Ship propeller side effects: pressure pulses and radiated noise

The present paper deals with the side effects of propellers cavitation, i.e. pressure pulses and radiated noise. These effects are gaining more and more importance for commercial ships for different reasons. Pressure pulses significantly affect comfort onboard, thus their reduction is of utmost importance for all ships carrying passengers. As regards the underwater radiated noise, in the last decade interest has shifted from navy applications to commercial ships, due to the concern for the rising background noise in the oceans. The propellers, generating noise directly in water, represent one of the main contributions to the overall underwater noise emitted from ships. Due to the complexity of the mechanisms of propeller noise generation, different complementary strategies have to be followed to properly analyze the problem, ranging from induced pressure pulses to broadband noise and cavitation. In the present work, part of the activities carried out in the framework of the collaborative EU FP7 project AQUO (Achieve QUieter Oceans by shipping noise footprint reduction, www.aquo.eu) are reported. The paper presents the investigations carried out on a specific test case represented by a single screw research vessel, which is analyzed with three different strategies: numerical calculations, model scale investigations and full scale measurement

Holistic control of ship noise emissions

The sustainability of anthropogenic activities at sea is recently gaining more and more attention. As regards shipping, emissions from ships into the environment of various nature (engine exhaust gases, anti-fouling paints leaching, ballast exchange, releases at sea of oil and other noxious liquid or solid cargoes, of sewage and of garbage) have been recognized as sources of pollution and therefore controlled and limited since a long time. The subject of noise emission has been identified only recently. To study the problem, the EU has funded, among others, the FP7 SILENV (Ship Innovative soLutions to rEduce Noise and Vibrations) project that run from 2010 to 2012. In the present work, the holistic approach followed within the project to characterize and control the ship as a source of noise is presented. Three types of noise emissions (in air, in water and inside the ship) are analyzed highlighting peculiarities and different strategies adopted to characterize the source, the impact on the receiver and the possible solutions to set limits to the ship emissions. The project outcome included a socalled “Green Label”: a set of new prenormative requirements defined for the three main areas mentioned above

MODAL RESPONSE OF CABINS AT LOW FREQUENCIES IN A RO-PAX VESSEL

Onboard ship noise represents a complex issue due to the interaction of different contributions such as engines, propellers, HVAC systems, and several propagation pathways throughout the ship structure from these sources to the various occupied spaces. Resonances can occur and strongly affect the acoustical comfort into closed spaces, particularly into cabins used to rest by the crew or passengers. Four different cabin geometries from a real passenger vessel have been simulated using 3D FEM techniques in order to obtain the modal response of each enclosure at low frequencies. The density of the modal frequencies was obtained for each space. Some of those frequencies could cause annoyance and could be caused by the engines or the propellers of the ship. The presence of high acoustic pressure zones in sensible locations of the cabins is evident in all the considered geometries. This approach can represent a valuable tool at design or retrofitting stages to achieve the best possible comfort levels by adopting control measures depending on each cabin geometry

METODO DI VALIDAZIONE DI TECNOLOGIE ROBOTICHE PER L’ISPEZIONE DI UN AMBIENTE NAVALE

Introduction The use of robotic technologies in ship inspections could lead to a significant reduction in costs, timing and problems related to the safety of the workplace. However, the application of autonomous systems in this field is still severely limited, both on a technological and regulatory level. It is necessary to demonstrate the effectiveness of these technologies in the field to support the human surveyor. Technical features There are currently no assessment methods available to verify the effectiveness of a ship inspection carried out using robotic technologies, checking that survey outcomes are at the same quality level of those commonly attainable by a human surveyor. Therefore, according to current regulations and traditional inspection techniques, validation methodologies based on the development of test equipment and protocols have been defined, aimed at verifying both the effectiveness of a robotic technology and of the involved personnel. As an innovative and pragmatic solution, it is proposed to experimentally test these technologies in a dedicated test environment, more economical and available than a ship, though able to simulate the reality on board in a controlled and repeatable way. Modular test equipment and protocols allow varying the parameters according to the needs and robotic technologies to be examined, by using different operating stations in which it is possible to simulate the actual conditions of a survey, detecting the degradation of the structure due to corrosion phenomena, fracture and mechanical damage. Possible Applications Shipbuilding engineering; Welded structures; Ship surveys and management; Applied robotic engineering and technologies; Training of inspection personnel. Advantages Simple and inexpensive method for verifying and validating the use of robotic technologies for ship inspections; Training centre for surveyors and pilots; Meeting place for the interaction and collaboration of different technology sectors, where to develop new knowledge and set rule requirements

On the possibility of estimating ship parameters using acoustical Lloyd's mirror effect

