Cargo Loss Prevention - What An Inspector Should do during Pre-Loading Surveys & Cargo Planning

Dear Petrol Inspector,

With regard to recent cargo shortages, I feel we should be enlightened more on how to take careful measures to ensure cargo losses are prevented as much as possible. We know what they say about "...prevention - being better than cure"

Claims for alleged shortages, after completion of discharge, are based on the difference between the bill of lading figures and the out-turn quantities. Even if both terminals (or STS vessels) carry out their measurements diligently, each will round off temperature and ullage readings in its favour, so differences must be expected, and extreme diligence is required by responsible personnel. In general the bill of lading quantity may be overstated and the out-turn quantity may be understated.

In the absence of other comparisons, the ship is the only common factor and, therefore, the measurements taken on board are critical. It is absolutely essential to the ship’s interests to ensure that all cargo inspections are carried out carefully and comprehensively. The cargo inspector must be accompanied at all times by the Chief Officer and his ullage measurements actively verified. 

Inspectors are as prone to error as any one else. Where more than one method of measurement is available use them for comparison and to confirm the accuracy of measurements made. This means that the vessel should have well calibrated UTI tapes and the relative accuracy of readings, if more than one tape is available, should be known.

The following steps in my experience can be taken to prevent incidents of petroleum cargo losses in future during Pre-Loading Surveys and then during Cargo planning.

Pre-Loading Surveys

1. In clean product trade, cargo tanks should normally be clean and dry for loading, unless loading without cleaning tanks has been agreed with the Operations Unit. BE AWARE THAT EVEN WHEN THE VESSEL IS ON A TIME CHARTER, THE OWNER REMAINS RESPONSIBLE FOR PRESENTING CLEAN TANKS FOR LOADING. HENCE TANK CLEANING INSTRUCTIONS FROM CHARTERERS SHOULD ALWAYS BE CONFIRMED WITH THE OWNERS.
2. Ensure that tanks have been gas-freed, mopped dry after water wash, and re-inerted (if vessel has inert gas or if that is part of the pre-loading instructions);
3. If any cargo tanks are not dry, determine the on board quantity (OBQ) of the previous cargo;
4. Prior to the OBQ survey all cargo line valves should be opened up to drain them and then closed.
5. Ensure that the vessel does not have any list and trim is allowed for during calculations.
6. Each tank should be dipped at the aft most dipping point, and the OBQ determined.

Cargo Planning

1. Obtain full details of the nominated cargo with reference to quantity, quality, carriage and discharge. If this is not available in the voyage orders, then ask for full details before the cargo is loaded.
2. Check and confirm that the vessel can load, carry and discharge the nominated cargo quantity safely, especially with regard to draft, trim and stability
3. To avoid large vapour losses, do not vent the cargo unnecessarily and ensure that the PV valves are in good condition and operate at designed pressures. These good practices should be followed, immaterial of the duration of the voyage.

You can comment on other preventive measures that you are aware of in your own experience.

Next time we shall dwell on "During Loading" and "After Loading"

Till then - 

...keep inspecting properly & professionally!

THE WEDGE FORMULA

Dear Petrol Inspector,

In my two decades or so of petroleum inspection, one of the daring challenges I have seen inspectors and Loss Adjusters face - most avoid, few face with trepidation is when (On Board Quantity – OBQ or Remaining On Board – ROB) on cargo tanks are so small that it is not captured by the MMC/UTI or the sounding tape or rod.

Most inspectors turn to the most popular, but now cliché "Unpumpable". They avoid the actual professional route to follow - and that is using the Wedge Formula.

What is Wedge Formula? 

Wedge Formula is a calculation to determine the small quantity of liquid or non-liquid cargo (On Board Quantity – OBQ or Remaining On Board – ROB) on cargo tanks. However, note that it should fulfil the following conditions:

• The vessel has huge trim, but there is no trim correction value available on the tank table,
• The liquid or cargo on tank is not touch one or more of the tank bulkheads,
• Observed sounding tanks at several points to ensure the cargo is not touch one of the tank bulkheads

See below diagram of a typical wedge condition:

In order to standardise the OBQ/ROB calculations on board the Crude Oil or white products carrying tanker vessels, the following geometric form of the Wedge Formula shall be used and this form of the formula assumes that the cargo tank is ‘box shaped’ with no internal ‘deadwood’ or pipeline systems, heating coils etc. which would impact the accuracy of the volume calculated from the sounding.

