Tomoki Ikoma

Time-domain analyses of elastic response and second-order mooring force on a very large floating structure in irregular waves

Real sea areas where very large floating structures are installed are random wave fields. Then, it is necessary to analyze the responses in the time domain due to directional waves. There exist hydroelastic deflections and slow drift motions in the responses of VLFSs in ocean waves. However, it is very time consuming to solve the equations of motion in the time domain and obtain these responses due to directional waves. It is unnecessary to solve such equations in the time domain, if mooring forces can be turned into on equivalent linear system. In this paper, we analyze the time-series responses without solving the equations of motion in the time domain. And, the corresponding model tests in uni- and two-directional irregular waves are carried out. The present method is validated by comparisons between the analytical and measured results of time histories, and moreover, the analytical method of a slowly varying wave drift force is also validated.

Slowly varying wave drifting force on a very large floating structure in short crested waves

Hydroelastic responses of very large floating structures (VLFS) have been studied activity in Japan. Most targets of them are hydroelastic behavior and an estimation method of linear hydrodynamic forces for the prediction of it. However a design of mooring system is very important to install a VLFS in a real sea area. Then, it is advisable that slowly varying wave drifting forces, which are the 2nd-order wave excitation, are predicted accurately. The authors have validated the theoretical method of the wave drifting force on the elastic floating bodies in long crested and crossing irregular waves by comparing with results of the model tests. It seems that irregularity and multi-directional quality greatly affect to the wave drifting force on a VLFS comparing to general scale floating structures. In this paper, characteristics of the slowly varying wave drifting forces on a VLFS are investigated systematically by numerical calculations with elastic behavior. A wave drifting yaw moment increases due to geometrical asymmetry of elastic behavior in short crested waves. Slowly varying wave drifting forces occurs due to interactions of wave frequencies in component waves. In case of the multi-directional irregular waves, there are effects due to the frequencys interaction of component waves in not only a same direction but also different direction. The terms of the latter effect are investigated in this paper.

Application of Wavelet Analysis for the Understanding of Vortex-Induced Vibration

Oceans are quite important fields for us because many resources lurk there which are oil and gas under seabed, mineral resources, water heat energy and so on. Development of submarine oil has been major in the North Sea and in the Gulf of Mexico. Today, submarine oil has been developed at ultra-deep water fields of offshore of Brazil and West Africa, which does deep over 1000m. Deepest field developed is more than 3000m in water depth of Brazilian seas. Riser system is necessary to develop and to production submarine oil. The riser is a tubing structure which is for drilling and production. Diameter of a drilling riser is greater than 50cm and that of a production riser is about 20cm. The riser is thin rope-like tube in oceans. Therefore, the tubing behaves elastically by marine currents and ocean waves and so on.

Feasibility Study of Floating Medical Support System on Big Disaster Installed in the River

An innumerable number of buildings in the coastal part of the east side of Japan were devastated by the Great East Japan Earthquake and Tsunami. Medical institutions also suffered the damage caused by tsunami, and the function to carry out the medical act has been spoiled. And the disabled concentrated on the medical institution which avoided tsunami damage. Under such circumstances, assistance from the water area began three days after the earthquake disaster, and much support supplies reached the stricken area. It was revealed that the support from the water area including the river was extremely effective against the stricken area. And then, the Tokyo Inland Earthquake disaster was assumed, floating medical support system installed in the quay of Arakawa River in Tokyo was proposed. Floating medical support system is movable; furthermore, it is built in the structural system of having very high isolation characteristics. Therefore, feasibility study such as selection of suitable setting position of this floating structure on Arakawa River, floor planning and flow planning at the time of always and a disaster, facilities and storage of various material, structural strength of the floating base, and the effects of disaster risk reduction was carried out about floating medical support system.

Development of a Ship-borne Wave Recording System

An on board system for wave data analysis and application to ship operation has been developed. The system consists of wave probes, accelerometers, a soft-ware of analysis and a wave data display tool. The former two are used to estimate the encountering wave surface elevations on ship hull. The main parameters of waves, i.e. the wave direction, the mean wave period or wave length and the significant wave height are calculated by using the encounter wave elevation data at three positions on ship side.The obtained wave parameters are used to show the situation of own ship at sea on the figures of operational guidance, so that the master can judge how to change the ship course or speed for preventing dangerous situations.This paper presents a simple method of wave analysis using cross spectra of wave data at three positions on ship sides and the examination of the method by numerical simulations and model experiments in directional waves. The test results on actual vessel at sea will be presented.