Page 301 - Practical Design Ships and Floating Structures

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substantial restrictions on the geometries handled, lack of computing power and unreliable analysis
tools. A short overview of previous work on automated optimization of offshore structures is given by
Birk (1998).
Today, automated optimization is mature enough to be a valuable tool in designing better floating
systems (Birk and Harries, 2000). This approach shifts the main focus from repeated interactive work
with CAD and CFD tools to the more important definition of objective functions, parametric
description of hull properties and assessment of final results. The following section concentrates on
rational design criteria suitable to compare motion behavior of different designs. For a detailed
description of the optimization framework used in this research work the interested reader is referred
to Birk (1998). Only a short review is presented below.

2 RATIONAL DESIGN CRITERIA

Automated assessment of design variants relies on rational criteria, i.e. an objective function which
provides the scale necessary to compare different designs. Appropriate models of system behavior and
environmental conditions have to be set up.
2.1 ModelIing System Behaviour

The performance assessment of each design variant starts with computation of selected response
amplitude operators of forces and motions. The application presented below utilizes results obtained
by the well established 3D-diraction-radiation software package WAMIT' (Newman and Sclavounos,
1988). The program is based on linear potential theory and neglects viscous effects. Although higher
order quantities and viscous effects may be important in special cases linear analysis based on potential
theory will be generally sufficient when comparing different designs. The complex ratio of output and
input signals s(w) and <(w) for each wave frequency w constitutes the transfer function or
response amplitude operator (RAO) HSc(m)=- 40) which gives a complete description of the
&w)
corresponding hydrodynamic characteristics. Values of HsC (0) are computed and stored for a larger
number of wave frequencies and an appropriate number of wave headings.
In many cases motions of selected points 5, are of special interest. Within linear theory they are easily
deduced from the translational and rotational motion vectors of the centre of gravity sT and gI), i.e.
.
s,=sI.+es,) The vector r describes the location of the reference point with respect to the centre of
gravity. Since all quantities are harmonic with wave frequency o amplitudes of velocities and
accelerations are computed by simple multiplication of E, by w and - o2 respectively.

2.2 Modelling environmental conditions

The hydrodynamic analysis yields transfer functions, which characterize the behavior in regular waves.
In reality, the elevation of the ocean surface is irregular and of random nature. Hence, rational
seakeeping criteria have to be based on a probabilistic description of random seas. For intervals
ranging from one to three hours the statistical parameters of irregular seas do not vary much, i.e. the
process is stationary (Barltrop and Adams, 1991). Gaussian distribution of wave elevations and
Rayleigh distribution of wave heights are assumed. The description of these short- term sea states is
commonly based on design spectra, representing the frequency dependent energy distribution of waves.
The relevant parameters are significant wave height H, and mean zero-up- crossing period To. The
significant wave height is linked to the variance u' of the random process by H: = 160' (Newland,