After completion of cargo custody transfer, transferred quantity can be determined by two different methods;
based upon vessel’s tank measurement; and
based upon a calibrated shore tank measurement or by a custody transfer flow meter,
Usually, the quantity of cargo measured, using the two methods mentioned above, differ in numbers.
One reason for the difference between the cargo quantity measured using different methods can be attributed to the fact that the vessel’s capacity tables are often calculated from the vessel’s building plans, rather than based on accurate physical tank calibration measurements.
For any particular vessel, the use of a numerical ratio between (i) the quantity measured using the vessel’s capacity table and (ii) quantity measured using shore facilities (calibrated tanks or flowmeters) provides an indication of vessel measurement differences, as a numerical ratio. This ratio is called Vessel Experience Factor (VEF) and it is a historical compilation of vessel-to-shore cargo quantity differences.
There are at least three approved methods for calculating VEF as published by both the American Petroleum Institute (API) and the Institute of Petroleum (IP).
The historical ratios, between quantity received as measured on the vessel (TCV – OBQ) divided by the Bill of Lading (shore delivered at loading) respectively, are calculated for at least five voyage, however a larger number of voyages is desirable.
First step in calculating the VEF is to determine the ratio between i) the quantity measured using vessel’s capacity table and (ii) quantity measured using shore facilities (calibrated tanks or flowmeters.)
Mean value of all the ratios is then calculated.
Third step identifies and all the values which fall within the range of +/- 0.0030 (or 0.3%) of the mean ratio of all voyages. These are the qualifying ratios.
And finally a mean of qualifying ratios provides the final VEF.
Following voyages shall be excluded while determining qualifying ratios;
First voyages after dry dock,
All voyages involving STS lightering operations,
Voyages where bills of lading are based only on shipboard measurement,
Voyages prior to any modification which have affected the ships carrying capacity,
Voyages where vessel has loaded several parcels or voyages where vessel only part loaded to less than 75% capacity.
Once calculated, a VEF can then be applied to the ships figures which can then be compared to the shore figures as a measure of accuracy and confidence.
VEF is a dynamic factor which might change over the time. Following factors can influence the VEF;
Change of trade (different grades and load ports)
Accumulation of sediment and scale
Dry docking preparation (de-scaling)
Structural alteration in cargo spaces.
VEF is used as a loss control tool to assess the validity of quantities derived from shore measurements. When agreed by interested parties, Bill of Lading may be determined based on vessel received adjusted by the VEF, in cases where shore based measurements are not available, or are known to be inadequate for custody transfer.
A variation of 0.0005 on the VEF can result in a quantity difference of about 1,000 bbls on a VLCC nominated to load 2,000,000 bbls.
========================================================================
For more details on VEF and how to protect your interest while loading and/or discharging, please contact,
Capt. Anurag Srivastava on anurag@ocsol.net