The Lloyd's mirror effect is an acoustic effect taking place when a source of noise moves parallel to a reflecting surface. In the case of ships, the underwater sources of noise, mainly the propeller, are at constant depth, i.e. a constant distance, from the free surface that can be considered a perfectly reflecting surface. In such a configuration, a destructive-constructive interference pattern is generated in the time-frequency domain by the combination of the direct and surface-reflected sound waves. As the shape of the acoustic patterns depends on the speed of the ship, the relative distance between the ship and the hydrophones and the sound celerity, it is in principle possible (it has been successively done for aircrafts) to derive the above mentioned characteristics simply analysing the corresponding Lloyd's mirror effect. This is particularly interesting when monitoring shipping noise traffic when data regarding ships cannot directly derived by the Automatic Identification System (AIS). In the paper a set of measurements carried out in the framework of two EU FP7 European Projects are analysed to discuss about the possibility of estimating ship parameters using acoustical Lloyd's mirror effect. During the measurements the main ship parameters have been monitored by GPS therefore representing a good reference test

Acoustical Impact of Ships and Harbours: Airborne and Underwater N&V Pollution

Noise and vibrations generated by ships affect a wide range of receivers: crew and passengers inside the vessel, inhabitants of the coastal areas and marine fauna outside it. Recent studies suggest that a large percentage of people living in urban areas close to harbors and a number of marine species, at different evolutionary levels (in particular mammals and cephalopods), suffer from ship N&V emissions in air and in water. The present degree of knowledge of the phenomena involved in the noise emissions inside and outside ships is quite different, as a result also of the time elapsed since the negative effects were realized and therefore studied. The development of the normative framework in the various areas reflects these differences, but there are expectations for improvements on all fronts that need to be supported by the scientific community presenting the latest research results in this particular field of acoustics

New Requirements for Noise Radiated from Ships: Pre-normative Formulations and Background

One of the aims of the SILENV project, funded by the E.U. within the 7FP, was the development of prenormative requirements regarding the impact of shipradiated noise into three different environments: onboard the vessel and outside it, in air and in water. The paper describes the background of the new formulations which generally take into account the existing requirements, the technical know-how in the field of ship construction about the characterization of noise source and transmission paths and the actual knowledge of the way noise affects the various categories of receivers involved. On these bases, objectives of the noise control are defined in the various areas and the final choices in terms of limits are reported, also highlighting the motivations at the basis of the requirements. Possible future trends in the redefinition of objectives for the control of noise and in normative developments are also discussed

Seafarers' work exposure to tonal noise components

The requirement of better workers' psychophysical health and the wish to guarantee a correct comfort also during work activities, induce to consider the presence of significant pure tones also for the working spaces. However, the current European legislation regarding professional exposure to the noise of the workers, and namely the EU Directive 2003/10/CE, does not take into consideration the possible presence of pure tones in working noise. The same lack can be noticed in the standard ISO 1999:2013. As regards seafarers, the reference codes are the International Maritime Organization (IMO) Resolution A.468(XII) for ships built before 1 July 2014, and Resolution MSC.337(91) for newer ships. While tonal components are mentioned in the first one, stating that, if these are "obvious", the ISO noise rating (NR) number should be also determined, in the latter and most recent IMO Resolution no reference to tonal components is present. Tonal components cause annoyance and, at the same time, they induce hearing damage and can alter the behavior of workers, with a potential risk for the safety of ships. In this study, noise measurements taken onboard different ships are analyzed in order to determine seafarers' work exposure to tonal components. The survey includes different work spaces. To assess the presence of tonal components in crew's work environments the methodology of environmental noise analysis, defined in the Annex B of Italian Decree D.M. 16/3/98, has been used, making reference to ISO 226:2003 for determining normal equal-loudness level contours. The method used in order to evaluate tonal components is well established for the environmental noise annoyance and therefore it can be operatively extended to the workplace noise exposure. Results show that the presence of tonal disturbing components is not sporadic and therefore worthy of further analysis and regulation

Evaluation of methods to measure acoustic transfer functions in cavitation tunnels

The interest for cavitation noise studies and prediction is increasing in the field of naval architecture, mainly because of attention to environmental issues. Model scale tests in cavitation tunnels are commonly considered as one of the most effective tools for cavitation noise studies. However, despite being carried out since long time, model scale experiments still present many challenges. These are mainly related to scale effects on propeller hydrodynamics, cavitation behaviour and scale effects on noise generation and propagation. Besides these phenomena the effect of the confined environment in which tests are carried out may be of great importance. In present work, an acoustic characterization of the SSPA large cavitation tunnel section #3 is presented, with the aim to obtain suitable transfer functions in order to take into account (at least partially) this phenomenon. The acoustic characterisation is performed considering two different underwater transducers and different signals and post processing techniques. The obtained transfer functions are shown and discussed in order to analyse advantages and shortcomings of the different procedures and to generally identify main problems related to this kind of activity