Ship Hull Vibration Research Articles

Ship's hull vibration damper

There is described an apparatus for inclusion in a ship comprising a mass (10), means (16) for vibrationally coupling the mass (10) to the ship's hull (12) and means (28) for damping the movement of the mass (10), the mass (10) being so sized and located as to, when in use, reduce the fundamental and/or harmonics of a transverse two node mode of wave-induced vibration in the ship's hull (12). There is also described an apparatus for inclusion in a ship comprising a mass (10) made up in part of the tackle or fitments of the ship, means (16) for vibrationally coupling the mass (10) to the ship's hull (12) and means (28) for damping the movement of the mass (10) so as to thereby, when in use, reduce vibrations in the ship's hull (12). In a preferred embodiment the mass (10) is in part comprised of a chain locker and that portion of the chain stored therein. By reducing vibrations in the ship's hull (12) significant stresses may be relieved and the performance of the ship enhanced. A ship incorporating the apparatus and a method of reducing vibrations in a ship's hull are also described.

A second-order theory for the response statistics of wave induced ship hull vibrations in random seas

The paper presents numerical results for the response statistics derived from the application of the Kac-Siegert representation of quadratic transformations of Gaussian processes to the statistical analysis of combined first-order and second-order ship hull vibration response in random seas. Specifically, results for the probability density functions and exceedance probabilities for the wave induced bending moment amidship are given.

PARAMETRIC STUDY OF SHIP-HULL VIBRATIONS

A parametric study carried out to investigate the vertical free vibrations of a ship hull is described. The analysis takes into account all the effects of the rotary inertia of mass, shear distortion, thrust force, shear lag and various damping components. The influence of these parameters on ship hull vibrations is explained. Special emphasis is placed upon a method for calculating the overall damping ratios.

Investigation of Effective Bending and Shear Stiffness of Thin-Walled Girders Related to Ship Hull Vibration Analysis

Application of beam theory in flexural vibration analysis of thin-walled girders is extended for the high-frequency domain by introducing the concept of effective values of beam parameters, that is, cross-sectional moment of inertia, shear area, mass, and mass moment of inertia. Formulation of these parameters is based on equivalence of deformation energy and inertia work, respectively, for a considered structure and its beam model, resulting in the same values of their natural frequencies. For illustration, the natural vertical vibration of a simply supported pontoon has been considered, where it was possible to obtain the analytical solution due to sinusoidal mode shapes. The effective values of cross-sectional moment of inertia and shear area show significant variation in frequency domain. Transfer of effective values of beam parameters, determined for simply supported structure, in the case of other boundary conditions is suggested, based on equal mode wavelengths, and checked for the free pontoon. The results show very low discrepancies compared with a three-dimensional finite-element model solution, so this procedure may be applied generally, as well as to the problem of ship hull vibration. In conclusion, the possibility of calculating the values of effective parameters for multicell ship cross sections, utilizing the theory of folded structure and the finite-element method, is pointed out.

A Study on the Strength of Collided Ship

The strength of ship structure in collision is one of the important matters in the term of saving crew lives and preventing a heavy disaster from the economic point of view, in the case of which the collided ship are earring dangerous cargoes such as liquified natural gas and so on.It must be nessesary to estimate the time history of dynami ccollision force and if necessarily, the penetration of the collided ship in order to design ship structure.In this paper, the time history of dynamic collision force was calculated by introducing mass, spring and dashpots system in the substructures which were localy striked and by considering motions and hull vibration of lateraly collided ship. And extending this method, the penetration of the collided ship was approximately estimated by using static destructive test result.From these result, the following conclusions were obtained.(1) When the ratio of damping coefficient to spring coefficient was not very small, impulsive force acted on the localy striked substructure at the first stage of collision.(2) The smaller ratio is, the larger penetration is.(3) In order to avoid over-estimation of the dynamic collision force and penetration, ship motions and hull vibration must be considered.

