Crude Calculation

Crude Calculations

1 Definitions

1.1 Free water

The measured volume of water in tank not in suspension with the

liquid in the tank at observed temperature.
Expressed in cubic meters or/and barrels.

1.2 Total Observed Volume (TOV)

The total measured volume of all petroleum liquids, BS&W, and

free water at observed temperature.
Expressed in cubic meters or/and barrels.

1.3 Gross Observed Volume (GOV)

The total volume of all petroleum liquids and BS&W, but excluding
free water at observed temperature.
Expressed in cubic meters or/and barrels.

GOV = TOV – Free Water

1.4 Net Observed Volume (NOV)

The total volume of all petroleum products but excluding free

water and BS&W at observed temperature.
Expressed in cubic meters or/and barrels.

NOV = TOV – Free Water – BS&W

NOV = TOV – BS&W

1.5 Gross Standard Volume (GSV)

The total volume of all petroleum products and BS&W, but

excluding free water corrected by the appropriate volume
correction factor (VCF) for observed temperature and density to a
standard temperature.
Expressed in cubic meters or/and barrels.

GSV = GOV x VCF

GSV = GOV at 15oC/60oF

1.6 Net Standard Volume (NSV)

The total volume of all petroleum products excluding BS&W and

free water corrected by the appropriate volume correction factor
Used as guide: Essomarine conversion tables and charts.
BP publication of Lionel Downer.
Shell publication January 1980 Cargo Calculations.
 Crude Calculation
(VCF) for observed temperature and density to a standard
temperature.
Expressed in cubic meters or/and barrels.

NSV = NOV x VCF

NSV = NOV at 15oC/60oF

1.7 Total Calculated Volume (TCV)

The total volume of all petroleum products and BS&W corrected by

the appropriate volume correction factor for observed temperature
and density to a standard temperature and all free water at
observed temperature.
Expressed in cubic meters or/and barrels.

TCV = GSV + Free Water

1.8 Bottom Sediments and Water (BS&W)

Quantity of sediments and water into the total amount of

petroleum products.
Expressed as a percentage as determined by tests.

1.9 Observed Temperature

Temperature of cargo measured at various places in the tank. Mean

temperature is determined.

1.10 Standard Temperature

Temperature of 15oC in the metric system or 60oF in the American

system. All volumes must be converted to this temperature before
comparing.

2 Relationship: density, specific density, API Gravity.

Gravity is mass-per-unit-volume relationship.

With petroleum products the relationship is expressed as specific gravity.

The following definition applies:

Mass of given volume oil at to Kg

Specific gravity = ---------------------------------- -----
Mass of equal volume water at to Kg

Temperature normally equal to 60oF or 15oC

Specific gravity is sometimes called relative density

Used as guide: Essomarine conversion tables and charts.

BP publication of Lionel Downer.
Shell publication January 1980 Cargo Calculations.
 Crude Calculation
Oil density at 60oF Kg/l
Relative density 60/60oF = ---------------------- -----
Water density at 60oF Kg/l

Specific gravity is being replaced by density expressed in kg per m3.

The following definition applies:

Mass of given volume of oil Kg

Relative density 60/60oF = ---------------------- ----
Volume of oil at toC m3

In the USA the API gravity is being used. API gravity is arbitrary, scale,
calibrated in degrees and related to the specific gravity by the following
relation:

141.5
API gravity = ------------------------- - 131.5
Specific gravity 60/60oF

3 Mass and weight relationship

3.1 Mass

The mass of a substance is the quantity it contains. It is

independent of external conditions. In oil measurement, is often referred
to as weight-in-vacuo.
The metric unit is the Kg where 1’000 Kg equals 1 metric tone.

Following relationship applies:

Mass of oil = Volume of oil at toC x Oil density at toC

3.2 Weight

The weight of a substance (called weight-in-air) is the mass

which a substance appears to have when weighted in air.
The weight is calculated using the following equation.

