5.11 GAUGING AND SAMPLING TANKS

As soon as the ship is cleared (unless permission is given by the authorities beforehand), the independent petroleum inspector and terminal operating personnel board the ship to measure and sample the cargo. The chief officer should have available for their review the ship's gauges from the loading berth, the loading certificate of quantity and quality, the ship's records of cargo transfer conducted en route and a summary record of the previous ten cargoes.
The ship's electronic gauging equipment, with latest calibration certificate, should be presented for examination. After the independent inspector and terminal representative have reviewed these documents and the vessel's draft, trim and list have been determined, measurement of cargo tanks can begin.

5.11.1 Cargo temperature
While cargo temperatures at the loadport tend to be fairly uniform throughout the oil column in the cargo tanks, they may vary significantly in tanks at the discharge port, regardless of whether the cargo has been heated or not. The temperature differences will vary according to several factors, including:
• Characteristics of the oil.
• Original loading temperature.
• Length of the voyage.
• Location of the tank in the ship.
• Design of the vessel (single or double hull).
• Ambient sea and air temperature.
• Ratio of freeboard to draft.
• Cargo heating en route.

A comparison of the ship's temperature measuring device should be made with a standard thermometer or the independent inspector's temperature unit. If a difference is noted, it should be cause to review the figures from previous cargo surveys. Any disputes should be dealt with as indicated in section 5.11.2.
The air and sea temperatures should be measured, preferably prior to the cargo survey (to prevent oil from the probe causing a sheen in the berth). The sea water temperature should be measured close to the keel of the ship. This is the temperature against which the final tank draining operations will take place. Recording it will provide valuable evidence in case of a subsequent claim regarding pumpability of the cargo.

5.11.2 Gauging tanks
The independent inspector should do the physical gauging. Closed gauging using electronic ullage and interface detection equipment is preferred. If the independent inspector or terminal representative insists, the inert gas pressure may be reduced so that open gauging can be conducted, provided there is no local regulation prohibiting it. If the independent inspector's equipment is used, the chief officer should verify its calibration. Shore terminal equipment should not be accepted for gauging ship tanks. If a dispute arises regarding equipment to be used, the chief officer may agree to use independent inspector's equipment only if a calibration record can be produced which is more recent than that of his own equipment. If the chief officer finds that inspector's measurements consistently differ from that taken with ship's equipment, then he should insist that two sets of measurements be made and recorded, using both ullage units and that both units be sent ashore for verification immediately after gauging is completed. The master must enter a protest letter against the alleged inaccuracy of the non-vessel gauging equipment.
Whenever a dispute arises, the facts must be recorded in detail, including the names and positions of all personnel involved and the serial numbers and descriptions of equipment used.
During the gauging at least two records of each of the readings should be made. The records should be compared at the end of the measurements and any tank where the records do not agree should be remeasured.
If the cargo contains H2S (sour crude), the highest H2S emissions will occur at the discharge port, rather than the loading port. Highest emissions at the discharge port take place during open ullaging or gauging of ship's tanks. If sour crude is to be gauged through open ullage ports, then the individual making the measurements should do so while wearing compressed air breathing apparatus, or:
• IG pressure should be reduced to atmospheric pressure (but not below) and
• Each ullage hatch should be opened (with flame screen in place) for one minute before gauging the tank.

The process of tank ullage measurement is covered in section 3.29.3.

5.11.3 Cargo calculation
Cargo discharge should not be started until all parties involved in the cargo measurement agree to the measured quantities on board. If the measured quantity differs by more than 0.3% from the loading port ship quantity, then the temperatures and ullages should be re-checked. If the cause for the discrepancy is not disclosed by this check, then the chief officer should request a calibration check of the equipment used to make the measurements. If the discrepancy still cannot be resolved, then the chief officer should take his own measurements using ship's equipment and the master should present a protest letter to all parties involved, setting down the facts and naming the surveyor. Promptly notify the vessel's P&I representative of the dispute.
The method of calculating cargo quantities is discussed in section 3.29.5. Remember that most cargo differences are due to inaccurate determination of the trim and list corrections.

5.11.4 Cargo sampling
The guidance of sections 3.28.2 to 3.28.4 should be followed at the discharge port as at the loading port. The chief officer should check that the samples are being properly drawn and labelled. Particular care is required with sampling of products to avoid cross contamination of the samples by the sampling equipment, such as a sample bottle cage.

5.11.5 In-line composite sampler
After gauging and sampling are complete, the chief officer should witness the independent inspector's inspection and sealing of the shore in-line sampler, verifying if possible that the sampler is properly set to sample the quantity to be discharged.

5.11.6 Free water measurement & letters of protest
If all precautions have been followed during the voyage, there should be no reason to expect to receive letters of protest for such matters as 'low cargo temperature', or 'transit loss or gain'. One matter beyond the control of the ship is that of 'free water' or 'free water increase' resulting from settling out of water from the cargo after loading.
Any free water detected at the loadport should already have been protested by the master, with the added note that 'this may increase during the voyage'.
It is a wise precaution for the chief officer to be aware of the amount of BS&W already documented on the Certificate of Quality. This is the difference between GSV and NSV stated on the certificate.
For practical purposes, the 'sediment' portion of the BS&W may be ignored. If the free water detected on the vessel on arrival exceeds the amount of BS&W at loading, then the risk of a claim against the ship is increased. Free water quantities are often overstated by over zealous inspectors due to inaccurate determination of the water cut on the gauge bob. (See section 3.29.3).
During the course of a voyage, waxes and sediments may precipitate our of the cargo, making the oil/water interface less detectable. Emulsified water just above the interface may cause 'blushing' or 'spotting' on the water-finding paste above the true oil/ water interface. If this blushing or spotting is measured as the water cut, it will cause the calculated free water volume to be overstated.
The chief officer must ensure that there is in fact a clear, sharp cut on the water-finding paste. Under some circumstances, it may be necessary to use two different water pastes on the same bob, different brands of paste having different reactions to the oil/water emulsion.
If a significant amount of free water is detected at the discharge port, the chief officer must ensure that samples of the water are drawn and sealed. One sample is retained on board the ship. In crude oils the free water can usually be identified as produced water (from the oil wells), if it has a higher salt content with different mineral composition than sea water.