Page 55 - Marine Structural Design

32 Part I Structural Design Principles

simple formulas, calculated results may be inaccurate.
6) If particular measures are required to preventheduce greenwater impact, past design
experience can be used, including increasing freeboard, using better bow shape and flare,
adding some protection measures, etc.

It should be noted that steps 1) through 3) may be replaced by a single step, Le, sophisticated
numerical analysis, if a reliable prediction method becomes available in future. Although great
effort has been made in recent years to develop such methods, there is no method considered
to be satisfactory. Therefore, use of model test results is recommended for design

A risk based approach may be more helpful for design decision making. The probability
analysis presented in Wang, Leitch and Bai (2001) can be expanded and modified to form
such a method. However, the probability (likelihood) of vessel heading involves a
considerable quantity of analysis work and some model tests may also be required. In addition,
the probability of vessel draft is also difficult to accurately determine because it is a function
of production rate, offloading rate (and frequency) ballast plan and rate, etc.

2.4 Ship Design for Classification
2.4.1 Design Value of Ship Response
The ultimate goal of determining the wave loads and the ship's response to these loads is to
obtain the design value of the ship's response. This involves making predictions of the worst
seas in which the ship could encounter within its lifetime. There are four factors, which are
going to influence the design value of the ship's response (Hughes, 1988):
. The severity of the sea-state, as characterized by the significant wave height, the frequency
of occurrence, and the duration of each level of severity. This data is used to determine the
ship's exposure time to each sea-state of different severity.
The shapes of the wave spectra for each sea-state.
The ship heading (direction) in a given sea-state.
The ship speed for a particular heading and sea-state.
The overall aim is to determine the largest response value resulting from the worst
combination of wave loads, which has a probability, a, of being exceeded during the ship's
life. This design value a, is a risk parameter determined by the ship designer and is used to
calculate the structural response of the ship. A typical value of a is 0.01.
There are two methods used to determine this design value as below.
The first method assumes that the largest waves appear in the most severe stationary sea-state,
which the ship is likely to encounter. This is called the "design wave method". Thus, this wave
value is used as the design value of the ship, along with a couple of less severe sea-states. This
method may not be considered to be accurate, because a larger wave may be encountered in a
less severe sea-state. However it is less time-consuming and is the preferred method unless a
more accurate determination of the design value is required.
The second method requires that all possible sea-states, which the ship is likely to encounter in
its lifetime, be evaluated. A complete analysis of all the sea-states is carried out and the