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 self-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
derrick system.
The machinery on the deck of an oil tanker is limited to that used for anchor
handling and mooring plus pumproomn 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 (Ex e) safety type.
Either electric or hydraulic drives are installed for the deck machinery of dry
cargo vessels.
Mooring equipment
Full load duties of warping capstans and mooring winches vary between
3-30 tonnes at 0.3 to 0.6 m/s and twice full load speed is normally provided for
recovering slack lines.
The size of wire rope used on mooring winch barrels is governed by the
weight of wire manageable by the crew; this is currently accepted as 140mm
circumference maximum. The basic problems associated with the use of wire
ropes is that they are difficult to handle, do not float and when used in
multi-layers, due to inadequate spooling, the top, tensioned layer cuts down
into the underlying layers causing damage.
To counteract this problem a
divided barrel can be used such that the wire may be stored on one portion and
a single layer of wire transferred to the second portion when tensioned. Low
density, high breaking strength synthetic ropes (polypropylene, nylon or
terylene) offer certain advantages over wire, its main disadvantage being a
tendency to fuse if scrubbed against itself or the barrel.
Fig :Electrically driven mooring winches with two split drums
Winches
Mooring winches provide the facility for tensioning the wire up to the stalling
capacity of the winch, usually 1.5 times full load thereafter the load is held by
the motor brake, or by the barrel brake when the power is shut off. The winch
cannot pay out wire unless the brake is overhauled or recover wire unless
manually operated, thus wires may become slack.
Image credit :Wärtsilä Encyclopedia of Ship Technology
Automatic mooring winches provide the manual control previously
described but in addition incorporate control features such that, in the
automatic setting, the winch may be overhauled and wire is paid off the barrel
at a pre-determined maximum tension; also wire is recovered at a lower tension
should it tend to become slack. Thus there is a certain range of tension,
associated with each step of automatic control, when the wire is stationary. It is
not practical to reduce this range to the minimum possible as this results in
hunting of the controls.
It should be noted that the principal reason for incorporating automatic
controls with the features described is to limit the render value of the winch and
avoid broken wires; also to prevent mooring wires becoming slack. Load
sensing devices are used with automatic mooring winches, e.g. spring-loaded
gearwheels and torsion bars are widely used with steam and electric winches;
fluid pressure sensing, either steam or hydraulic oil pressure, is also used where
appropriate.
Mooring winches are usually controlled at the local position, i.e. the winch,
For vessels of unusually large beam or where docking operations are a frequent
occurrence e.g. in ships regularly traversing the St. Lawrence Seaway, remote
and shipside controllers are of great advantage.
As mooring techniques vary
widely, the position and type of control must be engineered to suit the
application. It is considered, especially on vessels where mooring lines may be
long and ship position critical, that the greatest asset to the operator is
knowledge of the wire tensions existing during the mooring operation coupled
with an indication of the amount of wire paid off the barrel. It is quite feasible to record these at a central position and mooring lines would then only have to be
adjusted periodically as indicated by the recording instruments.
The majority of automatic mooring winches are spur geared to improve the
backward efficiency of the gear train for rendering, the gearing and bearings
being totally enclosed and lubricated from the oil sump.
On larger mooring
winches were a barrel brake is fitted, it is now common practice to design the
brake to withstand the breaking strength of the mooring wire. Worm geared
automatic mooring winches are uncommon as the multi-start feature required
to improve gear efficiency reduces the main advantage of the worm gear i.e. the
high gear ratio.
Summarized below some of the basic operation of deck machinery and maintenance guide :
Pump and motor systems are used for powering deck machinery such as
winches and windlasses. Pump and actuating cylinders are normally employed
for hatch covers. One or more pumps will be used to supply the volume of fluid
at the pressure required to operate one or more motors.
......
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 three essential components for a hydraulic circuit, are the hydraulic fluid
held in a reservoir tank, a pump to force the liquid through the system and a
motor or cylinder actuator to convert the energy of the moving liquid into a
working rotary or linear mechanical force. Valves to control liquid flow and
pressure are required by some systems.
......
Electric motors on vulnerable deck areas may be protected against ingress of
water by being totally enclosed in a watertight casing. Vents are provided on
some winches, which must be opened when the motor is operating in port.
......
The windlass cablelifter brakes must be able to control the running anchor
and cable when the cablelifter is disconnected from the gearing when
letting go'. Average cable speeds vary between 5 and 7 m/s during this
operation.
......
A large number of ships are fitted with deck cranes. These require less time to
prepare for working cargo than derricks and have the advantage of being able
to accurately place (or spot) cargo in the hold. On container ships using ports
without special container handling facilities, cranes with special container
handling gear are essential.
......
Hatch cover equipment like the other deck machinery, has to exist in a very
hostile environment and the importance of regular maintenance cannot be
over-emphasized. Drive boxes and electrical enclosures should be checked
regularly for water-tightness.
......
The duty of a deck winch is to lift and lower a load by means of a fixed rope on
a barrel, or by means of whipping the load on the warp ends, to top or luff the
derricks, and to warp the ship.
......
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