Influence of boundary conditions on the collapse behaviour of stiffened panels under combined loads

A series of finite element analyses are conducted to investigate the influence of boundary conditions and geometry of the model on the predicted collapse behaviour of stiffened panels. Periodic and symmetric boundary conditions in the longitudinal direction are used to calculate the ultimate strength of stiffened panels under combined biaxial thrust and lateral pressure. The calculated ultimate strength of stiffened panels are compared with those by different FEM (finite element method) code and are assessed. The periodic boundary condition in the longitudinal direction for two spans or bays model provides an appropriate modelling to a continuous stiffened panel and can consider both odd and even number of half waves and thus, is considered to introduce the smaller model uncertainty for the analysis of a continuous stiffened panel.     

Experimental assessment of the ultimate strength of a box girder subjected to severe corrosion

The objective of this paper is to describe the experimental assessment of the ultimate strength of a severely corroded box girder subjected to a uniform bending moment resulting from four-point loading. Three box girders that could simulate the behaviour of midship sections have been deteriorated in corrosive seawater environment to simulate different levels of corrosion degradation of ageing ship structures. During the deterioration process, various parameters have been controlled and the total weight lost was registered. Corroded plate thicknesses have been measured in 212 points and a statistical analysis has been performed. The resulting corrosion wastage has been fitted by a non-linear time variant degradation model. The experimental results of the ultimate strength test of a severely corroded box girder subjected to a four-point loading have been analysed. The load-displacement and moment-curvature relationship is discussed, different failure modes are identified, and the strain gauges readings are analysed.     

初始缺陷对加筋板结构极限强度的影响研究

采用非线性有限元软件Ansys研究包含3种初始缺陷加筋板结构在单轴压缩载荷作用下极限强度的变化趋势,即初始变形、残余应力和凹陷.以单筋单跨加筋板模型为研究对象,考虑初始变形,不同半径及不同速度的撞击球所形成的凹陷及残余应力对加筋板极限强度的影响.结果表明,初始缺陷的存在降低了结构的极限承载力,而且缺陷的叠加作用比单独缺陷存在时对结构极限强度的影响大;加筋板的极限强度随着凹陷深度的加深而减小;3种初始缺陷中,残余应力相对于初始变形和凹陷对结构极限强度的削弱影响更大.     

A general scaled model design method of stiffened plate subjected to combined longitudinal compression and lateral pressure considering the ultimate strength and collapse modes

A general and efficient scaled method for stiffened plates subjected to combined longitudinal compression and lateral pressure is proposed based on slenderness, simply by adjusting the number of stiffeners. The design method makes it easier to determine the dimensions of the scaled model for a given scale ratio compared with the previously proposed method. The emphasis is on the influence of the plate slenderness, the column slenderness, and the non-dimensional parameter of the lateral pressure on the ultimate strength. By maintaining the consistency of the plate slenderness and column slenderness, the proposed method is applicable for designing scaled models with materials of different yield stresses and Young's moduli. A similar effect of the lateral pressure on the ultimate strength of the prototype and scaled models is achieved by maintaining the non-dimensional parameter. In addition, the applicability of the scaled method to the initial deflection is considered, which provides a reference for similar models. The similitude of the scaled method is verified for several typical buckling modes, including the beam–column, tripping, web and overall collapse modes. Given the numerical results, the proposed general and fast scaled method can provide reasonable dimensions of scaled stiffened plates subjected to combined loads.     

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

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:

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基于广义切线模量理论的铝合金加筋板结构轴压极限强度分析

研究铝合金加筋板结构的极限强度,有利于充分发挥铝合金材料的强度潜力和结构的后屈曲承载能力。本文将广义切线模量理论应用于铝合金加筋板结构的轴压极限强度计算,并推导了适用于加筋板的协调参数、极限弯矩系数及结构系数等相关参数的解析算式,建立了铝合金加筋板结构的强度利用率函数,从而得到结构的极限强度,进一步完善了广义切线模量理论在加筋板结构极限强度研究领域的应用。通过将广义切线模量法计算结果与经验公式、有限元计算结果进行比较,验证了其适用性与可靠性,可用于工程分析计算。     

Ultimate strength assessment of stiffened panel under uni-axial compression with non-linear equivalent single layer approach

