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Determination of Quantity During Custody Transfer at FSO & FPSO Facilities

Let’s start with a quick glossary:-

  1. FSO : Floating, Storage & Offloading
  2. FPSO : Floating, Processing, Storage & Offloading
  3. Hydrocarbon : referring to crude oil and condensate
  4. Repeatability : tolerance for the percentage of variance between the initial and the next meter run (normally around 0.025%)


In hydrocarbon custody transfer at FSO and/or FPSO facilities, quantities delivered are determined from 3 sources namely, FSO/FPSO metering quantity, tank dipping and export tanker tank dipping (in the order of priority).

Normally, the FSO/FPSO metering quantity will be used as the Bill of Lading (BL) quantity, while FSO/FPSO tank dipping and export tanker tank dipping are used as figure comparison only.

However, in certain circumstances, the meter quantity could be declared as ‘null’ and ‘void’, and if this happens, then the FSO/FPSO tank dipping quantity would be taken for the BL figure.


The metering system usually include:

  1. Meter stream of 3-5 meter runs (could be more);
  2. Bi-directional ball type proper loop (pipe prover)
  3. An automatic sampling system (usually either flow-proportionate or time-proportionate sample grabbing system)


Typical meter used maybe Turbine or Positive Displacement (PD) meter where electrical pulses are generated to determine the quantity of liquid transferred. These electrical pulse are then ‘translated’ to volume by using the pre-set K-factor (pulse/KL).

Liquid Turbine meter measurement combines the turbine and electronics to measure total flow (or flow rate) within a piping system. Turbine meter is a volumetric measurement device which function by sensing the linear velocity of the fluid passing through the known cross sectional area of the meter housing to determine the volumetric flow rate. The fluid, as it passes through the meter, imparts an angular velocity (RPM) to the rotor, which is proportional to the linear velocity of the flowing fluid. Since the linear velocity of the flowing fluid thru a given area is directly proportional to the volumetric flow rate, it follow the speed of rotation of the rotor is directly proportional to the volumetric rate.

Positive displacement meter is a type of meter that requires the fluid being measured to mechanically displace components in the meter in order for any fluid flow to occur. A basic analogy would be holding a bucket below a tap, filling it to a set level, then quickly replacing it with another bucket, and timing the rate at which the buckets are filled or the total number of buckets for the totalized flow.


Due to flow dynamics and also different conditions during custody transfer (e.g. flow rate, temperature, pressure), meters may not be able to compute the quantity transfer with accuracy. Therefore, in order to reduce these errors, meters have to be proved using the prover loop.

This is done by allowing liquid to flow in the prover loop and after 1 complete run; the quantity computed by meter would then be compared with the calibrated prover loop volume which is know as PROVER BASE VOLUME.

Simple example:

(A) Calibrated Prover Loop volume (Prover Base Volume): 3.2916241 KL

(B) Meter computed volume: 3.2946184 KL

As such the meter correction factor woul be : A/B, which is 3.2916241 over 3.2946184 = 0.999091

Note that:

  • meter volume corrected for temperature and pressure
  • prover volume corrected for temperature and pressure with the correction for temperature and pressure for steel included.

The meter correction factor is similar to that of the vessel experience factor (V.E.F) where the factors maybe a ‘gain’ or ‘loss’.

There are 2 scenario for computation of meter factors

  1. In each run and after applying the necessary corrections, the meter factors are calculated. After 5 consecutive acceptable runs (REPEATABILITY), the 5 meter factors are average off to obtain an average meter factor for that particular meter/stream.
  2. The corrected prover base volume is then divided by the corrected/average meter volume from the 5 consecutive acceptable runs (REPEATABILITY).

Additional Note (meter proving)

  • 5 acceptable run (as per ASTM & SIRIM (Malaysia) requirement)
  • to ensure meter performance are within allowable tolerance (normally 0.025%)
  • If meter cannot be proved, used the last proven meter factor.
    • valid for 3 consecutive lifting/custody transfer per meter/stream
    • similar flow rate, temperature, & pressure are used.
  • Prover loop are required for annual calibration.

Meter factor = Prover base volume / meter’s indicated volume.


Normally a flow proportional type (may also be time proportional) automatic sampling system is included in the crude custody transfer metering system. The automatic sampler will be program to automatically ‘grab’ samples throughout the loading operation. Samples collected would then be thoroughly mixed by circulating the samples inside the sampler pot/can with an electrical pump/mixer.

Samples collected would then be distributed to relevant parties (as per buyer request) in sealed can and also be analyze for density (ASTM D 1298) and BS&W (ASTM 4007). The results of the analysis will be used for computation of the metered quantity.

Some terminal may adopt a certain procedure for the amount of sample collected by the auto inline sampler whereby the programmed / requested quantity should not be;

  • less than 80% of the sample amount requested
  • more than 120% of the sample amount requested.

If the amount collected exceeded the minimum or maximum quantity, then the auto inline sample is rejected and the hourly sample or tank fiscalized sample will be used for quality determination.

With all the data generated from the metering system, the meter quantity is then computed automatically by the meter and a meter delivery report is produced (maybe also referred to as Batch Report).


