Interaction of design and analysis, integration of CAD and FEA U. Rabien & R. Bronsart Germanischer Lloyd, Vorsetzen 32, Hamburg, Germany

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1 Interaction of design and analysis, integration of CAD and FEA U. Rabien & R. Bronsart Germanischer Lloyd, Vorsetzen 32, Hamburg, Germany ABSTRACT This paper describes an implemented strategy for information exchange between design and analysis systems as used in shipbuilding industry today. The design of the structural steelwork carried out at a shipyard is to be referred to as an information base for generating analysis models for the evaluation of structural strength and vibrational behavior. A system architecture is explained which allows the coupling of any CAD-system with any finite element analysis processor. The need for data transformation between different views on the object is discussed, a tool for supporting this complex activity is described by showing different examples. INTRODUCTION The design and construction of ships and offshore structures, as performed in cooperation by the shipyard, classification society, supplier and ship model basin, are certainly time-consuming processes. Because of the complexity of the products and the tight schedules involved, software tools are increasingly being used today for these tasks: CAD, CAE, CAM... For Germanischer Lloyd, the processing of data created by CAD is of increasing interest, in order to generate FEA models for the evaluation of structural strength or the prediction of vibration behavior [1]. The following advantages are desired: fast data transfer without unnecessary re-input, reduction of error possibilities during manual input, fast availability of the computation results, improved possibility of early correction or improvement of the design as well as enhanced testability of design relationships for components that in many cases are still considered as individual entities.

2 CAD systems provide their own data base management techniques using specific concepts and data models. These models are designed to represent all information necessary for production. Analysis on the other hand has a completely different view on the structural steelwork, though many data relevant for design and manufacturing are used as well: product structure, geometry, materials... [2]. Other information like loads and constraints have to be added. Therefore tools are needed to complete information and to store it using an extended model which is also capable to represent results e.g. deflection and stress values. SYSTEM ARCHITECTURE The system architecture with the corresponding data flow concept was designed so that various CAD and FEA systems can be used, see figure 1. Nowadays a large number of such software systems are used by the shipyards and classification society and each of these have their own system-specific data management method, as a result of the differing tasks, technical developments etc. Therefore a direct exchange of data is not accepted. Because of the global competition in development, a process of constant change must be expected in the field of information technology. This can only be answered adequately by means offlexibleconcepts. CAD CAD CAD - converting tool - - converting tool converting tool FEA FEA FEA Figure 1: architecture With these prerequisites in mind, data on both the CAD side and the FEA side are represented in a neutral data model, i.e. one that is independent of the system used [3], in order to achieve compatibility or at least keep the conversion effort for data transfer as low as possible. The syntax of the exchange files corresponds to the present state of international standardisation, ISO (STEP) [4]. With the aid of appropriate converters, it is possible to couple any CAD and FEA system. The central component in the process chain is the modeller, with which the definition of the steel structure from the viewpoint of the CAD application is transformed to the specific view of FEA applications. This requires a suitable transformation process acting on the relevant data model. Due to the complexity of marine products with respect to the inter-

3 Figure 2: CAD-Model imported from the smallsteerbear system nal structure and the number of parts, the integration of CAD and FEA tasks cannot be performed according to the same patterns that are found in mechanical engineering and related areas, where automatic modelling aids are often used. On the contrary, special modelling strategies must be implemented and these necessitate tailor-made software developments. The modeller is implemented as an interactive tool. It provides, however, algorithms for automatic processing of special tasks on different levels of modelling. The software is capable of handling more than one type of model at a time. For example, superposition of original CAD geometry with the derived intermediate geometry model or with the FEA model is supported. TRANSFORMATION OF DATA During the processing of the CAD models, thefirststep mainly concerns the interpretation of product structure information and transformation of shape representation. The imported geometry is used to first derive the geometrical basis in the modeller for further processing to obtain the FEA model. Figure 2 provides a partial view of the CAD model for a cargo hold structure shown as a wireframe. The plate components are imported into

4 the modeller from the STEERBEAR via afile,conform to the STEP physical file description. The figure also illustrates the user interface of the modelling tool which was kept as simple as possible. Due to the fact that the base model to start with (CAD structure) does not contain all information that is required for the target model (FEA model), it is generally not possible to perform the transformation automatically. Important information is still required and has to be added manually. On the other hand, much detailed information that is not required can be deleted. The transformation process is explained by using the example of an element of the ship steel structure, seefigure3. The CAD system specific CAD-Model -#- FEA-Model HE <t_equiv> Figure 3: Steps of transformation: CAD to FEA data structures contain all details of the plates and profiles: exact shape definition including holes, cutouts, seam preparation and stiffener endcuts. The functional information expressing the fact that e.g. certain plates together form a floor plate is normally not contained in the CAD model. Not all detailed components need to be modelled for the finite element analysis of the entire ship. This means that several floor plates can be grouped together to form a new idealized component that is equivalent in strength and vibrational behavior. The model transformation first uses the CAD description of the individual plate surfaces to generate a derived geometrical basis and the topology for the FEA model. The new geometrical representation forms an intermediate layer between the data model of the CAD system and the data model for the FEA system. This working form is an idealized surface-oriented model that is built up on the moulded-line or reference-surface geometry. The CAD geometry must therefore be modified so that no gaps between adjacent components occur by omission of the

