Investigation on the Post-Ultimate Strength Behaviour of Sandwich Plate

This paper focuses on the post-ultimate strength behavior of sandwich plates. With widely application of the laminate on the ship and offshore structures, the post-ultimate strength behavior is becoming more important for safety evaluation of structures. Since the post-ultimate strength behavior can reflect the collapse extent of sandwich plate when subjected to extreme loads. A sandwich plate was modeled by FEM, its load-displacement relationship was obtained and its collapse characteristics were analyzed. The load-displacement relationship indicates its post-ultimate strength behavior, which is shown as that the load carrying capacity has a rapidly reduction when the ultimate strength is exceeded, and that the failure modes of the sandwich plate are determined by the parameter of individual layer. The simulation results were validated against experimental results. Conclusions are drawn: the displacement of sandwich plate under axial compression increased slowly before reaching the ultimate strength, once the ultimate strength was exceeded, the loads exerted on the structures sharply decreased with slowly increased displacement until the plate cracked. The simulation results have a good agreement with the experimental results. The mainly failure modes of sandwich plates can be interpreted as delamination between skin & core and core compression fracture, which are typical failure modes in engineering. The stiffness of sandwich structures decreased due to the interlaminar cracking or skin fracture, further the load carrying capacity decreased, which is of significance for guiding the design of sandwich structures.

Challenges and Developments in Navigational Risk Assessment With Large Uncertainty

Concerns have been raised to navigational safety worldwide because of the increasing throughput and the passing ships during the past decades while maritime accidents such as collisions, groundings, overturns, oil-spills and fires have occurred, causing serious consequences. Formal Safety Assessment (FSA) has been acknowledged to be a framework widely used in maritime risk assessment. Under this framework, this paper discusses certain existing challenges when an effective safety assessment is carried out under a variety of uncertainties. Some theories and methodologies are proposed to overcome the present challenges, e.g., Fault/Event Tree Analysis (FTA/ETA), Evidential Reasoning (ER), Bayesian Belief Network (BBN) and Belief Rule Base (BRB). Subsequently, three typical case studies that have been carried out in the Yangtze River are introduced to illustrate the general application of those approaches. These examples aim to demonstrate how advanced methodologies can facilitate navigational risk assessment under high uncertainties.

Assessment of Residual Ultimate Strength of VLCC According to Damage Extents and Average Compressive Strength of Stiffened Panel

This paper presents the prediction of residual ultimate strength of a very large crude oil carrier considering damage extents due to collision and grounding accidents. In order to determine extents of damage, two types of probabilistic approaches are employed: deterministic approach based on regulations based on ABS [1], DNV [2], and MARPOL [3] and probabilistic approach based on IMO probability density functions (PDFs) (IMO guidelines [4]). Hull girder ultimate strength is calculated using Smith method which is dependent on how much average compressive strength of stiffened panel is accurate. For this reason, this paper uses two different methods to predict average compressive strength of stiffened panel composing hull girder section: CSR formulas and nonlinear FEA. Calculated average compressive strength curves using CSR formulas (IACS [5, 6]) and nonlinear FEA are imported by an in-house software UMADS. Residual ultimate moment capacities are presented for various heeling angles from 0° (sagging) to 180° (hogging) by 15° increments considering possible flooding scenarios. Three regulations and IMO guidelines yield minimum of reduction ratios of hull girder moment capacity (minimum of damage indices) approximately at heeling angles 90° (angle of horizontal moment) and 180° (angle of hogging moment), respectively, because damage area is located farthest from neutral axis.

Alternative Environmental Contours for Marine Structural Design: A Comparison Study

A new approach to estimating environmental contours has recently been proposed, where the contours are estimated in the original physical space by Monte Carlo simulations from the joint distribution directly rather than applying the Rosenblatt transformation. In this paper, the new and the traditional approach to estimating the contours are presented and the assumptions on which they are based are discussed. The different results given by these two methods are then compared in a number of case studies. Simultaneous probability density functions are fitted to the joint distribution of significant wave height and wave period for selected ocean locations and, for each area, environmental contours are estimated for both methods. The chosen locations are characterised by different wave climates. Thus, the practical consequences of the choice of approach are assessed. Particular attention is given to mixed sea systems, i.e. a combination of wind sea and swell. In these situations, the new approach for environmental contours may fail to identify realistic conditions along some parts of the contours while for other wave conditions the contours are quite similar. The paper also briefly discusses possible ways of amending the new approach to estimating the contours to obtain more realistic conditions all along the contour lines.

