Kazuhiro Iijima

Investigation Into Post-Ultimate Strength Behavior of Ship’s Hull Girder in Waves by Analytical Solution

To rationally assess the consequence of a ship’s hull girder collapse, it is necessary to know the post-ultimate strength behavior of the hull girder including the global deformation and motions under wave-induced extreme loads. In the foregoing research, the authors proposed a numerical analysis system to predict the collapse behavior in waves including the post-ultimate strength behavior. In this paper, an analytical solution to describe the post-ultimate strength behavior is proposed. The primary objective of the present research is to clarify the parametric dependencies of the severity of the collapse in a rational manner. The parameters may include those related to load-carrying capacity and those related to the extreme loads. By comparing the numerical results and the present results, the analytical solution is shown to be effective. Some important parameters to predict the severity of the collapse are derived based on the analytical solution.Copyright

A Comparative Study of Motion Performance of Four Different FOWT Designs in Combined Wind and Wave Loads

This paper summarizes our recent collaborative/competitive works on floating offshore wind turbines (FOWTs) among four universities including Osaka Prefecture Univ., Osaka Univ., Yokohama National Univ., and Nihon Univ. The tasks assigned to each member were to develop the respective FOWT designs which could support 5MW class wind turbine, then to fabricate a scale model based on their own concept, and finally to evaluate the performance by tank tests under prescribed environmental (wind and wave) conditions. Osaka Prefecture Univ. adopted TLP concept, Yokohama National Univ. semi-submersible concept, Nihon University SPAR concept while Osaka Univ. also adopted semi-submersible, however, with single-point mooring. All the measured data were collected and compared among the four designs. It turned out that: (1) All the proposed deigns suffice criteria in terms of motion performance which were assumed at the beginning of the study. (2) The TLP type shows the most favorable performance among the four while the SPAR type shows largest acceleration in almost all the range of environmental conditions. The large acceleration may pose a problem of maintainability. (3) The SPAR type suffers the gyration effects more than the other types. (4) The RAOs of motions under combined wind and wave loads are almost the same as those under only wave loads for all the concepts but the single-point moored semisubmersible. (5) The difference of the RAOs for the single-point moored semisubmersible may be ascribed to the larger coupling effects between the main floater and the mooring system under the combined loads.Copyright

Characteristics of Buckling and Ultimate Strength and Collapse Behaviour of Cylindrically Curved Plates Subjected to Axial Compression

In the present paper, to clarify and examine the fundamental buckling behaviours of cylindrically curved plates subjected to axial loading, a series of elastic and elasto-plastic large deflection analyses as well as elastic eigen buckling analysis are performed together with the comparisons with the buckling behaviours of circular cylinder. On the basis of the numerical results, the effects of curvature, magnitude of initial imperfection, slenderness ratio and aspect ratio on the characteristics of the buckling and post buckling collapse behavior of cylindrically curved plates and circular cylinders under axial compression are discussed. The buckling strength and ultimate strength formulae of the cylindrically curved plate are empirically derived based on the FEM series analysis by curve fitting using least square method. The validity of the ultimate strength formulations developed in this study has to some extent been verified through comparison with nonlinear numerical solutions.

Scaled model tests for the post-ultimate strength collapse behaviour of a ship's hull girder under whipping loads

A series of experimental investigations on the post-ultimate strength collapse behaviour of a ships hull girder under whipping loads are presented. It is a follow-up study of the authors based on numerical simulations. One of the important conclusions of the previous work is that given the same magnitude of the loads, the collapse extent is smaller for the loads with the shorter duration. For the validation, a scale model with a scale ratio 1/100, which follows a law of similitude in the ultimate bending strength as well as geometry is employed in tank tests. The whipping loads are produced by dropping a mass object. The time history of the whipping loads is pre-adjusted by tuning the object mass, cushion material and dropping height. The hull girder bending moment with a time duration ranging 0.5–1.5 s in real scale, however, with the same magnitude, is applied to the hull girder. It was observed that the collapse extent was smaller for the loads with the shorter duration.

Conceptual Design of a Single-Point-Moored FOWT and Tank Test for Its Motion Characteristics

A new design concept of a semi-submersible type floating offshore wind turbine (FOWT) moored by a single-point mooring is proposed. The FOWT model adopting 5MW class wind turbine is designed. The motion characteristics of the FOWT are evaluated by a series of tank tests. To this end, a scaled model with a scale ratio 1/100 is fabricated. The scaled mode tests are performed under winds, waves, and combined winds and waves to check its fundamental feasibility. It is observed that the motion characteristics under wind and waves are acceptable in general, and the combination of the single point mooring and the down-wind type rotor is effective in terms of weathervane. It is also shown that the difference between the two transfer functions to wave loads, one with and the other without wind loads, is small except pitch response at low frequencies.Copyright

Coupled Aerodynamic and Hydroelastic Analysis of an Offshore Floating Wind Turbine System Under Wind and Wave Loads