Furthermore this wedge formula calculation makes the enormous assumption that any ‘liquid’ found in a cargo tank is in the form of a regular wedge shape with its base at the aft bulkhead of the cargo tank.

It is obvious that such a series of assumptions normally can invalidate the absolute accuracy of the calculation immediately given, amongst other issues, the shape of the wing tanks (the turn of the bilge) and in particular those wing tanks at the fore and aft parts of the vessel.

The calculation method for the Geometric edition of the Wedge Formula: 

Assumption:

Given the small angle involved with the trim of the vessel, then the "Sine" of an angle can be considered as the same as the "Tangent"(Tan) of an angle and consequently: 

Step 1:
Correct the position of the sounding position with respect to the aft bulkhead of the cargo tank due to the trim of the vessel, distance = A
A = Tank Reference Height (Observed Height) x Tan X; 
    where X = the Trim angle of the vessel and;
           Tan X = (Aft draft – Forward draft) / Length Between Perpendiculars (L.B.P.) of the vessel.


Step 2: 
Determine the distance of the apex of the wedge from the aft bulkhead for obtaining information whether:

(1) Should a Wedge Formula be used at all (kindly note that a wedge formula is not applicable if:
     a. The liquid surface covers the total cargo tank bottom or the calculated apex of the wedge is at or          beyond the forward bulkhead of the cargo tank or:

     b. it is sludge ROB volumes only;

And

(2) Whether the wedge is a regular wedge (which can be checked by comparison with alternative soundings being taken).
       S = Observed Sounding;
       F (Distance of the apex of the wedge from the sounding position) = S x Tan X;
       E (Distance of the apex of the wedge to the aft bulkhead) = (F – A) + B;
          where B is the distance on deck from the point of sounding to the aft bulkhead.


Step 3:
Determine the depth of the wedge at the aft bulkhead of the cargo tank, depth = D;
D = E x Tan X


Step 4:
Knowing D (sounding depth at the aft bulkhead) and E (the distance from the aft bulkhead to the apex of the wedge), then the area of the longitudinal cross section of the wedge may be calculated,

Thus as the area of a triangle = (Base x Height) / 2 then;

(D x E) / 2 = cross sectional area of wedge.

Step 5:
Having obtained the cross sectional area of the wedge, the volume of the wedge is calculated by multiplication by the breadth of the cargo tank (please note that the breadth of the cargo tank should be measured at the bottom of the tank at the aft bulkhead position and not at deck level or elsewhere within the cargo tank).
Volume of theWedge = Cross sectional Area x Breadth of Tank

Throughout this calculation it is very important that all distances are in metres. Do not use centimetres for the observed sounding. 

Alternatives
Regardless above stated requirement, an I.S.O. standard method is also available in the event that any Cargo/Petroleum Inspector does not accept the geometric edition of the wedge formula. This method depends upon the accuracy of the vessel’s tank ullage calibration tables for the larger ullages / smaller soundings in the cargo tank. If the tank calibration tables are accurate for this region of the cargo tanks, then this method will give added accuracy to the general method of calculating tank residues after discharge.

This method is as follows:

Step 1: 
Calculate DA (the Corrected liquid sounding at the aft bulkhead position);
DA = D + {f(Y – (H x f))}
  where: D is the observed liquid sounding;
              f is the Trim factor ( TS / LS ); TS is the vessel’s trim, LS is the vessel’s Length Between                      Perpendiculars.;
             Y is the distance of the sounding point to the aft bulkhead;
             H is the reference height of the cargo tank;


Step 2: 
Calculate Ct (the Tank constant);
Ct = LS / ( 2 x TS x Lt ) (where Lt is the Length of the Cargo Tank).


Step 3: 
Calculate the ‘k‘ coefficient;
k = DA x Ct 
if k > 0.5 wedge is not required to be carried out;
if k = 0.5 wedge must be carried out.


Step 4:
if k > 0.5 then calculate the volume of the liquid contained in the cargo tank from the calibration tables using the Observed sounding, D, applying the trim corrections.