Fundamental Studies on Underwater Sound Radiated from a Vibrating Ship Hull

In the present paper the reciprocal theorem between the ship hull vibration and the fluid vibration, that is, underwater sound is studied.Using the reciprocal theorem of velocity potential in the fluid medium, the reciprocity of sound in the fluid systems is introduced. On the other hand, using the reciprocal theorem of mechanical vibrations, the reciprocity of ship hull vibration systems is introduced.To combine with these reciprocities, the reciprocal theorem on the total vibrating systems is proposed, and the relationship between radiation and scattering underwater sound problem is clarified by the theorem. This reciprocity is well known as Haskind's relation in water wave engineering.To verify its usefullness and accuracy, some numerical examples are shown for sound scattering problem of 2-dimensional semi-submerged circular cylindrical shell, and it is numerically demonstrated that the shell deflection amplitude in sound scattering problem is proportional to the Kotchin function in sound radiation problem.

Propagation of ship hull vibration and estimation of underwater sound radiation.

Propagation of ship hull vibration and estimation of underwater sound radiation.

フォワードスキューをつけた直進翼周辺の可視化

In recent years researchers have reported the ship hull vibration and the noise induced by the propeller are reduced by using the highly skewed propeller.In this investigation, a forward skewed propeller blade is tested expecting better characteristics to reduce the vibration and the noise. A protoype, a backward skewed blade and combined skew ones are tested as well.Measurements of hydrodynamical forces and flow visualizations by oil film method and tuft grid method are carried out on the blades. As a result, we found the forward skewed blade does not stall at the high attack angle and the location of the voltex from the tip of the forward skewed blade is closer to the root of the blade than that of the backward skewed blade.

Analytical Characteristics of Oscillating Pressure Distribution above a Propelle

The surface force, which is a cause of ship hull vibration, is due to oscillating pressure on the hull induced by a propeller. An analytical investigation is performed into characteristics of oscillating pressure on a horizontal plane fixed in the space above a propeller.

On the shear coefficient in Timoshenko's beam theory

Some existing formulations for the shear coefficient in Timoshenko's beam theory are discussed, especially through evaluation of the accuracy to which natural frequencies of simply supported, prismatic, thin walled beams can be obtained. The main conclusion drawn is that if a consistent expression for the shear coefficient, such as those given by Cowper [1] or Stephen [2], is used in Timoshenko's beam theory, then very high accuracies can be expected for the natural frequencies, even for wavelengths of the same magnitude as the transverse dimension of the beam. It is noted that no reduction of the moment of inertia due to shear lag effects should be made as these effects are included in the consistent formulas for the shear coefficient. Finally, some apparently paradoxical results indicating that a reduction in shear stiffness occurs in rare cases if more material is added to a section are discussed and explained as resulting from the use of integrated rather than pointwise deflection measures in the derivation of consistent shear coefficient expressions. The results are discussed in the light of the importance of the shear stiffness of the hull girder in ship hull vibration analysis.

A study on ship vibration using finite element method

When the beam theory was used to calculate ship hull vibration, greater discrepancies were found between theoretical calculations and actual measurements especially at higher modes. Thus the beam model cannot be considered as a practical one for higher-mode calculations. This paper presents the application of two-dimensional finite element model for the calculation of ship vertical vibration. Using the multi-element structural dynamic analysis program DDJ(DL) developed by ourselves, the hull vibration analysis of two ships (vessel A and vessel B) was carried out on the Model-709 Computer made in the People's Republic of China.

Literature Review : Wave-Induced Ship Hull Vibrations: a Review

Literature Review : Wave-Induced Ship Hull Vibrations: a Review

Blade rate radiation from merchant vessels

Acoustic radiation from merchant vessels at blade rate frequency and harmonics is caused by direct radiation from cavitation volume fluctuations and from forced vibration of the ship hull caused by unsteady propeller forces. Reciprocal measurements were made on a structural scale model of a 600‐ft tanker (1: 100 scale) to determine the relationship between forced ship hull vibration and radiated power. Unsteady propeller forces have been predicted theoretically in order to estimate the acoustic power radiated by ship hull vibrations. Analytical methods have been used to predict cavitation volume fluctuations and the acoustic power radiated by these fluctuations. For a typical single screw merchant vessel, it is shown that the acoustic power at blade rate and harmonics radiated directly from cavitation volume fluctuations will dominate over the radiation from the hull response to unsteady propeller forces.

