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5.38 CARGO OUTTURN

Next to delivering the cargo to the desired quality specification, the most important requirement for a tanker is delivering the manifested amount of cargo to the shore tanks.
The contributing factors to a good cargo outturn is discussed in several earlier sections of this book. They include:
A good tank sediment control programme.
Proper cargo heating procedures.
Tight cargo lines and efficient pumps.
100% COWing of crude oil tanks, where desirable and approved.
Correct trim and list while stripping tanks.
Accurate cargo measurement procedures and records.

5.38.1 ROB measurement
Many ROB measurements are done in a way which may not be reflective of the true conditions in the bottom of the tank. Accuracy of ROB measurements can be improved by:
Use of innages rather than ullages for ROB depth measurement. If ullages are used, a correction for vessel trim (cosine of the trim angle multiplied by the measured ullage), should be applied (though it is not in the vessel's advantage to correct the ullage). Measurement of sediment, semi-solid, or liquid ROB at five measurement points in each tank.
Application of the wedge formula to only the liquid phase of the ROB, taking the sediment thickness in the dry portions of the tank as also lying under the liquid.

Sampling the ROB (attempt to obtain a clear sample of any liquid phase).
The usual procedures for making ROB measurement neglect one or more of these recommendations, making the area of ROB volumes one of the most dispute filled parts of tanker operations.

5.38.2 Vessel experience factor
At each discharge port, the vessel should have available a summary record of previous cargo loading and discharge volume figures for examination by the independent petroleum inspector. These records are used to prepare a vessel experience review and vessel experience factor (or ratio). Separate experience factors are calculated for the loading port and discharge ports:
Ship loading port volume as a ratio of shore loading volume. Ship arriving volume as a ratio of shore receipt volume.
See section 3.29.7 for the details of experience factor calculation.
Over a period of time, for a number of similar cargoes (usually ten), the ratio should be relatively constant. For example, all of the ratios between the ship's arrival figure and shore tank received volume for Arabian Gulf crude oils should be in close agreement. A discrepancy of more than 0.3% is cause for further investigation.
If the ratio of vessel's arrival volume to the shore received volume is inconsistent with its previous discharge port VEF, then several possibilities exist. If the other ratios for the voyage are consistent with the ship's corresponding VEFs, then the shore tank receiving figures are suspect. If the ship loading to discharging ratio is also inconsistent with previous experience, then the ship arrival volume calculations are suspect. Vessel experience factors should provide reassurance that cargo measurements are consistent with previous experience, or indicate where an error in measurement or calculation may have been made.

5.38.3 Shore tank figures and shortages
While the cargo measurements made from the shore tanks when loading or receiving cargo are used for custody transfer of the cargo, there are many opportunities for inaccuracy in such measurements.

Tank bottom errors
The shape of a shore tank's bottom changes over time as the tank's foundation ring and bottom plating subside, tilt, or shift to conform to the supporting earth. Shore tank bottoms should be regularly recalibrated and new calibration tables issued. If a ship discharges into an empty shore tank the received volume may be more or less than the actual volume due to outdated calibration tables. Calibration tables should be stamped, or otherwise endorsed on each page with the seal of an independent petroleum inspection service and the date of the tank calibration. Calibration tables which are more than ten years old should be considered as a possible source of measurement error.

Measurement error
Can be due to use of incorrect tapes, taking too few temperature measurements, failure to check the measured ullage against the automated innage tape and the registered tank depth, or incorrect free water measurement.
Shore ullage tapes should be checked with the same diligence as with ship measurement tapes. Electronic ullaging/temperature units must have a recent calibration certificate, or be verified against certified thermometers.
Temperature measurements must be made at a sufficient number of points around the tank to obtain a representative sampling of each area. If an unheated cargo is discharged into a shore tank in a warm climate, thermal stratification in the tank can be expected and several temperature measurements at appropriate levels at each measurement point will be necessary.

Empty pipelines
If pipelines are not proven to be full ('packed'), at the start of the discharge, but are indicated as full in the receiving calculation, then the ship will not be credited with the volume of cargo needed to fill any empty portion of the shore pipelines. The shore terminal must cooperate with the independent petroleum inspector to verify that shore lines are filled before the ship begins discharging.

Floating roofs
Floating roofs that are uncovered ('open floaters'), are subject to measurement variations due to weather, including rain, snow, ice accumulations or run off and wind. Variations are larger if the tank is close to full. Floating roofs act as a known weight on the tank content surface. The roof displaces its own weight in product and forces the product higher up in the tank stilling well or gauge point neck. The amount of surface height difference is a function of the product density and the weight of the roof. The difference is calculated for the product normally stowed in the tank. If none of the weather conditions change during the discharge, there will be little error in measurement as long as the roof is afloat.
If weight is added to or subtracted from the roof during discharge, it will produce a substantial error in the calculated amount the tank receives. For example, discharging a warm cargo into a tank whose roof is snow or ice covered may cause some of the snow or ice to melt. If the roof water drains have been left open, the water will run off, the roof will float higher, the product level will fall in the stilling well and there will be an apparent loss on the delivery outturn! The more tanks affected, the greater will be the loss. The reverse effect will occur if a heavy rainfall is permitted to accumulate on a floating roof which was dry at the start of the discharge.

Shore error
On occasion, shore terminal operating personnel make mistakes. Valves are accidentally left open and the ship's discharge stream is directed to unintended areas. If this is discovered by the shore operators before the end of the discharge then they may correct the problem without advising of the error or making the necessary adjustment to the shore receipt figures.

Shore theft
At the end of any year, a shore terminal is expected to have generated positive variation gains from the cargoes handled. These gains are normally taken into the inventory of the terminal owners/operators as part of their operating 'profit'. Like a casino, an intelligently run terminal knows how to use measurement discretion and tank and pipeline features to realise gains without breaking the rules (see section 3.29). On occasion, however, a terminal may be less than honest in their handling of the cargo, to the detriment of the shipper and the ship owner. These rare events are, however, a principal reason for cargo interests employing cargo auditors to support the work of independent petroleum inspectors at a tanker delivery.

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