Improved method for non-linear FE analysis of polar class ship primary structures

In the IACS Polar Class Rules, primary structures are required to be assessed with direct calculation methods, which in most cases is either linear or non-linear finite element analysis. For linear analysis a clear set of requirements and acceptance criteria are provided in the rules. This approach is currently the most widely used assessment method although non-linear analysis would provide better insight into the behavior of the structure, giving designers tools to reduce weight and/or improve safety. The current rule requirements for non-linear analysis are high-level in nature and not easily applicable to practical design work. In order to implement non-linear analysis practically, a simple-to-use acceptance criteria is be needed. In this paper, a robust and simple-to-use assessment methodology and acceptance criteria is proposed. The proposed method provides means to ensure proper structural hierarchy. Appropriate safety margins are described through the plastic response of the local frame and the primary structural member. Allowable plastic deformation caused by the design ice load is determined by relating the permanent set of the structure to newbuild vessel production tolerances. An example analysis comparing the proposed method to existing linear and non-linear methods is presented. The proposed method ensures that the structure remains serviceable throughout the lifetime of the vessel.     

Identification of ice loads on shell structure of ice-going vessel with Green kernel and regularization method

Ice loads are important environmental loads that can influence the structural safety of ships during navigation in ice-covered waters. The identification of ice loads on ship hulls is the core of ice load monitoring. In this study, a new ice load identification model based on Green kernel and regularization methods is established. First, the forward model for ice load identification is developed through the discretised convolution integral of ice loads. Next, three commonly used regularization methods, including Tikhonov, truncated singular value decomposition, and least square QR-factorization (LSQR) are adopted to reduce solution errors. The LSQR method is thereafter selected as the optimal regularization operator, and its regular property is proved by numerical cases with ice-induced strains that contain noise. Finally, two load identification cases are conducted on an experimental rig to evaluate the feasibility of the model in ice load identification. The identified loads can determine the signal features of applied loads in the time domain with good accuracy. This identification model provides new insights for full-scale ice load monitoring.     

Vertical and horizontal bending moments on the hydroelastic response of a large-scale segmented model in a seaway

Wave-induced vertical bending moment (VBM) and horizontal bending moment (HBM) on a large-scale segmented model with a box-type backbone beam in short-crested irregular seas are systematically analyzed using sea trial measurement data. New insights into the relationship between nonlinear VBM and HBM of the ship sailing in short-crested sea waves are explored and presented. The results indicate that the HBM significantly contributes to the total sectional loads when the ship is sailing in a seaway and the HBM has a strong correlation with VBM in both magnitude and tendency. Therefore, design loads of HBM and the corresponding lateral structural strength issues should also be concerned in addition to VBM at the ship design phase.     

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。     

Experimental uncertainty of a physical model of a tanker moored to a terminal in a port

This paper presents the uncertainty modelling of experimental results for a physical model of a tanker moored to a terminal inside a port. The physical model was built for an oil terminal at the port of Leixões in Portugal. The model incorporates the new modified port layout, as well as a future 300 m extension of the port outer north breakwater to enhance operational conditions. The physical model tests were performed on a scale of 1:80 in the Portuguese Civil Engineering Laboratory (LNEC). A generic mooring system of four mooring lines and two fenders is simulated using a nonlinear spring system. Decay tests are carried out to evaluate the natural periods of the moored model. Then, tests are carried out for the moored model in waves. The major aim of the experimental study is to obtain novel results for the wave elevation and direction at various locations inside the port, the ship motions at six degrees of freedom, and loads on mooring lines and fenders including the modified port layout. As the physical model measurements are subjected to different types of uncertainties, a systematic uncertainty analysis is carried out here, following ITTC guidelines and recommendations, to quantify all possible sources of uncertainties. The results are discussed, and several conclusions are reached. Based on the experimental results, the presented physical model study may replicate the results for waves and motions with uncertainties less than 9% of the significant amplitudes. The assessment of the applied nonlinear spring model reveals load predictions on the moorings, with uncertainties less than 4% of the maximum mooring loads.     