A Review of Ship Hull Vibration: Part IV: Comparision of Beam Models

A Review of Ship Hull Vibration: Part IV: Comparision of Beam Models

Force method of ship hull vibration analysis

Many numerical methods--transfer matrix, finite difference and finite difference and finite element methods--which take piece-wise account of the variation of the input-datas over the length of the ship hull, such as area, weight, etc., are now available. Most of these methods give deflection and rotation as output in the eigenvalue problem or in the modal analysis, from which vibratory bending moments and shear forces for given exciting forces and moments are obtained after numerical differentiation of deflections and rotations of cross-sections. In order to avoid this numerical differentiation after the modal analysis, the force method can be employed, instead of the usual deformation method, for improved accuracy in ship hull vibration analysis. In this paper, the necessary differential equations of vibrations are derived and analogies between the force method and deformation method are established so that the existing programs in deformation quantities can be used. Moreover, the eigenfunctions of shear force and bending moment and the envelopes of their maximums for an impact force and moment under vertical vibrations are presented for an ice-going ship.

Literature Review : A Review of Ship Hull Vibration Part III: Methods of Solution

Literature Review : A Review of Ship Hull Vibration Part III: Methods of Solution

Literature Review : A Review of Ship Hull Vibration Part II: Modeling Physical Phenomena

Literature Review : A Review of Ship Hull Vibration Part II: Modeling Physical Phenomena

On the added mass of a rectangular cylinder moving in a rectangular channel

This paper deals with the effect of restricted waters on the added mass of rectangular cylinders moving in a free surface of rectangular channels, because the rectangular forms can be regarded a a representative of the main part of an actual ship. The moving directions of the cylinder are either parallel or perpendicular to the free surface. Moreover, the frequency of motion is either infinitesimal or infinite. That is, the former is concerned with the manoeuvring motion and the latter is concerned with the ship's hull vibration. The content of this paper overlaps a part of previous articles, but also includes new results obtained after those articles were completed.

Studies on lateral vibration characteristics of transverse member of large sized ships

One of the principal causes of the fractures around the slot of transverse web is considered to be the effect of lateral vibration of transverse member.In this report, at first, exciting tests were carried out with the bottom trans. model of 1/4 scale actual ship in order to investigate the natural frequencies and modes in detail.Secondly, during the sea trial of 224, 841 DWT and 284, 495 DWT oil tanker, comprehensive vibration measurements of ship hull and bottom trans. were carried out and those data were used to find response characteristics and excitation sources.Finally, the theoretical analysis of natural frequency and hydrodynamic pressure for the lateral vibration of trans. member were carried out and the former results were compared with the measured values.The results obtained are as follows : (1) Calculated natural frequencies and modes except for modes in water showed considerably good agreement with those of the measurement. Natural frequency of 2 nd order in water is less than or nearly equal to one of the O-node axial vibration of shaft system.(2) Hydrodynamic pressure distribution depends on natural mode of member itself, correlation between adjacent ones and the shape of bracket and face plate. Therefore, further investigation is necessary to estimate virtual added mass.(3) Engine and propeller induced axial forces of shaft system are mainly transferred to the bottom trans. and amplified through axial vibration of shaft system and ship hull vibration.

Application of Finite Element Method to Added Virtual Mass of Ship Hull Vibration (1st Report)

This paper presents a general method for the calculation of the three-dimensional hydrodynamic pressure by means of the finite element method and its application to the added virtual mass to the ship vibration. In applying this method to the ship vibration, the added virtual mass is obtained by assembling the pressures over the ship's wetted surface.Calculations were carried out on prisms of various cross-section, and comparisons were made between the values obtained by this method and those by the conventional analytical method. They were in good agreement for practical use.The paper also presents a calculation method of the added virtual mass to the vertical vibration of higher mode.