Analysis of ultimate capacity of the structural elements of single hull VLCC subject to corrosion

Abstract

During the last few decades, the emphasis in structural design has been moving from the allowable stress design to the limit state design, because the latter approach has many more advantages. A limit state is formally defined as a condition for which a particular structural member or an entire structure fails to perform the function that it has been designed for. In order to ensure critical for economy and safe design of a ship’s hull, it is necessary to accurately evaluate the capacity of the hull girder considering extreme loads. The objective of this project is to perform the strength analysis for tank structures, according to the Det Norske Veritas (DNV) Rules and Regulations; following the IACS Common Structural Rules (CSR). The general plan and methodology of the project are presented in the report (dissertation). The project is focused on the analysis of the structural strength of the hull and the ultimate capacity of structural elements and the hull girder of the single hull VLCC, taking into account the influence of the uniform and atmospheric corrosion. Based on the model that describes the phenomenon of the corrosion, a model (2) (see Chapter 4) has been introduced. This model is based on the approach which was taken from (Shengping Qin and Weicheng, 2002); it was used to develop a methodology to predict reduction of thickness of structural elements due to corrosion. The results obtained using the prediction methods were verified against the experimental results. The effect of the corrosion on the section modulus and the variation of the section area of the cargo deck were investigated, as a function of the time and the exponential depth of the corrosion. It was to get the relationship between weight and the ratio of the section modulus during 25 years the service of the tanker. The results of the calculations as a comparison between different models which determine the ultimate stress for the plate and rigid panels, including the results received using the PULS software (the theory of nonlinear plate) are presented in the dissertation. Based on the study of the parameters influencing the reduction in ultimate capacity of the plate, the effect of the corrosion on the reduction of the ultimate capacity was investigated for various regions of the tanker (bottom, deck, side, and longitudinal bulkhead). The analysis of the progressive collapse of the hull girder, based on the computation for the two candidate models, the Rahman code and the results of the Nauticus Hull (using the procedure of CSR) was carried out to evaluate the ultimate bending moment after 25 years of service. At the end of the work, the results of finite elements (using engineering software), the methodology to create the model, the mesh, the boundary conditions and loading conditions will all be presented and discussed. The results show the maximum displacement of three tankers and the distribution of the von Mises stresses in the critical area, during 25 years of ship service; due to the global loading conditions (full load in sagging condition).The dissertation is completed with conclusions from the analyses and recommendations.