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| SECOND
PART
1 MANUFACTURING PROCESS
by S. Zanelli The shipbuilding industry
has over the years developed the construction of ships with
hulls and structures in steel. For some decades now
other construction materials, such as metal alloys, have been
introduced. Aluminium alloys have been widely used in the construction
of light, small-sized vessels, while special metals such as
titanium have been used in particular cases. These are not currently
used on an industrial scale and are therefore not included in
this study.
1.1.2
The manufacturing process The manufacturing process
for steel or aluminium vessels is similar and therefore one
description is given for both processes. The greatest differences
are those inherent in the difference in vessel size: aluminium
vessels have a much smaller volume than steel vessels. The sheet
metal used is therefore thinner and can be cut, bent and soldered
more easily than that used in the construction of large steel
ships. Apart from requiring
great skill in metal-working techniques, the construction of
a ship also calls for professionalism and knowledge of numerous
technical sectors, such as erection of scaffolding for constructing
the hull and plating, electrical wiring, raising and moving
operations, sandblasting, cleaning and painting and all the
details of fitting-out. Figure 1 shows a summary
of the manufacturing cycle.
Figure1 - The manufacturing cycle in metalworking shipyards
The variety of manufactured
products which go to make up a fitted-out vessel is enormous
and entails a wide range of different materials; these include
semi-worked products such as metal sheeting and sections, finished
products such as engines and items for fitting-out which vary
according to the type of ship, be it a petrol or dangerous substance
carrier, a passenger ship or a cruise ship. The following in particular
are required: joinery and insulation materials, products for
sandblasting, cleaning and painting, gas and electrodes for
soldering and welding and oils for tool machines. On arrival material
is offloaded from the relevant means of transport (transport
is by sea or land depending on the size of the material), checked
and then stored in a warehouse. This study refers primarily
to the construction of a vessel which has a supporting structure
made in steel. Where this differs from construction using other
metal alloys, the difference is noted. Movements depend on
how the shipyard is laid out. Materials are divided
between the various departments on the basis of the work cycle.
In the first place the sheet metal and sections are delivered
to the naval workshop and then the rest of the material is delivered
to the relevant departments. Large shipyards have a general
warehouse which houses materials of general use. Sheet metal and other
bulky goods are moved using:
·
Gantry cranes on rails,
·
Cranes on wheels,
·
Fork lift trucks, · Wheeled trucks.
The naval workshop can
be considered the starting point in the manufacturing cycle.
Here the sheet metal and sections that will be used in constructing
the hull, the plating, and internal and external structures
are laid out, cut and formed. Cutting operations are
always preceded by tracings, and are carried out in different
ways depending on the material used, the size of the steel plate
and the shape that is to be cut. Mechanical cutting is carried
out by machine tools and is followed by cleaning of the sheet
metal to remove any off-cuts. Oxygen cutting, whether using
oxy-propane, oxy-acetylene or plasma is carried out by specialised
operatives either with the help of a pantograph or by hand using
a simple etcher. Each type of cut requires particular machinery
and tools along with appropriate systems for moving the piece
(small cranes, hoists etc.) towards the machine. After each cutting operation
the work area must be cleaned of off-cuts and waste. Prefabrication
of non-heavy rough pieces is begun in the naval workshop and
sheet metal and sections are put together here. Finishing operations
carried out in the naval workshop include the moulding of semi-worked
products which are then delivered to the various departments.
These pieces may be used for new assembly of both small and
large pre-fabrications as well as directly on the stocks.
The mechanical workshop
features in many manufacturing cycles as it produces all the
elements for installing machinery on board, such as supports,
joints, anti-vibration couplings, pipe work, valves and pumps. Numerous operations
are carried out here, such as cutting, moulding, welding, and
cleaning and sandblasting of the metal parts which form part
of the machinery on board. The mechanical workshop
also carries out maintenance work of on-site machinery, lifting
mechanisms and operating machinery. In large shipyards the
mechanical workshop has specified departments for working pipes
and for welding easily-transportable pieces.
