Page 150 - Ship Construction.DJ Eyres 6Ed

Ch14-H8070.fm Page 139 Wednesday, October 18, 2006 6:56 AM
Prefabrication 139
planned for each assembly and block unit, and fabrication and outfit instal-
lation is undertaken at a work station where the facilities and material are
supplied to the workforce.

Sub-assemblies

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When plates and sections have been machined (cut, bent, bevelled etc.)
they are ready for assembly into 2-dimensional ship units. Within the fabri-
cation shop there are often arranged a number of bays for different assem-
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blies, for example flat plate panels, curved shell units, matrix or ‘egg box’
structures and some minor sub-assemblies. All these may be termed sub-
assemblies if they are subsequently to be built into a larger 3-dimensional
unit prior to erection. A 2-dimensional plate panel assembly may be highly
automated (see also ‘Welding Automation – Chapter 10) with prepared
plates being placed and tack welded prior to automatic welding of the butts,
after which the plates are turned and back welded unless a single sided weld
process has been used. The panel is marked and the stiffeners placed and
welded automatically or with semi-automatic process. Marking and welding
of 2-dimensional panels is often robotically controlled. Minor sub-assem-
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blies such as deep frames consisting of web and welded face flat are also
attached at this stage. Curved shell plates are placed on jigs and welded and
the various stiffening members can be aligned and welded in a similar
manner to those on a flat panel assembly. Assembly jigs may also be used
for matrix or ‘egg box’ assemblies, for example structures of solid and
bracket plate floors with longitudinal side girders which are to go into
double bottom units.

Unit Fabrication

In most instances the 2-dimensional sub-assemblies will be built into
3-dimensional block assemblies. The size of the block assembly will have
been decided at an early stage of the planning process ideally at the structural
design stage. Constraints such as lifting capacities and dimensions that can
be handled are taken into consideration also the provision of breaks at
natural features ensuring the blocks are self supporting and easily accessed,
etc. Panel assemblies used in the block may well have dimensions restricted
by the plate length that can be handled at the machining stage and this can
subsequently influence block length. In the machinery area the block size
and arrangements can be decided by zone outfit considerations.
Each block should be designed for maximum downhand welding but may
have to be turned for this purpose. Also blocks are turned to effect outfit
installation particularly those containing machinery flats in the aft engine