KEY FACTORS OF A SHIP INTERNAL NOISE PREDICTION PROCEDURE

Transcription

1 23 rd International Congress on Sound & Vibration Athens, Greece July 2016 ICSV23 KEY FACTORS OF A SHIP INTERNAL NOISE PREDICTION PROCEDURE Simone Curletto and Cesare Tarditi Fincantieri S.p.A. Naval Vessel Business Unit, via Cipro 11, I Genoa, Italy simone.curletto@fincantieri.it, cesare.tarditi@fincantieri.it Although the underwater radiated noise reduction in sailing conditions represents the main target of the naval vessel acoustic design, for new military ships also the noise internal levels have to be maintained as low as possible in order to comply with comfort requirements, which have become more and more restrictive in the last few years. Even if the actions aimed at reducing the acoustic signature have an undeniable beneficial effect on internal comfort, additional attention has to be specifically paid to internal noise, while trying to avoid affecting typical military ship characteristics and performances. Present paper deals with an internally developed procedure aimed at predicting noise levels in the ship internal spaces, starting from the noise and vibration levels of the main sources installed on board. Even though this method is based on a theoretical approach, many parameters and coefficients - derived from a statistical analysis of past experimental measurements - have been introduced in order to improve the results reliability. Considering the ship structural configuration and the noise and vibrations propagation paths, the developed tool is able to predict the noise levels in the internal spaces as well as to suggest the most suitable acoustic insulation treatments on decks, ceilings and bulkheads needed to comply with the requirements. The possibility of separately simulating the acoustic behaviour and contribution of decks, ceilings and bulkheads, allows selecting only the acoustic treatments which are indeed necessary, ensuring, at the same time, a considerable saving in terms of weights and costs. The applied methodology will be duly explained focusing especially on the engineering advantages, which represent the top goal of the development and continuous improvement of the procedure. 1. Introduction In general, on naval surface vessels, three types of noise have to be considered: airborne noise in shipboard living and working spaces, which can produce discomfort and hearing damage to embarked personnel; ship underwater radiated noise, which can increase the probability to be detected; ship platform noise, which can reduce the detection effectiveness of sonar systems installed onboard. Present paper deals with an internally developed procedure aimed at predicting noise levels in the ship internal spaces. In order to obtain an estimation accuracy as adequate as possible, the selection of the basic key factors plays a primary role in the prediction of the noise levels in the ship spaces (first point of previous list). The procedure developed for estimating noise levels (and consequently noise reduction actions) is based on acoustical theoretical fundaments and also on experimental data collected during onboard past measurements. Practical experience, measured data and acoustical theory are combined to develop the ship silencing plan, and to define noise source levels and noise transmission losses as well as to predict the effectiveness of noise reduction treatments. When they are available, measured data are used to estimate noise source levels and transmission losses. Conversely, acoustical theory is used when measured data are unavailable. 1

2 2. Noise prediction key factors The identification of the parameters (key factors) to be modified in order to obtain the most reliable noise prediction represents the focal point of the procedure here described. It is quite clear that the better the choice of the key factors is the better the noise prediction procedure results can be, in terms of both accuracy and computational time. Following paragraphs describe the methodology used to identify the noise prediction key factors. 2.1 Noise critical spaces Basing on the vessel typology and the onboard comfort level required by the owner the most critical ship spaces (from noise point of view) are identified. Such a selection has to be performed mainly taking into account following aspects: Space Location. Considering vessel general arrangement and the position of the onboard main noise sources (typically installed in Engine and Auxiliary Rooms), ship s less comfortable spaces (conservative approach), with respect to the expected internal noise level, are selected as objective of the prediction procedure. Space Typology. Considering the noise requirements indicated in the Building Technical Specification of the vessel, different ship space typologies are characterized by different maximum allowable internal noise levels. In order to obtain a more complete prediction, typically one space for each typology is selected. Both space location and typology significantly affect the choice of the acoustic treatments to be applied to achieve the noise levels required by the owner and defined in mentioned specification. Figure 1: Noise spaces classification. Figure 1 shows a typical example of space classification. Considering the high complexity level of naval vessel configuration, the operational use of specific areas and the weight constraints, it is quite common to find working and living spaces close to zones in which the main noise sources are installed (noisy spaces). Basing on this last definition, the ship spaces can be classified as follows: Critical spaces. Ship spaces characterized by requirements of low internal noise levels (cabin, offices, etc.) and located close to the onboard noisy spaces. 2 ICSV23, Athens (Greece), July 2016

