modelling the outdoor noise propagation for different ship types

8/11/2019 MODELLING THE OUTDOOR NOISE PROPAGATION FOR DIFFERENT SHIP TYPES

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In this paper, the effectiveness of a general purpose commercial ray-tracing simulator inanalyzing ship noise propagation has been tested. Two different kinds of ships were modelled:a Fishing Research Vessel and a Multipurpose Ship. Both vessels were modelled as complexnoise sources, i.e as a combination of different punctual and area sources. The modelling wascarried out by dividing the work in steps (see Biot et al, 2011): acquisition and processing of the

input data, implementation of the ship model with the software, model validation by comparisonwith measurements and output of the results. Noise propagation was then simulated andnumerical results were validated by the comparison with measured sound pressure levels inseveral positions around the ship.

2. INPUT DATA

The accuracy of the model and its effectiveness in simulating noise propagation in harborsstrongly depends on the quality of the input data on the specific noise sources. The process,which can be easily standardized for other kinds of sources, is critical for ships because of theircustom-built characteristics. Dimensions and morphology vary deeply from one ship to another,making therefore difficult to define a general purpose procedure. Some general elements need

to be defined for the purpose of modelling the ship as a noise source and comparing theradiation patterns with experimental data. They are: sound pressure levels measured near thespecific sources, information about the ship geometry, the topographic and geometricalinformation about the area around the ship, the microphone positions and weather informationduring the surveys to be compared.The input data were provided by two partners involved, together with the University of Genoa, inthe SILENV Project: TSI - Tecnicas Y Servicios de Ingenieria concerning the Fishing ResearchVessel, and High Technological Park - HTP and Technical University of Varna - TUVconcerning the Multipurpose Ship. Information about the above mentioned institutions can befound in SILENV website (2012).The measurements were carried out on the ships berthed, with only the machinery inherent tothis operating condition working in stationary conditions.

Figure 1. Fishing Research Vessel measurement grid in the port side (a) and in the starboardside (b) (see SILENV European project, 2012)

(a)

(b)



3. AIRBORNE SHIP NOISE MODELLING

In general, commercial ray-tracing software contains specific modules for different noise sourcetypologies such as roads, railways, aircrafts, industries and, in general, for point, line and areasources. A specific module for ships, is, to the authors’ knowledge, not available. The different

parts of the ship were therefore to be created using some general purpose tools provided by thesoftware itself. These tools do not allow creating complex shapes, therefore it was necessary tosimplify the ship shape considering only parts which can significantly influence the noisepropagation.The geometries of the two ships, simplified this way, were imported in the simulation software,and the different elements were turned into objects available in the software’s database, e.g.buildings, walls or floating screens.The geometry of the Fishing Research Vessel was provided in a drawing with an unique layer.Fortunately, the different parts could be clearly identified because the vessel shape is made upof simple elements. In the case of the Multipurpose Ship, the several parts of the ship geometrywere drawn on different layers by considering its complexity. Thanks to this subdivision, also forthe Multipurpose Ship the diverse elements were easily identified.In figure 3 the 3D views respectively of the Fishing Research Vessel (figure 3a) and of the

Multipurpose Ship (figure 3b) are shown.Since methods to measure the noise emissions from these two ships were different, theassessment of the sound power levels of the sources was made using different methods on thebasis of the available information.In the Fishing Research Vessel, the sound pressure levels were measured next to the gridnodes at a distance of 1 m from the hull and were considered with the aim to calculate thesound power levels of the sources. In particular, three sources were identified: the funnel andtwo intake openings. These sources were considered as point sources (see Moro, 2010) whilenoise propagation was considered spherical.Thanks to the measures taken at 1 m from the sources of the Multipurpose Ship, it was possibleto assess the sound power levels of the sources by knowing the sound pressure levels. In thiscase, 7 main sources were identified in the superstructure and in the middle of the ship: thefunnel, 2 ventilation fans of the engine room and 4 ventilation fans of the cargo holds.

The fans were considered as ‘small industrial buildings’, where the sources are on the buildingsides. On the basis of their geometrical characteristics, these sources were defined as areasources. The funnel was considered as a point source and its propagation was consideredspherical. A point source was placed on the top of the funnel structure.

4. VALIDATION OF THE MODELS

The validation of the model was made comparing the noise levels measured on field and thenoise levels calculated by the simulator in the same positions. A-weighted SPL were measuredin both situations, but the operative procedure changed from Fishing Research Vessel to

Figure 3 – 3D view of the ships: (a) Fishing Research Vessel; (b) Multipurpose Ship



Table 1. Run settings

Angle increment [°] 1

Number of reflections 1

Source side reflection Selected

Max search radius [m] 500

Tolerance [dB] 0,0

Weighting dB(A)

Enable side reflection Selected

Set 5 dB rail bonus not selected

Create ground effectareas from road surfaces not selected

Multipurpose, in dependence on the characteristics of the ship and in respect of the actualpossibility to operate.As regards the numerical simulations, the run settings used to validate the model are shown in

Table 1. The same set of conditions were utilized for the following analysis of the noise field.Algorithms from ISO 9613-2:1996 standard were adopted to analyse outdoor noise propagationand diverse attenuation terms.

