HM 49/API Manual of Petroleum Measurement Standards Chapter 17, Marine Measurement Section 9 Vessel Experience Factor (VEF)
For any given vessel, a ratio can be established between the quantity of liquid bulk cargoes measured on board the vessel and the corresponding measurement by a load or discharge facility. This ratio, called a Vessel Experience Factor (VEF) is a historical compilation of shore-to-vessel or vessel-to-shore cargo quantity differences and 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 or Outturn quantities may be determined based on vessel received or delivered quantities adjusted by the VEF, in cases where shore based measurements are not available, or are known to be inadequate for custody transfer. In the event of a dispute regarding the application of a VEF, resolution shall be made by the commercial parties involved.
Vessel capacity tables (Gauge Tables) are often calculated from the vessel’s building plans, rather than based on accurate physical tank calibration measurements. There are usually differences between the quantity of a cargo measured in a calibrated shore tank or by a custody transfer meter, and the same cargo determined by vessel tank measurements. For a given vessel the use of quantity data from many voyages provides an indication of vessel measurement differences, as a numerical ratio. This ratio can also include other load and discharge factors. For each voyage a Vessel Load Ratio (VLR) and Vessel Discharge Ratio (VDR) can be calculated. The VLR or VDR is the quantity received or discharged as measured on the vessel (TCV – ROB or OBQ) divided by the Bill of Lading (shore delivered at loading) or Outturn Quantity (shore received at discharge) respectively. The mean of the qualifying VLRs or the VDRs over several voyages is called the VEF (VEFL and VEFD for load and discharge respectively.)
This standard provides a method for calculating VEF. The method uses an average of qualifying ratios, which fall within ±0.30 % of the mean. Certain voyages, including those considered to contain Gross Errors will be excluded from the mean calculation, as described in Section 8 of this standard. This method is preferred and should be used unless all parties specifically agree to an alternate method. See Annex D for an alternate method employing a statistical outlier rejection technique to discard unsatisfactory data.
A VEF cannot be calculated using voyages where load or discharge shore quantities are based on vessel measurement.
ISBN: 978 0 85293 621 4 2nd edition May 2012
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