A numerical procedure for the fully coupled aerodynamic and hydroelastic time-domain analysis of an offshore floating wind turbine system including rotor blade dynamics, dynamic motions and flexible deflections of the structural system is illustrated. For the aerodynamic analysis of wind turbine system, a design code FAST developed by National Renewable Energy Laboratory (NREL) is employed. It is combined with a time-domain hydroelasticity response analysis code ‘Shell-Stress Oriented Dynamic Analysis Code (SSODAC)’ which has been developed by one of the authors. Then, the dynamic coupling between the rotating blades and the structural system under wind and wave loads is taken into account. By using this method, a series of analysis for the hydroelastic response of an offshore large floating structure with two rotors under combined wave and wind loads is performed. The results are compared with those under the waves and those under the winds, respectively, to investigate the coupled effects in terms of stress as well as motions. The coupling effects between the rotor-blades and the motions are observed in some cases. The impact on the structural design of the floating structure, tower and blade is addressed.Copyright

Buckling/ultimate strength and progressive collapse behaviour comparison of unstiffened and stiffened curved plates subjected to axial compression

A ship structure is basically an assembly of stiffened panel elements and an estimation of the maximum load carrying capacity. The ultimate strength of these members is of the highest importance task for the safety assessment and rational design of the structure. In general, cylindrically curved unstiffened and stiffened plates are often used in ship structures, for example at deck plating with a camber, side shell plating at fore and aft parts and bilge circle part. It has been known that curvature increases the buckling strength of a plate subjected to axial compression, and is expected to increase also the ultimate strength. In the present paper, to clarify and examine the fundamental buckling and progressive collapse behaviours of unstiffened and stiffened curved plates subjected to axial compressive load, a series of elastic and elastoplastic large deflection analyses are performed applying non-linear FEM codes, ULSAS and ANSYS. On the basis of the numerical results, influences of curvature, magnitude of initial deflection, slenderness ratio and aspect ratio on the characteristics of the buckling/ultimate strength and progressive collapse behaviour of unstiffened and stiffened curved plates under axial compression are discussed.

Response Characteristics of Semisubmersible-Type-Megafloat in Waves and Accuracy of Hydroelastic Response Analysis Program VODAC

Technical feasibility and practical design method of SSMF, a Semi-Submersible-Type-Megafloat, was studied under research funding from Corporation for Advanced Transport and Technology. A SSMF which serves as a airport for local air transportation of isolated islands was assumed in the research. Typical deck size of the SSMF is 2200m long, 300m wide and 9m deep. In the design of the SSMF, column supported type structure was chosen to satisfy the strength and functional requirements in the environmental condition around Japanese islands in the pacific ocean. The deck structure is supported by 320 columns with draft of 16m. In the design of structural dynamics of the SSMF, a computer code VODAC was adopted to calculate hydro-elastic response. VODAC is an analysis program of hydro-elastic response of Very Large Floating Structure (VLFS) which has been developed in University of Tokyo. This paper presents a series of experiments and calculations carried out to investigate response characteristics of SSMF and to confirm capability and accuracy of VODAC. Basin experiments were carried out using a scale model, which is elastically and dynamically similar to the designed SSMF airport, and the response characteristics were clarified. A simple numerical model was also proposed as a simplified model of dynamic response of VLFS. This model is a simple beam on elastic foundation, but it is shown that basic response characteristics of the structure can be relatively accurately expressed by this model. Design parameters were discussed using this model. Relationships between major design parameters and dynamic response characteristics were clarified. Furthermore it was shown that this model is not just a qualitative model but gives relatively accurate estimation of the response. It is shown that this model gives upper limits of response of real structure and a good safe side estimation.Copyright

Strongly Coupled Method for Predicting the Response of Flexible FOWT With Mooring and its Experimental Validation

In this research, a numerical simulation method for a coupled system of a Floating Offshore Wind Turbine (FOWT) and its mooring system is developed. Flexibility of the platform and the nonlinear properties of mooring can be accounted for by the proposed method. A series of scaled model experiments which include the TLP and SPAR types of FOWT are also performed to evaluate the response of the FOWTs under combined wind and wave loads. Steady wind and regular waves are applied to the models. Measurements are made on strains in the structure, tension variation in the mooring as well as the rigid body motions of the platform. For validating the numerical model, comparison between the experimental and simulation results is made. An acceptable correlation between the experimental and the simulation results is obtained. It is shown that the flexibility of the platform may affect the tension variation in the mooring.Copyright

Designing process and motion characteristics of spar type offshore wind turbines

In this paper, we will discuss about the designing process and the motion characteristics of the spar type offshore wind turbines. When considering a spar type structure for offshore wind turbines, it is important to take a lot of elements into consideration which have not yet been considered in the case of oil and gas platforms. In this research, we used the following standards to conduct our tests. The limit of the heel angle was 5 degrees when the wind turbines are generating in the rated state. When designing the substructure for this research we have decided to go with a substructure that operates in depth of 100m or more. Following the conditions above we have designed the spar type offshore wind turbine used for this research. In order to compare the simulated result we have created a scale model and performed tank tests under various conditions. Also we observed unexpected motion characteristics in certain mooring arrangement. So we will touch these subjects in this paper.Copyright