Noise reduction for marine applications (NORMA)

A computational tool has been developed. The tool is based on the waveguide transfer matrix method as well as ordinary acoustical room noise theory. The tool calculates transmission of structureborne noise in ship hulls as well as radiation of airborne noise from the interior joiner elements. The tool also includes a capability to predict noise reduction from conventional noise reducing measures. It gives a concise ranking of transmission paths, dominating noise sources and the significance of the different joiner elements. Measurements have been performed onboard two high-speed vessels in order to validate the method. The results proved a good agreement between theory and measurements.

Model of structure-borne sound transmission through ship foundations: An analytical computation model of structure-borne sound transmission through ship foundations is based on a general wave-guide method for coupled plates. The results can be applied with a mount model to determine the effects of a number of variables on the isolation efficiency of the mount/foundation systems and thereby optimise those systems. Measurements on scaled-down foundations are currently going on in the laboratory. The model results are to be compared with those of measurements. The results achieved to date are promising. Model of structure-borne sound transmission through ship engine mounts: An analytical computation model of structure-borne sound transmission through ship engine mounts is based on a wave-guide method. The results can be applied with a foundation model to determine the effects of a number of variables on the isolation efficiency of the mount/foundation systems and thereby optimise those systems. A general finite element model of the dynamic stiffness of mount has been developed. The results of the analytical wave-guide model are to be compared to those of the finite element model and to those of measurements. The results achieved to date are promising.

Propulsion system design work has concentrated on the examination of all the propulsion system and economic options for the two concept fast ferries with inputs from and close collaboration with the two shipbuilders. Selection of the propulsion system components has been completed. This will, together with consideration of their noise and vibration transmission characteristics, allow choosing of the most suitable isolators. In the case of the Fjellstrand JumboCat the propulsion system will be diesel drive to water jets through a reduction gearbox. The NORMA propulsion system development activities will focus on optimised engine and gearbox mounting systems and on a technology demonstration of Active Noise Control (ANC) of a simulated diesel exhaust. In the case of the gas turbine installation the propulsion system installation design process addressed the key issues of gas turbine (MT30) structural noise and engine installation, MT30 exhaust noise, package enclosure design and weight, and gearbox design and noise characteristics. It is proposed to develop these tools to provide an installation design model to enable predictions of vibration transmission into a ship structure for a range of installation options.

The main source of hydrodynamic noise in a ship is the wall flow around the hull. Turbulent Boundary Layer excitation generates hull vibration through a random moving load (wall pressure fluctuation). As a preliminary step, a simple prototype experiment is performed to investigate the main feature of TBL-excited hydrodynamic noise, which aims at separating it from other noise sources, and understanding the basic correlations between the fundamental quantities involved. Most literature dealing with measurements of pressure fluctuations on solid surfaces generated by the turbulent boundary layer (usually smooth flat plates), refer to aerodynamic noise excitation. Most experiments were conducted in subsonic wind tunnels. Several models describing the space-time pressure correlation function are provided. In very recent work, these models are used to provide the aerodynamic loads necessary for the vibro-acoustic analysis. Useful know-how from the experience developed in aeronautics can be exploited, while keeping in mind the main differences between that field and the acoustic TBL excitation in marine applications: - Presence of the free surface; - Possible inception of cavitations. The results obtained from the preliminary experiment show that it is possible to build an experimental procedure that, applied to the ship model, will give general laws for the behaviour of the pressure spectra for a certain class of ship.

A systematic way of measuring the hydrodynamic noise impacts in water jets, such that post processing can lead to the most optimised solution. A method used for aero engines was intended, however because of manufacturing difficulties only 4 prototypes was manufactured which is to low number for a multivariate data analyses. The method was replaced by a reanalysing the pressure fields by CFD. It was found that the coherence was extremely constructive in the guidevane part, therefore all efforts was concentrated upon decrementing the ampliyudes in the guidevane-part. A number of candidates for softer interaction between rotor and stator were suggested. A CFD test for performance acceptance outraged four candidates. The result of the best choice was a significant reduction of vibrations. 14dB.

An analytical computation model of water jet tunnel uses the solution of the coupled problem water tunnel walls to predict the vibration and the resulting sound radiation from the tunnel. The tunnel is modelled as an elastic cylindrical shell. The result can be applied to noise modelling in frequency bands. The model is currently tested in laboratory, by comparing computation with measurements on a scaled-down water filled shell. Good overall matching was found.

