|WHY FAIRWAY ?
Fairway was conceived from dissatisfaction with existing methods for hull design and manipulation. Over the years, thorough investigations of available design methods and their implementations in commercially available software (including some now obsolete PIAS modules by SARC) have convinced SARC that the inherent shortcomings of existing techniques cannot be solved efficiently and elegantly. Rather than accepting and implementing fixes for the problems caused by these shortcomings (as has been tried in the majority of the commercially available software), SARC has developed Fairway.
SCOPE of Fairway
Fairway is the hull form modeling module of the PIAS/Fairway suit of naval architectural software. Fairway can be used for any activity with the hull form, such as :
- Start from scratch: a minimal set of lines can be generated for free hand design.
- Starting a new design from a previously defined hull form, using transformations.
- Design modifications during preliminary design and final design stages.
- Hull form transformation to match targets for displacement, LCB, etc..
- Automatic fairing with user-controllable accuracy, up to, and beyond production tolerances.
- Perform simple hydrostatic analysis, and farm out complex analysis to PIAS.
- Generation of shell plate expansions for double curved and developable surfaces.
- Automatic adjusting to the shape of a developable plate.
- Generation of lines plans and tactile scale models (Rapid Prototyping).
- Addition of additional, user defined curves: frames, buttocks, waterlines, diagonals, lines in oblique planes and free-hand 3-dimensionally curved lines.
- Import of hull form data from existing (complete or partial) linesplans from paper drawings, drawing files, tables of offsets, etc..
- Reconstruction of hull form data from photographs using photogrammetry.
- Export of hull form data to AutoCad (DXF), MicroStation (IGES), NUPAS, finite element (FE), computational fluid dynamics (CFD) calculations, etc.
In Fairway, newly developed techniques and methods have been combined with existing ones where useful, to yield a versatile and comprehensive tool for hull form design and manipulation. First we will discuss the merits of the ‘existing methods’ briefly, then we will introduce the Fairway approach to tackle the problems inherent to the ‘existing’ techniques.
|EXISTING METHODS FOR HULL DESIGN
Some older types of software use curve methods for hull representation and manipulation. Examples are SIKOB/Seasafe, SFOLDS and some PIAS modules (definition of existing hull form and hull form transformation). The major advantage of a curve method is its ability for simple definition of existing lines plans. With curve methods curves on the hull surface are defined, which implicitly define the hull surface. In older systems the curves are represented by polynomials, whereas more curves are represented by polynomials, whereas more recent systems mostly use B-splines.
The vast majority of contemporary general CAD systems (such as AutoCAD, MicroStation, Eagle and CATIA) and specific naval architectural software, such as AutoShip, FastShip, Mac/Maxsurf, Multisurf and some (now obsolete) PIAS modules, make use of NURBS surfaces (Non Uniform Rational B-Spline). For specific purposes (developable surfaces or for cylindrical segments) specific surface formulations may be used. For general purposes, B-spline surfaces are commonly used. One of the most expressive characteristics of the B-spline surface is that it hangs on a network of defining curves, which covers the complete surface. Manipulation of the surface is performed by manipulation of the intersections of the defining curves. These intersections can be called vertices, poles, knots, etc..
Limitations of existing methods of hull form modeling
In the course of time it appeared that the existing methods have the following disadvantages:
- There is one-way traffic on the road from surfaces to curves, for it is possible to derive specific curves from surfaces, but in general it is not possible to derive surfaces from an arbitrary combination of curves. Such an option is strongly desired, whenever additional curves (such as waterlines or extra frames) are required.
- The defining curves of B-spline surfaces in general do not run parallel to one of the three main planes of the vessel. Thus, the user must work with curves running more or less arbitrarily over the vessel’s surface. To model an existing hull design, or to work with great precision, the use of waterlines, ordinates, buttocks and diagonals is essential. To prove this, literature (and even commercial brochures) shows plenty of examples of objects rather similar to ships, but not real ships. Reconstruction techniques that are able to produce a B-spline-surface that approximates actual hull lines vessel are even proudly presented (!).
- The network of the surface method is too rigid. Each defining curve must cover the complete surface, so local refinements are impossible.
- Surface methods are suitable to design ships (hull form generation aspect), but because of the one-way traffic from surfaces to curves it is impossible to start with an existing lines plan.
- Neither with curve methods, nor with surface methods, it is possible to fair a vessel up to production accuracy in an efficient way.
- Curve methods are unsuitable for fairing, because the coherence between the curves is not guaranteed, which makes the addition of extra curves cumbersome.
- Surface methods are unsuitable because they cannot cope with the naval architectural sense of fairing. Besides, the rigidity of the network, combined with the desire to model local features, leads to an unmanageable tight network, when local refinements are taken into account.
To relieve the problems discussed here, one could suggest to use multiple B-spline/NURBS surfaces instead of a single surface, but with that 'solution' three other problems are introduced :
- A system of surfaces is constructed, which without further supporting topology may be not coherent.
- The arrangement of surfaces is the responsibility of the ship designer.
