Accuracy considerations in tank soundings and orb volume recording - Oil Record Book


Since there is an, often misconstrued, assumption that ORB measurements are bookkeeping records (as in the dollars have to add up), rather than engineering measurements, very significant disputes can arise when the numbers do not appear to “add up”.

As such, a methodology needs to be established with regard to shipboard tank measurement and recording that passes appropriate and reasonable engineering standards, which then can be applied to ORB volumes.This document establishes such a standard.

Recording conditions truthfully in the ORB can never get anybody in trouble therefore do not interpret and adjust records to make the ORB look “nice”

Basically it says the standard is record to the nearest tenth. So 3.0 + 0.02 = 3.0

The information is +/- 0.4 at best but is recorded to the nearest 0.1 so that results in an inherent inconsistency:

Worst case volume transfer accuracies are expected to be in the range of +/-0.4 cubic meters (+/-0.2 cubic meters level measurement plus +/-0.2 cubic meter conversion accuracy) in optimal conditions (no seaway, etc.). At the same time it is known that measurement accuracies of 0.01 cubic meters are not achievable even on the smallest tanks.

The above indicates that measurement recording to 0.1 cubic meters accuracy is barely achievable, and that on tank volume records, depending on the tank and conditions, the last digit (representing a tenth of a cubic meter, 100 liters) is inherently questionable.

It does seem like it would be helpful to have written guidance aboard that people could follow.

Right. Which is why the numbers should always add up. There are enough tanks onboard that rise as part of normal operations (bilges, purifier sludge tanks, etc.) that there’s no reason to not have the numbers in the ORB add up.

I don’t think that’s what the linked document says.

If there is a switch from a remote to a manual tank measurement (or vice versa) this should be recorded in the sounding log.

If this appears to result in confusion in the ORB a category “I” note should be provided in the ORB.

My understanding here is it’s saying discrepancies due to differences in measurement methods should be documented in the sounding log.

Maybe we’re talking about 2 different things. My scenario is all manual soundings, if you sound a tank at 34cm and it needs to be 32cm to make the change in volume in that tank equal the change in volume of the other tank that you pumped to, then you log it as 32cm. (Assuming this 2cm difference is equivalent to a ~ 0.1 to 0.15 m³ volume). This is made especially easy to do if the tank(s) in question rise due to normal operations like collection of bilges or purifier sludge discharge.

Common sense must be applied… You can’t log that you pumped 1.6m³ into a tank of 1.5m³ capacity for instance.

Additionally I don’t believe this to be a shady practice because as you stated previously in your quoted text that soundings are not an exact endeavor, especially true on a moving ship.

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This is an accountants’ vs engineers’ mindset issue. Accountants count and they expect things to balance at least to the penny, if not the mill – even if the amount is in trillions.

Engineers measure, and they know that the measurements are not going to balance exactly.

That document was written by people with one eye on politics/accountants. They state flat out that anything more precise than a cubic meter is wishing, and then they weasel around it.

If you want to get the accountants off your back, they must be forced to recognize that 1.6+/- 0.4 is the same number as 1.5+/- 0.4. Otherwise they will hang you on stuff like seeing 34 on the stick and writing down 32 in the log.

Here’s the problem, engineers do what they got to do. But Port State Control comes aboard and they are looking for an easy “hit”. And they love the Oil Record Book.

This is why having a set of instructions aboard ship can be helpful. When PSC finds what they think is a discrepancy, if the engineer can show that they have resolved the discrepancy in accordance with a set of written instructions PSC’s easy “hit” goes away. Now they have to show the instructions are wrong. In my experience they will just move on to something else.


There will also potentially be issues with measurement consistencies related to the presence of solid sludge in tanks, tank
evaporation and inherent measurement inaccuracies in tanks that are nearly empty. Such issues should be reasonably
recorded in the ORB once an operator becomes aware of them.
Recording conditions truthfully in the ORB can never get anybody in trouble therefore do not interpret and adjust records
to make the ORB look “nice”.
If there is a switch from a remote to a manual tank measurement (or vice versa) this should be recorded in the sounding
log. If this appears to result in confusion in the ORB a category “I” note should be provided in the ORB.
Since this document is the only known document that provides any type of guidance with regard to ORB recording
accuracies, it is considered to be applicable as of this date until further notice and is considered to be a state of the art

What’s the provenance?