4.1 Validation of the Fishing Research Vessel Model Regarding the Fishing Research Vessel, the calculated levels in the nodes of the two gridsplaced along the length for both sides were compared with the measured levels.For this comparison, 15 of the 21 measurements made in the port side (PS) and 3 of the 14measurements made in the starboard side (SB) were used taking into account the backgroundnoise criteria. In Table 2 the differences between measured and calculated SPL values for both

Table 2. Comparison between calculated and measured sound pressure levels

PositionHeight abovethe wharf

Distancefrom the hull

CalculatedSPL

MeasuredSPL Difference

[m] [m] [dB(A)] [dB(A)] [dB(A)]

PS 3 1.2 1.0 62.9 62.0 0.9

PS 4 1.2 1.0 68.9 68.4 0.5

PS 5 1.2 1.0 72.1 71.6 0.5

PS 6 1.2 1.0 70.4 70.4 0.0

PS 7 1.2 1.0 63.6 63.5 0.1

PS 9 1.2 15.0 60.1 60.1 0.0

PS 10 1.2 15.0 61.6 62.4 -0.8

PS 11 1.2 15.0 64.2 64.0 0.2PS 12 1.2 15.0 65.5 64.9 0.6

PS 13 1.2 15.0 65.3 65.0 0.3

PS 14 1.2 15.0 63.1 62.2 0.9

PS 17 1.2 25.0 60.2 61.4 -1.2

PS 18 1.2 25.0 62.0 63.5 -1.5

PS 19 1.2 25.0 62.8 62.6 0.2

PS 20 1.2 25.0 62.6 62.2 0.4

SB 2 1.2 1 64.3 64.3 0.0

SB 3 1.2 1 64.1 64.1 0.0

SB 6 1.2 1 63.7 63.1 0.6



ship sides are shown. The differences are very small both in port side and in starboard side: thehighest difference value is 1.5 dB in the third row. The Mean Absolute Error, that is the meanvalue of the absolute difference between predicted and measured values, calculated on thewhole set of Fishing Research Vessel data results to be equal to 0.1 dB(A), while the StandardDeviation is 0.7 dB(A).

Besides, the accuracy of the predictive model for the Fishing Research Vessel has beenevaluated according to two criteria: the fraction of data predicted within ±3%, called λ, and thePercent Absolute Mean Error, ε:

%1

1

∑=

−

=

N

i valuemeasured

valuemeasured valuecalculated

N ε

In reference to the whole data set, ε resulted to be equal to 0.0014% and λ to 100%. These values indicate a good accordance between measured and calculated SPL, thussuggesting the possibility to utilize the model to simulate airborne noise propagation from theFishing Research Vessel.

4.2 Validation of the Multipurpose Ship Model

On the basis of the on field measurements, three different test steps were made to validate theMultipurpose Ship modelling: the validation of the sound pressure levels close to the sources;the validation of the noise field along the ship length, taking into account a horizontal grid at 1.2m from the ground level; the validation of the noise propagation at different heights anddistances, taking into account the two cross grids placed near the ventilation fans of the engineroom and near the ventilation fans of the cargo holds.In the first test step the sound pressure levels close to the sources were verified comparing thecalculated and measured levels at a distance of 1 m from sources. The differences are smallerthan 1.2 dB. This test suggests that simplifications and hypothesis introduced in modelling thecharacteristics of the noise sources, and the correspondent Sound Power Level, are reasonableand the model properly represents the diverse onboard sources.The second test was carried out by comparing the calculated and measured values next to the45 nodal points of the horizontal grid, which is placed along the ship length at a height of 1.2 m

above the wharf. In this case, 8 out of 45 measures were not taken into account because of thebackground noise. The differences between the calculated and the measured values aregreater than 1.5 dB in the majority of cases, while only 6 differences are lesser or equal to 1.5dB. There are 15 differences in the range between 1.5 dB and 3.0 dB, and 16 differencesbetween 3.0 dB and 6.0 dB. No difference exceeds 6.0 dB. The Mean Absolute Error and theStandard Deviation, calculated on the whole set of data, are respectively equal to 0.0 dB(A) and1.3 dB(A).The third test allows to assess the simulated noise propagation along two planes perpendicularto the ship axis, the same ones shown in figure 2b. The differences between calculated andmeasured sound pressure levels are shown in Table 3 and Table 4 respectively for the grid nearthe fans of the engine room and for the mesh grid near the fans of the cargo holds. In this case,the Mean Absolute Error is equal to 0.8 dB(A) and the Standard Deviation to 2.7 dB(A).