5 material thickness, see figure 3. Material thickness and other parameters are then integrated as attributes to the basic geometry. MODELLING PROCESS For geometric manipulation, the modelling system provides a number of special functions for cutting, moving, deleting etc. in addition to the implemented automatic transformation algorithms. Other functions manage to classify andfiltercomponents of the CAD model with respect to a number of properties or criteria like: identifiers assigned to the component by the CAD system; membership of groups or subgroups; interactive selection at the graphic display and geometric filtering. A modelling example is given using an assembly of the double bottom of a container ship. It consists of transversal and longitudinal plates and related stiffeners, see figure 4. Based on this representation, the intermediate Figure 4: Double bottom structure of a container ship geometry model was derived. The followingfiniteelement generation was performed unsing the transformed geometry. In figure 5, the intermediate geometry of the double bottom assembly is shown as a wireframe representation, the finite elements are depicted as well, making use of a shrink algorithm. It is obvious that the program modules used here are not only suited for processing the CAD geometry but also for direct creation of the geometric intermediate layer with an interactive working method. If only global geometric information - such as ship lines, compartment subdivision and hatch

6 Figure 5: Derived geometry (wireframe) and finite elements (shrink mode) dimensions - is available, it is used as the basis for model generation. The transversal and longitudinal components can be arranged within the ship's hull, and varied or refined later using the powerful geometric tools provided by the modeller. Visualization of geometry models is a prerequisite for survey, editing, filtering etc. of CAD geometry models. Therefore different techniques of graphical representation are provided to meet the requirements of the user: wire frame, hidden lines, hidden surfaces, shading (flat, Phong, Gouraud), translucency... Figure 6 shows a part of the imported CAD geometry of a modern double hull tanker design in hidden surface representation. When generating FEA models as explained in the above sections it is necessary to show these elements on top of the geometry model. A FEA model of a gas tanker - cut off amidships using volume clipping techniques - is shown in Figure 7. It was found that the possibilities of graphic and geometric manipulation are indispensable during model testing and the assessment of the structural interrelationships. Colour and display intensity are used to indicate the spatial depth for a quick understanding of the object. It is thus much easier to detect errors, such as missing surface parts, in comparison to a boundary line representation.

7 Figure 6: Part of modern double hull tanker DATA MODELLING Following the system architecture, a neutral datamodel for the structural steelwork of ships and thefiniteelement analysis model was developed. In the ongoing standardization process within the ISO initiative [5], the main emphasis is placed on production related data. Input was given and still is given to add structural and functional information to the model. Through participation in the standardization committee, important extensions were achieved in relation to thefiniteelement description and structuring possibilities of afiniteelement analysis model (STEP part 104) [6]. PERSPECTIVES The increasing use of software systems for a wide range of tasks in the ship design process makes it necessary to couple the various systems in an effective manner. Therefore progressive solutions mustfitinto a more generalized concept. The most suitable path is via the organization of an effective information flow with system-neutral data representations according to the concept of the international standard ISO In information technol-

8 Figure 7: Visualization of FE-Model ogy, this area exhibits an interdisciplinary development towards uniform concepts that are utilized and supported by our software developments. Moreover, the necessity for increased integration has been recognized and has led to the definition of an innovative joint research project termed "Information Technology in Shipbuilding" (ITiS) funded by the German Federal Ministry of Research and Technology [7]. It is intended to establish the prerequisites and the corresponding technological base for the implementation and development of the growing number of software applications towards a computer integrated manufacturing (CIM) environment. REFERENCES [1] W. Fricke. Strength and Vibration Analysis of Modern Cargo Ships Using the Finite Element Method. Transactions North East Coast Institution, 107: , [2] T. Koch. STEP-based Modeling of Ship Product Definition Data. In C.B. Vieira, P. Martins, and C. Kuo, editors, Computer Applications in the Automation of Shipyard Operation and Ship Design, V7J, pages , Rio de Janeiro, September IFIP, North-Holland.

9 Marine Engineering 25 [3] ISO Product Data Representation and Exchange Part 1: Overview and Fundamental Principles. Draft International Standard TC 184/SC4 N181, International Organization for Standardization, January [4] ISO Product Data Representation and Exchange Part 21: Clear Text Encoding of the Exchange Structure. Draft International Standard TC 184/SC4 N180, International Organization for Standardization, January [5] M.L. Gerardi, P.L. Lazo, and M.A. Polini. NIDDESC Ship Structure Application Protocol; Version 0.9. Technical report, Navy Industry Didital Data Exchange Committee, January [6] K. A. Hunten. STEP Part FEA Reference Model. ISO TC184/SC4/WG3/P9 Document N21, International Organization for Standardization, Gaithersburg, USA, January [7] R. Bronsart et al. Innovative Anwendungen der Informationstechnik im Schiffbau (ITiS). Technical Report 233/1991 (in German), FDS, Hamburg, 1991.