Erland Johnson

Survivability analysis of a struck ship with damage opening - influence from model and material properties uncertainties

The conditions for damage stability and survivability of a ship struck in a collision in arbitrary sea-state are, from a structural point of view, determined by the size and shape of the damage opening in its side-shell. In the current investigation, explicit finite element analyses (FEA) are presented of a ship-to-ship collision scenario in which the damage opening of a struck ship is calculated for a selection of damage degradation models and realistic material properties, here referred to as model and material properties uncertainties. The model uncertainty is considered as a possible (user-related) insecurity in the selection of the most appropriate damage criterion for the analysis; the shear failure and the forming limit diagram (FLD) criteria were compared in the current investigation. The uncertainty in material properties is accounted for in the constitutive material model description and the material parameters used in the two damage criteria. The size and shape of the damage openings predicted by the FEA are used in damage stability analyses in which the struck ship is subjected to wave motions in an arbitrary sea-state and flooding into the damage opening. The survivability of the struck ship is estimated for all of the damage opening cases. One of the main conclusions is that the high degree of accuracy that a researcher on structure analysis strives for has to be considered together with the natural variation of the sea-state that defines the characteristics in the following damage stability analysis. Consequently, by adoption of a holistic approach in which structural integrity and damage stability research are combined using a systematic parameter (sensitivity) and collision-scenario-based analysis, simplified models and criteria can be developed more efficiently and with higher precision. It will also be clearer which variables are the most important to focus on when analysing the survivability or risk for capsizing.

A computational method for evaluating the damage in a solder joint of an electronic package subjected to thermal loads

Purpose – The purpose of this paper is to introduce a novel computational method to evaluate damage accumulation in a solder joint of an electronic package, when exposed to operating temperature en ...

Measuring axial forces in rail by forced vibrations - experiences from a full scale laboratory experiment

Abstract The longitudinal load in rail caused by thermal expansion must be regularly monitored in order to avoid buckling or rail fracture. Different methods of monitoring with benefits and drawbacks are used or suggested. In this paper one of the proposed methods is investigated by a full-scale experiment. The aim is to measure the change in wavelength of the bending wave caused by the longitudinal load. In contrast to other vibration methods, this method does not require knowledge of the boundary conditions. However, it requires very accurate measurements, advanced finite element (FE) calculations, and sophisticated data analyses. The full-scale experiment shows that this is a method with potential. On the basis of the results of the full-scale experiment the required accuracy of the different steps in the method are clarified. Influence of measurement accuracy, loosened clamps at the sleepers, FE mesh size, degree of wear of the rail, and inaccuracy in the material parameters is considered.

Study on the possibility of increasing the maximum allowable stresses in fibre-reinforced plastics

This article presents the results of a study aimed at investigating the possibility of increasing the maximum allowable stresses in fibre-reinforced plastics compared to current practice. The study consisted of a comparison between the maximum allowable stresses of a group of cross-ply laminates, determined by deterministic analyses with safety and model factors as stated in design rules, and their probabilistic responses estimated with a fracture mechanics based model that accounted for material degradation. The results suggest that for the studied cases, the maximum allowable stresses do not provide the desired reliability; thus, their increase cannot be motivated. More importantly, the investigation shows that a better understanding of the effects of matrix cracks, and therefore, the maximum allowable crack density in a composite laminate, would lead to better and safer composite structures.

Characterization of non-crimp fabric laminates: loss of accuracy due to strain measuring techniques

In mechanical characterization methods, the mechanical properties of multiple material test specimens are measured to determine the probabilistic characteristics of the material’s properties through statistical inference. Several of these methods require the measurement of deformations, and to do so, they rely on local strain measuring techniques, such as bonded strain gages and extensometers. In this study, we show that for non-crimp fabric laminates, local strain measurements acting as proxies of global laminate strain contain a random strain measurement error. Furthermore, we demonstrate that this strain measurement error can significantly reduce the accuracy of characterization methodologies for non-crimp fabric laminates. The strain measurement error pollutes the mechanical property measurements on laminate test specimens, leading to inaccurate statistical inferences. Because the strain measurement error is random, the inferences regarding the mechanical properties may occasionally be conservative or non-conservative with respect to the inference that would have been made if there was no strain measurement error. The results presented in this study are of importance because over-conservative mechanical properties can lead to unnecessarily heavy structures, and non-conservative ones may lead to unsafe structures, endangering life property and the environment. Both scenarios are discussed along with their likelihood and possible consequences.

