An Integrated Workflow for Rapid Structural and Hydrodynamic Analysis of Ships and Offshore Structures

This paper presents a unified designer-centred workflow between modelling and analysis for the evaluation of complex ships and offshore structures. Advancements in CAD-CAE technologies have enabled the shift of work once performed by dedicated
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  563 An Integrated Workflow for Rapid Structural and Hydrodynamic Analysis of Ships and Offshore Structures Vassilios Zagkas , SimFWD, Athens/Greece,  George-Theodore Spanos , SimFWD, Athens/Greece,  Abstract This paper presents a unified designer-centred workflow between modelling and analysis for the evaluation of complex ships and offshore structures. Advancements in CAD-CAE technologies have enabled the shift of work once performed by dedicated experts to a large number of designers and engineers. Here we describe the workflow connecting the designer’s personal problem solving environment to the set of tools enabling him to carry out combined Structural and Hydrodynamic verification using the same model and platform. This will be presented through a step by step design example utilizing BVB CAFE modeller and MIDAS NFX total solution from high-end structural analysis functions (contact, non-linear, explicit dynamics and fatigue analysis) to high-end fluid analysis functions (moving mesh, free surface analysis and mass transfer analysis). 1. Introduction The process of shipbuilding is quite a complex one and comprises of several stages, each of which requires a specific and a different kind of expertise. Over the years the ship-building process has been split into different modules, each of which is handled by a group of people possessing the expertise to perform the activities demanded of them. The end result of each module is then handed forward to the next group/department to make a contribution on their part to the ship design process. Now this work flow, although classic in nature, is gradually changing and so is the level of involvement of the indi-vidual group members. Collaborative design is gaining popularity where different parties communi-cate on the same design with real-time data, make design decisions and consider requests from all in-volved parties in the design process (designers, producers, buyers, suppliers etc.). Another very important issue these days, from a manager’s point of view is administrating the licens-ing costs. Commercial software on the market comes at a relatively high cost these days and efforts are being made to keep these costs to a minimum. If one considers the entire ship design process, it is not unusual for 3-5 licensed software products to be used for various purposes. Thus planning the en-tire workflow is of extreme importance wherein one needs to study the capabilities of each software product being used and the compatibility with other software products. Nowadays we have several FE codes on the market that are looking to offer the users a one-code solution. This means that as op-posed to using different software modules to perform different kinds of analyses, the user now has the possibility of using one single software product having the capability of performing a wide range of analyses relevant to different engineering fields. This in turn means a substantial saving for the user on the licensing front and also simplifies the work flow to a great extent! 2. The Integrated Platform 2.1. Conceptual Approach In the era of global competition, demands on the use of CAE are growing faster than ever in the process of product design simulations. These can be summarized below: Current State of Market Expected Effects Through CAE Increased Cost of Raw Materials Cost Reduction Requested High Quality Improvement of Quality Short Product Cycle Short Development Period  564 For design productivity and product competitiveness, the CAE tool must be capable enough for sophisticated analyses and yet be sufficiently easy for the product designers to use. This paper is based on a workflow fitted around MIDAS NFX platform which delivers to the user an integrated environment for modelling, meshing, solving and post-processing problems for a variety natural phenomena as: Linear Static Analysis, Modal/Buckling Analysis, Heat Transfer/Thermal Stress Analysis, Nonlinear Static Analysis, Explicit/Implicit Dynamic Analysis, Fatigue Analysis, Topology Optimization, CFD Aerodynamics and free-surface Hydrodynamics. Fig.1: Product development process,  NFX (2013)  The MIDAS NFX platform represents a change of paradigm in the CAE industry, work once performed by the combination of several dedicated platforms and few experts is now shifted to an integrated platform and a large number of designers, enabling them to easily verify and predict the product performance, thus connecting the design and analysis department. Here we present in different stages a real case study on 70t bollard-pull tugboat, imposed into hydrodynamic loads and lastly into a collision with part of an offshore installation. The process described enables the user to concentrate all effort and project results in one platform by utilizing diverse solver capabilities. 2.2. Hull form and CFD Pre-Processing Starting from a basis or even existing design the naval architect must always have the ability to import the hull geometry from other software in various extensions (dxf, iges, step, stl etc.). The most usable output found in most of CAE software that design offices, shipyards and classification societies have is the *.iges output. After the geometry is imported, MIDAS NFX provides a number of dedicated functions to repair the geometry. Fig.2: Hull surfaces imported and converted to B-Rep solid  565 For CFD based simulations the creation of the fluid domain is one of the most critical processes. Its quality will, in turn, affect the quality of the mesh and finally the quality of the results. For this purpose we have started by correcting minor edge problems of the imported models and then continued with merging the surfaces into B-Rep Solid. Converting the hull surfaces into a B-Rep Solid, enables its easy use as tool on a Boolean Operation, that will subtract its imprint from the fluid domain. The fluid domain itself can simply be a rectangular solid with specified dimensions based on the