Time domain simulation of hydroelastic response of ships in large amplitude waves

Abstract

The influence of non-linearities on wave-induced motions and loads has been the focus of many investigations in the past few years and continues to be an important issue. A number of two- and three-dimensional methodologies have been developed, by and large, partly accounting for various non-linearities. Non-linear radiation, and to an extent diffraction, is the main problem and its solution via a three-dimensional method using Eulerian-Lagrangian schemes is likely to be complex and time consuming for practical applications. On the other hand two-dimensional methods, in spite of issues associated with accounting for forward speed, offer more possibilities of making practical advances in dealing with non-linearities.

A two-dimensional hydroelasticity analysis for symmetric (i.e. vertical motions, distortions and loads) dynamic behaviour in waves, including the influence of nonlinearities, is presented in this thesis using two methods. In the first method the total response is decomposed into linear and non-linear parts. The linear part is evaluated using the conventional two-dimensional linear hydroelasticity analysis. The nonlinear hydrodynamic forces are due to changes in added mass and damping coefficients, as well as restoring and incident wave forces, all evaluated over the instantaneous wetted surface. Non-linear forces due to slamming (bottom impact and flare) and green water (treated in a quasi-static manner) are also added. One aim of the thesis is to investigate the influence/importance of each of the non-linear hydrodynamic forces. Furthermore, the effects of assumptions made when using these hydrodynamic forces, e.g. frequency dependence of added mass, neglecting the damping coefficients in some components and evaluation of derivatives, are investigated. The solution in the time domain is obtained using direct integration and convolution integration, the latter based on the impulse response functions of the hull in its mean wetted surface. In the second method the response, including nonlinearities, is obtained from the solution of one system of equations of motion, where the added mass and damping coefficients and the restoring, incident wave and diffraction forces are evaluated at the instantaneous draft. Non-linear forces due to slamming (bottom impact and flare) and green water (treated in a quasi-static manner) are also added.

Both methods are applied to the S-175 containership, for which experimental measurements of motions and loads in large amplitude regular head waves are available. Comparisons made between predictions and measurements (heave and pitch motions, vertical acceleration and vertical bending moment) indicate good overall agreement. The comparisons also show that the influence of flare slamming is important for the range of speeds and wave amplitudes investigated.

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