Page 387 - Practical Design Ships and Floating Structures

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Accuracy of production in steel hull fabrication is a target which may be used to demonstrate the new
approach. Manufacturers of cutting machines are able to guarantee an accuracy of positioning of the
cutting torch below 0,3 mm over a working area of for example 12m x 26m. This accuracy can be
used to apply positioning marks or reference lines to the plates for further use in parts fitting.
In order to benefit from this low machine deviation the distance between cutting nozzle and marking
torch must be kept constant. In reality however one may observe that due to changing geometric
conditions in the plasma cutting head over time (anode burn out) the actual cutting line does not follow
the theoretical one resulting in subsequent inaccuracies.
In consequence the efforts to increase cutting accuracy will yield no improvements if only the
mechanical part of the machine is addressed. The real subject is the development of more accurate
cutting nozzles of online accuracy control.
What is the effect of inaccuracies? Taking a buttweld in a 1 Omm plate with a design air gap of 2mm
the increase of gap width by 2 times 0,3mm results in an increase in weld deposit of 30%. The
unavoidable transverse shrinkage will increase correspondingly and will lead to stochastic buckliig of
the parts.
The only way to regain accuracy control is an on-line monitoring and control of cutting accuracy to
below 0,lmm in the large plate cutting shop.
3.2 Autonomous Subassemblies Production

Another ana in the production chain deals with the fabrication of small subassemblies consisting of
flat plates and stiffeners. These are different in size and shape but principally they all belong to one
"family". The production process of these can be regarded as series production and is likely to be fully
automated. The following diagram illustrates this new Subassembly factory concept.
From the cutting shops plates and profiles are moved into a parts recognition area where a camera
identifies the parts by reading barcodes applied before cutting. The geometry information for these
parts is taken fiom the CAD-database. In the next station the actual position of the plate on the
conveyor is identified, in order to adjust the co-ordinates of the part to its actual random position.
Meanwhile a manipulator has picked up the stiffener profile(s) and positiones it onto the plate for tack-
welding.
Once the subassembly is complete it is moved to the next station for robot welding. The last step is
final measuring of contours of the subassembly and delivery of actual geometry data into the as built
database.
One complete process step is avoided with corresponding gains in time and cost. By the use of such
system marking work prior to or after cutting is not necessary and higher total accuracy can be
achieved. The knowledge of the actual geometry of the subassemblies allows for process adjustments
further downstream.

3.3 Laser Welding
This technology has been under discussion in the shipbuilding scene in Europe since the early nineties.
Several large co-operative research projects dealt with the subject. The "Guidelines for Laser welding"
by the Euracs-Classification societies are one of the results. All these early works considered C02
Lasers as the source of power. Meanwhile these krs have become very reliable and less expensive.
As a result procedures for welding have been developed and some yards are using it in production.
Especially when working with relatively thin plate, like in passenger ship building, the advantages are
very obvious. The very low heat input reduces thermal distortions to a minimum; rework for
straightening becomes obsolete with consequential savings in process time and cost. Some yards quote
30% of all steelwork production hours as being rework.
However accuracy is not only to be achieved in the finished product; it is also a requirement in fitting
the parts prior to welding because of the low tolerance of indigenous laser welding regarding gaps and
steel composition. The idea to combine the sensitive Laser welding process with the more tolerant