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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 thrusters.

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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 powered equipment.

Hydraulic systems

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 considered closed.

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.

Electrical operation

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 disadvantage.

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 installation.

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 derrick system.

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.

Related guideline

  1. Hydraulic systems maintenance guideline

  2. Anchor handling equipment maintenance guideline

  3. Deck mooring equipment operation and maintenance

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