Evaluation of ultimate strength of stiffened panels under longitudinal thrust

A series of collapse analyses is performed applying nonlinear FEM on stiffened panels subjected to longitudinal thrust. MSC.Marc is used. Numbers, types and sizes of stiffeners are varied and so slenderness ratio as well as aspect ratio of local panels partitioned by stiffeners keeping the spacing between adjacent longitudinal stiffeners the same. Initial deflection of a thin-horse mode is imposed on local panels and that of flexural buckling and tripping modes on stiffeners to represent actual initial deflection in stiffened panels in ship structures. On the basis of the calculated results, buckling/plastic collapse behaviour of stiffened panels under longitudinal thrust is investigated. The calculated ultimate strength are compared with those obtained by applying several existing methods such as CSR for bulk carriers and PULS. Simple formulas for stiffened panels, of which collapse is dominated fundamentally by the collapse of local panels between longitudinal stiffeners, are also examined if they accurately estimate the ultimate strength. Through comparison of the estimated results with the FEM results, it has been concluded that PULS and modified FYH formulas fundamentally give good estimation of the ultimate strength of stiffened panels under longitudinal thrust.     

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.     

Compressive tests on short continuous panels

Results of eight tests on stiffened panels under axial compression until collapse and beyond are presented. The tests consider panels with different combinations of mechanical material properties and geometric configurations for the stiffeners including the use of ‘U’-shaped stiffeners. The specimens are three bay panels with associated plate made of high tensile steel S690. Four different configurations are considered for the stiffeners that are made of mild or high tensile steel for bar stiffeners and mild steel for ‘L’ and ‘U’ stiffeners. The influence of the stiffener's geometry on the ultimate strength of the stiffened panels under compression is analyzed.     

纵环加筋圆柱薄壳的塑性屈曲试验与分析

本文进行了纵环加筋圆柱薄壳在均布外压作用下的屈曲试验和分析研究。采用塑性形变理论和Misos屈服准则，在能量原理基础上导出了壳体简明的屈曲分析公式。给出了四个铝合金模型的屈曲破坏试验结果，描述了模型的破坏特征和破坏过程。试验及数值分析结果表明，在一定参数范围内，壳体将发生总体塑性屈曲破坏，且破坏表现为一个过程，局部屈曲将影响到总体屈曲临界压力。本文提供的总体屈曲简明分析方法具有较好的精度，可供初步设计时使用。     

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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双层板架的屈曲分析

将大型船舶的双层板架模型化为正交异性极,考虑横向剪切变形和材料非弹性的影响,提出了一种在弹性角约束边界条件下求解双层板架临界应力的方法。利用该方法,对几种特定边界条件下一系列的矩形权进行了屈曲计算,其结果与已有文献的解答相比吻合较好。     

大深度潜水器耐压壳体弹塑性稳定性简易计算方法

通过在耐壳弹性屈密塞斯（Mises)公式中引入非弹性段的材料弹性模量,从而计及了材料弹性模量与应力之间的非线性关系,考虑了材料非弹性影响。通过对材料拉伸试验曲线进行回归分析,得到了材料切线弹性模量的近似表达式,同时采用外径取代平均直径,致使本文的方法在预测中厚壳的屈曲压力时具有较好的精度,本文给出了潜水器耐压壳体屈曲压力的简易计算方法和计算公式。经试验验证,该方法简易准确,可供初步设计时使用。     

A new look at the external pressure capacity of sandwich pipes

Sandwich Pipes (SPs) have been developed to overcome the required flow assurance and pressure capacity issues in deep and ultra-deep waters. This research aims at studying the influence of certain structural parameters on the pressure capacity (also referred to as the plastic buckling pressure) of Sandwich pipelines. The use of high grade steel pipes, as the internal or external pipes, has also been considered as one of the design parameters in this study. Moreover, a comprehensive parametric study, considering a practical range of the parameters that influence the response of SPs (and considering 3840 SP configurations) was conducted. The results from this large array of pipes were used to formulate a practical equation, capable of estimating the plastic buckling pressure of SPs. The accuracy of the proposed equation was evaluated by comparing the results with the experimental and numerical results available in the literature. The comparative results demonstrated that the proposed equation could predict the buckling capacity of such pipes with a reasonable accuracy. Furthermore, the proposed equation was used, along with a general optimization procedure, to establish the most optimum and cost-effective combination of structural parameters for SPs suitable for use in various water depths.     

Accuracy of nonlinear finite element collapse predictions for submarine pressure hulls with and without artificial corrosion damage

Nonlinear finite element (FE) collapse pressure predictions are compared to experimental results for submarine pressure hull test specimens with and without artificial corrosion and tested to collapse under external hydrostatic pressure. The accuracy of FE models, and their sensitivity to modeling and solution procedures, are investigated by comparing FE simulations of the experiments using two different model generators and three solvers. The standard FE methodology includes the use of quadrilateral shell elements, nonlinear mapping of measured geometric imperfections, and quasi-static incremental analyses including nonlinear material and geometry. The FE models are found to be accurate to approximately 11%, with 95% confidence, regardless of the model generator and solver that is used. Collapse pressure predictions for identical FE models obtained using each of the three solvers agree within 2.8%, indicating that the choice of FE solver does not significantly affect the predicted collapse pressure. The FE predictions are found to be more accurate for corroded than for undamaged models, and neglecting the shell eccentricity that arises due to one-sided shell thinning is found to significantly decrease the resulting accuracy of the FE model.     

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.