Airborne Noise Flanking of Shipboard Vibration Isolation Systems

Transcription

1 Airborne Noise Flanking of Shipboard Vibration Isolation Systems Ray Fisher, Leo Boroditsky, Ron Dempsey, Nathan Jones and Mike Bahtiarian Noise Control Engineering, In., Billeria, Massahusetts Theoretial and antidotal results on quiet researh vessels indiate that airborne noise flanking of mahinery isolation systems adversely affets their underwater radiated signature. This is espeially true for diesel-generator drive systems installed on two-stage isolation mounts. Airborne noise from the diesel impinging on the nearby hull and tank struture may beome a signifiant fator of radiating underwater energy. This path and potential methods of minimizing its impat are disussed with respet to two-stage isolation systems. Airborne noise flanking is disussed for systems with open- and losed-bottom intermediate rafts and for a totally enlosed genset. Researh vessels built to stringent underwater-radiated noise standards usually have diesel-eletri propulsion drive systems. These researh vessels an be deployed for fish stok assessment or for bottom mapping using sophistiated sonar systems. The eletri motor is a low noise and vibration soure that drives a speially designed, large, and slow-turning propeller. The power omes from two-stage isolation mounted diesel-generators or gensets. This ombination of mahinery an be treated along with the use of hull insulation and damping to suh an extent that the aousti energy reahing the wetted hull is minimized. This artile overs how airborne noise flanking affets the vibration effetiveness of two-stage-isolation gensets on three vessels NATO s R/V Alliane 1, NOAA s FRV Bigelow 2, and the University of Delaware s R/V Sharp. 3 The 93-m Alliane was built to underwater noise standards developed by NATO for an aousti researh vessel. The 64-m Bigelow (Figure 1) and 44-m Sharp (Figure 2) were built to a radiated noise standard developed by the International Counil for the Exploration of the Seas 4 (ICES) for vessels used for stok assessment of fisheries. In this ase, the vessel s radiated noise is low enough so as to minimize fish reation to provide aurate ounts. The genset parameters for eah vessel are provided in Table 1. A simple drawing of the genset mounting system used on the Bigelow is shown in Figure 3 and the atual set-up for testing is shown in Figure 4. For this ase, eah of the four intermediate masses was speified to be 1% of the mass of the diesel/ generator system. The vessel has been designed, built and delivered 5 to meet the stringent radiated underwater noise standards set by the International Counil for the Exploration of the Seas (ICES). Airborne Flanking Theory Figure 5 shows the various paths for aousti energy from an isolation-mounted diesel to get into a ship s struture. (In this figure, the diesel is shown to be single-stage isolation mounted, not two-stage isolation mounted, as is the subjet of this artile.) The most obvious path is the first struture-borne Figure 1. NOAA Fisheries Researh Vessel Bigelow. Figure 2. University of Delaware Researh Vessel Hugh R. Sharp. path or strutural vibration path through the attahment point of the diesel to the ship s foundation. Energy transmitted through this path is redued by the use of single- or doublestage isolation, depending on the aousti soure level of the diesel and the underwater radiated goals to be ahieved. The other path, whih beomes more ritial as the mounting system beomes more effetive, is the seond strutureborne path. This path overs vibration transmitted to the ship s struture by inident airborne noise. This vibration gets ombined with vibrations from the first strutureborne path and an be transmitted throughout the vessel or radiated into the water. As with exitation of the hull by inident airborne noise, the tank top (struture in the immediate viinity of the diesel engine) an be exited by noise from the engine. With any twostage mount system, there is a potential for airborne noise flanking that bypasses the mounts and diretly exites the foundation and ship s struture. 6-8 Referene 7 shows that the ratio of power transmitted to the base via the airborne path P a to that transmitted through the mounting system P m is om- Table 1. Genset design parameters. Vessel Genset Power, kw rpm Genset Wt, t Raft Wt, t Vert Nat Freq, Hz Alliane GMT B Bigelow Cat Cat Sharp (2) Cummins KTA SOUND AND VIBRATION/DECEMBER 26 19