If available, counter check all data in the generated delivery report with the stream’s microcomputers. All data should be the same. If some data differs, may have to be re-compute metered quantity manually using the final data from the stream microcomputers.

There are typically 2 types of cases for delivery report.

CASE 1: 


Based on the sample above, used streams are named MT93B & MT93C.

(a) Manually calculated individual stream calculation to confirm the total stream gross volume.

(b) check for the batch flow weighted average temperature as follows.

X = (Stream MT93B × FWAT_Stream_MT93B) / Total Stream Gross Volume.

Y = (Stream MT93C × FWAT_Stream_MT93C) / Total Stream Gross Volume.

Therefore, X + Y = Batch Flow Weighted Average Temperature.

Note: Only for the utilized stream.

X = 26, 456. 041 (41.8) / 51, 125.838 = 21.630208

Y = 24, 669. 797 (41.8) / 51, 125.838 = 20.169792

FWAT = 21.6+20.8 = 41.8 Deg C.

(c) check for the batch flow weighted average pressure as follows.

X = (Stream MT93B × FWAP_Stream_MT93B) / Total Stream Gross Volume.

Y = (Stream MT93C × FWAP_Stream_MT93C) / Total Stream Gross Volume.

Note: Only for the utilized stream.

X = 26, 456. 041 (278) / 51, 125.838 = 143.856

Y = 24, 669. 797 (270) / 51, 125.838 = 130.283

Therefore X+Y = batch flow weighted average pressure

FWAP = 144+130 = 274 KPAG

(d) Check for correct meter factors input.

(e) Check the correct density, API @ 60 º F and BS&W input.

(f) Convert Total Batch Gross Volume to Gross Standard Volume as follows.

=(Batch Gross Volume × Batch V.C.F) / (1 – Batch compressibility factor × FWAP)

*However, please make sure step (c) are correct and same as per batch report since the total batch standard volume might be a bit difference due to metering system decimal error.

**if step (c) are correct, step (f) should also be correct.



(a) Check that correct meter factors input

(b) Calculate all streams gross standard volume individually as follows:-

Batch Finish – Batch Start = Batch Gross Volume × Meter Factor × VCF × CPL 

(c) Add all gross standard volume for all streams = Total Gross Standard Volume


  • Gross meter versus tank volume comparison is out of normal limits and reason to doubt meter integrity. (usually set at 0.5% meter and shore tank).
  • if all stream malfunction as follows:
    • unable to prove meter figure for (3 consecutive times, utilizing MF for 3 consecutive lifting.).
    • Significant leak at stream inlet or prover outlet.
    • the stream microcomputer malfunction due to presence of significance computer alarms which could affect the integrity of the volume.
    • any malfunction of the stream instrument (e.g. Pressure transmitter (PT) or Temperature Transmitter (TT))

If there are only 3 streams available and 2 of them are malfunction (as per above condition), then the meter are deem invalid as it is unable to load with 1 stream due to flow rate limitation (low).


For crude custody transfer, the nominated tanks are gauged ( including temperature and free water ‘cut’) and sampled.

The tasks conducted before crude custody transfer as follow:

(a) Gauging


Above are some of UTImeter used for Marine terminal gauging.

Gauging of nominated tanks is conducted by utilizing a sonic tape/ullage interface tape (MMC/UTI brand). Before gauging, the sensor probe is place with water finding paste to obtain free water ‘cut’, if any. Ullages are taken and average off, however, this is much dependent on the sea condition.

Temperature is taken using the same ullage interface tape and is taken according to the following procedure:

Level – 

0 – 3 meter = 1 point (middle)

3 – 5 meter = 2 points (top & bottom)

Above 5 meter = 3 points (top, middle, bottom)

*Note: Top – 1 meter below liquid level, Middle – middle of liquid level, bottom – 1 meter above the datum plate / tank bottom.

After confirming the ullages and temperatures, slowly allow the sensor probe/bob to touch bottom tank and immersed the probe/bob with water finding paste as follow:

  • Light product – immersion period between 5 – 10 seconds
  • Black/Heavy product – immersion period between 20-30 seconds.

Then, wound up the tape and inspect the probe/bob for any color changes i.e. from orange to red (indicates water ‘cut’).

(b) Sampling


Sampling in crude tank is conducted using a portable sample probe with detached reciprocating type mini manual pump.

Sample is taken using the portable sampler probe and is taken according to the following procedure:


0 – 3 meter : 1 Level (middle)

3 – 5 meter : 2 level (upper and lower)

Above 5 meter : 3 level (upper, middle, lower)


Upper – 1/6th of liquid depth below liquid surface. (5/6th of liquid level)

middle – middle level of the liquid

lower – 1/6th of liquid depth from bottom.

(c) Sample Analysis

Sample collected are then analyzed for required specification, typically density and BS&W for crude.

  • Density (ASTM D 1298) using density hydrometers. The observed densities at ambient temperatures obtained are converted to standard density @ 15 °C by using ASTM Table 53.
  • BS&W (ASTM D 4007) that is Centrifuge method.

