65th Anniversary: From 2D CAD to the Integrated Product Model

Thursday, August 05, 2004

One of the most important improvements in ship production has been the introduction of CAD/CAM. The initial use of computers for drafting, lofting and automation of steel cutting has been extended from the design office and the mold loft throughout the shipyard and beyond by integrating the Internet, sophisticated database applications and enterprise management software to radically improve the entire shipbuilding process. However, the result of this, the Integrated Ship Product Model, is poised to revolutionize ship management and maintenance throughout its lifecycle.

The Problem With Ships

Boats and ships differ from most other objects because they are formed of arbitrary curved surfaces instead of well-defined assemblies of geometric shapes. Manufacturing and hydrodynamics also requires that these shapes be "fair", smooth and free from any sudden changes in curvature. Traditionally, ships were designed using orthographic drafting and wooden splines and weights. Surface contours drawn in various views were laboriously resolved to develop a consistent surface. Then structure, machinery and other components were designed, and flotation, weight, structural and hydrodynamic calculations were done.

However, due to the large change in scale, the design drawings were not accurate enough to actually make parts that fit, so "laying down and picking up" was required. The hull surface was redrawn, refaired and laboriously resolved view to view at full scale, usually on a whitewashed floor in a loft, hence the term "lofting". Loftsmen developed patterns for piece parts and rolled and curved plates and made full size templates to hand cut and form parts. All of this represented a great deal of labor and schedule time. Ships are also very complicated objects. They contain miles of pipe and wire, ductwork, furnishings, large specialized machinery and perhaps even weapons systems. Virtually all engineering disciplines are involved in ship design. Even a small tug has piping for fuel oil, lube oil, seawater cooling, bilge water, oil contaminated bilge water, engine exhaust, fresh water, sewage, compressed air, hydraulics, and carbon dioxide and seawater for fire fighting. It also has ventilation ducting, AC and DC electrical systems, two locomotives worth of engines, gears and shafting and a small apartment/office/shop complex for the crew. A naval combatant is probably the most complex product ever manufactured: The drawings for a nuclear submarine weigh more than the vessel itself. Coordinating all of these parts so that a ship can be outfitted on schedule in very tight, oddly shaped spaces is a major challenge.

The computer first revolutionized the surface design lofting, and cutting processes. In 1962 the first computer programs were under development to automate this costly effort. By the late 70's several mainframe based large systems, notably STEERBEAR, were available that developed information for surface definition, piece part design and development of code to automatically drive Computer Numerically Controlled (CNC) torches that cut steel. Some of this software also included features to do various analyses, especially stability. The larger shipyards rapidly adopted these integrated systems throughout the 70s.

The late 80s saw the emergence of several hull surface definition programs for PCs, as small computers with graphic capability became readily available. Small shipyards adopted PC-DOS based processes, which interfaced specialized surface definition programs, off-the-shelf Computer Aided Drafting software, mainly AutoCAD, and stand alone CNC code generators. One typical such interfaced process was used at Munson Manufacturing of Edmonds, Wash., a builder of small aluminum workboats. In 1991, Munson used Baseline, for preliminary hull surface definition. The files were then transferred to ShipCAM, for detailed fairing, definition of developable surfaces, plate expansions and other lofting functions. This data was then transferred to AutoCAD for part detailing in 2D and to GHS for stability and flotation analysis. Structural, weight and mechanical analyses were performed with spreadsheets. Files of the parts nested together on a "burn sheet" were transferred by modem to Farwest Steel, for cutting. Shortly after, a truck with the CNC plasma cut parts arrived, ready to be erected. As a result, delivery times and labor costs were reduced.

The next challenge was to change shipyard practices to best take advantage of the new tools. The Coast Guard Yard, in Curtis Bay, Md., was a typical example of reengineering shipbuilding processes to take advantage of CAD/CAM. The Yard also used Albacore Research, Ltd.'s ShipCAM. It had been some years since their last new construction project, when they were awarded a run of 27 49BUSLs (small buoy tenders). The Yard was also the first federal organization to be ISO 9001 certified. Thus, when the Yard implemented a production CAD/CAM system it was systematically integrated into the production process, through the use of Total Quality Management techniques, looking for changes and streamlining processes. This proved to be another important advance, though one enabled by technology, rather than an advance in technology itself.