2 -2-3 Measured Bigelow w/damping Predited Bigelow w/damping Figure 3. CAT 3512 two-stage design, Bigelow. 1 Figure 6. Predited vs. measured tank-top response to inident sound. Figure 4. Fatory test of genset installation. the ss approah or equal 1, the airborne path is dominant 2 if k < 7r S/ A. By substituting the relationship between the mount system natural frequeny f and mass m in the above equation, the airborne path should ontrol when (f ) 4 < (S/m 2 ). For the area-to-weight squared ratios of many typial mid-sized marine diesels, this inequality holds true for systems with a natural frequeny on the order of 4 Hz. This is on the low side of most spring and elastomeri mounts used on today s vessels. As a further hek on the potential flanking aused by airborne noise, predited and measured vibration levels of the tank top struture on the Bigelow were ompared. This test was onduted with all equipment exept the ooling system and the gensets seured. The predited vibration response n of a framed plate to an inident airborne pressure p was predited aording to Referene 1: Ê È 1 1 ( 1 + f / f ) ˆ n / p = f Í f mp f Ë Á ph ÎÍ lx ly ( ff ) Ê p ˆ n / p = 21. f f mp f Ë Á 4h ª Figure 5. Aousti transmission paths. puted as: 7 P / P p r S / k A s s a m = ( ) where: r, = sound speed and density in air S = radiating area of mounted equipment and subbase k = spring stiffness A = absorption area in spae between engine and foundation s 1, s 2 = radiation effiienies of engine and foundation, respetively In the lower frequenies, the energy transmitted by the airborne path P a will be less important than that by the mounting system P m sine s 1 s 2 << 1. At higher frequenies, where n 2 2 / p 1 Ê p f = mp f Ë Á 4h f 1 f / f - ( ) ˆ f f where: m p = surfae mass of the plate f = frequeny of interest h = damping loss fator l x,l y = panel length and width f = ritial frequeny of plate Using the measured noise levels beneath the Bigelow genset, the measured and predited vibration levels on the tank top plating are ompared in Figure 6. At and above 1 Hz, measured and predited levels show good agreement. Sine the tank top plating is overed with damping tile, the loss fator used for the predition was an h of.1. The dip in the tank-top vibration between Hz and 125 Hz is urrently unexplained. So given the math in the levels shown in Figure 6, the tank-top vibration in this ase is due primarily to the inident airborne noise and not vibration levels transmitted through the double-stage mounting system. To redue aousti energy entering the struture, the airborne flanking path would need additional treatment. Flanking Investigation To further investigate and potentially redue the influene of airborne flanking of the resilient mounting system on the Bigelow, the authors installed a sreen or barrier between the underside of one diesel generator and the ship s tank top. A ¾-in.-thik plywood panel was temporarily installed under the port CAT 38 diesel generator. The plywood panel was laid on the intermediate masses approximately.3 m (12 in.) from 2 SOUND AND VIBRATION/DECEMBER 26

3 Figure 1. Transverse floor vibration. Figure 7. Plywood sreen under genset Figure 8. Vertial vibration levels on upper stage of mounts Figure 9. Tank-top vibration levels, with and without barrier. the tank top. Figure 7 shows the plywood resting on a foamsupported framework. The effetiveness of this treatment was determined by measuring both the sound and vibration level redutions on the strutures around the diesel generators. Vibration levels were measured above and below mounts, on the tank top, on the transverse floor under the genset, and on the hull side near the diesel. Noise levels were measured in the spae between the plywood panel and tank top plate. The orrelation between noise redution and vibration redution is important for this test. Noise levels above mounts were stable during whole test. Figure 8 shows vibration levels above mounts with and without the barrier installed. Figures 9 to 13 show the vibration levels around the genset and noise levels in the spae between the barrier and tank top. These graphs show measurable vibration and noise redutions for frequenies above 2 Hz. Underwater noise redutions are expeted to be the same order of magnitude as that shown for vibration Figure 11. Side shell vibration Figure 12. Vibration below mounts. Figure 13. Noise level below genset, with and without barrier Theory and measurements on the Bigelow showed onsiderable flanking existed on an open-bottom, two stage, rafted genset. On the researh vessel Sharp, the authors designed the raft to have a losed bottom to minimize this flanking path. 1 Not only was the raft bottom losed, but a flexible seal was built SOUND AND VIBRATION/DECEMBER 26 21

