Srini Sivaraman
Exxon Research & Engineering Co.
Florham Park, N.J.
Richard I. Wallace
Maunder Wallace Associates
Bebington, Wirral, U.K.
The electro optical distance ranging (EODR) method offers state-of-the-art technology for internal calibration of vertical, cylindrical tanks. The method provides a safe, accurate alternative to manual strapping methods.
With EODR, calibration can be achieved from ground level. The capacity of each course is determined from the measured average radiuses, taking into account corrections for such factors as deadwood.
EODR technology lends itself to the total automation of both field application and the development of capacity tables. It has been used in commercial applications in the U.K., France, Israel, Japan, and other countries.
An international standard on this method is under development.1
BACKGROUND
The manual method of calibrating vertical, cylindrical tanks (often referred to as the "strapping method") has been the primary calibration method since the turn of the century. 2-4 More recently, optical technologies have gained a wide degree of acceptance and commercial application.
The optical reference line (ORL) method provided the first major change in the application of new concepts to tank calibration.5-7 In this method, however, it is necessary to move the magnetic trolley, which carries the measuring rule, up and down the tank height.
Such a procedure poses some inherent limitations in its application under varying conditions. In other words the ORL method requires a clear, unobstructed, vertical path for the trolley.
The optical triangulation method is another technology that accomplishes calibration from ground level.8 The tank is traversed vertically and horizontally through optical methods, and the tank profile is established by triangulation.
The EODR method is an extension of the triangulation method, adding to the procedure measurement of the slope distance (Fig. 1). EODR lends itself to total automation and computer applications. Calibration is achieved from ground level using a single EODR instrument.
EODR METHODOLOGY
EODR methodology is an extension of the standard triangulation measurement techniques, but with the added advantage of measuring the distance between the instrument and the target point, referred to as the "slope distance." Measurements are made from a firm, stable point, close to the center of the tank.
The tank is traversed horizontally and vertically by optical means. Slope distances (D) and relative angular dispositions (O and o) are measured in relation to a reference target point (Fig. 1). These measurements are subsequently converted to appropriate course radiuses.
PROCEDURE
The calibration method requires that the EODR instrument be set up very near the center of the tank. The instrument is set on a firm base, leveled, switched on, and allowed to warm-up.
The instrument is checked by repeated measurement of angles and distances to two reference target points clearly marked on the tank shell, preferably some 100 GON (90) apart and on the same horizontal plane as the instrument (Fig. 1). The check measurements must agree. Once the repeated measurements agree, the calibration may proceed.
Discrete target points are positioned on the tank shell wall on two imaginary horizontal sections on each of the courses:
- One situated between 20% and 25% above the lower seam
- One situated between 20% and 25% below the upper seam (Fig. 1).
The target points are evenly spaced on each horizontal section.
The minimum number of points required on each horizontal section for any given tank circumference is given in Table 1.
Using the EODR instrument, both horizontal and vertical angles and the slope distance to each target point are measured and recorded. Once the measurements to the target points have been completed, the instrument is again checked by taking measurements to the two reference target points.
The repeated check measurements must agree with the original measurements to the reference target points.
The measurements taken in this manner will define the shape of the tank shell. Other measurements necessary to complete the calibration-such as tank bottom calibration, measurement of overall dip height, and items of "deadwood"-are measured as described in existing standards.
The EODR instrument may be used to calibrate the tank bottom, the simplest method being to use the theodolite capability of the instrument to establish a horizontal plane from which the undulations of the bottom can be measured.
CAPACITY TABLE
The reduction of each set of measurements to a radius is carried out by using the following formulas, which calculate the horizontal distance from the vertical line through the instrument to each point:
[SEE FORMULA]
The X and Y coordinates are subsequently used to compute average radiuses of the courses at each plane of measurement using standard regression analysis techniques.
Having established the average radiuses, the capacity of each course is calculated in exactly the same manner as that utilized in the strapping method. 2-4
Other corrections to capacity for the effects of such factors as hydrostatic head (liquid head), shell temperature expansion, deadwood, floating roof, and tilt, are applied during computation as in existing standards.
APPLICATIONS
One major field application of EODR is, of course, the internal calibration of vertical, cylindrical storage tanks or containers. This requires measurement to a large number of target points.
Another area to which EODR techniques would be applicable is the calibration of ship tanks. The standard methods, issued by International Standards Organization (ISO) and American Petroleum Institute/Institute of Petroleum, rely on direct tape measurements or the application of triangulation techniques using theodolites.
Adaptation of EODR methods to ship tank calibration will require considerable effort, but may assist in the elimination or reduction of one more area of uncertainty in the movement of bulk liquids around the world.
ADVANTAGES
Present calculation methods follow the dictates of the strapping method, resulting in the calculation of average volume per unit height for each course of the tank.
Use of the strapping method entails using either lifting chairs or lifting equipment, such as "cherry pickers," or erecting scaffolding to enable the calibrators to reach the correct measurement positions.
The EODR method is a ground-based system requiring the services of the calibrators only, eliminating any requirement for other personnel to assist in the operation.
This eliminates some, if not most, of the "add on" costs of calibration.
