By Jim Shaw
shaw11055@comcast.net
October 2011
Old style cargo handling often incorporated a swinging center-mounted derrick, rigged to port and starboard deck stanchions, to handle heavy lumber cargos which were shifted into the wings by forklift, a process that damaged the timber and eventually lead to the development of the open-hatch or box-hold bulk carrier. Photo courtesy of the J. L. Shaw Collection.
Cargo handling by merchant ships has been an evolutionary process that has seen the most change take place in only the last few decades. Old-timers on the docks still remember when most breakbulk cargo was handled by winches and derricks arranged in “union purchase” configuration. This would see two derricks rigged in such a manner that the span between their ends would be great enough to allow cargo to be handled from a ship’s holds to the dock by using cable joined between the winches. If cargo was being handled to the dock, plus a barge or lighter moored alongside, then a “Butterfly” rig might be adopted, with derricks rigged out over each side of the ship. For heavier loads a single mast-mounted or deck-mounted pivoting derrick might be used, with cable and tackle running from twin masts or deck stanchions.
In time, the rotating crane was modified for shipboard use and these units have come to replace masts and derricks on most modern cargo ships. Their main advantage is that rigging time is negligible and operation is quicker, but their greater weight also reduces a vessel’s deadweight capacity and maintenance is more involved. Being hydraulically actuated they also have a potential for fluid leakage, a no-no in today’s operating environment. This is leading to one more small revolution in cargo handling, the shift to all-electric cranes, which is also taking place in hatch cover and ro/ro ramp design.
Electric Cranes
Electrically operated deck cranes have started to enter the market as replacements for hydraulic units, principally to avoid oil contamination but also because they are easier to maintain. Finland’s Cargotec, known for its Hiab, Kalmar and MacGregor brands, has been developing a series of electric-drive cranes that can provide equal or better performance to hydraulic units at the same cost. The company’s new variable frequency drive (VFD) cranes have hoisting capacities up to 100 tons and an outreach of more than 40 meters.
In the Cargotec design all machinery is enclosed within the crane housing, thus ensuring safer operation and ease of maintenance. The main advantages of the electric cranes are that the potential for hydraulic oil leakage is eliminated and their drives are easier to maintain.
Since being placed on the market Cargotec has received orders from Grieg Shipping for 24 of the new units, all with a safe working load (SWL) of 75 tons, for installation on six open-hatch bulk carriers it is having built in China. At the same time, Ethiopian Shipping Lines has ordered 21 of the cranes for a series of seven 28,000-dwt multipurpose ships it will be taking delivery of in 2012/2013. These cranes will be mounted on each ship in sets of three with a capacity range of 60-tons, 80-tons and 100-tons.
Electric Ro/Ro Ramps
More use of electrically-driven machinery is also being seen in the roll-on/roll-off sector where Japan’s Shin Kurushima Dockyard Co Ltd is completing a 4,000-vehicle-unit pure car truck carrier (PCTC) for Japanese owner Mitsui O.S.K. Line. The vessel is being equipped with a stern quarter ramp, a side ramp and six movable interior ramps, all of which will be operated and secured by electric winches and actuators.
One of the main technical reasons allowing this development is the availability of improved electrically-driven screwjack drives that can replace hydraulic cylinders for operating cleating and locking devices. This same machinery is being used on a new series of Con/Ro vessels being built by South Korea’s Hyundai Mipo shipyard for French owner CMA CGM. In this case the liftable car decks, measuring about 5,200 square meters per vessel, will all be electrically operated with an electric jigger winch installed in the ship’s car deck panels.
Electric Hatch Covers
As with cranes and ramps, most hatch covers on modern ships make use of hydraulic rams to control opening and closing, the old moveable beams, timbers and battened-down canvas tarps having long since been shed. After years of development, Cargotec’s MacGregor subsidiary has recently developed an electrically operated system for large bulk carriers that uses side-rolling hatch covers.
The combined rack-and-pinion drive and lifter in the new “MacRack” system employs just one electric motor per hatch cover panel to both raise the panel and move it sideways to open or close the hatch, thus making separate hydraulic lifters obsolete. To accomplish the lifting move a lever mechanism converts the electric motor’s rotating motion into a vertical movement during the opening of the covers, providing the upward force needed to lift the panels. When closing, the mechanism lowers the covers and pushes them together to achieve the correct amount of rubber gasket compression and waterproof tightness.
Like the electrically operated cranes and ramps, the new electrically operated hatch covers provide a more environmentally-friendly ship.
Tuesday, October 25, 2011
