A computational investigation of the effects of both-sides general corrosion on the buckling/plastic collapse behaviour and strength of stiffened plates

This paper presents the results of an investigation into the post-buckling behaviour and ultimate strength of imperfect corroded stiffened steel plates used in ships and other marine-related structures. A series of elastic–plastic large deflection finite element analyses is performed on stiffened steel plates suffering general corrosion wastage with random distribution. General corrosion is introduced into the finite element models using a random thickness surface model. The effects of corroded stiffened plate parameters on the post-buckling and ultimate strengths are evaluated in detail. The stiffeners of different symmetrical or unsymmetrical cross-sections are introduced into the models for analysis. Some distinctions are explored and highlighted between the behaviours of steel plates suffering general corrosion in unstiffened and stiffened cases. Finally, a proposal is given in order to simulate the average stress–average strain relationship of stiffened steel plates having both-surface general corrosion wastage.     

Computation of ultimate strength of locally corroded unstiffened plates under uniaxial compression

Over the past decades there have been many losses of the merchant vessels due to either accidents or exposure to large environmentally induced forces. The potential for the structural capability-degrading effects of both corrosion and fatigue induced cracks are of profound importance and must be both fully understood and reflected in vessel's inspection and maintenance programme. The present study is focused on assessing the effects of localized pitting corrosion which concentrates at one or several possibly large area on the ultimate strength of unstiffened plates. Over 256 nonlinear finite element analyses (FEA) of panels with various locations and sizes of pitting corrosion have been carried out. The multi-variable regression method is applied to derive new formulae to predict ultimate strength of unstiffened plates with localized corrosion. The results indicate that the length, breadth and depth of pit corrosion have weakening effects on the ultimate strength of the plates while plate slenderness has only marginal effect on strength reduction. Transverse location of pit corrosion is also an important factor determining the amount of strength reduction. When corrosion spreads transversely on both edges, it has the most deteriorating effect on strength. It was also found out that the proposed formulae can accurately predict the ultimate strength of unstiffened plate with localized corrosion.     

含点蚀损伤船体加筋板在纵向压力下的极限强度

针对老龄化船舶结构上的点状腐蚀,利用非线性有限元方法进行计算,分析304个船体加筋板的极限强度,探讨带板柔度、加强筋柔度、腐蚀面积比和腐蚀深度比对纵向压力下含点蚀损伤船体加筋板极限强度的影响,拟合出点状腐蚀下船体加筋板极限强度折减公式并对其适用性进行验证,研究结果具有一定的工程参考意义和价值。     

点蚀板在纵向压力下的极限强度研究

本文着眼于老龄化船舶结构上的局部点状腐蚀和整体点状腐蚀,利用非线性有限元软件分析了超过100个船体板结构的极限强度。研究了蚀坑形状、蚀坑位置、蚀坑大小、蚀坑深度和板的柔度对含局部点状腐蚀船体板的极限强度的影响,蚀坑分布、板的柔度和腐蚀体积对含点状腐蚀船体板的极限强度的影响。拟合出了单面、双面局部点状腐蚀下的船体板极限强度折减公式,单面、双面点状腐蚀下的船体板极限强度折减公式。并得到同时适用于局部和整体点状腐蚀板极限强度的公式。     

Welding-induced residual stresses and distortions of butt-welded corroded and intact plates

The welding-induced residual stresses and distortions of butt-welds between non-corroded and corroded steel plates are studied experimentally and numerically, simulating a repair in an aged structure. Butt welds are made between plates with different corrosion levels and intact ones, and temperature distributions and distortions have been measured. Finite element analyses of the butt-welded corroded and non-corroded plates are performed to estimate the welding-induced distortions and residual stresses for different levels of corrosion degradation. The environmental conditions during the welding repair process are also accounted for. Experimentally evaluated mechanical properties of corroded steel plates as a function of the temperature during the welding process are employed in non-linear finite element analyses. The importance of the material properties as a function of the severity of the steel plates' corrosion degradation on the welding-induced distortions and residual stresses is also quantified. This study shows the importance of considering the ageing of steel in the repair process. The developed methodology allows critical operational decisions to be made with greater ease and confidence in analysing the welding-induced distortions and residual stresses when ageing structures are repaired to extend their service life. Furthermore, different welding sequence scenarios may be analysed to identify the most appropriate repair solution in minimising the repair cost.     

