Page 26 - Marine Structural Design

Part I

Structural Design Principles

Chapter 1 Introduction

1.1 Structural Design Principles
1.1.1 Introduction

This book is devoted to the modem theory for design and analysis of marine structures. The
term “marine structures” refers to ship and offshore structures. The objective of this book is to
summarize the latest developments of design codes, engineering practice and research into the
form of a book, focusing on applications of finite element analysis and riskheliability methods.
The calculation of wave loads and load combinations is the first step in marine structural
design. For structural design and analysis, a structural engineer needs to have basic concepts
of waves, motions and design loads. Extreme value analysis for dynamic systems is another
area that has gained substantial developments in the last decades. It is an important subject for
the determination of the design values for motions and strength analysis of floating structures,
risers, mooring systems and tendons for tension leg platforms.

Once the functional requirements and loads are determined, an initial scantling may be sized
based on formulae and charts in classification rules and design codes. The basic scantling of
the structural components is initially determined based on stress analysis of beams, plates and
shells under hydrostatic pressure, bending and concentrated loads. Three levels of marine
structural design have been developed:
Level 1: Design by rules
Level 2: Design by analysis
Level 3: Design based on performance standards

Until the 1970’s, structural design rules had been based on the design by rules approach using
experience expressed in tables and formula. These formulae-based rules were followed by
direct calculations of hydrodynamic loads and finite element stress analysis. The Finite
Element Methods (FEM) have now been extensively developed and applied for the design of
ship and offshore structures. Structural analysis based on FEM has provided results, which
enable designers to optimize structural design. The design by analysis approach is now applied
throughout the design process.

The finite element analysis has been very popular for strength and fatigue analysis of marine
structures. In the structural design process, the dimensions and sizing of the structure are