CAD/CAM
New naval developments at NAPA D
UE to budgetary constraints, naval authorities and shipbuilders are looking more towards the
commercial shipbuilding sector for best practice and production economies. In the case of software, this practice is all the more advantageous when one considers the size of the commercial shipbuilding industrial base compared to the defence world. As Christopher Ridgewell, CEng, MRINA*
explains, the NAPA system has proven to be a versatile tool to solve diverse problems, ranging from the optimisation of internal arrangement for probabilistic damage stability requirements, to hullform design and ship motion prediction, and Napa Ltd has recently been investing heavily in increasing the usability so that the power of the system can be more easily harnessed to improve the efficiency at the design stage. This article will review the recent and ongoing developments in the NAPA system applicable to naval ship design and operation. As the specification placed on the naval platform
becomes more diverse, it becomes necessary to analyse an ever larger number of variations in the design. This then leads to the optimisation of the design against competing requirements. A typical example of this could be the selection
of aircraft for an aircraft carrier. The choice of aircraft obviously has an impact on the potency of the projected force. However, in terms of ship design, the type of aircraft have a great impact on the ship’s length, which in turn impacts motion response, operability, and cost of construction. Multi-objective optimisation is therefore of great benefit in quickly assessing the impact of changing design variables such as ship length, on a variety of diverse design criteria such as stability, survivability, motion response, and cost. These optimisation methods have been shown to present promising design alternatives for naval applications that at initial review are non-intuitive and, therefore, would not be considered, for example, for submarine hullforms with fullness at the stern. In 2003, multi-objective genetic algorithms
(MOGA) were integrated into the NAPA system. By using macros in place of the graphical user interface, the design task can be automated. In this way the influence of different parameters on the design can be quickly studied. The MOGA algorithm monitors the output and modifies the input parameters to find the optimum solution within the given constraints. The use of MOGA and NAPA have already been utilised in the design of naval ships to study the influence of competing parameters such as mobility, shaft length, weight, and survivability [1]. Alternatively, optimisation can also be used to develop a design that can be built at minimum cost, whilst still meeting the rule requirements [2].
Flooding Simulation Recently the dynamic effects of cross-flooding on the ship’s stability during a damage scenario have been raised at the IMO [3]. In order to provide suitable tools to the designer, simulation within NAPA has made significant progress in the past year. By taking into account the physical properties of air as well as water flow through openings,
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Figure 1: Floating position after weapon detonation.
Figure 2: Calculation of the US Navy V Lines.
Figure 3: FEM mesh generation. WARSHIP TECHNOLOGY MAY 2007
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