Page 42 - Marine Structural Design

Part I

Structural Design Principles

Chapter 2 Wave Loads for Ship Design and Classification

2.1 Introduction
One of the major aspects of ship design is the calculation of wave-induced loads on the ship
structure. The difficulty in calculating this load arises from the fact that the sea is highly
irregular. Hence a number of techniques have been developed to tackle this problem. These
techniques enable the sea waves be defined in a mathematical form and this may then be used
to calculate the wave loads on the ship and ultimately the response of the ship to these loads.
When designing a ship, formulae provided by classification societies are used in order to
calculate the wave loads and ship response. However, a ship designer ought to have some
knowledge of the theory and techniques utilized for the statistical determination of wave loads.
Novel ship designs also exist, which require an extensive statistical estimation of the wave
loads to be undertaken in addition to using rule-based formulae alone.
As a basis for marine structural design, the objectives of this Chapter are threefold:
Present various ocean wave spectra and wave statistics
Discuss the wave-induced loads, slamming and green-water loads and hence the response
of the ship
Outline the design load calculations per ship classification rules.
For more information on wave loads acting on ship structures, reference is made to
Bhattacharyya (1978), Hughes (1988) and Jensen (2001).

2.2 Ocean Waves and Wave Statistics

2.2.1 Basic Elements of Probability and Random Process
Obtaining ocean wave data requires the use of different elements of statistics and probability.
Therefore, an introductory reference to statistics and probability is given prior to dealing with
wave loads.
In statistics, a random variable X is an event or an outcome among all possible outcomes. If all
possible outcomes form a continuous space, -Q) < x < co, and all events possible are a part of
this space, then the probability density function of an event occurring is the probability that X
lies within that portion of x. The probability density function is written as px(x). Thus in
Figure 2.1 the probability that X lies between x and x + dx, is px(x)dx. From this figure, we
may also define the mean value pux as: