Elsevier

Ocean Engineering

Volume 35, Issue 2, February 2008, Pages 281-286
Ocean Engineering

Methods for ultimate limit state assessment of ships and ship-shaped offshore structures: Part III hull girders

https://doi.org/10.1016/j.oceaneng.2007.08.008Get rights and content

Abstract

The present paper is Part III of a series of three papers on the methods useful for ultimate limit state assessment of ships and ship-shaped offshore structures. It is focused on the methods for the progressive collapse analysis of hull girders under bending moments, in contrast to the previous two papers (Parts I and II), respectively, dealing with methods for the ultimate limit state assessment of unstiffened plates and stiffened panels. An AFRAMAX-class hypothetical double hull oil tanker structure designed by IACS common structural rules (CSR) method is studied as an illustrative example. The ultimate vertical bending moment capacity of the hull structure is then analyzed by ANSYS FEA, ALPS/HULL, and IACS CSR methods, and their resulting computations are compared.

Introduction

In maritime industry, the ultimate limit state is now applied as a basis of structural design and strength assessment (ISO, 2007, ISO, 2006; IMO, 2006; IACS, 2006a, IACS, 2006b). This means that the ultimate limit state assessment is now a mandatory task for structural design of ships and ship-shaped offshore structures.

For the ultimate limit state assessment of hull girders that are composed of a number of structural components, it is desirable to perform the progressive collapse analysis. This is because the progressive collapse analysis can reflect the local failures of individual structural components and their interacting effects in the best way.

The aim of the present series of the study has been to evaluate methods useful for the ultimate limit state assessment of ships and ship-shaped offshore structures. Part I (Paik et al., 2007a) and Part II (Paik et al., 2007b) deal with methods for the ultimate limit state assessment of plates and stiffened panels, respectively, using ANSYS nonlinear finite element analysis (FEA) (ANSYS, 2006), DNV PULS (DNV, 2006), and ALPS/ULSAP (2006). The present paper (Part III) is focused on methods for the progressive hull girder collapse analysis.

As an illustrative example, an AFRAMAX-class hypothetical double hull oil tanker structure designed by IACS CSR method is studied. The ultimate vertical bending moments of the hull girder in sagging and hogging condition are analyzed using ANSYS FEA, ALPS/HULL (2006), and IACS common structural rules (CSR) (IACS, 2006a), and their computational results are compared.

Section snippets

Candidate methods

For ultimate vertical bending moment calculations of hull girders, the following three methods are used, namely

  • ANSYS nonlinear FEA (ANSYS, 2006);

  • ALPS/HULL (2006); and

  • IACS CSR method (IACS, 2006a).

For the ultimate bending moment calculations, IACS CSR method provides two approaches, namely single-step method and incremental-iterative (or multi-step) method. The single-step method is applicable only for ultimate sagging moment capacity prediction, while the multi-step method can be applied for

The object ship and its structural characteristics

The object ship is an AFRAMAX-class hypothetical double hull oil tanker designed by CSR methods as indicated in Table 1. While some details of the object ship are presented in Parts I and II, they are briefly described in the present paper for convenience. Fig. 1 shows the mid-ship section of the ship. Fig. 2, Fig. 3 show the stiffened plate structures of the object ship at deck and outer bottom, respectively. The entire ship structure is made of 32AH high tensile steel with yield stress σY of

Progressive hull collapse analysis

The ultimate vertical bending moment of the hull structure is now studied using ANSYS FEA, IACS CSR, and ALPS/HULL methods. It is noted that the hull structural dimensions applied for the present analysis were defined by excluding 50% corrosion margin values of individual structural components as specified by IACS (2006a), because the hull structure was designed to meet IACS CSR requirements.

Fig. 4, Fig. 5 show ANSYS FEA and ALPS/HULL models employed for the progressive hull collapse analysis

Concluding remarks

The aim of the present study has been to identify the accuracy and applicability of some candidate methods, which are considered to be useful for ultimate limit state assessment of ships and offshore structures.

For ultimate hull girder strength predictions, it is found that the IACS CSR method by either single- or multi-step approach gives accurate results compared with more refined ANSYS FEA and ALPS/HULL progressive hull collapse analysis method solutions for sagging. However, the ultimate

Acknowledgments

The present study was undertaken at the Ship and Offshore Structural Mechanics Laboratory (http://alps.ac), Pusan National University, Korea, which is a National Research Laboratory funded by the Korea Science and Engineering Foundation (Grant no. ROA-2006-000-10239-0). The authors are pleased to acknowledge the support of Samsung Heavy Industries. Part of the present paper was presented at MARSTRUCT 2007 Conference, Glasgow, UK.

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