Decision-Making Process

Marine product design process consists of designing individual components, designing sub-systems consisting of a number of components, system designing integrating a number of sub-systems and finally designing the whole product. At each stage and level of design, it is necessary to find a solution which must satisfy a certain performance level or a number of performance levels where the level is not absolute, but a maximum or minimum level such that performance achieved or predicted is below or above the pre-determined level, respectively. The design must also be subject to a number of pre-defined constraints. A standard overall ship design problem could be that the main dimensions and form of the ship need to be determined subject to constraints consisting of dimensional limits, stability and freeboard requirements, etc. with the performance or merit function which could be the ship’s first cost or net present value (NPV) of ship operation. Identification of the design variables and determination of complete specifications of the merit function(s) and constraints are required to define the design problem. One can observe that there could be a number of design solutions to the same problem and the designer has to take a decision as to which solution must be accepted. Thus, the design problem reduces to a decision-making process. The design decision process can be clearly divided into three stages: the ‘idea’ stage in which the designer does the concept formulation for the design, the ‘calculation’ stage where all required calculations are carried out with (or without) the help of computers and finally the ‘evaluation’ stage where the designer takes the required decisions for development of the product, as shown in Figure 15.1, which can be with or without the use of computers. Figure 15.1 also shows the iterative design process already discussed in Chapter 1 and shown in figure 1.1. The final design solution can be arrived at based on one of the three methods specified in the following:

1.Feasibility study where the aim is to find a feasible design solution which does not violate any constraint. The performance or merit function is not necessary to be defined. The example is selection of the ship’s main dimensions and form parameters which do not violate any design constraint such as weight, stability and other hydrodynamic and structural requirements. Merit functions such as NPV over ship lifetime or building cost, etc. are not considered. In case a merit function such as NPV over ship lifetime is to be the design basis, then the merit function value is estimated for all feasible design solutions individually and the best one is chosen. Such an approach is known as parametric study.