Page 123 - Marine Structural Design

Part I

Structural Design Principles

Chapter 6 Offshore Structural Analysis

6.1 Introduction

6.1.1 General
This chapter describes the primary considerations that the design engineer should bear in mind
during the initial design and subsequent structural analysis. In this chapter, the notation
‘Structures’ refers to all types of marine units ranging from floating ship-shaped vessels to
bottom founded platforms. Emphasis has been placed on ship shaped structures. However,
consideration is also given to column supported structures (e.g. semi-submersibles, tension leg
platforms (TLPs), spars, and mooring buoys) and also to steel bottom founded offshore
structures such as fixed steel jackets.
The UK HSE completed a study on offshore structures in the North Sea, which estimated that
around 10-15% of failures were related to inadequate design either at the initial design phase
or a subsequent upgrade in the design. Inadequacy in design includes lack of operational
considerations, failure to evaluate all structural elements and incorrect use of the design
formulae.
In the process of design, the primary concerns for the designer are risks to life, the structure,
the environment, and project economy. Hence, the relevant design codes and standards employ
the appropriate safety factors in order to minimize these risks without being excessively
conservative.
Throughout this chapter, emphasis is placed on the design process where the finite element
analysis will be employed. Reference is made to the formulae used in the design of marine
structures, although these are not reproduced within this Chapter. These formulae may be
found from Part II and Part III of this book together with the background information.
6.1.2 Design Codes
The designer is faced with a large number of rules, codes, standards, and specifications
describing the general policy for structure systems and the detailed design of structural
components. These documents are produced and distributed by:
National Governments
Certification Authorities
Technical Standards Committees
Companies, Universities, or Individual Expertise