Ship Computer Aided Design

MR 422

Contents

Associative Geometric Modelling

• Parametric Modelling.• Variational Modelling.• Feature-Based Modelling.• Relational Geometry.

Geometry Standards

Associative Geometric Modelling

• Represent and store generative relationships between the geometric elements of model in such a way that some elements can be automatically updated when others change, in order to maintain the captured relationships.

• The general concept can save much effort in revising geometry during the design process and in modifying.

Types of AGM : 1. Parametric Modelling.2. Variational Modelling.3. Feature-Based Modelling.4. Relational Geometry.

1. Parametric Modelling

• Geometric shapes are related by formulas to set of leading dimensions which become the parameters defining a parametric family of models.

• The sequence of model construction steps is stored in a linear history which can be replayed with different input or modified.

2. Variational Modelling

• In variational modelling , geometric positions, shapes, constructions are controlled by a set of dimensions ,constraints , and formulas which are solved and applied simultaneously rather than sequentially.

3. Feature-Based Modelling

• It is groups of modeling operations and it can be used in varying contexts.

• Holes, slots, fillets, and ribs are commonly utilized in mechanical designs.

• In ship design, web frames, stiffeners, and shell plates might be recognized as features and constructed by high-level operations.

4. Relational Geometry

• An object-oriented associative modelling framework in which entities are constructed with defined dependency relationships between them.

• Entity : point, curve, surface, and solid geometric elements

• Each entity in an RG model retains the information as to how it was constructed, and from what other entities, and consequently it can update itself when any underlying entity changes.

4. Relational Geometry

Directed graph ( digraph)Is the underlying logical structure of an RG model, in which

each node represents an entity, and each edge represents a dependency relationship between two entities.

The graph is directed, because each dependency is a directed relationship, with one entity playing the role of support or parent and the other playing the role of dependent or child.

• An entity’s ancestors means all its supports back to the beginning of the model.

• An entity’s descendants means all its dependents.

Geometry Standards

IGES (Initial Graphics Exchange Specification):

• neutral standard computer file format For exchange of geometric information between CAD systems.

• ASCII(text) file is the most used.• Organized (start, global, directory entry, parameter

data, and terminate)• No standardized way to relationships between entities.• IGES is a project of the American National Standards

Institute (ANSI)

Geometry Standards

STEP (Standard for the Exchange of Product model data)

is an neutral standard for capturing, storing, and communicating

digital product data.

STEP describes non geometric information such as design inlet and

decisions, materials, fabrication and manufacturing processes,

assembly and maintenance of the product and geometric

information is still a very important component of STEP

representations. • STEP is a project of the International Standards Organization (ISO).

Geometry Standards

• STEP is implemented in a series of APs (Application Protocols) related to the requirements and intersts of various industries.

• The APs developed for shipbuilding are:- AP-215 (ship arrangement) AP-216 (ship molded form) AP-217 (ship piping) AP-218 (ship structures)