Very Large Floating Structure In Waves Research Articles

Coupled analysis between catenary mooring and VLFS with structural hydroelasticity in waves

The rapid growth of marine renewables has led to the development of very large floating structures (VLFS) that are designed to operate in deep seas. It is significant to understand the mechanism of the coupled effects between deformable VLFS and catenary mooring system. This paper presents a time-domain hydro-elastic-moored model developed by integrating a quasi-static mooring module into a fully coupled Computational Fluid Dynamics (CFD) - discrete-module-beam (DMB) approach. The model is used to investigate the coupled effects between structural hydroelasticity and loose-type mooring systems on a deformable VLFS in waves. The mooring and hydroelasticity codes are validated separately and show favourable agreement with other numerical and experimental results. Then the coupled effects between the mooring system and structural hydroelasticity are evaluated by assigning various design parameters, i.e., VLFS structural stiffness and mooring stiffness. The numerical results, including dynamic motions, longitudinal vertical bending moments (VBMs) and mooring tension forces are presented and analysed. These results can be used to design a VLFS with mooring in medium-deep sea, and help with the conventional mooring design for a less-stiffness VLFS due to hydroelastic response.

Time domain calculation of connector loads of a very large floating structure

Loads generated after an air crash, ship collision, and other accidents may destroy very large floating structures (VLFSs) and create additional connector loads. In this study, the combined effects of ship collision and wave loads are considered to establish motion differential equations for a multi-body VLFS. A time domain calculation method is proposed to calculate the connector load of the VLFS in waves. The Longuet–Higgins model is employed to simulate the stochastic wave load. Fluid force and hydrodynamic coefficient are obtained with DNV Sesam software. The motion differential equation is calculated by applying the time domain method when the frequency domain hydrodynamic coefficient is converted into the memory function of the motion differential equation of the time domain. As a result of the combined action of wave and impact loads, high-frequency oscillation is observed in the time history curve of the connector load. At wave directions of 0° and 75°, the regularities of the time history curves of the connector loads in different directions are similar and the connector loads of C1 and C2 in the X direction are the largest. The oscillation load is observed in the connector in the Y direction at a wave direction of 75° and not at 0°. This paper presents a time domain calculation method of connector load to provide a certain reference function for the future development of Chinese VLFS

Study on a Structural Response Analysis Method for Very Large Floating Structures in Regular Waves (First Report)

The detailed design of Very Large Floating Structures (VLFSs) necessitates the structural response analysis of the complicated hull structure with three-dimensional models. Some two-step algorithms have been proposed, composed of the first-step classical Ritz's approach based on the free-free beam function expansions for the dynamic response analysis of the equivalent elastic rectangular plates and the second-step finite element one for the detailed three-dimensional structural analysis based on the first step output. However, they are rather intuitive. Scarce attention has been paid to their rationalzation.This paper deals with a one-step approach for the structural response of VLFSs in waves, as an early stage of an alternative rational multi-step approach. As an analysis tool, the general-purpose computer program previously developed for three-dimensional hydroelastic response of VLFSs in regular waves has been adopted, where NASTRAN for structural analyses is incorporated with the domain-decomposition type hybrid finite/infinite element method for water wave analysis and the modal analysis for structure water-wave interactions.Test calculations for a full three-dimensional finite element structural model of a 1200m box-like VLFS, composed of a deck, a bottom, side shells and bulkheads, have been conducted successfully and this has proved the applicability of the program to the structural response analysis. Comparative study for an orthotropic plate model and a sandwich grillage model has been conducted. Then, the sandwich grillage model, when appropriately adjusted, has shown to give a good approximation to the corresponding three-dimensional model.The results of these one-step analyses are also useful to validate the two-step ones.

