Experimental and numerical investigations of the ultimate torsional strength of an ultra large container ship

Structures of ultra large container ships (ULCS) are characterized by large deck openings and low torsional rigidity. It is essential to comprehensively figure out their collapse behaviors under pure torsion with both model experiments and numerical simulations, making an evaluation of their ultimate torsional strength. In this paper, a similar scale model of a 10,000TEU container ship has been designed and manufactured first, in which both geometric similarity and strength similarity are taken into account. Next the collapse behaviors of the test model are detailedly illustrated with both experimentally and numerically obtained results. Then discussions on warping or shear buckling deformations involved in the collapse process of the structure are conducted with extended numerical simulations. Finally, the ultimate torsional strength of the true ship is evaluated according to the similarity theory. Results show that it is the yielding and shear buckling of the side shells that causes the failure of the hull girder under pure torsion. Further nonlinear finite element analysis demonstrates that it may either have warping or shear buckling deformations in the torsional collapse process of the hull girder with a large deck opening, depending on the local rigidity distribution of side shells, which has a significant effect on the ultimate torsional strength of the hull girder.     

Experimental investigation of the strength and stability of submarine pressure hulls with and without artificial corrosion damage

A submarine may have to operate for a period of time with local corrosion damage in the pressure hull if a suitable repair method is unavailable or too expensive for implementation. This paper describes collapse tests on twenty ring-stiffened aluminium cylinders, which were conducted to study the effect of corrosion damage on hull strength and stability. Artificial hull thinning was found to reduce the collapse strength of experimental models through high local stresses in the corroded region, leading to early onset of yielding and inelastic buckling. Bending associated with the eccentricity due to one-sided thinning was found to further increase the local stresses in the hull. Overall collapse pressures were more severely affected by corrosion damage than interframe collapse pressures. The percentage reduction in overall collapse pressure, compared with intact experimental models, was found to be closely related to the percentage depth of thinning. The accuracy of conventional collapse pressure predictions for the experimental models was significantly better for intact than for corroded cylinders.     

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.     

船体梁极限强度的近似计算

船体梁的总纵强度是反映船舶结构安全可靠的最基本的强度指标。船体结构极限强度评估对于船舶结构初步设计、使用、维护和维修都非常重要，因此船体梁极限强度研究成为近几十年来船舶工程界的热点研究课题之一。到目前为止有两种典型的加筋板和船体梁的极限强度分析方法，它们是直接计算法和逐步破坏分析法。本文基于加筋板单元的平均应力应变曲线和逐步破坏分拆方法，提出了加筋板和船体梁极限强度的简化分析方法，考虑了初始挠度和残余应力对加筋板单元极限强度的影响。数值结果表明，采用本文简化方法得到的结果与有限元计算结果或其它逐步破坏分析结果比较符合。     

Comparison of aluminium sandwiches for lightweight ship structures: Honeycomb vs. foam

The use of lightweight aluminium sandwiches in the shipbuilding industry represents an attractive and interesting solution to the increasing environmental demands. The aim of this paper was the comparison of static and low-velocity impact response of two aluminium sandwich typologies: foam and honeycomb sandwiches. The parameters which influence the static and dynamic response of the investigated aluminium sandwiches and their capacity of energy absorption were analysed. Quasi – static indentation tests were carried out and the effect of indenter shape has been investigated. The indentation resistance depends on the nose geometry and is strongly influenced by the cell diameter and by the skin – core adhesion for the honeycomb and aluminium foam sandwich panels, respectively. The static bending tests, performed at different support span distances on sandwich panels with the same nominal size, produced various collapse modes and simplified theoretical models were applied to explain the observed collapse modes. The capacity of energy dissipation under bending loading is affected by the collapse mechanism and also by the face-core bonding and the cell size for foam and honeycomb panels, respectively. A series of low-velocity impact tests were, also, carried out and a different collapse mechanism was observed for the two typologies of aluminium sandwiches: the collapse of honeycomb sandwiches occurred for the buckling of the cells and is strongly influenced by the cell size, whereas the aluminium foam sandwiches collapsed for the foam crushing and their energy absorbing capacity depends by the foam quality. It is assumed that a metal foam has good quality if it has many cells of similar size without relevant defects. A clear influence of cell size distribution and morphological parameters on foam properties has not yet been established because it has not yet been possible to control these parameters in foam making. The impact response of the honeycomb and foam sandwiches was investigated using a theoretical approach, based on the energy balance model and the model parameters were obtained by the tomographic analyses of the impacted panels. The present study is a step towards the application of aluminium sandwich structures in the shipbuilding.     

