Page 11 - Shipboard_Petroleum_Surveys

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Chapter 2
There is still confusion in the tanker trade with regard to the terminology adopted in
the measurement and calculation of bulk oil. The historical definitions of ‘gross quantity’
being quantity at observed temperature and ‘net quantity’ being standardised quantity at
15°C or 60°F are erroneous.
For the purposes of loss control, the preferred approach is to deal in units of volume:
@ 15°C or US barrels @ 60°F. This approach reduces the scope for calculation errors
caused by uncertainty in establishing true density. It is difficult to obtain an accurate
density on board, particularly if stratification has formed in the bulk volume. Normal
practice is to obtain an official density of the supplied product from the terminal, which
will have sampled and analysed the designated cargo within the shore storage tanks or
from an installed in-line sampler, and to use this figure for all subsequent shipboard
measurements. The density obtained by the shipper of a cargo and by the eventual
receiver will vary to some extent, normally showing higher at outturn due to some loss
of light fractions during the loading, voyage and discharge. Such variation in density will
have a direct effect on the measured weight but normally will have very little effect on
the measured volume.
The principal terms used in the measurement of oil are as follows (Fig. 3).
Ullage distance
Ullage hatch
Hard-packed sediment
Fig. 3. Terminology for oil quantities
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