4 Figure 14. Noise level near gensets, Sharp vs. Bigelow. Sharp gensets TL, db Bigelow Sharp Otave Band Frequeny, Hz Figure 16. Isolation effetiveness, Sharp vs. Bigelow. Below raft Sharp Figure 15. Noise level below genset on Sharp and Bigelow. around the perimeter of the raft at its onnetion to the tank top. Figure 14 shows the sound pressure levels near the diesels for both the Bigelow and Sharp. These data show the sound levels near the gensets are omparable. However, the raft with the losed bottom signifiantly redues the sound impinging on the hull below. Figure 15 shows the noise levels inident on the tank top on the Bigelow and that omparable below the raft on the Sharp. The inident noise levels on ship struture are 5 to 25 db lower on the Sharp due to the losure of the bottom of the raft. This also leads to higher isolation performane of the two-stage system (Figure 16). In the region from 2 to 2 Hz, the noise below the raft is muh lower, and the transmission loss of the Sharp system is improved over that of the Bigelow. The transmission loss measurements were taken dokside with other equipment seured. One final omparison an be made. The gensets on the R/V Alliane are totally enlosed. This should effetively eliminate any airborne flanking from these units, assuming the enlosure itself does not beome a flanking path due to its large area and aeptane. Here again, aeptane is the vibration response of the enlosure panels due to the inident airborne noise. The measured noise outside the Alliane enlosure is provided in Figure 17 (data sanned from the figure in Referene 11). These noise levels are signifiantly lower than those measured below the raft on the Sharp. Furthermore, the enlosure is isolation mounted to the rails of the raft; so airborne flanking into the raft from the enlosure should not be a fator, at least at mid to high frequenies. The Alliane mount transmission-loss values are ompared to those of the Bigelow and Sharp in Figure 18. With little or no airborne flanking, this system provides the highest performane of all three. Conlusions Airborne flanking an redue the effetiveness of two-stage, isolation-mounted equipment, partiularly those with dieselemitting, high-airborne noise levels installed on effetive mounts. So any analysis of the effetiveness of a double-stage, TL, db Below raft Sharp Outside enlosure Alliane Otave Band Center Frequeny, Hz Figure 17. Noise outside enlosure, R/V Alliane. Bigelow Sharp Alliane Otave Band Frequeny, Hz Figure 18. Mount transmission loss, three vessels. isolation-mounted system should onsider both the transmission through the mounts and through any potential airborne flanking path. If there is no airborne flanking, the effetiveness should be ontrolled by the stiffness of the mounting system and the ratio of the weight of the raft to that of the equipment. Total enlosures or losing off the bottom portion of the raft or subbase will redue the sound inident on the ship s struture and limit the adverse impat of diret airborne flanking. Full genset enlosures will effetively eliminate airborne flanking. However, full enlosures entail severe weight, spae, and maintenane impats. Limited tests show that even a sreen or barrier beneath the genset an provide an effetive treatment to redue airborne flanking paths and improve vibration isolation for frequenies above 2 Hz. Novel tehniques and treatments of the rafts might further enhane the low-frequeny performane of the system. These inlude radiation damping, 12 use of a very thik absorptive insulation on the underside of the raft, or use of ut-onstrained layer damping to improve low-frequeny performane. 22 SOUND AND VIBRATION/DECEMBER 26

5 Aknowledgements The tests on the Bigelow were arried out as part of a joint NOAA and VT Halter Marine In. program to investigate ways to improve the aousti signature of the vessel. The authors appreiate the support and ooperation of the NOAA rew and VTH engineers and shipyard test and trials personnel in arrying out these tests; inluding Eri Rihards and David Terry at VTH and Stephen Madden of NOAA. The University of Delaware supported testing on the Hugh R. Sharp, and the authors appreiate the support and ooperation of the rew and the university; partiularly Matt Hawkins. Referenes 1. NATO Researh Vessel Alliane, NATO SACLANT undersea Researh Center, La Spezia, Italy, 2. National Oeani and Atmospheri Administration ship Henry Bigelow, 3. University of Delaware s R/V Hugh R. Sharp, newsenter/shiphristening.html. 4. Mitson, R., (ed.), ICES Cooperative Researh Report No. 29, Underwater Noise of Researh Vessels Review and Reommendations, International Counil for the Exploration of the Sea, Copenhagen, Denmark, Fisher, R., Aousti Design, Constrution and Testing of NOAA s Fisheries Researh Vessel, Lloyds 1 st Ship Noise and Vibration Conferene, London, June Petersson, P., and Plunt, J., Strutureborne Sound Transmission from Mahinery to Foundations, Chalmers University of Tehnology, Goteborg, Sweden, Hekl, M., Different Ways of Sound Transmission In and Around Resilient Mounts, International Symposium on Shipboard Aoustis, Elsevier Si. Publishing, NY, Fisher, R., and Boroditsky, L., Control of Diesel Indued Shipboard Noise by the Use of Isolation Mounts, inter-noise 21, The Hague, Aug Dittrih, M., and Van Vliet, W., Calulation Model on Aoustial Properties of Plate-Like Strutures Inluding Damping Layers, TNO-TH Report 822.4, Feb, Bahtiarian, M., The Aousti Design of the Researh Vessel Sharp, Aoustial Soiety of ameria, Providene, RI, June Manuso, G., and Sahi, F., Main Propulsion Diesel Generator Sets with Aousti Enlosure and Double Resilient Mounting for Low Noise Appliation, Shipboard Aoustis, Proeedings ISSA 86 (J. Buiten, ed.), Martinus Nijhoff Publishers, Frike, J. R., Lodengraf Damping, An Advaned Vibration Damping Tehnology, Sound and Vibration, July 2. The authors may be ontated at: rayf@noise-ontrol.om. SOUND AND VIBRATION/DECEMBER 26 23