A source of hydrodynamic noise in a trimaran SES is due to the free surface random flow generated inside the air cushion box. Since the direct measurements of the acoustic pressure is not possible because of the noise due to the fan used to produce the overpressure an indirect measurents was performed. The procedure implemented to obtain acoustic pressure inside the air cushion consists in the following steps: - Measurements of the wave elevation due to a constant pressure generated by a fan inside a rectangular box; - Derivation of the radiated acoustic pressure by analytical relations. The rectangular box had the lateral sidewalls immersed to avoid air leak while the front and the rear side just above the free surface level. The wave pattern was measured with the fan on and with the fan off. This way, the disturbances produced by the sidewalls were subtracted in the post processing phase to the air cushion wave field. Front and back skirt was provided when necessary to limit air leak. The results of the analyses done show that free surface excitation inside the air cushion box is of minor importance and thus its effect can be neglected in the evaluation of the total noise level on board.

The main emphasis of the work was to determine the most significant noise sources in the area of motors and exhaust systems and for which the implementation of an anti-noise system is considered to be useful. The second task was to simulate the noise sources and find out in which way the noise reduction could archive. Work was carried on an active silencer using a specific transducer able to stand high flow velocity and high temperature. The aim of this active silencer is to be able to control very low frequency noises by using the flow energy with a flap fixed inside the exhaust pipe and driven by an electrical rotation engine. Relevant prototypes have been manufactured and successfully tested.

Model of the excitation of ships hull by the turbulence layer in motion. The model computes both the dynamic state of water, including the free surface geometry, and the pressure pulsations acting on the hull. Real ship geometry is taken into account. The current work deals with the validation of the model to the laboratory case of water tunnel turbulence excitation.

The Fjellstrand concept design is a development of existing successful medium sized catamaran ferries. The new design will have a totally new and extended interior passenger accommodation space. This requires improved noise reduction and isolation, to achieve better than current noise standards. The propulsion system will be diesel drive to water jets through a reduction gearbox. The NORMA propulsion system development activities will focus on optimised engine and gearbox mounting systems and a technology demonstration of Active Noise Control (ANC) of a simulated diesel exhaust. A technical/design specification of a high-speed catamaran ferry has been made. A validation of the specification is done. Noise level targets are given. General arrangement and safety drawings are made. Structural drawings are made. A class design review is completed. Full-scale measurements of noise during construction period have been completed. Noise levels have been calculated by NORMA calculation model to find noise levels. Various NORMA noise reduction methods are included in calculation and it is found that the target can be met.

Proposals for revised comfort class rules: Based on the findings of this study it was decided to revise the comfort class criteria for high-speed vessels to a more realistic level than what appears in the current edition of the comfort class. It may be worthwhile considering an area-wise distribution of noise levels instead of dividing between different size vessels. Two alternative proposals are therefore presented. Based on the statistical analysis of the DNV database for noise levels onboard high-speed vessels two alternative proposals for maximum levels have been made and specific new limits have been proposed in the referenced technical note. Description of changes: - An area-wise division may be considered instead of a length division. Alternatively, the length division should be increased from 50m to 100m. The influence of size on noise levels is not felt to be significant before the vessels become very large. Both short and long vessels have problems reducing aftship levels. - If an area-wise distinction is allowed, the new maximum levels will be in line with the IMO requirements or slightly stricter for comfort rating 1 in the aftship. For the forward areas, the new maximum levels will be roughly in line with the previous requirements for vessels exceeding 50 m overall length (slightly less strict for rating 3). - If a length division is kept, the maximum noise levels for Passenger localities have been raised, except for comfort rating 3, which is already at the minimum requirement specified by IMO. The noise level requirements have been increased by 2 - 5dB(A) based on experience from measurements onboard new buildings, in order to make the requirements realistically attainable. The requirements are still strict compared to the experience from vessels in service today. However, it is believed that advances in noise reduction technology will make the requirements more easily achievable in the future. It is also felt that comfort rating 1 represents a noise requirement that most builders will have to stretch themselves to their utmost capability in order to achieve. It is also felt that vessels achieving the comfort class requirements will have a high level of comfort. Particularly, if the novel requirement to tonal noise is accepted. - The maximum levels for navigation bridge is increased from 60dB(A) to 63dB(A) for comfort ratings 1 and 2. It is felt that 63dB(A) on a bridge environment will be good enough. A noise level of 63dB(A) will enable clear and undisturbed communication and should not stress the crew. The bridge is often exposed to aerodynamic flow noise and noise from internal navigation equipment. Noise control for these sources may be difficult as well impractical. - The noise levels for service areas are reduced from 78dB(A) to 75dB(A) for comfort rating 3 in order to bring this requirement in line with the IMO maximum levels. For vessels longer than 100m the noise requirement is increased from 65 to 68dB(A) for practical reasons and to harmonise the requirement to the requirement for passenger localities. - New requirements have been added for outdoor passenger areas. Such areas have become common for vessels operating in hot climates. Hence, in order to keep in line with the market, noise requirements for such areas should be included in the comfort class. Noise on open deck areas is hard to attenuate and will often be influenced by noise from the sea in particular the wake, but also airflow. The suggested requirements have therefore been kept in line with the IMO requirements with a slight reduction for the higher comfort rating(s).

