Ship Computer Aided Design

MR 422

Outline

1. Classification and Regulation.2. Tooling and Manufacturing.3. Maintenance and Repair. 4. Levels of definition – Particulars– Offsets– Wireframe– Surface models – Solid models

Classification and Regulation

• Classification is a process of qualifying a ship or marine structure for safe service in her intended operation and to qualify for commercial insurance . It is extremely important to respect classification requirements during design process.

• Although final approvals depend on inspection of the finished vessel, it is extremely important to anticipate classification requirements at the earliest design stages.

• Much of the information required for classification and regulation is geometric in nature (design drawings and geometric models).

Tooling and Manufacturing

Manufacturing is the creation of individual parts from various materials through

diverse fabrication, treatment, and finishing processes, and the assembly of these parts into the final product.

• During assembly process it is extremely important that the parts fit within suitable tolerances. otherwise modifications or replacements would be needed and hence potentially enormous costs in materials, labour, and production time.

• Tolerances for the steel work of a ship are typically 1 to 2 mm throughout the ship, regardless of the ship’s size.

Tooling and Manufacturing

• most of the solid materials going into fabrication are flat sheets.

• The majority of the geometric information required is 2-D profiles ( frames, bulkheads, floors, decks, and brackets).

• Even for parts of a ship that are curved surfaces, the information required for tooling and manufacturing is still typically 2-D profiles: mold frames, templates, and plate expansions.

• 3-D information is required to describe solid and molded parts such as ballast castings, rudders, keels, and propeller blades, but this is often in the form of closely spaced 2-D sections.

Tooling and Manufacturing

• For numerically controlled (NC) machining of these complex parts, which now extends to complete hulls and superstructures for vessels up to at least 30 m in length, the geometric data is likely to be in the form of a 3-D mathematical description of trimmed and untrimmed parametric surface patches.

Maintenance and Repair

• Geometry plays an important role in the maintenance and repair of ships.

• When ship has been manufactured with computer-based geometric descriptions, the same information can be valuable during repair, restoration, and modification.

• The diversity of the CAD system used in different shipyards prompted the development of standard file format types like IGES and STEP.

• When defining geometric information is not available for a ship undergoing repairs ,a challenging process of acquiring shape information usually ensues.

Levels of Definition

The geometry of a ship can be described with 5 levels of definition , which are

1. Particulars2. Offsets3. Wireframe4. Surface models 5. Solid models

1. ParticularsReferring to the description of a vessel in the terms of a small

number of leading linear dimensions and other volume or capacity measures such as:

Length overall (LOA) , Water length line (LWL), Design water line (DWL) , Length between perpendicular (LBP or LPP )

Beam : the maximum lateral extent of the molded hull Draft : the maximum vertical extent of any part of the vessel

below water line Form coefficients : such as block coefficient, and prismatic

coefficient Displacement : the entire mass of the vessel and contents in

some specified loading condition Tonnage : measure of cargo capacity

2. Offsets

Represent a ship hull by means of a tabulation or sampling points from the hull surface.

An offsets-level description of a hull can take two forms:• The offset table ( a document of the numerical values

represents by points on station, waterline, buttock planes. )• The offset file computer readable form, represents the hull

by points that are located on transverse sections.

3. Wireframe

• Wireframes represent a ship hull or other geometry by means of 2-D and 3-D polylines or curves

• It shows only a finite number of the possible plane sections , to locate points on the surface that do not lie on any wires requires further interpolation steps

4. Surface Modelling

• Surface of product : mathematical formulas are developed and maintained .

Advantage over wireframe are• Highly precise and more compact , and easier to modify .• Wireframe can be easily computed from the surface . • These calculations are conducted with sufficient accuracy .• also more complete : points can be evaluated on vessel ‘s

surfaces at any desired location . • Such views will be 100% consistent with each other and with

3-D surface

5. Solid Modelling

• A solid is represented by describing its boundary surfaces , and those surfaces are represented , manipulated , and evaluated by mathematical operations similar to surface modeling .

• Solid modeling function is called Boolean operation .• The geometrical model contains full information about which

surface elements are the boundaries of which solid objects .