7.6.7 Crude Oil Washing
The practice of loading on top of retained oily residues (slops) and the implementation of enhanced cargo recovery techniques, such as Crude Oil Washing (COW), have a marked effect on the differences in these volumes. Typical ROB quantities have been considerably reduced since the introduction of double hulled vessels. Large quantities of cargo remaining in the tanks may suggest poor procedures or problems with vessel’s equipment. Liquid calculations should be checked to ensure that the wedge formula has been applied correctly where appropriate. While measured ROB does not represent a measurement loss it may be a real loss to the receiving terminal. High ROB (measured and unmeasured) can be due to inadequate heating on the vessel, possibly coupled with low temperatures at discharge. • If cargo heating has been specified and a problem is suspected, heating records should be obtained from the vessel. These should indicate whether charter party requirements have been followed. • If the cargo has not been correctly heated, wax may be deposited on the tank floor and sides, increasing measured and unmeasured ROB. Heated cargoes can suffer from high ROB if the cargo has not been discharged quickly once below the level of heating coils.
was too slow or that stoppages have occurred, allowing heavy cargo to begin to solidify. Time logs and any Letters of Protest should be reviewed. The physical characteristics of the product and the ability of the vessel to pump it are also factors affecting ROB. Problems can result from cargo Vaporising in the pumps (air lock) and loss of suction during final stripping. Trim and/or list may prevent the free flow of product to the suction point.
Crude Oil Washing (COW) operations can reduce the cargo volumes retained on board after discharge (ROB). Although Crude Oil Washing is a useful technique to improve cargo discharge, its effectiveness is dependent on many factors including the nature of the cargo, the efficiency of equipment, the number of tanks being washed, and the ambient air and sea temperatures during discharge: A thorough COW may reduce ROB to less than the OBQ. However, COW may cause additional cargo losses with volatile cargoes due to the Vapour generated. In cold weather improper handling of high pour or viscous cargoes may increase rather than decrease Clingage. A flushing medium may be employed to COW a vessel’s tanks or to displace a previously discharged cargo in the shore line. The medium, typically a light oil, is loaded aboard the vessel and stowed in a suitable tank for the intended purpose. The vessel should be gauged before and after the flushing medium is transferred to the vessel. The quantity received by the vessel (TCV) should be compared to the volume from the shore tank or meter and must be correctly accounted for to properly reconcile cargo quantities. Any flushing medium remaining on the vessel after discharge should be accounted for as ROB.
7.6.6 Undetected ROB
Undetected ROB is the result of cargo that remains clinging to the bulkhead (Clingage) or in immeasurable areas of vessel compartments. Clingage varies depending on the physical characteristics of the cargo and the conditions under which the discharge is performed. It also can vary depending upon the type of vessel, the number of tanks, and tank construction. Any quantification of loss due to Clingage can only be a subjective determination, but it is an identifiable reason for cargo loss. While Clingage cannot be precisely measured for the voyage in question, it may be estimated for similar cargo types and voyage conditions by reviewing ROB versus subsequent voyage OBQ (excluding water introduced in cargo/slop tanks for tank washing on ballast voyage). Clingage may be recoverable through COW or tank cleaning on a subsequent voyage.
Excess ROB in one tank may indicate that stripping
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