“It is considered” – by whom, and are they the authorities?

Is “cannot get anyone in trouble” wishful thinking? If so, that needs to be reconciled before things can work well.

I wouldn’t use that document, best case scenario the office should write one but I wouldn’t trust them to do it right.

The captain and C/E could write up a ship procedure, sign it and stamp it with the ship’s stamp to make it look official. If it’s a reasonable, common sense in alignment with how things are actually done and is procedure that can be followed easily it should keep the person that actually keeping the ORB out of trouble.

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They would certainly hang you on that if they knew. It’s not like you write “sounding was 34 but I’m gonna log 32”. If this is kept up with, it’s not a problem. If you try to reconcile once a week and find you “need” a sounding of 32 and the tank sounds at 43, now you’ve got a problem.

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isnt sounding a joke as tanks are never checked for volume just a calculation from the NA drawings.
Our ChEng’s always getting in trouble for missing fuel, until they learnt which tanks to pump fuel into to get the right readings on our POS asia built AHTS’s


Thank you. I meant to say earlier "What makes them think that the tanks actually hold what they say they will, or that the compensation curves for tank shape in non rectangular-prismatic ones are correct?

This is from the linked document in the OP:

These inconsistencies do not need to be specifically analyzed by shipboard crews, but inconsistencies that arise in larger quantities (more than a cubic meter in large shallow tanks, or more than 0.3 cubic meters in small tanks) need to be investigated and, if they cannot be resolved aboard the vessel, or reasonable ascribed to tank measurement problems (heavy weather, changed trim, etc.) need to be reported, recorded, and processed through management systems

As long as it’s just the ORB being examined and it is just error, regardless of the source of the error or if the actual source of the error is unknown, a written procedure of how to deal with discrepancies is going to reduce the risk of a problem as low as it practically can be lowered.

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This is not an unsolvable problem.

By way of analogy how are assets and liabilities measured?

With a bank account or debts etc dollars are measured to the nearest 0.01 dollar. So if an account has $6543.21 and I take out $1000.00 it leaves $5543.21. Accurate to the nearest $0.01.

But what about assets like a car. I list it as an asset at say, $6040.00 (by Blue Book) but when I sell it I only manage to get $5000 (turns out condition rated to high)

I presume that accounting practices have a standard method to account for the $1040? A write-off or whatever?

As long as the accountant can show that standard practices were used I think it would be difficult for an auditor to show wrongdoing.

Accountants “mind the GAAP” in accordance with FASB and IRS standards.

GAAP is the Generally Accepted Accounting Principles which are adopted by the Financial Accounting Standards Board.

Oil Record Book written procedures are essential, but it would be much better to have company wide or industry wide GAP (generally accepted procedures) rather than a “different ships, different long splices” approach. Then, the Chief and the Master simply have to “mind the GAP.”


I don’t get directly involved with the ORB but this comes up all the time with bunker amounts. Bunkers are traditionally measured to the nearest 0.1 metric ton. But in fact it’s not unheard of to have a 30 mt error.

We use the gauges and the meters at sea but before bunkering the tanks are manually sounded, corrected for trim and temp. If it’s a small difference we just adjust the burn rate till the error is gone but in some cases we document and just say that the ROB was adjusted after manual sounding. Never had any question that couldn’t be easily answered.

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One commonly found mistake is discrepancy between the tank volumes stated in the Stability Booklet, the IOPP Certificate and what is entered on the fly page of the ORB.

If the first two tally it is usually just a typo and easily fixed by asking the Ch.Eng. to make a correction, initial and log it in the ORB. But I have found a few cases where it has been confusion between Bbls. and Cbm.

Luckily the last is becoming less likely, since metric system is mostly used on all ships.
Confusion between Cbm and Tonnes is still a frequent problem though, especially when doing bunker survey, since bunker fuel is sold in m.t. and sounding tables are in Cbm.

PS> Newer vessels may not even have sounding pipes, so remote gauges and computer programs has to be accepted by all parties.