Hiroo Okada

A system for collapse and reliability analysis of ship structures using a spatial element model

This paper is concerned with the ultimate strength and reliability analysis of ships hull girders, which are composed of spatial membrane elements and subjected to combined static loads and extreme wave loads. First, a new method is proposed for generating safety margins of general plate structures. A linearized failure condition of an element is introduced which takes account of the yielding and buckling collapse under the plane stress condition. The failure criterion facilitates the generation of safety margins and the calculation of failure probabilities. Structural failure is defined as the generation of large deformations due to collapse. Probabilistically dominant failure modes are selected to applying the so-called branch-and-bound method. Finally, the proposed system is applied to a spatial plate structure for main hull girder of a 150 000 DWT bulk carrier under two loading conditions. Through comparison between numerical examples and other results using simple hull girder models, the applicability of the proposed system is demonstrated.

Probabilistic Collapse Analysis of Offshore Structure

This paper proposes a method for probabilistic collapse analysis of an offshore structure. Wave loads are estimated by using Stokes third-order theory and Morison’s formula. Plastic collapsing is evaluated by taking account of the combined load effect to generate the safety margins, using a matrix method. Probabilistically dominant collapse modes are selected through a branch-and-bound method. The proposed method is successfully applied to a jacket-type offshore platform.

A Method for Reliability-Based Sensitivity Analysis of Ship’s Hull Structures Using Combined Plate and Frame Structure Models

This paper is concerned with reliability-based sensitivity analysis of ship`s hull structures, which are modeled by combined spatial plate and frame elements, based on the ultimate strength analysis. In this paper, at first, a newly proposed system using combined plate and beam elements is presented to offer the expansion of the applicability of the structural reliability-based sensitivity analysis combined with collapse mode analysis. Next, the applicability of the proposed system is demonstrated through a numerical example applied to a simple box girder with longitudinal stiffeners. Finally, features of the structural reliability and sensitivity factors of a high speed ship based on collapse mode analysis are also investigated by successively applying the newly developed system.

A study on deterioration of strength and reliability of aged jacket structures

This paper deals with the deterioration of strength for tubular members with a perforation due to corrosion as a part of reliability-based design of aged jacket structures. Firstly, a simplified method is presented for the residual strength of perforated members subjected to combined compressive load and bending moment by applying the reduced Perry-Robertsons or Carlsens method. It is also shown that the above estimation results for effects of the perforation on the ultimate strength deterioration are in comparatively good correspondence to analytical results based on nonlinear finite element method using shell elements. Finally, structures are modeled as a spatial framework and reliability analyses are carried out based on collapsing behavior analysis considering the ultimate strength of cylindrical tubular columns with a perforated damage under the combined loads, using a matrix method combined with a plastic node method. These methods are applied to reliability analysis of a 65m-class damaged structure. Aged effects on the deterioration of reliability level of the structure are also demonstrated through numerical examples.

A study on fatigue behavior of structural steels under multiple repeated loads

This paper deals with fatigue behavior of structure steels under multiple repeated loads as a part of basic studies on fatigue life estimation. First, a testing machine is produced for investigating fatigue behavior of structural steels under multiple repeated loads. Next, fatigue tests are carried out for smooth specimens of structural steel SS400. The fatigue behaviors such as the change of hysteresis loop energy and increasing rate of temperature of specimens under completely reversed tension and compression are investigated with much circumstance. An estimation method of remaining fatigue life is presented by using above results. The applicability of the method is also demonstrated by comparison with experimental results under multiple repeated loads.

