Self-Discharging Cement Ships
Self-discharging cement ships allow independent operation because they do not require land-based ship unloading equipment. The closed systems of self-discharging cement ships offer significant advantages in the transport of cement, granulated blast furnace slag and fly ash:
- Clean loading and unloading, as the emission of cement dust is avoided
- No contamination or agglomeration of the load due to water ingress
- Weather-independent loading and unloading through closed transport systems
- Better planning and schedule reliability for delivery
- Fully automatic equipment for high loading and unloading rates
As is often the case, the devil is in the details when it comes to choosing the right unloading system.
The world fleet of self-discharging cement ships comprises more than 300 vessels ranging in size from 1,000 dwt (dead weight tons) to 30,000 dwt for sea transport and domestic cement supply via rivers, with the majority (approx. 80%) in the range above 4,000 dwt.
15,000 dwt self-discharging cement ship “Goliath”, operated by CSL Australia, in the port of Melbourne
When choosing the unloading system, the lowest possible investment costs are not the only objective. Throughput rates, energy requirements, flexibility, reliability and the remaining transport volume or weight of the unloading system also play a decisive role. Different unloading systems are available with some common features. All systems require a sloping ship's bottom, which is equipped with aeration elements to guide the material to a lowest point created by the slope. Apart from a mechanical system that can convey directly into trucks or conveyor belts on the quay, all self-discharging cement ships use a pneumatic conveyor to transport the material to the storage silo on land. The differences between the individual systems essentially consist in how the material is picked up from the lowest point in the hold and brought to the pneumatic conveyor system. This can be a suction system or vertical screw conveyors installed in the cargo hold. Another possibility is to dispense with lifting the material and place screw pumps or pressure vessels directly at that lowest point.
The choice of the supposedly best unloading system depends on many factors. For example, it is important to know whether the self-discharging cement ship should serve existing terminals. This would then mean that there are already existing conveying pipelines. This circumstance is one of the decisive factors when it comes to choosing the right system. The diameter of the conveying pipelines, but of course also the routing and distance to the storage silo installation have a significant influence on the conveying. If the terminal is also newly planned, the diameter of the conveying lines can be adapted to a pneumatic conveying system. Further decision criteria are, for example, the size of the ship and whether it is a new building or a conversion.
All unloading systems naturally have the disadvantage that they require space that cannot be used for cargo. This means that the less volume is used for the unloading equipment the better. But the centre of gravity also plays a role here. This means that simply placing the entire unloading system on the deck of a ship can be detrimental to stability. The additional weight of the unloading system also reduces the possible payload.
Due to the concentration of cement production capacities and the growing demand for cement terminals, which have to be located close to urban centres with high demand in order to keep logistics costs low, the sea transport of cement for the construction industry is constantly growing. Self-discharging cement ships are seen as a cost-effective and environmentally friendly method of cement distribution. However, there are major differences in the technology and design of self-discharging cement carriers. An independent consultant can provide assistance in this respect, in order to select the right system, taking into account all framework conditions and all requirements.