Loading and Unloading Oil Cargoes: Calculations

Many of the Deck Examinations on the new “Q” tests contain two types of questions about loading and unloading oil cargoes. Both require calculation. The first asks for a quantity. The second wants to know the time and date of final loading.

To determine the quantity of oil cargo loaded or discharged, remember two things. First, oil cargoes expand or contract depending upon temperature and second, “net” barrels are the number of barrels at a temperature of 60° Fahrenheit. If a liquid cargo is loaded at a higher temperature than the temperature at which it is discharged, there will be fewer barrels (or gallons) at discharge than at loading. If the cargo is loaded at a lower temperature than its discharge temperature, there will be more barrels when unloading. Regardless of whether loaded or discharged, net barrels are computed at a temperature of 60°F and a “net” barrel is 42 gallons.

The time and date of completed loading requires determining the rate of loading and applying that rate to the remaining cargo.

Barrels at Discharge

Your ship has loaded 9,000 barrels of fuel oil at a cargo temperature of 35°C (95°F). API gravity is 44°. The volume correction factor (VCF) is .0005. You are bound for New Jersey from Venezuela. How many barrels would you expect to unload if the cargo temperature is 55°F at the discharge port?

A. 9,180

B. 9,090

C. 8,910

D. 8,820

First of all, disregard the API gravity as it has no bearing on the calculation. (API gravity is a measure of the specific gravity of the product relative to water. In this case, the specific gravity of the fuel oil is 0.806 so it is lighter than water.) The VCF (Volume Correction Factor) is essential to the problem. When multiplied by the total number of barrels, the VCF gives the change in barrels for a temperature change of 1°F.

In this case the change in temperature is 40° (95° – 55°). Multiplying the total number of barrels (9000) by the VCF gives a change of 4.5 barrels for each degree of temperature change. 40° x 4.5 = 180 barrels.

Since the cargo is being discharged at a lower temperature than the loading temperature, there will be fewer barrels (9000 – 180 = 8,820). The answer is (D).

Gallons at Discharge

A nearly identical question asks for the answer in gallons.

Your ship has loaded 9,000 barrels of fuel oil at a cargo temperature of 35°C (95°F). API gravity is 44°. The volume correction factor (VCF) is .0005. You are bound for New Jersey from Venezuela. How many gallons would you expect to unload if the cargo temperature is 55°F at the discharge port?

A. 85,560

B. 504,900

C. 370,440

D. 485,110

The answer is (C), 8820 barrels at 42 gallons/barrel = 370,440 gallons.

“Net” Barrels

A tanker is loaded with 12,000 barrels of #2 fuel oil. The temperature of the product is 88°F. API gravity is 39°. The volume correction factor (VCF) is .0005. The net amount of cargo in barrels loaded is _________.

A. 12,168

B. 11,994

C. 11,832

D. 12006

The difference between the loading temperature (88°F) and 60° (the temperature for calculating net barrels) is 28°. 12,000 x the VCF (.0005) gives a change of 6 barrels for each degree of temperature change. 28° x 6 = 168 barrels. Since the change is from a higher temperature to a lower temperature, there will be 168 fewer net barrels. 12,000 – 168 = 11,832. The answer is (C).

Date & Time Loading Finished

You are loading 530,000 barrels of cargo oil. At 0945 on 13 April, you find that you have loaded 202,000 barrels. At 1130, you find that you have loaded 223,000 barrels. If you continue at the same rate, you will finish at _______________.

A. 1322, 13 April

B. 1920, 13 April

C. 1120, 14 April

D. 1305, 14 April

The first step is to determine the rate of loading. From 0945 to 1130, you have loaded 223,000 – 202,000 barrels, which is to say that in 1.75 hours (1 hour 45 minutes), you’ve loaded 21,000 barrels. 21000/1.75 works out to a loading rate of 12,000 barrels per hour.

At 1130, there are 307,000 (530,000 – 223,000) barrels left to load. 307,000/12000 = 25.58 hours or 25 hours and 35 minutes.

April 13 1130 + 25 hours and 35 minutes = April 14 1305 (Answer D).