Cargo Measurement and Calculation


GTM-8.17	Gas Tanker Manual GTM Cargo Measurement and Calculation	Form No.: GTM-8.17 Revision: 01 Date: 15 Oct 2024 Issued by: DPA Approved by: MD

17. CARGO MEASUREMENT AND CALCULATION

17.1 GENERAL

Liquefied gas cargoes are measured and calculated in a similar manner to that of other bulk liquid cargoes such as crude oils and petroleum products. However, as liquefied gases are carried as boiling liquids in a closed containment system, the quantity of vapour has also to be measured when calculating the total quantity onboard.

It is common practice for gas tankers on a regular trade to retain onboard a quantity of liquid (heel) in order to keep tanks cool on the ballast passage. In this way, the vessel arrives at the load port ready to commence loading with no cool down time necessary. At the load port, the new cargo is added to the heel. Equally, if the ship has arrived with uncooled tanks, a quantity is usually put onboard for tank cool down purposes. It is therefore extremely important that a full survey of all tanks is carried out before and after every operation.

17.2 MEASUREMENT

Temperature

Cargo being loaded may arrive at the manifold at various temperatures during loading. This may be due to cargo being taken from different shore tanks or the initial cooling of ship/shore lines. It is possible that because of this, some stratification in the vessel’s tanks can occur. It is very important, therefore, that temperatures are taken at all available points in order to accurately assess the actual average liquid temperature. Ships’ temperature sensors are usually provided at a number of different levels. This is equally important for vapour temperature where temperatures in the tank dome are usually higher than that of the vapour near the surface of the liquid. The positions of temperature probes must be accurately known in order that only those actually submerged in the liquid are used for liquid temperature and similarly for vapour temperature.

Density

Density is by definition measured in vacuum at 15°C.

Density x volume M3 (at 15°C gives metric tonnes in vacuum).

The measurement of liquid gas density requires laboratory facilities or equipment not available on ships. Modern terminals usually calculate this form from an analysis of liquid composition obtained from a gas liquid chromatograph. The results of this are provided to the ship in order to carry out the cargo calculations.

It is necessary to correct the density for the actual observed temperature of the cargo. For specialised chemical gases, the storage facility normally provides their own density table for the cargo showing the density for a range of temperatures. Some ports provide the density at a standard temperature of 60°F or 15°C. This has to be corrected to the density for the observed cargo temperature.

Density can be quoted as either being in air or in vacuum. For a density quoted in vacuum, subtract 0.0011 to obtain the density in air, i.e., 0.5074 in vacuum corresponds to a density of 0.5063 in air.

Cargo's quantities worked out in vacuum are always heavier than those worked out in air. Liquid gas quantification is more commonly expressed in terms of weight in air and indeed this is a requirement of most customs authorities. It is extremely important that when a density is provided to the vessel, it is ascertained whether the density is in air or vacuum.

Liquid Level

The liquid level is read direct from the tank level gauge on the tank dome. The remote readout must not be used for cargo calculations. It is necessary to apply corrections to this figure before entering the tables. These corrections are for tape shrinkage and float immersion. The float gauge tape passes through the cold vapour space and depending on the space temperature contracts, thus indicating a higher liquid level than actually present. Float immersion will depend on the density of the cargo and this will usually be different from the manufacturer’s initial determination. A small correction is necessary for both these items to obtain the correct gauge reading before entering the tables.

Liquid Volume

All ships are provided with a calibration table for each tank by means of which the tank’s liquid (and vapour) volume can be calculated from the measurement of the liquid level. These tables are obtained from careful measurement of the tanks during the ship’s construction. These tables normally refer to an upright vessel with no list. Corrections are therefore necessary for trim and list and these will be included with the tank calibration tables. Instruction for use will be included with the tables.

The cargo tank volume will have been calculated at ambient temperature and the tables calculated for a standard temperature of say 20°C. Cold cargo temperature will result in tank shrinkage and a reduction in volume. A correction therefore is necessary and this is normally expressed as the Tank Shrinkage factor.

Vapour Quantity

The volume of vapour is found by subtracting the volume of liquid from the tank's 100% capacity. This is at the calibration temperature for the tank before the Volume Correction Factor has been applied. It is necessary to apply a Volume Correction Factor (tank shrinkage factor) to this figure, and this correction is obtained using the average vapour temperature.

17.3 CALCULATION

On completion of measurement, calculation of the total cargo quantity can be carried out.

There is no internationally agreed standard for gas cargo calculations, and procedures can vary, particularly with the chemical gases.

In the absence of any instructions concerning calculations, the following procedure using the standard temperature of 15°C, which is widely used, should be followed.