An experimental study of wave-in-deck loading and its dependence on the properties of the topside structure

This paper concerns the largest and arguably the most threatening wave loading component experienced by a broad range of offshore structures. It arises when an incident wave crest exceeds the elevation of the underside of the deck structure, leading to direct wave-in-deck (WID) loading. The extent of this loading may be limited to the partial submergence of some of the lowermost deck beams, or could involve the large-scale inundation of the entire deck area. Either way, very large loads can arise which must be taken into account when assessing the reliability of the structure. In an earlier contribution Ma and Swan (2020) provided an extensive laboratory study exploring the variation of these loads with the properties of the incident wave. The present paper describes a second stage of this experimental study in which the variation of the WID loads with the properties of the topside structure is addressed. Specifically, it considers the porosity, position and orientation of the topside relative to the incident wave conditions, and seeks to explore both the variations in the maximum load and the loading time–history resulting from these changes.Given the highly transitory nature of a WID loading event, coupled with the fact that the problem is governed by flow conditions at, or very close to, the instantaneous water surface, the loading process is driven by an exchange of momentum from the wave crest to the topside structure. A recently developed WID load model, based on exactly these arguments (Ma and Swan 2020), is used alongside the laboratory data to provide a break-down of the load into its component parts. This provides an enhanced physical understanding of the resulting load time–history. The first part of the study is based upon an idealised generic topside structure, allowing a systematic variation in key parameters, particularly porosity. The second part addresses a realistic topside structure demonstrating the practical relevance of earlier work. Taken together, the analysis clearly establishes the importance of the topside porosity, clarifies the spatial effects associated with the evolution of a large ocean wave beneath the plan area of a structure and explains the unexpected occurrence of impact-type loading on topside structures having a high porosity. Most importantly, the paper highlights those properties of a topside structure which must be incorporated if the WID loads are to be accurately predicted.     

纤维增强复合材料疲劳渐进破坏过程的Monte-Carlo模拟

基于纤维增强复合材料的热/机械疲劳损伤机理,考虑纤维强度的统计特性和金属基体循环响应的特征,发展了纤维断裂、基体和界面局部循环塑性以及基体/纤维脱粘等细观模式控制的疲劳破坏分析模型,结合Monte-Carlo方法研究了复合材料在循环载荷作用下的渐进破坏过程和疲劳行为,将纤维的统计强度、基体热塑性以及界面特性与复合材料的疲劳寿命定量地联系起来。     

连续梁在行驶车辆作用下的动态反应

把桥梁和车辆看作两个分离体系,把车辆视为二维非线性模型,并考虑到桥面的路面不平度影响,应用虚功原理和模态叠加法分别建立振动方程,在车辆与桥梁接触点采用接触力和位移协调的条件,利用迭代技术求解二者之间的相互作用力。以一座三跨连续梁为例,计算了该桥的动挠度曲线和相应的冲击系数。结果显示车辆在边跨行驶时中跨跨中截面的冲击系数要远大于车辆在中跨行驶时的冲击系数,桥梁在车辆动荷载作用下的冲击系数与车辆动力特性、车速、桥梁动力特性以及路面不平度等密切相关,仅仅看作桥梁基频的函数是过于简化的。     

破损舰船剩余强度的可靠性评估方法研究

为了合理地评估破损舰船的剩余强度,基于可靠性方法,考虑剩余承载能力和外载荷的不确定性,给出了一种计算破损舰船剩余强度的方法.应用该可靠性评估方法和LR军规的确定性方法对某舰的剩余强度进行评估,计算结果表明两种方法的评估结论相吻合,且采用可靠性方法计算破损舰船的失效概率能更清晰地反映出舰船在破损情况下的残存能力,可以定量地给出海况、船体破损程度、浪向、航速等参数对残存能力的影响,是值得深入研究的方法.同时,还对破口尺寸的变化对剩余强度的影响进行了分析.