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We do so many things to make sure that ship owners get the maximum out of their investments on buying and running a ship. We make sure that there are least constants on the ship, the ballast is pumped out to the last drop and many other things like these.
All this to make sure that we have the capacity to load maximum cargo and ship owner has a chance to earn maximum from it. But here is the thing. It is sometimes difficult to get a hang of these calculations. There are so many tables to use and so many terms that float. Even when we hear some weight of cargo, let us say Tons of cargo, there are two things that we need to be aware of.
Even though on ships it is more common to measure the cargo weights in the air, sometimes you may find that the charterers would give the requirements for measuring weight in Vacuum. Coming back to the topic, can you guess for the same amount of cargo which weight would be more? Weight in air or weight in Vacuum? Well, the weight is Vacuum is always more than the weight in Air.
This is because, like with water, air and any other medium in which the weight is present would offer some kind of buoyancy which reduces the weight. In the vacuum, there is no buoyancy and hence the weight is more than the same weight when measured in air. Ok, so now here is the first thing that we can learn. How to convert weight in Vacuum to weight in Air? The first page of the ASTM table 56 provides the factor for converting weight in vacuum to weight in air and vice versa. Ok, now let us get back to basics of cargo calculations on tankers.
And it is not that complicated. We also measure the temperature of the cargo preferably at three levels and take the mean of these three temperatures to get the temperature of the cargo. Let us say we got the volumes from the ullage tables and the volumes for each tank are as per below. As the volume changes with the temperature, this cannot be the measure of how much cargo we have loaded or discharged.
We need to convert the volumes to the weight of the cargo in each tank. We need the density of the cargo to convert the volume of cargo to the weight. And as the density also changes with the temperature, we would need the density of the cargo at the cargo temperature to convert the observed volume to weight. Take a deep breath and read on. So let us say that cargo surveyor has provided us with the density at a particular temperature and correction factor. So, in this case, we just apply these densities to get the weight of cargo in each tank and thus the total weight of the cargo.
The cargo surveyor may provide a table of densities at different temperatures. This is even easier than the previous section that we discussed. The cargo calculations, in this case, are also easy. We just take the density of the cargo to the corresponding cargo temperature that we measured. If the cargo temperature is between two values in the density table, we just interpolate to the get the density at the desired temperature. The previous two methods are useful and applicable for cargoes the density for which changes proportionally with temperature.
But for petroleum products and crude oils, ASTM tables are used for calculating cargo weights. Let us say cargo surveyor provided the density at 15 deg C as 0.
Now at many places may be using the standard volume instead of weight. The standard volume of the cargo would also remain same as this is the volume at the fixed temperature 15 Deg C. But in any case, we still need the weight of the cargo as the stability calculations need the weight of the cargo in each tank and not the standard volume.
Getting the weight from standard volume is simple. We have the volume at 15 Deg C and we have the density at 15 Deg C. The density at 15 Deg C is always the density in Vacuum.
So if simply multiply this density with standard volume, we get the weight in Vacuum. So we then need to either convert the weight in vacuum to weight in the air as we discussed earlier or we can simply convert the density in Vacuum to density in Air. In above ullage report, I have applied the WCF to the Gross standard volume but we can easily make one additional column and apply the WCF to the standard volume of each tank to get the weight in the air for each tank.
And as you might have guessed correctly, these ports also do not measure the temperature in Deg C but in Deg F. So, in the same manner, we get the VCF Volume correction factor for other required temperatures that we have measured in each tank. And when we multiply the volume at observed temperature with VCF, we get the standard volume, this time the volume at 60 Deg F. We need to apply Weight correction factor WCF to the standard volume to get the weight of the cargo.
While the ASTM tables that we discussed in previous sections are the one that is used mostly, there are other ASTM tables that supplement these main tables. And even for the main ASTM tables, the information about which table need to be used for cargo calculation is provided by cargo surveyor.
We need to follow the information provided by the cargo surveyor because that would be the table that is used for shore calculations and we need to use the same to avoid ship shore quantity difference.
But we need to understand that at the very basic level, we calculate the volume from ullage tables and we need to be provided with density at the same temperature as the cargo. In this case, we need to get the volume correction factor VCF to convert the volume at the observed temperature to the standard volume which is volume at 15 Deg C or Volume at 60 F respectively. We then need to apply the weight correction factor WCF to convert the standard volume to weight.
The one with letter A is for crude oils and the one with letter B is for product oil. ASTM tables without any letter are common for both crude oils and product oils. Get your hands on ASTM tables and you will find that cargo calculations are not as difficult as it seems. Rajeev Jassal has sailed for over 19 years mainly on crude oil, product and chemical tankers.
He has done extensive research on quantitatively measuring Safety culture onboard and safety climate ashore which he believes is the most important element for safer shipping. Great article sir. Thanks for sharing ur knowledge. More power Sir! Always waiting for more articles. Hi Rajeev. I have crue oil density 0, and 42, 25 degC and VCF should be 0, but my formula gives much less. DO you have a correct formula to calculate correct VCF. In my formula I have a factor , Is this correct?
My email: mattijnasi gmail. Very nicely explained sir. Thank you very much for your kind effort for explaining cargo calculation in a very simple language. Pls next how to calculate : Vapour weight in LPG Thanks, Capt. Rajeev Jassal. Air buoyancy do exists It is same as how the same ship would weigh more in waters of different densities Aur offered some resistance and thus the buoyancy..
Good day Capt. Rajeev, Above well explained. Just wanted to know one thing how calculations are performed on chemical tankers. Different manufacturer may have different densities for same chemical.
This is true especially for vegetable oils Correct me if I am wrong. So how these calculations are performed on chemical tankers??? There are no standard tables for chemicals.
For most chemicals.. All of your blogs did a vast contribution to pass my orals exam from uk in my first attempt Thank you verymuch sir. Really helpful sir, I am preparing for Ch Mates phase I exams ans as I am from a dry cargo background, this artice really helped me out. Thank you so much for sharing your knowledge in such a simple manner. Thankyou sir , very well explained in simple words with deep explanation. This is the reason "Albert Einstein Quotes. If you can't explain it simply, you don't understand it well enough".
Fyi after loaded Would you please comment how it can be possible to checked the shortage? Irrespective of the reasons, vessel is not concerned about how much cargo shore received. This Is how to explain As expected, very well explained.
Thank you sir for help us junior officers to further our knowledge as aspiring Chief Officers. Thanks for this knowledge and experience based article which I was searching for long time.
Tabel ASTM 53 .pdf