Noise prediction of large ship 6700PCTC using EFEA-SEA hybrid technique

Transcription

1 Noise prediction of large ship 6700PCTC using EFEA-SEA hybrid technique Xinwei ZHANG 1 ; Shawn WANG 2 ; Jinxiang PANG 3 1 SDARI, China 2 Microcomputing LLC, USA 3 PROSYNX Technology Inc, China ABSTRACT In ship building industry, it has always been a big challenge to perform the global vessel noise prediction. Currently statistical energy analysis (SEA) is mostly used for noise prediction in industry, while some others have used energy finite element analysis (EFEA) for structure-borne noise calculation. It s desirable for engineers to apply the EFEA to calculate the structure-borne noise and SEA to analyze the airborne noise simultaneously. Both EFEA and SEA can be coupled in the software ViNAS. Based on such vibroacoustics computation software, a study of global noise prediction of large ship 6700PCTC (6700 pure car/truck carrier) was performed. The computation directly utilized the FEM model which was served for strength and vibration analysis. The properties of noise control treatment were attached to the plates surrounding treated cavities. The vibroacoustic responses of all cavities and plate elements were predicted. Verification data was measured on the ship and compared to the predictions. Unlike SEA, the EFEA doesn t need to make significant changes to FEM model to rebuild its own model. Therefore, this paper demonstrates that the EFEA-SEA hybrid technique has great engineering benefits to global vessel noise analysis. Key words: EFEA-SEA, ship noise, ViNAS I-INCE Classification of Subjects Number(s): INTRODUCTION In shipbuilding industry, usually the countermeasures to noise problems are proposed and executed after the delivery for most of the vessels. In the delivery stage, the cost and feasibility of countermeasures might not be satisfied by the yard and the owner. If the proper noise prediction could be carried out in the early design stage, the cost would be minimized and the improvement would be optimized. Thus, it s very important to conduct the vibration and noise prediction in the design stage. Ship is like a movable castle sailing in the ocean, normally with a set of giant power plant and massive steel structure. These characteristics determine that the vessels would be deeply influenced by both airborne and structure-borne noise. The excessive noise affects the work condition of seamen since they could accommodate in a vessel for days even weeks. Thus the noise level in the vessel must be controlled under certain level. After July 1st, 2014, the new IMO standard MSC.337 (91) (1) has been adopted, the noise level limit in accommodation area decreases 5 db comparing to old standard A468(XII) (2). It s paramount important to control the noise level on board after execution of new standard because the vessel that fails the noise test would be forbidden to be delivered to the ship owner even though the owner wants to accept the vessel. The 6700PCTC is China s first self-designed large PCTC (pure car/truck carrier) which was registered in Det Norske Veritas (DNV). For vessels such as bulk carrier or container vessel, the noise sources are normally located at stern (propeller) and engine room (main engine); the energy could be directly transmitted into main verified area, i.e., deckhouse. The cargo hold area only provides boundary conditions for the verified area, and doesn t belong to the verified area itself; so in many literatures only the FEM model of the aft vessel part was used to calculate noise. In this case, the acoustic power has to travel through the cargo hold 1 zxw010@hotmail.com 2 Microcomputing.team@yahoo.com 3 james.pang@prosynx.com 5520

