1,721,017 research outputs found
Full scale cruise ship dynamic identification using operational modal analysis on sea trial data measurements
No full textHigh operational vibrations in ships can lead to discomfort for passengers and crew and cause structural fatigue and cracking. Reducing these vibration levels under working conditions requires an understanding of both the characteristics of the excitation sources and the vibrational structural dynamic behaviour of the system in question. For accurate forced response vibration calculations, it is crucial to correctly identify resonant frequencies, mode shapes, and damping values through experimental methods, as these are essential for validating the results. Experimental Modal Analysis (EMA) is a widely used technique across many engineering fields that allows for the identification of modal parameters. However, EMA cannot be applied under operating conditions; it requires artificially exciting the system and measuring both the input force and the output response vibrations on an experimental grid of the structure. This identification technique can be challenging, costly, or even unfeasible for large and massive structures such as cruise ships. As an alternative, Operational Modal Analysis (OMA) can be used to experimentally extract modal parameters from response-only measurements. This paper presents the identification of two similar cruise ships using OMA, utilizing data collected during sea trials from endurance, crash stop, and propeller run-up tests. The testing procedures and the results obtained are presented and compared
On the experimental full scale vibrational response analysis of a large pleasure yacht
No full textThis paper presents an experimental investigation carried out on a large pleasure yacht during a sea trial using vibration signal processing methods to characterize its dynamic operational behaviour and to identify contributions of the main sources acting on the system including the onset of the propeller cavitation phenomenon. Synchronous averages were computed to isolate vibration components associated with a specific source acting on the system. Their use jointly with variance computation allowed the detection of the rise of cavitation. Spectral Kurtosis analysis suggested optimal-bandwidth of the filter for system response demodulation and so better identify the rise of high frequency energy of bursting contents induced from cavitation strictly linked to propeller rotation. Cyclic modulation spectrum was also adopted, which allows to extract the modulation features of cyclostationary signals, shows the rise of an interaction between high frequency contents and blade passage phenomenon approaching cavitating condition. The results seem to provide an interesting solution based on the vessel vibrational response to define source contribution to the targets for a better identification of the dynamic system. Moreover, the proposed diagnostic methods reveal reliable tools for real time condition-based monitoring of the marine propeller to distinguish cavitation and wake energy contribution using vibrational response measured on the hull instead of hydrophone arrays
A Quick and Practical Approach for Concept-design of Submerged Thin-walled Stiffened Cylinders
No full textGoal based and limit state design is nowadays a well-established approach in many engineering fields. Ship construction rules started introducing such concepts since early 2000. However, classification societies' rules do not provide hints on how to verify limit states and to determine the structural layout of submerged thin-walled stiffened cylinders, whose most prominent examples are submarines. Rather, they generally offer guidance and prescriptive formulations to assess shell plating and stiffening members. Such marine structures are studied, designed and built up to carry payloads below the sea surface. In the concept-design stage, the maximum operating depth is the governing hull scantling parameter. Main dimensions are determined based on the analysis of operational requirements. This study proposes a practical concept-design approach for conceptual submarine design, aimed at obtaining hull structures that maximize the payload capacity in terms of available internal volume by suitably adjusting structural layout and stiffening members' scantling, duly accounting for robustness and construction constraints as well as practical fabrication issues. The proposed scantling process highlights that there is no need of complex algorithms if sound engineering judgment is applied in setting down rationally the hull scantling problem. A systematic approach based on a computer-coded procedure developed on purpose was effectively implemented and satisfactorily applied in design practice
Low - Frequency analysis of super yacht free vibrations
