ABSTRACT: To utilize floating structure under heavy environmental condition in sea safely, motions of the floating structure and the mooring forces have to be reduced. To decrease these motions and mooring forces, fins attached structure has been proposed. In this paper, as a first step of the study, two vortex methods, the Random-Walk method and the Core-Spreading method, are applied to vertical oscillating plate under free surface. Hydrodynamic forces on the plate are calculated and compared with experimental results. INTRODUCTION It has been required that the ocean space is effectively used in Japan surrounded by the sea and various floating structures for development of ocean resources or oceanographic survey, etc. have been proposed. In recent years, the floating structures for utilizing ocean energy such as offshore wind farms or ocean thermal energy conversions are required because of depletion of fossil fuels, global warming and the accident in the nuclear power plant by the eastern Japan earthquake. Moreover, the floating structure seems to be useful as disaster prevention bases or shelters, because the damage of the floating structures in the tsunamis caused by the eastern Japan earthquake were relatively small. Therefore, various types of floating structure will be required in future. To utilize floating structure in safely economically under severe wind and wave conditions around our country, motions of the floating body and mooring forces acting on the body have to be reduced. To decrease these motions and mooring forces, fins attached structure has been proposed (Fig. 1). The attached fin seems to dissipate the energy of the motion as viscous damping and reduce the mooring forces due to so-called "wave devouring propulsion" effect. To put this method to practical use, the calculation method to design optimum shape and arrangement of the fin has to be developed.

ABSTRACT: Many analytical methods have been proposed to calculate hydroelastic responses of a very large pontoon-type floating structure in waves. In this paper, variational principles considering wave radiation condition at infinity related to motions of a plate in waves, which are very important in calculations of the elastic response of the pontoontype floating structure, are discussed. First, Sommerfeld radiation condition at infinity is extended to treat a case with an incident wave. Second, four kinds of variational principles related to motions of the elastic floating plate on a water surface considering the incident and radiated waves are proposed and clarified the mutual relationship of these variational principles. Third, numerical results for elastic response of a floating plate of rectangular and L-shaped plan geometry in waves, which are obtained by using these proposed variational principles, are shown. INTRODUCTION A Pontoon-type VLFS is one of the typical structural types of very large floating structures (VLFS). Various numerical methods have been proposed to predict the hydroelastic response of this structure in waves (Watanabe (2004); Chen (2006)). These methods are classified into the modal expansion method and the direct method. These analyses are carried out in the frequency domain or in the time domain. Finite element method is used for the structure in order to analyze actual complicated floating structure (Seto et al. (1998); Utsunomiya et al. (2002)). In relation to the elastic response of Pontoon-type VLFS in waves, four kinds of variational principles related to elastic motions of such a floating plate were derived and made clear the mutual relationship of them (Isshiki (2000), Isshiki and Nagata(2001)). However, in these variational principles, wave radiation condition at infinity was not included and the normal velocity on a vertical cylinder surface of finite size surrounding the plate was designated.

ABSTRACT: The hydrodynamic force acting on a fin oscillating beneath a free surface is investigated. To safely utilize floating structures under severe environmental conditions in the sea, the motion of the floating body and the mooring forces acting on the body should be reduced. To decrease these motions and mooring forces, fins attached to the structure are proposed (Yago et.al 2007). In this paper, a flat plate was used as a fin. As a first step, the hydrodynamic forces acting on the fin oscillating near free surface were resolved in new method. This method uses the modified Morison equation added to a wave damping component. INTRODUCTION The amount of natural resources is decreasing because of the industrialization of developing countries. Because the amount of natural resources in Japan is limited and most of them are imported from abroad, the utilization of resources in an exclusive economic zone (EEZ) is planned. Japan has an EEZ domain that spans more than ten times the country area and is considered to have abundant resources under the seabed. Rare metals, minerals and natural gas can be found on the sea bottom. To develop these EEZs, a large-scale platform for natural resources, renewable energy and fisheries is planned. In Japan, the technology for a large-scale platform is developed. In the Mega-Float project in 2000, a platform with a length of 1km was tested in the Tokyo bay, and a feasibility study for a floating airport was performed. Additionally, an offshore platform that is used for the utilization of an EEZ was installed in severe wind and wave conditions as compared to the platform installed in coastal region. The installation of a breakwater that protects the platform from severe wind and waves is difficult in offshore area with deep sea.

ABSTRACT: In this paper, from the viewpoint of thrust generation by waves, we discuss the fluid dynamics of two phenomena. One is a thrust generation by hydrofoil placed at the bow under water of a ship, and the other is a negative drift force acting on a floating oscillating water column (OWC) type wave energy converter "Backward Bent Duct Buoy (BBDB)" in particular wave frequency range. A foil among waves can produce thrust. This may be utilized into ship propulsion in waves and reduction of mooring force of an ocean platform. We discuss the possibility of applying this phenomenon to ship propulsion and reduction of mooring forces. In regard to BBDB, the mechanism of thrust generation and its characteristics are investigated by 2-D and 3-D tank tests and it is shown that this characteristic is very useful to reduce the wave drift force acting on BBDB in irregular waves. INTRODUCTION In this paper, from the viewpoint of thrust generation by waves, we discuss the fluid dynamics of two phenomena. One is a thrust generation by hydrofoil placed at the bow under water of a ship, and the other is a negative drift force acting on a floating oscillating water column (OWC) type wave energy converter "Backward Bent Duct Buoy (BBDB)" in particular wave frequency range. Generally speaking, resistance of a ship becomes bigger in waves than in calm water. This phenomenon is well known to naval architects as the resistance increase of a ship in waves. According to the contemporary report, he really built two boats of 13 ft and 24 ft in length named Autonaut as shown in Fig. l, and the boat could move of their own motions due to waves against wind and wave at a speed of three to four knots.