Unit Erection

Experience of previous ship erection schedules and difficulties given the yard’s physical and equipment constraints leads to standard practices being established. These are taken into consideration at the structural design stage as are the desirability of minimising positional welding and fairing. In general it is common practice to make a start in the region of the machinery spaces aft, working from the bottom upwards, and also forward and aft, this area requires larger amount of finishing work. In particular the boring of the stern for the tail shaft is preferably undertaken when the after sections are fully faired and welded.

Typical erection sequences for a bulk carrier are shown in Figure below.

In erecting the ship units it is important to employ the correct welding sequences. These are arranged to avoid excessive ‘locked in’ stresses; and overlapping frames, longitudinals, stiffeners etc. may be left unwelded across unit seams and butts until these are completed.
In erecting units, tolerances are a problem, more so on 3-dimensional units than with 2-dimensional units and particularly at the shaped ends of the ship. Quality control procedures in the manufacturing shops to ensure correct dimensioning and alignment are very necessary if time-consuming, expensive and arduous work at the berth is to be avoided. Improvements in this area have been made with the use of accurate jigs for curved shell units, planned weld sequences, and use of lower heat input welding equipment dimensional checks on piece parts, and the use of laser alignment tools for setting up datums and checking interfaces.

Outfit Modules

Units of machinery, pipework and other outfit systems required for a specific zone can be planned and built up into modules and installed as such into a block fabrication. Pipework in particular lends itself to this form of assembly and can, with careful planning from the CAD stage, be arranged in groupings so that pipe bank modules can be arranged for a particular zone. Modules can range from a small pipe bank supported by light framing of pipe hangers, or a complete auxiliary machinery unit on its seating which has even been test run prior to installation, to a large modular unit which together with several similar units constitutes the bulk of a complete engine room.

Not all outfits can be incorporated into modules and a large number of piece parts have to be provided for fitting in any given zone at a particular time within the assembly shops. To maintain production engineering standards a concept of ‘palletisation’ has been developed whereby the piece parts for that zone are generated at the CAD/CAM stage, bought in and/or fabricated etc. and made available at the work station when the particular assembly is ready to receive them.
An ‘open top’ arrangement for block or smaller ships being outfitted under cover can facilitate installation of the items and modules.
Superstructure blocks are fabricated separately and pre-outfitted with accommodation before erection as a complete unit. Modular cabin units are a common feature of modern shipbuilding, Figure below shows a typical self supporting cabin/toilet module complete with pipework, ventilation, electrical fittings, wiring, and all built-in furniture.

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 handed are taken into consideration also the provision of breaks at natural features ensuring the block 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 block are turned to effect outfit installation particularly those containing machinery flats in the aft engine room areas where pipework etc. can be fitted on the underside of the flat with the block in the inverted position and then it is turned to install equipment above the flat . A block’s centre of gravity is calculated and lifting lugs so provided that these operations can be undertaken, and finally the block can be suspended for erection at the building dock or berth and drop into place in the correct plane.

Sub-Assemblies

When plates and sections have been machined they are ready for assembly into ship units. Within the fabrications shop there are often arranged a number of bays for different assemblies, 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 large unit prior to erection. Panel assembly is usually highly automated 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. Minor sub-assemblies such as deep frames consisting of web and welded face flat may also be 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.

Liquefied Natural Gas Ships

There are over twenty approved patent designs of containment vessel for LNG ships, the majority of which fall into the membrane or independent tank categories. Those types which have been or are more commonly found in service are described below. A feature of LNG ships is their double hull construction within which are fitted the cargo tanks and the secondary barrier system.

Independent Type A Tanks

Early LNG ships were fitted with self-supporting tanks of aluminium alloy having centreline bulkheads. The balsa wood insulation system was attached to the inner hull (secondary barrier) and each insulated hold contained three tanks.

Independent Type B Tanks

The Kvaerner-Moss group have designed an independent Type B tank containment system which has been well accepted and is installed in a good number of LNG ships. Tank consist of either an aluminium alloy or 9 percent nickel steel sphere welded to a vertical cylindrical skirt of the same material which is its only connection to the full see Figure 25. The sphere expands and contracts freely all movements being compensated for in the top half of the skirt. The outer surface of the sphere and part of the skirt is covered with a polyurethane foam insulation. The system is fitted with a partial secondary barrier consisting of a drip tray under tank and splash shields at the sides. Above deck the spheres are protected by substantial weather covers.

Semi-Pressurised (Or Semi-Refrigerated) Tanks

The capacity of pressurised ships ranges up to about 5000 m³ the cargoes carried being similar to fully-pressurised ships. The independent Type C tanks are generally constructed of ordinary grades of steel suitable for temperature of –5°C and are designed for a maximum pressure of about 8 kg/cm². The outer surface of the tank is insulated and refrigeration or reliquification plant cools the cargo and maintains the working pressure. Cargo tanks are often horizontal cylinders mounted on the saddle supports and many designs (see Figure 26) incorporate bio??? Tanks to better utilise the underdeck space and improve payload.

Fully-Refrigerated Tanks

The capacity of fully-refrigerated ships ranges from 10,000 m³ to 100,000 m³ the smaller ships in the range being multi-product carriers whilst the larger vessels tend to be single product carriers on a permanent route. Tanks fall almost exclusively into the prismatic, independent Type A category with tops sloped to reduce free surface and bottom corners sloped to suit the bilge structure in most cases they are subdivided along the centreline by a liquid-tight bulkhead which extends to the underside of the dome projecting through the deck which is used for access and piping connections etc. The tanks sit on insulated bearing blocks so that surfaces are accessible for inspection are located by anti-roll and pitch keys in such a manner that expansion and contraction can take place relative to the ships structure. Anti-floatation chocks are provided to prevent the tank floating off the bearings if the hold were flooded. Tanks are constructed of a notch ductile steel for the normal minimum operating temperature of –43°C the boiling of propane.

Technigaz

The Gaz Transport system uses a 36 percent nickel-iron alloy called ‘Invar’ for both the primary and secondary barriers. Invar has a very low coefficient or thermal expansion which makes any corrugations in the tank structure unnecessary. The Invar sheet membrane used in only 0.5 to 0.7 mm thick which makes for a very light structure. Insulation consists of plywood boxes filled with pearlite (see Figure 24).