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VEF Calculation - Procedure

- Introduction

For a particular vessel, an approximately constant ratio can be established between the quantity of oil measured on board the vessel and the corresponding measurement by a shore terminal.

As the vessel comprises of several tanks, the random errors associated with ullage and temperature  measurements are minimised and give an excellent overall repeatability.  However, as the accuracy of the ship's calibration tables can vary by +/- 2%, it is necessary to use a ratio, called Vessel Load Ratio (VLR) or Vessel Discharge Ratio (VDR) which is the quantity received and measured in the ship divided by the Bill of Lading or shore Outturn Quantity respectively.



Consequently, in order to monitor the accuracy of Bill of Lading or Outturn quantities, it is possible to use the vessel quantities for comparative purposes.

This appendix provides two alternative methods for calculating Vessel Experience Factors.

  • Method 1 is for use in the field an provides a simple technique to monitor Bill of Lading or Outturn figures.

  • Method 2 should be used when the highest mathematical precision is required.

The parties involved should agree on the method to be used.

- Data Qualification / Rejection

Going by general statistical rules, the greater the number of sets of data which are included, the greater will be the confidence in the VEF established.

In practice, an adequate level of confidence will be achieved when the final VEF calculation is based on data from a specified minimum number of qualified voyages.  Using Method 1, data from a minimum of five (5) qualified voyages is needed to calculate a VEF.  However, a larger number is desirable.  Using Method 2, data from a minimum of ten (10) qualified voyages is needed to calculate a VEF with the greatest accuracy.

Because on occasions the available data can give rise to non-typical VLRs or VDRs or because essential information is missing, both calculation methods employ techniques to ascertain the validity of a particular cargo lift VLR or VDR before inclusion as a qualified voyage in the VEF calculation, else it is rejected.

Method 1 excludes -

  • Voyages where shore measurements were not available.

  • Voyages prior to structural modification which affected the vessel's cargo capacity.

  • Voyages when the VLR or VDR is not within +/-0.3% of the ration of the totals of vessel and shore quantities, for all the voyages under consideration.

Method 2 -

Employs rigorous statistical methods for establishing the reliability of individual load (or discharge) ratios and for estimating the confidence limits (probability of 95%) for the range of acceptable values.

Calculation of Vessel Experience Factor - Loading  (Discharge proceeds in a like manner)

Method 1 - Uses a sequential voyage log (form).

  1. List the voyages data (for voyages not disqualified basis the above paragraphs) under the following column headers - Voyage Sequence (beginning with the last); Cargo Name; Terminal; Date; Sailing (Arrival) TCV; Optional OBQ (ROB); Load (Discharge) TCV, Shore TCV, VCF Tables Used.

  2. Calculate Total Vessel Quantity and Total Shore Quantity from the individual voyage quantities recorded in Load TCV and Shore TCV.

  3. Calculate the average vessel/shore ratio from the total quantities.

  4. Calculate individual VLRs for each voyage and record in the next column - Vessel Ratio (Vessel TCV / Shore TCV).

  5. Check whether VLRs qualify - Y/N in the next column (Y/N).  (VLRs outside +/-0.3% from the average Vessel/Shore ratio determined in step 3).

  6. Transfer qualifying voyage numbers from Column 1 to the next column (Qualified Voyage No.) and the corresponding vessel and shore TCV figures to the next two columns.  Total these two columns.

  7. Divide the new total vessel quantity by the new total shore quantity.

  8. Calculate to five (5) decimal places and report to four (4) decimal places the Vessel Experience Factor as established in step 7.

Use this VEF to monitor the Bill of Lading or Outturn Figure.  Agreed limits may be applied to monitor the Bill of Lading or Outturn.


  • List last voyage first

  • Use same units for all entries

  • Do not mix load and discharge figures

  • The average TCV ratio is equal to the Total Vessel TCV divided by the Total Shore TCV

  • The form should be prepared taking information from the Voyage Analysis Report (VAR), if available

Purchase SNsVEF where API, IP and Statistical Methods (Method 2) are included in the coding.

Sections of text taken from API - MPMS Chapter 17 Marine Measurement

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