Ken Takagi

An anti-motion device for a very large floating structure

We proposed a simple anti-motion device, which is a box-shaped body attached to an edge of VLFS. The performance of this device is investigated theoretically and experimentally in the present work. The theory is based on the eigen-function expansion method and the extension of this method to the oblique sea case is presented. The experiment is carried out in a wave tank with small-scale models. Both experimental results and the theoretical results show that the anti-motion performance of this device is very good at the design period in the case of beam-sea. The theory also demonstrates that this device reduces not only the deformation but also the shearing force and the moment of the platform and it also works well in the oblique sea.

Literature Review of Methods for Mitigating Hydroelastic Response of VLFS Under Wave Action

Presented herein is a literature review on the design and performance of antimotion structures/devices such as breakwaters, submerged plates, oscillating water column breakwaters, air-cushion, auxiliary attachments, and mechanical joints for mitigating the hydroelastic response of very large floating structures (VLFS) under wave action. Shapes of VLFS that could minimize the hydrodynamic response of the structure are also discussed. The analytical, numerical, and experimental methods used in studying the effect of these antimotion structures/devices toward reducing the hydroelastic responses of VLFS are also reviewed.

Flipper type crawler system for running on the irregular seafloor

Flipper type crawler system gives advantages when running on the irregular seafloor and when working on the seafloor. We developed a small size ROV (remotely operated vehicle) equipped with the flipper type crawler systems and conducted the experiments using it in a water tank and on the seafloor to observe its advantages. In the experiments in the water tank, the ROV was tested to run on the tilting table changing an angle and also to run over bumps nailed onto the top board of the tilting table. In the experiments on the seafloor, the ROV could evacuate from the condition that the crawler system was slightly submerged in the sand. Also the ROV could run and climb up the bumps. This paper describes the developed flipper type crawler systems and its advantages by describing the results of the experiments in the water tank and on the seafloor.

Study on the bow shapes above the waterline in view of the powering and greenhouse gas emissions in actual seas

In the shipping industry, as well as in other industries, a reduction in the greenhouse gas emissions is expected to help to reduce global warming. Shipbuilding companies are now developing ship hull forms to reduce greenhouse gas emissions with an emphasis on the vessel performance in actual seas where winds and waves are part of this environment. In this paper the bow shapes above the waterline, which may affect the added resistance in waves, are focused upon since the ship motion and wave reflection at the bow contribute to the added resistance in waves that can be reduced by an improvement in the bow shapes above the waterline. The relationship between the bow shapes above the waterline and the added wave resistance is investigated through tank tests in waves with model ships with different bow shapes. From the results of the investigation, the influences of the bow shape on the powering and greenhouse gas emissions is evaluated.

Experimental and theoretical study on the motion of ROV with crawler system

Mobility characteristic of ROV with crawler system in steady running was theoretically and experimentally investigated. It was found experimentally that flipper type crawler system gave advantages when running on the irregular seafloor and when working on the seafloor. On the other hand, the experiment suggests that the static or quasi-static analysis is not enough to estimate the motion of ROV with a crawler system. This paper also shows theoretical approach on evaluating dynamic effects of the ROV motion. The crawler motion is formulated by the lumped-mass method. A computer code has been developed and some numerical results are shown.

Navigation Simulation of a VLMOS for Wind Power Generation

Using one year measured wind data and wave hind- cast data, we investigate the statistical property of the wind and waves in the EEZ of Japan and its surrounding area in which we assume VLMOS navigates. It is confirmed that the stochastic distribution of the wind speed and the significant wave height is approximated by the Weibulle distribution. In addition, we perform a navigation simulation of VLMOS in which we used 144 h wave forecast data to increase the generated power and not to meet the high sea. Using the simulation results, the stochastic distribution of wave height experienced by VLMOS is discussed. It is found that deformation of the stochastic distribution obtained from the sailing simulation has a relation with its capacity factor and it is quite reasonable. The result of the fatigue assessment shows that VLMOS seems to have enough fatigue strength for twenty year operation even if its capacity factor exceeds 40%.

Dynamic motion of crawler-type ROV

A crawler system as a mobility function on the seafloor has the possibility to expand the activity of research and development on the irregular terrain and on the sand seafloor. To improve the mobility function, a flipper-type crawler system was developed. In addition, the dynamic motion of the crawler system investigation is important to realize the activity there because characteristic parameters in water such as buoyancy and hydrodynamic forces will considerably affect and lower the mobility. Actually, wheelie and slip were observed in preliminary experiments despite having been able to run stably on land. This paper also describes the theoretical and experimental studies evaluating the mobility performance and the effect of parameters on it.

Symmetric Wing Sections for the Switchback Motion of the Very Large Mobile Offshore Structure

We propose a new concept of VLMOS, which is composed of slender beams, demi-hulls and struts. The struts are wing-shaped, so that the wing induces a lateral force to counter the lateral wind drag force. The shapes of strut wings are symmetrical in the direction of the cord and their both ends have sharp edge. The main topic of this paper is to analyze the characteristic of the strut wings. We have carried out the model experiment and the calculation to estimate the performance of the strut wings.

Hydroelastic Behavior of a Long Flexible Structure in Irregular Waves

A simple 2D analysis on a long flexible structure, which is used for the floating type offshore wind power plant, has been performed to know the basic hydroelastic behavior of the structure. The method is not time consuming to cover wide-range combinations of the incident angle and the period. Using this method, the hydroelastic behavior of the structure in oblique irregular waves is investigated. It is found that the high stress region is very sensitive for the variation of the wave direction and the period. The variance of the stress is dramatically reduced, if we take the effect of the wave short-crestedness into account. The estimation of the long term stochastic property and the fatigue damage assessment has been also performed. It is found that the wave short-crestedness plays an important roll to lower the fatigue damage.

A Fatigue Design for Large Container Ship Taking Long-Term Environmental Condition Into Account

In order to promote the reliable evaluation for the fatigue strength of ships, the validation of a direct load and strength computation was performed based on the tank test and the full scale measurement. The fatigue damage in short term sea state under various operation parameters was indicated quantitatively by the tank test in irregular wave utilizing the elastic model. The long term fatigue damage was evaluated based on the full scale measurement [1]. The fatigue damage inferred from the measured stress on deck structure is quite small compared with the direct computation utilizing a full ship finite element model. That is mainly caused by the difference of environmental wave condition. Moreover, the effect of operational condition through whole life was indicated by the direct computation quantitatively. To make more rational fatigue design, it is important to take the long-term wave condition into account.Copyright