The throughput of a ship unloader is still a much-discussed topic. The main reasons for this are the different views and definitions of manufacturers and terminal operators.

For the manufacturer of the ship unloader the throughput capacity is important for which he has to design the conveyor system. For a mechanical ship unloader, this is the peak throughput that the ship unloader achieves under optimal conditions and is important because the conveyor system behind the ship unloader must be able to handle this peak throughput.

The terminal operator has a different view on the unloading throughput. What counts for him is the time between the arrival of the full ship and the departure of the empty ship. This is the time he has to pay for. If a ship with a load of 50,000 tons of material spends 100 hours in the port, the operator of the unloader will set the throughput capacity of the machine at 500 t/h. If he has bought an unloader with a throughput capacity of 800 t/h without really understanding what this throughput capacity means, he will be very disappointed. On the other hand, the manufacturer of the unloader is often frustrated that the owner of the machine does not create the right conditions for the unloader to achieve the throughput capacity for which it is designed.

The owner of a ship unloader usually understands quite well that if he buys a car that can drive at 200 km per hour, this does not mean that he can drive from city centre A to city centre B at that speed. Traffic lights, speed limits, driver skill and possible traffic jams all play a role in the time it takes to get from A to B and even then he needs to know how to drive, otherwise he will get lost. The owner of the ship unloader must be aware that very similar factors apply to ship unloading. The maximum efficiency of the ship unloader is only one of many factors that determine the time needed to unload the ship.

It is also important that the manufacturer of the unloader is aware that the success of his machine is not only measured by its maximum throughput capacity, but also by its ability to unload a ship in the shortest possible time. This is not only a question of the right design, but also of customer support.

Both the owner and the manufacturer of the ship unloader must be aware of what is involved in a ship unloading operation and who is responsible for what. Only then can they agree on a definition of throughput performance, how it should be tested and what to do if the unloader does not perform as agreed.

Factors to be taken into account during a ship unloading operation

When ordering a bulk carrier (see pict. 1)  from a terminal operator, the charter contract with the shipping company describes the size and characteristics of the vessel. These should correspond to the capabilities of the ship unloader. This agreement also specifies the previous cargo of the vessel and the degree of cleanliness before loading the vessel. It should also contain a definition of how clean the holds must be after unloading.

Pict. 1.: Handysize bulk carrier of approx. 45.000 dwt

 

During the voyage, the owner of the terminal already contacts the master of the ship and agrees a discharge plan. This is the order in which the ship's holds must be emptied, it is agreed with all parties involved. A berthing plan must be drawn up, which determines the position of the ship along the pier ( see pict. 2).

Pict. 2.: Bulk carrier berthing at the pier

As a rule, the shipping company agrees on the berthing / discharging activities with the port. Once the vessel has moored, customs formalities must be completed as soon as possible, and the draught marks ( see pict. 3) of the vessel must be approved by a certified surveyor.  Sometimes quality samples of the material must be taken from each hold.

Pict. 3.: Draught marks at the side of a ship

At least half a day before the actual arrival of the ship, the unloader should be positioned on the pier at the place where the hold that will be unloaded first will be. A test run must then be carried out to check that the ship unloader and the conveyor system to the silo are functioning properly.

As soon as the ship arrives, a logbook of the unloading process must be kept, which should contain all start and stop times of the unloading process and other relevant information. The start is defined by the moment when the inlet feeder of the unloader is immersed in the material. Stopping is defined by the moment when the feeder is lifted substantially above the material or when it stops operating.

When the unloader has started unloading the first load compartment, it is possible to unload at the nominal throughput rate for several hours.  The only interruptions can be caused by rain, mechanical failures of the unloader or the conveyor system to the silo or the silo itself. These must be properly documented in the logbook. During the entire unloading process, the unloading efficiency is very much determined by the experience and skills of the operator (pict. 4 Inlet feeder in material). Sometimes the unloader can only operate at reduced throughput due to lumps or foreign objects in the material. This must be documented and immediately reported to the terminal operator and the shipping company. Under such conditions, optimum unloading performance is not possible.

Pict. 4.: Inlet feeder of a pneumatic Unloader inside the material

When large bulk carriers are unloaded, it will generally not be possible to unload a cargo hold from top to bottom, as this would cause unacceptable stresses in the hull. As a rule, the upper halves of all cargo holds must be unloaded first, followed by the lower halves. Therefore, the ship unloader has to be moved from hold to hold several times during the unloading process, which takes some time. During or before moving the unloader, the hatch covers of the new cargo hold must also be opened.

If this standard procedure is to be deviated from and the ship is still able to allow a complete emptying of a cargo hold via its ballast system, this must be taken into account when designing the unloader as there is a risk that outrigger arms are too short and do not reach the bottom of the hold.