Weight of oil = Volume of oil x Weight correction factor

Used as guide: Essomarine conversion tables and charts.

BP publication of Lionel Downer.
Shell publication January 1980 Cargo Calculations.
 Crude Calculation

3.3 Weight Correction Factor (WCF)

The weight correction factor must be obtained from the

appropriate table in the Petroleum Measurement Tables.
In the American system the WCF can be obtained from table II
“Long tons per barrel at 60oF against API gravity at 60oF”.
In the Metric system WCF can be obtained from table 56 “Kilograms
per liter at 15oC and liters at 15oC per metric ton against density at
15oC”
The WCF can be easily obtained by subtracting 0.0011 Kg/l from
the density at 15oC.
Unfortunately it is much easier to subtract than to consult Table
56. This has led to the problem that it is no longer obvious which
density is used: the density or the WCF.

4 Unit Conversions

Units can be easily interchanged by using the appropriate table

from measurements Table Volume XI “Entry with Relative Density”.
It is extremely important however to be aware of the following
fact: “Like can only be compared with like”.

Comparison between volumes and capacities must be done at same

temperature.

5 Cargo Calculations

5.1 Metric system

Schematic layout of calculations.

Notes
Used as guide: Essomarine conversion tables and charts.
BP publication of Lionel Downer.
Shell publication January 1980 Cargo Calculations.
 Crude Calculation
Given: TOV
Free Water
Temperature
Observed Density

Can be calculated: Weight in air in MT (and LT if needed)

Weight in vacuo (= oil mass)

Tables used: 53A

56
54A

The relationship between barrels and cubic meters is very precise if

same temperatures are used.

1 bbl = 0.158987 m3 1 m3 = 6.28981 bbls

Temperatures: see “Calculation Representative Cargo Temperatures”

Free water: m3 x 1.025 = MT (if sea water)

Example:

TOV: 100000 m3 at 30oC Density: 0.8520 at 30oC

VCF from Table 54A: 0.9873
WCF from Table 56 : 0.8509

GSV = 100’000 x 0.9873 = 98’730 m3

Weight in vacuo = 98’730 x 0.8509 = 84’117.96 MT (oil mass)
Weight in air = 98’730 x 0.8509 = 84’009.36 MT

General remark concerning metric calculations:

In the metric system both weight in air and weight in vacuo can be
easily calculated. But this can lead to confusion if calculation form is
not clearly labeled.

Table

53A Generalized crude oils. Correction of observed density to density 15oC

54A Generalized crude oils. Correction of volume to 15oC (against density 15oC)
56 Kilograms per liter at 15oC and liters at 15oC per metric ton against density
at 15oC

Used as guide: Essomarine conversion tables and charts.

BP publication of Lionel Downer.
Shell publication January 1980 Cargo Calculations.
 Crude Calculation
5.2 American system

Schematic layout of calculations:

Notes:

Temperature: see “Calculating Representative Cargo temperatures”

Oil weight is calculated (weight in air): oil mass can only be
calculated by converting API to density and to perform the metric
system calculation.
For calculation in LT table II must be consulted for calculations in
metric ton table 13.
If in LT the free water must also be converted to LT.
(LT = MT x 1.016047).
The calculations for LT are the most accurate.

Example:

TOV = 100’000 m3
TOV = 628’981 bbls at 86oF API = 34.49

VCF from table 6A: 0.9877

WCF from table 11: 0.13309
WCF from table 13: 0.13520

GSV = 628’981 x 0.9877 = 621’244.54 bbls

Weight in air = 621’244.54 x 0.13309 = 82’681.4 LT
621’244.54 x 0.13520 = 83’992.3 MT

(If formula for LT/MT – conversion is being used, a tonnage of

84’008.2 MT is found a difference of about 16 MT)

Used as guide: Essomarine conversion tables and charts.

BP publication of Lionel Downer.
Shell publication January 1980 Cargo Calculations.