Underwater noise requirements for high-speed crafts: The proposed limits were derived based on comparison of measured underwater noise levels from selected high-speed vessels and criteria used for other marine activities. The best-documented underwater noise criteria were the proposed ICES criteria. A mammal warning limit of 180dB and the fisheries research limit were superimposed on these measured data. The fisheries research limit has been converted from the indicated constant bandwidth noise levels to 1/3-octave band noise levels. The mammal warning limit is indicated at a frequency of 1kHz, thereby avoiding the frequency ranges of high sensitivity for most of the commercial types of fish except herring. Not surprisingly, the measured data exceed the fisheries research limits in the low frequency part of the spectrum, since the fisheries research limits are based on a 20 m avoidance reaction distance. A 20m limit would be not only unrealistic but also unreasonable for a vessel not engaged in fisheries research. Increasing the avoidance reaction zone to 200m would appear to be more appropriate and this would result in a limit, which is 20dB higher than the recommended fisheries research limit. The corresponding curve for the 200m requirements was found to be realistic. The selected water jet driven vessels satisfy this requirement with clear margins. The selected propeller driven craft exceeds, however, the suggested limit in the low frequency end of the spectrum and at the 1/3 octave band including the propeller blade passing frequency the noise level is only 5dB below the mammal warning limit of 180dB. The indicated limits are therefore proposed as suitable underwater noise criteria for high-speed vessels. External airborne noise limits for high speed vessels: Similarly measured data for a number of vessels have been studied and compared to criteria applied to other modes of transportation. The analyses have resulted in the following recommendations: - LAeq, T=24 h = 55dB(A) at facade of dwelling and / or; - LAFmax = 70dB(A) close to dwelling; - Measured in accordance with ISO 2922.

Two whole ship noise prediction models, one for each of the concept designs have been developed. The models are based on calculations performed using the wave-guide computer code developed in the project, but also incorporate results from other partners. The results have been used to demonstrate the success-rate of then complete project.

A water jet with reduced noise signature reduces the noise level in the ships aft part, various designs of the rotating blades as well as the stator blades will lead to a design choice of minimized pressure pulse generation within given performance constraints. The development of a low noise water jet will be provided using CFD.

The expansion of the fast ferry market requires new designs with major performance improvements to enable economic development of existing and new routes: - Drag reduction to enable economic trade-offs of the benefits of - High speeds or reduced fuel consumption; - Increased range or reduced transit times; - New hull designs to reduce wakes and open up more inshore routes. The Mistral SES concept design has been developed to meet these requirements by Chantiers de LAtlantique. The trimaran hull encloses air cushions with rubber skirts between the inner and outer hulls. Lift fans in the outer hulls generate the air cushion. The Mistral is designed to carry 1200 passengers and 250 cars, and achieve 60 knots with 2 waterjets each powered by 30MW gas turbines. A technical specification of CAT Mistral fast ferry will be validated, and a design approval of this new concept design will ensure that it is a technical feasible and safe baseline ferry project. So, all necessary drawings and documents (general arrangement, mid-ship section, watertight integrity and preliminary damage study for example) are realised.

Deliverables

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