Ship grounding and hull-girder strength

Abstract

A mathematical model for analysis of ship hull loading due to grounding on relatively plane sand, clay or rock sea bottoms is presented. The analysis model of the grounding event is separated into two phases. In the first phase the ship is subjected to an impulse caused by the sudden contact with the ground. This initial impulse is assumed to be completely inelastic and leads to a rapid change of the forward speed such that after the impact the ship has heave and pitch velocities which make its motion compatible with that of the contact point along the ground. In the second phase the ship is sliding with continuous contact to the ground. This means that the kinetic energy which is available after the end of the first impulse is, in this second phase, transformed into potential energy and into friction in the contact zone between the ground and the ship. In both phases the forces exerted on the ship bow are assumed to be governed by a Coulomb friction law. Grounding forces, sectional shear forces and bending moments caused by the grounding are determined and related to the ultimate capacity of the hull girder. First approximations to the ground reaction and the sectional forces are found to be proportional to the initial forward velocity of the ship. The results show that larger ships are more exposed to hull failure due to grounding than smaller ships. Finally, the mathematical grounding model is verified by model tests and controlled full-scale grounding experiments.