What Is VEF and Why Is It So Important in Custody Transfer?

VEF (Vessel Experience Factor) is a coefficient used to adjust the quantities measured on board a vessel based on its historical performance relative to the quantities measured at the terminal.

According to API MPMS Chapter 17.9, it is calculated using the vessel’s most recent valid voyages and applying statistical and operational criteria to obtain a representative and reliable factor.

The VEF development process involves several stages. First, the vessel’s 20 most recent voyages are listed. If 20 voyages are not available, at least five valid voyages are required for the VEF to be valid. Voyages that do not meet the acceptance criteria are then identified, including STS operations; operations involving an FPSO when the relevant certificates for the measurement equipment used in the transfer are unavailable; evident errors, as defined by the standard; and the first voyage after the vessel enters dry dock, but only when structural modifications or changes to the measurement systems were made. These voyages are not removed from the list, but they are excluded from the calculation of the average loading ratio, which establishes the qualifying range.

The loading ratios for the remaining voyages are then analyzed to determine whether they fall within that range. Those outside the range are discarded in a second filtering stage. Finally, the VEF is calculated using the voyages that pass both filters, provided that the valid sample includes at least five valid voyages.

It is important to note that the VEF does not correct measurement errors. Its purpose is to reflect systematic and recurring vessel behavior and identify whether the vessel overestimates or underestimates quantities, provided that sufficient and representative historical data are available.

In practice, the VEF normally used in commercial transfers is the loading VEF (VEFL), even for discharges or lightering operations, because the data required to calculate a discharge VEF (VEFD) are often unavailable.

Proper collection of historical data, appropriate voyage selection, and strict compliance with API MPMS 17.9 are essential to obtaining a reliable VEF and reducing uncertainty in custody transfers.

BZ Consulting, “The value of experience.” 💼✨