10 prepar ation and fabrication

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10 PREPARATIONS AND FABRICATION

10.1 AIMS OF THE LECTURE

a) To outline the tasks to be carried out for preparation and fabrication of plates and

sections.

b) To provide popular methods for preparation and fabrication of plates and sections

c) To explain the role of uses automation equipment for fabrication processes

d) To examine the use of machinery, equipment, and facilities for preparation and

fabrication of plates and sections.

10.2 MAKE UP OF SHIPS STRUCTURE

The structure of ships hull is built from plates and sections; they are welded from small parts

to large structure.

Before being welded the plates and sections have to be prepared through several treatment

and preparation processes.

The plates and sections are then fabricated and assembled to form large structures as units

and blocks. Illustration of the process is as follows:

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10.3 PLATES AND SECTIONS TREATMENT

When steel plates and sections come to the shipyard from the steel mill by means of truck,train, barge, or ship and then stacked in accordance to their identities on the open air stock

yard. Due to handling and storing the plates and sections might bend, and rust, therefore

before being further processed they need to be straightened, cleaned through the following

flow:

a)In the stock yard

From the stock yard plates and sections are transferred to the treatment shop using magnetic

gantry crane for plates and mobile crane and sling or fork lift truck for sections, and by means

of roller conveyor plates and sections are directed to their designated treatment machines.

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b)In the treatment shop

Plates are firstly straightened using roll machines or are also called mangles; this machine

usually has three top rollers and two bottom rollers, these rollers are also supported by several

smaller rollers. The gaps between the top rollers and bottom rollers can be adjusted to suit the

thickness of the plates. The plates are fed into the mangle and due to forces apply on the

surface of the plates they are straightened upon leaving the mangle.

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After being straightened the plates are cleaned from rust and mill scale using shot-blasting

machine. Plates are fed on the conveyor horizontally into the shot-blasting machine; iron grits

are sprayed using pressurised spray guns onto both surfaces of the plates; the spray guns are

attached to the carrier that move right angle to the flow of the plates reciprocally. Upon

leaving the shot-blasting machine the plates are sprayed with water and after that with hot air;

the clean and dry plates are then transferred into the primer painting chamber to be

temporarily protected with primer paint.

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Sections are usually not straightened but directly sent to the shot-blasting machine. The

process is more or less similar to the plate blasting and then the sections are also primer

painted.

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10.4 PLATES AND SECTIONS PREPARATION

a)Lofting

In the conventional shipyards when the production drawings have been produced in the

drawing office the preparation works will follow start from lofting shop; it is a huge roomwhere the lofts men or markers translate the lines plan into full scale drawings as the

reference for making full size patterns of the ship components, and templates for fairing and

cutting the shell plates. In large and modern shipyards the lofting works are carried out by

optical marking that photographs small scale drawings of the ship to the steel plates.

Now with the advancement of computer system, most of the loft works can be directly carried

out from the drawing office in line with the production drawing tasks. Ship components data

are inputted to the computer and the dimensions of the plate are defined; the programme will

automatically nests the components in most optimum ways. The nesting data will be

transferred to the CNC cutting machine for planing, marking and cutting the plates.

b)Marking and Planing

At the preparation stage the plates are planed to obtain the right dimensions and to prepare

the edges using CNC cutting machine. Ship components are also cut on the CNC cutting

machine using the data transferred from nesting programme, sometimes marking is also

carried out either manually or using the same CNC machine to give identity to the

components plates which will be needed in the processes to follow.

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Manual cutting using oxy-acetylene burner sometimes can still be found in some shipyards

for cutting simple sections. For larger beams CNC beam cutting machine may be used.

c)

Bending

For curved components some plates and sections need to be rolled or bent; Plates are usually

bent using rolling machine or pressing machine, and in order to obtain precision curvature

line heating are sometimes applied onto the plates; for brackets, knees and other specific

components flanging and swaging are carried out using pressing machine.

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Sections and beams are sometimes also need to be bent, for this purpose hydraulic

section/beam bender machine is used, but in some small shipyards since the utilisation of

beam bender is relatively low, to minimise cost of investment curved beams are made fromwelded plates.

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d)Profiling

Cuts on the ship structures are sometimes required, to produce these cuts profiling tasks are

carried out.

In modern shipyards where mass production approach is implemented multi-head CNC

profiling machine is mostly used especially when accuracy of the cuts is required, but forsmall number and simple cuts manual profiling still can be found in small and conventional

shipyards.

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10.5 SHIP FABRICATION

Fabrication is aprocess of joining paltes and sections to form simple structure.

Upon passing through the preparation processes the plates and sections are ready to be

fabricated into various simple structures before being further constructed into blocks in theassembly shop.

During the fabrication plates and sections are welded together to form two dimensional

simple structure such as attaching stiffeners onto the double bottom floors and beam knees,

welding flanges onto the manholes or lightening holes etc., known as sub-assembly.

For larger sub-assemblies known as panels the fabrication is carried out in the panel lines

shop, the panels are then joined together to form blocks and units.

