The vessel, a bulk carrier, arrived in Singapore and began to bunker 900 tons of high sulphur fuel oil. The crew was alerted to the fact that something was wrong by the rattling of the float valves situated inside the fuel tank vent head bonnets.
It was found that air was escaping from the vents at a greater rate than would normally be expected. In addition, the bunker supply hose lying on deck was seen to be jerking violently. Further, when tank soundings were subsequently taken, the sounding tape and brass bob were found to be covered in bubbles of fuel oil.
All of the foregoing symptoms indicated that air had been introduced into the fuel oil resulting in the development of froth and foam on the surface – the so-called “Cappuccino effect”.
This malpractice increases the volume of the fuel and results in artificially high volumetric flow meter readings. When post bunkering soundings are taken it also gives the impression that the amount of fuel in the bunker tank is more than it actually is.
As the air bubbles gradually dissipate, the fuel oil soundings decrease. If a lot of bunkers has been stemmed and subjected to the “cappuccino effect”, the consequential shortfall may be significant. The vessel in question stopped bunkering immediately and tank soundings were taken. Sometime later the tanks were sounded again when it was found that the level had dropped by 15 cms. A visual inspection of the contents of the tanks revealed clear evidence of froth and foam on the surface of the fuel.
Upon completion of bunkering, the difference between the vessel’s figures and those of the bunker barge amounted to 46 tons. Although this resulted in a heated dispute, the Barge Master eventually accepted the vessel’s figures and signed the bunker receipt.
What Causes the “Cappuccino Effect”?
There are several ways in which air may be introduced into fuel oil: