Ship Vibration Research Articles

Computation of springing transfer functions

Ship vibrations excited continually by regular waves are predicted by a potential method. It takes into account the interaction of the oscillatory flow with the steady flow due to the ship's forward speed, including steady ship waves and squat, but it excludes non-linear oscillatory wave forces. A three-dimensional Rankine source patch method is used both for the steady and the oscillatory flow. Vibration damping by an immersed transom, wave radiation, bilge keels, propeller, and hatch cover friction are approximated. Application to a large containership demonstrates that, in this example, damping is mostly attributable to the transom.

船体振動 - 主機関起振力による船体振動

船体振動 - 主機関起振力による船体振動

Analysis of the flexural vibration of ship’s tail shaft by transfer matrix method

A ship’s tail shaft has serious flexural vibration due to the cantilevered nature of the propeller’s blades. Analysis of the nature frequency of flexural vibration is vital to be able to provide effective shock absorption for a ship’s tail shaft. A mathematic model of tail shaft flexural vibrations was built using the transfer matrix method. The nature frequency of flexural vibration for an electrically propelled ship’s tail shaft was then analyzed, and an effective method for calculating it was proposed: a genetic algorithm (GA), which calculates the nature frequency of vibration of a system. Sample calculations, with comparisons by the Prohl method under conditions bearing isotropic support, showed this method to be practical. It should have significant impact on engineering design theory.

Evaluation of main engine and propeller excitations of ship hull and superstructure vibration

Increasing requirements are coming to be set against ship vibration and noise from considerations of both safety and comfort. It is therefore required to perform, at an early stage of the design, vibration prediction by numerical simulation to identify possible problems and define solutions. Finite element methods are widely accepted as a standard tool for such studies. The problem is of prime importance for cruise ships and also for the new generation of fast Ro–Ro/Ro–Pax ferries and large container carriers. For those complex ships, the propeller is the main source of vibration and noise. Therefore, the first part of the paper will mainly focus on propeller excitation. From the propeller, excitation forces are transmitted into the ship via the shaft line and in the form of pressure pulses acting on the ship’s shell. The second part of the paper focuses on a typical cargo ship (container); identification of main excitation forces, as well as investigation of influence of analyzed excitation on ship vibration level. The following excitation forces have been analyzed: dynamic pressure on the transom deck, propeller-generated hydrodynamic forces, as well as internal and external engine-generated forces. Rigidity and attenuation characteristics of lubricating oil film have been taken into account in the analysis of excitations transmitted by propulsion system bearings (stern bearing, propeller shaft bearing and engine main and thrust bearings). The stiffness dynamic characteristics of the power transmitting system foundations have been also taken into account. Multivariant computations of ship vibration using the formerly identified excitation forces have been carried out. The results of analyzes have been verified by comparing the computation results with measuring tests.

선박진동 기술의 현황과 전망

선박진동 기술의 현황과 전망

2118 領域分割型CG法による並列振動解析に関する研究(G01-2 計算力学(2),G01 計算力学)

Large ship structures are designed considering vibrations induced bypropeller or engine excitation, to prevent structural failure due to metal fatigue and to guarantee comfortable habitability. Vibration analysis of large complex structures such as ships needslarge-scale computation and much time for modeling, computation and post-processing. Therefore, parallel computing is expected to be applied for the ship vibration analysis. In the present study, the authors have developed parallel vibration analysis software by application of DDM based on CG method. As one of options for DDM analysis procedure, we have proposed a 'domain decomposition before mesh generation' and have showed the data structure. Moreover, the effect of pre-conditioning on the convergence is discussed.

Hull Disturbing Forces Due to Propeller Sources

An acoustic boundary element model of a modern twin-screw cruise liner is used to show how fluctuating pressures above the propeller, and hull forces that cause ship vibration, change relative to each other with the nature of propeller sources. The low frequency effects of fluctuating loads on the propeller in different directions are compared with the effects of unsteady cavitation. The implications of the results for specification of ship requirements and estimation of ship vibration are discussed.

Minimization of the vibration transmission through the propeller-shafting system in a submarine

Ships and submarines are efficient sources of underwater radiated noise. In the low-frequency range, the main sources of ship vibration and noise are the working machinery such as the diesel engines, gearboxes and generators, and the propeller/propulsion system. Rotation of the propeller results in discrete tonals at the blade passing frequency and its harmonics. In addition, in the case of a submarine, hydrodynamic forces acting on the propeller due to the fluctuating pressure field are transmitted through the propeller-shaft and thrust bearings, resulting in axial excitation of the submarine hull. A hydraulic dynamic absorber, also known as a resonant changer, is used to minimize the vibration transmission through the propeller-shafting system, in order to prevent excitation of the hull axial resonances. The present work is concerned with minimizing the vibration transmission to the hull by optimizing the virtual spring, mass, and damper parameters associated with the resonant changer in a submarine. The dynamic response of the propeller-shafting system, including the propeller shaft, thrust bearing, thrust block foundation, and resonant changer, are characterized using four pole parameters.

A Review of Recent Ship Vibration Papers

The vibration analysis of large and complex structures such as ships is difficult, because a vibrating ship hull (or its components) exhibits quite different vibration characteristics. Technological developments offer new concepts, definitions and comprehensive studies in the ship vibration field. The aim of this paper (bibliography) is to list references to papers, conferences, and proceedings dealing with ship vibrations that were published between January 2000 and April 2003. It contains 64 bibliographic references. The following types of ship vibrations are included: sea conditions, structural analysis, machine components, ship propulsive system, ship machines and special studies. Moreover, basic and brief information (reference, publication type, simulation models, utilizing methods and/or models, examined subject and/or problem about grouped references has been given in the tables. So, this review paper not only lists references to papers at the millennium, but also helps researchers in their studies on ship vibration to find the necessary references quickly and practically.

Hull vibration excitation due to monopole and dipole propeller sources

An acoustic boundary element (BE) model of a modern twin-screw cruise liner is used to show how fluctuating pressures above the propeller, and hull forces that cause ship vibration, change relative to each other with the nature of propeller sources. The effects of fluctuating loads on the propeller, represented by dipole sources in different directions, are compared with the effects of monopole sources at one and two times maximum propeller blade passing frequency. Convergence of results for the cruise liner model is demonstrated using different element distributions. The results are specimen transfer functions that relate propeller sources, having general spectral characteristics, to hull excitation. They are presented in non-dimensional form to facilitate comparison with results for other hull shapes and to demonstrate the effect of hull scale. A simple analysis of dipole and monopole sources is used to interpret results and to demonstrate why solution of the Helmholtz equation, which includes the effects of a finite speed of sound underwater, is needed for accurate evaluation of hull disturbing forces. It is argued that such modelling techniques can facilitate more accurate future interpretation of model-scale experimental data from towing tanks or water tunnels, as well as allowing improved specification of ship acoustic requirements.

大型機関と船体との連成振動応答予測法 機関据え付け点インピーダンス計測値の適用

A ship is equipped with a large engine that increases ship vibrations. Particularly, it may give rise to the serious large vibration problems due to the coupling effect of the large engine and the hull. So, it is important for us to grasp vibration characteristics of each support point of a large engine. However, a measuring method of vibration char-acteristics of support points of a large engine has not been established until now because a ship engine is too large. Therefore, we tried to apply Mechanical Impedance Method to prediction of mechanical impedance at each support point by the former paper. This paper confirms utility of the experimental analytic method for measuring impedance characteristics of support points of a large engine that we suggested. And we showed the effectiveness of Mechanical Impedance Method by using the experimental results of the engine and hull models on prediction coupling vibration.

Identification Method of Damping for Ship Structural Vibration (2nd Report)

In order to prevent harmful vibration in ships at the early stage of design, anti-vibration design is required on the basis of accurate estimation methods. To investigate vibration response of superstructure, very large scale of 3-dimensional finite element analysis including hull structure is sometimes conducted, however, significant improvement of accuracy is not yet achieved due to the difficulty in estimating damping and exciting forces.In the 1st report, a new identification method of damping factor is proposed by calculating dynamic response directly on the basis of Rayleigh's damping; [C] = α [M] β [K]. Then the capability of this method is verified by the measured data of frame structure model.In this 2nd report, this method is applied to the measured data of a 280, 000DWT VLCC's superstructure and its ability is confirmed in the following procedures : 1) Precise transfer functions of superstructure vibration were obtained by the sweep exciter test of the superstructure. This test was performed with the aid of the active mass damper controlled by electromagnetic force recently developed), which enabled to oscillate the superstructure with fine frequency increment.2) 3-dimensional FE model of superstructure including hull structure is analyzed to investigate the influence of FE modeling manner on the calculated natural frequency, in particular the influence of FE model realization of local structures such as radar mast, funnel and etc. Consequently the guideline of FE modeling of superstructure is proposed to estimate the accurate natural frequency of superstructure vibration.3) By means of the above accurate superstructure model with hull structure, the new identification method described in the 1st report was adapted to identify the measured response of the VLCC's superstructure. As a result of this identification, calculated response showed a good agreement with the measured data in both ballast condition and full load condition.4) Obtained damping ratio is ζ=0.00896 and 0.01025 in ballast condition, ζ=0.01297 and 0.01626 in full load condition. These values almost agree to the assumed lower limit of the published damping ratio.

Identification Method of Damping for Ship Structural Vibration (1st Report)

In order to prevent harmful vibration in ships at the early stage of design, anti-vibration design is required on the basis of accurate estimation methods. In the field of vibration response prediction, however, significant improvement of accuracy is not yet achieved due to the difficulty in estimating damping.It is difficult to deal with damping of ship in a theoretical manner, since the damping mechanisms are complex and related to the dissipation of energy occurring in different ways. Therefore, damping should be obtained by the experimental approach.With regard to damping factor measurement, response function is acquired from vibration measurement of actual ship by exciter test and it is processed by modal analysis method. In spite of these measurements, it is not an easy work to apply modal analysis method to ship response analysis, in particular an analysis of huge scale structural model, such as 3-dimensional finite element analysis of whole ship structure. The reason for the difficulty of response analysis by the modal analysis is because it needs a lot of skill to identify major vibration modes from various vibration modes obtained by modal analysis of large structural model.In this paper, a new identification method of damping factor is proposed to overcome the above difficulties by calculating dynamic response directly on the basis of Rayleigh's damping, [C] =α [M] +β [K], as well as by correlating measured data and calculated one using iterative method. This paper describes that the theoretical background of this identification method to obtain identified coefficient of damping α, β is explained firstly and secondly performance of convergence is confirmed by a simple hull girder model, and finally this method is verified by the measured data of frame structure model.

Active Mass Damper for Ship Superstructure Vibration by Electromagnetic Control

It is important to prevent excessive vibration in the superstructure of a ship, in order to provide comfortable habitability to ship's crew and to keep normal operative condition for navigation instruments. Vibrations observed in ships has the following features :(1) The vibration amplitude largely scatters as long as the vibration is induced by the propeller, because propeller exciting force fluctuates due to the change of flow velocity into the propeller.(2) The vibration does not always have a single frequency but have several frequencies, since there are several exciting sources in a vessel such as main engine, propeller and auxiliary machines.Therefore, the vibration of ship's superstructure cannot be reduced by installing a simple anti-vibrating device, but more sophisticated instrument is required.To satisfy the above requirement, an active mass damper driven by electromagnetic force has been developed. An active mass damper can reduce effectively the vibration of fluctuating amplitude as well as the combined frequencies induced by several vibration sources. Firstly, vibration characteristics of this damper were confirmed by the exciter test using model frame structure. Then the capability of the damper was verified by the measurement of a VLCC and a large bulk carrier. Vibration amplitude of these vessels were confirmed to be reduced to about 1/2 to 1/5 of the original level by this active mass damper.

Couplage fluide-structure en construction navale

Click to increase image sizeClick to decrease image sizeFluid structure interaction is a key phenomena in the numerical simulation of the dynamic behaviour of ship structures. Considerable research efforts have been continuously carried on, but though many advanced tools are available, a numerical mock up approach is far from being used. This is mainly due to insufficient accuracy or reliability of these tools. Numerical simulations are therefore used complementary to experimental methods performed in large basins and correlated with long records of measurements. In most cases, ship structure dimensioning is rule based, using classification societies requirements. The study of ship vibrations induced by oscillating forces due to auxiliary and main engine and pressure variations due to the propeller is well mastered and the numerical simulation is then a standard design tool. The increasing size, speed and complexity of new ships such as high speed crafts raise new dynamic problems and IRCN contributes to the development of numerical tools allowing a direct approach of the design of such ships.

(38)A Countermeasure of Ship vibration due to M/E excitation by Automatic Control of Hydraulic Top bracing

(38)A Countermeasure of Ship vibration due to M/E excitation by Automatic Control of Hydraulic Top bracing

A Countermeasure of Ship vibration due to M/E excitation by Automatic Control of Hydraulic Top bracing

As the countermeasure of global vibration problems such as main engine structure, deck house, etc., the structural strengthening or the addition of vibration absorber have been so far invoked. However, it is very difficult to apply these methods to solve the global vibration problems in the sea trial stage since the tremendous costs and time are required. Also, the expected reduction effects can not be sometimes obtained due to various limitations. So, it is required a more effective and economic method to solve global vibration problems in the sea trial stage. In this paper, it is introduced that the vibration of main engine and deck house can be very effectively controlled by the automatic adjustment of pressure of hydraulic top bracing of main engine in case the main engine causes of the vibration problems are considered as main engine excitations or resonance of main engine structure. The proposed method is proved as very effective and economic through vibration measurement during sea trial.

Vibration analysis of ship beams

This paper is essentially addressed to the vibrations of ships which were extensively studied both experimentally and analytically by modeling a ship as a beam of uniform cross section and predicting its vibration characteristics by Euler’s theory of elastic beams. To begin with, the free vibration characteristics of a ship beam are predicted from Timoshenko’s theory of elastic beam of variable cross section by use of Galerkin’s method. The results are evaluated by use of those obtained recently from Euler’s theory [e.g., S. Abrate, J. Sound. Vib. 185 (4), 703–716 (1995)]. Next, a numerical algorithm that is based on the method of moments [e.g., G. A. Altary and M. C. Dökmeci, Z. angew. Math. Mech. 77, 1–17 (1997) and references therein] is developed so as to predict the vibration characteristics of a ship beam at both low and high frequencies. The equations derived include the effects of warping and shear deformations of cross section and, in fact, they incorporate as many higher order effects as deemed necessary in any case of interest. They govern all the extensional, flexural, and torsional as well as coupled motions of ship beam. Also, the results are compared with those of earlier ones. [Work supported in part by TÜBA.]

Active Mass Damper Demonstration for Ship Vibration Reduction

ABSTRACTModern naval vessels, which mount a large number of advanced electronic equipment, require such equipment to be well protected against external disturbance, including particularly vibration. The conventional means of minimizing vibration is to increase structural rigidity, but this entails the increase of structural mass. If vibration could be suppressed by means of a lightweight compact device, it should immensely benefit naval vessel design.To this end, a two‐axis active mass damper driven by a hydraulic actuator has been developed, which effectively reduces the vibration of ship superstructures. A prototype model damper unit has been tested in a laboratory, which proved to corroborate closely the high performance of the design estimated from simulated calculations.It is shown that;1. The acceleration generated in the model structure by a sinusoidal exciting vibration was reduced to about 1/5 upon actuation of the device, and2. A similar reduction of acceleration, to about 1/5, was obtained when the exciting vibration wave pattern reproduced the actual movement of a ship at sea.

Propagation Characteristics of Vibration in Ship's Hull Structures

Propagation Characteristics of Vibration in Ship's Hull Structures