measurement cargo

7/29/2019 measurement cargo

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CARGO MEASUREMENT AND CALCULATION

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 theconclusion 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 anindependent 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 isobvious 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 measurementswhere the gauger is allowed to use his discretion. The basic rule for shore terminal loadingsor 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 onthe 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, theterminal 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 tankercargo '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 thesurface of the cargo to a datum point in the cargo hatch or in a sounding pipe. This point iscalled 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, thereforeclose attention and care are essential. Errors are normally the combined result of threefactors:

- 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 isfloating 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 theeye 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 aspossible be eliminated from this procedure, so that the ship's cargo measurement willcorrelate properly with the shore figures and vessel's previous loading experience.

3.29.1 Checking equipmentBefore 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 thezero mark on the bob corresponds correctly to the zero point for the tape to which it isattached. 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


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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 portsreduces 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 checkingthem against a standard, certificated thermometer is acceptable.

Electronic thermometers are preferable to cup thermometers with respect to both absoluteaccuracy 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 averagingcalculators 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 temperaturesensors should be standardised annually over their complete working range (underlaboratory 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 correctlevels.

Necessary lights for night gauging, personal protective equipment, breathing apparatus (ifnecessary), hydrocarbon vapour test equipment and the necessary forms for recording themeasurements should all be ready in advance of taking ullage readings.

3.29.2 Confirming draft, trim and listPrior to any cargo ullages being measured, the draft should be accurately read all aroundthe 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 tounexplained, 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 isplaced 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 orlist 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 whenthe 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. Thesecentre tanks are often wider than they are long. Corrections to list are particularly importantin such tanks.

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


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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 becorrected. For small amounts this may require correction by use of the wedge formula.

3.29.3 Measuring cargo ullageTanks 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 containersor 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 aship'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 notbe 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 bewithdrawn slightly and then lowered again, noting the exact reading when the audible signalis first heard.

- Repeat withdrawing and lowering the probe until three readings are obtained which allagree 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 inspectorand 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 agreemust be re-measured.


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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, eitherby 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 bobhas 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 inthe 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 ismade 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 forthe 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 loadingremain 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 thepetroleum 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 hisautomatic gauges.


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3.29.4 Ullage measurements at sea berthsIf 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 measurementsmay 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 mostcorrect 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 weatherand sea conditions at the time of measurement should be carefully noted on the ullagerecord and in the logbook.

3.29.5 Temperature measurementsTemperature measurements should be made with procedures and equipment conforming as

closely as possible to the recommendations of the American Society for Testing andMaterials (ASTM) and the American Petroleum Institute (API). Errors in cargo tanktemperature 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 beavoided. 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 combinedwith 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 hasstabilised and the reading can then be recorded in memory (by pushing a button). If this


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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 metersof liquid depth require three temperature measurements, one each at 1 meter from top and

bottom and one atthe middle.

Another standard, Institute of Petroleum IP 204/86 recommends temperaturemeasurements 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. Ifthe 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 theaverage. If not, more temperature observations are recommended (but in practicalapplication are rarely taken).

3.29.6 Calculating cargo quantityWhen the cargo density at 15 C (or the API gravity at 60 F, if using Imperial units), alltank 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/stabilitycomputer.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), aredetermined 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.

PipelinesAllowance should also be made for pipeline or duct keel volumes to be included in thecalculations 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 thecorrection factor of cargo in the pipelines. For pipelines external to the cargo tanks, theaverage cargo temperature may be used at the loadport. At the discharge port, the averagecargo 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.

ExampleConsider a vessel with duct keel of volume 1000 barrels filled with crude oil of 30.0 APIgravity 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).


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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 andpressure. 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 listcorrections. 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, includingS&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 bycentrifuge, but standard petroleum sale contracts call for S&W measurement by filtrationand 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 forthe 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 waterand corrected by the appropriate temperature correction factor for the observedtemperature 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 basison which the cost of the cargo of oil is determined.

Total calculated volumeTotal 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 againstwhich transit differences will be measured at the discharge port. Since the vessel has nocontrol 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 tobe added back to the GSV in order to accurately compare the TCV 'after loading' to the TCV'before discharge';


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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


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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 thelaboratory 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 thetwo 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 replacingthe 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 densitieswill produce a less accurate result due to the different quantities in each cargo tank.

3.29.7 Comparison with shore figuresThe 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 beclose 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 OBQor 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 VEFshould preferably be based on documents that follow accepted industry standards andpractices, 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 mutualagreement between all interested parties. All calculation procedures, such as those fortemperature, 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 cubicmeters 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 loador 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 anexample, 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 cargocapacity.


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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 ofthe previous experience or is greater then permitted by the charter party, then a closeexamination 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 waterin the cargo is also cause for a master's letter of protest, including a note that 'free watermay increase during the voyage'.


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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 reportWhen 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 signthe 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 independentinspector 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 printednames of the independent inspector (along with his company's name), the name of theofficer witnessing the cargo measurements and the name of the chief officer.

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