Marine deck equipment — Cranes, winches & deck equipment
Marine deck equipment is important. Almost every ship needs a crane of some sort for taking provisions and stores aboard in addition tankers need cranes for handling hoses and some dry cargo ships need gear for loading and discharging cargoes.
The largest bulkers are almost always gearless but ships of Panamax and below may have gear ranging from simple cranes on smaller general cargo ships through to cranes capable of lifting 1,000 tonnes or more on heavy lift vessels.
As far as SOLAS goes there are no regulations concerning cranes but their placement will have been taken into account at the design stage and by the class society and all cranes will be marked with a safe working load that should never be exceeded. Crane operation is a skilled task and while most of the time cranes will be operated by crew, there are some ports where local labour will demand the right to operate them under local employment conditions.
From a safety point of view, the crew will need to ensure the qualifications of non-crew operatives and ensure that there is some element of supervision at all times. Older vessels may still be equipped with derricks but these are gradually becoming obsolete.
Cranes vary enormously in design and operation with the ship usually being fitted with a type that suits the most common cargoes. At one time most of the cranes found on ships were operated by wires for raising and lowering the jib but today it is equally common to see hydraulic rams used instead.
Some vessels – particularly those carrying containers or packaged lumber – are fitted with travelling gantry cranes that can run the length of the cargo deck. For heavy lift ships, cranes are designed to work in tandem when handling very heavy loads.
On a ship that uses its cranes for several different types of cargo, specialist equipment is likely to be found onboard. As well as hooks and spreaders for lifting general cargo and crates, there may be clamps for lifting newsprint or reels and grabs and buckets for handling cargoes such as grain and coal.
Marine deck equipment - Winches
A winch is a marine deck equipment device for handling wires or ropes and works by spooling the wire or rope on a drum with a horizontal axis. The winch can be powered by electric or hydraulic motors; steam winches were once common but are now obsolete.
Winches on ships are fixed and used for specific purposes. As previously mentioned cargo derricks are now much less common and so the winches needed to provide power for these have also been more or less abandoned.
The most common use of a winch is for mooring meaning the winches are mostly located on the fore and after decks at both sides of the ship. Tugs and offshore vessels such as AHTS, seismic survey and OSVs will also be equipped with work winches designed for the very heavy duty work these ships are used for.
The power source for any winch systems can be, as required by the customer, low-pressure hydraulic, high-pressure hydraulic, frequency-converter electric drive or pole-change electric drive. The choice of drive type often depends on type of application and the actual winch operations. All types have their pros and cons.
In recent years, some criticism has been levelled at hydraulic winches because of their potential to leak oil and frequency converter drive winches have found favour for their energy saving and soft start potential. For ships trading to the US, the choice of hydraulic fluid for winches should be one that meets the requirements of the VGP.
Not all winch makers produce a full range of different power types. One that does is Rolls Royce which describes the merits of each as follows. The low-pressure drive’s key characteristics are foremost, reliability and robustness. In addition, the low-pressure drive gives dynamic braking, low noise level and is easy to operate.
Further advantages are stepless speed regulation and high torque. Because it has few mechanical parts, the low-pressure drive is less exposed to wear and tear, giving low maintenance costs.
The hydraulic system for the high-pressure drive is of an open loop, constant pressure type. One pump can simultaneously supply a number of winches and other hydraulically driven devices. The high-pressure drive has excellent stalling and effective low speed performance. It is easy to install, operate and maintain.
Three speed pole-change drive and nearly maintenance-free electric motor is of squirrel-cage rotor type, without mechanical contact between the rotor and the stator. The motor is equipped with standstill heating, temperature sensors and a fail-safe brake. The winch control is precise and easy.
Speed steps in both directions are obtained by a single lever. Electric systems are easy to install and provide quick start in all environmental conditions. Frequency converter drive models represents the latest and the most advanced electric drive technology available.
The stepless control allows the use of very low speed for clutch control and anchor nesting. The drive system also offers good stalling performance, and smooth, low noise operation provides for a good living and working environment. Winches are relatively simple pieces of machinery and while they can fail for various reasons, it is usually poor maintenance and age related wear and tear that is the prime cause.
The company’s new concept of using permanent magnet motors gives the benefit of both systems in that it can apparently provide the precise control and high torque which is not easy for an electric motor to replicate but which is easily achieved using hydraulics while at the same time removing the pollution potential of hydraulic leaks.
Rolls-Royce found that there was no existing PM motor available that could virtually mirror the performance of a low pressure hydraulic motor so has drawn on its experience in PM technology used in its thrusters to develop one that can. The result is the compact XT140 permanent magnet motor which is designed so as to be suitable for retrofitting, replacing existing motors and making electric winches a possibility across a wide range of applications.
An alternative to traditional winch design appeared in 2004 when TTS now part of Palfinger) developed the winch bollard. This device comprises of rotating grooved cylinders mounted over a bollard arrangement. It replaces all of the equipment normally used for mooring and can be operated by one man using a moveable foot operated pedal, the Winch Bollard provides direct mooring without the use of stoppers.
An integrated emergency stop and fail safe brakes optimise safety and the unit features stepless speed control and automatically reduces line speed when the load is increased. Deck space requirements are also reduced as the unitised device occupies less space than the conventional mooring equipment it replaces.
In most ships the ropes from a conventional winch are routed through a fairlead to prevent damage to them. The winches themselves are usually trouble free if well maintained but failures to equipment that is constantly exposed to the worst environment and weather does inevitably occur occasionally. Most accidents that do occur when mooring – and there are too many of them – are a result of poor working practices and damaged ropes.
Marine deck equipment - Windlass
Located separately but close to the mooring winches or as an integral part of them, the windlass is the device used for lowering and raising the anchor it does this in tandem with the bow or chain stopper which prevents chain slippage when anchored and when raising the anchor.
The anchor chain passes from the anchor and through the hawse pipe, over the windlass and down into the chain locker where the end is secured. The windlass is driven by a motor, either electric or hydraulic, and features a brake and clutch for use in different operations. Anchoring is usually done in one of two ways; letting go or walking back. Before anchoring the anchor chain must be released from its lashings on the bow stopper.
When letting go, the anchor is lowered in controlled steps to a certain depth before being released under gravity. Limiting the height from which it is dropped is essential to stop the chain over running. Walking back means lowering the anchor with gear. The principle difference between walk back and let go methods is that in walk back power is used. Anchoring safely is a skill and the speed and direction of the ship during the process is crucial.
Once anchoring is completed the stopper must be closed on the chain to prevent movement. The stopper is in effect a rachet arrangement that prevents the chain from paying out when at anchor and acts as a brake when raising it. This prevents strain on the brake and clutch of the windlass which could burn out in an uncontrolled situation.