3 Partially critical spaces. Ship spaces characterized by less demanding noise requirements or ship spaces characterized by low internal noise levels and sufficiently far from the onboard noisy spaces. Not critical spaces. Ship spaces located so far from onboard noisy spaces to be hardly affected by noise sources (required noise levels easily achieved). 2.2 Noise critical sources Taking due account of all possible noise sources installed onboard, the estimation of the internal noise levels is performed on the basis of following noise sources selection criteria: Structure borne noise levels. As indicated in Figure 2, basing on the structure borne noise (SBN) levels and the mass, each source could be considered critical, partially critical or not critical from noise point of view. All the noise sources characterized by elevated SBN levels and heavy mass (such as propulsion diesel engines, diesel generators, reduction gearboxes and, even if conceptually different, propellers) are considered critical. These sources are used as primary input of the prediction procedure. Not critical sources are usually excluded by the estimation procedure because they represent equipment characterized by both low SBN levels and light mass (e.g. small pumps, small fans, etc.) and thus negligible if compared, for example, to a propulsion engine from vibration energy point of view and the consequent contribution to the airborne noise. Partially critical sources are equipments represented by a combination of SBN levels and mass which, without any noise control action (single or double elastic mounting, foundations stiffening, equipment balancing, etc.), could become critical and thus should have to be considered as input of the prediction procedure. This last classification includes equipment such as pumps, air compressors, gas turbines, etc.. mass POTENTIALLY CRITICAL CRITICAL SBN Levels NOT CRITICAL POTENTIALLY CRITICAL Figure 2: Noise sources classification. Airborne noise levels. Noise sources characterized by elevated airborne noise (ABN) levels are selected as input of the prediction procedure. Typical equipments included in this category are, for instance, propulsion diesel engines, diesel generators, reduction gearboxes and gas turbines. Usually all other sources can be considered negligible because their noise levels are well below the noise level of the main sources. In Figure 3 an ABN levels comparison between typical main and secondary sources is represented. Considering a typical engine ICSV23, Athens (Greece), July

4 room arrangement where, as an example, a propulsion diesel engine and several pumps are installed in a limited space, it is evident that the contribution to the global ABN level is mainly due to the engine (blue line). The ABN levels generated by one, two or ten pumps (red, green and violet line respectively) are indeed lower than the engine ones. Distributed sources. Although these sources are characterized by moderate SBN and ABN levels, they are able to propagate the noise all over the ship. Typical example of distributed source is the HVAC. In order to lower the impact of these sources on the noise level in the ship spaces, it is necessary to impose suitable noise requirements to the suppliers during the purchasing phase. Electronic sources position. Although they are generally characterized by low noise levels, electronic components (mainly related to the combat system) are often installed inside ship spaces with stringent internal noise requirements. In order to match the maximum allowable noise level in those spaces, it is again necessary to impose suitable noise requirements to the suppliers during the purchasing phase. ABN Level [Sound Pressure Level ref. 20e-6 Pa] ABN levels comparison f [Hz] PE pump pumpx2 pumpx10 3. Noise prediction procedure Figure 3: Example of sources ABN levels comparison. The developed prediction procedure is based on the theoretical fundamentals of the noise and vibration propagation through the ship structures. Once the ship spaces, where noise estimation procedure has to be applied, and the onboard noise sources have been identified, the SBN/ABN propagation paths can be defined (see Figure 4) in order to obtain the noise expected level. In general, the ABN level in the ship spaces can be predicted by adding the two following components: ABN component from SBN. Vibrations generated by the considered noise sources propagate through the ship structures (such as foundations, pillars, etc.) and reach the ship space under investigation. Taking into account the attenuation due to the resilient mounts between source and foundations and those due to propagation and intersections, the residual SBN levels induce vibrations on deck, bulkheads and ceiling of the receiving space. Considering the radiation efficiency of the space boundary structures and the space absorption value (related to 4 ICSV23, Athens (Greece), July 2016

5 the insulation treatments), the ABN component due to the source of SBN levels is estimated (blue arrows in the figure). ABN component from ABN. Residual ABN component inside the ship space under investigation (red arrow in the figure) can be obtained considering the ABN generated by the sources lowered by both the attenuations due to the ship structures between the space where the source is located and the ship space under investigation and the attenuations produced by the insulation treatments. As shown in Figure 4, developed prediction procedure is able to simulate the acoustic behaviour of deck, ceiling and bulkheads separately and allow therefore to evaluate the suitable acoustic insulation (damping, floating floor, panels or mineral wool with different densities) to be installed separately on the different surfaces in order to comply with ship internal noise requirements. This can be beneficial since the procedure can reliably define the effectively needed acoustic treatments, providing cost and weight savings. Figure 4: SBN and ABN propagation paths. In order to improve the results reliability, although the prediction procedure is based on a theoretical approach, it is possible to set many parameters and coefficients deriving them from a statistical analysis of past experimental measurements. It can be noted that a simplified estimation of such parameters, based on a pure theoretical approach, could produce significant underestimation or overestimation errors. As an example, resilient mounts dynamic stiffness behaviour and insulation treatments noise attenuation efficiency should be properly modelled to avoid important underestimations. Figure 5 describes a typical evolution of the dynamic stiffness as a function of frequency. Often, the mounts datasheets describe only the static stiffness values and the dynamic behaviour is missing. When unavailable, the diagram of stiffness versus frequency can be derived starting from a wide database of measurements performed on different equipment and mounts typology. As regards acoustic insulation performances, the degradation of noise attenuation value due to the possible unavoidable reductions of the treatments integrity typical especially on military ships ICSV23, Athens (Greece), July

