Cement is described as a “minor bulk” cargo, but is nevertheless important to the construction industry. Major projects, like the construction of the new Hong Kong airport, or some of the vast new port projects in the Chinese coastal provinces have a vast appetite for cement, which is sourced internationally. Traditionally transported in bags, finished cement is today carried largely in bulk, a powder cargo sufficiently fine to be handled pneumatically, moving through large diameter pipes on a column of air.
Modern cement carriers are specialised ships that will carry no other cargo, and while classified as dry bulk carriers, are fully enclosed vessels which more resemble tankers. They are distinctive craft, with complex cargo handling equipment in the pneumatic plant, an extensive system of conveyors and often a high tower on the foredeck Cement carriers tend to be divided into deep sea vessels, which will carry cement on long international voyages and short sea traders, which effectively form a distributive network, feeding cargo from deep sea terminals to ports close to where the cargo is required.
Deep sea vessels tend to around 20,000 tons, a size which is dictated both by the loading ports and the storage facilities ashore in discharge terminals. The cargo, while fairly inert, is a demanding one, requiring absolutely dry conditions in the holds, which are totally enclosed. It also behaves like a liquid in that it can shift if the ship takes a list, so much care is taken to maintain the optimum stability. Dust could be a serious problem, which encourages the most clinical handling to prevent the cement ever getting into the air. Air leaving the holds as it is displaced by cargo, is filtered and re-circulated.
The cargo holds are hopper bottomed, which helps the powder flow into the bottom of each, the ship’s powerful pneumatic system being used to load and discharge, by changing the direction of the airflow. Mechanical systems are also used to distribute the cargo within the holds to ensure that they are filled, and mechanical conveyors offer an alternative handling system to the pneumatic blowers, to suit the wishes of customers. Nobody has to touch a bulk cargo of cement!
Compared to old cement ships, which were basically bulk carriers with bags handled manually in a laborious operation which could take days, the modern cement carriers are models of clean, fast and damage-free cargo handling. Some proprietary systems will load a ship at 1,500 tonnes per hour and discharge at a slightly slower rate. Best of all, such ships are virtually self contained and are able to handle cargo in all weathers, where a ship loading cement in bags would have to shut down at the slightest sign of rain. A good cargo outturn, with fresh, clean and undamaged cement can be expected from these useful, sophisticated members of the bulk carrier family, in their specialised trade.
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