VESSEL EXPERIENCE FACTOR ISSUES CLARIFIED

Oct. 15, 1990
S. Sivaraman Exxon Research & Engineering Co. Florham Park, N.J. R.I. Wallace Marine Consultant Surveyor Wirral, U. K. Vessel experience factor (VEF), when developed through proper application of industry guidelines and field procedures, provides a factor that improves the overall accuracy in the determination of a marine vessel's cargo quantity. VEF as currently computed is applicable to a vessel as a whole and not to individual compartments or tanks.
S. Sivaraman
Exxon Research & Engineering Co.
Florham Park, N.J.
R.I. Wallace
Marine Consultant Surveyor
Wirral, U. K.

Vessel experience factor (VEF), when developed through proper application of industry guidelines and field procedures, provides a factor that improves the overall accuracy in the determination of a marine vessel's cargo quantity.

VEF as currently computed is applicable to a vessel as a whole and not to individual compartments or tanks.

The development and application of VEF is sometimes fraught with confusion and a lack of appreciation as to its potential value for marine vessel quantity determination. VEF is not a factor to compensate for procedural errors and system inconsistencies such as improper or out-of-date calibration.

On the other hand, it is a factor to account for residual inherent measurement imprecision associated with ships' calibration and residual random measurement variations (both ship and shore).

Properly developed, VEF can facilitate more accurate determination of vessel quantity for custody transfers when necessary and where warranted. Beyond any doubt, however, it must be recognized that a properly calibrated shore tank/shore meter will continue to be the primary measurement system for custody transfer.

BACKGROUND

Historically, in the determination of VEF, voyage data on ship and shore measurements were applied with very little emphasis on calibration and measurement procedures and practices. As a result, VEF ended up as a compensatory factor for procedural errors, improper calibration, and system deficiencies.

The factor, even today, is looked upon with suspicion, and its potential value for overall enhancement of vessel quantity determination is still not understood clearly. With the American Petroleum Institute (API) and the Institute of Petroleum (IP) guidelines, VEF is now emerging as a meaningful factor.1 2

An attempt has been made here to clarify the VEF development process. Major parameters that affect VEF are clearly identified and discussed, on their technical merit, to emphasize that VEF can indeed be a meaningful factor for improving the accuracy of volume determination aboard marine vessels.

To promote a better understanding of VEF development, an analogy has been drawn to the meter factor development.

VEF development is similar to meter factor development. A meter factor is established on a periodic basis for meters in good working order, using a frame of reference such as a volumetric tank prover or a master meter.

Typically, three proving runs, excluding the wetting run, are undertaken, and the meter factor is computed for each run. The average of the meter factors is determined with the stipulation that the variation between the maximum and minimum meter factor shall be within 0.05%. Field measurement procedures must be strictly followed during proving.

A VEF is established using a similar methodology. For developing a VEF, a minimum of five qualified voyages is recommended by API guidelines.1

For each qualified voyage, a vessel ratio of loaded to empty is computed. The VEF is then established with a permissible variability of 0.3% (i.e., any single vessel ratio should be within this range when compared to the calculated VEF).

As in meter proving, all measurements and field procedures should be followed carefully for all voyages under consideration.

It is important to ensure that the necessary measurement systems and procedures are in place at the ports under consideration for VEF development. Very specifically, the following parameters should be reviewed during the development process.

Parameters to be considered for VEF development are:

  • Shore volume determination. The shore volume may be determined using either a shore tank or a shore meter.

    If the shore tank is the reference medium against which the ship's quantity is compared, it is extremely important that the shore tank carry a capacity table that is recent (i.e., no more than 15 years old and no structural modifications of any kind undertaken since the last calibration). It is equally important to ensure that the tank calibration was undertaken in accordance with the established industry standards.

    VEF has been known to be established with tank capacity tables as old as 30-35 years with tanks not having been calibrated with proper corrections and, in many instances, with procedural deviation. Use of these tanks as the basis for VEF development will result in distortions and loss of the true value of VEF.

    If a shore meter is used, it is equally important that the meter be proved at regular intervals in accordance with existing guidelines and the meter's factor established in accordance with existing standards.

