MEASUREMENTS OF AIRBORNE NOISE EMITTED BY A SHIP AT QUAY

Transcription

1 MEASUREMENTS OF AIRBORNE NOISE EMITTED BY A SHIP AT QUAY Davide Borelli, Tomaso Gaggero, Enrico Rizzuto and Corrado Schenone University of Genova, Genova, Italy enrico.rizzuto@unige.it The problem of the impact of airborne noise emitted by commercial ships moored at quay is gaining attention by Regulatory Bodies. The question is often raised by citizens sending complaints to the municipalities of port cities in which the harbour is located inside the town. The problem may be even more important in the case of pleasure crafts accessing protected areas and mooring there. Limit levels for airborne noise emissions started therefore to be fixed in normative as well as contract requirements. In the light of what above, an effective measurement procedure to characterise the noise emissions from ships and compare them to limits plays an important role. The procedure needs to take into account aspects linked to the ship source characteristics (dimensions, complexity, directivity) as well as related to the surrounding environment, which is often represented by the shipyard where the ship is built. The latter aspect includes the presence of obstacles and reflecting surfaces around the ship, as well as significant components of background noise. The paper recalls the background of a measurement procedure that was defined within the project SILENV of the 7 th Framework Programme specifically for the purpose of the characterisation of ships as airborne noise sources and reports a trial application of this procedure to a ship moored at a yard quay. Instrumentation used and data post processing are discussed, together with practical problems encountered and solutions found in order to carry out measurements. 1. Introduction: noise emissions in air by ships Air noise pollution from ships moored at harbour may very easily affect people living in the surrounding area. The problem of the impact of airborne emissions has been raised on several occasions by citizens living in adjacent areas, which may be located even very close to the mooring point. As mentioned f.i. in [1], cargo ships spend a considerable amount of their operational time in port for loading/unloading processes. While moored, all ships need to run a number of plants related to the functionality of the vessel. For ferries and cruise vessels, in particular, relevant plants are the ventilation plants (for garages) and heating, ventilation and air conditioning (HVAC) systems (for cabins). A similar situation applies to yachts of various sizes, including mega-yachts. In almost all cases (except when electric power is provided from shore), this requires to run power supply units (diesel generator sets). Other sources of noise, generally characterised by transient components and more dependent on the specific ship type, are due to cargo handling equipment (run from the ship or from ashore): this ICSV22, Florence (Italy) July

