Cement Carriers - An Integrated Cement Transport Solution

Prior to assess the importance of the various factors related to the operation of the cement carriers, it is worth referring to the special properties of the cement while being transported in bulk. Finished cement is a powder cargo which needs to be sufficiently fine in order to be handled pneumatically. Although it is fairly inert, cement is a demanding cargo, requiring absolutely dry conditions in the holds, which have to be totally enclosed. It also behaves like a liquid in that it can shift if the ship takes a list, so special care needs to be taken to maintain the optimum stability. Dust could be a serious problem, but it is largely prevented due to the sophistication of the cargo handling equipment of the pneumatic cement carriers.

The common advantage of all the self-loading /self-unloading cement-carriers is that their system is totally closed, free for environmental problems, able to dissipate any excess of dust or contaminated air during the loading and unloading operation. To ensure that no cement particles will get into the atmosphere, the air leaving the holds as it is displaced by cargo, is filtered.

Another important figure of each cement carrier is the maximum loading and unloading rate they can achieve. The loading / unloading rates achieved by means of integrated conveying systems can reach the level of 1500mts/hr and 1200mts/hr accordingly at a distance of approximately 300m. This is mainly attributed to the fluidization of cement due to suspension of its particles with the air-stream. Nevertheless the final rates can differ significantly depending on the actual conditions. The loading rate is mainly determined by the capacity of filters which ensure the sacking of air supplied from shore facilities for the acceleration of the air-cement mix and normally would be capable to receive 250-500 cbm air/min. It is also determined by the capacity of air-slides which facilitate the distribution of cargo into the holds. The unloading rate is mainly determined from the type of the cargo handling system installed on the vessel, that is pneumatic or mechanical. Specifically for the pneumatic vessels, the unloading rate depends mostly on the capacity of air compressors which are used to create adequate conditions in the pressure tanks for the escape of cargo towards the silos. Obviously greater velocities can be obtained by use of greater compressors. Similarly, greater compressors can prevent the slow down of unloading operations due to ineffective pipeline design (too many bends and other appendages).

In terms of consumption the mechanical systems are considered more efficient than the pneumatic ones. The pneumatic system requires rather high energy consumption for pneumatic conveying, both onboard the vessel and ashore for further transport to the silos. The onboard consumption normally is 0,9-1,2 Kwh/mts of bulk cement, and ashore same naturally relates to the distance of transportation and normally varies from 1,5-2,0 Kwh/mts, however for high rates and long distances up to 600 m we shall calculate 3,5-4,0 Kwh/mts. The mechanical system normally consumes 0.5 Kwh/mts for the unloading system based on a normal unloading rate of 3-400 Mts/hr, but if raised to 600 mts/hr the effective energy consumption is reduced to 0,4 Kwh/mts, which for larger quantities, may be of interest.

An important consideration with regard to the efficiency of the cement plant system is the optimization of power supply for the satisfaction of energy requirements. Usually the arrangement consists of a number of electrical generators which must be paralleled in order to provide the necessary power. Alternatively, a special provision can be made so as the shaft generator to be possible to substitute one or more generators in which case the system may result in less consumption during the unloading operation.

As already mentioned, the flexibility of the cargo handling system is of outmost importance for both shipowners and charterers as they must always specify the exact possibilities of the shore facilities and the vessel's system prior to fix an agreement in order to ensure efficient combination. The mechanical conveying system requires that the receiving facilities are fully adapted for such system, as unless there is a mechanical receiving and convey system ashore, to carry the cement further then unloading is impossible. On the contrary the pneumatic cement system is more flexible and allows the vessel to call other ports than the dedicated.

Cement Carriers - The World's Fleet
The last fifteen years the self loading / unloading cement carriers' fleet has been coping with a continuous aging of its tonnage worldwide. This is mainly due to the fact that cement was traditionally considered a 'minor bulk' cargo transported mainly with conventional bulk carriers in big bags or sacked.

Diagram 1. Age of cement carriers

The evolution of cement terminals accommodating more sophisticated storage facilities has encouraged the old cement shipowners to invest the recent years in a number of new buildings which have now started to be launched in the market. Such initiative is further supported by the fact that certain first class charterers have introduced an age limitation of 25 years for the use of such cement carriers, causing a heavy demand for the relatively few younger tonnage. To the same direction, the up-swing in Korea-Japan have made the Japanese owners - who used to sell their older Cement Carriers to European buyers - to upgrade their vessels and give them a life-extension of 5 years more, and this has created a lack of older and cheaper tonnage in the rest of the world, which again have created a demand for new buildings and conversions.