Crude Oil or Product Tanker Calculations

Alternative Innage Procedure

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

Open Free-Water Measurement

The use of water-indicating paste in conjunction with innage or ullage procedures provides a measurement of the free water in a vessel's tanks. The recommended procedure for free-water gauging is by the innage method. If the level of the water being measured is high enough to show a cut on or above the tape clip, a larger gauge bar should be used. How- ever, if measurement conditions dictate, it may be necessary to utilize the ullage method or other methods as agreed upon by all the parties. For measurement under adverse conditions.

Vessel tanks should be gauged for free water using water- indicating paste or other equipment agreed upon by the par- ties involved. Measurements should be taken independently of any other innage or ullage measurements and should be properly recorded. Free water should be measured at both the loading and the discharging port.

Using an Innage Tape and Bob To Measure Free Water

1. Apply the water-finding paste on the bob or bar sufficiently high to measure the anticipated level of water (see note 2).

2. After grounding, .the innage tape and bob should be lowered into the tank until the bob is a short distance from the bottom, as determined by the tape reading at the reference point.

3. The tape should then be unwound slowly until the tip of the bob just touches the bottom or datum plate. If the tape is lowered too far, the bob will tilt and an incorrect gauge will be obtained.

4. Once the bob touches bottom, keep it there long enough for the paste to react to the water (see note 3).

5. Withdraw the tape and read and record the highest, clearly defined water cut (see notes 4 and 6).

6. Repeat steps a through e until two identical readings are obtained.

7. Record the cut as "clearly defined," "speckled," or "slightly discolored." Measurement of free water on vessels that are out of trim is addressed in MPMS section 9.7.

Note 1: There are many brands of water-indicating pastes available that change color on contact with free water. It should be noted, however, that all brands may not react the same in the presence of water. Accordingly, the following qualities should be known before selecting a water paste:

1. Clarity of color change.

2. Ability to "shed" oil.

3. Shelf life.

4. Ease of application to the bar and ability to "grip" the bar.

5. Dense enough not to wash off when passing through the oil.

Note 2:   It is recommended that two different pastes be applied on the bar for each free water innage gauge at the beginning of gauging. After it has been established which paste yields the highest, continuous clear water cut, the other can be discontinued.  'When applying the two pastes to the bar, cover a little less than one-half of the entire surface of the round bar with each paste.  Make sure that the measurement scale remains free of paste. The coating of paste on the bar should be thin but opaque.

Note 3:   Allow the paste-coated bar to remain in the gauging position for a minimum of ten seconds for gasoline, kerosene, and similar light products, and one-to-five minutes for heavy, viscous products (or as otherwise specified by the manufacturer).  This amount of time is required to shed the petroleum that adheres to the paste. in heavy viscous petroleum, apply an even film of light lubricating oil over the paste to facilitate the shedding of the petroleum from the paste (see 9.1).

Note 4:   When the bob or bar is removed to read the water cut, do not blow or wipe the petroleum off the paste as this may distort the clarity of the water cut. If the water cut is obscured by the petroleum (black oils), wash the surface of the paste with a suitable solvent. The solvent should be poured or lightly sprayed on the paste-covered bar well above the anticipated cut and allowed to rinse down over the cut area. Pouring directly on the paste may distort the clarity of the water cut.

Note 5: Wipe the bar clean after gauging each tank and re-apply paste before gauging subsequent tanks.

Note 6:   If the paste on one side is spotted or lower than the other, record the highest level reading as the official measurement of free water level.  Oil adhesion may cause low readings, but will not cause high readings.  Spotting may indicate a layer of emulsified oil and water or that the product did not completely shed off the paste.

If water cuts indicate that an emulsion layer may be present, read and record both the clear cut and the height of the spotting measurement.

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