The UTI tape can measure only what is underneath it.
If there is an unpumpable wedge of –
1) viscous cargo ( possible if the Framo wear rings are worn out )
2) due to use of portable framo pump
3) excessive back pressure at terminal, and no internal stripping was done.
The ROB quantity inside tank can be calculated as follows using the wedge formula.
Most surveyors and ships have ready made computer software to do this. The forward part of the aft trimmed tank is empty, and you cannot use the trim tables for ullage volume, that is why you use this formula.
F= distance of ullage pipe from aft bulkhead (metres).
B= breadth of tank (metres)
L= LBP of ship (metres)
T= trim (metres)
S= sounding of liquid (metres)
V=volume of liquid (cum)
C=corrected sounding (metres)
O= trim in degrees
D=depth of tank (metres)
C=S+(F - DTanO) TanO ( no UTI tube extension tube )
V=L X B X C/ 2T
Remember—Most of the UTI tubes have a perforated tube extension to the bottom, to prevent the 30 minute moratorium, after completion of loading static cargoes ( in absence of inert gas ).
Wedge formula is applicable for the tape which falls straight through a main deck hole , by gravity-- NOT a tape which lies inside a UTI extension tube at an incline.
The above formula assumes that the vessel is box-shaped. No allowance is made for the internal structure or changing shape at the bottom of the wing tanks at ford and aft parts of ship . In the case of end wing tanks, the residues could be in error by a considerable amount.
For a square tank, as is usual on chemical tankers with several tanks, it hardly matters if the ship is listed or trimmed--common sense dictates.
The Chief officer must know the exact position of his MMC / UTI tape inlet, and the fixed ullage float.
Use the wedge formula with commonsense. The out of work mariners who come as cargo measurement surveyors, hardly know the meaning of this term. All this is heavy money.
-------CAPT AJIT VADAKAYIL ( 28 years in command )