  • Vessel capacity table. The vessel under consideration must carry ullage tables based on a recent calibration carried out in accordance with established industry standards. Vessels that carry old tables which are not updated after structural or deadwood modifications or changes of measuring position should not be used for VEF development.

    Use of outdated and incorrect vessel tables will result in distortions in VEF.

    Vessels should preferably be calibrated using the latest API calibration standards, API Chapters 2.7 and 2.8 (due for industry publication sometime in 1990/91).

  • Vessel loaded condition. No attempt must be made to develop a VEF on individual vessel tanks or for part cargos using existing guidelines. As far as possible for each voyage, the vessel should be loaded to maximum fill conditions with, preferably, the same grade of cargo.

    The current guidelines do not make any specific or direct reference to the voyage immediately following a dry dock operation. This issue is presently being investigated.

    Until such time that data become available as to the effect of the first voyage after a dry dock on VEF, where possible, this voyage may be treated as a wetting run similar to a meter factor development and excluded from VEF calculation.

    Also, the effect of heated cargos on VEF development is under investigation and therefore heated cargoes should be treated with caution.

  • Vessel trim/list corrections. As and when trim/list corrections are required, corrections must be applied in accordance with standard procedures. Use of vessel data without trim/list corrections will result in VEF distortions.

  • Reference gauge heights. Both the shore tank and the vessel's tank reference gauge heights must be checked and verified to reflect the reference heights shown on the capacity tables. If there is a difference between the actual and the capacity table reference heights, then until such time as the error is corrected, the use of shore/ship data for the specific ship and the specific shore tank should not be considered.

  • System integrity. This one single factor is by far the most important factor that distorts ship/shore reconciliations at the port where VEF development is considered. One has to ensure that the procedures are in place to fill the lines (line packing) as well as to verify physically that the lines are full.

    In the absence of established line fill and verification procedures, the ship/shore data used for VEF development may be suspect.

    Also, the integrity of all valves must be maintained to ensure no leakage or bypass of cargo to adjoining tanks. Emphasis now is on soft seated valves which maintain a good seating contact and offer reliable shut off.

  • Miscellaneous parameters. Temperature measurement and sampling procedures, as well as laboratory analysis procedures for the determination of density, must be in accordance with accepted industry standards. It is preferable to adopt the same procedures and analytical methods for all voyages under consideration.

    One should verify that the computational procedures for volume conversion to a standard temperature (e.g., 60 F.) are undertaken through use of the API/ASTM/IP petroleum measurement tables. The same volume correction factor tables should be applied for both the ship and the shore.

    It is further suggested that the same dimensional units be maintained in the initial phases of VEF development (i.e., for the first five voyages).

CONTROLLED SHIP/SHORE SURVEYS

For the first five voyages, it is strongly recommended that the ship/shore measurements, line-packing procedures, computational procedures, systems integrity, and review are undertaken by a specialized team of experts. This is similar to meter factor development which is normally accomplished with proving contractors and/or in house personnel specialized in proving systems.

The survey approach for the first voyage will eliminate any uncertainty or doubts relating to data integrity and ensure that the VEF subsequently developed will be indeed reliable. The survey methodology will also reduce human errors.

When all of the voyage data are available, the vessel's VEF may be computed using either the API guidelines or the alternative IP method. These are two differing, independent approaches and have their own characteristics (IP's basic field method is the same as the API method).

API AND IP METHODS

The API method is simply an averaging method with a set control limit of 0.3% above and below the mean value.1 That tolerance will eliminate all individual vessel ratios that fall outside the range.

On the other hand, the detailed IP method is based on a statistical approach using critical values for a given population to determine and eliminate outlying data. Subsequently, the mean value is computed for the rest of the population.

There is one distinct difference between the two methods. The limit of 0.3% in the API guidelines is based on vessel ratios computed on volume.

Furthermore, because of this preset limit, there is always the probability that some good data may, by simple averaging techniques, become outlying data.

The IP method does not have such preset limits because a population is examined statistically for its critical values. Hence, it is a rigorous method. The IP method requires some basic understanding of statistics.

OTHER CONSIDERATIONS

Some of the commonly encountered questions regarding VEF are:

  • What should one do if no voyage data exist?

  • Does one really need four or more voyages?

  • Does it make a difference if one combines ports where the vessel is loaded and ports where the vessel is unloaded?

