Page 113 - Marine Structural Design

Part I

Structural Design Principles

Chapter 5 Ship Hull Scantling Design by Analysis

5.1 General

Classification rules have traditionally been the mainstay of ship design practices. These rules
are primarily semi-empirical in nature and have been calibrated to ensure successful
operational experience. They have obvious advantages - simple in format and familiar to most
ship designers. Nevertheless, the ship sizes have increased dramatically and the ship designs
have changed remarkably in the past 20 years. The conventional design approach that relied on
the "Rule Book, has been seriously challenged with the development of unconventional ship
types and complex ship structures such as high speed vessels, large opening container ships
with considerably increased capacity, large LNG-carriers, drilling ships, FPSOs, etc. The
conventional design rule formulae involve a number of simplification assumptions and can
only be used within certain limits. Moreover, scantlings based on rules are not necessarily the
most cost efficient designs. Hence, the application of rational stress analysis using FEM has
gained increasing attention in the shipbuilding industry. With the rapid growth of information
technology, computational complexity is no longer a big issue and numerical efficiency is not
the main concern in the design procedure. The actual design approach includes the overall
strength analysis by accounting for both static and dynamic loads and evaluation of the fatigue
life of all critical structural details. This approach provides a well-designed and uniformly
utilized structure, which ensures a higher degree of reliability than past structures.
A rational analysis procedure is presented in this Chapter, starting fiom design loads, strength
criteria, FEM analysis, up to the assessment of the obtained calculation results. FEM analysis
is discussed in detail, including modeling, load application, application of boundary conditions,
element selection, and post-processing. The summarized procedure of strength analysis can be
seeninFigure 5.1.

5.2 Design Loads

The design loads acting on the overall ship structure consist of static and dynamic loads. Static
loads include dead and live loads, such as hydrostatic loads, and wind loads. Dynamic loads
include wave induced hydrodynamic loads, inertia loads due to vessel motion, and impact
loads. The various loading conditions and patterns, which are likely to impose the most
onerous local and global regimes, are to be investigated to capture the maximum local and
global loads in structural analysis. Sloshing and slamming loads should also be taken into
account where applicable. When designing ocean-going ships, environmental loads are usually
based on global seastate criteria due to their mobility. While for offshore structures,
environmental loads are calculated in accordance with specifically designed routes and/or site
data.