Page 93 - Practical Design Ships and Floating Structures

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design process have been analysed and a top-down hierarchy of design elements has been identified.
Within the novel approach the high-level descriptors used in the shipyard’s daily optimisation process
can be translated directly into a geometric definition. Physical properties of the hull shape are
maintained automatically and the resulting curves and surfaces yield excellent fairness. The naval
architect’s craftsmanship of geometric modelling has become part of the internal generation process
without compromising his or her freedom of creativity. The flexible set of parameters and the hlly-
automated adoption of patch arrangement makes the program a powerful tool. The designer can thus
concentrate on the optimisation ofthe ship to improve its performance and thereby its value as well as
the competitiveness of the designing yard.
In the sections to come a classification of design language is presented. Subsequently, the modelling
approach and the hierarchical character of design parameters and their implementation in the
generation process of complex hulls are outlined. Global variations induced by single parameter
changes as well as local changes typically applied in the hydrodynamic optimisation process illustrate
the applicability of the parametric design methodology.

2 DESIGN LANGUAGE

The geometric design of ship hull forms consists of several subsequent procedures based on a wide
range of abstraction levels. The description of specific properties of a ship may vary from a global
expression like apostpan max container carrier with draft restricfion to a mathematical detail like the
weight of the second-to-last control point on thatpame should be slightly decreased. Both descriptions
are needed to carry the necessary information from one partner to another without burdening the
communication with excessive data.
The view on a ship, and therefore the language applied for its description, depends basically on the
context which governs the particular situation. While at the global level of description the appearance
of a ship dominates the vocabulary, at the stage of hydrodynamic optimisation the communication on
the basis of functional descriptors, e.g. form parameters, is more usehl. The CAD-system which is
applied to model the ship shape again requires a completely different language since it is based on
patch arrangements, vertex coordinates and weights defining the hull by means of a specific
mathematical method.
The modelling process of a hull can be performed independently at any of these three levels. The
selection of features used to describe the hull form at any of these levels has to follow a topological
description. For a better distinction let us introduce three topology levels that we call
TopoIogy of Appearance,
Topology of Design and
Topology of Representation.
All three levels are applied within the design process in close relation, see Figure 1.
Applying state-of-the-art CFD-programs represents today’s standard for performance evaluation of
ships in the early design stage. Due to their reliable ranking, see e.g. (Harries and Schulze 97), they
can be utilized in the optimisation process. Shape variation and decision-taking is generally performed
on the basis of functional descriptors - at the level of topology of design, i.e., the second level in
Figure 1. However, the numerical simulation programs (CFD) require a complete geometric
description stemming from the lowest level, Le., the third level in Figure 1. Even though the simulation
expenses in terms of computing time play a significant role, the implementation of the desired shape
variation is of even higher magnitude since it is usually brought about by hand. Consequently, the
limited period of design refinement restricts the number of iterations possible and systematic variations
cannot be performed in general.
The different languages applied to the description of ship characteristics represent a major bottleneck
in hull form development. Design decisions are usually taken on a more abstract level and the lack of
automated mapping of functional descriptors into a corresponding mathematical representation makes
optimisation a time-consuming, highly interactive process. In most cases no comprehensive