Crude Oil or Product Tanker Calculations

Open Innage/Ullage Equipment An innage tape shall only be used with an appropriate innage bob.  An ullage tape shall only be used with an ullage bob.  The measurement units on the innage/ullage equipment used should be consistent with the measurement units in the vessel's capacity tables; that is, the equipment should be graduated in 1-mm, 1/8^th-in, or 0.01-ft increments

Water Gauge Bars  A water gauge bar may be used to assist in determining the height of free water in a tank. Typically a water gauge bar is a 12-in. (30-cm) or 18-in. (45-cm) round bar.  The longer length of these bars can reduce the incidence of water cuts occurring on clasps and areas not scaled between the tape and bob.  A square bob is not recommended because the corners on the bar may cause dips and slants to occur on the paste, resulting in false readings.

Indicating (Product/Water) Paste Indicating pastes can be used to determine the levels of water in a tank or assist in the reading of product levels.  The pastes should be applied and used according to the manufacture's specifications and the shelf life should be noted before each use.

Inspection of Innage/Ullage Equipment Before a tape is used, it should be checked for breaks, kinks, and illegible markings.  The tape hook should be inspected for wear and distortion.  Innage bobs should be inspected for wear and damage of the tip and eye hole.  The bob/tape interface should be measured to determine accuracy.  If these inspections indicate any inaccuracies, the equipments should not be used. (API MPMS Chapter 3.1A has more information for tape and bob accuracy requirements.)

Tank Capacity Tables

Tank capacity tables show the volume corresponding to each measured innage or ullage.  Measurements should be taken in the same units used in the capacity tables.  If measurements must be taken in other units, the conversion factors should be used.

When tank capacity tables are not calculated to the minimum prescribed graduations (1/8^th in., 1 mm, or 0.01 ft), and when gauge readings fall between the values in the tables, interpolation will be necessary.

The observed reference height should be compared with the reference height given in the tables and recorded.

Procedures for Open Manual Gauging Manual gauging consists of either innage or ullage gauging of the liquid level with appropriate gauging equipment.  Considerations in determining whether innages or ullages are to be taken are whether the capacity tables are presented in inn age or ullage format, the amount of liquid in the tanks and the nature of the material.  Measurements should be taken in the units in which the tank capacity tables are presented. 

When taking custody transfer measurements, all cargo tanks, ballast tanks, bunker tanks, void spaces and cofferdams should be inspected and/or gauged as appropriate.  Measurements should always be taken at the reference points noted in the capacity tables. The reference height should be stenciled or otherwise permanently marked near the gauge point. The observed reference height should equal the reference height of the tank. If it does not, the procedures described in Appendix B.3, should be followed.  An accurate permanent record of the gauge measurements should be made when the readings are taken.

It may be noted that some tanks and void spaces on the vessel may not have reference heights.  Also, in some instances, it may be necessary to take gauges of water, ROB, OBQ, etc. at points other than those designated as official gauge points.  In those cases, full details of such gauging must be noted in the gauger's log and on the appropriate cargo measurement documents.  If foam is present on the surface of the liquid, no gauge should be taken until the foam has subsided or been cleared from the surface of the liquid beneath the gauging hatch.  Before a tank is gauged, time should be allowed to permit the oil to free itself of entrained air, gas, and water.  When the surface of the oil is at rest, at least two identical readings should be obtained before a measurement is recorded.  Best accuracy is usually obtained when oil is motionless in the tanks. Measurement procedures when the liquid in the tank is in motion, as occurs when a vessel is pitching and rolling, as described in MPMS 9.3 should be followed.

Measurement procedures to be followed when the vessel is out of trim are described in MPMS 9.7. In all cases, trim and list should be recorded to determine the necessary corrections to be applied.

Open Ullage Procedure

Ullage measurement is the determination of the distance from the gauge point to the surface of the material being measured.  This may be accomplished by using ullage or innage tape and bob combination.

Using an Ullage Tape and Bob

Ullage measurement using ullage equipment should be conducted as follows:

1. Obtain approximate tank ullage using vessel ATG or other method.

2. After safely grounding the tape and opening the gauge hatch, slowly lower the tape and bob into the tank until the bob just touches the surface of the liquid (see Figure 6).

3. After the bob bas stopped swinging, lower the tape slowly until a small portion of the bob is in the liquid and an even inch, centimeter, or hundredth of a foot graduation on the tape is at the reference gauge point.

4. Record the tape reading at the reference point.

5. Withdraw the tape from the tank and read the ullage bob scale at the liquid cut and record the-reading. Care should be exercised during the withdrawal procedure to ensure that the tape and bob are not allowed to re-enter the liquid and thereby give a false reading.

6. The sum of the tape readings at the reference gauge point and the ullage bob reading at the cut is the ullage gauge (see Table 1).

Using an Innage Tape and Bob

When innage equipment is used to take an ullage, the procedure in the previous section should be followed except the bob reading must be subtracted from the tape reading. See the example in Table 1.

Alternative Ullage Procedure

An innage gauge may be converted to an ullage gauge by subtracting the innage from the reference height shown on the capacity table.

Open Innage Procedure

Innage measurement is the determination of the height of the liquid in the tank. This may be accomplished by using ullage or innage tape and bob combination and may be a direct reading or calculated value.

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