In addition to the elimination of these costs, the EODR method removes the difficulties experienced in the strapping method, including the maintenance of true and perfect contact between the strapping tape and the tank shell, particularly at higher elevations.
This problem can create random errors in strapping method measurements. These errors will be quite significant at higher elevations, giving rise to some of the uncertainty in strapping method results. The EODR method does not rely on physical contact and will minimize, if not eliminate, such errors.
Furthermore, it consistently verifies the stability and position of the instrument, as the method requires that measurements of slope distance and angles to the reference points be made upon completion of each series of measurements on a plate course (in addition to the initial measurements of these values).
In other words, verification takes place twice on each course.
When compared to the ORL method, the advantages of EODR are apparent, at the present stage of development, in the method's ability to calibrate tanks that have wide wind girders or side-mounted spiral staircases, which cause problems in ORL measurement. Other advantages will become apparent when the EODR method reaches a higher stage of development.
The main advantage of the EODR method over the standardized triangulation methods is that it measures the slope distance as well as angular information.
Measurement of the slope distance gives EODR methods a built-in check system in that the calibration results provide information that allows calculation of the container capacity by two different routes: using only angular measurements and using both angular and slope distance measurements. Apart from its inherent accuracy, this is probably one of the method's most advantageous points.
While it must be admitted that EODR and ORL rely on the same information data base, the calculation procedures differ to the extent that they provide the means of checking the calibration and assessing the uncertainty that attaches to it.
The EODR method is capable of providing information that allows the tank shell to be profiled. This, however, requires that more measurements be taken than indicated by the forthcoming standard.
This capability can be of great use when surveying tanks equipped with floating roofs or covers. Distortions in the tank shell are identified and may be removed or due allowance made for their presence.
In the future, this capability may lead to a revision of the calculation method and a more accurate capacity table.
PROSPECTS
The EODR method will, with adaptations, permit external calibration. It is hoped that a further standard dealing with the external method of calibration of vertical, cylindrical tanks will follow.
The accuracy of the method is dependent on the accuracy of the EODR instrument.
But advances in technology are being announced, and the accuracy of EODR instruments now available will be surpassed in the future.
Automatic measurement procedures and "on board" data logging facilities will allow many more measurements to be taken more easily, thus improving the information available from the calibration and reducing the uncertainty of the calibration table.
At present, available EODR instrumentation can include the electronic data-logging capability. With this, the instrument can record the various measurements made.
In its simplest form, the instrument's data logger memory can be down-loaded to a computing facility.
But future plans-depending on the further miniaturization of the electronic circuitry within a computer-may include an instrument setup capable of not only recording the measurements, but also carrying out the calculation and producing a tank calibration table "on site."
The system currently being standardized is an excellent method of calibration, offering safety in operation and a final calibration table that will equal, in uncertainty, any produced by other calibration methods.
The future of the system lies in its application to any shape of container. In its present stage of development, it basically follows the dictates of the established calibration methods.
It provides a method of establishing the parameters from which a vertical, cylindrical tank's calibration table may be computed. It is also, however, capable of providing information that can be used to define the actual shape of the tank, if sufficient measurements are taken.
This can have financial benefits for tank operators.
Eventually it may become the primary method of calibration, once its capabilities have been developed to the stage the authors believe possible.
ACKNOWLEDGMENT
This article is based on a draft standard prepared by ISO Technical Committee 28, subcommittee 3, Working Group 1 (ISO TC28/SC3/ WG1).
Acknowledgments are extended to the following members of the working group who have actively participated in the development of the draft standard: Dr. Baubinder (Austria), M. Claude Favre and M. P. Canavaggio (France), A. Sakai (Japan), E. Guicciardi (Italy), A. J. Nederlof (The Netherlands), R. I. Wallace (U.K.), and S. Sivaraman [Convenor] (U.S.).
REFERENCES
- International Standards Organization, ISO/CD 7505-4 Petroleum and petroleum products, Calibration of vertical cylindrical tanks, Part 4, Electro-optical Distance Ranging Method.
- International Standards Organization, ISO/CD 7507-1 Petroleum and petroleum products, Calibration of vertical cylindrical tanks, Part 1, Strapping method.
- American Petroleum Institute, API Standards 2550, Measurement and Calibration of Upright Cylindrical Tanks.
- Institute of Petroleum, Petroleum Measurement Manual (PPM), Part 11, Tank Calibration, Section 1, Vertical Cylindrical Tanks, Measurement Methods.
- International Standards Organization, ISO 7507-2, Petroleum and petroleum products, Calibration of vertical cylindrical tanks, Part 2, Optical Reference Line method.
- American Petroleum Institute, Manual of Petroleum Measurement, Chapter 2, Tank Calibration, 2.2B, Calibration of Vertical Cylindrical Tanks using the Optical Reference Line Method.
- Institute of Petroleum, PMM, Part 11, Tank Calibration, Section 7, Vertical Cylindrical Tanks, Optical Reference Line Method.
- International Standards Organization, ISO 7507-3, Petroleum and petroleum products, Calibration of vertical cylindrical tanks, Part 3, Optical Triangulation.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