A Study on Efficient Methods for Estimating Load Effects and Reliability and Their Application to Preliminary Structural Design of Rectangular-VLFS: 1st Report

This paper is concerned with efficient methods for estimating load effects and reliability and their application to preliminary structural design of box-type VLFS supposed as a floating airport, which is to be put in the west area of the Inland Sea of Japan (“Setonaikai”). In this study, as the first report, structural responses in oblique waves are investigated by using the efficient method based on Weighted Residual Method combined with thin plate and shallow water approximation presented by Tsubogo (2004). Effects of wavelength and wave incident angles on characteristics of the structural response are clarified from results obtained by numerical calculation. These results can be used to evaluate the load effect and the reliability in next report.Copyright

A method for estimating strength and reliability of stiffened plates for pontoon-type VLFS

This paper deals with a simplified method for estimating strength and reliability of stiffened plates for pontoon-type very large floating structures (VLFS), which are supposed floating airport, as a part of studies on reliability based preliminary design systems. Firstly, limit state conditions are shortly presented for the buckling and ultimate collapse strength of stiffened plates under combined compression, shear and lateral pressure in the deck, bulkhead and bottom parts of VLFS, especially, by using a simplified estimation formula. Next, the validity is shown by nonlinear finite element method. Finally, the reliability level is investigated through numerical examples for a 5,000 m-class VLFS under a trial design.

A Method for Structural Design of Pontoon Type VLFS Based on Collapse Behavior and Reliability Analysis

This paper deals with a simplified method for the preliminary design of pontoon-type very large floating structures (VLFS), which are supposed floating airport, based on collapse behavior and reliability analysis in irregular waves. Firstly, a simplified estimation method is presented for the probabilistic load effect model of VLFS under irregular sea-state conditions. Next, limit state conditions are shortly presented for the buckling and ultimate collapse strength of stiffened plates under combined compression, shear and lateral pressure in the deck, bulkhead and bottom parts of VLFS, especially, by using a simplified estimation formula. Then, the validity is shown by non-linear finite element method. Finally, dominant limit state modes of 5,000m-class VLFS under combined loads with bending moment, shear force and lateral pressure are obtained by applying the above methods. Then, the features of the collapse behavior and reliability level are investigated by using above calculation results. Effects of design parameters such as yield stress, plate thickness, stiffener and bulkhead space are also investigated using sensitivity analysis.© 2004 ASME

A Study on Structural Design of Huge Barge Structures Based on Collapsing Behavior and Reliability Analysis

This paper deals with the limit state and reliability analysis of huge barge structures (HBS) which are supposed floating refuse storage and incineration plant based on collapsing behavior analysis in irregular waves as a part of studies on structural reliability-based design methods. First, a limit state and reliability analysis method is shortly presented for the buckling and ultimate collapse strength of deck, bulkhead and bottom panels of HBS. Next, a simplified method is briefly introduced for collapsing behavior and reliability analysis of HBS under extreme sea loads by using a developed system combined with a finite element method and plastic node method using hexahedral element models. Moreover, a simplified estimation method is shortly introduced for the probabilistic load model considering the hydro-elastic response of the structure in irregular waves. Finally, dominant limit state modes of 1,000m-class HBS under combined loads with bending moment, shearing force and lateral pressure are obtained by applying the above methods. Then, the features of the collapsing behavior and reliability level are investigated by using above calculation results. Effects of statistical values such as reduction of thickness due to corrosion, yield stress and design parameters are also investigated using sensitivity analysis.Copyright

A Study on Structural Response Behavior and Strength of Long Submerged Floating Structures With Tension Legs in Waves

In this paper a submerged floating structure with tension legs is dealt with, the hydro-elastic response of which in long waves is investigated as a part of basic studies on marine space utilization structures. The analytical method is applied to the submerged floating structure. Then, the hydro-elastic characteristics of a floating structure model with a submerged plate and tension legs in regular long waves are investigated numerically. Next, in order to demonstrate the validity of theoretical results an experimental study is carried out for a basic model in regular waves. It was found that experimental results for the frequency response amplitude are in good correspondence to theoretical results in the low frequency range. Finally, characteristics of hydro-elastic responses of a 1,000m-class submerged floating structure under trial design are also investigated applying the above method.Copyright