Actually you are reading this article means you are interested on Tanker vessel’s operation. Either you have sailed or you are planning to sail on Tanker.
I have seen so many sailors are avoiding Tanker vessel due to it’s dangerous cargo but to me, where is more dangerous cargo there is more safer environment. Actually, most of the accident is due to human error and very rare cases for the natural error.
And professionally all the Tanker man is more vigilance than when they are in Dry ship, so less error at tanker leads less accident too.
You might be thinking why I am discussing about Error, whereas my Topic was about Tanker Cargo Calculations. There are so many blogs regarding this topic and most of the time once you read you understood but when it comes for impose you can’t.
Do you know why? Why you can’t explain / calculate without error when it requires. Is mainly you don’t read or understood WITHOUT ERROR.
In this post I will try to explain it easiest way so that you can understand WITHOUT ERROR.
Cargo Calculations is the most vital operation of a Shipboard cargo Officer.
There is a simple formula to calculate any liquid:
So as to calculate any liquid’s Mass at first we need to know :
- DENSITY (Which varies with Cargo/ Temparature)
Here I will discuss with the volume at first,
There are two methods to get Volume, either by ULLAGING/ SOUNDING.
Ullage/ Sounding just reciprocal to each other. Ullage is the volume which is left empty in a tank so that there is space for the liquid in the tank to expand. The air space between the oil surface and the top of the tank is known as ullage. And sounding is just opposite.
According to the Sounding / Ullage of the Tank you will need to scroll down on your Tank table and then you will be found Observed Volume. Below Picture will clear your vision more:
Here, you need to scroll according to your sounding and then you will get initial steps of Volume, we called it “OBSERVED VOLUME“.
Before being scroll you must have to consider some stability issues like Trim / List. As you have seen upper portion of this picture shows some column TRIM By the head / Even Kill/ Trim by the stern (-1 to -3).
Here, you will get the volume only according to your present Trim (In the shipping practices you can get maximum these values of TRIM, 1 degree by the TO -3 degree by the Stern).
Now, you must be thinking of List, where is the list correction / how to correct? Usually you will get the List Correction on beginning of every Tank Table.
This is a sample picture of list Cor’n in a Tank Table. You have to correct the list according to your list. In here it shows upto 3 degree (either PORT or STBD side) but still if you need much bigger value than you can interpulate with these value.
So, what comes next?
We have got our Initial Sounding/ Ullage then we have got our Corrected ullage / Sounding and with that Corrected Ullage / Sounding we have got our ” Observed Volume”. Now, we need to know is there any correction required for this volume? If so then why?
So, till now what comes to us? below ullage calculation sheet will let you understand well.
Here you see, initially we have got our Sounding/ ullage. Then we corrected for the Trim/ List. And according to the Corrected Ullage/ Sounding we have got our GOV.
Ok, let’s think of the volume. There is several stpes to correct the Volume. This picture will let you understand various steps of volume correction.
First you need to deduct the level of water, so you will get ‘Gross Observed Volume (GOV or G/KL)”
So, now remaining only density to calculate. Sometimes, you will get a DENSITY CHART TABLE (from shipper). Changes of Density for every Temparature changes.
If you have such table then, your job is very simple.
TRIANGLE ASKED : Volume x Density = Mass.
So, you need to interpulate according to your temparature which will give you corrected density And then multiply with Volume = MASS.
BUT BUT BUT here is still some issues which you have to consider that is WCF (Weight Correction Factor).
If initially you have taken the Sounding and your density is in air then no need to do any correction. But if density is in Vacuum then you need to do below Corn for all the cargo. And this is constant for all.
WCF = Density in Vac – 0.0011
And same but opposite coorection goes for the Ullaging too.
So, with all of this information we can go till the bottom line of our cargo calculation as follows:
##BY ASTM TABLES ##
Anyways, If you don’t have such table then you must need to consult with ASTM TABLE (American Society for Testing and Materials, is an international standards organization).
Now let’s think of the GOV to gross standard Volume (GSV)? GSV will be aquired only after correction of volume due to temparature changes. For the product cargo ASTM Table 54B. Today I will explain from Table 54B.
Thank you for your interest on Marine Legends.