Special correspondent Heinz-Gert Kessel discusses the merits of tower cranes for use in shipyards.
To cope with quickly changing market situations and changing demand for different types of vessels, shipyards need economical and flexible crane logistics.
The shipbuilding industry is still recovering from the 2008 market downturn and the falling oil price has also had a negative impact. China’s shipbuilders alone reported nearly a 70 % drop in orders during the first seven months of 2015. South Korea, however, fared better, mainly by concentrating on special vessel construction of, for example, offshore rigs, and high-end ships such as liquid natural gas carriers.
With financial backing from governments and the availability of low cost labour, new shipbuilding capacity has been developed in South East Asia. This is adding to the currently huge amount of over- capacity. It will be 2017 before the shipbuilding industry can be expected to pick up again, according to the Castrol Marine Trade Barometer.
Weighed down by delayed offshore projects, South Korea’s big three shipyards, Samsung, Daewoo and Hyundai, are trying to limit the impact by selling all non-core assets and are implementing rigorous restructuring programmes.
Under these circumstances it is necessary to take a new approach. An alternative is needed to the long term investments of custom-designed goliath cranes or large luffing jib cranes for the heavy block construction method. Medium capacity cranes responsible for the bread and butter work in the shipbuilding industry must be evaluated to find the most cost effective crane mix in a shipyard.
To help control investment costs, top end high capacity models of modular series-production tower cranes with sophisticated drive units could be essential. Shipyard owners could gain further flexibility with this type of construction industry crane from the ability to adapt them with custom-built components. When no longer needed the standard cranes could find application in other industries.
Examples of custom components include rail mounted purpose-built portals, different proven foundation types (anchor, travelling carrier, cruciform base) and the add-on potential of rental back-up of complete crane or crane components.
The great flexibility of the tower crane concept could make it a sophisticated alternative to the classic mobile crawler crane. Crawlers need lots of working space and they negatively impact the quay from ground pressure and surface damage caused by the crawler tracks. To evaluate the benefits and limits of modular tower cranes in contrast to level luffing jib shipyard cranes the inherent design characteristics of both types should be considered.
First to be considered are the advantages of the custom built level luffing jib crane (LLC) for the shipbuilding industry. According to Tadashi Ito at crane manufacturer IHI in Japan, the standard LLC has a capacity of 50 to 60 tonnes at 30 to 40 m radius. Tower cranes with 10 to 20 tonnes capacity are only used for small parts assembly work in the Japanese shipbuilding industry. A recent shift in Japan has been to favour large LLCs with more 100 tonnes capacity.
To cover the lifting radius required by two ships lying aside a pier during outfitting work, or to reach the top of high-rise drilling rig structures of offshore vessels, LLCs with long jibs between 90 and 110 m have being realised by several crane manufacturers. On a single jib crane the load is raised and lowered at the same time as the working radius is changed. For the precise lifting work required in the shipbuilding industry a frequent choice is to use a rope drum balanced level luffing mechanism.
It means that the weight of the rope stored between the A-frame and the top of the boom can become so heavy that it generates an uneconomically excessive pull on the boom tip. In addition, a large number of moving parts has to be maintained. Wire rope storage in the form of a triple pulley block leads to significantly greater hoisting rope lengths, which adds to the replacement cost of the ropes.
An alternative could be another level luffing method - the drum balanced type. In this case the hoisting wire rope is connected to the luffing drum. To achieve a horizontal load path the hoist rope is pulling in and out when the luffing drum is rotating. An alternative to this mechanical method of level luffing is a sophisticated electric steering system, as found in modern luffing jib tower cranes used in the construction industry.
In this case, however, energy consumption will rise because the luffing and hoisting winches are both in operation during a luffing movement. In addition, experienced service personnel are required for the complex electrical steering system. The rail mounted undercarriage and the risk of wind from the front when working in steep boom configuration mean that the traditional LLC crane has a large minimum working radius in comparison to the luffing jib tower crane. Having said that they don’t need a complicated, sometimes maintenance-intensive, buffer system at the A-frame to prevent the boom from overturning.
Most LLC will be delivered with a weatherproofed machinery deck or, at least, walk-in electric control cabinets. These are now standard for construction tower cranes. A jib that can be lowered with its tip to the ground for easy inspection and maintenance work is also requested. For low maintenance, tubular sections adapted to the requested height are becoming more and more popular over the traditional block-column design.
Portal and travelling bogies can be especially designed according to the requirements of the individual track for the crane. Also considered should be the accepted wheel loads and the desired gantry height and gauges. In Europe it is often the case that cranes following the rigid design of the totally tailor made LLC, to the FEM A5 standard for heavy duty cycle work, have an extraordinary long service life of 25 years or more.
Next to be considered are the advantages of the modern modular tower crane for the shipbuilding industry. In addition to the well-known medium sized saddle jib tower crane, modern modular design top slewing luffing jib and high capacity saddle jib cranes are an interesting alternative to small and medium sized LLC shipyard cranes. These are basically used for fit out and repair work where loads up to 30 or 40 tonnes have to be moved.
On one hand the fast load cycles typical of construction tower cranes are a benefit. On the other hand speed is the natural enemy of accuracy. New ways of providing precise control for load placement are important for ship yard applications. These include improving the sensitivity of stepless frequency inverter controlled motor drives, for example, Liebherr’s Micromove.
