Twenty-First Symposium on Naval Hydrodynamics (1997) / Chapter Skim
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A Practical Prediction of Wave-Induced Structural Responses in Ships with Large Amplitude Motion
A Practical Prediction of Wave-Induced Structural Responses in Ships with Large Amplitude Motion
Pages 438-452
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1. Introduction In the traditional ship design the waveinduced ship motions, hydrodynamic forces and structural responses are evaluated directly or indirectly by applying the linear potential flow theory, especially the strip theory.
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The nonlinear part comes from the convolution of the impulse response function of the ship-fluid system and the nonlinear hydrodynamic force caused by slamming and nonlinear modifications of added mass, damping, restoring and wave forces. Unlike the previous ones, it can be used with high-speed strip theory or threedimensional flow theory and the frequency dependence of added mass and damping has been taken care of, to some extent.
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Ao, Bo and Ro represent the generalized linear fluid added mass, damping and restoring coefficient matrices. Forts is Be generalized linear wave force vector.
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The way we deal with the nonlinear hydrodynamic force implies that although large amplitude relative motion is introduced, the memory effect of the free surface is still treated in a linear manner. The slamming force in Eq.~8)
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Each plate is connected to the hull segments at four points via two stiff aluminum beams. A pinned joint is adopted to transfer only vertical forces due to wetdeck slamming without taking up the bending moments or shear forces between the two hulls.
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Fig. 1 Body plan of the catamaran model r 3 elastic weet deck plates r Elastic hull beam _ ~ .
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By investigating the measured bending moments and shear forces in the seven slender steel beams of the catamaran at two different forward speeds and three different regular wave headings, we find that most of them display a linear behaviour for most wave periods. However, severe nonlinear effects are found in the vertical shear force of the fore longitudinal elastic hinges when the ship model is running at forward speed ~ mlsec.
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pryingJsqueezir,E, Fig. 4 Calculated vibration shapes of the first four symmetrical flexible modes.
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The calculated linear vertical shear forces in the fore longitudinal elastic hinges as well as the measured time histories at wave periods 2.2 sec., 2.0 sec.
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40 30 20 o - l n -20 -30 -40 theory experiment s experiment p Fig. 9 Calculated total vertical shear force and the measurements in the fore longitudinal elastic hinges.
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Wave height = 0.087 m. wetted surface under the steady wave profile instead of that under the calm water as the mean wetted surface if it is available.
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to calculate the slamming force. This is a van Kannan type of solution where the pile-up water around the intersection between the free surface and the ship hull is not accounted for.
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The nonlinear part comes from the convolution of the impulse response function of ship-fluid system and the nonlinear modification in hydrodynamic forces. The method is then applied to a high-speed catamaran model running in regular head waves.
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(1989) , Hull Girder Loads in a Seaway Including Nonlinear Effects, Schiffstechnik, 36, pp.
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In order to make the present method even more attractive for practical use, the future development of it should aim at the solution of the following problems: · Wet-deck slamming: The figure below shows how wet-deck slamming dominates the hull girder loads of a 700 t SES in higher sea states (from calculations verified by tank tests)
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