Ultimate strength of container ships subjected to combined hogging moment and bottom local loads,Part 2: An extension of Smith's method

This paper is the second of two companion papers concerning the ultimate hull girder strength of container ships subjected to combined hogging moment and bottom local loads. The nonlinear finite element analysis in Part 1 has shown that local bending deformation of a double bottom due to bottom lateral loads significantly decreases the ultimate hogging strength of container ships. In this Part 2, extending Smith's method for pure bending collapse analysis of a ship's hull girder, a simplified method of progressive collapse analysis of ultimate hogging strength of container ships considering bottom local loads is developed. The double bottom is idealized as a plane grillage and the rest part of the cross section as a prismatic beam. An average stress-average strain relationship of plate/stiffened plate elements employed in Smith's method is transformed into an average stress-average plastic strain relationship, and implemented in the conventional beam finite element as a pseudo strain hardening/softening behaviors. The extended Smith's method is validated through a comparison with nonlinear finite element analysis.     

Test of extensively damaged hybrid ship hull

A hybrid ship hull model made with a steel truss and composite sandwich panels was tested and analyzed with the goal of gaining insight into how well this structural concept holds up after major damage. One of the ideas of the concept was to mount the sandwich panels to the steel truss such that they can be blown out in a controlled fashion to ventilate a large internal blast. The hull should be designed to have sufficient strength for the ship to reach a port safely even after such extensive damage. A 6-m model of such a hybrid ship hull, consisting of a stainless steel truss and 60 composite sandwich panels, was manufactured and mechanically tested. A number of panels were then removed one by one and the hull was retested to the design load after each panel had been removed. The removed panels simulated major damage. After nine panels had been removed, from all the different areas of the hull, it could still carry the design load, although with considerable nonreversible deformations of the hull girder. The hull was eventually loaded to final failure, which occurred at 25% above the design load.     

Time-variant bulk carrier reliability analysis in pure bending intact and damage conditions

Time-variant reliability analysis of a corroded bulk carrier in intact and damage conditions is performed by First-Order (FORM), Second-Order (SORM) Reliability Methods and Importance Sampling simulation. Annual failure probabilities are determined up to 25-year ship lifetime, accounting for time-variant corrosion wastage of structural members contributing to hull girder strength. Statistical properties of hull girder capacity are determined by Monte Carlo simulation, applying three correlation models among corrosion wastages of structural members contributing to hull girder strength, namely no correlation, full correlation and full correlation among wastages of structural members belonging to the same category of compartments. A modified incremental-iterative method is applied, to account for instantaneous neutral axis rotation, in case of asymmetrical damage conditions, as for collision and grounding events. Incidence of intact/damage condition, as well as correlation among corrosion wastages, on annual sagging/hogging time-variant failure probability is investigated and discussed. Time-variant sensitivity analyses for intact and damage conditions are also performed, to investigate the incidence of random variables' uncertainties on the attained failure probability. Finally, the bulk carrier section scheme, benchmarked in the last ISSC Report, is applied as test case.     

Reliability analysis of a bulk carrier in ultimate limit state under combined global and local loads in the hogging and alternate hold loading condition

The alternate hold still-water loading in hogging combined with wave loading is critical for the safe design of bulk carriers. The ultimate longitudinal strength of the hull girder of bulk carriers in this condition has been found to be considerably reduced by the action of local lateral pressure loads. In the present paper, an interaction equation based on the ultimate hull girder strength assessment obtained by nonlinear finite element analyses is adopted to consider the relationship between ultimate longitudinal bending capacity and average external sea pressure over the bottom. This interaction equation is used as the basis for the failure function. The annual probability of failure is obtained by FORM analysis considering two typical load cases, namely, pure longitudinal hogging bending moment and combined global hogging bending moment and local lateral pressure loads. The effect of heavy weather avoidance on the failure probability is evaluated. The results show that the local lateral pressure has a significant influence on the annual probability of failure of bulk carriers in the hogging and alternate hold loading condition.     