Steel stiffened panels are widely used in engineering design and construction. However, numerical modeling and analysis effort for a three-dimensional (3D) stiffened panel may be notable, especially for the ultimate limit state of ship structures. Therefore, a homogenization method is outlined that transforms 3D stiffened panel into an Equivalent Single Layer (ESL) concerning the same mechanical behavior. ESL stiffnesses are obtained with a unit cell analyses based on stiffened panel where periodicity is imposed with boundary conditions based on a first-order shear deformation theory (FSDT). Stiffnesses were determined from the first derivative of a membrane force and bending moment obtained with numerical simulations. The effect of initial imperfection shape was included in the analysis to account for local and global buckling behavior. ESL with non-linear stiffness was implemented in Abaqus UGENS subroutine, allowing incremental evaluation of stiffness. Ultimate strength prediction of a steel grillage model with ESL finite element analysis was in excellent agreement with detailed 3D FEM analysis. The key in this analysis was consideration of non-linear ESL stiffness as linear analysis was unable to detect the point where ultimate strength capacity of the grillage was reached.     

大深度载人潜水器极限承载能力非线性有限元分析方法研究

大深度载人潜水器载人舱的极限承载能力是载人潜水器安全性的保障,在设计制造中非常关键。本文基于非线性有限元方法,对大深度载人潜水器球形载人舱极限承载能力进行了分析计算,提出了大深度载人潜水器球形载人舱极限承载能力非线性有限元计算中的影响因素;并与极限承载能力设计新公式计算结果进行对比,验证了本文非线性有限元分析方法和设计新公式的广泛适用性,为大深度载人潜水器建造规范和检验指南的制定奠定了基础。     

Assessment of IACS-CSR implicit safety levels for buckling strength of stiffened panels for double hull tankers

The present study aims at applying structural reliability methods to assess the implicit safety levels of the buckling strength requirements for longitudinal stiffened panels implemented in the IACS Common Structural Rules (CSR) for double hull oil tankers. The buckling strength requirements considered are used in the initial stage of the hull girder scantlings’ design to control the buckling capacity of longitudinal stiffened panels subjected to the compressive loads induced by the hull girder vertical bending. The following buckling collapse failure modes are explicitly considered in the design formulation: uniaxial buckling of the plating between stiffeners, column buckling of stiffeners with attached plating and lateral-torsional buckling or tripping of stiffeners.The paper presents the procedure used to assess the implicit safety levels of the strength requirements for the three buckling collapse failure modes above mentioned, which includes the optimization of the scantlings of the plate panels and longitudinal stiffeners in order to reflect the minimum strength required by the formulation. A first order reliability formulation is adopted, and stochastic models proposed in the literature are used to quantify the uncertainty in the relevant design variables. A sample of five oil tankers representative of the range of application of the IACS-CSR design rules is considered. The effect of corrosion in the implicit safety levels is quantified based on the three corrosion levels of the Net Thickness Approach (NTA) adopted in the design rules. Sensitivity analyses are also performed to quantify the relative contribution or importance of each design random variable to the implicit safety levels.     

Buckling strength assessment of corrugated panels in offshore structures

There is an increasing trend of living quarters in offshore structures being constructed using corrugated panels in order to save construction time and cost. Different from corrugated bulkheads in ship structures, the corrugated panels used in the living quarters are designed in triangular or trapezoidal profile with unequal flanges and with a corrugation angle between 45° and 90°. Industry needs have prompted the American Bureau of Shipping to develop design recommendations for the buckling strength assessment of these corrugated panels. This paper describes the main features and the principles of the recommendations based on ABS experience, along with the technical background. The modeling uncertainty associated with the recommended criteria has been established by comparing the predictions with laboratory tests and finite element analysis results. Two design examples are provided to demonstrate the application of the recommendations.     