2 includes e.g. grabbers, conveyors, gantry cranes or ramps for vehicles. In the following, the attention will be focussed on stationary emissions from typical plants on board. Commercial ports are in some cases located in or close urban areas (see e.g. most of Mediterranean ports). This applies even more often for passenger terminals. harbours for pleasure crafts or small ferries are often located in areas with high touristic and possibly naturalistic value. This geographic vicinity between the ship sources and the potential receivers of noise creates the need for an assessment and a control of the annoyance. The impact of the radiation depends significantly on the local shape of the coastline (presence of bays, fjords) on the orography of the surroundings (presence of elevations, hills) and the urban layout (distribution of buildings, houses, population). In principle, a full impact assessment should account for all the elements of the noise transmission chain, i.e. source, propagation and receiver. Generic predictive models for noise propagation are available, but applications to the specific case of ship source and propagation in harbours are still lacking [2]. As a matter of fact, the specificity of the ship source and of its noise radiation and the implications on the impact evaluation are not fully covered in the existing technical and normative framework (see 2). The present paper deals in particular with the characterisation of the ship as a source of noise and presents the first application of a newly proposed experimental procedure. 2. Existing Normative and technical framework for airborne emissions from ships As mentioned above, in the noise impact assessment of airborne emissions from ships, the receivers at the end of the transmission path and the transmission path itself, are both external to the vessel and, therefore, independent from the vessel characteristics. This makes the normative framework on external noise quite different from the case of the noise internal to the ship, for which, source, transmission path and receiving position are all on board [3]. In this latter case, the whole noise transmission chain is subjected to the same Normator, which, for sea-going ships, is the the International Maritime Organisation (IMO): the IMO Noise Code [4] is a detailed technical and normative reference which establishes, for ocean-going vessels, noise measurement procedures to be followed and limit values for noise levels in db(a) and for exposure levels in db(a) equiv to be fulfilled in various locations on board. As regards noise emissions in air outside the ship, the existing situation in terms of technical standards for measurements and requirements on emission levels is less defined (see also [5]). As a matter of facts, ocean going vessels are not covered by either type of documents. An ISO standard [6] is available for the characterisation of noise emissions in air from inland vessels, both in sailing and moored conditions. For stationary vessels, in particular, a description in terms of time-averaged A-weighted sound pressure levels, L paeq, is adopted. Recreational crafts of different types with length up to 24m are covered in another ISO standard [7] (only in the sailing condition). A second part of the same standard specifies a comparative procedure to assess the maximum sound emission of powered mono-hull recreational crafts using the concept of reference craft. Both the above ISO standards refer to measurements carried out at ground level. The aim, in particular in [7], is to carry out acceptance and/or monitoring tests, but the outcome of the measurement can also be used for comparison with a limit value set by a normative requirement. Normative requirements in terms of maximum source levels are actually set for inland vessels in the European directive 2006/87/EC: these limits are of 75 and 65 db(a) under way and in moored conditions, respectively; both are to be verified at ground level, at a distance from the ship side of 25 m. Specific features of the ship source, making difficult a proper characterisation, are related to the large dimensions and strong directivity. ICSV22, Florence, Italy, July

3 Figure 1. Grid of points for the characterisation of airborne radiated noise (SILENV). Ships are in fact very large sources, particularly if compared with the characteristics of the sites where they are measured or where their impact needs to be assessed. They also feature a strong directivity, as pointed out by experimental as well as numerical investigations carried out within the SILENV Project [8, 9]. These characteristics suggest that measurements carried out at ground level in a single position may not be enough to characterise completely the source. 3. Pre-normative requirement for ocean going vessels (SILENV) In the light of what above, the objective of a more complete acoustic characterisation of the moored ship source, to be possibly used for an accurate prediction of the external sound propagation, was pursued in the SILENV Project [10]. A set of different measurement layouts, additional to those recommended by the existing Standards, have been identified and described in [11]. Measurements made according to these layouts are to be carried out on a grid of points (on each side of the ship, due to the possible asymmetry of the radiated sound field) in order to capture the noise generation from local sources, e.g. ventilation systems, vents, funnels which are located at a certain height in the side or above the deck and even at the top of superstructures (Fig. 1). The grid of point should be placed at a fixed distance from the side, with small tolerances, in order to make possible a direct comparison among source levels, without the need for applying a propagation model to refer the measured to a common reference distance. A limit value of 70 db(a) was set for the levels surveyed on the grid, derived from existing requirements [11]. 4. The test case (illustrative of the procedure) A measurement campaign was carried out on a target ship moored inside a shipyard. The aim was to check the feasibility of the experimental procedure that was proposed in 8. The shipyard layout is shown in Figure 2. As it can be seen the ship was berthed port side. The above explained measurement procedure, with survey points distributed both in the horizontal and in the vertical direction, was adopted. In particular 13 measurements positions, distributed along the ship side every 10 m at the heights of 1.2 m, 17 m and 26.6 m and at a distance from the ship-side of 10 m were investigated (see Figure 2a and Figure 3). In order to reach the positions at the heights of 17 m and 26.6 m part of the sensors were placed on a crane, normally used for construction work, (see Figure 2a in purple). In fact, it was possible to reach the upper balconies and place there the instrumentation. In Figure 2b the positions were the equipment were placed are indicated with red dots. The crane is able to rotate around its central axis and to move alongside the ship on rails. Rotating the crane allows to fix the right distance of the survey points from the ship side: such operation was done just once, as the crane can move parallel to the ship side maintaining a constant distance. As the crane in itself represent a strong and near source of noise due to ventilation, during the recording phase in each position the engine of the crane was turned off. ICSV22, Florence, Italy, July