2 area, and transmit to uppermost living area. Besides the upper accommodation area, there are also 45 fans that blow air to the cargo hold area through tunnels. Such characteristics determine that it might be necessary to use the global finite element (FE) model to predict the noise of 6700PCTC, thus the computation effort might be tripled comparing to other normal type of vessels. This paper firstly describes the theory of SEA and EFEA, and then applies SEA and EFEA hybrid technique to calculate the noise of 6700PCTC; finally shows the plots of predicted results for acoustic cavities and plate elements. 2. BASIC THEORY OF SEA AND EFEA 2.1 Statistical Energy Analysis (SEA) The first researchers of SEA were Lyon & Maidanik (3) in1960's - they found that the modal energy is independent to one another within a subsystem, and the total energy is the sum of each independent modal energy. The SEA developed so fast in shipbuilding industry in 1980 s (4-6). Nowadays SEA has been the most frequently used method in industries including shipbuilding industry. The SEA decomposes the structure as a series of linear coupled subsystems. The assumption has been made that power flow among subsystems is induced by structural resonances or acoustic modes. For a single subsystem, the power input to this subsystem equals the power output from the subsystem. The input power can be classified as excitation power and power coupled from other subsystems; the output power can be classified as dissipated power and power coupled to other subsystems. For a set of N subsystems, the equation of power equilibrium is (1) where is the input power of subsystem i, ω means the angular frequency, η i is the internal loss factor of subsystem i, η ij is the coupling loss factor from subsystem i to j, is the mean energy value for subsystem i, and n i is the modal density of subsystem i. 2.2 Energy Finite Element Analysis (EFEA) Nefske and Sung (7) firstly built the control function of power flow using FEM in 1980 s. In 1990 s Wohlever and Bernhard deducted the control function of energy flow in the same manner as the traditional mechanics (8). Bouthier and Bernhard (9, 10) further fulfilled such theory, so the complex geometry and damping characters in the real engineering project could be considered. In 1999, Germanischer Lloyd (GL) calculated the SBN propagation throughout the whole ship using GL.NoiseFEM, an energy finite element formulation (11). In the new century, Wang and Bernhard (12) further developed the EFEM 0 theory, where the finite volume method and hybrid methodology are introduced, and modeling efficiency could be increased tremendously. The big engineering project in this paper fully utilizes this method. The energy equilibrium in the elastic media can be expressed as following equation: where e means the energy density, σ means the stress vector of structure, means the displacement vector of any point, π in is the input power, and π diss is the dissipation power. Based on the above formula, several constituent equations can be deducted. For energy balancing, For damping dissipation, For energy flow conduction approximation, Thus the energy density equation would be: (2) (3) (4) (5) 5521

3 where η is the damping loss factor, c g is the group velocity, and e is the energy density. Finally, for a three beam model, a similar format to SEA of simplified EFEA equation can be summarized as (12): (6) (7) By solving the above equation, one can get the energy density value of each element. The EFEA can utilize the FEM model directly, so as to save tremendous time in the real engineering project. 3. THE FEM MODEL OF THE WHOLE VESSEL The main design parameters of the 6700PCTC are displayed in Table 1. Parameter Table1-The main design parameters of the 6700PCTC Dead weight at Breadth Depth Design Length BP scantling draft MLD MLD Draft Speed Value 19000t m 32.26m 14.95m 9.00m 19.35kn Main Engine MAN-B&W 6S60ME-C8.2 Propeller Parameter Power MCR Speed MCR Diameter Number of Blades Value 12200kW 98r/min A 3-dimensional finite element model was built in Patran software and used for the strength and vibration analysis. All major structural components such as decks, longitudinal and transverse bulkheads, girders, web frames and shell plating were modeled by plane stress elements. Deck beams as well as pillars and stiffeners are represented by truss elements. The deckhouse was modeled by shell and beam elements. In order to achieve a realistic representation of stiffness and mass of the main engine, a comprehensive FE model of the main engine was incorporated into the hull FE model. This also allowed us to realistically applying the main engine excitation forces. The main engine was idealized by shell and beam elements. The main engine top bracings were represented by truss elements with equivalent stiffness. For reference, an overall view of the global model for the design is presented in Figure 1 and the main engine is presented in Figure 2. Figure 1 Global view of 6700PCTC FEM model Figure 2 Main engine 5522

