LINE PRESSING & LINE DISPLACEMENT - 2 WAYS TO ENSURE PRODUCT LOSSES ARE MINIMISED.....Part 2

Dear Petrol Inspector,

Yesterday we started a series of 2 great ways to ensure petroleum product losses are minimised when discharging from vessel to shore tank farms.

We dwelt on Line pressing (packing) yesterday and today we shall deal on Line Displacement.

Ideally, this procedure is carried out when:

1. Integrity of the line in terms of pressure check is not guaranteed;
2. Line contains a different grade of product;
3. Line contains water;

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, then apply.

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 or duration for the displacement.

• It has been observed from experience that most inspectors will say "...discharge for 1hr". However, shore facility distances vary - hence quantity and duration for displacement will vary.

• TOV of product displaced should be at least 120 percent of the combined capacity of all designated vessel and shore transfer lines. 

• 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:

- Check all calculations for accuracy.

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

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

- Repeat the line displacement procedure.

• 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.

Till then - Keep inspecting properly!