Volume 3, issue #2 - 22-01-1998

Dec. 1, 1997 Minton Treharne & Davies (MTD), an analytical and technical consultantcy bureau, believes that crude oil cargo loss through evaporation on a typical voyage from the Persian Gulf to north-west Europe is much less than has been indicated in some quarters.
MTD disputes suggestions that the VOC (volatile organic compounds) release from a 300,000 dwt VLCC fully-laden with Iranian light crude typically amounts to 0.6 % over the course of a passage to Europe. It believes the figure is closer to just 0.01 %-0.02 %.
Engine designer MAN B&W has referred to measurements which have reportedly pointed to losses of around 0.6 % of total cargo volume in this way. Figurative references made by MAN B&W arose from discussion of its joint development with Norwegian energy group Statoil of a shipboard system for recovering VOC, for use as fuel in shuttle tanker and crude carrier propulsion machinery. MTD acknowledges that the arrangements for burning VOCs may be viable in applications where crude oil is delivered to a vessel more or less directly from the well, without extensive treatment. This includes shuttle tankers operating in areas of the North Sea and, before long, to the west of Shetland where field development is based on the use of offshore production vessels. The company confirms that the oils involved may well contain levels of light ends that can be utilised as proposed.
However, crude oil sourced in the Persian Gulf represents a different scenario, according to the consultantcy firm, since the crude has been degassified prior to storage in shore tanks, and thus prior to loading into tankers.
Enrichment of crude oils, or spiking, by the addition of butane was practised many years ago but stopped, primarily after the explosion on board the British Crown at Umm Said,- observes MTD.
It says the Institute of Petroleum-s PM-L-4A marine loss database panel has correlated crude oil shipment information for many years. Analysis of the voyage data indicates that comparison between the net bill of lading and net outturn figures show a continual decrease in loss from 0.34 % in 1989, to 0.25 % in 1994 and 0.22 % in 1996.
Overall evaporation loss from all phases of the voyage, including loading and discharge, would therefore seem to be considerably less than the 0.6 % figure referred to earlier.
While it is appreciated that the majority of evaporative losses occur during loading and discharge operations, especially in the presence of crude oil washing, the total loss is to the order of 0.13 % or nearly five times less than reported, says MTD.
A study undertaken by Exxon and reported by Captain R C Uhlin at an IP meeting in London in 1984, according to MTD, indicated that the total loss due to evaporation was 0.13 %, of which 0.033 % occurred during loading, 0.079 % during discharge, and 0.015 % on passage. The Cardiff company also suggests that "the apparent intention to cease carriage under inert gas, and to rely upon a tank atmosphere in the ullage space that is 'too rich' to sustainburning may well contravene existing safety regulations under Marpol 73/78".
Furthermore, contends MTD, "The constant removal of cargo vapour, and thus light ends, during the voyage will reduce the quality of the crude oil, increase the density, and deprive the receiver/refinery of lighter end products".