1
Centre of Maritime and Ocean Studies
Marine Cargo Operations MCAO1010
Basics of Cargo Calculations on Tankers
1.0 Understanding Basics of Volume and Weight
Volume changes with temperature but the weight remains the same.
2.0 Measurement of liquid cargoes
The volume of oil in a tank is ascertained by measuring the distance from a fixed point on the deck
to the surface of the oil. The distance is known as the ullage and is usually measured by means of a
gas tight Ullage interface and temperature (UTI) tape.
Fig 1: UTI tape
Prepared by: Capt. Sukhjit Singh Date: January 2020
Revised by: Capt. Sukhjit Singh Revision Date: April 2020
2
Centre of Maritime and Ocean Studies
Marine Cargo Operations MCAO1010
Fig 2: Ullage and Sounding
A set of ullage tables (figure 3) is supplied to every ship, which indicate for each cargo
compartment, the volume of liquid corresponding to a range of ullage measurements for each tank,
for example ullage of 418 cms and vessel at even keel will correspond to 2318.7 CuM of volume.
The temperature observations are also taken usually at three levels within the cargo sounding (top,
middle and bottom). Average of this temperature is used as Observed temperature for Cargo.
Fig 3: Tank Ullage tables
Prepared by: Capt. Sukhjit Singh Date: January 2020
Revised by: Capt. Sukhjit Singh Revision Date: April 2020
3
Centre of Maritime and Ocean Studies
Marine Cargo Operations MCAO1010
Step 1 Gauging
Measurement of Ullage or Sounding
Measurement of Temperature
Measurement of Interface
The water in the cargo settles after loading and is called free water. Interface is the separation level
between the cargo and the free water.
Step 2 Density
Density in one of the following forms is provided by the Supplier
Density at 15° C – This is the mass per unit volume of oil at 15° C.
Relative density at 60° F – This is the density of oil at 60° F divided by the density of water
API gravity – Bears a purely mathematical relation to relative density
API gravity at 60° F = (141.5 / Relative Density at 60° F) minus 131.5
Step 2 Volume
Volume of oil on board is measured at Observed temperature. Since the density and volume of oil
change with change in temperature, a Standard temperature has to be used for calculations.
The standard temperatures used are 15° C and 60° F.
Tank volumes
TOV : Total Observed Volume of cargo at observed temperature including free water
GOV: Gross Oil Volume at observed temperature is TOV minus free water.
GSV ( Gross Standard Volume ) : Gross oil volume at standard temperature
Tank volumes – units
Tank Volumes are usually calculated in US Barrels OR Cubic Meters
1 Cubic Meter = 6.28981 US Barrels
Step 3 Use of ASTM tables for Correction Factors
ASTM (American Standard of Testing and Materials) tables are used because they represent a
uniform international standard for petroleum measurement. ASTM tables can be broadly divided
into three main groups:
1. Tables used for volume Correction Factors (VCF)
2. Tables used for Weight Correction Factors (WCF)
3. Tables used for conversion of volume and density measures
Prepared by: Capt. Sukhjit Singh Date: January 2020
Revised by: Capt. Sukhjit Singh Revision Date: April 2020
4
Centre of Maritime and Ocean Studies
Marine Cargo Operations MCAO1010
VCF or Volume Correction Factor (Obtained from ASTM tables)
GOV x VCF = GSV (Vol at standard temp)
VCF is obtained from one of the ASTM tables (figure 4) dependent on the units of supplied density
Table 24 is entered using the
Table 54 is entered using the Table 6 is entered using the
Relative Density and the
Density at 15 C and the API and the observed
observed temperature Deg
observed temperature Deg temperature Deg Fahrenheit
Fahrenheit (For correction of
Celsius (For correction of (For correction of US Barrels
US Barrels at Observed
Cubic meters at Observed at Observed temperature to
temperature to US Barrels at
temperature to Cubic meters US Barrels at 60 F.)
60 F.)
at 15 C.)