Small pre-fabrication
involves the assembly, where necessary by welding, of sheet
metal and pipe work to form small-sized pieces such as walkways
etc. The welding is either done electrically or using gas in
inert atmospheres. The main aim of large-scale
pre-fabrication, which can be done both in parallel or in series
with small pre-fabrication, is the construction of semi-worked
items which make up elements in the base structure and plating.
When assembled with the appropriate walls of insulation and
sound-proofing this type of pre-fabrication can produce almost
complete “blocks” which are then used to construct the ship. Interiors are made of
wood which comes from the ship-yard carpenters and from other
materials which are previously worked in the appropriate department.
The materials used must retain their characteristics in a marine
environment. Different blocks of
the ship are then constructed contemporaneously in different
“islands”, thus providing for better work organisation and most
importantly reducing the overall construction time. Work from the ground
is carried out using safety platforms, parapets, scaffolding, elevator platforms and cranes, all of which are essential
for working at high level on the blocks. As work progresses
it becomes necessary to work in restricted spaces, particularly
when carrying out interior work on small cabins carved out of
the block. 1.1.8
Sandblasting and painting After assembly, which
is once more accomplished by welding, the parts are then sanded,
using jets of sand directed at the metal surfaces. This simultaneously
cleans and primes. The cleaned surface is then covered with
a protective paint, or primer, which preserves it against the
oxidising effect of damp air and other aggressive agents. These
operations are carried out in a different place to that where
the item was constructed, and it is therefore necessary to move
the blocks on trolleys or cranes to the sheds which are specially
equipped for such operations. 1.1.9
Construction on the stocks The completed blocks
are then assembled on the stocks with the help of a bridge crane.
The stocks may consist either of two large slipways or by a
dock with an opening towards the sea. The various blocks are
fixed together and then welded, thus creating the vessel. In addition to assembly,
during this phase many of the other operations which render
the vessel operative are carried out. During the phase of
on the stocks construction all the internal and external finishing
operations are carried out on the hull in order to guarantee
that the vessel will float and to complete the parts that will
be under water following the launch. Tests are carried out
on the hydraulics, the electrical wiring system, and, under
pressure, on the pipe-work and tanks, particularly for those
ships which are to be used for the transport of pressurised
liquids. The final operations
prior to launch are those of cleaning and painting of the hull.
It should be pointed out that all the construction operations
described above, such as welding during the assembly of the
blocks and cleaning and painting, involve the use of platforms
and scaffolding for the workers and hoists for the machine tools
used. Launching operations
depend on the type of stocks used. In practice, when the vessel
is mounted on a slip-way the stops are removed and the ship
is left to slide into the sea. When the vessel has been assembled
in dry-dock, the dock is filled with water. Operations of loading
and installing machinery and fitting-out can be carried out
both on solid ground (on the stocks) and after launching. In
general it is better to carry out these operations on solid
ground, particularly for work which must be done in limited
space or where communication hatches leading to adjacent areas
are restricted. These works include finishing operations and
all other auxiliary services carried out with the aim of making
the ship fit for navigation. Some of the most important of these
are the services for dealing with situations of flood or fire,
those intended for the crew or passengers (bunks, kitchens etc),
navigational services, such as command and route-finding equipment, and services connected to the distribution
of electricity produced by the auxiliary generator which runs
on engine or turbines. During fitting-out plant
and machinery are completed and tested. The cabins are painted
and the last finishing operations are finalised. 1.1.13
Sea trials and delivery The last phases of the
“manufacturing cycle” are sea trials, which are essential for
testing the propulsion system, operability, and control equipment
and safety systems for navigation, both for crew and passengers.
In general a preliminary trial is carried out prior to the official
one. Before delivery the
on-board equipment is loaded. To summarise the description
of the production of a ship it is possible to sub-divide the
activity into two major processes; actual construction of the
vessel and its fitting-out. Operations begin with construction
but as work proceeds the two activities are carried out together.
The greatest difference between these two operations is in the
place in which they are carried out; construction operations
are often done in open areas, with the help of cranes and high-level
platforms. Only rarely is it necessary to work in closed places
such as cabins inside the vessel, which are narrow, badly lit
and with few exits. All the fitting-out operations however are
carried out inside the ship in restricted spaces. |
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