model particulars and the requested analysis. The size of the air domain has a breadth of 30 m, a length of 110 m and has 10.1 m height. The sea domain shares the same breadth and length and has 20.6 m depth. The domain’s inlet is 26 m far from the models forward part and the aft part has 50.3 m distance from the domains end, giving enough space for the wake to develop properly. Fig.3: Fluid domain and hull Boolean operation After having defined the dimensions of the fluid domain, we subtracted the hull model’s imprint through the Boolean Operation feature that MIDAS NFX has to offer. The domain is divided into two parts, namely the air and the sea domain. Both parts finally shared the same mesh size. On the sea and air domain interface, it is necessary that we define a size edge control so that more accurate results are obtained in that area. Great attention needs to be given concerning the CAD detail of our model and thus, its imprint’s corresponding one. Subsequently, we follow an iterative procedure which involves trial and error and more specifically, checking the mesh quality and altering or simplifying the imprint’s geometry. The upper procedure involves mainly rebuilding or joining the most complex edges of our model’s geometry. We have to consider the edge size of the domain as an aiding factor in our attempt to achieve a fine mesh. After we got an acceptable mesh quality, we move on optimizing it. The problem in our specific mesh topology was situated in the middle lower area of the hull and in the upper hull chines. To overcome it, we reduced the number of unnecessary edge size controls and concluded in using a main size control in the interface of the two domains and on the upper chines of the hull. The finest mesh quality was achieved testing a number of mesh sizes combined with the corresponding edge size and was finally obtained for mesh size 0.124 m and edge size of 0.62 m (2:1 ratio). Fig.4: Fluid domain meshing; sea and air 2.3. Structural Modelling and FEM Pre-Processing To complement our workflow we have in this stage introduced BVB CAFE, a dedicated tool for rapid structural modelling of ships and offshore structures. Models created here integrate well with MIDAS NFX and can be readily used for applying loading conditions and solving them.  566 BVB CAFE gives the opportunity to the designer to define interactive rulers that will assist him during the design process. The rulers are used here for the rapid generation of the structure by defining the frame spacing of the ship and the various deck levels. User is able to define appropriate label for his rulers as well as ID and step that will be replicated according to sequence of the input data table. Rulers can also be used to add guided quickly at a specific frame or at specific deck level. The imported iges model can be directly used in BVB CAFE for preparing the structural entities. Following the import of the hull shape the second design stage would be to create the main structural components of our design, meaning, main transverse and longitudinal bulkheads, double-bottom, and decks. Whether we are working with an existing design or a new concept an innovative function makes such definitions very quick and easy. This is the definition of planes by the intersection of guides and their surrounding geometry. In our case a guide is positioned in a Frame value or at Deck level and by selecting the surrounding hull geometry we apply a polygon strake that defines the longitudinal or transverse bulkhead. Further to x, y and z plane guides, w guides allow the definition or arbitrary planes. Further to that all structural elements such as deck and hull stiffeners are modelled by the parametric entities provided within BVB CAFE. After the completion of detailing with respect to stiffeners, beams, etc. we have a complete structural “half-model”. The model was then meshed using the powerful meshing algorithm in CAFE, Fig. 5. A global mesh size of 100 mm was used. Fig.5: Fluid domain m, sea and air While creating entities in CAFE, we appropriately assign different thicknesses to different areas of the model. To perform a visual check of the assigned thicknesses, the user can activate a “plate thickness” filter and by means of a contour plot easily identify the areas with different thicknesses. Similar filter can also be used to plot the profiles, materials, etc. Fig.6: Model contours displaying variations in thickness Basic mesh regeneration and node merging is performed in CAFE itself. For more complicated mesh corrections we shall resort to the functionalities of midasNFX, which we shall also use as our solver. After completing basic mesh adjustments, the model is exported as a NASTRAN Data file (*.dat). This file format is read into midasNFX. The entities with different thicknesses are treated as separate “properties” in midasNFX, Fig.7.  567 Fig.7: CAFE model directly imported into MIDAS NFX We then proceed to use the wide palette of functions that midasNFX offers in order to fix various areas in the mesh. Fig.8 shows examples of some problematic areas. Fig.8: Mesh refinement within MIDAS NFX After mesh refinement, a quick check for element quality and topology is done by the provided dedicated tools. The model is then tested for connectivities, contacts, etc. by means of a modal analysis. Performing the modal analysis guarantees that there are no free faces or singularities in the model by utilizing simple fixed constraints. The “half-model” is then mirrored about the central axis to obtain a complete model of the aft hull of the tug boat. The circular structure at the thruster foundation is modelled in a simplistic manner by adding rigid mass elements. Fig.9: Modal check and mirrored topology 3. Analysis & Results 3.1. CFD Results and Pressure Mapping Following the pre-processing of the two models, one for the hydrodynamic analysis and one for the structural analysis, it is now time to solve them and post-process. Starting the with the CFD case what we want is to get the pressure distribution on the aft part of the tug in order to utilized the pressure as an input to the structural model solution. For that purpose we have used a Transient CFD function and an inlet of 2.42 m/s defined by the following expression 2.42+0.6*sin(3.66*t) to give a sine wave
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