Table 54A – For crude Table 6A – For crude Table 24A – For crude
oils oils oils
Table 54B – For Table 24B – For
Table 6B – For Products
products Products
Fig 4: Use of ASTM Tables for VCF
For example, if supplied density of MOGAS @15 °C is 774.0 and observed average temperature of
the cargo inside tank is 34.0 °C. The VCF obtained using Table 54B (54A if it was Crude oil instead of
product (MOGAS)) is 0.9788
Fig 5: Example from Table 54B
Prepared by: Capt. Sukhjit Singh Date: January 2020
Revised by: Capt. Sukhjit Singh Revision Date: April 2020
5
Centre of Maritime and Ocean Studies
Marine Cargo Operations MCAO1010
The density usually supplied is density at 15° C in Vacuum.
In above example GOV x VCF = GSV at 15° C
Hence to obtained weight of cargo in air, GSV @15° C must be multiplied by density at 15° C in Air.
Therefore, the density in Air @ 15° C is weight correction factor.
WCF or Weight Correction Factor
GSV (Vol at standard temp) x WCF = Mass
Tables for WCF
Table 11 – For converting US Barrels at Standard temperature to Long Tonnes using API.
Table 13 – For converting US Barrels at Standard temperature to Metric Tonnes using API.
Table 29 – For converting US Barrels at Standard temperature to Long Tonnes using Relative Density
at 60 F.
Table 56 – For converting Cubic meters at Standard temperature to Metric Tonnes using Density
at 15C.
Table 57 – For converting Cubic meters at Standard temperature to Long Tonnes using Density at
15C.
Table 13 and Table 56 are most commonly used tables based on units used in industry and trade.
Weight Correction Factor can also be obtained from Density at 15° C in vacuum by using below
formula.
WCF = Density at 15C in vacuum - 0.0011
Flowchart in Figure 6 summarises the basics of cargo calculations procedure on-board oil tankers.
NOTE: at Load port during tanks are gauged prior to start loading and OBQ (On Board Quantity
determined). This is quantity inside tank prior to loading (mostly applicable for crude oils), As there
may be ROB (remaining on-board) quantity from previous discharging due to various reasons.
During cargo calculations on completion of loading, this OBQ must be subtracted from the total
quantity to determine the cargo loaded during the loading operations, if applicable.
Prepared by: Capt. Sukhjit Singh Date: January 2020
Revised by: Capt. Sukhjit Singh Revision Date: April 2020
6
Centre of Maritime and Ocean Studies
Marine Cargo Operations MCAO1010
Working with Density at 15°C Working with API Gravity at 60°F
Using Ullage, find Total Observed Volume TOV (in CuM) (list Using Ullage, find Total Observed Volume TOV (in US
or trim corrections must be applied, if any) Barrels) (list or trim corrections must be applied, if any)
From Interface, find Volume of Free Water (in CuM) From Interface, find Volume of Free Water (in US Barrels)
TOV - Water = Gross Observed Volume (GOV) of Cargo TOV - Water = Gross Observed Volume (GOV) of Cargo
(in CuM) (in US Barrels)
Use Density at 15°C and Observed Temperature (°C) to find Use API Gravity at 60°F and Observed Temperature (°F) to
VCF from Table 54 A for Crude oils and Table 54 B for find VCF from Table 6 A for Crude oils and Table 6 B for
Product oils Product oils
Gross Standard Volume (GSV) (CuM @ 15°C) = GOV x VCF Gross Standard Volume (GSV) (US Bbls @ 60°F) = GOV x VCF
Weight Correction Factor (WCF)(or the Density at 15C in air)
Find Weight Correction Factor (WCF) from Table 13
= Density at 15°C in vacuum - 0.0011 or Use Table 56
Weight in Air (Metric Ton) = GSV x WCF Weight in Air (Metric Tons) = GSV x WCF
Fig 6: Summary of cargo calculation procedure
Prepared by: Capt. Sukhjit Singh Date: January 2020
Revised by: Capt. Sukhjit Singh Revision Date: April 2020