Shipyards traditionally tend to use luffing jib cranes because they can work close together. Even tandem lifting operations are possible and they can be luffed away to overslew tall vessels and other obstacles. A common feature, in addition to the main hoist, is a runner winch for light service work. The growing number of large luffing jib construction tower crane models increases the choice for the shipbuilding industry.
In terms of energy consumption, trolley speed, manufacturing costs and availability from rental crane fleets, the series-system saddle jib type tower crane is still unsurpassed. To combine the benefits of saddle jib and luffing jib cranes, designer Franc Jost developed the JLT series of articulated trolley jib shipyard tower crane where the front part of the boom can be raised. In this way, overall tower height can be reduced and, with that, the wind attack area, which means less corner pressure on the quayside and a shorter pendulum length for the hoisting rope.
As Mariano Echavarri from Comansa explains, saddle jib cranes can be even less wind sensitive, as demonstrated on a job site in the USA where a flat top crane can be still operated where the wind speed is 45 mph (73 km/h) while a luffer in a shipyard has to stop work at 25 mph (41 km/h).
If requested the tower crane upper can be based on a custom-built closed tube tower or even the shipyard can manufacture its own tower and portal, to reduce initial costs. Individual design and fabrication of a closed tubular tower system is expensive when carried out by the crane manufacturer. In addition, the standard tower system is already certified to be used in connection with the upper crane. This saves delivery time and money for the client.
A modular tower system allows the tower height to be changed easily according to new requirements at the shipyard. Krøll has increased the hook height of several big, 60 tonne, capacity tower cranes at DSME some 20 years after delivery, simply by providing additional tower sections. Jorn Lorentzen adds that according to Krøll’s experience the lattice crane tower is much lighter and can be split into pieces for easy erection and transportation in contrast to a tubular tower which also requires an expensive plate-type structural portal.
On the other hand, a crane tower made up of lattice tower sections needs more maintenance than a tubular tower of fixed length. That applies not only to the anti-corrosion painting of the structure but also monitoring the connection devices, regardless of whether slug bolts or screws are used. For heavy duty cycle work, for example, 2 million hoist cycles, then fatigue issues rule out the use of lattice structures.
A case study by Wolffkran, however, found that in most yards cranes were only in operation for about 70 % of the working year. Hence, it should be sufficient for most shipyard work to apply for FEM A3 tower crane standard and thereby avoid capacity derating of standard tower cranes. When applied as a rail mounted unit, tower cranes in the shipyard industry should have rigid and high-powered travel bogies. This is because, in contrast to the construction industry, shipyard cranes are intensively used to travel with their loads over hundreds of metres.
Shipyard tower cranes have custom-designed cabins and access platforms for optimum safety during maintenance work. A permanent operator lift integrated into the tower should also be included. A standard construction tower crane may also have to be specially painted with a marine finish to cope with the humidity and salty atmosphere. In this respect Krøll’s standard corrosion protection uses zinc and aluminium primers. Most tower cranes use electric power. If necessary diesel power packs can be added, especially when the crane is shifted around the yard to different job sites or in cases where the supply of enough electricity may be a problem.
While LLC shipyard cranes generally only use an individual rail track, tower cranes are highly versatile in their temporary installation. In addition to a rail mounted undercarriage, they can be erected on foundation plates welded directly to the vessel deck for temporary use on a cross frame foundation during fitting out work. They can also be based on a concrete foundation. In this case, thanks to the very small out of service radius, a wide working area can be served outside the footprint of the crane.
When a traditional LLC in a shipyard is found to be in the wrong place, expensive relocation work becomes necessary and it is normally ruled out for that reason. To move a tower crane as a single unit, it can be prepared with special lugs on the tower head or the slewing ring section. They allow the crane to be shifted by a floating crane or a goliath crane.
A method designed in South East Asia is to use a 1.50 m low portal under which a self propelled modular transporter unit can be moved in to shift the fully rigged crane around on the shipyard. Wilbert has improved this idea. Günter Kronewitter, Wilbert spokesperson, uses the luffing jib WT 2405L e.tronic as an example. Capacity is 32 tonnes at 51 m outreach in two fall operation mode and it still lifts 16 tonnes at 78 m radius. It can be used as a stationary tower crane while the ship yard traffic can pass under the standard Wilbert portal which has pylons instead of conventional travelling bogies.
In contrast to a standard LLC shipyard crane the original climbing tower crane design allows flexibility in the tower height and, at just 10 m, a very small minimum working radius. It is unnecessary to compensate for a large minimum radius by travelling during working. In addition, thanks to the moving Wilbert ballast system, corner pressures are reduced.
According to Claus Hartmann from Lion Cranes, at the proposal stage the idea of releasing a new tower crane from an existing, sometimes inadequate, rail track system could have great potential to cut initial shipyard crane installation costs and increase versatility. The same crane, for example, could be used for prefabrication and yard work at a remote area of the shipyard while it might also be transferred, when rigged, to the quayside for use there in a prepared location as stationary crane.
Shipyards are increasingly faced with a wide range of short-term projects. A versatile and modular crane design will become more attractive to a rental market. Delivery times are long for custom-designed LLC cranes and the second hand market is limited. A modular tower crane requiring only limited adaptation to the special project requirements for ship yard applications could be a sound alternative.