Because VEF is a factor developed by comparing ship and shore figures for five voyages, in the absence of any existing data, one should start off with the very first voyage for VEF development under controlled conditions. The vessel ratio thus developed can be used immediately as the VEF for that vessel.

However, in order to ensure that this single or the first factor is indeed a reliable factor for initial use, one should follow all requirements and comply with procedures detailed in this article. Subsequently, as additional voyage data become available, leading to five or more voyages, actual VEF computation can be accomplished in accordance with the guidelines.

As far as the total number of voyages required, based on current guidelines, a recommended number between 5 and 10 is deemed acceptable and adequate. If more than 10 data points were to be used, the magnitude of random variations will be minimized.

However, with the larger data set, there is also a potential for a greater number of outlying data points. If reliable additional data are available, they can be used to update the VEF. In the opinion of the authors, a set of reliable, 5-voyage data is preferred over a data set of 15, 20, or more data points gathered with a lower level of control and precision.

Because VEF is dependent on residual random measurement variations and calibration procedures, as well as on product characteristics, it may be necessary to compute VEF at load and discharge ports. The control limits of 0.3% offer a wide range within which the VEF can vary depending on unique port measurement and product characteristics.

A VEF computed for each port may further identify residual measurement bias between ports (for a given product characteristic).

The key to development of VEF under any combination of ports and vessels is to ensure the implementation of procedures and measurement systems at each location in accordance with existing standards and guidelines with meticulous emphasis on the elimination of procedural errors, ensuring proper and up-to-date capacity tables and system integrity.

MASS AND VOLUME VEFS

Currently, VEF guidelines are also being considered for the development of VEF on a mass basis. It is important to appreciate the real difference between application of guidelines for VEF on a mass basis and VEF on a volume basis.

For mass-based VEF, one additional parameter, cargo density determination, plays an important part. Sampling and laboratory procedures introduce additional variability that could affect VEF.

Because API guidelines specify the preset limit of 0.3% based on volume data analysis, if the same limit were to be applied for mass without regard to sampling and analytical variations, it could result in potential distortions and loss of accuracy.

Because the IP method is a statistical manipulation of dimensionless numbers, the detailed IP method by itself does not discriminate between volume and mass ratios (nor is there any preset limit based on volume). Technically, the IP method can operate on volume or mass ratios without loss of accuracy. However, it is suggested that, as a general rule, volume and mass ratios not be combined.

It is also the opinion of the authors that further investigation and analyses are required before one could use any given method for the determination of VEF on mass basis.

VEF APPLICATIONS

VEF developed under controlled surveys and in accordance with guidelines on a volume basis, has immense significance for marine applications. Some of the most notable applications are:

  • More accurate on board volume determination for vessels (ships and barges) involved in custody transfer where systems warrant the use of vessel data (i.e., multiple movements into or from shore tanks preventing accurate assessment of delivered quantity).

  • Overall accuracy of ship and shore and ship and ship reconciliations.

  • Better technical basis for contract negotiations to establish ship and shore reconciliation criteria, and for acceptance of vessel quantity as a backup for custody transfers.

VEF is not a factor to compensate for procedural errors and errors associated with lack of system integrity. Developed properly, with emphasis on calibration and measurement procedures, it can provide a factor for enhancement of accuracy of volume determinations on board vessels. Furthermore, it is suggested that VEF should be developed on a volume basis.

The concepts for the development and application methodology presented in this article are based on the authors' interpretation of the industry guidelines and do not in any way, reflect the views of the individual companies or standards organizations. The subject matter is presented purely on its technical merit simply to promote awareness and a better understanding of the development and application of VEF.

ACKNOWLEDGMENT

Thanks go to Phil Baratta at the Exxon Bayonne marketing terminal for his valuable contributions in the preparation of this article.

REFERENCES

  1. API Chapter 17.1 addendum, "Guidelines for Marine Cargo Inspection-Vessel Experience Factor," American Petroleum Institute, Washington, D.C.

  2. Institute of Petroleum, Petroleum Measurement Manual, Part XVI, Procedures for Oil Cargo Measurements by Cargo Surveyors, Appendix C, Procedures for calculating Vessel Experience Factors, Institute of Petroleum, London.

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