Remaining fatigue life assessment of corroded mooring chains using crack growth modelling

Failures caused by the combined actions of fatigue, corrosion and wear are important safety concerns for mooring chains used on floating structures in the oil and gas industry. Prediction of remaining corrosion fatigue life based on surface condition could therefore be a useful tool for the continued safe operation of corroded chains. This paper investigates the use of crack growth modelling for estimating the remaining corrosion fatigue life of mooring chains that exhibit significant pitting corrosion damage. A crack growth modelling approach is used to produce remaining fatigue life estimates for a selection of severely pitted mooring chains. Using fatigue crack growth rate test results for grade R4 high strength mooring chain steel, empirical crack growth laws are presented for free corrosion and cathodic protection conditions at load ratio R = 0.1. Two different methods for establishing equivalent cracks from surface scans of corrosion damage are presented. The mooring chains are proof loaded as part of their manufacturing process. Residual stresses introduced during this process have therefore been determined by finite element analysis and accounted for in the fatigue crack growth predictions. One of the equivalent crack models, accounting for the single dominant corrosion pit, provided quite accurate fatigue life predictions when compared with full scale test results.     

Effect of pitting corrosion on the ultimate strength of steel plates subjected to in-plane compression and bending

Corrosion pits with a circular cone shape are typically observed on coated hold frames of aged bulk carriers which carry exclusively coal and iron ore. In order to ensure the safety of these types of bulk carrier, it is necessary to understand the effect of pitting corrosion on the local strength of hold frames. In order to investigate this effect, a series of nonlinear finite-element (FE) analyses has been performed with pitted plates subjected to in-plane compressive loads and bending moments. It has been shown that the ultimate compression load or bending moment of pitted plates is smaller than that of uniformly corroded plates in terms of average thickness loss, and that predictions of the ultimate strength using the average thickness loss at the minimum cross section would be conservative. In order to establish a method of evaluating strength reduction due to pitting corrosion, it is important to identify the failure mode that would be most detrimentally affected by pitting corrosion. It was found that the reduction of the ultimate compressive load or bending moment due to pitting corrosion is smaller than that of the tensile strength in terms of equivalent thickness.     

考虑腐蚀影响的船舶结构可靠性研究现状与展望

对近年来考虑腐蚀影响的船舶结构可靠性计算各主要研究层面的研究现状做了介绍与评述.通过这一综述可以发现:此领域研究工作重心已由考虑线性、非线性均匀腐蚀的影响逐步向局部腐蚀尤其是点腐蚀方向转变.今后的研究重点为以下三个方面,即:①发展符合基本腐蚀机理、与实测腐蚀数据较为吻合的点腐蚀多指标模型;②确定蚀点几何参数在各类荷载条件下对构件强度的影响;③深入评估腐蚀与疲劳的相互作用对结构可靠性计算的影响.     

Compressive strength of tubular members with localized pitting damage considering variation of corrosion features

Localized pitting corrosion often occurs on marine and offshore structures in the form of patch corrosion with great uncertainties in the location, size and shape. The variation of corrosion features affects ultimate strength of tubular members significantly, but it is still not well understood. This paper presents a numerical study on tubular members of diverse slenderness ratios to clarify the localized pitting effect on ultimate strength. Numerical analyses were performed based on novel models of pitted members that were calibrated against benchmark column tests. Corrosion pits were randomly introduced on the local outside surfaces of members via stochastic simulation, forming corrosion patches varied in the location, size and shape. Numerical results obtained were regressed to propose a unified empirical formula to predict ultimate strength. It turned out that the shape of the corrosion patch has a significant influence on the ultimate strength. The shape change of the patch can alter failure modes of medium length columns. The reduction of ultimate strength is closely related to the shape ratio of the patch besides the volume loss of corroded material. The unified empirical formula incorporating the shape ratio and the volume loss shows a good ability to predict the experimental results.     