Ultimate strength analysis of a bulk carrier hull girder under alternate hold loading condition,Part 2: Stress distribution in the double bottom and simplified approaches

This is the second of two companion papers dealing with nonlinear finite element modelling and ultimate strength analysis of the hull girder of a bulk carrier under Alternate Hold Loading (AHL) condition. The methodology for nonlinear finite element modelling as well as the ultimate strength results from the nonlinear FE analyses was discussed in the companion paper (Part 1). The purpose of the present paper is to use the FE results to contribute towards developing simplified methods applicable to practical design of ship hulls under combined global and local loads. An important issue is the significant double bottom bending in the empty hold in AHL due to combined global hull girder bending moment and local loads. Therefore, the stress distributions in the double bottom area at different load levels i.e. rule load level and ultimate failure load level are presented in detail. The implication of different design pressures obtained by different rules (CSR-BC rules and DNV rules) on the stress distribution is investigated. Both (partially) heavy cargo AHL and fully loaded cargo AHL are considered. Factors of influence of double bottom bending such as initial imperfections, local loads, stress distribution and failure modes on the hull girder strength are discussed. Simplified procedures for determination of the hull girder strength for bulk carriers under AHL conditions are also discussed in light of the FE analyses.     

Ultimate strength of container ships subjected to combined hogging moment and bottom local loads part 1: Nonlinear finite element analysis

This paper is the first of two companion papers concerning the ultimate hull girder strength of container ships subjected to combined hogging moment and bottom local loads. In the midship part of container ships, upward bottom local loads are usually larger than the downward ones. This leads to the increase of biaxial compression in the outer bottom plating and the reduction of the ultimate hull girder strength in the hogging condition. In this Part 1, the collapse behavior and ultimate strength of container ships under combined hogging moment and bottom local loads are analyzed using nonlinear finite element method. Buckling collapse behavior of bottom stiffened panels during the progressive collapse of a hull girder is closely investigated. It has been found that major factors of the reduction of ultimate hogging strength due to bottom local loads are (1) the increase of the longitudinal compression in the outer bottom and (2) the reduction of the effectiveness of the inner bottom, which is on the tension side of local bending of the double bottom. The obtained results will be utilized in the Part 2 paper to develop a simplified method of progressive collapse analysis of container ships under combined hogging moment and bottom local loads.     

圆筒型FPSO上部模块支撑结构疲劳分析

与船型FPSO相比,圆筒型FPSO没有明显的总纵弯曲,上部模块与船体结构之间通常采用刚性支墩来连接,水平运动所产生的弯矩和装/卸载引起的船体垂向变形对模块支撑结构的影响较为显著。因此,以“希望6号”圆筒型FPSO上部模块支撑结构为研究对象,基于DNVGL船级社规范,介绍一种简化疲劳分析方法。以FPSO运动加速度和船体变形载荷作为载荷输入条件,利用SESAM/GeniE软件进行有限元分析,得到结构在所有组合工况下应力的扫描计算结果。根据作业海域各个方向波浪发生的概率,运用简化疲劳分析方法计算得到所关注节点的疲劳损伤和各个工况对结构节点疲劳损伤度的贡献。结果表明,所关注节点的疲劳强度均满足设计疲劳强度要求;同一节点的疲劳损伤对不同浪向的敏感度不一样。该简化疲劳分析方法同样适用于承受周期性载荷的FPSO上部模块主结构和其他型式海洋结构物的疲劳分析。     

A comparison of hydrodynamic impacts prediction methods with two dimensional drop test data

The effect of hydrodynamic impacts on a ship's bow structure is an important design consideration. In addition, to possible failure to local structure, such impacts can generate slam-induced whipping loads, the magnitude of which when combined with ordinary wave induced loads, could lead to first passage failure of a ship's hull girder. This paper presents the results of a recent study that compares select methods of predicting hydrodynamic impacts with available test data of 2-D wedge shaped section shapes. Results based on peak pressures at different impact velocities are presented. The results are compared with traditional prediction theories of Wagner and Chuang.     

Probabilistic analysis of the hull-girder still water loads on a shuttle tanker in full load condition,for parametrically distributed collision damage spaces

Box-shaped damage affected volumes are applied, with parametric variation of their positions and extensions, to an otherwise intact shuttle tanker in full load condition. The vessel's deadweight is numerically calculated and the lightweight is estimated using semi-empirical formulations to a great extent. For each damage configuration, the final position attained by the ship is investigated and the corresponding global loads are assessed, in terms of still water vertical bending moment and shear force. The ship is considered to be floating in the absence of waves and a quasi-static version of a generalized adaptive mesh pressure integration technique code, for progressive flooding of floating objects, is used to model the progression of the floodwater and the vessel's attitude. The probabilistic model suggested by the Marine Environment Protection Committee of the International Maritime Organization (MEPC-IMO) is considered for the collision induced probabilistic distribution of the damage boxes. A total of 90 damage cases are considered and comparisons of the maximum loads and the location where these take place is carried out relative to the intact case. The minimum design values for still water bending moment and shear force, included in the Common Structural Rules for Bulk Carriers and Tankers of the International Association of Classification Societies (IACS), are introduced in the analysis and their envelopes are compared with the numerically obtained values in light of a probabilistic assessment. Several conclusions are taken regarding the effects of the damage parameters variation, and important findings are presented specifically when accounting for the MEPC-IMO probabilistic distribution, in opposition to a uniform distribution of the damage cases.     

