Owing to the enormous increase in size of bulk carriers and tankers,
shipyards with restricted facilities, berth size particularly, have examined
various means of building these large ships in sections which are to be joined
off the berth.
In most cases the problem becomes one of joining the two hull sections
afloat or in a dry-dock of sufficient size where available. Where the sections
are to be joined afloat, extremely accurate fit up of the sections is aided by
the possibilities of ballasting the two ship halves. The two sections may then
be pulled together by tackles; and for the finer adjustments hydraulic
cylinders may be used, extremely accurate optical instruments being employed to
mark off the sections for alignment. One method adopted is that where a
cofferdam is arranged in way of the joint, a caisson is brought up against the
ship’s hull, and the cofferdam and caisson are pumped dry. To balance any
tendency for the vessel to ‘hog’ during the pumping of the cofferdam it is
necessary to shift ballast in the fore and aft sections. Once the spaces are
dried out welding of the complete joint may be undertaken, the resulting weld
being X-rayed to test the soundness of such a critical joint. On completion of
the paint scheme in way of the joining the caisson is removed.A similar method makes use of a rubber ‘U’ form ring rather than a caisson which needs modification for each ship size.
If a dry-dock is available the sections may be aligned afloat and even welded above the waterline, the rest of the joint or the complete joint being secured by strongbacks. The welding of the rest of the whole joint is carried out in the dock.