Propagating buckles in corroded pipelines

Rigid–plastic solutions for the steady-state, quasi-static buckle propagation pressure in corroded pipelines are derived and compared to finite element predictions (ABAQUS). The corroded pipeline is modeled as an infinitely long, cylindrical shell with a section of reduced thickness that is used to describe the corrosion. A five plastic hinge mechanism is used to describe plastic collapse of the corroded pipeline. Closed-form expressions are given for the buckle propagation pressure as a function of the amount of corrosion in an X77 steel pipeline. Buckles that propagate down the pipeline are caused by either global or snap-through buckling, depending on the amount of corrosion. Global buckling occurs when the angular extent of the corrosion is greater than 90°. When the angular extent is less than 90° and the corrosion is severe, snap-through buckling takes place. The buckle propagation pressure and the corresponding collapse modes also compare well to finite element predictions.     

锈蚀钢筋混凝土大偏心受压柱的数值模拟

基于锈蚀钢筋混凝土偏心受压构件承载力试验研究结果,运用ANSYS有限元分析软件,对锈蚀钢筋混凝土大偏心构件进行分析,通过温度场模拟钢筋锈蚀作用,得到锈蚀构件破坏特征及承载力随锈蚀程度的变化规律。经对比试验结果,该有限元模型数值计算与试验结果能较好吻合,温度场模拟钢筋锈蚀较理想。对不同直径钢筋影响受压柱承载力和裂缝分布规律进一步分析,结果表明:同等条件下,随着钢筋直径增加,钢筋锈胀力减小,裂缝开裂分布区域发生变化。     

CFRP修复的FPSO点蚀船体梁极限强度分析

采用非线性有限元法对中拱和中垂工况条件下碳纤维增强聚合物(Carbon Fiber Reinforced Polymer, CFRP)修复的浮式生产储卸油装置(Floating Production Storage and Offloading, FPSO)点蚀船体梁极限强度进行仿真分析。对比FPSO的完整船体梁、点蚀船体梁和CFRP修复的点蚀船体梁的中拱极限弯矩和中垂极限弯矩，分析CFRP对FPSO点蚀船体梁的修复效果，并分析胶层失效规律。结果表明，CFRP可为船舶的高效修复提供一种新的方式。     

Cyclic behavior and strain energy-based fatigue damage analysis of mooring chains high strength steel

This study investigates the low-cycle fatigue behavior of mooring chains high-strength steel grade R4 under different strain amplitudes and strain ratios at room temperature. A fatigue test program has been carried out on small low cycle fatigue specimens cut from large mooring chains. The experimental results characterize the cyclic stress-strain relationship, the mean stress relaxation behavior, and the cyclic plasticity parameters of the material. Strain energy density is correlated with fatigue life through a simple power-law expression and very well represented by Basquin-Coffin-Mansion relationship. Further, a non-linear elastic-plastic material model is calibrated to the experimental stress-strain curves and used for the estimation of energy dissipation in the specimens under applied cyclic loads. The predicted fatigue life using the calibrated material parameters demonstrates a close agreement with the experimental fatigue life. Numerical simulations are carried out to analyze local plastic straining and assess crack initiation at the pit site of corroded mooring chains considering the multiaxial stress state. An energy-based approach is employed to estimate the number of cycles needed for a crack to initiate from an existing corrosion pit.     

碳钢、低合金钢全浸没海洋环境下最大点蚀深度简化模型

腐蚀是导致老龄船舶结构失效的主要原因之一。结合基本点蚀原理,文章对碳钢、低合金钢海水全浸没点蚀的主要影响因素、具体点蚀进程做了简要解释与评述。基于Melchers点蚀深度模型及其相关实验数据,文中提出一种简化形式的新型点蚀最大深度模型,并采用该模型对三组我国船舶结构常用碳钢、低合金钢的青岛海域全浸没点蚀试验观测数据进行分析。通过对比验证,证明采用Weibull函数表示点蚀最大深度随时间变化关系是合适的。此外,依据青岛、厦门、榆林海域碳钢试验数据,文中还对海水环境因素,如:溶解氧浓度、平均温度、盐度、PH值等,以及钢材成分变化对新型最大点蚀深度模型各参数的影响进行了初步探讨,得出了相应的函数关系式及相关结论。     

