Masahiko Fujikubo

Elastic local buckling strength of stiffened plate considering plate/stiffener interaction and welding residual stress

Abstract An analytical formula for estimating elastic local buckling strength of a continuous stiffened plate subjected to biaxial thrust is derived considering the influence of plate/stiffener interaction and welding residual stresses. Through a comparison of calculated results with those by FEM eigenvalue analysis, high accuracy of the proposed formula is demonstrated. A series of buckling strength analyses is performed on the deck and bottom plating of actual ships. It has been found that: (1) In the case of stiffened plates in ordinary ships, an increase in the elastic local buckling strength as a result of stiffener torsional rigidity is almost compensated for by the influence of welding residual stresses. (2) For a thin plate under transverse thrust, higher elastic buckling strength can be expected than that specified in the classification societies’ rules by selecting an appropriate size for the stiffener.

New simplified approach to collapse analysis of stiffened plates

Abstract A new simplified model for collapse analysis of stiffened plates is developed in the framework of the idealized structural unit method (ISUM). By idealizing material and geometrical nonlinearities, larger structural units are defined as an element in ISUM than in conventional finite element analysis (FEA). The proposed stiffened plate model consists of ISUM plate elements and beam-column elements. The formulation of the plate element is performed by introducing accurate shape functions to simulate the buckling/plastic collapse behaviour of plate panels. Combining plate and beam-column elements allows for both local buckling of the plate panel and overall buckling of the stiffener. Fundamental collapse modes of plate panels and stiffened plates are investigated by conventional FEA. According to the observed characteristics, the new simplified model is formulated. Comparisons with FEA demonstrate the accuracy of the simplified model and its high applicability to typical stiffened plates in marine structures.

Plastic node method considering strain-hardening effects

Abstract Extending the basic theory of the plastic node method (PNM), a general theory for the elasticplastic analysis of structures considering strain-hardening effects is proposed. In the plastic node method, plastic deformations of a finite element are concentrated at the nodes, while the inside of the element remains elastic. A strain-hardening rate for the plastic nodal displacement is proposed by equating the plastic work done at the plastic node with that evaluated in the actual elastic-plastic stress distribution within the element. For framed structures, this nodal-point strain-hardening rate gives a reasonable definition of strain-hardening rate at a plastic hinge subjected to combined loads. Two yield criteria for beam-column and shell elements are employed, one of which is based on a full plastic condition, while in the second criterion an intermediate plastic state between the initial yielding and the full yielding of sections is taken into account. Applying the extended theory of plastic node method, several examples including an elastic-plastic large deflection analysis of plates are shown, and the validity and usefulness of the proposed method are demonstrated.

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

Analysis method of ultimate hull girder strength under combined loads

The objective of this study is to propose an analysis method of ultimate hull girder strength under combined bending and torsion. The hull girder is modelled by a series of thin-walled beam elements and the average stress–average strain relationship of plate and stiffened panel elements under axial loads considering the effect of shear stress is implemented in the beam elements. First, a torsional moment is applied to the beam model for a whole model within the elastic range. Then, the ultimate bending strength of cross-sections is calculated applying Smiths method to beam elements considering the warping and shear stresses. The proposed simplified method is applied to the progressive collapse tests of scale models under combined loads. On the other hand, nonlinear explicit finite element method (FEM) is adopted for the analysis of the test models. The effectiveness of the simplified method is discussed comparing with the results of experiments and FEM analysis.

Idealized Structural Unit Method for Collapse Analyses of Stiffened Plate Structures

Abstract A new model for collapse analysis of stiffened plate structures is formulated within the Idealized Structural Unit Method (ISUM). By idealizing material and geometrical nonlinearities, larger structural units are defined as an element in ISUM than in conventional Finite Element Analysis (FEA). The stiffened plate model consists of ISUM plate elements and beam-column elements. Comparisons with FEA show its accuracy in predicting the collapse behaviour of typical stiffened plates. Its applicability to system analyses is demonstrated by investigating box girder specimens.

Generalization of the plastic node method

Abstract In this paper, the plastic node method previously developed by the authors is further extended so that plastification can be examined at any point in the element including the nodal points. The accuracy of collapse loads obtained by the plastic node method should be improved by selecting suitable checking points for plasticity considering the characteristics of stres distribution assumed in the element. The theoretical background of the plastic node method is also argued, especially paying attention to the mechanism of the plastic deformation in the elements. As a result, it is proven that this method can generate plastic hinges, plastic hinge lines and plastic slip lines in the selected elements. Applying this generalized plastic node method, several examples are analyzed including the elastic-plastic analysis of solid bodies, and the validity and usefulness of this method are demonstrated.

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.

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

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.