Figure 3.6 gives a brief diagrammatic description of the total design process. The tasks in each subsystem can be performed either sequentially or concurrently. Designing a complex marine structure or vehicle is a time-consuming process involving a large num- ber of people with different skill sets. There could be a number of feasible solutions. It is imperative that the internal design groups must agree on a few feasible design solutions. Subsequently, at a higher system level, different stakeholders have to agree on a single feasible solution for which a detailed design can be worked out. The design process can be based on point-based or set-based design paradigm.
3.4.1 Sequential Design Process
Starting with a design solution, one moves from one activity to another such that the requirements of the activity or subsystem are satisfied. One moves in a sequential man- ner, which means that the activities of a later subsystem depend on the outcomes of one or more previous subsystems. If the performance requirement of the subsystem is not satisfied, one has to go back to the beginning for design modification. Multiple iterations are necessary to arrive at a feasible design space. The technical subsystem, as shown in Figure 3.4, can be shown as a sequential design process as per Figure 3.7a.
The basic tenets of a widely accepted sequential approach to ship design have been cap- tured successfully by the so-called design spiral given by Taggart (1980), which is a point- based design system. Figure 3.8 shows the traditional design spiral where the spokes of the spiral indicate various subsystems whose requirements must be met. The spiral converges to a product at the centre. Mistree (1990) makes two important observations regarding this spiral: 1. The spiral converges towards a product, but the process is divergent with regard
to information, i.e. increasing detail of definition. Perhaps this divergence aspect is represented in the spiral given by Buxton (1971) in which the direction of move- ment along the spiral is outwards. At any stage the design information is a combi- nation of ‘hard’ or exact information and ‘soft’ or qualitative information. As the design progresses, the ratio of hard to soft information increases.
2. The spiral approach is sequential and iterative, and it has truly represented the state of the art (state of research) and the state of the industry (state of practice) of any product design in the past years. The computer, in this definition, has been used more as an efficient calculator where certain performance calculations can be done by scientific algorithms as and when required. This method has been improved over the years where the interaction between the designer and the com- puter has been more direct (CAD).
Such a solution necessarily leads to a satisfactory solution though not an optimum solu- tion. Being slow and laborious, this method may be effective when the marine industry
is doing well and the manufacturer’s order book position is good. Since the design is based on a large amount of previous data, such a design is almost always built and small improvements can be incorporated from product to product.
3.4.2 Concurrent Engineering in Design
A useful technique of system design is doing various individual tasks at a time, and through a communication process between such tasks, an integrated solution can be achieved. This process is known as concurrent engineering. It is a common-sense approach to product development in which all elements of a product’s life cycle from conception through dis- posal are integrated into a single continuous feedback-driven design process. According to
Mission requirement Establish constraints First estimate of main parameters D = T + freeboard + margin Estimate freeboard No Is D ≥ T + freebord ? Yes Δ = LBTCB (1 + s)ρ
Estimation of form stability parameters Estimate power Yes Yes Yes Stop Change parameters No No (a) Stability satisfied ? Estimate vibration frequency
Estimate seakeeping qualities/manoeuvrability Estimate GT and NT
Are all requirements satisfied ? No Is Δ = lightweight + deadweight
Estimate lightweight
Mission requirement
Establish constraints Selection of main parameters Hull form design
Freeboard
Arrangement and outfit Capacity and tonnage Trim and stability Damaged stability
Powering, speed and machinery Structures and strength Weight estimate Vibration Seakeeping Manoeuvrability Result (b) FIGURE 3.7
Mistree (1990), ‘Concurrent engineering is a systematic approach to the integrated, concur- rent design of the product and related processes including manufacturing and support’. This approach intends to make the developers consider from the outset all elements of the product life cycle, including quality, cost, schedule and user requirements. Figure 3.7b shows this approach modified from the earlier approach shown in Figure 3.7a.
Concurrent engineering or design requires communication between different subsystem design modules for proper integration. This communication can be effectively achieved through information technology (IT) tools. Therefore, IT tools become an integral part of any systems engineering application. Without adequate information, integrating all sys- tems and subsystems into a unified whole is impossible. Complex data and knowledge bases have to be evolved to access and use data and generate further information on the product in a concurrent manner. The concurrent engineering approach is embedded in the iterative design process, which also includes the decision-making process. Though a unidirectional flow of information is shown, it can be observed that the design must have the life cycle information of the product while designing a new product.
This design methodology by Mistree (1990) is represented by the frustum of a cone (Figure 3.9). Let the surface generator of this frustum represent the locus of one of the design activities (as depicted in the traditional design spiral) such as stability, shape generation, etc. As the design process advances in the traditional sense, one works outside this frustum, i.e. the design activity is sequential and iterative. Alternatively, if the design process is viewed as taking place inside this frustum, all design activities can be sighted at any inside point. Thus, the notion of concurrence can be accommodated by working inside. Each interplay between design considerations can be modelled effectively with information flowing through a ‘ring of interactions’. Concurrence should, therefore, replace iterative processes where a balance
Manufacturing Concept design Preliminary design Basic/contract design Detailed design Sustainability Economic Mission requirement Operational Technical FIGURE 3.8 Design spiral.
between conflicting requirements and objectives is sought. In this sense, at any phase of design, the information available/generated is disk-like but of irregular shape. In the limit, at the end of the design process, the information disk is geometrically circular and complete. Thus, the design time is reduced and perhaps a more efficient design is obtained.
3.4.3 Point-Based Design
Point-based design is a sequential design process. To reduce rework time due to iterations, concurrent engineering principles can be adopted at this level; when all lower-level sub- systems can be analysed simultaneously, all results can be compared with respective cri- teria. Expectedly, this process reduces the design time. This gives a feasible design, which can be modified to start the iteration process from the beginning to get another feasible solution. One can generate a design space consisting of a number of feasible solutions and select the one that satisfies the design objectives the best.
3.4.4 Set-Based Design
In the set-based design process (Singer et al. 2009), developed and used by the Toyota Motor Corporation, defining a design space where all feasible and possible designs will
Stability Structure Hull form (a) Structure (b) Hull form Stability FIGURE 3.9
Frustum of a cone representation of ship design, spokes indicates various design activities, a few being shown as illustrative.
reside is necessary. The design space for each major subsystem is defined such that all design requirements of the subsystem are met within this space. It is necessary to consider all design alternatives and explore trade-offs between different sets of designs. After all the design spaces for all subsystems have been defined, they must be integrated to get the overall design solution. This is done by the intersection of the design spaces such that the space common to all subsystems is identified. This common space may contain a number of solutions. A solution is then found from among these solutions such that system per- formance is robust in all respects, the solution is less sensitive to minor changes in design and it can absorb a good bit of uncertainty in data used without compromising with per- formance. To reduce the design time, concurrent engineering concepts can be adopted at different stages in an overall manner to obtain one feasible solution. Figure 3.10a and b shows the conceptual difference between point-based and set-based design processes.