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.     

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.     

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.     

破损船体的剩余极限强度分析

基于ABS和DNv规范关于剩余强度规范要求,对船舶碰撞与搁浅的破损部位和范围的假定,考虑了发生碰撞与搁浅破损后,其船体梁剩余有效剖面的非对称性和可能发生不同程度倾斜的情况,采用结构共同规范(CSR)中的逐步破坏分析方法和船体梁非对称剖面计算模型,分别计算了散货船和油船破损船体在两种破损模式和不同倾斜情况下的剩余极限强度,并对不同破损范围的影响进行了分析和比较。研究结果为协调结构共同规范(HCSR)的制定提供了有价值的参考。     

基于完整船体极限强度和搁浅剩余强度的协调共同结构规范对比分析

文章基于Smith法,根据国际船级社协会发布的2013版协调共同结构规范(HCSR)中破损模型、失效模式和载荷模型,考虑材料屈服、结构单元屈曲及后屈曲的特性,应用FORTRAN程序设计语言编写船体极限强度计算程序,以某76000吨散货船为算例,对完整船体的极限强度进行计算,对搁浅状态下破损船体的剩余强度进行计算并校核承载能力。通过在中拱和中垂工况下与其他规范的对比验证,2013版HCSR指定的剩余强度校核公式及船体梁载荷计算公式中选取的安全系数要求更高,校核更严格。     

Hydroelasto-plasticity approach to predicting the post-ultimate strength behavior of a ship’s hull girder in waves

Dynamic collapse behavior of a ship’s hull girder in waves is investigated; post-ultimate strength behavior is the focus. Firstly, a simulation method is proposed. Assuming that a plastic hinge is formed during the collapse of the hull girder, the whole ship is modeled as two rigid bodies connected amidship via a nonlinear rotational spring. The post-ultimate strength behavior, such as the reduction of load carrying capacity due to buckling and yielding, is reflected in the model. Hydrodynamic loads are evaluated by using nonlinear strip theory to account for the effect of large plastic deformations on the loads. A scaled model for validation of the simulation is designed and fabricated. Then a series of tank tests is conducted using the scaled model to validate the simulation results. Post-ultimate strength behavior characteristics in waves are clarified by using the numerical and tank test results. It is shown that the hull girder collapses rapidly after reaching ultimate strength, and then the plastic deformation grows until unloading starts at the collapsed section. Finally, several parametric dependencies of the extent of the collapse behavior are discussed based on a series of the simulations.     

Research on the dynamic buckling of a typical deck grillage structure subjected to in-plane impact Load

The dynamic buckling of the main deck grillage would result in the total collapse of the ship hull subjected to a far-filed underwater explosion. This dynamic buckling is mainly due to the dynamic moment of the ship hull when the ship hull experiences a sudden movement under impact load from the explosion. In order to investigate the ultimate strength of a typical deck grillage under quasi-static and dynamic in-plane compressive load, a structure model, in which the real constrained condition of the deck grillage was taken into consideration, was designed and manufactured. The quasi-static ultimate strength and damage mode of the deck grillage under in-plane compressive load was experimentally investigated. The Finite Element Method (FEM) was employed to predict the ultimate strength of the deck grillage subjected to quasi-static in-plane compressive load, and was validated by comparing the results from experimental tests and numerical simulations. In addition, the numerical simulations of dynamic buckling of the same model under in-plane impact load was performed, in which the influences of the load amplitude and the frequency of dynamic impact load, as well as the initial stress and deflection induced by wave load on the ultimate strength and failure mode were investigated. The results show that the dynamic buckling mode is quite different from the failure mode of the structure subjected to quasi-static in-plane compressive load. The displacements of deck edge in the vertical direction and the axial displacements are getting larger with the decrease of impact frequency. Besides, it is found that the dynamic buckling strength roughly linearly decreased with the increase of initial proportion of the static ultimate strength P₀. The conclusions drawn from the researches of this paper would help better designing of the ship structure under impact loads.     

