ASSESSMENT OF GROUNDING STRUCTURAL RESISTANCE OF SHIPS

Abstract

The grounding of Exxon Valdez (I 989) near the coast of Alaska focused attention on the effect of a major oil spill; there has been a renewed interest in gaining a better understanding of the mechanics of bottom damage and overall failure of the ship hull during grounding.

The study in this thesis relates the ability of a ship to operate in risky areas to the ultimate strength of its bottom and its ultimate hull girder strength. It is believed that these parameters are better determined using simple analytical formulae and simple mathematical models (finite element models). If the grounding structural resistance of a ship is represented in this manner, this may be helpful for design purposes, operation and salvage purposes. For instance, the external work causing the untolerable damage can then be determined and related to the kinetic energy of the ship in order to find the critical ship's speed in a specific navigation zone, known by it high grounding risk level. It is then only important to know the construction and scantlings of the ship's bottom, and the type of sea bed in the concerned area (rocky, soft, clay, etc... ).

The main analytical tools of tl1is research are taken from the theory of plasticity. The problem is considered to be a quasi-static elasto-plastic model. The methods proposed are extensions to other basic approaches which have been applied to ice­ strengthening studies and collision studies. It is attempted to examine orthotropic panels at impending collapse, in order to evaluate the collapse load, the allowable permanent set and the energy of deformation. The selection of the suitable collapse mechanism depends on the aspect ratio and the orthotropic factor of the panel. It is also attempted to introduce a kind of "energy charts" for various plate strips and

gross panels; in these charts the energy of deformation of the considered structure would be plotted versus its maximum deflection. If prepared for a large variety of structures, the energy charts would facilitate the application of the proposed methods.

The adoption of a certain approach depends on whether the grounding is on a soft bed or a rock; this is referred to soft grounding and rigid grounding analyses, respectively. ln each case tlw analysis of the energy dissipation is done following