Statistical characterization of surfaces of corroded steel plates

The statistical characteristics of corroded steel plate surfaces exposed to marine environments are of interest for assessing longer term structural safety and integrity using probabilistic methods. This requires information about the variability of corrosion loss and pitting over surfaces. The present paper reports on the observed statistical character of the surfaces of 10 large (1.2 m × 0.8 m × 3 mm thick) steel plates exposed in temperate climate marine immersion, tidal and splash zones for 2.5 years. For the analysis the plates were cut into smaller segments that were mechanically scanned to obtain digitised surface topographies. These were then analysed to estimate the correlation structure and the standard deviation of the surface topography. Considerable differences were found for these and for the mean corrosion loss between different exposure zones. For any one segment the surface topography was found to be highly statistically dependent, implying that smaller coupon sizes can provide adequate estimates of corrosion loss. From this it may be inferred that the deepest pits are not statistically independent as commonly assumed in extreme value statistical representations.     

点蚀损伤板在复杂受力状态下的极限强度研究

采用非线性有限元软件模拟船底板单面点腐蚀,通过对边缘载荷系数比、蚀坑分布、直径、深度的改变,经过一系列的数值计算分析,结果表明横向载荷和侧压对板的极限强度影响不可忽略,计算板的极限承载能力时需要考虑计及侧压与横向载荷。在复杂受力状态下,就单考虑蚀坑深度时,板表面的蚀坑深度在0~0.5t区间变化时,极限强度相应的折减率比大于1/2板厚时更大,极限强度对蚀坑深度更加敏感。腐蚀面积不能有效表征腐蚀程度,当DOP系数相同时,点蚀后板的最小横截面决定了极限强度的大小。     

氯盐和硫酸盐侵蚀下RC梁受弯性能分析

考虑到氯盐对钢筋锈蚀会引起钢筋横截面损失和粘结强度下降,硫酸盐对混凝土侵蚀会引起混凝土强度改变,因此,建立受腐蚀钢筋混凝土（ reinforced concrete ,RC）梁的有限元分析模型,利用大型有限元ANSYS分析软件,对腐蚀梁的受力性能进行模拟分析,模拟结果与实验结果吻合,说明采用非线性有限元方法分析锈蚀梁受力性能正确有效。     

An approximate method to generate average stress–strain curve with the effect of residual stresses for rectangular plates under uniaxial compression in ship structures

The average stress–strain curve for rectangular plates under uniaxial compression in ship hull structures is generated on the basis of the existing design formulae. The dual-term equation for ultimate strength of uniaxially compressed plates is employed to generate the average stress–strain curve for long plates. Two alternatives proposed by Faulkner and Frankland, respectively, are considered. An approximate method to take into account the effect of residual stresses is proposed. By investigating the variation of the stress distribution at different stages of compression, the average stress–strain relationship of the rectangular long plates with the effect of residual stresses is derived from equilibrium. The behaviors of the wide plates are also discussed. The assumption proposed by Valsgard for the idealized stress distribution of wide plates under uniaxial compression is adopted. The average stress–strain relationship for wide plates is derived from the existing design formulae. Two equations proposed by Hughes and recommended by API for critical stress of the central portion of the wide plate are considered. The effect of residual stresses is also taken into account for wide plates.     

Probabilistic modelling of the ultimate strength of ship plates with non-uniform corrosion

This paper presents the results of a parametric study of probabilistic modelling of the ultimate strength of ship plates with non-uniform corrosion represented by random fields. The load-shortening behaviour of the plates with non-uniform reduction of thickness due to corrosion under longitudinal compression is obtained using a general-purpose nonlinear finite element analysis program. A nonlinear time-dependent corrosion model is used to define the probabilistic characteristics of the random fields based on corrosion data measured in plate elements at different locations of bulk carriers. Based on the probabilistic models derived by Monte Carlo simulation, equations to predict the mean and the 5 % characteristic value of the ultimate strength of plates with non-uniform corrosion are developed. Finally a regression equation is proposed to take into account the effect of non-uniform corrosion patterns in the predictions of the ultimate strength of plates with uniform corrosion.