Measurement data — Tank No.3 port
Product — Propane
Gauge reading of liquid depth.......................................…..................... : 10.020 metres
Ship's trim ............................................................................................. : 2.0 metres by stern
Ship's list.........................................................................…................... : 0.5° to port
Mean temperature of liquid ................................................................... : - 43°C
Mean temperature of vapour................................................................. : - 38°C
Vapour space pressure....................................................….................. : 59 mbarg
Molecular weight of liquid ..................................................…................ : 44.097 given by
Density of liquid at 15°C ......................................................….............. : 511 kg/m3 shore
From ship's calibration tables for Tank No.3 port
Correction for trim..................................................................……......... : - 127 mm (Figure 8.1)
Correction for list.....................................................................…........... : + 46 mm (Figure 8.2)
Level gauge correction.......................................................................... : + 1 mm
Float immersion correction.................................................................... : + 0 mm
Corrected liquid depth = (10020 - 127 + 46 + 1 + 0)/1000..........…....... = 9.940 metres
Volume of liquid at calibration temperature...................................…..... = 5441.88 m3
100 per cent tank volume at calibration temperature......................…... : 9893.63 m3
Volume of vapour space at calibration temperature.............................. = 4451.75 m3
Factor for tank contraction from calibration temperature to -43°C....….. : 0.99773
Factor for tank contraction from calibration temperature to -38°C...…... : 0.99791
Liquid calculation
Volume of liquid at calibration temperature.................................…......... : 5441.88 m3
Tank thermal correction factor (-43°C)...........................................…...... : (x)0.99773
Volume of liquid at -43°C..................................................................…... = 5429.52 m3
Volume reduction factor from -43°C to 15°C from ASTM-IP Table 54 .... : (x)1.145
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Volume of liquid at 15°C....................................................................…... = 6216.8 m3
Density of liquid at 15°C (from shore)................................................…... : (x)511 kg/m3
Mass of liquid...............................................................................…......... : 3176785 kg
Vapour calculation
Volume of vapour at tank calibration temperature ..........................…..... : 4451.75 m3
Tank thermal correction factor ( -38°C)..................................….............. : (x)0.99791
Volume of vapour at -38°C ........................................................….......... : 4442.45 m3
Density of vapour at -38°C =
........................…....... = (x)2.389 kg/m3
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 (weight-in-vacuo) to weight-in-air for liquid
of 511 kg/m3 density at 15°C from short table of ASTM-IP Table 56.…... : (x)0.99775
Total weight-in-air...........................................................................…...... = 3180226 kg
3180.23 metric tonnes
The above procedure and calculation requires to be duplicated before and after cargo transfer in order to obtain the weight of cargo transferred.
OTHER CALCULATION PROCEDURES AND MEASUREMENT UNITS
The above discussion and calculation uses Sl units. These are often used in liquefied gas quantification. It is still possible, however, to find ships with tank calibrations and instruments working on Imperial units. In this case, the basic calculations remain the same but the user must ensure that all data conforms to the same system. For the Imperial system, this requires temperatures in degrees Fahrenheit, relative density (or specific gravity) in 60°/60°F form, volumes in cubic feet and weight in long tons. In some countries, cargo determination may be made in US barrels and in yet others, densities may be quoted at +20°C.
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Even in the metric systems of calculation, procedures can vary and some of these variations have been identified above. It is possible in one cargo shipment to find that units and calculation procedures differ at the loading terminal, on the ship and at the receiving terminal. In order to resolve this difficulty, it is normal, in many gas trades, to standardise on the ship's volume measurement and calculation procedure and to apply this at both ends of the voyage. Provided, therefore, that the units and procedure used are fully documented by the ship and are understood by both loading and receiving terminals, discrepancy problems of a mathematical nature can be avoided.
In most gas trades it is usual for the liquid density to be provided by the loading terminal and this is then used in the calculations both at loading and discharge. However, in all cases, it is important that the terms of this density (applicable temperature, true-density or so-called density-in-air) are understood and indicated in the calculation records.
Independent cargo surveyors often use computerised methods for calculating cargo quantities. This is often carried out on small portable instruments. This method of working is fast, efficient and is suitable for cargoes where a full analysis is available. Calculators or computers of this type can be connected to printers so that a record of the workings is permanently available.
8.7 CARGO DOCUMENTATION
The transport of liquefied gas is subject to similar commercial documentation as found for oil cargoes. The documents accompanying cargoes of liquefied gas normally include those described in this section.