4 The 22nd International Congress on Sound and Vibration a) b) Figure 2. a) Shipyard layout; b) Position of the measurements points on the crane (red dots). Figure 3. Ship and measurements grid: horizontal step = 10 m; h1 = 1.2 m; h2= 17 m; h3= 26.6 m. 5. Measurement chain The instrumentation used for the survey was composed by three IEC Class 1 compliant sound level meters (Brüel & Kjær Type 2250), equipped with ½-inch free-field microphones (Brüel & Kjær Type 4189, compliant with the same standard) and calibrated with IEC Class 1 compliant calibrators (Brüel & Kjær Type 4231). Since the measurements were taken in outdoor spaces, a windscreen was used and the appropriate correction was taken into account. In the following, the results of the measurements are reported in Figures 4 (A-weighted spectra in various longitudinal locations: the various curves are associated to different vertical position) and Figure 5 (levels in db(a) in the same positions). Looking at Figure 5 it is noted that levels increase moving towards the bow and towards upper recording positions. Such a trend is due to the strong noise coming from another unit moored behind the target vessel on the other side of the basin (see Figure 2). As a matter of fact, for positions 4 to 8 (see Figure 4 and 5) the noise coming from the other vessel is partially masked by the presence of the target vessel superstructure, while for sections 1 to 3 and fore of number 8 the two higher measurement positions are directly in view of the second ship. In the same way the levels increases moving forward as the distance from this other vessel decreases, as it can be seen from Figure 5. ICSV22, Florence, Italy, July

5 Pos. 2 Pos. 3 Pos. 4 Pos. 5 Pos. 6 Pos. 7 h Pos. 8 Pos. 9 Figure 4. A-weighted spectra for the different positions. ICSV22, Florence, Italy, July

6 2 db(a) Longitudinal position, step 10m Figure 5. Spatial distribution of the measurements points. This interpretation is confirmed by Figure 6, where, for selected longitudinal positions, additional measurements are reported, carried out on board the target vessel, from the main deck, but on the starboard side (opposite the quay and facing the other vessel) and pointing the sound level metres on this secondary source. From the analysis of Figure 6 it is possible to conclude that the measurements carried out are dominated by the other vessel, which determines the shape of the spectrum and also the peak at low frequency. A smaller peak at high frequency is, on the contrary, present only on port side measurements (and only for stations 1-4 and for the lower elevation) and is therefore not related to the secondary source. It is here noted that the additional measurements reported in Figure 6 cannot be defined as background noise in a specific sense, as they are not carried out in the same locations as the main measurements and in absence of the target source. They cannot be used to correct the measurements as usually done. The survey of proper background measurements was actually not possible in that situation, as it would have implied a complete shut down of the target vessel, which was not realisable for safety reasons. The same applied to the second vessel, which was not possible to shut down, either. 5.1 Noise from secondary sources As above-mentioned additional measurements have been carried out on the starboard side of the yacht (at sections 2, 5, 11) in order to better evaluate the influence on the measurements of the noise emitted by the other vessel. By superimposing the spectra measured at the survey position to the spectra measured on the starboard side of the yacht (where the contribution to the noise of the other vessel is predominant) it can be noted that, mainly for the higher positions, the spectra almost coincide. Such surveys highlighted how much the measurements, especially of the higher positions, were affected by the noise emitted by the other vessel. ICSV22, Florence, Italy, July