Bi-fidelity Kriging model for reliability analysis of the ultimate strength of stiffened panels

A method based on a Bi-fidelity Kriging model is proposed for structural reliability analysis. It is based on adding low-fidelity data samples to the model to predict high-fidelity values, thus saving computational effort. Distance Correlation develops the correlation between the low and high-fidelity functions, initially proposed to assess the correlation between two variables. The bi-fidelity Kriging response surface model's efficiency as a surrogate model will be assessed for structural reliability problems that demand high computational costs, such as nonlinear finite element analysis structural models. The efficiency assessment is performed by comparing the accuracy of the failure probability predictions based on the Subset Simulation and First-order reliability method using the Bi-fidelity Kriging model as a surrogate for the performance function. The idea is illustrated by considering a representative component of marine structures analyzed by finite element analysis to create bi-fidelity scenarios to assess structural reliability with many variables. The results show that the proposed multi-fidelity method can provide an accurate failure probability estimation with less computational cost.     

Time-variant ultimate longitudinal strength of corroded bulk carriers

Many bulk carrier losses have been reported of late, and one of the possible causes of such casualties is thought to be the structural failure of aging hulls in rough weather. In aging ships, corrosion and fatigue cracks are the two most important factors affecting structural safety and integrity. This paper uses a set of the time-dependent corrosion wastage models for 23 different member locations/categories of bulk carriers previously developed by the authors, based on the available corrosion measurements for existing large bulk carrier structures. Differences due to the location and corrosion severity of every member type are taken into account. The nominal design corrosion values for the primary members are suggested based on the annualized corrosion rates obtained in the present study. The effect of time-variant corrosion wastage on the ultimate hull girder strength as well as the section moduli is studied. The criteria for repair and maintenance of heavily corroded structural members so as to keep the ultimate longitudinal strength at an acceptable level are discussed. Important insights and conclusions developed are summarized.     

Progressive collapse analysis of ship hull girders subjected to extreme cyclic bending

This paper introduces a novel analytical method to predict the buckling collapse behaviour of a ship hull girder subjected to several cycles of extreme load. This follows the general principles of the established simplified progressive collapse method with an extended capability to re-formulate the load-shortening curve of structural components to account for cyclic degradation. The method provides a framework for assessing residual hull girder strength following a complex series of unusually extreme load events where the wave induced bending moment rises close to, or even surpasses, the monotonic ultimate strength. These load events may be sequential, such as might be caused by a series of storm waves, or they may occur as a collection of discrete events occurring over a longer period. The extreme cyclic bending amplifies the distortion and residual stress initially induced by fabrication in the flanges of the girder, which results in a deterioration of the residual ultimate strength. Validation is firstly completed through a comparison with previously published experimental work and secondly via comparison with numerical simulation on four ship-type box girders using the nonlinear finite element method.     

Improved design formulas for the ultimate strength of platings with circular openings and manholes under uniaxial compression

The paper focuses on the ultimate strength of perforated platings with circular openings and manholes, eventually reinforced by ringed or carling stiffeners, in order to develop a comprehensive and rationale format, useful to assess the ultimate capacity of perforated plate panels under uniaxial compression. In this respect, a large number of FE simulations is performed by Ansys Mechanical APDL, in order to provide new design formulas for the ultimate strength of platings with circular openings or manholes and, subsequently, for perforated plate panels reinforced by local stiffeners. The design formulas are developed by properly varying the opening size and the scantlings of ringed or carling stiffeners, in order to provide a comprehensive set of design curves. Hence, the incidence of the opening longitudinal position on the ultimate capacity of perforated platings, without and with local stiffeners, is also investigated. Finally, the proposed equations are applied in a straightforward design example. Based on current results, the new design formulas allow a reliable assessment of the ultimate capacity of platings with circular openings or manholes and the incidence of local stiffeners on the plating ultimate strength, so providing a rationale design format that could be easily embodied in current Rules and guidelines.     

Numerical investigation on the ultimate strength behaviour and assessment of continuous hull plate under combined biaxial cyclic loads and lateral pressure

The ultimate strength of continuous hull plate under combined biaxial cyclic loads and lateral pressure is investigated in the present paper by using nonlinear finite element method. Geometric nonlinearity due to large deflection and material nonlinearity induced by kinematic hardening and isotropic hardening are both accounted for. A parametric study is designed and completed to examine the longitudinal ultimate compressive strength behaviours in the cycles on the basis of a large number of load-shortening curves. Effects of series of parameters especially the plate slenderness ratio, transverse cyclic compression, lateral pressure and cycle number are analyzed with details. It is found that the strength characteristic and collapse mode are highly affected by the coupling influence of the mentioned factors as well material behaviours. A dimensionless unified formulation as a function of the decisive factors is empirically proposed to accurately assess the ultimate strength of continuous hull plate in various cycles.     

