PRESCRIPTIVE BASED DESIGN VS FUNCTIONAL BASED DESIGN.

The maritime safety committee at its 74th session and 82nd session, approved guidelines on alternative design and arrangement for SOLAS CH-II-2 and SOLAS CH II-1 and III respectively.MSC issued guidelines, which serve to outline the methodology for engineering analysis required by above SOLAS chapters on alternative design and arrangements for which the approval of an alternative design deviating from the prescriptive requirement of SOLAS is sought.
      Alternative design and arrangement means measures which deviate from the prescriptive requirement of SOLAS CH II-1, CH-II-2 or CH-III, but are suitable to satisfy the intent of that chapter. For this first of all engineering analysis is carried out. It shows that alternative design and arrangements provide the equivalent level of safety to the prescriptive requirements of SOLAS CH II-1, CH II-2 and CH-III. This approach should be based on sound science and engineering practice incorporating widely accepted methods, empirical data, and calculation, correlation and computer models as contained in engineering textbooks and technical literature.
            A design team acceptable to the Administration should be established by the owner, builder or designer and may include a representative of owner, builder or designer and experts having the necessary knowledge and experience in safety, design and/or operation as necessary for the specific evaluation at hand.
     The design team should:-
1) Appoint a coordinator serving as primary contact.
2) Communicate with the administration for advice on the acceptability of the engineering analysis of the alternative design and arrangement throughout the entire process.
3) Determine the safety margin at the outset of design process and review and adjust it as necessary during the analysis.
4) Conduct a preliminary analysis to develop the conceptual design in qualitative terms.
5) Conduct quantitative analysis to evaluate possible trial alternative designs using quantitative engineering design.
6) Prepare documentation, specification, and a life cycle maintenance programme.
To understand it better let us take the example of part F of SOLAS II-2 :-
                                       PRELIMINARY ANALYSIS

                               FIRE SCENARIO INFORMATION

             QUANTIFY PRESCRIPTIVE SYSTEM PERFORMANCE

                 QUANTIFY PROPOSED SYSTEM PERFORMANCE

  EVALUATE PERFORMANCE OF PRESCRIPTIVE VS PERFORMANCE

         PERFORMANCE OF PROPOSED DESIGN ACCEPTABLE

                                    ALL SCENARIOS EVALUATED

                                          SELECT FINAL DESIGN
But performance based design is not about:-
1) This approach should not be used to justify the use of material or equipment, where material or type approval exists.
2) This approach should be used to demonstrate equivalent or higher level of safety.
Advantage of functional based design:-
1) Cost effective design
2) Design for unique application
3) Formal method for incorporating novel designs.
4) Greater understanding of loss potential.
5) Allows designs to be assessed which are too complex to have relevant regulations applied.
6) Conventional guidance can restrict design flexibility.
7) Can address issues beyond life safety e.g. protection of assets.
Disadvantages of functional based design:-
1) More time consumed.
2) More cost.
3) Greater documentation.
4) Special training will be required for the employees to understand new system.