The co-mingling operation can also cause “apparent” losses, as the density of the mixture will not be a mathematical average of the densities of the components as the molecular component is different. Calculation of cargo densities in such circumstances is discussed in Appendices 3 & 4 of the SIGTTO publication “Quantity Calculations for LPG and Chemical Gases.” Copies are available directly from SIGTTO.

17.4 CALCULATION PROCEDURE (TYPICAL)

a. Determine by measurement the average liquid and vapour space temperature (degrees C) and the vapour space pressure (barg or mbarg).

b. Read the tank liquid level and calculate the liquid volume (V1) at tank conditions using the ship’s calibration tables for that tank and making all necessary corrections for temperatures, list, and trim.

c. Determine the liquid density, noting the temperature at which it is determined and using ASTM* table 53 to convert this to liquid density at 15°C.

d. Using the liquid density at 15°C and the measured average liquid temperature, enter ASTM* table 54 to derive the appropriate volume correction factor to convert to the volume at 15°C.

e. Calculate the liquid mass. Volume x Density.

f. Calculate the vapour volume at tank conditions by subtracting the apparent liquid volume (liquid quantity before applying tank shrinkage factor) from the tank total volume.

N.B. ASTM tables 53 & 54 have been revised for densities in the range 610.0 to 1076.0 kg/m³. However, below this range covering LPG, no revision has been carried out and ASTM-IP tables 53 & 54 are to be used.

g. Using the average vapour temperature, correct the apparent volume of vapour for tank shrinkage.

h. Determine the vapour density at vapour space conditions using the following formula:

        Density of Vapour = Ts x Pv x Mm / (Tv x Ps x I)

Where:

Ts is standard temperature of 288 K
Tv is average temperature of vapour in K
Pv is absolute pressure of vapour space in bars
Ps is standard pressure of 1.013 bar
Mm is molecular mass of vapour mixture in Kg/Kmol (sometimes called molecular weight)
I is ideal gaseous molar volume at standard temperature (288K) and standard pressure (1.013 bar). This is 23.645 m³ / Kmol.

i. Calculate the vapour mass by multiplying vapour volume and vapour density.

j. Add the liquid mass and the vapour mass to give the total cargo mass in the tank.

k. Convert the total to weight in air.

17.5 EXAMPLE CALCULATION

The following numeral example demonstrates in detail the typical procedure for the calculation of the contents of a ship’s tank.

Measurement Data

Tank No. 3 Port

Product - Propane

Gauge reading: 10.020 metres
Ship’s Trim: 2.0 metres by stern
Ship’s List: 0.5 degrees to port
Average liquid temperature: -43°C
Average vapour temperature: -38°C
Vapour space pressure: 59 mbarg
Molecular weight of liquid: 4.097
Density of liquid: 511 Kg / m³

From Ship’s Calibration Tables for 3P Tank

Tank gauge reading: 10020 mm
Correction for trim: -127 mm
Correction for list: +46 mm
Level gauge correction: +1 mm
Float immersion correction: 0 mm

Corrected liquid depth: 9940 mm

Liquid Calculation

Volume of liquid (uncorrected): 5441.88 m³
Tank shrinkage factor: x 0.99773
Volume of liquid at -43°C: 5429.52 m³
VRF from -43°C to 15°C (table 54): x 1.145
Volume of liquid at 15°C: 6216.8 m³
Liquid density at 15°C (from shore): x 511 Kg / m³
Mass of liquid: 3176785 Kg

Vapour Calculation

Volume of liquid (uncorrected): 5441.88 m³
100% tank volume: 9893.63 m³
Vapour volume (uncorrected): 4451.75 m³
Tank shrinkage factor: x 0.99791
Volume of vapour at -38°C: 4442.45 m³

Density of vapour at -38°C:

        Ts x Pv x Mm = 288 x 1.059 x 44.097
        Tv x Ps x I = (273-38) x 1.013 x 23.645 = 2.389

Mass of Vapour: 10613 Kg

Total Mass

Mass of liquid: 3176785 kg
Mass of vapour: 10613 kg
Total Mass: 3187398 Kg

Weight in Air

Factor for converting mass to weight in air for liquid of 511 Kg / m³ density at 15°C (Table 56): x 0.99775
Total weight in air: 3180226 Kg = 3180.23 MT

17.6 CARGO DOCUMENTATION

The transportation of liquefied gases is subject to the same commercial documentation as applies to oil cargoes. Documents accompanying a liquid gas cargo will generally include the following:

Bill of Lading

This is the most important document. It is a receipt for the cargo on board and is normally signed by the Master on behalf of the ship-owner or time charterer. It will state the quantity of cargo shipped, that it was received onboard in apparently good order and condition and will indicate the terms and conditions under which the ship will carry the cargo to its destination. In some ports which operate early departure procedures, the Agents will sign the Bills and the Master will be required to furnish the agents with a letter authorising them to do this.

