- The bunker barge may inject compressed air into its tanks prior to joint soundings being taken to increase the apparent volume of the fuel oil before it is transferred.
- Compressed air may be injected into the fuel oil during the transfer, either in the vicinity of the discharge pump or into the tank or into the discharge line. This may be by using the compressed air equipment designed to blow through the pipelines after discharge, or via a separate system. The stripping of bunker tanks using a positive displacement pump means that air will be drawn into the fuel oil when pumped. Consequently, excessive stripping by the bunker barge may also result in the “cappuccino effect”.
Fuel oil in a double bottom tank displaying frothing and foaming consistent with the cappuccino effect
Signs of the “Cappuccino Effect”
The presence of one or more of the following may indicate that fuel supplied to the vessel contains an excessive amount of air:
- Look for foam and/or frothing on the surface of the fuel oil on the barge prior to bunkering, and on the vessel, while bunkering is taking place and on completion.
- While taking soundings on the bunker barge or on the vessel, check for bubbles on the sounding tape or brass bob.
- If possible, check the bunker barge supply pump and supply pipework prior to pumping and look for any suspect connections, remembering that an air injection line may be quite small.
- Ask the bunker barge crew for details of how they carry out tank stripping and line blowing (where employed) and inspect the line blowing arrangements before bunkering commences.
- Once bunkering has commenced, open the manifold sampling point at regular intervals placing a clean glass receptacle underneath. Check the samples for frothing or an excessive amount of air bubbles.
- Unusual gurgling noises from the supply line or at the manifold may also indicate the presence of air.
- Variations in line pressure at the manifold may be an indication that air has been introduced into the line.
- Monitor the supply hose for unusual movement. When a large quantity of compressed air is passed through a hose containing fuel oil, the hose may tend to jolt or shudder.
- Listen for unusual noises from the fuel tank vent head; ball or float valves may vibrate or rattle if excessive air is present. Also, check whether the amount of air being vented from the fuel tanks appears to be greater than normal.
- Check the density of the fuel oil received. A lower density than expected (taking account of any temperature change) may indicate the presence of air.
If it is suspected that air may have been introduced into the fuel oil, board the bunker barge and ask to see the line blowing arrangements and the air compressor. If these have recently been in use, the compressor and its connections will be warm, while the compressed air delivery line will be cold. Empty compressed air bottles may also provide an indication that the fuel oil has been injected with air.