Page 56 - Marine Structural Design

Chapter 2 Wave Loads for Ship Design and Classij2ation 33

different sea-states are weighted according to the likelihood of being encountered by the ship.
This method is computationally more expensive but is a more realistic analysis, see Chapter 4.
Once the method to be used has been chosen, and the design wave load is determined, the
ship's required structural strength may be evaluated.
2.4.2 Design Loads per Classification Rules

General
Structural analysis may be divided into three parts:
establishing the design load,
defining the acceptance criteria,
conducting the strength assessment.
It is relatively easy to establish the acceptance criteria thanks to many years of accumulated
knowledge and expertise from owners, builders, class societies and researchers, see Part I1
and Part I11 of this book for more details. The strength assessment is also rather simple once
the loads and acceptance criteria are defined. However, the most challenging task is to
calculate the different loads that the ship is subjected to. This difficulty arises from the fact
that the ship may be exposed to various sea and wave conditions, and different loading
patterns of the cargo.
Classification societies have proven techniques for calculating the loads on a ship and
evaluating the structural integrity of ship hulls.
Load Components
A detailed design consists of two steps:
the nominal design for initial scantlings,
a more detailed analysis where finite element analysis is used to evaluate the combination
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of a number of load cases and their effects on the ship structure.
In a ship structural design, three load components are considered:
hull girder load, which consists of the still-waterlwave induced bending moments and
shear forces,
external pressure, which consists of a static, hydrodynamic, and an impact slamming load,
internal pressure caused by the liquids carried in tanks onboard the ship. This pressure
depends on the hydrostatic pressure, the changes in pressure head due to pitching and
rolling motions, and the inertial force of the liquid column resulting fkom accelerations of
the fluid.
The following sub-sections describe the evaluation process of these different loads.
Hull Girder Loads
Wave data measured from the North Atlantic are used to determine wave loads. Thus, the
nominal design value of a ship represents the long-term extreme value for the North Atlantic
Sea in 20 years, which corresponds to a probability of exceedance of lo-'. The global spectral
ocean wave models provide data about different wave spectra and different wave heights.