VESSEL'S EXPERIENCE FACTORS

Petroleum and Petrochemical Bulletin
The purpose of this bulletin is to clarify the position of IFIA member companies regarding the use and
application of VEFs (Vessel Experience Factors).
IFIA member companies are frequently asked to apply VEFs to vessel figures for the purpose of
Custody Transfer where measurements determined by shore tank or meter are not available.
Following the first issue of this bulletin in March 2002 the new standard, API MPMS Chapter 17.9/EI
HM 49 Vessel Experience Factor (VEF) was issued in November 2005, “the Standard”. Appendix C in
API MPMS Chapter 17.1 was withdrawn in March 2008.
The VEF is primarily a loss control tool to help assess the validity of quantities derived from shore side
(tanks, meters, etc.), off-shore installations or from other vessels. It should be recognized that
quantities derived using vessel figures and the VEF will never have the measurement rigor of those
from traditional static shore tank or meter measurements. For these reasons the Standard states that
VEF adjusted vessel figures may be used for custody transfer where traditional measurements are
either "not available, or are known to be inadequate". Although not well defined, "known to be
inadequate" implies a general acceptance by all parties that due to poor design or obvious malfunction
a shore tank or meter measurement cannot be relied upon.
While in many situations when traditional measurements are questioned, a VEF adjusted vessel
quantity may be the only available alternate measurement, the lack of statistics for precision and
uncertainty cannot be overcome. Therefore, in these situations, it should not be automatically
assumed that VEF adjusted vessel figures will provide any greater accuracy than the traditional
measurements which they might replace.
Limitations of the Vessel Experience Factor
There are a number of factors which limit the overall accuracy of quantity calculations based on
application of a VEF to vessel measurements. These factors are recognized in the Standard but a
number of issues remain:
• The voyage data used to compile a VEF must, by necessity, come from information supplied
by the vessel. The inspector is rarely able to verify the accuracy of this data.
• Most vessels only maintain records of load port data. Therefore, most vessel experience
factors are load port experience factors (VEFL), even though they are frequently applied to
discharge measurements. The Standard requires that data necessary for calculation of both
load and discharge VEFs should be maintained by vessels. As vessels are commonly not
aware of the shore outturn this data is not normally available. Data and information may be
available from other reliable sources but not necessarily in sequential order.
• Barge towing companies often do not maintain load or discharge records. Data may be
available from other reliable sources, and which would include inspection companies and/or oil
companies, but, again, not necessarily in sequential order.
VESSEL EXPERIENCE FACTORS
Bulletin 02-01
Rev. 1
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• There is considerable debate about whether VEFs calculated from “full” cargoes should be
applied to part cargoes. The Standard allows for development of partial VEFs for “a specific
set of compartments, or amount less than 75% of a vessel capacity”. However, this requires
regular operation of the vessel in the same condition and it is extremely rare for there to be
sufficient data to compile a partial VEF. The Standard states “if data to determine a partial
cargo VEF is not available it is recommended to apply the full cargo VEF to each partial
cargo”. Although the recommendation is clear the application of VEFs to part cargoes remains
a contentious issue.
As a consequence of the known limitations, the use of vessels figures adjusted by VEF to determine a
Custody Transfer quantity (Bill of Lading or Outturn) is a commercial decision and can only be made
with the agreement of the interested parties. This is clearly noted in the Standard. However it should
be recognized that some regulatory authorities impose the use of a “correction” factor to vessel
measurements that is not in keeping with the Standard and outside the control of inspection
companies and their principals.
Application of the Vessel Experience Factor
Noting the above, it is recommended that IFIA member companies should calculate Custody Transfer
figures by applying a VEF to vessel measurements as follows:
• In accordance with the Standard, vessel experience factors will be applied for purposes of
determining Custody Transfer quantities only “when agreed by interested parties”.
• A full-vessel VEF will only be used for partial cargoes, as recommended in the Standard,
where i) a partial VEF is not available or ii) the interested parties instruct that a full-vessel
VEF is to be applied.
• When data to determine a discharge VEF (VEFD) in accordance with the Standard is not
available the Standard states that “a VEFL may be used”. However, in these circumstances,
IFIA member companies may, with agreement of all interested parties, use data from other
reliable sources to calculate a discharge VEF. Such data may not be sequential.
• Where no agreement is obtained from interested parties with regard to application of a VEF for
purposes of determining a Custody Transfer quantity, IFIA member companies should issue
vessel figures with no factor applied.