20th January 2012 05:05 GMT

2011 saw a sharp rise in claims of entrained air in bunkers. Entrained air, or the 'cappuccino effect' as it is more commonly known, is an increasingly cited phenomenon by fuel buyers that occurs during bunkering. However, there appears to be a lack of understanding regarding what constitutes entrained air in bunkers. The confusion in the industry today seems to stem from whether the presence of air bubbles seen in fuel upon delivery is normal or excessive.

Entrained air may occur for example due to the practice of tank stripping, which is the process of pumping from the bottom of a bunker tank in order to empty it which can produce entrapped bubbles in fuel. Consequently if tank stripping is not controlled effectively this can make a fuel delivery appear to be of a larger volume (and the derived fuel mass) than it actually is.

Air-blowing (also known as purging) after bunkering is also a standard industry practice to aid in cleaning of the bunker delivery hose before being detached from the receiving ship to help avoid any oil spill. Although this practice is common, there have been cases where this process has been abused. Therefore it is of utmost importance to distinguish between genuine instances of aerated bunkers, and occurrences where there is intent to abuse existing standard industry bunkering procedures.

The potential economic losses when entrained air is deliberately introduced into fuel can be substantial and it is understandable that ship owners might be concerned about cappuccino bunkers especially in today's climate of high fuel costs and depressed earnings capacity by ship operators. However, it is important for fuel buyers to understand that tank stripping and air blowing at the end of the bunker delivery are common industry practices, as far as they are effectively controlled and not done excessively.

Bunker surveyors will measure the quantity of fuel of the nominated bunker tanks of the barge prior to delivery at which point entrained air if encountered and observed would be reported. Together with the closing sounding of the bunker delivery barge an industry acceptable accurate quantity figure can be derived, which would constitute the delivered bunker quantity. Fuel surface air bubbles may be observed by the receiving ship after bunker delivery, however, these should be considered superficial in nature after taking into account tank stripping and hose clearing. Frequent gauging of the receiving tanks during bunkering is one of the methods that can give a clear sign of any air being introduced as accumulated air bubbles can be seen on the gauging tape.

Notwithstanding the presence of an independent surveyor during bunkering the following warning signs would indicate something is amiss with a bunker delivery to the crew onboard the receiving ship.
-    Bunker delivery hose jerking.
-    Gurgling sound when standing in vicinity of bunker manifold.
-    Fluctuation of pressure indication on the manifold pressure gauge.
-    Fluctuations of level indication gauge of bunker tanks.
-    Excessive bubbles observed on the sounding tape prior and after delivery.
-    Slow delivery rates, when air is being introduced during pumping.

If several of the above warning signs are observed, the ship's crew should, preferably with the help of an independent surveyor, launch a thorough investigation to get to the root cause of any potential malpractice of entrained air during bunkering operations.

It is worth reiterating that it is fairly common for the bunker delivery hose to jerk and for superficial air bubbles to be observed following hose clearing and these should not be deemed automatic indicators of foul play. If the opening gauge is performed correctly, the quantity would have been measured prior to the hose clearing and/or tank stripping. Hence, any froth observed after would not have any bearing on the quantity delivered as such.

Working with surveyors to measure bunker quantity is one of the key measures of preventing misunderstanding. It is also important that any concerns are flagged as soon as they are noticed and shortly after delivery. It goes without saying that surveyors are independent parties whose first priority is to ensure the accurate measurement of fuel quantity.