- Continuity and discontinuity conditions between the surfaces have to be specified explicitly by the ship designer.
|Beyond NURBS: the technical backgrounds of Fairway
During the development of Fairway we have been led by the desire to produce a general computer tool for hull form definition, hull form generation and production fairing, where the mentioned disadvantages do not occur. SARC has developed a new type of model representation. The technical merits and potential of this new model representation is elaborated in the following sections.
Fairway model structure
- A curve consists of one or more curve segments. The curve representation is a NURBS line.
- A curve segment is defined by internal knots. These knots can be points where the curve passes, tangents at specified points or B-spline vertices.
- Curves describing the hull surface are connected in a network through their intersection points (external knots). This network may be highly irregular.
- External knots of a curve are connected with those of one or more other curves. The connections of curves and external knots form the network. Contrary to B-spline surfaces this network is very flexible, and may consist of actual waterlines, buttocks, 3-D lines, etc..
- Curves of the network need not run over the entire hull surface, so they may be defined only in those regions where they are really necessary, for instance for local refinements. Thus, contrary to B-spline surfaces, the network may be highly irregular.
- Surface ‘patches’ are constructed to fill the ‘gaps’ in the network. These patches derive their shape from neighboring curves. Automatically ‘regular’ regions in the network are detected where it is appropriate to use one surface that extends over multiple patches.
- Surface patches are connected with matching tangents, unless the patches share a curve designated as knuckle.
- The shape of new lines can automatically be derived using these surface patches and/or intersections with existing lines, depending on the design stage, user preference, etc..
- Those surfaces are curved in two directions, unless the user explicitly specifies regions to be developable.
- Coherence of points, curves and surfaces is automatically maintained with special solid modeling techniques.
Working with Fairway
Fairway allows actions to be performed on points, curves, tangents, surfaces or on the entire solid. Furthermore, any action can be performed at any given time. Thus, Fairway can be looked upon as ‘toolbox’ for ‘work’ on, or with the hull form. The potential of this ‘toolbox’ approach is given in an incomplete outline of manipulations:
- Fairway offers support in working towards a user defined sectional area curve. The target area of frame sections can be compared to the actual area, both graphically and numerically. Frames can be shrinked or expanded to meet the target area, without exceeding preset width, raise of floor, etc..
- Hydrostatics for the design draught are available on screen.
- A curve segment definition can be set to the following types: curved (B-spline or NURBS), straight or conic section (circular , parabolic or elliptical).
- The user can specify the nature of the connections between adjacent segments (fair, tangent or knuckle).
- Double curved and developable surfaces can be mixed and plate expansions of both developable and double curved plates can be created, including templates.
- Manipulations can be done graphically and numerically, allowing excellent control over coordinates and shapes.
- B-spline curves can automatically be faired with a local scheme, where the user specifies a mean deviation between the original points and the faired curve. The user can also specify the relative weight of each individual point. The mean deviation is analogous to the stiffness of the physical spline (the smaller the deviation, the stiffer the spline), while the relative weight can be considered as a model for the weights of the so-called ducks.
- These fairing options allow complete control over the curves. This automatic fairing can be done to satisfaction of the user: a model for preliminary stability calculations requires less accuracy than a model for production drawings and plate expansions.
- The degree of fairness can be checked by means of the curvature, plotted perpendicularly to a curve.
- User defined sample lines can be projected onto the defined model, with user defined parameters.
- Geometrical relations between curves can be defined, so one curve governs another, using factors and offsets.
- Boolean operations between defined solids (merging , subtracting , etc.) are being implemented at the moment of writing.
Fairway input and output options
For input of existing hull models, Fairway accepts a wide range of input data formats, either directly or through pre-processing. Once a hull form is defined in Fairway file format, regardless of the source of the input data, Fairway can generate numerous output formats:
- Manual input of an existing hull form through copying points of an existing lines plan, either from paper using a digitizer tablet, or from screen using bitmaps.
- Import of a hull form from a wire frame model in drawing file (DXF or IGES). Such a model can be converted into a solid model in Fairway file format , which is unique for commercially available software.
- Other CAD/CAE software can draw sections from a Fairway hull model interactively: the software ‘asks’ for a hull line and the hull server responds with the desired line geometry, generated on-the-fly if required.
- Both surface and line entities can be converted into files, for example to AutoCAD (DXF), MicroStation (IGES), NUPAS, finite element (FE) or computational fluid dynamics (CFD) calculations.
- Lines plans on paper or in offset tables.
- Plots, DXF files and coordinate tables of plate expansions, including tables of distortion.
- Tactile scale models (Rapid Prototyping).
- Rendered graphics of hull models for presentation and checking.
- A unique option is reconstruction of a hull form through photogrammetry. Using a range of overlapping photographs and some additional data, a complete or partial hull form can be reconstructed for further manipulation.
Fairway comes with full user support. Most user questions can be answered over the phone, but when necessary, models may be exchanged for in-depth support. All SARC employees are involved in development, sales, training and project engineering, so expert user support is always available. A manual is included in the Fairway package. New software manual editions can be freely downloaded from our webserver, thus enabling users to keep up-to-date with new functions and developments.
For more background information the reader is referred to the book Computer Support for the Design, Engineering and Prototyping of the Shape of Ship Hulls, which is available on www.sarc.nl.