In summary, before crude lifting to export tanker, FSO/FPSO nominated tanks are:

  • Gauged ( Ullage, temperature, free water)
  • Sampling of crude
  • Analysis of crude sample.

It is also prudent to check the condition of non-nominated tanks and their related valves. This includes ullages of cargo in tanks and related valves are properly shut/sealed, sea chest and slops overboard valves are properly shut/sealed.


(a) Ullage corrections – the ullage taken must be corrected for the following.

  • Gauging tape correction – applied as the gauging reference point is read on the tape, while the actual reference point usually at the top of the vapor lock valve/gauging pipe. Gauging tape correction is done by minus the additional height of the gauging tape before applying ullage value in the tank calibration table. Gauge perimeter are spell out in the tank calibration table.
  • List correction – applied when the FSO/FPSO is listing (tilting) toward either portside (left) or starboard (right) side. This can be verified thru clinometers which will give the value in degrees. List correction is to be referred to tank calibration table.
  • Trim correction – applied when FSO/FPSO is ‘trimming’ by the stern (Aft) or by head (forward). This is the difference between the forward draft marks and aft draft marks.
    • Note: if the aft draft mark is more than the forward draft marks then it is trim by stern.
    • If the forward draft mark is more than the aft draft mark, then it is trim by head.
    • Trim correction is to be referred to tank calibration table.

(b) The gross volume (based on aforementioned corrections) are to be corrected to standard volume using V.C.F from ASTM table 54.
(c) Nett standard volume are obtained after less BS&W % based on sample analysis.
(d) To convert to barrels @ 60°F – multiply with factor obtained from ASTM Table 52 with reference to the sample density.
(e) To obtain weight in metric tons – volume KL @ 15°C multiply with weight correction factor (WCF) from ASTM Table 56 with reference to standard density. (factor in KG/Litre)
(f) To obtain weight in long tons – volume KL @ 15°C multiply with weight correction factor (WCF) from ASTM 57 with reference to sample standard density. or weight to weight conversion by applying factor from ASTM Table 1.


After completion of lifting operation, the FSO/FPSO nominated tanks are then re-gauge (ullage, temperature) .
Quality of crude are based on before lifting quality results.


  • Same calculation method as per before lifting.
  • Tank figure quantity deliver = Quantity before lifting – Quantity after lifting
  • Once this completed, the FSO/FPSO Tank Measurement Report (Tank Figure) is issued.


(a) On board quantity (OBQ) inspection – before the loading operation, the nominated and non-nominated tanks are inspected for condition for any quantity therein. In view of tanks of the inert gas in tanks, visual inspection is not possible. As such tanks are gauged using brass sounding rod. (Advisable to put water finding paste at end of rod to verify free water presence in tank)
Compute OBQ based on tanks calibration table. (Note: If free flowing liquid, apply trim. If sludge, no trim correction)
Issue OBQ report (when applicable)
(b) For non-nominated tanks, if with cargo in transit, then is gauged and temperature reading taken. Compute quantity to gross standard volume applying density as per cargo manifest or shipping documents from the Loading Port.
(c) Check pump room condition so that its clear of cargo and subsequently sealed the sea chest and slop overboard valves. (NEVER ENTER PUMP ROOM ALONE, ALWAYS BRING A BUDDY)


Same as before loading where the nominated tanks are gauged and temperature readings are taken whilst the condition of the non-nominated tanks are re-check for any discrepancy or quantity variance.


Similar to FSO/FPSO calculation.
Once the above has been done, then ship ullage report is generated.


(a) Loading On Top (LOT) – sometime conducted on board export tanker when cargo space is not available and such cargo may have to be loaded in slops tank.
This is subject to agreement between Buyer and charterer.
If this operation is conducted, the calculation of loaded are as follow:
The said tank (slop) is gauged & gross quantity at ambient determined from tank calibration table.(Sample also taken and retained)
Tanks are re-gauged and temperature reading taken. Total quantity determined from tank calibration table.
∴ Volume loaded  = (Volume_after_loading – Volume_before_loading)×V.C.F (based on density from terminal)
the figure will be at standard volume.
(b) Loading Commingled Cargo – where cargo is loaded commingled when there is insufficient ullage on board export tanker and cargo has to be loaded inside cargo tanks with cargo loaded from previous load-port. 
This is subject to agreement between Buyer and charterer.
If this operation is conducted, the calculation of loaded are as follow:
Gauge the nominated tanks and take temperature readings. Determine the tank volume from tank calibration table and compute on board quantity as follows:
A = Tank volume × V.C.F (from load-port)
Re-gauged tank and take temperature readings. Obtain total loaded tanks volume from tank calibration table and compute loaded quantity as follows:
B = Total volume × V.C.F* (interpolated density)
B – A = Loaded volume (KL @ 15°C)
Calculation for interpolated density (ID):

  • X = [Initial tank volume × initial tank density of previous load-port] / final tank volume
  • Y = [ (Final tank volume – Initial tank volume )× Final density of current load-port] / final tank volume

∴ ID = X+Y


Categories: Oil & Gas Notes

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