An Efficient Direct Calculation Approach for Fatigue Assessment of Container Ships Concerning Bending and Warping Stresses

Container ships are particularly susceptible to torsional loads. The distribution of torsion-induced warping stress in a container ship hull is more complicated and difficult to be expressed by beam theory formulas. In practice, finite element (FE) analysis is typically used to calculate the stress response to wave-loading conditions. However, it is time consuming to compute hull girder stresses for all relevant sea conditions through FE analyses. In this paper, an efficient and robust approach is proposed by combining beam theory and FE analyses in the determination of hull girder stresses. The parameters required by beam theory can be regressed through matching stress records from a FE analysis with the corresponding sectional and pressure loads from the hydrodynamic simulation. Stress records obtained using the proposed method are utilized in fatigue assessment of a case study container vessel. The results show that the accuracy of the regression approach is satisfactory compared with the full FE analyses.

Experimental and Numerical Investigation on the Load Carrying Behavior of Large Ship Windows

Aboard ships windows are exposed to static as well as dynamic loads, e.g. impact loads. Failure can lead to serious consequences. Therefore two research projects were initiated in order to analyze the load carrying behavior of windows. In addition to quasi-static ultimate load tests and drop tests with water filled rubber bags special attention is paid to the Finite Element (FE) modeling. In particular the response — stresses and deformations — to quasi-static lateral loads can be calculated with good agreement to test results. Hence FE calculations can be useful to determine and compare failure mechanisms of different window designs. An ultimate load range can be estimated by taking into account the breaking strength range of glass. A comparison between FE calculations and results of the impact tests showed that these are sensitive to conditions which could hardly be measured during the test, e.g. the shape of the approaching water-filled rubber bag. Varying of parameters eventually yielded that window response to impact loads can also be calculated sufficiently, at least, to evaluate different window designs. Further investigations on this topic are in progress.

Structural Analysis of a Hybrid Substructure With Multi-Cylinder for 5MW Offshore Wind Turbines

The substructure for offshore wind turbines is strongly influenced by the effect of wave forces as the size of substructure increases. Therefore, it is very important to reduce the wave force acting on substructures. In the present study the hybrid substructure, which is composed of a multi-cylinder having different radius near free surface and a gravity substructure at the bottom of multi-cylinder, is suggested to reduce the wave forces. The fluid domain is divided into two regions to calculate the wave forces acting on the hybrid substructure with multi-cylinder and the scattering wave in each fluid region is expressed by an Eigen-function expansion method. The comparison between the mono pile and the hybrid substructure is made for wave forces. Using the wave forces obtained from this study, the structural analysis of hybrid substructure is carried out through ANSYS mechanical. In order to investigate the resonance between the wind turbine and the hybrid substructure, the modal analysis is also carried out.

Dropped Object Analysis Using Non-Linear Dynamic FE Analysis

For the last few decades, necessity of direct non-linear FE analysis has been increasing for the accidental events at the vessel/offshore structures. One of major areas for the accidental design, dropped object analysis using non-linear analysis is indispensable for the verification of structural safety at the design process. This paper is concerned with the methodology, conditions, and design consideration of dropped object analysis using dynamic FE analysis. By comparing the results from direct FE analyses to those from simplified energy method described in DNV-RP-C204, necessities and advantages of direct non-linear analysis can be verified. In this paper, the effect of analysis condition is investigated using parametric study. The results are influenced by the application of failure criteria according to the rule requirements, application of material properties, dropping position, condition of the object, and so on. This study can suggest appropriate determination of the methodology and condition for the dropped object analysis using direct FE analysis.

A Procedure for Non-Linear Structural Collapse Analysis

It is a normal practice nowadays in structural engineering, including ships and offshore industry, to perform non-linear finite element analysis to assess the structure’s capacity for design or evaluation purposes. However, experience has shown that the quality and accuracy of the non-linear FE analysis results are highly dependent on the skill of the person performing the analysis and the analysis procedure used. The difference between results obtained by different people can be significant. In some cases, the results can be misleading. It is considered that a unified procedure is necessary. This paper is moving a step further and trying to develop a standard procedure which can provide a guideline for structural collapse analysis of stiffened panels under any load combinations. The paper provides the technical background on the analysis procedure and the key steps such as model extent, mesh density, initial imperfections, and boundary conditions. Analysis examples are provided in the paper for reference and discussions.