22 000m3液化气船整船和舱段三维有限元强度分析

对２２０００ｍ＾３液化气船进行了整船和舱段三有限元强度计算分析,建立了整船和船体主舱段的三维有限元结构模型。并通过节点力的自动加载技术和惯性平衡处理技术建立有限元模型的节点载荷,在中拱和中垂弯矩作用下,计算出本在压载和满载工况下的船体应力和变形,是后通过对本舱舱段的边界处理技术,计算出受船体总强度的船体舱段局部强度,对船体强度出判断,为改进船体结构设计提供依据。     

基于谱分析法的FPSO剩余疲劳寿命研究

疲劳破坏是船舶与海洋工程结构破坏的主要模式之一。多年来,船舶结构的疲劳断裂问题一直是造船界广泛关注的问题[1]。对于由大型油船改装而成的FPSO而言,预测并延长其服役寿命是很关键的。本文通过谱分析法对船体疲劳损伤度进行计算,分别对油船和FPSO阶段进行计算从而得到FPSO剩余疲劳寿命。通过建立3D有限元模型,采用热点应力方法来确定评估处应力传递函数,分别计算各个短期海况损伤度并通过线性叠加来计算总的损伤度以及剩余疲劳寿命。根据疲劳评估结果,更加高效地实施船体结构的检测及维修。     

Ultimate collapse tests of stiffened-plate ship structural units

An increasingly popular approximate method for assessing ship hull girder ultimate strength is to combine the individual elasto-plastic load-carrying characteristics of each single stiffened-plate unit comprising the ship hull cross section. In order to evaluate methods (numerical and experimental) for developing the load-carrying characteristics (load–shortening curves), a full-scale testing system was designed and constructed to provide data for stiffened steel plate units under combined axial and lateral loads. The system included an assembly of discrete plate edge restraints that were developed to represent symmetric boundary conditions within a grillage system. Twelve full-scale panels including ‘as-built’, ‘deformed’ and ‘damaged’ specimens were tested in this set-up.

The specimens failed by combined plate and flexural buckling, stiffener tripping or local collapse, depending on the magnitude of lateral loads and local damage. Load-shortening curves associated with different failure modes were found to be distinctly different and it was found that a small lateral load could change the failure mode from flexural buckling to tripping. Current design criteria should directly consider effects of the lateral loads on the failure modes and the collapse loads of stiffened plates.     

三体船分段模型波浪载荷试验研究

三体船的主体与连接桥结构的波浪载荷特性是船舶结构设计者非常关注的问题,目前对于这些载荷特性进行较为全面的模型试验研究在国内很少。该文在拖曳水池和方形水池开展了某三体船的分段模型试验,详细介绍了分段模型的设计原则,对于纵向载荷和横向载荷的测量采用不同的分段布置形式。通过对试验数据的分析及与理论计算的对比,对三体船横摇运动非线性修正方法加以改进,同时研究了该三体船的船舯横剖面和连接桥纵剖面的波浪载荷特征,得出了一些结论,为船舶结构设计提供了确定设计载荷的依据。     

Some analytical results for the initial phase of bottom slamming

The deformation of boat hull bottom panels during the initial phase of slamming is studied analytically using a linear elastic Euler–Bernoulli beam as a representation of the cross section of a bottom panel. The slamming pressure is modeled as a high-intensity peak followed by a lower constant pressure, traveling at constant speed along the beam. The problem is solved using a Fourier sine integral transformation in space and a Laplace–Carson integral transformation in time. Deflection and bending moment as functions of time and position for different speeds, bending stiffnesses, etc. are given. In particular the effect of slamming load traveling speed on structural response of the simplified bottom structure is investigated. It is found that rather large deflections and bending moments are encountered at certain speeds of the pressure, which suggests that bottom panels may benefit from tailoring their stiffness and mass properties such that loads are reduced. This would vary with boat particulars and operation (deadrise angle, mass, speed, sea state, etc). The importance of the high-intensity pressure peak often encountered during slamming is also studied. It is seen that for relatively slow moving slamming loads the pressure peak has little influence. However, for faster moving loads its influence can be significant.     