均匀受压含裂纹损伤加筋板的极限承载能力分析

  目的  长期服役于恶劣海洋环境中的船舶与海洋工程结构,不可避免地会产生裂纹和点蚀,这些损伤会对结构的极限承载能力产生较大影响。为探讨裂纹、点蚀同时存在时对结构承载能力的影响,  方法  采用非线性有限元法开展含裂纹、点蚀损伤的加筋板在轴向压载作用下的极限强度研究。在讨论网格尺寸对含裂纹、点蚀损伤加筋板极限强度影响的基础上,开展裂纹点蚀坑相对位置、点蚀数目、裂纹长度对含裂纹、点蚀损伤加筋板剩余极限强度的影响。  结果  计算结果表明,裂纹长度、点蚀的增加会使加筋板的剩余极限强度下降明显。  结论  这些结果可用于指导全寿期船舶与海洋工程结构的设计与维护。     

Collapse strength of hatch cover of bulk carrier subjected to lateral pressure load

A series of elastoplastic large deflection FEM analyses has been performed on hatch covers of bulk carriers subjected to lateral load. Two types of hatch covers are considered which are the folding type and the side-sliding type. For the folding-type cover, one size is considered which is for Handy size bulk carrier, whilst for the side-sliding-type cover, two sizes are considered which are for Panamax and Cape size bulk carriers. For each type, two hatch covers are selected which are designed in accordance with the old ICLL rule and new IACS rule, respectively. Calculated collapse strengths are compared with individual design loads, and a strength assessment is performed. On the basis of collapse behaviour observed in FEM analyses, a simple method is proposed to evaluate the collapse strength of a hatch cover subjected to lateral load. It is confirmed that the collapse strength is accurately predicted by the proposed method.     

Analysis of the accident of the MVNakhodka. Part 2. Estimation of structural strength

In the early morning of January 2, 1997, a Russian tanker, the MVNakhodka, broke in two in the Sea of Japan. The fore part of the vessel drifted and was stranded on the coast of Japan, and the aft part sank. The coast of Japan was seriously polluted by spilled heavy oil. Following this disaster, the Japanese Government established a Committee for the Investigation of the Causes of the Casualty of theNakhodka. This paper deals with the structural strength of MVNakhodka at the time of the accident. First the structural characteristics of theNakhodka are described, and the reduction in thickness of the structural members are estimated based on the data measured on the fore part of the vessel which drifted ashose. Then the ultimate longitudinal strength of the hull girder at the time of the accident is evaluated by applying Smith's method, and the possibility of break-up collapse due to excess loads is discussed. The mechanism of fracture at the bottom plate is also discussed based on the observed fracture surfuce of the cross section. Finally an FEM (finite element method) simulation of the break-up of the hull girder is performed. It is shown that buckling/plastic collapse took place at the deck plate near Fr.153, which was followed by the successive buckling collapse of the side shell plate of the hull girder. Right after the collapse of the deck structure, the bottom plate fractured just in front of the transverse bulkhead at Fr.153. This article is based on an article that appeared in Japanese in the Journal of the Society of Naval Architects of Japan, vol. 183 (1998).     

Simulation on progressive collapse behaviour of whole ship model under extreme waves using idealized structural unit method

To ensure the safety of navigating ship, working loads and structural load-carrying capacity are two important aspects. In the present paper, a total simulation system combing load calculation and structural collapse analysis is applied to simulate progressive collapse behaviour of a single-hull Kamsarmax type bulk carrier. A three dimensional singularity distribution method is adopted to calculate pressure distribution with time history. A mixed structural model, collapse part simulated by ISUM elements and remaining part by elastic FEM elements with relative coarse mesh, is proposed for collapse analysis. Progressive collapse behaviour obtained by ISUM is good agreement with that by nonlinear software package, MARC. However, the calculation time of ISUM analysis is about 1/70 of MARC analysis. The applicability to structure system, high accuracy and sufficient efficiency of ISUM had been demonstrated.