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Title: Measurement Of Petroleum On Board Marine Vessels
Author: John A. Jack Szallai
Source: 2004 International School of Hydrocarbon Measurement
Year Published: 2004
Abstract: Marine measurements are, generally, used to confirm the validity of shore side custody transfer measurement from meters or gauging of shore tanks. Marine measurements can also be used for custody transfer if no other valid means are available or the shore side custody transfer system is not available. Measurement of petroleum on board marine vessels, ocean or inland, are generally based on the American Petroleum Institutes Manual of Petroleum Measurement Standards Chapter 17 with cross references to other pertinent chapters. The actual physical measurement of petroleum on board marine vessels is not vastly different than for a shore tank. The differences arise from the fact marine vessels are floating structures that are mobile. Their physical structure which permits them to change their orientation relative to a flat plain requires additional steps be taken and different adjustments be made to the physical measurements in order to obtain the proper volumes. It must be recognized at the beginning of this discussion that marine vessels ARE NOT designed or built to be accurate measurement facilities. It has been said that measurement of bulk liquids is an art and not a science. This is truly applicable to measurement of petroleum on board marine vessels. Marine vessels ARE NOT strapped or physically calibrated like a shore tank. The calibration or ullage tables for a marine vessel are developed from the naval architects drawings rather than physical measurement. Therefore any changes, adjustments and/or mis-alignments in the construction of the marine vessel will not be reflected in the calibration tables. This results in a measurement bias for each vessel. This bias is the basis for the Vessel Experience Factor (VEF) which will be discussed later. Additionally, marine vessels do not maintain a constant orientation, i.e., a shore tank is fixed and built to be level. Admittedly, some shore tanks lean, have bottom flexing, etc. but marine vessels will change vertically (list) and horizontally (be trimmed down by the stern or the head) on a regular basis. Compounding this change orientation is the fact these changes are not consistent. Since the calibration tables for a marine vessel are developed for a vessel being on an even keel (no trim) and upright (no list), adjustments have to be made to the marine measurements when the vessel is not on an even keel and/or upright. Since the marine vessel is constantly going through changes in trim and list these adjustments apply most of the time. Factors such as the vessel resting on the bottom of the channel, twists in the hull, hogging and sagging, or sludge and sediment build-up in the bottom of the vessel will influence marine vessel measurements. In the short period of time allotted we will address the basic measurement procedures outlined in API MPMS Chapter 17 and discussion the influence of the physical limitation of the vessel in obtaining accurate measurements on board marine vessels.

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