Douglas Raitt,
20th January 2012 05:05 GMT

Comments on this Blog
25th January 2012
Interesting article and particularly given that I am involved in the investigation of a number of "cappuccino effect" issues at the moment. Most of the bunker delivery shortfalls tend to be around 20mt to 50mt but I have one particular case that involves a shortage of 250mt and this represents around 30% of the total product alleged to have been supplied. There are rumours that Nitrogen is somehow introduced into the fuel at or prior to delivery and I would be interested to know if there have been any similar reports.
Douglas Raitt - Lloyd's Register Asia
31st January 2012
Hi Steve, to our knowledge there have not been similar reports. It is hard to see why anyone would resort to such a tactic as Nitrogen is a high premium product and there would appear to be little benefit in using nitrogen as opposed to (free) air in such situations. Air, in any case, consists of 79% nitrogen after all. If this in an actual scam it would have to be quite sophisticated in order to have the required nitrogen on hand. However, it's difficult to fathom a good reason why anyone would go through all that trouble. It may be the case if the nitrogen takes much longer to migrate out of the oil (till the receiver is well over the horizon), but even so, any sample drawn out of the bunkers would still have the appearance of an aero chocolate bar.

Of course, since any Residual Fuel Oil can be 0.2% - 0.8% (even over 1%) elemental nitrogen, anything up to those limits should not be taken as a sign that nitrogen has been deliberately introduced into the fuel.
18th March 2012
Many thanks for your reply/comments Douglas and yes, I would agree that using Nitrogen seems to be a rather pointless and indeed difficult means of inflating the product. We are presently carrying out tests using various gases and chemicals as we now have a very similar case involving a shortage of 330mt on a delivery of 1750mt. This is clearly becoming a somewhat serious issue that may well lead to significant consequences involving bunkering operations at Singapore.
Jerry Wang - Ascenz Solutions Pte Ltd
30th March 2012
Emerson mass bunker flow meter can take care of aeration issue in bunkering.
Oleg Micevic - Cross Keys Group Pte Ltd
11th April 2012
Dear Steve and Jerry,

I dont think it is necessary to have a mass flow meter to detect a loss of 20% of delivered bunker fuel.
Also, with losses on that scale it would be probably wiser to investigate the individuals involved in this transaction on all sides and report it to the relevant authorities.
Robert Markoja - CiDRA Corporate Services Inc.
12th April 2012
This is a very interesting blod. I have had 2 inquiries within 3 days regarding air entrainment in bunker fuels. Both inquiries were from companies interested in determining the amount of air in the bunker fuel that became apparent to them after a couple of days following natural de-aeration or settling of the oil. The CiDRA SONARtrac GVF-100 Gas Volume Fraction meter can be used to assess the amount of entrained air in bunker oil during transfer to ship. The GVF-100 clamps on the outside of a pipe and is able to provide real time entrained air measurements in the range of 0-20% air / gas by volume. This equipment has been used to correct positive displacement flow meters used for crude custody transfer and with coriolis meters used on 2-phase separators for oil field applications.
Shishir Dutt - Tolani Maritime Institute
14th April 2012
Dear Robert ,
Can you please comment on the initial cost , working principle , size and operation of this GVF-100 gas volume fraction meter . Which company has designed the same and it will be great if you can cite some examples where it has actually been on continuous use during each bunkering operation
17th April 2012
Hi Robert and Jerry. Whilst I am sure the equipment may help in reducing losses due to air or gas inflation of fuel delivered to a vessel, the crux of the issue is that it is something which is clearly happening, mainly at Singapore I believe, and absolutely nothing is being done about it. I have six shortage cases on my desk at the moment and doubtless there are many more out there. What is needed is a forum whereby shipowners and shipcharterers who purchase fuel and then encounter such shortfalls, can identify the suppliers, supplying barges and surveyors involved. Of the six cases on my desk, two involve the same bunker barge and it may be that there are many other similar cases out there with that particular barge.

If anyone has any similar cases or experiences please email me at steve@findlaymarine.com
Louie Tordillo
2nd April 2015
Foaming and air entrainment may be classified into two distinct events for clarity. Failure of air molecules to break out of the liquid surface results in foaming. Air entrainment happens due to common factors like a leak in the fluid line before the pump, limited reservoir settling and etc. Foaming is more common in lubricating oils but on rare occasions can also occur in fuels, mostly on heavier grades. I suggest you test for foam using ASTM D892, Sequence 1 and compare the results from the supplier source tank and the delivered product samples.
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