Ship Grounding: Analysis of Ductile Fracture, Bottom Damage and Hull Girder Response

Abstract

The present work is concerned with problems related to grounding of ships. This ranges from prediction of fracture to analysis of hull girder loads during grounding. The thesis is composed as a collection of articles and consists of two parts. The first part gives an introduction to the topic and summarizes the findings from the PhD. work. The second part presents the articles.

A significant part of the thesis is dedicated to the work on fracture. Reliable prediction of fracture initiation and propagation is essential in order to estimate the hull indentation resistance and the oil spill during grounding. In order to better predict the onset of fracture two relatively advanced criteria are investigated. These are the BWH instability criterion, Alsos et al. (2008b), and the RTCL damage criterion, Töornqvist (2003), respectively. These criteria are implemented into LS-DYNA by the author and verified by finite element simulations. In addition, problems related to mesh sensitivity close to the point of fracture is discussed. An attempt to improve the existing way of dealing with this effect is made.

The response of stiffened panels subjected to extreme lateral loads is investigated by testing. The panels are indented to and beyond the point of fracture. The experimental results are further compared with numerical simulations where fracture is predicted using the BWH and the RTCL failure criterion.

An important issue in problems related to ship grounding is the size and shape of the sea floor. As information about this is generally limited, this is discussed in view of the resistance to indentation which is typical for grounding at various sea floor geometries. From this, characteristic scenarios are defined. The resistance to indentation is analyzed for a typical oil tanker for various positions of contact. The interaction between global hull girder bending effects and local indentation actions is also addressed.

Ships which run aground with forward speed may suffer hull damage over the entire girder length. Analysis of this is made complex by the temporal and spatial extent which this implies. Thus, complete finite element simulation becomes very demanding. A new, simplified approach to estimate the contact actions and the subsequent damage due to grounding is therefore presented. Excellent agreement is obtained between the simplified procedure and numerical simulations.

The application of the simplified sliding force method is illustrated by simplified simulations of dynamic/powered grounding. It has been shown that the process is indeed dynamic and that large hull girder bending moments and shear forces are formed, especially during the initial impact. These shear forces and bending moments may become larger than the minimum still water rule requirements set by the classification society. This correlates well with observations made by Pedersen (1994) and Simonsen (1997a).