. In particular the vertical extension is needed
to overcome the above mentioned ‗shadowing‘
effect of the main deck. This is an important aspect
because as pointed out by actual cases of
complaints sent out by citizens near port area, very
often the position where these negative noise
effects were detected is often placed at higher
levels than the ship deck (higher floors of buildings,
or on hills surrounding the harbour). This highlights
the importance of carrying out measurements also
25
International Conference on Advances and Challenges in Marine Noise and Vibration
MARNAV 2012, Glasgow, Scotland, UK, 5-7 September 2012
(a)
would rely much on the subjective evaluation of the
surveyor. To overcome this problem, a regular grid
of points (with constant density) is placed on the
measurement surface surrounding the ship.
As regards the resolution of the grid, a balance is to
be found between accuracy and measurement
efforts. The distance between the measurement
points is influenced by the dimension of the source
and by the position of the measurement surface:
the farer the surface is placed from the source, the
coarser may be the grid of points on the surface
(see ISO 3746:2010). In the light of these considerations, the spacing for the points was set as:
d= 6 m for L<100 m
d=10 m for L>100 m
(b)
The first row from below is to be set at 1.2 m from
the ground. These values seem to be a good
compromise
between
accuracy
of
the
measurements and their duration.
4.4
Figure 8: Grid of points depending on the
longitudinal profile of the ship.
at high levels from the quay, in order to capture the
most relevant noise components.
As the longitudinal profile of ships may include
portions of the weather deck at different levels
(particularly in the presence of aft superstructures),
the grid does not need to be rectangular in the
longitudinal planes, but may follow the ship profile,
provided it is extended by 10 m in vertical and
longitudinal direction from each corner (Figure 8).
Figure 8 sketches a part of the grid for two types of
ship: a passenger ship (a) and a merchant ship (b).
These two kinds of vessels respectively represent
regular profile and irregularly shaped ships.
Characterisation tests must be carried out on both
sides of the ship, due to the possible asymmetry of
the sound field radiated.
4.3
RESOLUTION OF THE GRID OF
MEASUREMENT POINTS
Due to the complex distribution of the noise sources
on
board,
the
measurements
aimed
at
characterising the noise levels emitted by ships are
to be carried out with a spatial resolution able to
capture all the contributions and in particular to
quantify properly the strongest ones.
In principle, only a small number of measurements
concentrated in the proximity of the (strongest)
sources are needed, but both the sources location
on board and their relative strength are not known
‗a priori‘. Accordingly, an approach based on a
small number of selected measurement positions
on the reference surface would be efficient but
26
PRACTICAL CONSIDERATIONS
For example, the number of measurement points
for a passenger ship with L=300 m, B= 35 m and
D=60 m, is around 500. This kind of measurements
will be typically carried out in the shipyard where
the vessel is built. A possible strategy consists in
using a microphone array suspended from a crane
(see Figure 9).
This procedure allows to measure an entire vertical
‗column‘ of points at the same moment, with
considerable savings of time. The other columns
can be easily surveyed by moving the crane on the
rails along the ship length without changing the jib
configuration.
4.5
LIMIT LEVEL
Based on the analysis of on field measurements
and on the limit set for inland vessels at harbour in
the directive 2006/87/EC (65 dB(A) at 25 m), a
provisional limit of 70 dB(A) at 10 m from the ship
was set as SILENV requirement. This value is not
to be overcome in any of the measurement points.
In respect to the above directive, a key difference is
that measurements at different height are to be
carried out, thus ensuring that all connection paths
from the source to the possible receivers are
covered. The value in itself is, on the basis of the
measurements available, technically achievable by
state of the art designs, but on the other hand,
seems to be restrictive enough to filter away
particularly annoying situations. It is here noted that
the same limit, to be verified at the border of
industrial areas, is typical of European regulations
at national level (see f.i. Italy‘s DPCM 1997). The
same value is also set as SILENV limit for open
spaces on board ships (people on decks may
however be shielded more by e.g. funnel emissions
International Conference on Advances and Challenges in Marine Noise and Vibration
MARNAV 2012, Glasgow, Scotland, UK, 5-7 September 2012
than other people ashore, so the requirement is not
to be considered as redundant)
4.6
SIMPLIFIED PROCEDURE
What above described represents the suggested
procedure for a proper characterization and
limitation of the noise radiation from a moored ship.
Within the SILENV requirement, it was decided to
set an alternative criterion, that can be accepted
when it is impossible to carry out measurements on
the parallelepiped surface.
According
to
this
simplified
procedure,
measurements can be carried out in an horizontal
line of points at ground level (at least 1.2 m over the
quay) at 25 m from the ship side (see Figure 10).