The Bill of Lading is usually issued in three “originals” of equal standing, each separately stamped and signed. One of these goes to the shipper, one to the carrier (ship-owner or time charterer), and one to the intended receiver of the cargo. A copy will be retained onboard and normally the Master will only deliver the cargo on presentation of the receivers “original”.

Refer to the commercial section of the Fleet Operating Manual for company policy on Bills of Lading.

Certificate of Quantity

This is issued by the loading terminal and is the cargo quantities declared as loaded, usually established by an independent surveyor.

Certificate of Quality

This provides the product specification and quality in terms of physical characteristics and component constituents. It is again issued by the loading terminal.

Certificate of Origin

This is a document issued by the manufacturer or shipper, countersigned by the customs authorities and attesting to the country in which the cargo was produced.

Time Sheet

This records all timing details of the ship’s movements and operations from the ship's entry to its final departure from the port. This is usually prepared by the vessel’s agents and is countersigned by the Master. Its purpose is to provide an agreed statement of facts relating to timing of events and any delays.

Cargo Manifest

This document is again usually prepared by the vessel’s agents at the loading port and lists the cargo according to the Bill of Lading(s) and the disposition of the cargo within the ship. Its purpose is to provide readily available data for Customs authorities etc., at the discharge port.

Certificate of Tank Fitness

This is issued by independent chemists or surveyors where particular tank conditions are required prior to loading.

Certificate of Inhibitor Addition

Certain gases require an inhibitor added for transportation, and the certificate will show the quantity added and the length of time the inhibitor will last for.

17.7 MOLECULAR WEIGHTS

The following are the molecular weights for various standard gases. In the absence of data from the loading terminal, these figures should be used for calculating the vapour mass.

Ammonia: 17.03
Butadiene: 54.10
Butane: 58.12
Butene: 56.10
Chlorine: 70.91
Ethane: 30.10
Ethylene: 28.05
Methane: 16.04
Methyl Chloride: 50.49
Propane: 44.10
Propylene: 42.08
R12: 120.92
R22: 86.48
Sulphur Dioxide: 64.07
Vinyl Chloride: 62.50

17.8 SAMPLING

Cargo is normally sampled by shippers’ or receivers’ personnel, or by authorised petroleum inspectors. The responsible officer must, however, be present when sampling is carried out to ensure that samples are taken from correct sampling points and that this is performed in a correct and safe manner. He is to also make a proper record of the samples taken as these may be of considerable value subsequently. A good rule is to request samples to be taken from the liquid shore connections at the start of loading to safeguard against possible contamination of shore transfer lines.

The following precautions are to be observed when sampling cargo liquid or vapour:

17.9 LIQUID SAMPLES

(a) The sample container must be completely clean and compatible with the cargo to be sampled and is to be able to withstand the extremes of temperature and pressure anticipated.
(b) Sample containers must be purged of air by pure nitrogen before use with flammable cargoes.
(c) If the sample is to be representative, its container has to be purged thoroughly with cargo from the sampling connection. Sufficient cargo must be passed through the container to cool it down to liquid temperature. If the cargo is a mixture (which is often the case), the most volatile components will evaporate more rapidly than the heavier fractions as the container is cooled down; this will leave the sample with a higher concentration of the heavy fraction than present in the cargo, and it will therefore be unrepresentative. To counteract this, sample containers are to be turned with the vent valve downwards during cool down, to drain off the liquid that first collects. For the same reason, samples from the bottom of cargo tanks at the beginning of, or just after, loading may not be representative. It is recommended that the cargo is circulated using the cargo pump, if possible, before taking bottom samples.
(d) It is imperative that sufficient ullage or vapour space is left in the sample container to allow for the liquid expansion that will occur when the temperature increases to ambient. Ullage is obtained by holding the full sample container upright after disconnecting it from the sample connection and draining some liquid by opening the bottom valve for a moment.
(e) Unless the sample container is free of cargo vapour, it should not be stored in an unventilated space.
(f) Gloves, goggles and protective clothing must be worn when sampling cold cargoes.
(g) If the cargo is toxic, a suitable respirator, or preferably self-contained breathing apparatus, must be worn. If sampling in an enclosed space, a respirator is unsuitable, due to the possibility of asphyxiation, then breathing apparatus is necessary.
(h) If electrical equipment is used when taking samples, this is to be of the certified-safe type.

17.10 VAPOUR SAMPLES

(a) The precautions given in 17.9 (a), (b), (c), (f), (g) and (h) are to be observed when sampling cargo vapour or inert gas.
(b) Plastic sample bags are sometimes used for collecting vapour samples. These must be handled carefully, never used for liquid samples and always purged after use.