CAD/CAM


Figure 4: Onboard-NAPA: calculating residual strength.


model, the stress cycles within the structure can be evaluated along with the fatigue properties of the material, to determine the operational life of a proposed design. Thus the hull, loading condition, and structural arrangement can be optimised to maximise the heavy weather manoeuvring for a given design life.


Product verification As naval constructors utilise the best practice found in the commercial shipbuilding sectors, the concepts of product verification and the role of the classification society and Naval Authority become more intertwined [4]. One of the key concerns of any project is the


progressive flooding due to damage can be simulated through the time domain. The methodology of the approach has been developed in close cooperation with the Helsinki University of Technology where scale-model testing and other data has been used to validate the calculations. The practicality of this method in the design


process has also attracted the attention of the passenger ship sector. By using this method, a classification society and shipbuilder have verified the minimum size of air pipes required to give adequate cross-flooding rates within a transversal compartment of a passenger ship. This type of approach is also particularly useful in assessing the flooding response of frigates that could be vulnerable to asymmetric damage and larger ships such as aircraft carriers, where the time to right can play an important factor in the recoverability of the ship’s combat systems.


Survivability Analysis A recent project to develop a survivability analysis tool has also been making good progress. The program has been developed within the NAPA system as a Manager application by the naval architecture consultancy Gadlab Engineering Oy, with support from the Finnish Navy and Napa Ltd. The objective of this work is to develop tools that


will allow the survivability to be assessed at the earliest stages of the design iteration. By using the


Figure 5: Onboard-NAPA: flooding simulation.


explosive and fragmentary properties of various conventional weapons, the damage to the structural integrity of the steel structure is assessed by using a physical SDOF model. Once the structural damage has been calculated,


the program automatically calculates various properties available in NAPA such as damage stability, progressive flooding, and residual strength, as well as determining the damage to the ship’s systems. In this way the survivability of the design to a variety of threats can be quickly assessed both during the design cycle and in the operational phase of the vessel. As well as the challenges of survivability and


recoverability, susceptibility also brings new challenges for the platform designer. Notable amongst these are the use of materials such as aluminium for the hull structure. Aluminium solves many problems associated with magnetic signatures; however, recent cases of failures in aluminium naval structures have shown that fatigue life limitations must be accounted for when designing the ship for the expected mission profile. The solution of this complex problem is


simplified by using an integrated design system such as NAPA. By using strip theory, the loading amplitude and frequency along the length of the ship for the statistical sea states and headings can be calculated for the actual loading condition. Using this information together with the NAPA structural


Figure 6: NAPA Power: route optimisation.


access to valid design data. Obviously with this requirement the need for security is also of paramount concern. This is an area that Napa Ltd has been focusing on for some time, and has recently incorporated tools that will assign encryption and access rights to selected components within the database. The access to a database can also be recorded. In this way, confidential data can be kept secure and tracked, whilst at the same time the benefits of working from a single model are achieved. From the early project inception, design approval, to final stability certification, all stakeholders in the project are able to work securely with common data. The required safety standards are also becoming


more aligned to current best practice found in the civilian sector. Many navies require that their ships are designed in accordance with the requirements of SOLAS and MARPOL. When one considers the number of personnel onboard naval ships, the need for high standards of safety becomes apparent. Where the design verification by a classification


society is considered, NAPA is already utilised by American Bureau of Shipping, Bureau Veritas, Det Norske Veritas, Germanischer Lloyd, Hellenic Register of Shipping, Lloyd’s Register, Nippon Kaiji Kyokai, and Registro Italiano Navale. This leads to significant benefits in the verification process. For example, the structural arrangement of a NAPA Steel model can be exported directly to the Lloyd’s Register RulesCalc software package or to the STEP standard used by DNV Nauticus and GL Poseidon. Many of the statutory requirements can be time-


consuming and difficult to implement efficiently in the design. Typical examples are the SOLAS requirements for probabilistic damage stability and the MARPOL requirements for the protection of fuel tanks. Napa Ltd has been working hard to develop a suite of Manager applications within NAPA that can be used to check the ship’s geometry against a variety of the latest statutory requirements (see box). This list is not exhaustive and is continuously


expanding. Recent additions have included the calculation of powering requirements for ice navigation, visibility checks using actual loading conditions, and calculation of the US Navy V lines. The cornerstone of the Manager application


is the intuitive tree structure, whilst at the same time maintaining the flexibility of NAPA. It is hoped that these tools add real value to the design process and support the designer in effectively


WARSHIP TECHNOLOGY MAY 2007 63


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