once it is established that cargo is entering the correct tanks and that there are no leaks from hose connections or any other difficulties, the rate is increased to the maximum. It is recommended that at an early stage the cargo officer should satisfy himself that the correct grade of cargo is being loaded, either by checking the specific gravity of a sample or at least by visual means. In modern tankers the ship’s instrumentation may facilitate remote monitoring of temperatures during loading but in any event it is essential to measure accurately and to record the temperature in each tank during loading. It is wise not to use an average of the tank temperatures since this leads to inaccurate cargo figures.
The loading rate should be monitored and it is recommended that ullages and the corresponding tank volumes be recorded in the deck log at least at hourly intervals. Any changes in the loading rate or any stoppages must also be recorded. During the final stages of loading the rate should be reduced to a minimum in order to permit measurement of the quantity of cargo so far loaded and to calculate the correct finishing ullage in the last cargo tank.
On completion of loading
Before the cargo hoses are disconnected, the ship’s figures must be calculated in order to check that the correct quantity of cargo has been loaded. Whilst it is in the ship’s interests to measure the cargo onboard ship, it is customary for various witnesses to attend this operation and in some cases to make independent calculations. These witnesses may include representatives from the loading terminal, the shippers and the charterers. It is of prime importance that the
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measurements of ullage, temperature and where appropriate, water dips are agreed by all concerned, although it must be accepted that the methods of calculation employed thereafter may not always be consistent. It is generally accepted that the latest edition of the API/IP Petroleum Measurement Tables are more accurate than the old tables, but it should be borne in mind that all tables are based on the average characteristics of a range of oils. Where a surveyor is attending on the ship’s behalf he should collaborate with the ship’s officers in order to ensure that no inconsistencies arise in the calculations.
Ship’s tanks may be calibrated using imperial or metric units of volume and the quantity of cargo may be expressed in various units including long tons, tonnes or barrels. Whichever units are applied, it is essential to compare like with like. The use of standard volume may be considered preferable as it is less susceptible to misinterpretation by observers or laboratories. The appendix to this article shows the various terms used in the measurement of liquid cargoes and the abbreviations in common use.
At this point it may be worth considering in some detail the degree of accuracy which may be expected when ullaging tanks, measuring temperatures, taking samples or quantifying free water.
Ullaging
This is the measurement of the distance from the datum point at the top of a tank to the surface of the liquid cargo. This is usually done by means of a steel tape fitted with a weighted brass bob. Many tankers have fixed gauging equipment in each tank and electronic
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