The hydroelastic response of very large floating structure suppoted by footing-columns (2nd report)

A Very Large Floating Structure (VLFS) is expected as a new utilization of the ocean space. Rigidity of VLFS is, however, relatively small, so that an hydroelastic response of VLFS in waves should be considered. Many researchers studied on the elastic response of a pontoon type VLFS, in addition, on that of column support type VLFS. The VLFS supported by columns shows the smaller elastic response than that of the pontoon type VLFS. In the 1st report of this paper, we have discussed about hydrodynamic responses of a column with footing which has a wave-free frequency on wave exciting force and also examined the hydroelastic responses of a VLFS supported by a large number of the columns in regular waves.In this paper, we investigate on the charcteristics of hydroelastic behaviors of a VLFS supported by a large number of footing-columns which have two wave-free frequencies. And then, we disccuse about the hydroelastic behaviors of a VLFS in irregular seas based on spectral analyses. Furthermore we compare the hydroelastic responses of VLFSs among supported by columns without wave-free frequency, supported by columns with one wave-free frequency and supported by columns with two wave-free frequencies. In the analyses, the hydrodynamic forces of the single column are calculated by using 3-dimensional source sink method, and the hydrodynamic characteristics of VLFS are obtained by using hydro interaction theory.

Structural modeling for global response analysis of VLFS

For the structural design of very large floating structures (VLFS) of several thousand meters long, a hierarchical system of structural analysis must be developed, in which an idealized structural modeling is employed for the global response analysis while the local structural response is analyzed using a zooming technique. In this study, two types of grillage beam models, a plane grillage model and a sandwich grillage model, are considered and their applicability to the global response analysis of a pontoon-type VLFS in waves is examined. The influence of structural modeling on the buckling strength evaluation of VLFS is also studied.

Comparisons of hydroelastic behaviors of a pontoon-type VLFS and of a column-supported VLFS in waves

A very large floating structure (VLFS) for various applications such as a floating airport is generally classified into a pontoon-type, a column-supported type or a semisub-type. It is expected that large elastic deformations of such structures are induced by the dynamic effects of waves, because the horizontal dimension is extraordinary large compared to the thickness of the structures or of their upper deck. However, there exist few researches as to how the structural type of a VLFS affects its elastic deformations in waves. Recently, we have developed a prediction method of hydroelastic behavior of VLFSs. In this paper, the characteristics of wave-induced elastic deformations of a pontoon-type VLFS and those of a column-supported VLFS are compared using the developed method. It is found that the characteristics can be categorized in terms of the area ratio of the water surface area of the corresponding VLFS to the plain projected area of the whole structure rather than be categorized in terms of the structural type. This, in turn, implies that the relevant parameter that determines the elastic deformations of a column-supported VLFS is not the number of columns but the area ratio

Numerical Calculation of Hydroelastic Responses of Pontoon type VLFS

Very Large Floating Structures (VLFS) of several thousand meters long are being considered for various applications such as floating airports, offshore cities and so on. The VLFS recently designed have a thin mat-like configuration and very large horizontal size. A typical design, for example, is 5 Km long, 1 Km wide and 5 m high. Therefore this type of structure will be very flexible and the elastic deformation due to wave action will be more crucial than the rigid body motions.For the analysis of the hydroelastic behaviors of VLFS in waves, the conventional numerical techniques can be applied in principle. However, the direct application of such techniques is practically impossible, because the wave length is very small relative to the horizontal size of the structure and it requires enormous computational burden. There are many calculation methods proposed to overcome this difficulty. However, it is still very difficult for the case of the length of structure is more than one hundred times of incident wave length.Then the author propose a new numerical calculation method based on the 3-dimensional eigenfunction expansion method for the diffraction and radiation problems and the modal expansion method for the elastic response representation.This paper describes, at first, a new calculation scheme proposed then the numerical results compared with the model experiments carried out at various institutions which indicate good agreement each others. Finally, based on the calculations, it shows the various features of hydroelastic behaviors of typical VLFS including the effects of water depth, elastic stiffness and so on.