Table 3. Third test - near the fans of the engine room

Position I.1 I.2 I.3 II.1 II.2 II.3 III.1 III.2 III.3

Height abovethe wharf [m] 1.2 1.2 1.2 3.0 3.0 3.0 6.0 6.0 6.0

Distancefrom the hull [m] 1.0 11.0 19.0 1.0 11.0 19.0 1.0 11.0 19.0

CalculatedSPL [dB(A)] 63.0 70.2 64.5 69.7 70.7 64.8 74.9 71.4 65.4

MeasuredSPL [dB(A)] 68.9 67.2 62.8 69.8 67.9 65.2 75.6 68.3 64.6

Difference [dB(A)] -5.9 3.0 1.7 -0.1 2.8 -0.4 -0.7 3.1 0.8



Table 4. Third test - near the fans of the cargo holds

Position I.1 I.2 I.3 II.1 II.2 II.3 III.1 III.2 III.3

Height abovethe wharf [m] 1.2 1.2 1.2 3.0 3.0 3.0 6.0 6.0 6.0

Distance fromthe hull [m] 1.0 11.0 19.0 1.0 11.0 19.0 1.0 11.0 19.0

CalculatedSPL [dB(A)] 62.4 64.2 63.9 64.9 66.3 65.7 72.5 71.2 69.3

Measured SPL [dB(A)] 62.2 65.6 65.9 65.1 66.2 64.5 71.8 65.5 64.0

Difference [dB(A)] 0.2 -1.4 -2.0 -0.2 0.1 1.2 0.7 5.7 5.3

the highest difference was observed in correspondence of the receiver I.1 and it is negative. Inthe first row all deviations had minus sign, even if the differences referring to receivers I.2 andI.3 were very limited and lesser than the difference for receiver I.1. The mean deviation between

measured and calculated levels regarding the 9 nodal points of this grid is equal to 0.5 dB(A),whereas the standard deviation is equal to 2.8 dB(A).Table 3 shows that the highest difference was observed in correspondence of the receiver I.1and it is negative. In the first row all differences are negative, much smaller in the case ofreceivers I.2 and I.3 than for receiver I.1. Among the grid points number 2 (all placed at adistance of 11 m from the hull) at the three different heights, the mean error is 3 dB, whereas inthe grid points number 3 (19 m far from the hull) at the three different heights, the mean error is1 dB. The larger errors in the cross grid near the cargo holds (Table 4) were observed in thethird row (upper level), and particularly in positions III.2 and III.3. Except for these positions,differences are generally quite small. The Mean Absolute Error between measured andcalculated levels in reference to the 18 nodal points of the two grids is equal to 1.1 dB, whereasthe Standard Deviation is equal to 2.7 dB.Finally, the whole set of data relative to the second and third test steps was utilized to assess

the accuracy of the predictive model according to the two criteria above introduced: the fractionof data predicted within ±3%, λ, and the percent absolute mean error, ε, result respectively0.005% and 75.5%.Also for the Multipurpose Ship the model seems to work quite well, in spite of the complexity ofthe geometry and the number of noise sources.

As a summary analysis, Figure 4 presents the comparison between measured and predictedsound pressure levels for all the test steps regarding the Fishing Research Vessel (figure 4a)

Figure 4. Comparison between measured and predicted sound pressure levels concerning

Fishing Research Vessel (a) and Multipurpose Ship (b).

(a) (b)



and the Multipurpose Ship (figure 4b). The dotted lines indicate a difference between predictedand experimental values within ±3%, whereas the continuous line is the bisector. When a pointis on the bisector the difference between calculated and measured values is equal to 0.0 dB(A).General results indicate that, in spite of the limits of the software, an accurate calibration of themodel can carry to reliable predictions of noise levels emitted from ships.

5. RESULTS AND DISCUSSION

Once the model has been tested and validated, it was utilized to analyse propagation paths and3D sound field around the two modelled ships. Sound pressure levels were represented bymeans of chromatic maps (figure 5), where each color indicates a 5 dB(A) range. The same thechromatic scale has been adopted for both vessels.For instance, in figure 5a and figure 5b two horizontal maps relative to the Fishing ResearchVessel, respectively at 4.0 m and at 8.0 m above the sea level, are represented. A vertical map,corresponding to the longitudinal cross-section of the vessel, is reported in figure 5c. The non-axisymmetric noise field is a consequence of the odd position of the sources. Thanks to thevertical map it is possible to see how the superstructure of the ship can significantly obstruct the