The lesson from this is that CAD/CAM in particular and computers in general afford significant opportunities for improvement, and the wider one looks for improvements, the more opportunities. The precision offered by CAD/CAM has been especially important in modular construction, because if the parts are exactly defined and guaranteed to fit, they can be made anywhere and outfitted ahead of time. Other opportunities include concurrent engineering, palletization and group technology, improved techniques for controlling and scheduling work and better accuracy control. These improvements are also due to the fact that data in computer form is readily converted to whatever "format" is needed. This was the fundamental idea of CNC: the data in a CAD file could be plotted with a pen or with a torch without significant rework. By this time, the most pressing problem, designing, defining and cutting steel piece parts, was largely solved. However, though there was significant progress, the problem of outfitting largely remained - as one builder said, "We can always make money on steel, but outfit eats our lunch." By 1995, interfaced processes, mainly based on AutoCAD, were wide spread in small shipyards, which were also beginning to use specialized piping design packages and other software from other industries. Simultaneously, the large integrated programs, notably TRIBON, were adding ever more functionality for piping, HVAC and electrical system design, and integrating design software with analysis packages. By the late 90s, desktop hardware and software had improved so that the mainframe systems had migrated to Windows 98 or Windows NT platforms and were becoming still more powerful and easier to use. At the same time, AutoCAD and other CAD packages and the related applications for shipbuilding had become more powerful and more tightly integrated. The result was a number of software suites that addressed the issues of piping and other outfitting as well as steel, either in single large, standalone applications, like TRIBON or Intergraph or in combined interfaced programs linking specialized programs, standard CAD software and other main stream Windows applications. However, the power and standardization available from the Windows operating systems meant that much of the interfacing between these separate programs was largely invisible to the user. This brought the capability for both large and small shipyards to develop a true product model. The product model is much more than CAD geometry in 3D, though that is its core. One key concept is that data within the product model are not drawing entities, lines and circles, or even solids, but objects combining drawing entities with links to databases. In AutoCAD, such objects are called blocks and have embedded data called attributes. These attributes combined with their databases, give the objects what Gribskov has described as "behaviors"; a watertight door object is picked out of a CAD catalog and "knows" that it lives in a cutout, so it makes the right cutout for itself automatically. A pipe fitting object "knows" that it only connects to a certain specification and size of pipe. This not only makes design easier and eliminates errors, but improves the way a designer thinks, since he is thinking in terms of components, not drafting conventions, and is essentially building the system in virtual reality.

Another key concept is that a part only is in one place (the true one) in the product model, but since the data is electronic, it can be sorted, "sliced and diced" and viewed in any way desired. AutoCAD uses a concept called "Paper Space", "Model Space" and "Xrefs" to explain this idea. All objects representing real material are developed in 3D in Model Space. A given drawing file can contain objects or links - external references - to other drawing files containing objects. Thus what appears on the screen may be the sum of many different drawings combined. The drawing is then plotted in Paper Space. This is a conceptual sheet of paper with "windows" that look into Model Space, but the windows can be set up to see as much or as little of Model Space, at any scale, and with any combination of selected object visible (by controlling the visibility of the CAD "Layer" they are on). The dimensions, notes or any other material can be added as desired, and the result is not only drawings that automatically update as the model changes, but also the ability to almost automatically derive any drawing desired. In a piping drawing, as much or as little of the background can be shown, and drawings that only show an area of the ship at a certain stage of construction (for advanced outfitting, for example), can be developed without rework.

The final concept is the ability to link off to other databases through the CAD model. This means that any desired data can be linked to a database, which is linked to the drawing, so a drawing can contain active data for information such as cost, delivery date or technical data. This is essentially the same basic idea as the Internet - by having "links link to links" a user can navigate to any data desired, and a wide range of business applications, from accounting to Enterprise Resource Planning can use the data. ShipConstructor is a typical interfaced ship building suite. It gives designers a wide range of tools for automatically generating 3D solid models of steel, pipe and HVAC (using yard standard details and parts catalogs) while only working in 2D. It also manages data so that the solids are translated into 2D piece parts and are automatically sorted for nesting and CNC cutting. Pipe is automatically broken into spool pieces. The system also provides for automatic derivation of "block" and "unit" drawings and maintains structural weight data, steel stocking and parts lists and other materials management information through Structured Query Language (SQL) compatible databases such as Microsoft SQL Server.

CAD information is rich and mutable. It can change from graphics to text to CNC data and beyond. It can link to costs, schedules, technical manuals and even on line auctions. Ship builders found opportunities in the CAD product model, and have radically improved productivity. The next step is for ship owners to start looking at what they can do with the rich data available from a product model, especially with the Internet. How can better linked, easily accessible data improve ship management and maintenance? The possibilities to realize the real potential of the CAD product model are only limited by our imaginations.

Chris Barry is a naval architect and has worked at design firms and shipyards in California, the UK and the Pacific Northwest. He is currently with the Engineering Logistics Center of the U.S. Coast Guard in Baltimore. The opinions expressed are those of the author and do not reflect official policy of the Coast Guard. This material is based on papers by the author and numerous others that have appeared in the Journal of Ship Production of the Society of Naval Architects and Marine Engineers, (www.sname.org) and on projects and reports that have been developed under the aegis of the NSRP, (www.nsrp.org).

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