Crude Oil or Product Tanker Calculations

Survey Documentation -  Determining Fullness of Pipelines between vessels and shore tanks

Vessel Requirements

The capacity of the vessel's lines designated for the line displacement operation should be available for reference.  Vessel personnel should also indicate whether the designated lines are full or empty.

The condition of the vessel's lines will have a direct effect on the accuracy of the line displacement between vessel and shore.  Vessel personnel should provide every assistance to accurately determine the fullness condition of its lines.

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Procedures

GENERAL PROCEDURES

Before loading or discharging marine cargoes, the following information should be obtained, recorded, and agreed upon to ensure that procedures to determine the fullness condition are as thorough as possible:

1. The capacity of the designated pipelines.

2. The stated condition of the designated pipelines (full, slack, or empty).

3. The time of last cargo movement through the designated pipeline(s).

4. The last (or current) product in the designated pipeline(s).

5. The temperature of cargo to be loaded or discharged.

6. The sources of all the above information.

Agree on the method of pipeline fullness verification to be used and on the extent - if any - of corrective actions that may be required.

If automatic equipment is used to record measurements, it must meet the precision requirements described in API MPMS 3.1A and API MPMS 7.

Review measurement accuracy limits outlined in API MPMS 3.1A, and agree on the tolerance to be met for the method selected.

The procedures described in API MPMS 3.1 A and API MPMS 17.2 for selecting, calibrating, and using measurement equipment should be strictly followed.

PROCEDURE FOR INTERNAL CIRCULATION METHOD

Terminal personnel should circulate product through the designated pipeline system after the system has been isolated.  Circulation should pass through the designated dock manifold and be performed immediately before and after custody transfer. If circulation cannot pass through the designated dock manifold, then an evaluation should be made of the validity and accuracy of using this method.

To apply the internal circulation method, per- form the following steps consecutively.

Step 1 - Gauge the delivering and receiving tanks before line circulation.  Measurements should be taken using reliable automatic gauging equipment or manual measurement equipment.

Step 2 - Circulate sufficient product to displace at least 120 percent of the capacity of the designated pipeline system.  Close tank valves and re-gauge tank(s) using the same method as that used for opening gauges.

Step 3 - Using the tank calibration table, convert the measurements taken before and after internal circulation to volumes.  Total Observed Volume (TOV) should be used to quantify volumes transferred.

Step 4 - Compare measured volume delivered to measured volume received to determine whether their difference exceeds the agreed tolerance.  If the difference exceeds agreed tolerance, repeat line circulation and the measurements.  If the excessive difference persists, the integrity of the system should be investigated.

Step 5 - For purposes of line fill determination, the designated pipeline system may be considered full if the difference between measured volume delivered and measured volume received is within the agreed tolerance.

Note: This does not necessarily ensure that voids do not exist in the line.

PROCEDURES FOR LINE DISPLACEMENT METHOD

The vessel should be on an even keel with no list.  Otherwise, a trim or list correction should be applied and noted on the report.  If a wedge calculation is necessary, refer to API MPMS 17.1.

Note: Even-keel measurements are preferred because of the precision limitations involved in trim, list and wedge calculations.

The vessel quantity survey should be conducted prior to line displacement.

The number of vessel tanks used for line displacement should be minimized, and their location should be selected to minimize changes in trim or list.

Deck lines, risers, bottom lines, and drop lines should be in the same fill condition both before and after line displacement between vessel and shore.

Agree on the quantity to be displaced.  TOV of product displaced should be at least 120 percent of the combined capacity of all designated vessel and shore transfer lines. (definition of TOV.)

Agree on the cargo flow rate.

Line fill differences should not be addressed independently of the overall voyage analysis of the entire cargo transfer.

To apply the line displacement method, perform the following steps consecutively

Step 1 - Gauge and measure the temperature of delivering and receiving tanks before line displacement.  Measurements may be taken using either automatic measurement equipment or manual measurement equipment.

Step 2 - Transfer the volume required for displacement between vessel and shore.  Re-gauge the vessel and shore tanks using the same types of equipment and technique that were used for the opening measurements.

Step 3 - Using shore and vessel tank calibration tables, convert the measurements taken before and after line displacement to volumes.  TOV should be used to quantify volumes transferred.

Step 4 - Compare measured volumes of delivering and receiving tank(s) to determine whether their difference exceeds the agreed tolerance.  If vessel and shore volumes differ by more than the agreed tolerance, any or all of the following options may be exercised until all parties agree that line fullness has been determined to their satisfaction:

1. Check all calculations for accuracy.

2. Re-gauge shore tank (or verify its meter reading) and re-gauge designated vessel tank(s).

3. Re-gauge all vessel tanks and reconfirm vessel line condition.

4. Repeat the line displacement procedure.

5. Proceed with cargo transfer and determine after transfer whether the line fill difference has had any unacceptable effect on the cargo transfer volume.  The initial line displacement volume will normally be part of the cargo transfer and must therefore be accounted for in the total transferred quantity.

 Step 5 - For purposes of line fill determination, the designated pipeline system may be considered full if the difference between measured volume delivered and measured volume received is within the agreed tolerance.

Note: This does not necessarily ensure that voids do not exist in the line.

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