noise propagation, specially the noise due to the funnel Simulations evidence the markeddependence of noise propagation on sources directivity: if sources present a strong directivity,sound levels at a certain distance can change significantly according to the relative position.This behaviour deeply influences noise maps and must be carefully considered during sourcescharacterization.Figure 5d and figure 5e refer to the Multipurpose Ship and show horizontal maps respectively at3.2 m and at 9.5 m above the sea level, i.e. 1.5 m and 7.8 m from the wharf floor. The verticalmap in figure 5f reports a cross section next to cargo hold fans. This last map indicates that thesuperstructure and the deck act as a screen for the ground area close to the ship, whereas theelevated zone, not shaded by the ship itself, is fully exposed to the noise emissions.In general, the ship is a kind of source characterized by a very complex geometry, also forrather small vessels. Many reflecting surfaces of different dimensions are onboard, whichgenerate a lot of reflective waves. In the same time, a simplification of the ship shape is

necessary in order to create a quite simple ship model in the simulator. Therefore a selection ofthe surfaces is needed during model drawing, introducing only those ones closer to the sourcesand with bigger dimensions. A standardized procedure can help a correct modelling of the realgeometry and avoid too drastic simplifications. This task should be taken into account inimplementing specific tools for ship noise modelling.The shape of the level curves is not generally obvious: simulations indicate for the MultipurposeShip a symmetrical sound field in respect of the vessel length, whereas the fishing ResearchVessel shows a marked dissymmetry in emitted noise levels, depending on the odd position ofthe secondary sources (ventilation terminals, intake ducts, vents, etc.), even if for both ships thefunnel is the major source of noise. As expected, noise levels seems to increase with tonnage,but not in a linear way.Results confirm the relevant influence of interactions (reflection, diffraction) between structuresand sources on ship noise propagation. Reflection on onboard surfaces as well as diffraction on

their hedges markedly affect noise propagation from the ship, particularly for the zones close tothe hull. Shadow zones are generated by the obstructing surfaces close to the sources, so thatthe noise field depends on the interactions between sources and obstructing volumes of thevessel, which screen the noise in dependence of their relative position.The specificity of ship noise is clearly evidenced, where a number of different sources,superstructures, decks, hull create a very complex condition, in which all these elements deeplyinteract. Therefore an accurate modelling should carefully consider diverse aspects:determination of Sound Power Level values in accordance with ISO standards, evaluation ofdirectional characteristics of the sources, measurements of Sound Pressure Levels valuesalong the noise propagation paths at different heights and distances from the hull, analysis ofgeometrical characteristics of the ship and of the surrounding area, and weather information astemperature, humidity, speed and direction of the wind. Some aspects are obviously common



with other fields, some other are specific for ships, as the need to make SPL measurements ata certain height from the ground or the complexity of the source geometry.

6. CONCLUSIONS

The modelling of a ship as a noise source is not a simple task because of the complexity of theship geometry, featuring numerous reflective surfaces, and also for the difficulty in thecharacterization of the onboard sources. The lack of specific tools for ship geometries in thecurrent commercial software does not facilitate technicians in facing such an issue.The attempt to simulate airborne noise propagation from two different ships, a Fishing ResearchVessel and a Multipurpose Ship, confirmed the above mentioned criticalities and, in the sametime, evidenced the importance of following a systematic approach during the various phases ofthe numerical modelling and, in particular, when selecting and defining the input data. In fact,several simplifications are needed and quite a complex set of input data are required for thesimulation, more than what normally occurs for the other kinds of sources.The complexity of the ship as noise source strongly increases the difficulty of the modelling. Inparticular, the following elements should be carefully considered: sources characterization,sources modelling, ship geometry simplification, measurements aimed at model validation.

- Specific measurements are needed close to onboard sources in order to adequately

characterize them; no specific standard to this aim is available at the moment.

- A proper simplification of the ship shape is necessary in order to create a correct vesselmodel in the simulator. In particular:o Among the many reflecting surfaces of different dimensions that are onboard, only

those ones closest to the sources and with biggest dimensions are to be introduced inthe model

o Ship drawings in the model should have different layers which define the differentelements of the vessel, such reducing the risk of incorrect simplifications

- Measurements along the propagation paths in reference points far from the hull, whencombined effects of different sources act, are recommended to validate the model.

All together, present results indicate that ship noise modelling can be implemented by usingcurrent commercial software, in spite of the need to realize a deep simplification of the shipgeometry and onboard sources. However, the complexity of the modelling suggests that effortsshould be made to develop specific software or to implement particular algorithms for ships’noise in current simulators, as it has been already done for other kinds of sources (roadvehicles, trains, aircrafts).

ACKNOWLEDGMENTS

This work was developed in the frame of the collaborative project SILENV—Ships orientedInnovative soLutions to rEduce Noise & Vibrations, funded by the E.U. within the Call FP7-SST-2008-RTD-1 Grant Agreement SCP8-GA-2009-234182.

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modelling the outdoor noise propagation for different ship types

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