7 Pos. 2 h17000 On board measure stb side Pos. 5 h17000 On board measure stb side Pos. 11 h17000 On board measure stb side Figure 6. Comparison of noise levels measured on board the yacht with the measurements carried out on the crane. 6. Comments and Conclusions The characterization of noise emitted by ocean-going ships at quay is not defined by a standard, nor carried out in the practice. On the other hand, a proper characterisation is needed to assess the annoyance to the population exposed to port noise. The paper reports a first trial application of a newly proposed experimental procedure for the characterization of the airborne emissions from a vessel at wharf. Results are reported and interpreted, but the main target of the work was to prove the feasibility of the proposed procedure. The experience gained permitted to test the method proposed within the SILENV project, highlighting critical issues and possible solutions. A main remark is that results are heavily affected by the presence of a strong secondary source (another vessel, moored on the other side of the basin at about 100 m). The surveyed levels are dominated by such secondary source so, from a practical viewpoint, these levels can be considered ICSV22, Florence, Italy, July

8 as an upper envelope of the actual emission of the target vessel, that is certainly lower of what is measured. Concerning the operating procedure, the use of the crane for moving the microphones on the grid of survey points allowed an easy and precise positioning of the sensors, creating however the need for turning off at all stations the crane power supply to prevent noise interference. This suggests to use battery operated equipment to avoid repeated shut downs. Possible effects due to sound reflections on crane s surfaces, where not assessed. Nevertheless, the duration and the complexity of the test procedure is adequate for a shipyard environment and the equipment required for the measurements is not too costly or too complex to be used in practice by a small number of operators. The amount of data is manageable and postprocessing and interpretation is feasible in a reasonable time. As regards the disturbance by noise from secondary sources, an accurate selection of the measurement site proved to be crucial. The possibility of removing external sources or at least of controlling external and target sources for a proper evaluation of background noise should be sought. In alternative, intensity measurements could be considered. REFERENCES 1 Badino, A., Borelli, D., Gaggero, T,. Rizzuto E. and Schenone C. Acoustical impact of the ship source, Proceedings of the 21 st International Congress on Sound and Vibration, Beijing/China, July (2014). 2 Badino, A., Borelli, D., Gaggero, T., Rizzuto, E. & Schenone, C. Analysis of airborne noise emitted from ships. In Rizzuto, E. & Guedes Soares, C. (eds.), Sustainable Maritime Transportation and Exploitation of Sea Resources, CRC Press/Balkema, Leiden, The Netherlands, (2012). 3 Badino, A., Borelli, D., Gaggero, T., Rizzuto, E. and Schenone, C. Normative framework for ship noise: Present situation and future trends, Noise Control Engineering Journal, 60 (6), , (2012). 4 IMO Resolution MSC.337(91), Adoption of the Code on Noise Levels on Board Ships, International Maritime Organization, London, United Kingdom Di Bella, A. Evaluation Methods of External Airborne Noise Emissions of Moored Cruise Ships: An Overview, Proceedings of the 21 st International Congress on Sound and Vibration, Beijing/China, July (2014). 6 ISO Standard 2922:2000/Amd.1:2013, Acoustics - Measurement of airborne sound emitted by vessels on inland waterways and harbours (2013). 7 ISO Standard :2008, Acoustics - Small craft Airborne sound emitted by powered recreational craft Part 1: Pass-by measurement procedures (2008). 8 Draganchev H., Valchev S., Pirovsky, C., Georgiev, M. and Mihaylov, B. Experimental and Theoretical Research of Noise Emitted by Merchant Ships in Port, Proceedings of the 19 th International Congress on Sound and Vibration Vilnius, LT, July (2012). 9 Badino, A., Borelli, D., Gaggero, T., Rizzuto, E. and Schenone, C., Modelling the Outdoor Noise Propagation for Different Ship Types, Proceedings of the 17 th International Conference on Ships and Shipping Research NAV2012, Naples, Italy, October, (2012). 10 SILENV-Deliverable 5.2 Green Label proposal Badino, A., Borelli, D., Gaggero, T., Rizzuto E. and Schenone C. Control of airborne noise emissions from ships, Proceedings of the International Conference on Advances and Challenges in Marine Noise and Vibration MARNAV2012, Glasgow, UK, (2012). ICSV22, Florence, Italy, July