When the lower half of a cargo hold has been emptied, there comes a phase in which the inlet feeder can no longer pick up the material at high throughput rates. This is the moment when the residual emptying begins, when one or more front loaders and their operators are required. These should be available at the pier. Since the cleaning crew often works in shifts, it is important that the time of residual emptying coincides with the start of a shift. This can be achieved by first unloading one corner of a hold and then continuing with the rest of the hold. The front loader can be lifted into the hold by means of the ship unloader (interrupting the unloading process) or by means of the ship cranes.

The extent of the residual emptying is determined by the amount of material in the hold that cannot be directly reached by the inlet feeder of the unloader, in combination with the size of the front loader. The entire clean-up operation must be planned and organised in detail. Picture 5 shows a front loader during final resting of material. When the front loader can no longer effectively reach material, the loading space is considered to be clean. Ideally, the ship unloader is then moved to the next hold. The hold is then still not completely empty. A sweeping team must clean the hold of material adhering to the sides and frame.  This is a slow and labour-intensive job. It should never be part of an unloading guarantee.

Pict. 5.: Front loader inside the cargo hold

When all holds are clean, the ship is considered empty as far as the unloading process is concerned. The last remnants of material are swept into a container and lifted from the ship. The final draught marks are checked by the certified surveyor. The amount of material unloaded is determined and documented. The ship is then ready for departure.

After the unloading process, the ship unloader is brought into a parking position, completely cleaned and the necessary maintenance work must then be carried out.

With the help of the unloading logbook an evaluation must be made. This evaluation serves as a basis to determine the achieved throughput rates of the ship unloader and to search for methods to improve and optimize the overall unloading efficiency.

The unloading process is thus a combination of the performance of the equipment, the skills and experience of the operating personnel and the organisation. It is useful for a ship unloader manufacturer to support a new owner to the best of his ability and to ensure that he is informed about all aspects of the unloading process.

Ultimately, however, it is the purchaser of the ship unloader who is responsible for the organisation and management of the unloading process. This is a skill that must be learned. In general, the learning phase for an efficient ship unloading operation takes 6 months to a year.

Definitions of ship unloading throughput

There is no uniform definition of ship unloading throughput that is valid throughout the industry. On the contrary, apart from the different points of view of terminal operators and manufacturers, there are also different definitions from the different manufacturers themselves.

Offers should already include a definition of throughput capacity. Manufacturers of mechanical ship unloaders often use the "rated throughput capacity" in their offers, manufacturers of pneumatic unloaders use "maximum throughput capacity". The rated throughput capacity and maximum throughput capacity are comparable.

Contracts should include a precise definition of throughput capacity, which should be agreed between the two parties. In most cases this is the throughput capacity definition given in the quotation, but sometimes customers require a guarantee for the average throughput capacity.

The most used throughput performance definitions are the following:

Peak Throughput Performance
Peak throughput is the maximum possible throughput of a mechanical ship unloader. This peak throughput capacity can be achieved if the inlet feeder in the upper part of the hold is clearly immersed in the material, the vertical arm is in (almost) vertical position and the horizontal arm is in horizontal position or higher. Peak throughput is important because it is directly related to the conveyor equipment behind the unloader. The peak throughput is irrelevant in relation to the general performance of the unloader, as it is only temporary and is highly dependent on operator experience and material properties. The use of peak throughput as a selling point is misleading. Testing it is impractical. The maximum possible throughput is, among other things, the result of the design of the conveying system, which is determined in accordance with the applicable standards for calculating the performance of screw conveyors and experimentally determined factors for vertical conveying.

Rated throughput capacity
The rated throughput capacity is the throughput that an unloader can maintain for several hours under good conditions. This is not a constant throughput capacity, but an average, as the throughput capacity changes according to the positions of the unloading arm and depends on the operator's skills. The nominal throughput is very suitable for performance guarantees as it can be easily defined, and the circumstances of the test can be well controlled by the manufacturer of the unloader. Nominal throughput performance tests should always be carried out during unloading of the upper half of the material in a cargo hold, normally starting in a fresh cargo hold. The length of the test will depend on the measurement methods available and the accuracy required. No interruptions should be allowed during the test.

A guarantee for the nominal flow rate is normally based on experience with similar devices under similar conditions. It should include a degree of certainty. The rated flow rate is not a calculated value. However, experience has shown that, on average, approximately 80% of the peak flow rate can be taken as the rated flow rate.

Throughput capacity over the entire shipload
The throughput over the entire ship's cargo is the weight of the cargo unloaded from a ship divided by the time required to unload this cargo. The cargo is considered unloaded when all holds are machine clean. The unloading time includes cleaning with front loaders, the lifting of front loaders into and out of the cargo holds, the movement time of the unloader between cargo holds and mechanical failures of the equipment supplied by the manufacturer. Excluded from the unloading time are all other downtimes.

This average performance is directly related to the efficiency of a ship unloader.

The efficiency is the ratio between the throughput capacity over the entire shipload and the rated throughput capacity and is between 60 and 70 % depending on the material, size of the ship, qualification of the operators, etc.