Jørgen Amdahl

Rapid assessment of ship grounding over large contact surfaces

A simple method for the rapid assessment of ship bottom structures subjected to grounding over seabed obstructions with large contact surfaces is proposed in this paper. It has been recognised that the shape and size of the seabed obstruction is of crucial importance in relation to the characteristics of bottom damage. Most studies on ship grounding are concerned with ‘rock-type’ sharp obstructions, where plate tearing is the dominating failure mode. However, very few studies are found related to grounding over blunt obstructions with large contact surfaces such as ‘shoals’. Denting rather than tearing is more likely for the bottom plating as observed from actual grounding incidents. The sharp obstruction may cause earlier penetration and result in very unfavourable consequences such as compartment flooding. In contrast, the bottom plating may not fracture when moving over a blunt-type sea floor. But it may threaten the global hull girder resistance and give rise to even worse consequences such as hull collapse. The proposed simple method is established on the basis of a series of closed-form solutions for individual structural members developed from the plastic mechanism analysis. The primary deformation modes for the major bottom structural members are sliding deformation of longitudinal girders, denting and crushing of transverse members and indentation of bottom plating. The effect of friction is considered and estimated in a simple manner. The vertical resistance that governs the vertical ship motion is derived. It is found that the vertical resistance is free of friction. The proposed simple method for bottom strength is verified against large-scale non-linear finite element analyses, where a good correlation is obtained.

Plastic mechanism analysis of the resistance of ship longitudinal girders in grounding and collision

A theoretical model for ship bottom longitudinal girders being crushed during accidental raking process has been proposed and developed on the basis of nonlinear finite element analysis. Focus is placed on establishing the basic folding mechanism, and identifying the major energy dissipation pattern. Using the plastic methods of analysis, the simplified analytical expression for total energy dissipation, which is composed of bending and membrane energy, has been formulated. Subsequently, the mean horizontal resistance is derived. It depends on two free parameters, namely crushing distance and wave angle. It is noted that the optimality condition could not be achieved by minimising the energy expression or mean crushing force with respect to these free parameters. Therefore, empirical expressions, which agree reasonably with the results from finite element simulations, have been employed. Three series of parametric/sensitivity studies have been carried out by using LS-DYNA code. Comparisons between the proposed simplified method and simulations have been made. In some cases, excellent agreement is obtained. However, some discrepancy between theory and numerical simulations is also noticed and needs further investigation. The theory presented is not only pertinent to longitudinal girders in ship bottom structure during raking but is also relevant to side stringers in sliding collision situations. The simplified method developed will contribute substantially to the establishment of efficient methods for fast and reliable assessment of the outcome of accidental collisions and grounding events. Such methods may in turn be incorporated into decision support tools for crisis handling in emergency situations, e.g. for tankers in disabled conditions.

Ice-load estimation for a ship hull based on continuous response monitoring

The expected growth of maritime and offshore activities in arctic areas has led to an increased interest in understanding the risks associated with operating in ice-infested waters. Transportation systems such as large tankers, gas carriers, and bulk carriers that are able to operate throughout the year are of particular interest. One of the key factors for this scenario to occur is the ability to design vessel hulls that possess sufficient strength to be able to resist the pressures generated by the ice fields without any critical damage taking place. This paper is concerned with the estimation of ice loads acting on the hull of the coastguard vessel KV Svalbard, based on strains that were measured during the winters of 2007 and 2008. A finite element model of the bow structure is utilized in order to correlate the loading with the measured strains. The influences of ice thickness and vessel speed on the measured strain levels are also investigated. Methods for extrapolation of the hull response into the future for predictive purposes are also addressed.