Tanker Cargo Calculations

What is Gauging ?

• Measurement of Ullage or Sounding 
• Measurement of Temperature
• Measurement of Interface

Ullage and Sounding

Usually ullages are used for calculation. 

In rare cases soundings may be used 

What is 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 .

Trim Correction

To obtain the actual cargo quantity in a tank, Trim correction must be applied to the measured ullage

List Correction

To obtain the actual quantity of cargo in a tank, List Correction must be applied to the measured ullage

The basics of Cargo Calculation

Various forms of Density

 Density in one of the following forms is provided by the Supplier

a) Density at 15 C – This is the mass per unit volume of oil at 150 C. 

b) Relative density at 60 F – This is the density of oil at 600 F divided by  

the density of water

c) 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 

Standard Temperature

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. 

15 C is not the same as 60 F. 

In fact 15 C = 59 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

GSOV : This is the Gross oil volume at standard temperature + the free water

Tank volumes – units 

Tank Volumes are usually calculated in

US Barrels  

OR

Cubic Meters 

1 Cubic Meter = 6.28981 US Barrels

Correction Factors

VCF 

or

Volume Correction Factor

Also called Volume Reduction Factor  Obtained from ASTM tables 

GOV x VCF = Vol at standard temp.

WCF 

or

Weight Correction Factor

Also called Weight Reduction Factor  

Obtained from ASTM tables 

Vol at standard temp x WCF = Mass

ASTM Tables

Tables for VCF

Table 6A – For crude oils

For correction of US Barrels at Observed temperature to US Barrels at 60 F.

Table 6B – For Products

For correction of US Barrels at Observed temperature to US Barrels at 60 F.

These tables are entered using the API and the observed temperature

Table 24A – For crude oils

For correction of US Barrels at Observed temperature to US Barrels at 60 F.

Table 24B – For Products

For correction of US Barrels at Observed temperature to US Barrels at 60 F.

These tables are entered using the Relative Density and the observed temperature

Table 54A – For crude oils

For correction of Cubic meters at Observed temperature to Cubic meters at 15 C.

Table 54B – For Products

For correction of Cubic meters at Observed temperature to Cubic meters at 15 C.

These tables are entered using the Density at 15 C and the observed temperature

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.

Tables for Interconversion of units

Table 1 – Interrelation between Various units of measurement.

Table 2 – For Temperature conversion. Centigrade to Fahrenheit  and vice versa.

Table 3 – Conversion of API gravity at 60 F to Relative density at  60 F and Density at 15 C 

Table 52 – For converting Barrels at 60 F to Cubic meters at 15 C  and vice versa.

ASTM tables can be broadly divided into three main groups

Tables used for volume Correction Factors ( VCF )

Tables used for Weight Correction Factors ( WCF )

Tables used for conversion of volume and density measures

Other Factors

Line Content

Line content refers to the cargo in the bottom line of the ships cargo tanks.

•The ullage tables normally state whether the line content is included within the tabulated volume of each tank or not. 

•If not stated, it is implied that the line content is included within the tabulated volumes for each tank.

•Line contents must be added to or subtracted from the GOV.

Add line content:

•If any lines passing through empty tanks are filled with cargo.

•Contents of all lines if the line content is not included in the tabulated volume of each tank

Line content may have to be subtracted in certain cases