Digging deeper will not appreciably reduce the rate for either method. A grab will free fall the extra drop very quickly & the crane hoisting speed will not feel the extra time. Slewing time is greater than the hoisting period.

For continuos unloading there should be no drop off in throughput. Yet.

When you reach the hold floor the fun starts, & the 'Bobcat Syndrome' kicks in. Unloading rate then equates to the capacity of the skid loader fleet. Regardless of what you paid for a 1.2MW suction unloader, or similar, it has to wait for the bobcats to shovel the muck to where it can reach. This sad condition is brought about by the naval architects' failure to recognise the operational differences between bulkers & tankers.

If the sidewalls of the holds were profiled to direct material under the coamings into the working envelope of the digging device there would be no need to subject personnel to the hazards imposed by trying to dodge a big chunk of iron while driving a shovel around in a dark & dusty satanic workplace.

Perhaps naval architects have got their act together since I last peered into a ships hold, but I haven't heard about it.

For your calculations work out the 'live' cargo, divide it by 2000tph, decide how many bobcats you use, work out their capacity based on shovel size & average journey. Divide the 'dead' cargo by the total bobcat rate & add that time to the 'live' time. Nowt to it! Unless it rains through your open hatches & all cargo is 'dead'. ■

Hello Chris Kelly,

Mr. Johngately has already given good information and suggestions on the subject issue. I would like to add the following information.

In case of ship unloader there are three types of capacities for practical needs:

- Cream unloading rate. This rate arises by unloading the top portion of the material, when sea level is high resulting into least cycle time i.e. least travel path of grab bucket. This capacity is needed to size the conveying system onward of ship unloader. Such onward conveying system will include machines like stacker etc. as applicable.

- Free digging rate is defined as the rate, which would occur when the ship is at mean sea low level and the unloading is at the centre of material (vertically as well as horizontally).

- The average unloading rate is the average overall unloading rate for the ship. This unloading rate is of particular interest for the port authorities as well as the people concerned with the unloading. This rate will provide the information about the total berthing time for the ship and also the time period for which the unloading system would be operating.

The free digging rate is to be calculated by calculation of individual time in various movement / operations of the grab bucket. The free digging rate is somewhat theoretical in nature considering specific position for the ship and reclaiming of material in specific zone. However, this provides the basis for comparison of unloading rates by the machines. The free digging rate can best be calculated by the manufacturer of machine who has data about travel speeds and time interval in various operations. It will also depend upon degree of automation for the machine. It also depends upon ship size and its hatch arrangement. You can also calculate the free unloading rate if you have the necessary information about the machine. This demands information about the machine in depth and various time intervals resulting due to automation in machine. Regarding reclaiming for the residual part of the material, it is to be calculated as per the system / equipment being used etc. in hatch.

The experience and data accumulated during course of time indicates that the average unloading rate is approximately 55% of free digging rate, when the ship size is up to 50000 tonne dead weight. This rate can be nearly 45% if the ship size is larger than the aforesaid size. This information is as per a German publication. I do not have the specific information about the difference between the cream unloading rate and free digging rate. Possibly, the cream unloading rate could be around 15 to 20% more than the free digging rate.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

hi all

I just stumbled onto this forum whilst researching for an assignment, i am studying cargo handling systems management and was wondering if anyone could provide some rough guides for the time taken to unload grain using the ships gear.

The assignment involves a ship under time charter that will be unloading grain to various Pacific Islands. The port facilities are rudimentary - pretty much a dock with road access, so the ship will need to have the gear onboard to unload.

I was thinking of using grabs and unloading into hoppers to fill the trucks, the assignment asks for approximate discharge rates to be given to allow planners to estimate port times but I am having trouble finding this out..

can anyone provide me with some help please?

Cheers

Jason ■

This is a golden oldie; not me; the thread.

You are inadvertently looking for a self unloading ship, on the cheap. A highly commendable quest. "Once more unto the beach!" as Shakespeare really meant to write.

To nail you down a bit let us assume that 2 out of 5 islands do not have a truck loading hopper available. Not unreasonable since they are considerable fabrications with a headroom clearance of up to 4.5 m. An on board hopper is necessary. Different island's hoppers would also be a difficult target for discharge with ships cranes & the spillage would be to the islanders account once it has cleard the ships rail. (FOR)

If the vessel had enough freeboard you could construct a hopper to attach over the deck coming, between the hatch & rail. This would feed grain down to a truck, via a chute, on the quay while the ships crane only had to supply the hopper. Your slewing distance would be just about halved & spillage would be the responsibility of the ships master.

Your freeboard might be inadequate for that solution since a time chartered vessel that can dock in a harbour with no cranes is hardly likely to tower over the shoreline. In such a likely event you could consider fitting a luffing conveyor under the onboard hopper to outreach into the trucks. A simple luffing winch could be mounted on the hopper wall.

Your outloading rate is then quite simply governed by the size of the grab. You are not going to use a 30 tonne grab because the ship cannot lift it & even if it could the truck could not accept the load. Let us forget the vessel unloading rate for a minute & imagine the islanders having to clean up dockside spillage of chargeble grain before they can verify, by truck weighing, for accepting the bill of lading. There is your realistic port time; plus tides, of course, to clear the reef.

You could work on a figure of about 120 tph per outloading point for stevedoring rates.

BUT this would only be relevant to the unloading. Remaing cargo will have to be trimmed before the ship casts off. This is in the interests of safety of the vessel & crew & also to enable grab access at the next port of call.

As you are probably realising, the phyical unloading rate is not as important as it first appeared, especially if you are the last port of call & do not have to laboriously trim the holds for your neighbours. A fact of island life. Lets have some more input though, from other members because you never know what can turn up. ■