散货船CSR屈曲应力计算方法对结构设计的影响

屈曲强度是船舶结构设计中需要着重考虑的因素之一,对于散货船来说更是如此.散货船CSR对构件屈曲强度的校核方法与标准作了明确的规定,其中在直接计算法中屈曲应力的计算采用了新的方法--位移法.文章以载重量为118 000 t散货船货舱区的外底板和底部纵桁为例,分别采用CSR的位移法和原规范的平均应力法计算其屈曲强度,并作了比较,进而说明CSR位移法对散货船结构设计的影响,具有实际参考价值.     

基于试验获取主应力应变曲线的油船极限强度评估研究(英文)

A geometrically similar scaling was made from small-scale specimen to full-scale stiffened panels and then their collapse behaviour is investigated. It is considered that the stiffened panel compressive ultimate strength test was designed according to geometrical scaling laws so that the output of the test could be used as representative of the stiffened panels of the compressive zone of a tanker hull subjected to vertical bending moment. The ultimate strength of a tanker hull is analysed by a FE analysis using the experimentally developed master stress-strain curves which are obtained by the beam tension test and the compressive test of the stiffened panel, and are then compared with the result achieved by the progressive collapse method.     

Ultimate strength assessment of a tanker hull based on experimentally developed master curves

A geometrically similar scaling was made from small-scale specimen to full-scale stiffened panels and then their collapse behaviour is investigated. It is considered that the stiffened panel compressive ultimate strength test was designed according to geometrical scaling laws so that the output of the test could be used as representative of the stiffened panels of the compressive zone of a tanker hull subjected to vertical bending moment. The ultimate strength of a tanker hull is analysed by a FE analysis using the experimentally developed master stress-strain curves which are obtained by the beam tension test and the compressive test of the stiffened panel, and are then compared with the result achieved by the progressive collapse method.     

复杂载荷作用下船体板格屈曲强度计算公式研究

分析复杂载荷作用下船体板格结构屈曲强度的影响因素,基于ANSYS软件APDL模块参数化建模,对这些因素进行相关性分析,排除无关参数,保留有关参数,得到屈曲强度的定性表达式。然后变更有关参数,得到不同的有限元模型,通过计算得到不同模型下的屈曲强度,分析大量数据,最终得到屈曲强度的定量表达式。该公式可以在工程上用于复杂载荷作用下船体板格结构的屈曲评估,具有重要的实用价值。     

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).     

藕节形耐压壳体强度与稳定性分析及优化设计

结合有限元理论,利用基于试验点的设计方法对藕节形耐压壳体进行优化设计。基于试验点的优化设计的相关理论,利用正交试验设计确定25组试验点。利用参数化建模的方法建立25组试验点对应的藕节形耐压壳体的三维模型。基于有限元理论,利用Workbench对其强度及稳定性进行分析,得到各试验点参数对应的最大应力、临界失稳载荷及壳体质量。根据有限元分析结果,构造最大应力、临界失稳载荷及壳体质量关于球心间距半径比、厚度及切弧角的响应面;利用遗传算法,结合构造的响应面,对藕节形壳体进行优化设计并利用Workbench对其进行校核,验证所设计壳体的安全性。     

Reliability analysis of ship hulls made of composite materials under sagging moments

An approach is presented to assess the reliability of ship hulls made of composite materials under sagging moments. Buckling, first-ply failure, and ultimate collapse are regarded as the three possible failure modes of ship hulls of composite materials under sagging moments. Reliability estimates were carried out by a combination of the first-order second-moment method and the response surface methodology. A ship hull of composite materials under sagging moments was evaluated and the results showed that the effects on reliability estimates of the model uncertainty in the longitudinal strength of the ship hull, the model uncertainty of the sagging moments, and the sagging moments were significant, whereas the influences of the stochastic characteristics of material elastic moduli were relatively unimportant.     

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.