The Design Spiral

The design spiral is one of the most important pieces of the technical know-how of a Naval Architect and/or Ocean Engineer. So what exactly is the Design Spiral and why is it so important for a typical ship designer in the marine industry or any personnel concerned with the design development of a marine structure?

SHIP DESIGN SPIRAL

Designing a ship is perhaps the most rigorous of all engineering problems. It takes a lot of technical expertise, often across many disciplines to design such structures.A ship not only floats on the surface of the ocean but also makes its way around the world, along different routes through rough weather - "in one piece". The Design Spiral is a systematic approach to achieving near perfect designs for a given ship design problem. A naval architect traces his way along the design spiral through the different stages of design given below:

1.CONCEPT DESIGN 

2.PRELIMINARY DESIGN

3.CONTRACT DESIGN 

4.DETAILED DESIGN

As we move along the spiral into its core, we move from a zone of less information to extremely specific instruction for the construction of ship or in other words- "Realization of the design into a product". Given below are the definitions of each stage of design spiral along with the typical effort/man days required for a large merchant ship.

Concept design - 20 man days

This stage witnesses the translation of the mission requirements into naval architectural and engineering characteristics.The fundamental elements of the ship such as length, beam, depth, draft, fullness, power, or alternative sets of characteristics, all of which meet the required speed, range,cargo cubic, and deadweight are used to perform feasibility studies.

Light-ship weight estimation is done.

Alternative designs are analyzed  during this phase to determine the most economical design solution.

Preliminary design - 300 man days

This stage essentially improves and elaborates the major ship characteristics affecting cost and performance. The factors such as length ,beam, horsepower deadweight, would not be expected to change after the completion of this stage. It also provides basis for the development of contract plans and specifications.

Contract design - 5000 man days

This stage describes more precisely  features such as hull form based on a faired set of lines, powering based on model testing, seakeeping and maneuvering characteristics, the effect of number of propellers on hull form, structural details, use of different types of steel, spacing and type of frames . Paramount, among the contract design features, is a weight and center of gravity estimate taking into account the location and weight of each major item in the ship. The final general arrangement is also developed during this stage. This fixes the overall volumes and areas of cargo, machinery, stores, fuel oil, fresh water, living and utility spaces and their interrelationship,as well as their relationship to other features such as cargo handling equipment, and machinery components.

Check on Quality standards of hull and outfit and the anticipated performance for each item of machinery and equipment is done. Also describe the tests and trials required.

Detailed design  - 60000 man days

Comprises of detailed working plans .These plans are the installation and construction instructions to the ship fitters, welders, outfitters, metal workers, machinery vendors, pipe-fitters, etc. At this stage the interchange is from engineer to artisan, that is, the engineer's product at this point is no longer to be interpreted, adjusted, or corrected by any other engineer.