Page 95 - Marine Structural Design

Part I

Structural Design Principles

Chapter 4 Scantling of Ship’s Hulls by Rules

4.1 General
In this Chapter, the term “scantling” refers to the determination of geometrical dimensions
(such as wall-thickness and sectional modules) for a structural component/system. The initial
scantling design is one of the most important and challenging tasks throughout the process of
structural design.
Mer signing the contract, scantling design is the next step and continues throughout the
design process until the design is approved by the owner, the shipyard, the classification
society, and other maritime authorities. Hull form, design parameters for auxiliary systems,
structural scantlings, and final compartmentation are decided on, during the initial design
phase. Hull structural scantling itself is a complicated and iterative procedure.
In recent years, the procedure for dimensioning the hull structure is changing rapidly. First, the
full benefit of modem information technology is applied to automate the routine scantling
calculation based on classification rules. Meanwhile, the application of rational stress analysis
and the direct calculation approach using finite element analysis have gained increasing
attention in recent years.
In order to develop a satisfactory ship structure, an initial scantling design is generally
performed, to establish the dimensions of the various structural components. This will ensure
that the structure can resist the hull girder loads in terms of longitudinal and transverse
bending, torsion, and shear in still-water and amongst the waves. This process involves
combining the component parts effectively. Furthermore, each component part is to be
designed to withstand the loads imposed upon it from the weight of cargo or passengers,
hydrodynamic pressure, impact forces, and other superimposed local loads such as the
deckhouse and heavy machinery.
Generally, this Chapter introduces the design equations for tankers based on IACS
(International Association of Classification Societies) requirements and classification rules
(e.g. ABS, 2002).

4.2 Basic Concepts of Stability and Strength of Ships
4.2.1 Stability

Two resultant forces act on a free floating body, the force of weight acting downwards and the
force of buoyancy acting upwards. The force of weight (W), acts through a point known as the
center of gravity (CG), and the force of buoyancy (B) acts through what is known as the center
of buoyancy (CB). By Archimedes’ Principle, we know that the force of buoyancy equals the