The longitudinal spacing follows the general rule
(see section 5.3 above). No obstacles must be
present between the ship and the measurement
rows.
The limit set in this case is 60 dB(A). It is lower
(stricter) than the limit set on the complete grid for
two reasons: it is to be verified at a larger distance
from the ship and it takes into account that at
ground level important contributions due to sources
at higher levels may be underestimated.
As in the case of the complete grid measurements,
the procedure is to be repeated for both ship sides.
5.
5.1
REQUIREMENTS FOR SHIP SAILING
PASS BY TESTS
Figure 9: Possible measurements arrangement
Figure 10: Measurement points for the simplified
procedure.
The existing standards for pass by tests of inland
vessels (ISO 2922:2000) and pleasure crafts (ISO
14509-1:2008) have been reviewed earlier in the
paper. In both documents a single microphone
placed at a distance of 25 m is used. The same
reference is adopted in the directive 2006/87/EC to
set the limit of noise radiation for inland vessels. In
the following, the case of seagoing vessels is
considered. Differences that can arise with the
previously mentioned types of vessels are related
with the dimensions of seagoing ships, in general
larger, and in the arrangement of test. Inland
vessels and pleasure crafts can be tested with a the
microphone positioned ashore, while passing at the
required distance from the channel side or the
quay. For a large seagoing ship it might be
necessary to carry out the test offshore (possibly
combining this type of tests with sea trials); in any
case it would be required for safety reasons to keep
a larger distance from the recording position, placed
either at sea or on quay.
Must be noted that the noise indicator for pass by
tests is the maximum level, Lmax, expressed in
dB(A), that is the higher sound pressure level
measured at the microphone position while the ship
is passing by. The SPL logging can be set at
different intervals, being one second the most usual
option.
A further note regards the possible presence at 25
m of the same shadow zones that have been
observed and commented for the noise radiated by
the ship still at harbour. In comparison with the
surveys of the ship in static conditions, on the other
hand, pass by tests should be less exposed to the
presence of obstacles (buildings and other
deflecting or reflecting surfaces) and noise coming
from external sources (particularly for tests at sea).
Unfortunately experience on the subject of pass by
tests was lacking in the project, so the final decision
was to propose for Lmax the same limit that has
been set for inland vessels, that is 75 dB(A) at 25
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International Conference on Advances and Challenges in Marine Noise and Vibration
MARNAV 2012, Glasgow, Scotland, UK, 5-7 September 2012
m. Nevertheless the possibility of setting a larger
distance, for example using Eq. 2 to avoid shadow
zones, and a larger number of microphones, placed
in a vertical array, may be considered.
5.2
SINGLE EVENT LEVEL AS A SAILING SHIP
INDICATOR
A further note regards the possible adoption of
Single Event Levels (SEL) as quantity for reporting
the results of the pass by test (and for the
corresponding limits), as from long time for the
airport noise. The quantity is actually defined in
general terms also in the ISO 2922:2000 (see
Eq.3), but could be adopted with the operative
definition provided e.g. in CFR 14 for aircraft noise:

SEL  10 log
1 2 p 2 ( )
d
T0 1 p02
(1)
where T0 is equal to 1s. The integration time (2-1)
is determined with reference to the time history of
the dB(A) signal: a typical pattern is reported in
Figure 11, showing a rising trend on the approach
of the vessel and a decreasing part when the vessel
starts to get farer. The portion of the curve
comprised between the max dB(A) value LMax and a
horizontal line drawn 10 dB(A) below it identifies the
integration range.
The SEL indicator takes into account the maximum
noise level and the duration of the event and allows
to compare noise from ships having different
velocities and power levels, on the basis of the
energy content of their emissions during the passby test. For ship events, the SEL value is always
higher than the LMax value.
From a general viewpoint, the SEL technique
seems to be well suited for the purpose of
characterising a sailing ship. On the other hand, the
lack of data prevents at the moment from
quantifying a proper noise limits for ships in terms
of SEL.
SEL integration
domain
Figure 11: Definition of Single Event Level
28
6.
CONCLUSIONS
The paper present the background of SILENV
Green Label requirements regarding the airborne
noise radiation by ships. Such requirements have
been based on analogous ones already available
for other classes of vehicles or static sources, but
significant changes have been made on the
characterisation procedure. These changes have
been necessary to adapt the analysis to the case
of seagoing ships and to make results more
representative of the impact of airborne emissions
from ships. The present version of requirements is
subject to further development following the
generation of a consistent database on the subject.
ACKNOWLEDGEMENTS
This work was developed in the frame of the
collaborative project SILENV—Ships oriented
Innovative soLutions to rEduce Noise & Vibrations,
funded by the E.U. within the Call FP7-SST-2008RTD-1 Grant Agreement SCP8-GA-2009-234182.
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