We study hydrodynamics associated with motions of ships and offshore structures and their responses to ocean waves, current and winds. We develop new hydrodynamic theories to evaluate the forces on floating bodies, new hydro-elasticity theories for comparatively soft floating structures and new stochastic theories to know the statistical values in ocean waves, as well as completely new devices which have a special function.
(1) Higher order hydrodynamic forces
Slow drift oscillation of a moored offshore platform in ocean and low frequency hydrodynamic forces acting on the platform are the hottest topics which have received much attention during the last 15 years by marine hydrodynamicists. It is a resonance of the dynamic system due to several excitations. Difference frequency second order wave drift force is one of the most important excitations to the resonant response, the slow drift oscillation. Because it is a resonant response, the damping has a crucial role to determine the amplitude. Dominant damping comes from advance speed dependence of the second order wave drift force as well as the fluid viscosity. We have been investigating these higher order hydrodynamic forces acting on the platform in directional irregular ocean waves theoretically by the perturbation approach confirming by the model tests in a wave basin.(2) Dynamics of a moored floating structure in multi-directional ocean waves
Time domain simulation is very practical to predict statistical values of non-linear and non- Gaussian responses of a moored floating structure in directional ocean waves. We develope computer codes for time domain simulation of directional ocean waves which the Walsh function is applied in order to reduce computational time, and computer code for time domain simulation of non-linear responsed of non-linear motion of a floating body in multi-directional waves. We investigate the effects of wave directionality, multi-peaks of the wave spectrum and time duration on non-linear responses of a moored floating body.(3) Dynamics of a very large elastic floating structure
We investigate hydroelasticity of a very large offshore structure which may include not only difference frequency hydrodynamic forces, but also sum frequency forces. Since the panel number is limited to get hydrodynamical forces, we have to develope practical algorythms to get hydrodynamical forces which is robust and reliable, and requires less computational time.(4) Stochastic theory of the motions of the moored floating bodies in ocean waves
The horizontal motions of the moored floating bodies due to ocean waves can be approximated to the sum of the linear and the quadratic responses to the random waves. The quadratic responses change the shape of the p. d. f. s of the total responses, especially the tail of the p. d. f. s. Then rare events, which must be accurately estimated for the safety design, are underestimated by the conventional way. New theories which can accurately predict the p. d. f. of the total response, even the tail of it, are being developed and confirmed their accuracy through time series simulation and the model tests in a wave basin.(5) Ocean wave control
Ocean wave control by floating breakwaters or submerged breakwaters and ocean wave utilization are being studies. Several promising types of devices have been developed and theoretical models to predict their performance have been proposed. For example, the floating breakwater with minimized mooring line forces was developed and the reverse problem of ocean wave focusing was shoved.
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