measuring devices are also available. Ullaging is best carried out when the ship is on an even keel and with no list: otherwise inaccuracies may creep in despite the application of trim corrections. A ship whether afloat, alongside a jetty, at anchor or at sea, is a moving platform. Whilst it is not implied that ships necessarily roll heavily when berthed, nonetheless slight movement will affect the accuracy of measurement. In any single tank, a difference of one inch in the ullage may involve a volume of several hundred barrels.
Some factors may affect the calculation of onboard quantities particularly residues on tank floors and structures, which will vary with the age of the vessel and previous cargoes carried. It is not unusual for ullages to be recorded for the purpose of determining ROB and OBQ when the trim of the vessel, at the time of survey, is such that the ullaging tape or sounding rod is not perpendicular to the ship’s tank bottom on contact. In such cases it follows that the depth of ullage obtained must also be inaccurate. Clingage is a further area for consideration because whilst crude oil washing (COW) reduces clingage with most crude, there are a few types of crude where the reverse is true.
Water dips
Free water beneath a crude oil cargo is normally measured with a sounding rod. Using water-sensitive paste, the presence of water can be detected by a change in the colour of the paste. Interface tapes may also be used for the detection of free water. Unfortunately, neither of these can be used to distinguish accurately between an emulsion and free water. Each method involves the risk of inaccuracies which can only be determined by proper sampling and analysis techniques.
Temperature
The temperature of liquid in a vessel’s tank is generally obtained by the use of a cup case thermometer, although some vessels are now equipped with electronic temperature sensing devices. Cup cased thermometers are unreliable and errors of ±2 to 3°C are not unknown. When taking temperatures, great care should be exercised to ensure that the thermometer is not affected by the environmental temperature after it has been removed from the oil.
The vertical positioning of the thermometer in a vessel’s tank particularly at the discharge port is critical because significant temperature variations can develop within the cargo tanks during the voyage. Furthermore, as temperatures vary from tank to tank, calculations of quantity must be calculated using individual temperature corrections for each tank. The use of an
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arithmetical average for the whole ship is, as previously mentioned, inaccurate and contributes to ‘paper losses’. An error of1°C in temperature produces an inaccuracy in the volume at standard temperature of approximately 0.1%.
Sampling
The ship when calculating cargo quantities, has to rely upon certain data supplied from the shore, in particular the density of the cargo which is calculated after the analysis of samples. Shoreline samples may however contain inaccuracies and cannot always be accepted as being representative of the cargo loaded. It is recommended that with crude oils, the standard sampling ‘thieves’ should not be used but that clean sample bottles be used for individual samples from each level, (i.e. top, middle and bottom of each of the ship’s tanks) and clearly labeled. Regrettably, sampling is often undertaken using a one litre ‘thief’, each sample being decanted into a larger sample can.
During such an operation volatile fractions may be lost to the atmosphere and the density established from the final mix does not represent the true density of the cargo in each tank. This, in turn, may later have a significant effect upon the calculation of weight and bottom sediment and water. The importance of sampling as a measure to counter contamination claims is dealt with later in the article.
Measurement errors
Studies by a major oil company revealed that a measurement error of ±0.21% may occur when calculating the measurement of volumes and an error of ±0.25% when calculating weights. Thus, measurement errors may easily account for what has previously been termed a ‘measurement error loss’ or ‘measurement tolerance’.
Completion of documentation
Once the calculation of the ship’s figures has been completed, the shore installation will provide a shore figure. It is generally this figure which is used on the bill of lading. For the reasons given in the section dealing with cargo measurement, it is most unlikely that the two figures will precisely coincide. In practice, and in the vast majority of cases, the discrepancy is small and of no great significance and the master of the ship will have no difficulty in reconciling the figures nor in signing the bills of lading. In each case, the gross figures should be compared and the ship’s experience factor should also be taken into consideration.
On those occasions when there is an exceptional difference between the bill of lading figure and the
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