At the fabrication stage most of the works are carried out by welding, fillet welding in

particular. For simple and small parts manual electric arc welding is widely being used, for

large and repetitive tasks automatic welding are implemented.

a)Panel line

Panel line is a process of constructing flat or curved stiffened panels such as deck panels,

bulkhead panels etc. It normally contains a series of fabrication processes and operations for

producing of these panels.

The plates are first fed into the line, aligned, clamped and manually tack welded together.

The plate seams are then welded on one side and the plate turned over. The second side

seams are welded. Some panel lines use one sided welding technique and therefore no plate

turning is required.

The panel is flame planed to size and marked out for web and stiffeners which are to be

fitted. Stiffeners are pushed from the side, positioned, clamped and welded on the panel one

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by one. The stiffened panel is then transferred to the assembly shop to be joined with other

components to form unit, or block.

Special devices are usually used for supporting the plates joining process such as for fitting

and fairing aids as follows:

Wedge Devices

Threaded Devices

Hydraulic Devices

Pneumatic Devices

Gear-Pulley Devices

Magnetic Devices

Strongbacks

Jigs, Mocks and Fixtures

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10. 10. Pneumatic

10.0 Magnetic 10.1 Strongbacks

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c)Modern approach in panel line

With the advancement of computerised mass production technology, many of world leading

shipyards have implemented automatic conveyor panel lines system, where robotic welding

are also employed in some shipyards. The system would normally consist of a series of work

stations dependent on the shape of panels to be constructed; in general it passes through thefollowing stages:

1. One side welding Gantry

The purpose of this gantry is to weld together two or more plates to make up panel.

Sometimes the gantry can weld different thickness of plates. The system consists of two

roller conveyors placed at the beginning and the end of the weld section. The gantry is

located in the centre and is equipped with an approaching and welding system.

2. Brushing and marking gantry

The purpose of this gantry is to mark the material surface in order to identify the welding

position for supports, squaring, boring, etc., Usually such operations are done semi-

autonomously by a group of tracing operators. The system envisages a computerized

console on which the tracing data has been previously inserted. Manual operations are

restricted to the actual panel positioning and gantry positioning. Marking can be done

either longitudinally or transversally with either paint or ink. Before marking, a brushing

or shot blasting system is placed nearby the marking area in order to clean it.

3. Transversal tack-welding gantry

The transversal tack-welding system is used to position the stiffeners before weldingoperations take place. Such positioning can be done manually, semi-automatically or

completely automatically, while welding of such stiffeners is done manually. The gantry,

equipped with a magnetic material pick-up system especially designed according to the

various types of profiles and associated to an accurate positioning system, enables the

pressure system to provide safety and precision, thus facilitating tack-welding operations.

4. Automatic welding gantry

The automatic welding gantry will weld the reinforcements which have been tack-

welded by the transversal tack-welding gantry onto the panel. The gantry is equipped

with a trolley that moves parallel to the profile to be welded. The trolley houses twowelding heads as well as a joint-track system. The operator may insert the data for the

voltage and current values from the control console as well as continuous or block

welding. Various types of welding procedures (MIG-MAG, SAW) may be applied to this

gantry.

5. Longitudinal welding gantry

The longitudinal welding gantry is used to position the longitudinal stiffeners. This type

of positioning is normally done in manual mode due to the large number of stiffeners on

a panel. The gantry is in any case equipped with a rotating beam which picks up the

reinforcements, rotates them and positions them on the panel. Tack-welding and/or


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welding operations can be done manually in this station as well as in the succeeding

working stations.

6. Service gantry

The purpose of the service gantry is to facilitate all tack-welding or welding operationson the panels or simply to complete those operations carried out previously. The service

gantry is generally equipped with 4 semi-mobile trolleys with controls and welding

torches. The welding rectifier is instead positioned on board the gantry which can move

down the entire length of the panel.

d)Curved panels

The process of constructing curved panels is almost similar to the flat panels; the only

different is the utilisation of jigs or mocks for supporting the plates.

Curved plates are laid on the jigs or mocks which previously have been arranged to have

same curved pattern as the plates, the plate seams are butt welded (it is recommended that

one sided welding is employed).

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The plate is marked for the positions of stiffeners by the aid of templates, which usually have

been prepared in the mould loft. The stiffeners are then positioned, tack welded, and fully

fillet welded as on the flat panel.

In automatic panel line, curved panel is laid on the hydraulic movable bench, or on the jigs or

mocks, then robotic or computer programmed welding machines are employed to weld thestiffeners and the webs.

10.6 CONCLUDING REMARKS

a) The structure of ships hull is built from plates and sections; they are welded from small

parts to large structure.

b)

Before being welded the plates and sections have to be prepared through severaltreatment and preparation processes.

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c) The treatment include: straightening, cleaning, and primer painting; and preparation

include: lofting, marking, nesting, cutting, bending.

d) Prepared parts are fabricated into sub-assembly and panels before being assembled into

larger structures.

10 prepar ation and fabrication

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