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 the Tanker vessel due to its dangerous cargo but to me, where more dangerous cargo there is more safer environment. Actually, most of the accident is due to human error and very rare cases for 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 to 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 for 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/ Temperature)
FIRST STEP – Volume
There are two methods to get Volume, either by ULLAGING/ SOUNDING.
Ullage/ Sounding just reciprocal to each other. Ullage is the volume that 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 find 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 then Head 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 the 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 up to 3 degree (either PORT or STBD side) but still if you need much bigger value than you can interpolate with these values.
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”.
So, till now what comes to us?
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 are several steps 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)”
LAST STEP – DENSITY
So, now remaining only density to calculate. Sometimes, you will get a DENSITY CHART TABLE (from the shipper). Changes of Density for every Temperature changes.
If you have such table then, your job is very simple.
TRIANGLE ASKED : Volume x Density = Mass.
So, you need to interpolate according to your temperature which will give you corrected density And then multiply with Volume = Weight (MT).
##BY ASTM TABLES ##
Anyways, If you don’t have any Density table then you must need to consult with ASTM TABLE (American Society for Testing and Materials, is an international standards organization).
STEP-1 – get GSV from GOV:
Now let’s think how to get GOV to gross standard Volume (GSV)?
Remember, GSV is acquired only after TEMPERATURE CORRECTION. Internationally, cargo density is calculated at 15C.
Now, as you heat up liquid cargo it’s volume will increases. So when cargo temperature changes, volume also will be changed. This changed Volume known as Gross Standard Volume (GSV). Means, Voulme after temperature correction.
Now, this correction factor prior to temperature changes varies for different types of cargo. So, ASTM Published different Table for different types of cargo.
For PRODUCT OIL CARGO – Table 54B & For Crude Oil Cargo – Table 6B.
Now I will discuss for the product oil cargo below:
For example, the shipper gives you Density (d) = 0.8650 (In Vac). And your observed temperature 29C.
Now, you need to scroll on the ASTM Table 54B. Nowadays various software is available for such. Below’s screenshot will help you understand better.
So, The Volume Correction Factor (VCF) is 0.9886.
We know, GSV = GOV x VCF.
Now, we have GSV, which is sometimes called as Net/ KiloLetres.
Final STEP – GSV to Weight (MT) :
Now, your job is almost done. You need to apply the correction of density only. Then multiplying with GSV, will lead you to get Weight (MT).
GSV x Density in Air = Weight in Air (MT).
GSV x Density in Vacuum = Weight in Vacuum (MT).
Initially, you have received Density in Vacuum. Density in Air as follows :
Density in (Air) = Density in (Vac) – 0.0011 (WCF)
This is called Weight Correction Factor (WCF). And this is a fixed figure.
So, with all of this information we can go to the bottom line of our cargo calculation as follows:
Now, hopefully your all error’s regarding Cargo Calculation is now clear. If you are first time in the Tanker vessel above long elaboration may mazed your brain. So, below are summarization of the Calculation procedures:
liquid Cargo Calculations (With ASTM Tables) :
1) Observed Ullage – apply corrections – get Corrected Ullage
2) Observed Interface – apply corrections – get Corrected Interface
3) From Corrected Ullage, find Total Observed Volume TOV (in cubic metres)
4) From Corrected Interface, find Volume of Water (in cubic metres)
5) TOV – Water = Gross Observed Volume (GOV) of Cargo (in cubic metres)
6) Use Density at 15C and Observed Temperature (C) and find Volume Correction Factor (VCF) from Table 54B
7) Gross Standard Volume (GSV) = GOV x VCF (cubic metres)
8) Weight Correction Factor (WCF) = Density at 15C in vacuum – 0.0011 (or the Density at 15C in air)
9) Weight in Air (Metric Ton) = GSV x Density at 15C in air
10) Weight in Vaccum (Metric Ton) = GSV x Density at 15C in vacuum
Before I come to the conclusion I must thank you for your valuable time to read this detailed elaboration. Still, if you have any doubt, you are requested to comments below or you may sent WhatsApp messages too.
Thank you for your interest on Marine Legends.