No full textAfter a long period of crisis and uncertainty the yacht market seems to exit from the tunnel. Many shipyards are developing new interesting projects, especially for vessels over 50 m in length. The high intrinsic value of these vessels fosters technical offices to deepen the research on some critical subjects to increase competitiveness. The onboard comfort, with particular reference to vibrations, is probably one of the key aspects of this trend which nowadays drives most of the design choices relatively to structure lay out and materials for interior outfit. A great help comes from Classification Society new rules and regulations for the evaluation of noise and vibration. The so called "Comfort Class Rules" contain the general criteria for noise and vibration measurements in various yacht areas and suggest maximum limit values as a function of the required quality level of the yacht. In this paper a study carried out in cooperation between the University of Genova and a leading Italian shipyard on the dynamic behaviour of superyacht structures is presented. A detailed FEM analysis of the global and local natural frequencies of a 60 m superyacht was carried out in order to evaluate the reliability of simplified procedures based on reduced numerical models. The investigation has been carried out by two different FEM codes and the numerical results have been compared with experimental data measured during the construction of the vessel
Experimental and numerical analysis of absorber materials for steel decks
No full textNowadays, comfort is becoming the principal goal for designers. But what does onboard comfort really mean? It is possible to define it as a sense of physical or psychological ease, often characterized as a lack of hardship, and it is therefore a subjective sensation. In fact, the perception of a comfort condition is due to a complex mechanism in which the different senses are involved and interact with each other. When trying to improve the comfort onboard, it should be considered that the interaction between the surrounding environment and the people on board is realized through the perception of stimuli related to hearing (noise), smell, breathing and transpiration (air quality and temperature), the sight (aspect of the environment), physical contact (vibration) and the sensation of movement of the body in space (ship motions). Until now, the most important Classification Societies impose severe rules and regulations only for the evaluation of noise and vibration maximum levels for different zones of the ship. Many other aspects that influence the comfort on board are currently under study. The incentive to provide the market with more comfortable products gave rise to high awareness for the analysis of sound transmission and absorption of the main materials used in shipbuilding. There are two driving parameters to describe the behavior of sound absorber materials: Transmission Loss (TL) and Insertion Loss (IL). In case of metals, with particular reference to steel, it is well known that the internal damping is very low. In contrast, there are so-called viscoelastic materials, which show high dissipation of mechanical energy. In this paper, a measurement campaign in real scale, carried out to investigate the dynamic behavior of different materials used to absorb vibration and sound propagating through steel decks, is described. During the tests, a comparison of velocity level measured under the naked metal plate obtained from experimental data and from finite element analyses has been carried out. Furthermore, the TL and IL values of the four different floor configurations obtained from experimental data are compared to understand which type of floor shows the best damping behavior
A STRATEGY FOR THE REDUCTION OF STRUCTURAL WEIGHT OF COMPOSITE VESSELS
No full textHull weight is a main structural concern in pleasure craft design, being in such project the maximum speed one of the most important contractual limits imposed by owner to shipyards. This is particularly true for composite structures, where the structural layouts and the beams' dimensions may vary significantly among very similar projects, resulting in very different structural weights. Moreover, for this type of vessels, the structural weight is a significant percentage of the total displacement, and, consequently, reducing the structural components leads to important total weight reduction. The present work aims to orient the preliminary design stage of composite pleasure yacht structural design towards an optimal configuration of the structural layout in terms of hull weight, within the current framework of rules provided by classification societies
The role of structural and insulating material in the structure-borne noise propagation of superyachts
No full textThe insulation and noise and vibration damping characteristics of complex systems used by humans, such as automobiles, airplanes and ships are becoming increasingly important from the early design stages to ensure an adequate level of comfort. Especially in the naval fields, the design of insulation plans is crucial to prevent the propagation of noise and vibrations on board of megayachts. Since it is quite complex to identify and correctly model the sources due to their deterministic and random nature, during the early design stage, the focus is on insulation rather than reducing the acoustic levels of the sources themselves. In this article, a method for analyzing sea trials on a superyacht will be proposed with the aim of analyzing how much the insulation materials can limit the levels of vibrational energy on board during the path between sources and receivers. A new a descriptor based on harmonic transmission energy through the structure is proposed for path insertion loss experimental identification. In particular, by using spectral analyses and synchronous averaging of sound levels in various areas of the superyacht, it will be possible to study the vibroacoustic insulation of structural materials and their damping coefficient
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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