Step 5: 
if k = 0.5 then calculate DX (the wedge sounding).
DX = DA / 2


Step 6: 
Enter the cargo tank calibration tables with DX, without applying trim corrections to equivalent volume VO.


Step 7: 
Calculate the liquid wedge volume V1;
V1 = VO x 2 x k

In addition to above methods it should be noted that if the procedures as specified in the vessel’s COW manual are being followed for the determination of the ‘Dryness’ of a cargo tank, namely, the sounding of the residues in four (4) differing locations within the cargo tank, then the foregoing methods of calculations can be avoided.
Assuming the shape of the individual cargo tanks is fairly regular / constant in a fore and aft direction and, notwithstanding the fact that the vessel will be significantly trimmed by the stern, then the four measurements, as suggested in the COW Manual guidelines, as obtained by sounding can be used to calculate an average sounding so as to obtain a single sounding. The single average sounding can be used directly in order to obtain an equivalent volume from the vessel’s tank ullage calibration tables. Such a method will provide a clearer indication as to the type and nature of the residues on the cargo tank floor as well as provide much clearer indications as to the profile of the residues within the cargo tanks.

I understand that due to the technical mathematical computation involved in all the steps, it could be cumbersome - but there lies the difference between the truly professional inspector and the lazy charlatans.

Which are you?

Till next time -

....Keep inspecting properly & professionally!

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!

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

Dear Petrol Inspector, 

Today, let us look at 2 great ways to ensure petroleum product losses are minimised when discharging from vessel to shore tank farms. They are Line Pressing and Line Displacement.

Let us start with Line Pressing.

1. Line Pressing: Another common name for this is Line packing. This procedure assumes that the designated pipeline system is tight and able to withstand pressures applied during line press operations without loss of line pressure as determined by pressure readings from a calibrated pressure gauge from the vessel. This procedure is invalid with any pipeline system that does not meet this tightness recommendation. To apply the line press method (or the line pack method), perform the following steps consecutively. 

Step 1 - Close the valve at the shore tank. Open the vessel's manifold, COT tank (better just one) and pump valves, and gauge the tank before line press. Measurements may be taken using either reliable automatic gauging equipment or manual measurement equipment. 

Step 2 - Start the pump and run it until the discharge pressure stabilizes and/or reaches a predetermined pressure. The predetermined pressure should be higher than the maximum static pressure available on the system. 

Step 3 - Isolate the pipeline to prevent backflow and stop the pump. 

Step 4 - Once the pump has been shut down. record the pressure and re-gauge the COT using the same gauging method as that used for the opening measurements. Record the tank product level. 

Step 5 - If the tank product levels before and after the line press are within 1/8th inch (0.003 meters) of one another, pipelines may be considered liquid-full and no correction is necessary. 

Step 6 - If the COT liquid levels before and after the line press differ from one another by more than 1/8th inch (0.003 meters), relieve the line pressure into the tank until the pressure at the highest elevation is slightly above product vapour pressure and then repeat the test. If the tank product levels before and after the second line press differ from one another by less than 1/8th inch (0.003 meters), pipelines are now full of liquid because condensible vapours have been re-liquified and no further correction is necessary. 

Step 7 - If the tank liquid levels before and after the second line press differ from one another by more than 1/8th inch (0.003 meters), either the line fill condition may be corrected or - with the agreement of all parties - gas volumes may be estimated by procedures and calculations. 

Step 8 - For purposes of line fill verification, the designated pipeline system may be considered full if the difference between the volume gauged before line press and the volume gauged after line press is within measurement precision. Note: This does not ensure that voids equal to compressed gas volumes less than measurement precision do not exist in the line.

Now let me give you time to digest this. We shall talk about the Line Displacement tomorrow. 

Till then - Keep Inspecting properly!

We are Back - Bigger and Better!

Hi Fellow Inspectors, We have been away too long The Petroleum Inspector ought to be an expert in measurement and monitoring of petroleum products (like PMS, DPK, Crude oil etc) either being transferred via STS or ship to shore or vice versa. He/she is in control of millions of dollars worth of product, but most times paid peanuts. One of the reasons this happens is that the Surveyor is either poorly experienced or is not a professional. Over time, we shall be getting infos on various ways to get more professional and enhance avenues for Surveyors or prospective surveyors to get increased experience. So welcome back to this blog as we sail into proper petroleum products monitoring, in other to make good money!