Abstract

A theoretical study of the energy released for deformation of structure during collisions between shipe and fixed or floating offshore structures is presented. Special emphasis is laid on a consistent treatment of the hydrodynamic pressures on the ship hull and the overall fleXibility of the platform structure involved. The hydrodynamic forces acting on the 'ship hull during the collision are calculated by means of unit response functions. These functions are determined by cosinetransformations of sectional dampings. The flexibility of the involved structures is determined as elasticplastic load-deformation behaviour at the zone of impact, together with the overall flexibility of the platform structure or the anchoring system. The numerical solution of the equations of motion is obtained by a time-simulation procedure.

The study shows that if the traditional concept of a constant added mass is to be used, then this constant will depend on the duration of impact and the form of the ship. It is also shown that in the case of a relatively long duration of the collision, the traditionally used added mass in the expression for the kineticenergy of the ship striking the platform should be chosen as a larger value than normally prescribed. An approximate method is given to determine this added mass. Finally, the study shows that it is important to take into account the overall flexibility of the platform.

Introduction

It is the aim to present a mathematical model which can describe the mechanics of a collision where a ship sways arid yaws into an offshore structure, which can either be a fixed steel jacket or an anchored platform: Special emphasis is placed on obtaining a consistent procedure for the outer dynamics, in which all essential features are maintained, so that quantitative results of sufficiently high accuracy can be produced. Therefore, the model is derived so that it takes into account memory effects of the fluid-structure system, the overall flexibility of the involved structures and the local elastic-plastic deformation near the zone of impact.

Before the mathematical model is presented in detailthe physical problem will be described and reference will be made to some of the existing literature on the subject. In order to calculate the damage that occurs during a collision between a ship and an offshore platform the traditional approach is to estimate the amount of energy available for deformation during the collision and thereafter determine how this energy is absorbed by the structure of the ship and the platform.

When dealing with the problem of determining the amount of energy released for deformation, the most common approach is to assume that the ship will be totally stopped by the platform and that all its kinetic energy will be released for deformation. The energy that must be absorbed by the structure involved is then determined by;

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