Homogenous Cargoes

Often charterers are unable to arrange for full homogenous cargoes for the larger VLCCs and ULCCs and therefore these classes, which tend to be thought of as homogenous cargo carriers, may nevertheless be required to carry more than one type or quality of crude oil.  The segregation requirements are likely to be such that single valve separation and pipeline admixture will be perfectly acceptable.  Load on top will be practised with most crude cargoes but not all crude oils are compatible with each other and after some cargoes, tank washings may have to be segregated or put ashore in accordance with charterer’s instructions. Very few of the present generation of VLCCs or ULCCs are in fact equipped to heat cargo and those that are have a fairly low heating capability.   Those crude oils which require heating will most often be carried in vessels under 100,000 tons. When cargo heating is required it must be carried out correctly, with neither overheating or underheating taking place.  Either could result in cargo loss or damage.  Overheating can damage the cargo and boil off valuable light fractions, while underheating will increase the precipitation of some of the heavier components, increase the viscosity of the cargo causing discharge difficulties and thus probably cause outturn losses.  When a waxy crude cools too far, the wax starts to precipitate and falls to the tank bottom.  On the cold tank bottoms it will harden and because in this position the deposits are below the heating coils, these can have little effect on such deposits.  For the same reason when discharging any high heat cargo it is important to quickly strip the tank bottoms while the cargo is still liquid and pumpable. If the temperature of any cargo is permitted to fall below its pour point it will start to solidify.