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Deck machineries for cargo ships
The various items of machinery and equipment found outside of the
machinery space of modern cargo ship. These include deck machinery
such as mooring equipment, anchor handling equipment, cargo
handling equipment and hatch covers. Other items include lifeboats and
liferafts, emergency equipment, watertight doors, stabilisers and bow
The operations of mooring, cargo handling and anchor handling all
involve controlled pulls or lifts using chain cables, wire or hemp ropes.
The drive force and control arrangements adopted will influence the
operations. Several methods are currently in use, and these will be
examined before considering the associated equipment.
Three forms of power are currently in use: steam, hydraulic and electric,
Each got its advantages and
disadvantages for particular duties or locations.
Steam powered deck machinery
With a steam powering and control system the steam pipelines are run
along the deck to the various machines. Steam is admitted first to a
directional valve and then to the steam admission valve. Double-acting
steam engines, usually with two cylinders, are used to drive the
machinery. Additional back pressure valves are used with mooring
winches to control tension when the machine is stalled or brought to a
stop by the load. Arrangements must also be made, often associated with
the back pressure valve, to counteract the fluctuations in main steam line
pressure as a result of other users of steam.
The steam-powered system was widely used on tankers since it
presented no fire or explosion risk, but the lengths of deck pipework
and the steam engines themselves presented considerable maintenance
tasks which have generally resulted in their replacement by hydraulically
The open-loop circuit takes oil from the tank and pumps it into the
hydraulic motor. A control valve is positioned in parallel with the motor.
When it is open the motor is stationary; when it is throttled or closed the
motor will operate. The exhaust oil returns to the tank. This method can
provide stepless control, i.e. smooth changes in motor speed.
The live-line circuit, on the contrary, maintains a high pressure from
which the control valve draws pressurised oil to the hydraulic motor (in
series with it), as and when required.
In the closed-loop circuit the exhaust oil is returned direct to the
pump suction. Since the oil does not enter an open tank, the system is
Low-pressure systems use the open-loop circuit and are simple in
design as well as reliable. The equipment is, however, large, inefficient
in operation and overheats after prolonged use.
Medium-pressure systems are favoured for marine applications, using
either the open or closed circuit. Smaller installations are of the
open-loop type. Where considerable amounts of hydraulic machinery
are fitted the live-circuit, supplied by a centralised hydraulic power
system, would be most economical.
Early installations used d.c. supply with resistances in series to provide
speed control . This inefficient power-wasting method
was one possibility with d.c., but a better method was the use of Ward
Leonard control. The high cost of all the equipment involved in Ward
Leonard control and its maintenance is, however, a considerable
Machines operated on an a.c. supply require a means of speed control
with either pole-changing or slip-ring motors being used. Slip-ring
motors require low starting currents but waste power at less than full
speed and require regular maintenance. Pole-changing motors are of
squirrel cage construction, providing for perhaps three different speeds.
They require large starting currents, although maintenance is negligible.
Apart from the advantages and disadvantages for each of the drive
and control methods, all electric drives have difficulty with heavy
continuous overloads. Each system has its advocates and careful design
and choice of associated equipment can provide a satisfactory
Handling Deck machineries: safety precautions
The operation of mooring a vessel has traditionally required the attendance of
a large number of deck crew fore and aft. Supervision of the moorings was also
necessary to maintain correct tension through changes due to the tides and the
loading or unloading of cargo.
The installation of constant tension mooring
winches, which maintain tension in ropes through any rise and fall, has
removed the need for constant attendance and equipment is available for tying
up which is designed for operation by as few as two men.
Large container ships
may have four mooring winches on the after deck; each of the seif-tensioning
type with its own rope drum. Controls are duplicated and are situated at each
side of the vessel, giving a clear view of the operation. Mooring ropes are paid
out directly from the drums as they are hauled by the heaving lines from the
quay. With the loop in place on the bollard, the capstan is set on auto-tension
after slack is taken up and the ship is correctly moored. A common
arrangement forward is for two similar winches plus rope drums for
auto-tensioning on each windlass.
The introduction of steel hatchcovers not only speeded up the operation of
opening and closing the covers but also reduced the number of personnel
required for the task. Rolling and folding covers may be operated by a pull wire
or hydraulically. Covers for large container ships may be lifted bodily by crane
and there are now hatchcoverless container ships in service.
Cargo handling may be by winches and derricks or cranes. Some geared bulk
carriers have overhead cranes arranged to travel on rails.
Most deck machinery is idle during much of its life while the ship is at sea. In
port, cargo equipment will be in use for one or more days but the machinery for
anchoring and mooring is used for a very limited time. Deck machinery with a
restricted and intermittent duty may be designed with drives with a rating
limited from 30 minutes to one hour. Despite long periods of idleness, often in
severe weather conditions, machinery must operate immediately, when
required. Cooling vents, open when machinery is working, must be closed for
the sea passage.
It is essential that deck machinery should require minimum maintenance.
Totally enclosed equipment with oil bath lubrication for gears and bearings is
now standard but maintenance cannot be completely eliminated and routine
checking and greasing should be carried out on a planned basis.
There are many instances where remote or centralized control is of great advantage,
for example, the facility for letting go anchors from the bridge
under emergency conditions; the use of shipside controllers with mooring
winches; or the central control positions required for the multi-winch slewing
The machinery on the deck of an oil tanker is limited to that used for anchor
handling and mooring plus pumproorn fans and equipment for handling the
gangway and stores. Power was universally provided in the past by steam.
Hydraulic equipment is now common, sometimes with air motors for gangway
duties. The availability of safe electrical equipment means that electric motor
drives can be used where appropriate.
Liquefied gas carriers and product or chemical tankers have similar deck
machinery installations but the drive motor for deepwell pumps may be an
induction motor of the increased or enhanced safety type.
Either electric or hydraulic drives are installed for deck machinery of dry cargo vessel.
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