波激振动和砰击颤振对大型油船疲劳强度影响研究

文章给出线性波激振动和砰击颤振引起的大型油船结构疲劳贡献度的计算方法。基于三维线性水弹性理论可计算得到包括刚体和2节点的振动模态下的垂向弯矩的传递函数,采用谱分析法对某30万吨油船进行结构疲劳强度计算,分析波激振动对船体结构疲劳损伤影响;基于二维非线性波浪载荷程序（含砰击颤振的波浪载荷）,采用时域方法对该油船进行船体梁弯矩计算,分析砰击颤振对船体梁弯矩及疲劳载荷的影响;结果表明对于该类大型油船波激振动和砰击颤振对船体的结构疲劳损伤的影响不可忽略。     

水下爆炸冲击波与气泡载荷作用下船体结构的动响应

由冲击波引起的结构动力毁伤与由气泡引起的结构动力毁伤机理不同.基于双渐近（DAA）理论,建立-套有限元方法与边界元方法相结合的数值计算程序.分别研究非接触水下爆炸冲击波载荷与气泡载荷作用下三维船体结构的动响应,阐述水下爆炸载荷与三维船体结构之间的流固耦合理论.通过算例,详细讨论冲击波与气泡载荷作用下船体结构的总体响应和局部响应的-些特征与机理.计算结果表明,在非接触水下爆炸中,冲击波主要是对船体结构造成局部毁伤,而气泡则会对船体结构造成总体与局部的双重毁伤.     

Fatigue strength improvement for ship structures by Ultrasonic Peening

Ultrasonic Peening has attracted recent attention as a fatigue strength improvement method. The main features of Ultrasonic Peening are generation of compressive residual stress and smoothing of weld toe radius. However, for ship structures, various loads act on the hull structure over a ship??s life, including the construction period, and the compressive stress field of structural members generated by Ultrasonic Peening may change. In order to verify the technique's benefits to ship structures, the influence of load history on the improvement effects on fatigue strength by Ultrasonic Peening needs to be clarified. In this paper, the improvement effects on fatigue strength by Ultrasonic Peening for welding joints were confirmed by experiments with several joint type specimens. In addition, fatigue tests modeling launching, when the stress conditions of a ship's structure changes significantly, were carried out in order to clarify the influence of load history in a ship??s life. Consequently, some cases that have the possibility of decreasing or increasing the improvement effects on fatigue strength by Ultrasonic Peening were clarified, and some efficient methods of Ultrasonic Peening for ship structures were suggested.     

Statistics and damage assessment of ship grounding

In this paper, general statistics of ship grounding incidents are considered and the damage extent distributions for Ro–Ro ships are presented from the results of a comprehensive damage data survey conducted using Lloyd's Register's damage database. Theoretical models and semi-empirical formulae based on parametric studies are used to study the damage extents of grounded ships. Two real life grounding accidents are assessed. One is a single hull VLCC grounded onto a single rock and the other is a cargo ship grounded onto multiple rocks. A simulation based on a simple multi-rock scenario has been conducted on a 304-m single hull tanker. Correlation is made between the present calculation method results, statistical results and IMO requirements. The paper concludes with the main findings from the study.     

Plate tearing and bottom damage in ship grounding

A theoretical method for plate tearing by a rigid wedge is developed in this paper. The studied model is an idealization of ship-grounding and collision damage. The analysis model postulates that the plate curls up into two curved surfaces behind the wedge tip and that the plate material ahead of the wedge is tensioned and ruptured due to the direct pushing. Based on a parametric study, a semi-empirical formula is proposed for determining grounding force in the event of a ship running onto rocks in a high-energy grounding. The bottom strengths of single hull structures and double hull structures in ship-grounding incidents are compared. Finally, simple formulae for determining damage resistance and the extent of damage in ship grounding, expressed in terms of the ship principal particulars, are developed.     

双壳型船体结构稳态温度场和温度应力

用简化解析方法和有限元数值方法,分析了双壳型船体货舱区域在运载高温液货时的稳态温度场;根据船体结构的温度分布,用有限元法计算了其温度应力,同时与货物压力、海水静动压力、总纵弯矩等载荷作用下的结构应力做了比较。研究结果表明：在货舱结构温度场分析中用简化分析方法和有限元数值方法所得的计算结果相当一致;高温液货大幅度增加船体结构的纵向应力和横向应力,同时加剧结构不连续处的应力集中;槽型舱壁可以有效地释放