Large Floating Offshore Structure Research Articles

A Study on Prediction Method of Time History Response of Very Large Floating Offshore Structure by Sea Shock Force

The objectives of present paper are to derive the approximate solution for comparatively easily computing the sea shock forces on developing numerical simulation method of sea shock force and to clear the effect of sea shock forces on very large floating offshore structures by the present approximation. Therefore the approximation solution and the boundary integral equation method on frequency domain are derived to understand fundamental characteristics of sea shock force. Further the accuracy and the usefulness of the present approximation is confirmed by comparing the results by the present approximation with the numerical results by the boundary integral equation method.Sea shock forces on very large floating structure are concretely understood by transforming numerical results in frequency domain into numerical results in time domain and basic data of earthquakeresistant design which is very important for implementation design is obtained.

Hydroelastic Responses of Pontoon Type Very Large Floating Offshore Structure

Hydroelastic Responses of Pontoon Type Very Large Floating Offshore Structure

Hydroelasitic Responses of Pontoon Type Very Large Floating Offshore Structures

Hydroelasitic Responses of Pontoon Type Very Large Floating Offshore Structures

Hydroelastic Responses of Pontoon Type Very Large Floating Offshore Structure

To design a very large floating structure, such as a floating airport, we have to estimate the hydroelastic response on any point of the very large floating structure exactly. We developed the estimating method of the hydroelastic response of a very large pontoon type floating structure. In this method, to obtain the hydrodynamic forces we used the pressure distribution method with Shallow draft assumption, considering the elastic motion of a floating body. The elastic response of very large floating structure was estimated by using the modal analysis of 2-D beam model.On the other hand, to observe the elastic response of a large pontoon type structure we carried out experiments in head sea and head+beam sea condition. The experimental model was a pontoon type large elastic floating body. This model consists of many box type rigid floats that were connected by longitudinal and lateral elastic beams. To measure vertical motions of multiple points of the elastic model we applied the optical and image processing system. We could measure the elastic motions and deformations of the model in 2-directional waves.To validate the corresponding of numerical estimating method we compare the numerical results with experimental results. From these results, we found good agreement and confirmed the accuracy of this numerical estimating method of the hydroelastic of a very large pontoon type floating structure. Furthermore, we discussed the hydroelastic responses and bending moments in shorter wave length with changing stiffness of the floating structure.

On the Response Characteristics of Motion and Deflection of Moored Long Flexible Floating Body in Waves

The responses of a large floating offshore structure, which has quite large length-to-breadth ratio and is considered as a flexible structure, are dealt with in this paper using both experimental and theoretical approaches.The experiments were carried out in regular waves, long-crested and short-crested irregular waves with two different types of flexual rigidity. According to the results of experiment, it is found that the flexibility of structure affects strongly the characteristics and amplitudes of motion response as same as deflection. The transfer functions of these response are calculated based on modal analysis using ‘wetted mode’. Also the influence of mooring system and directional distribution of irregular waves are discussed. The estimated response spectra in irregular waves and statistical values which are predicted from the response spectra are compared with measured one. Finally, the vertical motion characteristics along longitudinal direction in waves are investigated from viewpoint of deign of such floating airports, some studies and works necessary in future are proposed.

Experiments on Responses of Very Large Floating Offshore Structures in Directional Spectrum Waves

Experiments on Responses of Very Large Floating Offshore Structures in Directional Spectrum Waves

Experiments on Responses of Very Large Floating Offshore Structures in Directional Spectrum Waves

Many plans to construct of very large offshore structures of floating type are proposed, such as floating cities, airports, and so on. The size of floating structures is expected to become larger and the natural frequencies of structural vibrations become lower. In these conditions, wave exciting force and moment induce structural vibration. The flexibility of structures must be considered in the phase of not only operation but also construction. Therefore, it is important to study the response including the structural deformation.This paper deals with the experiments on the responses of very large floating offshore structures in directional spectrum waves. To clarify the structural response in waves, response functions of flexible models for experiments are estimated. For this purpose, the response function is divided into two transfer functions. One shows the relation between the wave height and the wave exciting force, and the other evaluates the structural deformation due to the wave exciting force in a modal space. The natural frequencies and mode shapes are evaluated by structure analysis program for simple models on the condition of floating. The two transfer functions are verified by experiments respectively and utilized to estimate the response function in waves. The estimated response functions show that the position in the structure differs the amplitude of response. So, there are some frequency regions in that the larger response causes at the lee side than the weather one. This characteristics are also shown in the results of experiments in transient water waves.Three types of models are used for experiments in directional waves. Difference among the three models is only the structural flexibility. The structures are moored by a single buoy of a TLP type. The experiments are performed in many waves. The measured responses are displacements in the inertial coordinate, structural deformations and tensions of the mooring system. In the experiments, the most flexible structure is designed as follow. When the structural natural frequency is same as the wave one, a shape of the structural deformation agrees with the wave length. So the response of the large flexible offshore structures are easy to be affected by characteristics of waves, directional distributions and frequency properties. And, it is also shown that the characteristics of response for flexible structure is different from ones of rigid structures.

Experiments on Responses of Very Large Floating Offshore Structures in Directional Spectrum Waves (second report)

Many plans to construct very large offshore floating structures are proposed, such as floating cities, airports, and so on. In the first report, the authors studied the response of the flexible structures in directional waves. The main results show that the flexibility of the structures affect their responses due to structural deformation. However, these responses are mainly in wave frequency regions. The low frequency or steady responses sometimes become sgnificant and cause fatal accidents. This paper is about the experiments conducted to investigate the responses of large structures in wide frequency region.In the first report, the structures are moored by a single buoy of a TLP type. Therefore, the large and slow oscillations are observed which cause rotation around yaw axis and displacement in sway direction. These oscillations cause difficulty in using such offshore structures for any purpose. In these experiments, described in this paper, another buoy is added to the mooring system in order to reduce such motions. Both ends of the structure therefore are moored by two buoys.The main purpose of this paper is to measure the responses of the structures in an oblique condition relative to the wave direction. In the experiments, two setting angles, 0 and 22.5 degrees are used to obtain comparative results. The results show that in the oblique condition the level of some slow oscillations decreased while the interactions between motions increased especially in low frequency responses.