6 (such as holes for pipes or ducts penetrations, thickness decrease for space constraints, etc.) is another very important parameter to be considered. Figure 6 represents the real attenuation values versus the potential ones as a function of the percentage of interruptions in the acoustic insulation treatment. Such a behaviour should be carefully considered for any airborne noise acoustic barriers to be applied on deck, ceiling or bulkheads of the considered ship space. TYPICAL RESILIENT MOUNT DYNAMIC STIFNESS (axial) Dynamic stiffness [db] f[hz] Figure 5: Resilient mounts dynamic stiffness typical behaviour. NOISE ATTENUATION EFFICIENCY Real Attenuation [db] ,00% 0,01% 0,05% 0,10% 0,20% 0,50% 1,00% 2,00% 5,00% Potential Attenuation [db] Figure 6: Noise attenuation efficiency performance in relation to the percentage of holes and/or interruptions of an acoustic insulation treatment. 6 ICSV23, Athens (Greece), July 2016

7 4. Conclusions In present paper, an internally developed procedure able to predict noise levels in the ship internal spaces has been explained. Starting from the ship structural configuration and the noise and vibrations propagation paths, the noise levels in the internal spaces can be estimated and the acoustic insulation treatments on decks, ceilings and bulkheads needed to comply with the requirements can be defined. Although the prediction is based on a theoretical approach, many parameters are (or can be) derived from a statistical analysis of past experimental measurements database. Taking into account naval vessel peculiarities (such as limited available areas, need of weight containment, noise critical spaces close to noisy spaces, systems and plants redundancy, etc.), noise prediction key factors have been selected in order to optimize the methodology estimation accuracy. The possibility of separately simulating the acoustic behaviour and contribution of decks, ceilings and bulkheads for each ship space, and consequently optimizing the selection of the actually necessary acoustic treatments, provides an undeniable engineering benefit in terms of weight, space and cost reduction. REFERENCES 1 Beranek, L. Ed., Noise and Vibration Control, McGraw-Hill Book Company, New York, NY (1971). 2 Harris, C.M. Ed., Handbook of Noise Control, 2nd Ed., McGraw-Hill Book Company, New York, NY (1979). 3 Cremer, L., Heckl, M., Structure-Borne Sound, 2nd Ed., Springer-Verlag, Berlin, Germany (1973). 4 Listewnik, K., Some aspects of noise measurement of ships, Proceedings of the 20 th International Congress on Sound and Vibration, Bangkok, Thailand, 7-11 July, (2013). 5 Badino, A., Borelli, D., Gaggero, T., Rizzuto, E., Schenone, C., Acoustical impact of the ship source, Proceedings of the 21 st International Congress on Sound and Vibration, Beijing, China, July, (2014). 6 Piana, E.A., Marchesini, A., How to lower the noise level in the owner's cabin of a yacht through the improvement of bulkhead and floor, Proceedings of the 21 st International Congress on Sound and Vibration, Beijing, China, July, (2014). 7 Borelli, D., Gaggero, T., Rizzuto, E., Schenone, C., Analysis of noise on board a ship during navigation and manoeuvres, Ocean Engineering, 105, , (2015). 8 Esparcieux, P., Silhouette, L., Noise and vibrationnal source identification techniques and their application to warship acoustic diagnostic, Proceedings of the 18 th International Congress on Sound and Vibration, Rio de Janeiro, Brazil, July, (2011) 9 Borelli, D., Schenone, C., Di Paolo, M., Application of a simplified "source-path-receiver" model for HVAC noise to the pre-liminary design of a ship: A case study, Proceedings of the 21 st International Congress on Sound and Vibration, Beijing, China, July, (2014). 10 Boroditsky, L., Fischer, R., Ship hull structure response on airborne noise excitation, Proceedings of the 19 th International Congress on Sound and Vibration, Vilnius, Lithuania, 8-12 July, (2012). 11 Biot, M., Moro, L., Mendoza Vassallo, P.N., Prediction of the structure-borne noise due to marine diesel engines on board cruise ships, Proceedings of the 21 st International Congress on Sound and Vibration, Beijing, China, July, (2014). ICSV23, Athens (Greece), July