Sang-Rai Cho

Ultimate strength formulation for stiffened plates subjected to combined axial compression, transverse compression, shear force and lateral pressure loadings

A robust ultimate strength formulation is proposed for stiffened plates subjected to combined axial compression, transverse compression, shear force and lateral pressure loadings. Before deriving the formulation, a simplified numerical method was developed to trace the structural behaviour of stiffened plates under combined loadings. A rigorous parametric study was then performed using the developed numerical method to predict the ultimate strength of various stiffened plates under various combinations of loadings. The formulation was derived by a regression study using the parametric study results. The accuracy and reliability of the proposed formulation were compared with those of commercial packages, such as ABAQUS and DNV PULS, and experimental results.

Experimental Investigations on Slamming Impacts by Drop Tests

AbstractWhen ships are sailing with large motions in rough waves, the slamming phenomenon occurs and the ships suffer from impulsive pressure loadings. Recently, ships are becoming lager and faster than before and it becomes more possible that the ships experience larger impacts on their bows and sterns. Many researchers have been pe rforming the investigations on slamming experimentally and theoretically for a long time. Most of the research reported in the open literature focused on how to accurately estimate the amplitude of the peak pressure of slamming. According to the results of a recently published work, not only the amplitude of peak pressure but als o the width of the peak may play an important role in predicting the extents of damage of impacted structures. The uncertainty of impulsive pressure loadings due to slamming has been indicated by many researchers. However, probabilistic treatments of the impulsive pressure loadings are few. In this study, drop tests were conducted on wedges having dead-ris e angles of 0° and 10°. Not only the amplitude of peak pressure but also the width of peak pressure were measured. Furthermore, the variations of those values are also provided for the probabilistic approach of the slamming problem.

Low Temperature Plastic Hardening Constitutive Equation for Steels of Polar Class Vessels

In this study, a plastic hardening constitutive equation for steels of polar class vessels at low temperature is proposed. The equation was derived using the experimental data obtained from tensile tests at room and low temperatures. Tensile tests at low temperature are both costly and time consuming because an expensive cold chamber is necessary and it takes too much time to cool down a specimen to set temperature. Using the proposed plastic hardening constitutive equation the plastic hardening characteristics of steels for polar class vessels at low temperature can be easily predicted from the tensile test results at room temperature.

Ultimate Strength Assessment of Bollard and Its Foundation Considering Production Costs

Common structural rules of JBP(Joint Bulker Project) and JTP(Joint Tanker Project), which will come into effect in 2006, invoke the necessity of the ultimate limit state(ULS) design for ship structures. Even though the many applications of ULS analysis have been performed for ship structures, there have few studies carried out for deck machineries and their supporting structures. Recently four major Korean shipbuilders(DSME, HHI, HHIC, SHI) jointly developed and proposed a new design standards for mooring fittings and also proposed the SWL (Safety Working Load) obtained based on the first yield criterion. In this study, various ultimate strength analyses were performed for bollards and their foundation structures whose yield strengths were quantified by the research consortium. Prior to performing the ultimate strength analyses, the numerical calculation method was substantiated with the test results provided in the joint work report. Based upon the results of this study, it can be concluded that the reinforcements to increase the yield strength are not always resulted in the enhancement of the ultimate strength. Furthermore, the additional production costs for the reinforcements can not be rewarded by the ultimate strength. Therefore, another alternative arrangements should be developed in the view point of ultimate strength.

On the Fracture of Polar Class Vessel Structures Subjected to Lateral Impact Loads

Single frame structures with notches were fractured by applying drop impact loadings at room temperature and low temperature. Johnson-Cook shear failure model has been employed to simulate the fractured single frame structures. Through several numerical analyses, material constants for Johnson-Cook shear failure model have been found producing the cracks resulted from experiments. Fracture strain-stress triaxiality curves at both room temperature and low temperature are presented based on the extracted material constants. It is expected that the fracture strain-stress triaxiality curves can offer objective fracture criteria for the assessment of structural fractures of polar class vessel structures fabricated from DH36 steels. The fracture experiments of single frame structures revealed that the structure on low temperature condition fractures at much lower strain than that on room temperature condition despite the same stress states at both temperatures. In conclusion, the material properties on low temperature condition are essential to estimate the fracture characteristics of steel structures operated in the Northern Sea Route.

Ultimate Strength Analysis of Curved Stiffened Shells of Container Bilge Strakes

For medium size container ships, curved bilge strakes should be stiffened with angle bars. However, ultimate strength design formulations for those structures are not available. Design equations for flat stiffened plates are employed in design without considering the effects of curvature of shells. in this study non-linear finite element analyses were performed and the curvature effects were quantified. According to the analysis results the effects of curvature can not be neglected when the curvature is larger than a certain amount. Therefore design formulations should be developed for curved stiffened shells, which is presently designed using the formulations for flat stiffened plates.

Numerical Investigation on the Plastic Response of a Small-Scale Laterally Impacted Tanker Double Hull Structure

Numerical simulations are presented, on the dynamic response of a one-tenth scaled tanker double hull structure struck laterally by a knife edge indenter. The small stiffeners of the full-scale prototype are smeared in the small-scale model by increasing the thicknesses of the corresponding plates. The dynamic response is evaluated at an impact velocity of 7.22 m/s and the impact point is chosen between two frames to assure damage to the outer shell plating and stringers. The simulations are performed by LS-DYNA finite element solver. They aim at evaluating the influence of strain hardening and strain rate hardening on the global impact response of the structure, following different models proposed in the literature. Moreover, the numerical model is scaled to its full-scale prototype, summarizing the governing scaling laws for collision analysis and evaluating the effect of the material strain rate on the plastic response of large scaled numerical models.Copyright

Development of Simple Design-Oriented Procedure for Predicting the Collision Damage of FPSO Caisson Protection Structures

Protection structures for FPSO caissons are required to be strong enough not to contact caisson pipes even when the protection structure is damaged by impact by attendant vessels. In the present structural design process, non-linear commercial packages are employed for the collision analyses. However, non-linear collision analyses using commercial packages are still time-consuming and expensive to operate especially at the initial design stage. In this study, validation of the adopted commercial package was firstly performed using collision test results on unstiffened tubulars. Then, a rigorous parametric study was conducted on simple protection structures by changing the collision velocity and the scantling of the protection structure. A simple analytical expression was derived assuming that the kinetic energy of the striking vessel is dissipated by plastic elongation of tubulars and rotation of plastic hinges. Using the parametric study results, an equation for the modification factor was obtained by which the effects of local denting and dynamic behavior can be considered. The developed procedure was also substantiated using numerically predicted extent of damages of an actual protection structure.Copyright

Fracture Estimation of Stiffened Plates under Impact Loading using Micromechanics Plasticity Model

This paper first reviews the physical meanings and the expressions of two representative strain rate models: CSM (Cowper-Symonds Model) and JCM (Johnson-Cook Model). Since it is known that the CSM and the JCM are suitable for low-intermediate and intermediate-high rate ranges, many studies regarding marine accidents such as ship collision/grounding and explosion in FPSO have employed the CSM. A formula to predict the material constant of the CSM is introduced from literature survey. Numerical simulations with two different material constitutive equations, classical metal plasticity model based on von Mises yield function and micromechanical porous plasticity model based on Gurson yield function, have been carried out for the stiffened plates under impact loading. Simulation results coincide with experimental results better when using the porous plasticity model.