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Considering the documents covered below, the Bill of Lading is the most important and is the basis against which the cargo receiver can assess if the proper quantity has been discharged. The shipmaster, before departure from the loading terminal, should ensure that the Bill of Lading quantities accurately represent the cargo loaded. The shipmaster should also be sure that cargo calculation records made at loading and discharge are properly prepared.
Bill of Lading
A Bill of Lading is a document signed by the shipmaster at the port of loading. It details the type and quantity of cargo loaded, the name of the ship and the name of the cargo receiver. The cargo quantity written on the Bill of Lading can be the shore tank figure or the quantity as given by shore-based custody transfer meters. However, in many gas trades it is commonly found that the ship's figure is used and this is calculated after completion of loading, usually with verification from an independent cargo surveyor.
The Bill of Lading has three functions. It is:
• The shipmaster's receipt for cargo loaded
• A document of title for the cargo described in it
• Evidence that a Contract of Carriage (such as a voyage charter party) exists
As such, the Bill of Lading is a vital document in the trade. By signing the document, the shipmaster attests to the apparent good order and condition of the cargo loaded.
By signing the Bill of Lading, the shipmaster agrees to the quantity of cargo loaded and any subsequent claim for cargo loss will hinge on the quantity stated on the document. In some circumstances, where the Bill of Lading quantities do not match the ship's figure, the shipmaster may be expected to issue a Letter of Protest at the loading port.
The most important function of a Bill of Lading is as a document of title. Whoever possesses the Bill of Lading rightfully owns the cargo and can demand a shipmaster to discharge that cargo to him. Therefore, unless a Bill of Lading's whereabouts is carefully controlled, it may fall into the wrong hands. For this reason, the old practice of issuing three original Bills of Lading has been largely superseded and now it is common to find only one being issued. On completion of loading, the original Bill is then mailed from the loading port to the rightful cargo receiver.
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Should a cargo be sold on the water — that is before it reaches its destination — the Bill of Lading must be endorsed by the original cargo buyer to show the new cargo owner. A new cargo owner requiring a shipmaster to discharge against presentation of an endorsed Bill of Lading is normal practice. However, due to delays in banking or trading chains, an endorsed original is not always to hand at the discharge port. Accordingly, as an alternative to presenting the original Bill of Lading to the ship master, a receiver may issue a Letter of Indemnity (LOI) to the ship. The terms of the Letter of Indemnity should be agreed between the ship charterer and the ship owner. As the name suggests, such a letter indemnifies the shipowner against any subsequent claims to the cargo and against wrongful discharge.
Certificate of Quantity
A Certificate of Quantity is issued by the loading terminal as, or on behalf of, the shipper and the cargo quantities declared as loaded may be verified by an independent cargo surveyor. The certificate is of assistance to the shipmaster in determining the quantities to be inserted in the Bill of Lading. However, the quantities as stated on the Bill of Lading remain the official record of the cargo as loaded.
Certificate of Quality
A Certificate of Quality provides the product specification and quality in terms of physical characteristics (such as vapour pressure and density) and component con-
stituents. It is issued by the loading terminal as, or on behalf of, the shipper or may be issued by an independent cargo inspection service. Again, the data contained in the document assists the shipmaster in signing the Bill of Lading.
Certificate of Origin
A Certificate of Origin is a document issued by the manufacturer or shipper, countersigned by the customs authorities, which attests to the country in which the cargo was produced. It may be required by financial authorities in the importing country so that they may assess import taxes or grants. Unlike the previous two certificates, it is not complementary to or supportive of the Bill of Lading but its distribution to shipper, carrier and cargo receiver is similar.
Time Sheet
The Time Sheet records all salient port-times, from a ship's port entry until final departure. The Time Sheet is usually prepared by an independent cargo surveyor or the ship's agent and is checked and countersigned by the shipmaster and the shore terminal. Its purpose is to provide an agreed statement of facts relating to the timing of events and delays during the ship's port call and is used to facilitate demurrage claims.
Cargo Manifest
A Cargo Manifest is usually prepared by the ship's agent at the loading port or by the shipmaster and lists all cargoes according to the Bills of Lading. Its purpose is to provide readily available data for customs authorities and ships' agents in the discharge port. The appropriate preparation of the Cargo Manifest is controlled by the SOLAS convention.
Certificate of Tank Fitness
A Certificate of Tank Fitness is usually issued by a specialist chemist from a cargo surveying company and is issued where particular tank cleanliness conditions are required prior to loading.
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