Tomoaki Utsunomiya

An eigenfunction expansion-matching method for analyzing the wave-induced responses of an elastic floating plate

An eigenfunction expansion-matching method is extended to the analysis of the wave-induced responses of an elastic floating plate using modal expansions of the structural motion. The plate is considered as a strip and assumptions of potential flow, wave linearity and small structural deformation are employed. Wave-induced responses such as the displacement, bending moment and shear force of the structure can be obtained with a few computations using this method. A comparison of the solutions based on this method with experimental results has shown good agreement.

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.

Evaluation of modal stress resultants in freely vibrating plates

In the dynamic analysis of a very large floating structure (VLFS), it is crucial that the stress resultants are accurately determined for design purposes. This paper highlights some problems in obtaining accurate modal stress-resultant distributions in freely vibrating rectangular plates (for modeling box-like VLFSs) using various conventional methods. First, it is shown herein that if one adopts the classical thin plate theory and the Galerkins method with commonly used modal functions consisting of the products of free-free beam modes, the natural boundary conditions cannot be satisfied at the free edges and the shear forces are completely erroneous, even when the eigenvalues have already converged. Second, it is shown that the problem still persists somewhat with the adoption of the more refined plate theory of Mindlin and the use of both (a) NASTRAN (that employs the finite element method) and (b) the Ritz method. The former method requires extremely fine mesh designs while the latter requires very high degrees of polynomial functions to achieve some form of satisfaction of the natural boundary conditions. Third, it is demonstrated that a modified version of the Ritz method, involving the use of a penalty functional for enforcement of the natural boundary conditions, also did not solve the problem when the plate is relatively thin. In fact, the method produces artificial stiffening to the plate. It is hoped that this paper will inspire researchers to develop an efficient technique for determining accurate stress resultants in a freely vibrating plate, apart from taking the brute force approach in having an extremely fine finite element mesh or using a very high polynomial degree.

Research on floating wind turbines: A literature survey

This article presents a literature survey of research and development on floating wind turbines. The various, proposed conceptual designs for floating platforms used for floating wind turbines are described and the working principles of these various floater concepts are outlined. This is followed by an overview of the research work that has been undertaken pertaining to floating wind turbine technology by several research institutes and the academic community. The research work undertaken to date is reviewed categorically according to the proposed floater concept (spar-buoy type, TLP type, semi-submersible type, pontoon type and others) as per sections 3–7. Based on the research work undertaken thus far, recommendations for future work are suggested.

Experimental Validation for Motion of a SPAR-Type Floating Offshore Wind Turbine Using 1/22.5 Scale Model

In this paper, motion of a SPAR-type floating offshore wind turbine (FOWT) subjected to wave loadings is examined. The proposed prototype FOWT mounts a 2MW wind turbine of down-wind type, whose rotor diameter is 80m and hub-height 55m. The SPAR-type floating foundation measures 60m in draft, having circular sections whose diameter is 12m at the lower part, 8.4m at the middle (main) part and 4.8m at the upper part. The FOWT is to be moored by a conventional anchor-chain system. In order to design such a FOWT system, it is essential to predict the motion of the FOWT subjected to environmental loadings such as irregular waves, turbulent winds, currents, etc. In this paper, the motion of the FOWT subjected to regular and irregular waves is examined together with the application of steady horizontal force corresponding to steady wind. The wave-tank experiment is made in the deep sea wave-basin at NMRI (National Maritime Research Institute), using a 1/22.5 scale model of the prototype FOWT. The experimental results are compared with the numerical simulation results for validation of the simulation method.Copyright

Trapped modes around a row of circular cylinders in a channel

Trapped modes around a row of bottom-mounted vertical circular cylinders in a channel are examined. The cylinders are identical, and their axes equally spaced in a plane perpendicular to the channel walls. The analysis has been made by employing the multipole expansion method under the assumption of linear water wave theory. At least the same number of trapped modes is shown to exist as the number of cylinders for both Neumann and Dirichlet trapped modes, with the exception that for cylinders having large radius the mode corresponding to the Dirichlet trapped mode for one cylinder will disappear. Close similarities between the Dirichlet trapped modes around a row of cylinders in a channel and the near-resonant phenomenon in the wave diffraction around a long array of cylinders in the open sea are discussed. An analogy with a mass–spring oscillating system is also presented.

SHEAR BUCKLING OF CORRUGATED PLATES WITH EDGES ELASTICALLY RESTRAINED AGAINST ROTATION

This paper presents an estimation of the elastic shear buckling capacity of corrugated plates with edges elastically restrained against rotation. The corrugated plate possesses higher shear buckling capacity compared to an unfolded flat plate. It has been used to replace the concrete web in PC box girders in recent bridge constructions in Japan. In this study, the corrugated plate is modelled as an orthotropic Mindlin plate. Elastically rotational restraint on boundary edges is taken into account in the form of rotational springs in the analysis. The prediction of buckling capacities of corrugated plates is carried out by using the Rayleigh–Ritz method, which was proved to be consistent with those as predicted by existing formulas for the limiting cases of simply-supported and clamped edges. The present study covers the more general case of elastically rotational restraint on the boundary edges showing transition curve of plate buckling capacities from the case of simple support to the case of clamped support.

DEVELOPMENT OF 3D ELASTODYNAMIC INFINITE ELEMENTS FOR SOIL-STRUCTURE INTERACTION PROBLEMS

This paper presents three-dimensional (3D) infinite elements for the multi-layered elastodynamics problems. There are three types of elements, namely horizontal, vertical and corner elements. They have been developed using wave functions in function spaces that can simulate real wave problems properly. The elements are extended forms of the axisymmetric infinite elements developed previously. Since these elements can simulate multiple layers and multiple wave numbers, the response of a 3D general structural system considering soil-structure interaction effect can be determined effectively. Numerical analyses are carried out for a rigid massless disk and square footings on the surface of various layer conditions for verification purposes. The calculated results are compared with existing analytical and numerical data and they were found to be in good agreement.

Motion analysis of a floating offshore wind turbine considering rotor-rotation

Development of offshore wind energy is desired as a countermeasure against global warming. This article presents motion analysis of a floating offshore wind turbine during the rotor-rotation under wind loads. A 2 MW downwind turbine is mounted on a floating foundation of the spar-type. The wind loads acting on the rotor blades are calculated using the blade element momentum theory. The multibody dynamics system theory is employed to consider the effect of the rotor-rotation. As a result, the motion of yaw, sway and roll are generated due to the effect of the gyro-moment for the rotor-rotation.

Dynamic Response Analysis of a Floating Offshore Wind Turbine During Severe Typhoon Event

In this paper, dynamic response analysis of a Floating Offshore Wind Turbine (FOWT) with Spar-type floating foundation is presented. The FOWT mounts a 100kW down-wind turbine, and is grid-connected. It was launched at sea on 9th June 2012, and moored on 11th for the purpose of the demonstration experiment. During the experiment, the FOWT was attacked by severe typhoon events twice. Among them, Sanba (international designation: 1216) was the strongest tropical cyclone worldwide in 2012. The central atmospheric pressure was 940 hPa when it was close to the FOWT, and the maximum significant wave height of 9.5m was recorded at the site. In this paper, the dynamic responses of the platform motion, the stresses at the tower sections and the chain tensions during the typhoon event, Sanba (1216), have been analyzed, and compared with the measured data. Through the comparison, validation of the numerical simulation tool (Adams with SparDyn developed by the authors) has been made.Copyright