4 4. THE EFEA MODEL OF GLOBAL VESSEL AND THE PREPROCESSING The EFEA model was derived from the FEM model; the whole model has nodes, shell elements, beam elements, and 498point elements. The noise calculation software is called ViNAS. Figure 3 shows the EFEA model, which is preprocessed directly from FEM model. Figure 3 Global view of 6700PCTC EFEA model The whole vessel has 697 cavities, and all of them are represented by SEA-like fluid subsystem. The boundary shell elements of these cavities are grouped and represented by EFEA elements. The properties of noise control treatment (NCT) are classified as three types: transmission loss only, absorption only, and both transmission loss and absorption. These properties of NCT are attached to the appropriate sides of cavities boundary elements. Totally 18 loads were applied to the global model. The propeller, air compressor, boiler, fans, main engine, and generators are the most influential sources. The water borne source of propeller is represented by diffuse acoustic field. The structure-borne noise sources emitted from main engine and generators are represented by constraint acceleration level in the equivalent mass models. The airborne noise sources from fans, main engine exhaust, main engine surface, and generators are represented in sound power level and applied in appropriate source SEA cavities. The noise sources are displayed in Figure 4. Figure 4 18 types of loads applied on the global model 5. THE POST POSTCESSING AND THE RESULTS The calculation was performed for standard octave band center frequencies from 63 Hz to 8000Hz. Figure 5 and 6 illustrate the energy density of EFEA elements and sound pressure level (SPL) of SEA cavities at 1000Hz, respectively. The fringe plots of other frequencies can be displayed in the similar way. 5523

5 Figure 5 Energy density of EFEA elements Figure 6 Sound pressure level in different SEA cavities The engine control room is one of the most important spaces in the vessel. This room is chosen to be target room for this study. Figure 7 shows the SPL curve of the engine control room for each center frequency. The real ship measurement data shows that the overall SPL in octave band is 68.8 dba; and the calculated SPL is 69.5 dba. The difference is within the engineering requirements. Figure 7 Sound pressure level in engine control room 5524

6 6. CONCLUSIONS The comparison between measurement data and model prediction by ViNAS is good and within engineering requirement. From the real engineering project described in this paper, it shows that the modeling process of EFEA is much simpler than SEA - users don't need special knowledge to build SEA subsystems, therefore the project efficiency could be increased dramatically. The EFEA-SEA hybrid model (EFEM 0 ) utilizes FEM model directly, thus the structural details are reflected directly and the calculation precision is improved in comparison with old method. The SEA-like cavity subsystems enable users to calculate not only structure-borne noise but also airborne noise. The EFEA theoretically has less restriction on high damping and strong coupling. Considering the strong coupling effect of welded large-scale structures, the EFEM 0 might be the superior method to compute the mid-to-high frequency vibroacoustics in shipbuilding industry, and also has big potential for other industries. REFERENCES 1. Maritime Safety Committee, Resolution MSC.337(91), Adoption of the code on noise levels on board ships. International Maritime Organization, International Maritime Organization, Resolution A.468(XII), Code on noise levels on board ships. 1981, p Lyon RH, Maidanik G. Power flow between linearly coupled oscillators. Journal of the Acoustical Society of America, 1962, 34(5): Yoshika T, Hattori K, Sato T, Prediction of noise level on board ship using statistical energy analysis. Proceedings of Internoise 81(C), Tratch JrJ. Vibration transmission through machination foundation and ship bottom structure. M.Sc. Thesis, Massachusetts Institute of Technology, Shimomura Y. The effect of a liquid storage of tank on sound transmission through ship structure. Ocean Engineering Thesis, Massachusetts Institute of Technology, Nefske DJ, Sung SH. Power flow finite element analysis of dynamics systems: basic theory and application to beams, Statistical Energy Analysis. ASME Publication NCA-3, New York, Wohlever JC, Bernhard RJ. Mechanical energy flow models of rods and beams. J. of Sound and Vibration, 1992, 153: Bothier OM, Bernhard RJ. Simple models of energy flow in vibration membranes. J. of Sound and Vibration, 1995, 182(1): Bothier OM, Bernhard RJ. Simple models of energetics of transversely vibrating plates. J. of Sound and Vibration, 1995, 182(1): Matthies, H., Cabos, C. A method for the prediction of structure-borne noise propagation in ships. Sixth International Congress on Sound and Vibration. Copenhagen (1999), p Wang S, Bernhard RJ. Prediction of averaged energy for moderately damped systems with strong coupling. J. of Sound and Vibration, 2009, 319: