Back to Main Page ----- Back to Chapter 3


The purpose of gauging ship tanks is to provide a verification of the quantity said to have been loaded from the shore tanks and to determine the quantity of water on board at the conclusion of loading. The quantity of cargo used for the bill of lading is determined by shore tank measurements. Normally, the shore tank measurements will be witnessed by an independent petroleum inspector and perhaps by a loss control auditor employed by the shipper. Occasionally, a representative of the cargo insurer is also present. While it is obvious that the seller of the cargo would benefit by exaggerating the quantity loaded from the shore tank, any gross exaggeration is certain to be detected. The terminal's best chance of regular gains on such transfers is simply to take advantage of those measurements where the gauger is allowed to use his discretion. The basic rule for shore terminal loadings or receipts, is to 'Maximize the opening volume ... and minimise the closing volume.' This is done at tank 'opening' gauges by choosing the next lower degree of temperature and/or the next smaller ullage graduation whenever the observation falls between graduation marks on the thermometer or gauging tape. When 'closing' the shore tank after loading, the next higher degree of temperature and/or next greater ullage graduation is chosen if the measurement offers the opportunity to choose between readings. This practice applies regardless of whether the tank is receiving or delivering oil. If it is applied consistently, the terminal will produce a significant net gain of product for the terminal owners over any period of time, without violating any law or contract.
Numerous petroleum loss control studies have demonstrated that a large portion of tanker cargo 'losses' in transit were never losses at all, but rather inaccuracies in the measurement of shipping and receiving shore tanks.
The volume of cargo in the ship's tanks is determined by measuring the distance from the surface of the cargo to a datum point in the cargo hatch or in a sounding pipe. This point is called the ullage point and the distance is called the ullage, (see figure 3.29.6)
Making ullaging measurements is not difficult, but errors are frequently made, therefore close attention and care are essential. Errors are normally the combined result of three factors:
- Improper measuring technique.
- The use of non-standard or worn measuring equipment.
- Insufficient accuracy when ullaging and making temperature measurements.

There are many opportunities for inaccurate measurement on the tanker. The ship is floating in a liquid and filled with a liquid, a condition likely to produce motion of the cargo surface during measurement. The tank hatch measurement area may be worn from years of service and the datum point eroded. If the gauging tape is old, it may be elongated, or the eye between bob and tape-end snap worn away so as to increase their overall length. Measurements taken in bad weather or in darkness by personnel who are tired will include more errors that otherwise. Nevertheless, it is important that as many inaccuracies as possible be eliminated from this procedure, so that the ship's cargo measurement will correlate properly with the shore figures and vessel's previous loading experience.

3.29.1 Checking equipment
Before gauging ship's tanks, all equipment should be checked for correct performance and measurement accuracy. The ullage tape should be checked with a steel rule to verify that the bob's point is the zero point of the tape, or in some tape/bob combinations, that the zero mark on the bob corresponds correctly to the zero point for the tape to which it is attached. This check must be done for both manual and electronic ullage tapes.
If cup case thermometers are to be used, they should be verified against a standard thermometer and any with significant errors discarded. Minor errors should be indicated as a correction to the thermometer reading. To further reduce the effect of thermometer inaccuracies, each cup-case thermometer should be designated/ dedicated to a specific tank. Using the same thermometer in the same tank at loading and discharging ports reduces apparent tank losses or gains. Even when new, it is possible for standard cup-case thermometers to have errors of as much as 3 C.
Calibration of thermometers is best done in a constant temperature water bath, but placing all thermometers on a cloth on a table in an unventilated room for ten minutes and checking them against a standard, certificated thermometer is acceptable.
Electronic thermometers are preferable to cup thermometers with respect to both absolute accuracy and the accuracy with which the average temperature in a tank can be measured. The probe can be lowered accurately to a number of desired depths and a series of stabilised measurements made each 20 seconds. Some types have automatic averaging calculators and will indicate the average temperature of the tank on command.
Electronic temperature sensors in ullaging units may also be checked on board by comparison with a certified thermometer, however all portable electronic temperature sensors should be standardised annually over their complete working range (under laboratory conditions) and the calibration noted on a calibration decal attached to the unit.
Automatic ullage tapes which give an audible signal for cargo surface and cargo/water interface measurement should be tested before each use by immersing in a clear container of oil and water and confirming that the correct sound tones are produced at the correct levels.
Necessary lights for night gauging, personal protective equipment, breathing apparatus (if necessary), hydrocarbon vapour test equipment and the necessary forms for recording the measurements should all be ready in advance of taking ullage readings.

3.29.2 Confirming draft, trim and list
Prior to any cargo ullages being measured, the draft should be accurately read all around the ship and the trim and list also determined and agreed by all parties.
More cargo measurement errors at the loadport are caused by failing to determine and apply trim/list corrections than any other reason. This failure could expose the ship to unexplained, or 'apparent' transit losses, (or gains), at the discharge port.
The completion of the loading operation is usually a hectic time for the crew, as pressure is placed on the ship to sail as soon as possible. Nevertheless, the chief officer should not be persuaded to commence any cargo ullaging until all operations that might affect the trim or list of the vessel have been completed.
The position of each gauging point in relation to the tank will have an important bearing on the accuracy of cargo measurements. On only a few vessels will the gauging point be in the 'centre' of the tank where it is unaffected by trim or list.
The most accurate measurements will be when the vessel is on an even keel with no list. Any deviation from this condition will require corrections to the tank calibration table quantities. Such corrections are variable in their accuracy and are the least accurate when the cargo surface is not contained in a 'box' shaped tank. Forward wing tanks and the upper hopper spaces of OBO vessels are particularly prone to error in their list or trim corrections.
On many newer vessels, the traditional wing cargo tanks have been replaced by protectively located segregated ballast tanks (PL-SBT), leaving only the centre tanks for cargo. These centre tanks are often wider than they are long. Corrections to list are particularly important in such tanks.

A 1991 built 135,000 DWT crude tanker has all the official gauge points sited aft and to the port side of each tank. The trim and heel correction for one of these tanks is as follows:

Tank No.4 C
1.0 m. aft= 4.5 cm ullage correction.
1 deg. list = 27.0 cm ullage correction.

Consider a mere 1/10 degree list which would be essentially undetectable by the average clinometer and the list correction will still be 2.7 cm.
On this vessel, with 7 main cargo tanks, each with 1 cm. of ullage = 63 barrels, the 'error' in measured quantity from failing to detect and correct for a 1/10 degree list is 1190 barrels.
When correcting for trim and list, it is imperative that any free water found also be corrected. For small amounts this may require correction by use of the wedge formula.

3.29.3 Measuring cargo ullage
Tanks containing static accumulator oils should not be sampled or ullaged until thirty minutes after they have stopped receiving cargo. However, gauging can begin with the first set of tanks loaded, and finish by measuring the trim tanks last.
On non-inerted ships, or inerted ships where IGS pressure has been reduced for gauging and sampling, only one tank at a time should be opened. Gauging should be completed first, followed by sampling. Synthetic lines should never be used for lowering sampling containers or thermometers; use cotton line for this.
Steel tapes should be earthed to the tank hatch before the bob is placed in a tank containing static accumulator oil.
Measurement readings should be made by the independent inspector and recorded by a ship's officer.
On inerted ships, or where electronic, closed gauging equipment is used the following procedures should be followed:
- Lower the probe carefully into the tank by slowly unwinding the tape. The probe must not be allowed to descend under its own weight (as with steel tapes).
- When the audible signal indicates that the probe has reached the liquid level it should be withdrawn slightly and then lowered again, noting the exact reading when the audible signal is first heard.
- Repeat withdrawing and lowering the probe until three readings are obtained which all agree within 5 mm.

Each tank measurement must be witnessed by a ship's officer and a terminal representative. The ship's officer should record the readings for the independent inspector and the terminal representative should make a second copy as the reading is told off. Later the two sets of readings will be compared and any tank for which two records do not agree must be re-measured.

Oil/water interface detector.

The ship's officer should have with him (in his deck notebook), a list of the total gauge depth of each cargo tank. This is used to verify that the tape is lowered to tank bottom and no further, when taking the water measurements. Tanks must be 'thieved' for water, either by use of an electronic sensor, or by coating the bob of the ullage tape with litmus paste and lowering it to the tank bottom. When it is retrieved, the height to which the coated bob has changed colour is the measurement of the level of water depth at the bottom of the cargo. If the bob is allowed to fall over and lie flat on the bottom, then one inch of water in the bottom of the tank would appear to be six inches when the bob is retrieved. And if it is not lowered completely to the bottom, then water received from the shore terminal may not be detected by the ship tank gauges. The officer's job is to see that the measurement is made correctly. A crew member should be assigned to follow behind the sampling crew to clean and secure each sampling opening after the tank has been gauged and sampled.
The slop tank(s) should be measured last. Determination of the oil/water interface is made much easier if an electronic probe unit is used. Measurements of temperature must be for the oil level only. Do not take temperature measurements of the water layer.
The independent inspector should complete his cargo measurement observations by verifying and recording the seal numbers on any cargo valves which were sealed before loading. He should also verify that the numbered seals placed on sea suctions before loading remain intact when loading is completed.
If the ship is fitted with automatic tape or electronic (radar), tank ullaging equipment of sufficient accuracy, the ullages should be taken with the fixed units. The tapes or sounding devices must be in good condition and their calibration records available for review by the petroleum inspectors. If both manual gauges and automatic gauges are taken and recorded, the chief officer should later use the ullage records to determine the accuracy of his automatic gauges.

3.29.4 Ullage measurements at sea berths
If the vessel is exposed to swell or other conditions affecting the measurements (such as high winds while open gauging), an ullage based on the average of three measurements may be used in lieu of three consecutive measurements within 5 mm.

Typical 'cut' on paste-coated bob.

When exposed to swell in a sea berth, waves will form in the cargo causing an apparent reduction in the measured ullage as the wave crests traverse the ullaging point. The most correct reading(s) will be those taken when the vessel's roll amplitude is at a minimum. Ullages taken in sea berths normally overstate the amount of cargo in the ship. The weather and sea conditions at the time of measurement should be carefully noted on the ullage record and in the logbook.

3.29.5 Temperature measurements
Temperature measurements should be made with procedures and equipment conforming as closely as possible to the recommendations of the American Society for Testing and Materials (ASTM) and the American Petroleum Institute (API). Errors in cargo tank temperature measurement of 1 C can produce calculated volume errors of 0.1% for crude oils of average density and 0.2% for light products and light crudes.
Use of only a single thermometer lowered to the centre point of the cargo should be avoided. Cargo tanks may have a temperature difference of 5 C between top and bottom. If cup-case, mercury thermometers are used, temperatures should be observed at three or more levels in the tank.
The preferred method is the modern thermal probe with digital output (usually combined with the ullage tape). Using a digital probe permits temperature measurement intervals as small as one meter. Some probe units sound a tone when the temperature reading has stabilised and the reading can then be recorded in memory (by pushing a button). If this feature is not included, then the probe must be held at each level for 20 seconds before recording the measurement.
ASTM D1086 (heated cargo) guidelines suggest that cargo tanks with more than 4.5 meters of liquid depth require three temperature measurements, one each at 1 meter from top and bottom and one at
the middle.
Another standard, Institute of Petroleum IP 204/86 recommends temperature measurements at 1/6, 3/6, and 5/6 of cargo innage depth. If the temperature of the middle sample is within 1 C of the average of the three temperatures, the average may be used. If the middle temperature is outside the one degree tolerance, then additional temperature measurements should be made at 9/10, 7/10, 3/10 and 1/10 of cargo depth and the average of all seven used, provided the middle temperature is within one degree of the average. If not, more temperature observations are recommended (but in practical application are rarely taken).

3.29.6 Calculating cargo quantity
When the cargo density at 15 C (or the API gravity at 60 F, if using Imperial units), all tank ullages, average cargo temperatures (by tank) and water innages have been recorded, the calculations of cargo volumes loaded can be completed. This will be done manually by the independent inspector, but may also be done with the aid of the ship's loading/stability computer.
Each tanker has a set of certified calibration tables for the cargo and slop tanks. The tables indicate the amount of cargo in each cargo tank for ullages at one centimetre (or inch), intervals. Volumes for ullage readings between even centimetres (or inches), are determined by linear interpolation or by referring to a millimetre (or fractions of an inch) table for the tank. After the observed volumes are determined, the rest of the calculation follows.

Allowance should also be made for pipeline or duct keel volumes to be included in the calculations if they are not included in the calibration tables. For pipelines which run through the cargo tanks, the average temperature of the cargo in all tanks will suffice for the correction factor of cargo in the pipelines. For pipelines external to the cargo tanks, the average cargo temperature may be used at the loadport. At the discharge port, the average cargo temperature may still be used, or the volume may be assumed to be at standard temperature (ie. 60 F or 15 C). The difference will be minimal.

Consider a vessel with duct keel of volume 1000 barrels filled with crude oil of 30.0 API gravity after loading then:
1000 bbls. @ 60 F = 1000.0 bbls.
1000 bbls. @ 50 F = 1004.4 bbls.
1000 bbls. @ 70 F = 995.5 bbls.
The 'error' involved for each 10 F difference in pipeline cargo temperature is insignificant.

Units of measurement
Different countries will normally utilise metric or imperial units as follows:
Metric (cubic meters):
TOV - Free water = GOV x vcf (tables 54A/54B) = GSV @ 15 C (cubic meters).
Imperial (barrels):
TOV - Free Water = GOV x vcf (tables 6A/6B) = GSV @ 60 F (barrels).

Total observed volume (TOV)
Total observed volume is the total measured volume of all petroleum liquids, including sediment and water (S&W) and free water, measured at the observed temperature and pressure. It is determined by converting the observed ullage readings to volumes using the vessel's tank calibration tables and adjusting the volumes with the necessary trim and list corrections. This is for designated cargo tanks only. The slop tanks may or may not be included, but this must be clearly indicated in the report.

Gross observed volume (GOV)
Gross observed volume is the total measured volume of all petroleum liquids, including S&W, but excluding free water, at observed temperature and pressure.

Net observed volume (NOV)
Net observed volume is the total measured volume of all petroleum liquids excluding S&W and free water, at the observed temperature and pressure. S&W can be estimated by centrifuge, but standard petroleum sale contracts call for S&W measurement by filtration and chemical titration respectively.

Gross standard volume (GSV)
Gross standard volume is the total measured volume of all petroleum liquids and S&W, but excluding free water and corrected by the appropriate temperature correction factor for the observed temperature and API gravity (or density), to a standard temperature, 60 F. (or 15 C.) and also corrected by the applicable atmospheric pressure correction factor.

Net standard volume (NSV)
Net standard volume is the measured volume of all petroleum liquids excluding S&W and free water and corrected by the appropriate temperature correction factor for the observed temperature and API gravity (or density), to a standard temperature such as 60 F (or 15 C and also corrected by the applicable pressure correction factor.
The NSV is the commercially important result of the calculation process, since it is the basis on which the cost of the cargo of oil is determined.

Total calculated volume
Total calculated volume is the GSV plus any free water measured at the observed temperature and pressure.
The TCV is important to the chief officer because it will be the reference quantity against which transit differences will be measured at the discharge port. Since the vessel has no control over the amount of water contained in the cargo, such water will often settle out during the voyage, resulting in a much higher free water quantity at the discharge port than at the loading port. Since free water is not corrected for temperature, this quantity has to be added back to the GSV in order to accurately compare the TCV 'after loading' to the TCV 'before discharge';

Diagram of the process of a cargo quantity calculation.

Total received volume or total delivered volume
The total received/delivered volume is the TCV less any OBQ/ROB found in the designated cargo tanks after discharge is completed. This is the quantity against which the vessel experience factor is applied to determine the accuracy of the bill of lading.
In some countries, the density of the cargo used in these calculations is determined in the laboratory in an artificial vacuum. This method removes the factor of atmospheric pressure from the cargo calculation process, but has two results:
- Quantities determined from densities determined 'in vacuum' (or 'in vacuuo') are greater than the result obtained by the standard method indicated above, and
- The method is not used in most countries and will conflict with the quantities determined on arrival at the discharge port by the normal density measurement method. Comparing the two figures without adjustment or explanation will indicate an apparent (but false), in-transit cargo loss.

In 1980 the petroleum measurement tables were updated with tables 6A and 6B replacing the old table 6. Several countries, including many OPEC members, still use the old table 6 to calculate the bill of lading. The effect of this is to overstate the bill of lading quantity and create an erroneous ship to shore difference.
Calculation of the ship's cargo quantities should be done by individual cargo tank. Calculating cargo by using the total tank volumes and average temperatures and densities will produce a less accurate result due to the different quantities in each cargo tank.

3.29.7 Comparison with shore figures
The master should issue a 'Letter of Discrepancy' for any and all ship to shore differences as well as any free water found on the vessel after loading.
The vessel experience factor (VEF) is used to compare ship and shore figures. Before an accurate ship to shore comparison can be made, the vessel's VEF must be determined. It is important to be aware of just how accurate the VEF is for each individual ship. For example, if the ship frequently loads above and below shore quantities, the resultant factor may be close to 1.0000 and will not be accurate. The method used to determine a vessel's VEF is frequently under review by the API, but currently is as follows:
The VEF is a compilation of the history of the TCV vessel measurements, adjusted for OBQ or ROB, compared with the TCV of shore measurements. Separate VEF's should be developed for both the load and discharge operations. Information used to calculate the VEF should preferably be based on documents that follow accepted industry standards and practices, such as inspection company reports.
Whenever possible, manual vessel measurements should be used for volume determinations. Automatic gauging systems with accuracy/measurement tolerance equal to or better than those of manual measurements may be used for custody transfer by mutual agreement between all interested parties. All calculation procedures, such as those for temperature, volume correction factor, ROB/OBQ, wedge or trim factor, must be consistent. Do not use long or metric tons. Quantities shall be expressed in either barrels or cubic meters at standard temperatures, but shall not be combined.
Information from all load or discharge terminals should be used to calculate the respective VEF's. If the information is available, the VEF can be based on the data from the same load or discharge terminal.
Only voyages that qualify should be used in the compilation of a VEF. A minimum of five qualifying voyages is needed to calculate a VEF, however, a larger number is desirable. The definition of a qualifying voyage is one that meets the following criteria:
1 Any voyage that is within +/- 0.0030 of the average ratio of all voyages listed. (As an example, if the average is all voyages listed is 1.00105, all voyages within the range from 0.99805 through 1.00405 would qualify).
2 Excludes all voyages prior to any structural modification which affected the vessel's cargo capacity.
3 Excludes load or discharge data where shore measurements were not available.
4 Excludes all voyages involving ship-to-ship lightering.
5 Excludes the first voyage after dry docking.
6 Excludes all voyages after non-liquid cargoes.

The VEF should be calculated to five decimal places.
When the shore loading figures and the result of the ship's cargo calculations are available, they should be closely compared. There should be close agreement between the ship figures divided by the VEF and the shore figure. If the ratio of ship/shore falls outside the range of the previous experience or is greater then permitted by the charter party, then a close examination of both shore and ship cargo gauges and calculations must be made. If the source of the discrepancy cannot be determined (and corrected), the master must deliver a letter of protest to the shippers' representative against the difference The presence of water in the cargo is also cause for a master's letter of protest, including a note that 'free water may increase during the voyage'.

Example of vessel experience report.

The vessel cargo temperatures should be checked against the shore tank temperatures. If there is a difference of more than 2 C the following actions should be taken:
- If the vessel cargo temperature is higher than the shore, then:
Give a protest letter to the shore terminal, and
Enter the temperature difference on the vessel's cargo papers.
- If the vessel's temperature is lower than the shore, then:
Enter the temperature difference in the vessel's records, and
If the temperature difference is more than 4 C, request the shore terminal to check the accuracy of the shore temperature measurements. If no error is found or correction made, submit a protest letter to the terminal.

3.29.8 Cargo quantity report
When the cargo figures have been agreed, the petroleum inspector will prepare a report which he will ask the master and/or chief officer to sign. This appropriate officer should sign the report, indicating 'For ullages and temperatures only' and 'protest noted' if one has been made.
The ship will have a corresponding owner's/charterer's document which the independent inspector will be asked to sign. He may refuse to do so, as most will not sign any but their own documents. In any event, the vessel's records of cargo loaded must include the printed names of the independent inspector (along with his company's name), the name of the officer witnessing the cargo measurements and the name of the chief officer.

page top
Hosted by uCoz