Fossil Fuel Supply for India's Coal-fired Power Plants

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Types of Ship and Barge Unloaders

There are essentially two types of unloaders: Viz intermittent unloaders and continuous unloaders. In case of intermittent unloaders, the unloading operation is cyclic involving many elements in the operation. An example of this is the grab bucket type unloader. The working cycle of such an unloader involves lowering of the bucket in to the ship’s hatches, opening and closing of the grab buckets for collection of coal, filling up of buckets, lifting the buckets back from the ship’s hatches and leading the bucket to the receiving hopper for unloading the coal, opening the grab bucket, discharging the material and resetting the bucket for the next operation. Because of the cyclic nature of operations, the throughput gets fixed by the cycle time.

In case of continuous unloaders the number of such operations is far less and therefore the cycle time is less and so the throughput is expected to be higher than intermittent unloaders. Throughput also gets affected by the unloading of coal owing to the shape of the pile. The unloading rate is not constant throughout the height of the pile. Because the amount of material available for unloading is less at the top of the pile, the unloading rate that can be achieved at this location is smaller.

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As unloading operation continues, at the middle portions of the pile more coal would be available, and therefore the rate proportionally increases and peaks in some areas. As the amount of material reduces with progressive unloading from the top of the pile, the unloading rate reduces again and at the end, clean up of material is required to be carried out using dozers. In this way of unloading operations, there is a certain average rate and a peak rate possible. Peak rate is the free digging rate.

The ratio of average rate (also called the through the ship capacity) and the peak capacity varies with the type of unloader as it is governed not only by the pile configuration but also by the nature of operation of the unloader viz cyclic in the case of intermittent unloaders and almost continuous type of operations in case of continuous unloaders. For an intermittent unloader the ratio can be 50 to 60 percent and for a continuous unloader it can be about 75 percent. In case self unloading vessels fitted with belt conveying systems for unloading, this ratio can be as high as 90 percent.

Unloading Rate

The required unloading rate is central to adoption of different systems. Required unloading rate is calculated by the total annual requirement for the plant, number of ship loads, number of barge loads and other logistics, dead time hours or the time not available for unloading at the port or the terminal each time a ship or a barge calls at the port or the terminal, berth occupancy, number of hours of operation in a year leaving about 30 days for overhauling of ships. This rate is the average unloading rate required. There is also another method of fixing the unloading rate.

Based on the actual quantity of coal required per day, chartering rate, berth occupancy, turnaround time etc, and a prefixed daily unloading rate is fixed and the unloading system has to achieve this.

As discussed above, available draft gives a strong guide line for basic selection of the transportation system. The unloading rate, as fixed above gives a very good idea about the quantum and therefore the practicality of adopting ship or ship and barge system. For smaller unloading rates such as up to 3 million tonnes per year ship and barge system could be practical provided draft available in the selected area allows barge movement at least during the high tides.

An example of this are the Trombay ship and barge unloading systems. For higher unloading rates, ship and barge system may not be practical and cost effective. In such cases, it is better to adopt direct unlading of large, medium, or small sized ships as required depending upon the required unloading rate.

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