Analysis of a struck ship with damage opening - influence from model and material properties uncertainties

The conditions for damage stability and survivability of a ship struck by collision in arbitrary sea-state are, from a structural point of view, determined by the size and shape of the damage opening in its side shell. In the current investigation, explicit finite element analyses (FEA) are presented of a ship-to-ship collision scenario where the damage opening of a struck ship is calculated for a selection of damage degradation models and realistic material properties; here referred to as model and material properties uncertainties. The model uncertainty is considered as a possible (user-related) insecurity in the selection of the most appropriate damage degradation model for the analysis: the shear failure and the forming limit diagram (FLD) criteria. The uncertainty in material properties is accounted for in the constitutive material model description and the material parameters used in the two criteria. The size and shape of the damage openings predicted by the FEA are used in damage stability analyses in which the struck ship is subjected to wave motions in arbitrary sea-state and flooding into the damage opening.

Effective use of composite marine structures: Reducing weight and acquisition cost

Composite structures are a way to reduce the operational costs of a vessel or to increase its potential revenue. However, depending on the design of the vessel, its operational profile, and the business model of the owner, the benefits brought by a composite structure may not justify its acquisition cost. This paper presents a number of investigations aimed at reducing the acquisition cost of marine composite structures and maximizing their benefits through a more effective use of composite materials (in other words, weight reduction of the composite structure). The investigations cover three areas of opportunity for doing so: material safety factors, material characterization, and numerical optimization of large composite structures. The following conclusions are drawn from the investigations: motivating a reduction of material safety factors through probabilistic analyses is unpractical at best, and questionable at worst; improving the material characterization of textile composites is easy, relatively costless, and can modestly reduce structural weight through better material property values; numerical optimization of large composite structures is cumbersome, but feasible, and holds the greatest potential increasing the economical attractiveness of composite marine structures.

Reduction in ultimate strength capacity of corroded ships involved in collision accidents

ABSTRACT The objective of the study is to investigate the effects of sudden damage, and progressive deterioration due to corrosion, on the ultimate strength of a ship which has been collided by another vessel. Explicit finite element analyses (FEA) of collision scenarios are presented where factors are varied systematically in a parametric study, e.g. the vessels involved in the collision, and consideration of corroded ship structure elements and their material characteristics in the model. The crashworthiness of the struck ships is quantified in terms of the shape and size of the damage opening in the side-shell structure, and the division of energy absorption between the striking and struck ships for the different collision simulations. The ultimate strength of the struck ship is calculated using the Smith method and the shape and size of the damage openings from the FEA. In conclusion, the study contributes to understanding of how corroded, collision-damaged ship structures suffer significantly from a reduction in crashworthiness and ultimate strength, how this should be considered and modelled using the finite element method and analysed further using the Smith method.

Experimental and numerical investigation of a taut-moored wave energy converter—a validation of simulated buoy motions

This study presents an experimental and numerical investigation of a taut-moored wave energy converter system with a point-absorber type of wave energy converter. The wave energy converter system consists of a buoy, a unique three-leg two-segment mooring system with submerged floaters, and a power take-off system designed for the current experiment as a heave plate. The main objective of the study is to validate a numerical simulation model against experiments carried out in an ocean basin laboratory. Two physical models in model scales 1:20 and 1:36 were built and tested. The detailed experimental testing programme encompasses tests of mooring system stiffness, decay tests, and different sea state conditions for ocean current, regular, and irregular waves. A numerical model in the model scale 1:20 was developed to simulate coupled hydrodynamic and structural response analyses of the wave energy converter system, primarily using potential flow theory, boundary element method, finite element method, and the Morison equation. Several numerical simulations are presented for each part of the experimental testing programme. Results for the wave energy converter buoy motions under operational conditions from the experiments and the numerical simulations were compared. This study shows that the simulation model can satisfactorily predict the dynamic motion responses of the wave energy converter system at non-resonant conditions, while at resonant conditions additional calibration is needed to capture the damping present during the experiment. A discussion on simulation model calibration with regard to linear and non-linear damping highlights the challenge to estimate these damping values if measurement data are not available.

Cost and weight of composite ship structures - a parametric study based on DNV rules

A wider use of composites in larger, commercial vessels has been limited by initial costs and fire regulations, but both of these obstacles are diminishing. Increasing fuel costs and more stringent emission requirements have heightened the value of lightweight structures. Due to the higher acquisition costs and other entry barriers, composite designs must be as cost efficient as possible in order to compete with traditional steel or aluminium designs. The purpose of this article is to investigate which fibre-reinforced polymer materials and types of structures are most suitable for different parts of a ship design in order to minimize weight or cost. This is done by designing and comparing individual composite panels while varying a wide range of input parameters and strictly following the ‘Det Norske Veritas (DNV) Rules for Classification of High Speed, Light Craft and Naval Surface Craft’. The results are presented as weight and cost comparisons between materials and structures and also degree of utilization for the different design criteria; carbon fibre structures are on the average 20%–30% lighter than glass fibre structures but are consistently more expensive. The results also indicate that sandwich panels in most cases are lighter than single-skin panels, and that for sandwich structures, the mechanical properties of the core material are commonly the critical design criterion. The minimum amount of reinforcement stipulated by the rules is also found to be a critical factor.