International Shipbuilding Progress - Volume 52, issue 1

Authors: Selvam, R. Panneer | Bhattacharyya, S. K. | Haddara, M.

Article Type: Research Article

Abstract: A new frequency domain system identification method for estimation of hydrodynamic derivatives embedded in linear steering equations for ship maneuvering in calm seas is presented. The frequency domain multiple input-single output models developed for identification involves determination of constant, 'zero-frequency' hydrodynamic derivatives. The method is robust, non-iterative and computationally light and it does not require any starting estimates. In this method, the time domain operations are converted to linear operations in the …frequency domain. The responses of the ship in a few standard maneuvers have been simulated in the numerical examples and the proposed method is applied to this data in order to estimate the hydrodynamic derivatives for all possible 'identifiable' combinations. Show more

Citation: International Shipbuilding Progress, vol. 52, no. 1, pp. 5-27, 2005

Authors: Silva, S. Ribeiro e | Santos, T.A. | Soares, C. Guedes

Article Type: Research Article

Abstract: A method is proposed to deal with dynamic stability of ships in head seas based on a time-domain, non-linear numerical model of ship's motions in 5 degrees-of-freedom (sway, heave, roll, pitch, and yaw). A quasi-static approach is adopted for heave, roll, and pitch motions in waves, in which the calculation of significant variations on restoring coefficients is made using a pressure integration technique over the instantaneous submerged hull. Dynamic and hydrodynamic effects in waves for a …given encounter frequency are included in the response calculations, which are based on a strip theory. Comparisons between numerical and experimental results demonstrated the adequacy of the technique proposed to predict parametric roll in regular waves. The results demonstrated also that the method is capable of predicting wave-induced parametric resonance not only in regular but also in irregular waves. Finally, in the stochastic excitation a certain threshold value is assumed for parametric roll response and then the probabilities of exceedance are calculated as a function of the prevailing weather and ship's operational conditions. Show more

Citation: International Shipbuilding Progress, vol. 52, no. 1, pp. 29-56, 2005

Authors: Fonseca, N. | Soares, C. Guedes

Article Type: Research Article

Abstract: The paper presents a comparison between experimental data and numerical results of the nonlinear wave induced heave and pitch motions and structural vertical shear forces and bending moments on a containership. The experimental data was obtained from tests with a model of a containership advancing in head regular waves with different steepness, ranging from small to large amplitude waves. Strong nonlinear effects were detected on the experimental data, identified by: the variation of the transfer functions …with the wave amplitude, the existence of higher harmonics and the asymmetry of the response records with respect to the positive and negative peaks. The experimental data is compared with numerical results from a nonlinear time domain strip method. The method assumes that the radiation and diffraction hydrodynamic forces are linear and the nonlinear contributions arise from the hydrostatics and Froude-Krilov forces and the effects of green water on deck. The effects of vertical viscous forces due to flow separation are also investigated. The associated forces are represented by a semi-empirical formulation and depend of cross flow drag and lift coefficients. Nonlinear vertical motions and loads on ships advancing in waves have been investigated before, both experimentally and numerically however, the present paper focuses on several new aspects of the nonlinear vertical structural loads in regular waves. The new aspects include the identification and quantification of third harmonic amplitudes, the influence of the regular wave amplitude on the amplitudes of the first three harmonics and on the sagging and hogging peaks, and the influence of the steady structural loads, that exist also with the ship advancing in calm water, on the asymmetry of the vertical loads in waves. Finally, the influence of viscous forces associated with the large amplitude motions on the wave induced motions and structural loads are also investigated. Show more

Citation: International Shipbuilding Progress, vol. 52, no. 1, pp. 57-89, 2005

Authors: Rosén, A.

Article Type: Research Article

Abstract: The paper presents a method to reconstruct the momentary pressure distribution in hull-water impacts. Measurements of the propagating pressure pulse in one position of the hull at a particular time instant, are associated with measurements in other positions at other instants, by a set of assumptions and interpolation techniques. Hereby the complete pressure distribution can be reconstructed with the use of only a limited number of transducers. The method is evaluated with data from full-scale trials …with a planing craft in waves. Reconstructed pressure distributions are applied as loads in a finite element model of the hull. The resulting structural responses are calculated and compared with corresponding structural measurements with good accuracy. This confirms that the reconstructed distributions well resembles the real distributions. The method offers a powerful tool to better handle impact pressure measurements, and enables more detailed experimental analysis of the phenomena. Areas of application are for example in full-scale design evaluations and in evaluations of time-domain simulations of loads and motions for planing craft in waves. Show more

Citation: International Shipbuilding Progress, vol. 52, no. 1, pp. 91-107, 2005