Originally Posted by RomainM

Dear All,

There is a question popping regularly, but to be sure everyone get what i'm talking about, I'll take a practical example:

I'd like to unload dry cargo (e.g. ore, coal...) from a ship by grab, the laytime that I provide specifies that my equipment shall discharge 10,000 t/day, because this how it is designed. Now, here is the question, what happen when I reach the end of a hold? I am of course obliged to get a wheel loader aboard to gather the material in order to help the grab. I reckon this impact considerably my discharge rate, but I'd like to know if there is a common approximation of the time lost during this phase depending on ship size or a percentage I can apply to my nominal rate.[/INDENT]

So I hope someone could help, it will be much appreciated

Thanks in advance.

Kind regards.

Romain

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As many of my friends will tell you "it depends".

It is simply a case of preparation and more preparation for the end

result to avoid paying the penalty of delaying the bulker from exiting

the port to the breakwater or the 12 mile limit with the harbor pilot.

Time is money: especially with regard to bulk cargoes of low monetary

value requiring the cargo carrier to make more trips per year to make money.

Ideally you will have a vacuum system that can pull the ore out of the hold providing

it is "dry" when you are nearing the bow or the stern.

A large enough vacuum system will be able to move huge amounts of material

from the hold wet or dry.

A remotely operated vacuum system with suction pipes lowered by a small crane

or suction truck can easily work in the hold be it bow or stern without interfering

with a grab or grabs in use.

A suction truck will be limited by its volume capacity which will fill quickly.

A vacuum system operating through a series of cyclones dropping the ore into a

pile or a takeaway conveyor will only be limited by its power as you need massive amounts of

air flow in suction service to create the pressure gradient against the resistance created by

the mass of the ore.

Its all going to depend on your pocketbook as the system can be designed to do the job providing

and I mean providing the ore is uniform as this will affect the volume of ore moved and any blockage will

delay your progress. ■

Teus T produced a formula which gave the day rate. I am not sure how he came by the formula. It is linear and therefore does not appear to account for standing wall demolition, coaming obstruction and clean up. However it gives far more realistic times than most methods used. Ship unloading is the most interesting aspect of bulk handling and practitioners range from those who insist that the time-scale is simply a matter of dividing the cargo capacity by the specified unloading rate to those who propose that the ship be moved underneath the unloader when the berth is too short: and I don't suppose we've seen it all yet!

This situation is largely the fruit of naval architectural inertia. VLCC's have only the degassing situation to contend with and this often starts well before the ship has berthed so that after emptying the crude the developing ullage is effectively already purged. No such luxury exist with solids handling. In my understanding bulkers are not even degassed in port: let alone deep water.

Grabs are the best way to unload ships, always have been and, until there is a quantum leap in naval architecture, will remain so. Screw unloaders snap when the going gets rough and suction unloaders are notorious for rotary valve failures. Others exist and some even get used beyond Disneyland. If there is only a single machine available then the user deserves all the downtime he gets.

Returning to the theme: the cheapest way to shorten unloading time, talking Panamax, is to use 3 hatch covers instead of the absurd 2 that are used at present. That would enable complete coverage of the holds floors and reduce clean up time to an absolute minimum. There was an illustration posted on these forums a few months back but the principle was beyond most folks understanding. If you talk Handymax then you can discard the laytime since the on-board cranes are the ships' property. ■

Dear Romain, John,

The average achieved unloading rate per day is related to the demurrage costs of the vessel.

As these demurrage costs increase with the dwt size, it is obvious that bigger ships require higher unloading rates.

This leads to a formula, where the average daily unloading rate is a function of the vessel size.

To estimate the design capacity of the unloader(s) (often referred to as the free digging rate), an “unloading efficiency” number is used.

Design capacity = average capacity / unloading efficiency

In these estimates, there are a number of factors involved:

-The type of the commodity (cement, ore grain, containers, etc)

-Ship type (self-trimming holds, box type holds)

-Unloader type. (vacuum unloader, screw unloader, grab unloader)

These unloader types have their own specific properties is free digging capabilities and clean-up performances, which are also dependent of the commodity.

Also the type of auxiliary clean-up equipment depends on the unloader type and commodity.

Here the “unloading efficiency” number is applied, which is a number that is based on experienced unloading logs.

-Import- or export terminal (Export terminals have normally a higher loading rate than import terminals have an unloading rate)

The formula, John refers to is meant for cement:

-Average unloading rate = 120 * dwt^0.37 (export terminal)

-Average unloading rate = 180 * dwt^0.37 (import terminal)

The formula is just a regression line through the usual demurrage times for the various sized ships.

If extra procedures have to be followed, s.a. degassing, should be accounted for in the charter.

Have a nice day. ■

Firstly, many thanks to Teus for insight into the derivation of the formulae. Cement, alumina or similar materials all worked well for me when using the formula for import terminals.

Romain, if you want to derive your own formula tailored to your particular product I suggest you read through the various port newspapers. I remember when I was in Durban the Mercury always gave the arrival and departure times of all shipping. So if you know what the vessel is carrying you have what you need in the press archives. I never thought of it before because when I was in Durban in the early 1970's I was not yet engaged in bulk handling at all. Probably Richards Bay newspapers would be more accurate these days. Get the on-line archives of some multi cargo ports, anywhere, and you will get a pretty accurate picture.

Thanks again Teus! ■